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POLLUTED INSULATORS :

A REVIEW OF CURRENT KNOWLEDGE

Task Force 33.04.01

June 2000



POLLUTED INSULATORS :

A REVIEW OF CURRENT KNOWLEDGE

PREPARED BY

Task Force 33.13.01 (fo rm erly 33.04.01 )

of Working Group 33.13

(DIELECTRIC STRENGHT OF INTERNAL AND EXTERNAL INSULATION)

MEMBERS OF TASK FORCE 01 OF WORKING GROUP 33.04 :

D.A. SWIFT (Convenor, United Kingdom), J.P. REYNDERS (Secretary, South Africa),C.S. ENGELBRECHT (Compiler of documents, South Africa), J.L. FIERRO-CHAVEZ(Mexico), R. HOULGATE (United Kingdom), C. LUMB (France), R. MATSUOKA (Japan),G. MELIK (Australia), M. MORENO (Mexico), K. NAITO (Japan), W. PETRUSCH (Germany),

A. PIGINI (Italy), G. RIQUEL (France), F.A.M. RIZK (Canada)



1999-09-01 I

TABLE OF CONTENTS

1. INTRODUCTION.................................................. ........................................................... ................................................. 1

1.1 THE POLLUTION PROBLEM .................................................... ........................................................... ............................. 11.2 PREVIOUS REVIEW DOCUMENTS...................................................... ........................................................... ................... 11.3 R ELEVANCE OF IEC 815 (1986) ..................................................... ........................................................... ................... 21.4 I NSULATOR TYPES AND DEFINITIONS OF SPECIFIC CREEPAGE LENGTH & SPECIFIC AXIAL LENGTH............................... 21.5 APPROACH FOR INSULATOR SELECTION AND DIMENSIONING .................................................. ....................................... 3

2. POLLUTION FLASHOVER PROCESS.................................................... ............................................................ ......... 5

2.1 I NTRODUCTION ........................................................... ........................................................... ....................................... 52.2 MODELLING ...................................................... ........................................................... ................................................. 6

2.2.1 Hydrophilic surface ............................................................................................................................................. 6

2.2.2 Hydrophobic surface.......................................................................................................................................... 10

2.3 E NVIRONMENTAL ASPECTS................................................... ........................................................... ........................... 102.3.1 Climates or atmospheric variables and typical environments ........................................................................... 10

2.3.2 Type of pollution ................................................................................................................................................ 13

2.3.3 Mechanisms of contamination accumulation on insulators...............................................................................21

2.3.4 Mechanisms of wetting.......................................................................................................................................24

2.3.5 The natural cleaning processes.......................................................................................................................... 29

2.3.6 Critical wetting conditions................................................................................................................................. 29

2.3.7 Effect of various aspects of the insulator on its pollution accumulation ...........................................................29

2.3.8 Physical and mathematical models of pollution deposit .................................................................................... 33

2.4 ICE AND SNOW ............................................................ ........................................................... ..................................... 332.4.1 Flashover on insulators covered with ice. .........................................................................................................34

2.4.2 Flashover on insulators covered with snow....................................................................................................... 35

3. INSULATOR CHARACTERISTICS ......................................................... ............................................................ ....... 37

3.1 I NTRODUCTION ........................................................... ........................................................... ..................................... 373.2 MATERIALS USED FOR OUTDOOR INSULATORS ................................................... ......................................................... 383.2.1 Porcelain and glass............................................................................................................................................ 38

3.2.2 Polymers ............................................................................................................................................................ 38

3.3 I NSULATOR PERFORMANCE................................................... ........................................................... ........................... 393.3.1 Ceramic insulators............................................................................................................................................. 40

3.3.2 Polymeric Insulators..........................................................................................................................................50

3.3.3 Effect of insulator orientation............................................................................................................................52

3.3.4 Influence of a non-uniform pollution deposit.....................................................................................................56

3.3.5 Electric field at the surface of insulators ...........................................................................................................57

3.3.6 Cold switch-on and thermal lag.........................................................................................................................59

3.3.7 Contaminated insulators under transient overvoltages .....................................................................................59

3.3.8 Air density correction factors for polluted insulators........................................................................................ 68

3.3.9 General trends for ice covered insulators.......................................................................................................... 693.3.10 General trends for snow covered insulators ...................................................................................................... 71

3.4 SPECIAL INSULATORS ............................................................ ........................................................... ........................... 733.4.1 Hollow insulators...............................................................................................................................................73

3.4.2 HVDC wall bushings.......................................................................................................................................... 75

3.4.3 Circuit breaker and isolator insulation..............................................................................................................75

3.4.4 Insulators in desert conditions...........................................................................................................................76

3.4.5 Semiconducting Glaze insulators....................................................................................................................... 76

3.5 CONCLUSIONS ................................................... ........................................................... ............................................... 77

4. ENVIRONMENTAL IMPACT.......................................................... ........................................................... ................. 80

4.1 VISIBLE DISCHARGES ............................................................ ........................................................... ........................... 80

4.2 AUDIBLE NOISE ........................................................... ........................................................... ..................................... 804.3 R ADIO INTERFERENCE........................................................... ........................................................... ........................... 814.4 TELEVISION INTERFERENCE ............................................................ ........................................................... ................. 82



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4.5 CORROSION OF METAL HARDWARE - TELEVISION INTERFERENCE.................................................... ............................ 824.6 CRITERIA FOR RADIO NOISE LIMITS OF INSULATORS..................................................... ................................................ 834.7 CORROSION OF METAL HARDWARE - MECHANICAL STRENGTH REDUCTION......................................................... ........ 844.8 FIRES ..................................................... ........................................................... .......................................................... 85

5. POLLUTION MONITORING.......................................................... ........................................................... .................. 87

5.1 I NTRODUCTION .......................................................... ........................................................... ...................................... 875.2 AIR POLLUTION MEASUREMENT ..................................................... ........................................................... .................. 88

5.2.1 Directional dust deposit gauge .......................................................................................................................... 88

5.3 EQUIVALENT SALT DEPOSIT DENSITY (ESDD)................................................... .......................................................... 895.3.1 Advantages.........................................................................................................................................................89

5.3.2 Disadvantages.................................................................................................................................................... 89

5.3.3 Further developments ........................................................................................................................................ 89

5.4 NON-SOLUBLE DEPOSIT DENSITY (NSDD) ........................................................ .......................................................... 905.4.1 Optical measurement .........................................................................................................................................90

5.5 SURFACE CONDUCTANCE ..................................................... ........................................................... ............................ 905.5.1 Advantages.........................................................................................................................................................90

5.5.2 Disadvantages.................................................................................................................................................... 90

5.5.3 Further developments ........................................................................................................................................ 90

5.6 I NSULATOR FLASHOVER STRESS..................................................... ........................................................... .................. 915.6.1 Advantages.........................................................................................................................................................91

5.6.2 Disadvantages.................................................................................................................................................... 91

5.7 LEAKAGE CURRENT ................................................... ........................................................... ...................................... 915.7.1 Surge counting ................................................................................................................................................... 92

5.7.2 I highest ....................................................... ........................................................... ................................................ 92

5.8 CONCLUSIONS .................................................. ........................................................... ................................................ 92

6. TESTING PROCEDURES FOR INSULATORS ......................................................... ................................................ 93

6.1 I NTRODUCTION .......................................................... ........................................................... ...................................... 936.2 CATEGORIES OF TEST METHODS ..................................................... ........................................................... .................. 93

6.2.1 Testing under natural pollution conditions........................................................................................................ 93

6.2.2 Artificial pollution laboratory tests.................................................................................................................... 956.3 TEST PROCEDURES FOR PORCELAIN AND GLASS INSULATORS TO BE USED IN HIGH -VOLTAGE A.C. OR D.C. SYSTEMS ...956.3.1 Standardised test procedures.............................................................................................................................95

6.3.2 Non-standardised test procedures...................................................................................................................... 96

6.3.3 Non-standardised test procedures for laboratory tests on naturally polluted insulators .................................. 98

6.4 TEST PROCEDURES FOR POLYMERIC INSULATORS TO BE USED IN HIGH-VOLTAGE A.C. OR D.C. SYSTEMS.....................986.5 TEST PROCEDURES FOR INSULATORS COVERED WITH ICE OR SNOW........................................................... .................. 98

6.5.1 Laboratory test methods with ice .......................................................................................................................98

6.5.2 Laboratory test methods with snow.................................................................................................................. 100

6.6 ADDITIONAL INFORMATION ON PARTICULAR POINTS OF POLLUTION TESTING ...................................................... ...... 1006.6.1 Ambient conditions during testing ................................................................................................................... 100

6.6.2 Leakage current measurement......................................................................................................................... 103

6.6.3 Testing of insulators for the UHV range up to 1100 kV...................................................................................104

6.6.4 Comparison of test results obtained with different pollution test methods ...................................................... 1046.6.5 Comparison of test results obtained from test stations ....................................................................................104

7. INSULATOR SELECTION AND DIMENSIONING .................................................. .............................................. 106

7.1 I NTRODUCTION .......................................................... ........................................................... .................................... 1067.2 SELECTION OF I NSULATOR CHARACTERISTICS .................................................. ........................................................ 106

7.2.1 Selection of profile...........................................................................................................................................107

7.2.2 Selection of insulator dimensions..................................................................................................................... 107

7.2.3 Deterministic method ....................................................................................................................................... 108

7.2.4 Probabilistic method. ....................................................................................................................................... 108

7.2.5 Static and dynamic methods in the probabilistic approach. ............................................................................ 109

7.2.6 Present status of the probabilistic approach.................................................................................................... 110

7.2.7 Dynamic method .............................................................................................................................................. 113

7.2.8 Truncation of the distribution .......................................................................................................................... 114

7.2.9 Conclusions......................................................................................................................................................115



1999-09-01 I I I

7.3 SELECTION OF INSULATORS FOR APPLICATION UNDER ICE AND SNOW ........................................................ ............... 1157.4 SELECTION OF INSULATORS FOR D.C. ENERGISATION..................................................... ............................................ 116

7.4.1 Introduction ....................................................... ............................................................ .................................. 116

7.4.2 Selection of a site severity correction factor........................................................ ............................................ 116

7.5 I NSULATOR POLLUTION DESIGN OF PHASE-TO-PHASE SPACERS ....................................................... ......................... 1177.5.1 Introduction ....................................................... ............................................................ .................................. 117

7.5.2 Design Practice.............................. ............................................................ ...................................................... 117

8. PALLIATIVES AND OTHER MITIGATION MEASURES............................................................ ........................ 118

8.1 I NTRODUCTION ........................................................... ............................................................ .................................. 1188.2 MAINTENANCE PROCEDURES .......................................................... ........................................................... ............... 118

8.2.1 Live-insulator washing of ceramic insulators ...................................................... ............................................ 118

8.2.2 Live-insulator washing of polymeric insulators................................................... ............................................ 128

8.3 USE OF GREASES AND RTV COATINGS ...................................................... ........................................................... ..... 1298.3.1 Introduction ....................................................... ............................................................ .................................. 129

8.3.2 Hydrocarbon and silicone greases ............................................................ ...................................................... 129

8.3.3 RTV rubber coatings .................................................... ............................................................ ........................ 130

8.3.4 Summary .................................................. ............................................................ ............................................ 130

8.4 BOOSTER SHEDS ......................................................... ............................................................ .................................. 1318.5 METHODS FOR INCREASING INSULATOR RELIABILITY UNDER ICE AND SNOW CONDITIONS......................................... 132

8.5.1 Some measures to prevent flashovers during ice conditions................................ ............................................ 132

8.5.2 Some measures to prevent flashovers during snow conditions ......................................................... ............... 133

9. THERMAL EFFECTS OF CONTAMINATION ON METAL OXIDE ARRESTERS (MOA) ............................ 134

9.1 I NTRODUCTION ........................................................... ............................................................ .................................. 1349.2 OPERATIONAL EXPERIENCE AND FIELD TESTS.................................................... ...................................................... 1349.3 ARTIFICIAL POLLUTION TESTS OF LIGHTNING ARRESTERS..................................................... ................................... 135

9.3.1 Test Techniques........................................ ............................................................ ............................................ 135

9.3.2 Laboratory Test Results ......................................................... ............................................................ .............. 135

9.4 STANDARDISATION OF A LABORATORY TEST...................................................... ...................................................... 139

10. ADITIONAL INFORMATION AND RESULTS ....................................................... ............................................ 142

10.1 I NSULATOR PROFILES AND DIMENSIONS .................................................... ........................................................... ..... 14210.2 R ANKING OF INSULATORS ..................................................... ............................................................ ........................ 158

10.2.1 Ceramic Insulators......................... ............................................................ ...................................................... 158

10.2.2 Polymeric insulators .................................................... ............................................................ ........................ 162

10.3 I NSULATOR PERFORMANCE AS A FUNCTION OF POLLUTION SEVERITY ........................................................ ............... 16410.4 AGEING OF I NSULATORS ....................................................... ............................................................ ........................ 165

11. REFERENCES....................................................................... ............................................................ ........................ 1 67



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1. INTRODUCTION

1.1 The Pollution problem

The performance of insulators used on overhead transmission lines and overhead distribution lines, and in outdoor substations

is a key factor in determining the reliability of power delivery systems. The insulators not only must withstand normaloperating voltage, but also must withstand overvoltages that may cause disturbances, flashovers and line outages. Thereduction in the performance of outdoor insulators occurs mainly by the pollution of the insulating surfaces from air-bornedeposits that may form a conducting or partially conducting surface layer when wet.

The presence of a conducting or partially conducting layer of pollution on the insulator surface will dictate flashover performance. It is impractical in many situations to prevent the formation of such a layer and consequently insulators must bedesigned so that the flashover performance remains high enough to withstand all types of anticipated voltage stresses despitethe presence of the pollution layer. In certain situations where pollution is extremely severe, further preventative or curativemeasures - such as periodic washing or greasing - may be necessary.

It is clear that the environment, in which the insulator must operate, together with the insulator itself, will determine theseverity of the pollution layer on the insulator. Translating the environment into parameters that can be used to design theinsulation, therefore, presents one of the fundamental problems in designing external insulation with respect to pollutedconditions. This is due to the vast range of possible conditions such as those found in coastal, industrial, agricultural anddesert areas; also in areas with ice and snow or at high altitude. Combinations of these conditions may also occur. A further complicating factor is that environments have an inherent statistical behaviour that is to a large extent unpredictable.Furthermore, the increase of available electrical energy in an area, through the construction of a new substation, may trigger industrial growth that can contribute to the pollution and affect thus the behaviour of the insulation. It is, therefore, difficult toquantify the effect of the environment on insulator performance.

This document attempts to address these problems by serving as a review of current knowledge on insulator pollution with theintention of providing information for the selection and maintenance of insulators in polluted environments. A very extensivelist of references is provided.

It is recognised that ageing may influence the performance of insulators, particularly in the case of polymer insulators.However, this report is restricted to discussing the pollution performance of insulators, since Cigré Study Committees 15 and

22 are mandated to deal with material and insulator ageing.

1.2 Previous review documents

The first large-scale review of pollution effects on insulators was published in 19711. That document describes theories of theflashover process as well as artificial and natural test methods for assessing insulator performance in pollution conditions.Various parameters that influence insulator performance, such as surface conductance and insulator dimensions, are alsodiscussed. Furthermore, several methods for measuring pollution severity are described and preventative procedures such asgreasing are reviewed.

In 1979, a major review on insulator pollution was published as two separate reports: one on the measurement of pollutionseverity2 and the other as a critical comparison of artificial pollution test methods3.

The report on pollution severity measurement analysed the main methods in use in terms of the pollution flashover process.The conclusion was that there is no single best method but rather that the best results are obtained when several methods areused in parallel. Factors pertaining to the equipment - i.e. cost, availability, etc. - and the power delivery system - i.e. extent,voltage level and type, etc. - were identified as being important for selecting a pollution site severity measurement method. Itwas noted that the cost of optimisation also should be weighed against the cost of a detailed site severity assessment beforesuch measurements are undertaken.

The report on artificial pollution test methods gave an analysis of available test methods with the intent of indicating whichmethods are best suited for international standardisation. This report also recommended the natural conditions bestrepresented by each test method.

Another report4 combined the experience of electric utilities, manufacturers, and research laboratories in a comprehensivesummary on the design and maintenance of outdoor insulators in polluted environments. In addition to providing adescription of the flashover process, this report also contains discussions on pollution severity measurement, test procedures,

design practice, and maintenance procedures.



1999-09-01 2

1.3 Relevance of IEC 815 (1986)

The present edition of IEC Publication 815 (1986)5 is based on knowledge obtained mainly from experience withconventional porcelain and glass insulators on a.c. systems. It applies only to these insulators, and only when they are used ina.c. applications.

Minimum specific creepage distances are specified in this document for different pollution severity levels. These pollution

severity levels do not consider all aspects of the environment that can affect the performance of various insulator profiles.Apart from some restrictions on insulator profile and corrections for diameter, IEC 815 thereby implies that no other factorsneed to be considered when designing insulators for use in polluted conditions.

It is now recognised that a broader approach for insulator design and selection is required to address the optimised design of porcelain and glass insulators as well as polymeric insulators for a.c. and d.c. systems world-wide. Other areas where IEC815 lacks information have been identified.

This review document is based on the following list of areas where IEC 815 is perceived to be weak, and where input isneeded for its revision:

• Performance of polymeric insulators

• Insulator orientation

• Extension of applicability to voltages above 525 kV a.c.• Design for d.c. application

• Insulators with semiconducting glaze

• Surge arrester housing performance, particularly with reference to polymeric materials

• Longitudinal breaks in interrupter equipment

• Radio interference, television interference, and audible noise of polluted insulators

• Effect of altitude

• Effect of heavy wetting

The revision of IEC 815 was started in 1998 and it is expected that the work will be completed by the end of the year 2005.The revision will appear as five parts under the number ‘IEC 60815’.

1.4 Insulator types and definitions of Specific Creepage Length & Specific AxialLength

For the purpose of this document, insulators are divided into the following four broad categories:

1. Ceramic insulators for a.c. systems

2. Polymeric insulators for a.c. systems

3. Ceramic insulators for d.c. systems

4. Polymeric insulators for d.c. systems.

Ceramic insulators have an insulating part manufactured either of glass or porcelain, whereas polymeric insulators have acomposite insulating part consisting of a polymer housing such as Silicone Rubber (SR), Ethylene Propylene Diene Monomer

(EPDM) and others, fitted onto a glass fibre core.In Section 10, details are given of some of the available types of insulators. The tables presented therein are used throughoutthis document to identify insulators and provide data for analysis.

For the purpose of this review, the electrical stress over an insulator is considered in two ways; one is related to the leakage path length and the other to the axial length of the insulator.

In IEC 815, the leakage path of an insulator is specified by the ‘Specific Creepage Distance’ defined as the leakage distanceof the insulator in mm divided by the maximum system phase-to-phase voltage in kV. The Leakage Distance is defined as theshortest distance, from on end of the insulator to the other, along the surface of the insulating parts. In this document, theSpecific Creepage Length (SCL) defined as the Leakage Distance of the insulator divided by the actual voltage across the

insulator - i.e. the phase-to-ground voltage in most instances.

The corresponding Specific Axial Length (SAL) of an insulator is defined as the axial length of the insulator divided by theactual vol tage across the insulator . The axial length refers to the shortest distance between fixing points of the live and



1999-09-01 3

earthed metalware, ignoring the presence of any stress control rings, but including intermediate metal parts along the length of the insulator - as is shown in Figure 1-1.

Axial Length

Figure 1-1: Definition of axial length of an insulator as is used in this review.

1.5 Approach for insulator selection and dimensioning

The process of insulator selection and axial dimensioning together with its influencing parameters is shown in Figure 1-2.The flow chart in this figure forms the basis of this review document for which an overview is given below.

The process of insulator selection starts with the collection of the basic data consisting of information on:1. Insulator application

2. Insulator characteristics

3. Power system parameters

4. The environment

5. Constraints.

6. Field performance

1. The application of the insulator is an important aspect from the pollution performance viewpoint as it determines boththe radial dimension and the orientation of the insulator. Section 3 addresses the application of insulators under a variety of

headings.2. An integral part of the basic data is the characteristics of the available insulators. These are discussed throughout thisreport, but especially in Section 3. Information may also be obtained from manufacturers.

3. Power system parameters that form part of the basic data consist of:

• The electrical environment in which the insulator is applied, i.e. a.c. or d.c. voltage; maximum system voltage; andlightning, switching and temporary overvoltages and their effects on insulator performance. These aspects arecomprehensively addressed in Section 2.2 and Section 3.

• The performance required from the insulator. This is determined mainly by power quality criteria such as the power system’s sensitivity to outages.

4. Each environment where the insulators are to be installed has a different set of conditions under which the insulator mustoperate reliably. An insulator that has a good performance under one set of conditions might have a bad performance in adifferent set of conditions. It is therefore necessary to characterise the environment in terms specific to insulator performance.In Section 2.3, the environmental aspects and how they affect the pollution flashover process are discussed. Methods tomonitor the environment are described in Section 5.

5. Constraints may also influence the selection of insulators. For example, limitations on the width of the right of way maydictate the use of structures for which special insulator designs are required. In such cases, the range of available insulatorsmay be restricted. Cost and the need to minimise the visual impact may also be important factors that have to be built into theselection process.

6. Field performance of insulators in service is an invaluable source of data for future applications. Unfortunately, thesedata are not always available, and, as noted earlier, their applicability to different environments must always be questioned.

Nevertheless, service experience is usually a very important component of the basic data since it forms the basis for determining whether the selection of a particular insulator leads to acceptable performance. Service experience also may

indicate whether certain artificial pollution tests are appropriate for a specific environment, and it may also contributeinformation on insulator characteristics.



1999-09-01 4

Methods to assess insulator field performance are given in Section 5. References to service experience are given throughoutthe document, but especially in Sections 2 and 3.

5) Constraints4) Environment3) Power System parameters

2) Insulator Characteristics

Basic data

Alternative solutions

Field testsnecessary?

Field test station

Test program

Test results

Lab testsnecessary

?RepresentativeTest technique

Lab testing

Test results

Design Procedure

Deterministic?

PreliminaryDesign

Acceptable

Failure rate?

PreliminarySolution

Cost optimisation

PreventativeMeasures

?Identifymeasures

Insulator selection

Insulator monitoring

NoYes

NoYes

Yes No

Yes No

Yes No

6) Insulator field performance

1) Insulator application

Figure 1-2: An overview of the process of insulator selection, as based on a published6 diagram.

Once these basic data are collected, the various options for insulator selection can be identified for further study. Dependingon whether or not information is available on service experience, insulator characteristics and the environment, the need for further field tests should be determined. However, it should be noted that these tests normally take 2-4 years. An overview of the available methods for site severity measurement and field tests is given in Section 5.

Since the time required for field tests is very long, such tests are usually augmented with laboratory tests. A brief overview of laboratory test methods and some examples of field test stations are given in Section 6.

When the basic data and field and laboratory test results have been compiled, the actual design procedure - as described inSection 7 - can begin. The choice between a deterministic or statistical approach will depend on the criticality of the design.

Economic and time constraints may dictate a shortened selection procedure with the possible concomitant reduction inconfidence in the design.

In the event that a reliable insulator design is not achieved, mitigation methods may be necessary. Examples of such methodsare given in Section 8.

Improvement in the design procedure requires verification of performance that also will provide further service experience for future designs.



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2. POLLUTION FLASHOVER PROCESS

2.1 Introduction

The pollution flashover process of insulators is greatly affected by the insulator’s surface properties. Two surface conditions

are recognised: either hydrophilic or hydrophobic. A hydrophilic surface is generally associated with ceramic insulatorswhereas a hydrophobic surface is generally associated with polymeric insulators, especially silicone rubber. Under wettingconditions - such as rain, mist etc. - hydrophilic surfaces will wet out completely so that an electrolyte film covers theinsulator. In contrast, water beads into distinct droplets on a hydrophobic surface under such wetting conditions.

In the Electra No. 64 publication2, the pollution flashover process for ceramic insulators - that is, insulators with a hydrophilicsurface - is described essentially as follows:

a) The insulator becomes coated with a layer of pollution containing soluble salts or dilute acids or alkalis. If the pollutionis deposited as a layer of liquid electrolyte - e.g. salt spray, stages (c) to (f) may proceed immediately. If the pollution isnon-conducting when dry, some wetting process (stage (b)) is necessary.

b) The surface of the polluted insulator is wetted either completely or partially by fog, mist, light rain, sleet or melting snowor ice and the pollution layer becomes conductive. Heavy rain is a complicating factor: it may wash away the electrolytic

components off part or all of the pollution layer without initiating the other stages in the breakdown process, or it may - by bridging the gaps between sheds - promote flashover.

c) Once an energised insulator is covered with a conducting pollution layer, a surface leakage current flows and its heatingeffect starts to dry out parts of the pollution layer.

d) The drying of the pollution layer is always non-uniform and, in places, the conducting pollution layer becomes broken bydry bands that interrupt the flow of leakage current.

e) The line-to-earth voltage is then applied across these dry bands, which may only be a few centimetres wide. It causes air breakdown to occur and the dry bands are bridged by arcs, which are electrically in series with the resistance of theundried portion of the pollution layer. A surge of leakage current occurs each time the dry bands on an insulator sparkover.

f) If the resistance of the undried part of the pollution layer is low enough, the arcs bridging the dry bands are able to burncontinuously and so may extend along the insulator; thereby spanning more and more of its surface. This in turndecreases the resistance in series with the arcs, increases the current and permits the arcs to bridge even more of theinsulator surface. Ultimately the insulator is completely spanned and a line-to-earth fault is established.

Figure 2-1: Schematic representation of the pollution flashover process across a hydrophilic surface.

The key processes involved in the flashover process are shown in Figure 2-1. The environment, in which the insulator mustoperate in, influences the first two processes - pollution deposit and wetting - whereas electrical aspects govern the last two

processes. This Section, therefore, discusses the flashover process from these two viewpoints.



1999-09-01 6

To date, no clear description exists of the complete insulator flashover process for insulators with a hydrophobic surface - butthe key aspects, as defined, will still be present to a greater or lesser extent.

The aforementioned points do not include the effects of ice and snow on the electrical strength of insulators. Such additional points are discussed in provided in Section 2.4.

2.2 Modelling

2.2.1 Hydrophi li c sur face

It is assumed that, in general, the flashover process across ceramic insulators applies to hydrophilic surfaces - i.e. where thissurface is covered with a film of electrolyte. The models are, therefore, based on the study of an arc in series with a resistance- representing a dry band arc and a wet polluted surface respectively.

2.2.1.1 d.c. Model

Mathematical modelling of the pollution flashover of ceramic insulators has already been the subject of an extensive review published in Electra 7. Therefore, only a brief summary of the results will be given here.

For modelling of pollution flashover under d.c. energisation, the basic approach8 involves the determination of the minimum

voltage needed to sustain a dry band arc of a given length in series with the corresponding pollution surface resistance. Thearc length is then varied in order to obtain the critical position that corresponds to the highest value of the supply voltage. Thelatter is taken as the insulator withstand voltage for the pollution severity concerned9. An alternative approach10, still for thed.c. case, is to consider that the dry band arc will continue to elongate as long as:

E E a p< (2-1)

where E a is the arc voltage gradient and E p is the mean voltage gradient of the pollution layer.

The static arc characteristic for a current ‘i’ is of the form:

E i N a

n =0

(2-2)

where N o and ‘n’ are constants.

Assuming a constant surface resistance r p per unit leakage path, the critical arcing distance xc was found to be:

x L

nc = + 1(2-3)

were L is the leakage path length. The corresponding critical voltage ‘U c’ was determined as:

U N r Lcn

p

nn= • •+ +

0

11 1 (2-4)

The critical d.c. current ic - i.e. the maximum leakage current not leading to flashover - can be obtained from :

i N

r c

o

p

n

=

+1

1

(2-5)

Several refinements have been introduced to the d.c. model. In another paper 11, an insulator model was introduced with twodifferent surface resistances per unit length r p1 and r p2 - corresponding to the stem and the shed of a longrod insulator. A

circular insulator disc model was also investigated 12. The contribution of arc current concentration at the roots to the pollution layer surface resistance was included 13 14. Other refinements include the consideration of the arc electrode voltagedrops 13, effect of temperature on the pollution layer resistance14 and the influence of multiple parallel arcing that takes placeon many insulators - especially on those of large diameter 15.

The d.c. model has been used to study the polluted insulator : test source interaction 16. This contributed to the interpretationof the experimental results and to the determination of the minimum requirements for d.c. sources in polluted insulator tests17.



1999-09-01 7

Unfortunately, the d.c. model has been frequently used to account for polluted insulator performance under a.c. energisation11

13 14, despite there being important basic differences - as is shown below.

2.2.1.2 a.c. Model

At the instant of voltage and current peak, the circuit equation of an a.c. arc burning in series with the insulator pollution

surface resistance is identical to that of the d.c. circuit equation. However, it has been amply demonstrated experimentallythat, for the same pollution severity, the peak a.c. withstand voltage far exceeds the corresponding value under d.c. conditions.It has also been observed experimentally that arc-propagation across the insulator surface can take several cycles and,therefore, the arc is subject to an extinction and re-ignition process at around current zero 18 19. This means that the d.c.criterion for arc propagation, i.e. E a < E p , referred to previously will not be sufficient to predict insulator flashover under

a.c. energisation. An arc can start propagation when this criterion becomes fulfilled, but if the voltage is not sufficient tocause re-ignition after current zero, the arc will extinguish without leading to flashover.

It has been demonstrated, both theoretically18 and experimentally20, that for the current ‘i’ in a resistive circuit the re-ignitionvoltage ‘U ’ can be expressed as:

U A x

im=

•(2-6)

where ‘x’ is the residual arc length and ‘A’ and ‘m’ are constants

Inserting this relationship in the circuit equation results in:

A x

i

N x

i R im

o

n px

•=

•+ (2-7)

Where R px is the pollution surface resistance corresponding to an arc length ‘x’.

Since the voltage drop of a burning arc is much smaller than the re-ignition voltage, an acceptable - although not accurate -approximation would be to put n ≅ m. This simplifies the analysis and yields a critical arc length x c :

x Lmc = +1

(2-8)

For constant r p , the corresponding critical voltage ‘U c’ is:

U B r Lc p

m

m= • •+1 (2-9)

where ‘B’ is a constant.

Expression 2-9 is similar to that of equation 2-4 for the d.c. case, although instead of n ≅ 0.8 - valid for the d.c. staticcharacteristic of a free-burning arc - m ≅ 0.5 in the a.c. arc re-ignition expression 2-6. Also, the constants in equations 2-9and 2-4 are quite different. In fact, numerical evaluation of these expressions shows that for a high pollution severity - i.e.

relatively low values of r p - the critical a.c. voltage (rms) is much higher than the critical d.c. voltage. This differencediminishes, however, at lower pollution severity and ultimately - with no pollution at all - the a.c. sparkover voltage peak value is nearly equal to the corresponding d.c. voltage.

The a.c. model 21 has been used to investigate the source: polluted insulator interaction and has revealed the effect of the

parallel capacitance on insulator performance. It proved, therefore, to be quite useful in determining the minimum sourcerequirement 22. Recently, the model has been further used to investigate the effect of altitude on the performance of a.c.insulators under pollution conditions23; see also the discussion in Section 3.3.8.

2.2.1.3 Evaluation of the pollution flashover mechanism under transient overvoltages

Consider an impulse voltage with a time to crest (tcr ) much smaller than the time to half value (th ). The main influence on theleakage current flashover stress is given by t

h 24 25 (see Figure 2-2). At very short times to half value (t

h less than 200

µs), no

pre-arc will occur and mainly streamer discharges develop. Then the flashover voltage is determined by the requirement for astreamer discharge to occur and may attain a value close to that for dry conditions.



1999-09-01 8

For very long times to the half value - i.e. longer than 3000 µs, a long pre-arc could be formed. In this case, the leakagecurrent flashover stress will be determined by the pre-arc only and reaches a value of approximately 0.7 kV/cm.

With a virtual impulse duration longer than 100 ms, a further decrease of the flashover voltage will be observed. This is notcaused by a new flashover mechanism. It is due to the fact that the pollution layer will be heated for a longer time duration bythe current flowing and so the surface conductivity will be increased.

In the range between 200 and 3000 µs of th - i.e. SI range, the performance is more complicated; as is analysed below.

Figure 2-2: Flashover strength vs. the voltage-time duration for a cylindrical model insulator under pollution conditions25

.

2.2.1.4 Evaluation of the discharge process under switching overvoltages

Di scharge without dry bands (appli cation of SI only).

Based on the analysis of experimental data as well as on simplifying assumptions for the very complex flashover mechanismfor a leader discharge 26 27, a flashover model has been developed 28. Whereas the a.c./d.c. flashover is governed by the pre-arc 29 10 7, this is not the case with the SI stress - now the leader discharge becomes more important. Because of itscomparatively short lifetime and low energy dissipation, the leader can not produce any dry bands - which is contrary to thecase of an existing pre-arc. Furthermore, the leader gradient is much higher than the gradient in the pre-arc; i.e. the currentflowing in the bridged layer can not be neglected, as in the case of a.c./d.c. stresses. From this consideration it follows that,instead of the usual voltage (U) - current (I) characteristic for a.c. and d.c. cases, only the strength (E) - current (I)characteristic is applicable for SI28, for the instantaneous discharge parameters (Figure 2-3).

Analogous to the a.c./d.c. flashover criterion, a critical condition for the SI flashover arises. For this condition, the dottedstraight line in Figure 2-3b - which represents the negative slope of the layer resistivity per unit length - becomes a tangent of the E-I leader characteristic - as given by the full curve in Figure 2-3b.



1999-09-01 9

Figure 2-3: Flashover models for a.c./d.c. (a) and SI (b)28

.

Discharge with dr y bands (appli cation of SI with pre-stress).

As reported by Garbagnati et al 152, the SI strength can be reduced due to the presence of dry bands. If the flashover strengthis drawn versus the dry band length, typical U-curves are obtained.

Figure 2-4: Approach for the evaluation of the minimum flashover strength in the presence of dry bands28

.

In the presence of a short dry band, having a length ‘ar ’ (Figure 2-4), the flashover under positive SI first occurs from this dry band in a very short time (air breakdown in the µs range). This is followed by the flashover along the contaminated layer of

the length ‘ag’ during a much longer time period (leakage - current flashover in the ms range).



1999-09-01 10

For dry band lengths smaller than 1 m, the strength of the air gap corresponds to the positive steamer gradient, i.e. 450 kV/m.For longer dry band lengths, the mean breakdown strength corresponds to the minimum possible breakdown voltage per unitlength of a long air gap under positive SI.

To check if the proposed approach works, even for insulators of practical interest, the results of calculation are compared withavailable experimental data. Because non-uniform contamination is to be regarded as the worst case, only the presence of dry

bands of critical lengths shall be considered in the following case.

As an example, Figure 3-28 shows the results obtained for a post insulator, where the experimental data of Garbagnati et al 152

are used. As is evident from the broken-line curve, the calculated values meet the measured ones quite well up to the longestinvestigated insulator length of 12 m.

Another example is reported in Figure 3-33. Here, the calculated minimum curve agrees satisfactorily with the experimentalone presented by Garbagnati et al 152 for practical insulators up to 12 m length.

2.2.2 Hydrophobic sur face

The superior performance of new polymeric insulators under pollution conditions is generally attributed to its water repellent -i.e. hydrophobic - properties. Because the surface does not wet, water forms as isolated drops rather than as a continuoussurface film. Hydrophobicity can be lost due to different ageing mechanisms - heavy wetting, blown sand, corona and spark

discharges and possibly solar radiation. For the same pollution surface density, the surface resistance of a polymeric insulator is generally some orders of a magnitude higher than that of a similar porcelain or glass insulator. It also follows that theleakage currents associated with polymeric insulator discharges are generally some orders of magnitude lower than thecorresponding levels for ceramic insulators.

Due to the dynamic nature of a polymeric surface and the resulting complex interaction with pollutants and wetting agents,there exists today no quantitative model of pollution flashover for polymeric insulators that is similar to the one expounded inSection 2.2.1. for ceramic insulators. However, a qualitative picture for the pollution flashover mechanism is emerging 30. Itinvolves such elements as the migration of salt into water drops, water drop instability, formation of surface liquid filamentsand discharge development between filaments or drops when the electric field is sufficiently high.

2.3 Environmental Aspects

From the discussion of the previous sections, it is clear that there exists a direct relationship between the likelihood of flashover and the conductivity of the polluted surface layer. In this section, attention will now turn to the formation of thisconducting layer on the insulator surface and the important aspects that determine its conductivity. These aspects are:

• The quantity of pollutants on the insulator surface; this is determined by the contamination deposit-process.

• The types of pollutants present, plus the wetting conditions.

• The natural cleaning properties of insulators.

• Whether the polluted surface layer is in the form of distinct droplets or as a continuous film.

An influence common to all of the above is the climate in which the insulator is installed.

2.3.1 Climates or atmospher ic variables and typical environments

The conditions surrounding a H.V. insulator leading to the pollution deposit, and the wetting or cleaning of the insulator, arecaused by a set of atmospheric variables which interact among themselves and with the insulator surface. The most importantatmospheric variables are: wind, rain, humidity, temperature and pressure. Atmospheric conditions can vary in both time andspace. Similar identifiable patterns of occurring atmospheric conditions may be grouped into climates.

Climate is, therefore, the result of the interaction of atmospheric conditions with the surface of the earth and may be classifiedas local, regional or global. A pertinent feature of meteorological information is that it is expressed in average values,obtained from statistics taken over a long period of time (e.g. 30 years).

A general classification of climates is given in Table 2-1.



1999-09-01 11

Table 2-1 A general classification of climates31

.

TYPE OF

CLIMATE

DESCRIPTION 1 DESCRIPTION 2 DESCRIPTION 3

Tropical Often called Equatorial climates. Here theweather is hot and wet around the year.

These climates are found within about 5°of latitude North and South of theEquator

Hot tropical climates with a distinct wetand dry season. They occur roughly

between 5° and 15° North and South of theEquator. In parts of South and South-EastAsia the division between the wet and dryseasons is so clear that they are calledTropical Monsoon Climates

Dry Hot deserts with little rain at any seasonand no real cold weather althoughtemperature drops sharply at night. TheSahara desert and much of the Arabian peninsula are the best examples of thistype.

Tropical steppe or semi-desert with a shortrainy season during which the rains areunreliable and vary much from place to place. Good examples are found in parts of India and the Sahel region of Africa.

Deserts with a distinctly cold season.These occur in Higher Latitudes inthe interior of large continents. The best examples are parts of centralAsia and Western China.

WarmTemperate

Rain occurs at all seasons but summer isthe warmest time of the year andtemperatures range then from warm to

hot. Winters are mild with occasionalcold spells. Much of Eastern China andthe South Eastern States of the USA fallin this category.

Winters are generally mild and wet,summers are warm or hot with little or norain. This type of climate is often called

“Mediterranean” because of its wide extentaround that sea. It occurs in smaller areaselsewhere, for example central Chile,California and Western Australia.

Cold The cool temperate oceanic types of climate: Rain occurs in all months andthere are rarely great extremes of heat or cold. This climate is found in much of Northwest Europe, New Zealand andcoastal British Columbia.

Cold continental climates with a warmsummer and cold winter. Much of Easternand Central Europe and Central and EasternCanada and the USA have this type of Climate

Sub-Arcticor Tundra

The winters are long and very cold.Summers are short but during the longdays temperatures sometimes risesurprisingly high. This type of climateoccurs in Central and Northern Canadaand much of the Northern and CentralSiberia.

Arctic or Icecap

In all months temperatures are near or below freezing point. Greenland and theArctic continent are the best examples of this type but it also occurs on someislands within the Arctic and Antarcticcircles.

Highmountainand Plateau

Where land rises above or near the permanent snow line in any latitude theclimate resembles that of the Sub-Arcticor Arctic. The largest extent of suchclimate is found in Tibet and the greatmountain ranges of the Himalayas

2.3.1.1 Local climate

For its general characteristics, the local climate depends on the regional climate and - ultimately - upon the global climate-system. It is, therefore, useful to remember that the local climate of a particular place is a variation on the regional climate.Indeed, the mechanisms acting to create a local climate are essentially the same as those creating the global climate32. Thismeans, that it is possible to apply this knowledge to create models to:

a) Understand the physical process of the interaction between climate and insulator.

b) Predict the pollution phenomena.

Work has been done to correlate the general climatic specification and meteorological data with the pollution flashover

performance of insulators, as is reported in Section 7, where the impact of climate on selection and dimensioning is discussed.



1999-09-01 12

The aim of such a study is to find the basic relationship between the atmospheric variables and the pollution phenomena.Information on the time-variation of atmospheric variables is necessary. The information sources will, of course, vary asneeds differ. The Meteorological Service usually only provides general information, i.e. average values; However, when anapplication is submitted to its research department, specific information can be obtained.

Depending on the study being made, either regional or local climate data will be used. For example, to study the insulationdesign or maintenance of a transmission line 100 km long (place) and for an expected life of 50 years (time), regional climateinformation will be used.

2.3.1.2 Typical environments

To assist in the selection and design of external insulation, typical environments have been defined. Some examples are:

• Marine: Areas where the insulator pollution is dominated by the presence of the sea. The pollutants present on theinsulators are, therefore, mostly NaCl and other marine salts that are easily soluble. On insulators close to the coast it isgenerally found that the inert component of the pollution is low.

• Industrial: Areas in close proximity of polluting industries - such as steel mills, coke plants, cement factories, chemical plants, generating stations or quarries - are classified as industrially polluted. In these areas, the pollution types can bevery diverse. The pollutants present may vary from dissolved acids - such as found close to power stations or chemical

plants - to slow dissolving salts - such as gypsum or cement - found close to quarries or cement factories. Generally, the pollution has a high inert component in areas close to industries.

• Desert: In desert environments, the pollution tends to be sand based. The desert sands may contain high amounts of salt, e.g. 18 % in Tunisia 62, resulting in a very conductive layer when wetted. The pollution on the insulator tends to behygroscopic with a very high inert component. Inland desert areas are typically very dry, dusty, windy and hot. Thelarge fluctuations between day and night raises the relative humidity to levels as high as 93% during early morning up tosunrise thereby leading to very heavy dew that causes frequent flashovers in some cases. If desert areas are close to thecoast, the pollution problems are compounded61.

• Mixed: If industrial areas are situated close to the coast or desert, then the pollution can be described as mixed.

• Agricultural: Localised insulator pollution may also be caused by agricultural activities such as crop spraying, ploughing etc. When lines cross land ready for harvesting the structures may serve as perches for large birds - therebyleading to flashovers due to bird streamers.

The environment may also be classified according to the nature of the contamination-source, as was done in a survey33 oninsulator in-service performance. The classification was as follows:

• Areas with no signs of pollution-related problems. These areas are defined as ‘clean’ areas.

• Areas with isolated pollution problems of limited extent that can usually be traced to a particular pollution-source.These areas are defined as experiencing ‘local’ pollution. Local pollution is often found in areas where the generalatmospheric condition is pollution free but local industrial or agricultural activities cause the problem.

• Areas with widespread pollution problems that can not usually be traced to a localised pollution-source. These areas aredefined as experiencing ‘regional’ pollution. Regional pollution can often be found in extended industrially developedareas - typically with numerous chemical plants, steel mills, and cement or fertiliser factories. Regional pollution mayalso be found along coastal areas, especially if the weather pattern includes a dry season that allows the accumulation of

pollution on the insulators.

These types of classification can only be used to describe the environment in general terms. Therefore, a detailed study of theactual pollutants present is required to achieve an optimal insulator selection. Service experience has demonstrated that the

performance of ceramic insulators - in all but the severest environments - is adequate if the insulators have been properlydimensioned. However, several factors may adversely influence performance even though insulator selection was appropriateat the time of design.

Firstly, the environment may change during the lifetime of the insulators. This can be particularly troublesome inindustrialising areas, where the region may have been classified originally as ‘clean’ and then - at some point - additionalsources of pollution become located near an installation. This could be the case if new factories are constructed, or if an area

becomes developed for agriculture after the insulators have been selected.

Secondly, it has been observed that - in some areas thought to be clean - pollution effects become apparent several years after the insulators have been installed, even if no new industrial or agricultural activity takes place. This is simply a matter of the

insulators gradually accumulating pollution with time, often on a time-scale of several years. Other changes in theenvironment could be related to changes in the nesting habits of birds, which have been known to cause pollution flashovers.



1999-09-01 13

Third, extreme changes in weather have been known to cause major outages because of unusual meteorological patterns.Major storms that may occur with relatively low probability can suddenly cause severe coastal pollution. In inland areas, longdry periods with little rain may also cause an unusual build-up of pollution.

2.3.2 Type of pollution

To degrade the service strength of an insulator, the pollution must either form or adversely influence a conductive layer on itssurface. Pollution can, therefore, be classified either as being an active type - i.e. pollution that forms a conductive layer - or as being an inert type - i.e. pollution that adversely influences the conductive layer 4.

The amount, or severity, of the pollution layer on an insulator is normally expressed in terms of the Equivalent Salt DepositDensity (ESDD). This quantity is obtained by measuring the conductivity of the solution containing pollutants removed fromthe insulator surface and then calculating the equivalent amount of NaCl having the same conductivity 322. ESDD is expressedin mg salt per cm2 of the insulator’s surface area.

2.3.2.1 Active type of pollution

Active pollutants are classified according to the ease by which the conductive layer is formed. Two types are apparent:

1. Conductive pollution2. Pollution that must dissolve in water to become conductive

Typical examples of each of these pollutant types are given in Table 2-2.

Table 2-2: Examples of the different active pollution types4 36

37

39

69

.

CONDUCTIVE POLLUTION DISSOLVING

Metallic deposits such asMagnitite, Pyrite

Gasses in solution:SO2, H2S, NH3

Salt SprayBird Streamer

Ionic Salts: NaCl, Na2CO3,MgCl2, gypsum CaSO4

Others,

Fly ash, cement

2.3.2.1.1 Conductive pollu tion

Metallic deposits

Metallic deposits are normally found close to mining activity and related industries. The electrical strength of the insulator isseverely affected if the density of the pollutants on the insulator surface is such that the individual particles are in contact or if the gaps between the particles are bridged by an electrolyte.

Bird Droppings

It has been reasoned that bird droppings can explain a large number of unidentified outages of transmission lines with systemvoltages up to 500 kV 34 35 36. When large birds release their excrement, a long continuous length of highly conductive fluiddroppings (volume conductivity 10 - 30 mS/cm) can shorten the air gap between the tower structure and the conductor. Thenthe remaining air gap is too small to withstand the phase-to-earth voltage. Most of these flashovers occur during the time

period prior to birds’ commencement of daily activity.

A secondary effect is that the insulators are covered with the bird excrement, which is a pollution layer with a very high saltcontent. If the birds utilise the tower frequently, this may become a very thick layer.

Pollutants in dissolved state

A more common conductive pollution type is where the pollutants are already dissolved in the wetting agent, as in acid rainand salt-fog conditions. Some of these pollutants - such as gasses dissolved in water, e.g. SO2 - are difficult to detect bytaking measurements from the surface of the insulators, because this contaminant returns to the gaseous state as soon as theinsulator surface dries4.



1999-09-01 14

2.3.2.1.2 Poll uti on that needs to dissolve

Various studies have been made to find a relationship between the dissolving characteristics of salt contaminants and theinsulator flashover voltage 52 39 37 69. From these studies, the following parameters have been identified as being important:

• The solubility of the salt.

• The rate at which the salt goes into solution.Figure 2-5 39 shows the effect of salt-solubility on the limiting flashover voltage of an insulator for three different equivalentsalt deposit densities (0.01 mg/cm2, 0.03 and 0.10 mg/cm2). The limiting flashover voltage is the minimum value achievedunder a cold fog test for a polluted insulator. Eight salts were investigated. From this figure, it is clear that there is very littledependence of pollution flashover voltage on the solubility of the contaminating salt.

0

2

4

6

8

10

12

14

0 20 40 60 80 100

Solubility (g/100 g H2O)

L i m i t i n g F l a s h o v e r V a l u e ( k V , r m s )

Na2SO4 MgSO4

NaCl

Mg(NO3)2

MgCl2

Ca(NO3)2

CaCl2NaNO3

ESDD= 0.01 mg/cm2

0.03 mg/cm2

0.10 mg/cm2

Figure 2-5: Relationship between Salt solubility and limiting flashover values (LFOV) 39.

Different salts also have different rates at which they go into solution; generally the higher the solubility of the salt the quicker it will go into solution - but this is not always the case. This is shown in Table 2-3 where the salt is classified according to itssolubility and speed by which it goes into solution.

Table 2-3: General classification of salts according to their solution properties.

LOW SOLUBILITY SALTS HIGH SOLUBILITY SALTS

FAST DISSOLVING SALTS MgCl2 , NaCl

SLOW DISSOLVING SALTS MgSO4, Na2SO4, CaSO4 NaNO3, Ca(NO3)2, ZnCl2

Highly soluble salts that dissolve quickly need a short time in contact with water to go into solution. Therefore, a highlyconductive layer can form quickly on the insulator during all wetting processes. However, with higher wetting rates - e.g. rainetc. - the pollution will also be purged more easily from the insulator due to its high solubility.

Low solubility salts that also dissolve slowly need a large quantity of water to speed up the solution process. This is illustratedin Figure 2-638. The relationship between ESDD and the quantity of distilled water used to make the measurement is shownfor insulators that came from two environments; one in an agricultural area, Huang Du, and another is from an environmentclose to a steel plant. In both of these areas, the main pollutant is gypsum.



1999-09-01 15

Figure 2-6: Relation between ESDD and Quantity of distilled water38

.

This figure shows that for the naturally polluted insulators, an increase in ESDD occurs for an increase in the quantity of distilled water used for making the measurement. This is in contrast to an insulator polluted artificially with NaCl - i.e. a fastand highly soluble salt - that does not show the same tendency.

Various studies have shown that insulators contaminated with highly soluble and fast dissolving salts - such as NaCl - havelower clean-fog withstand voltages than insulators contaminated with low solubility salts which are slow dissolving39 40 41 -such as gypsum (CaSO4.2H2O) - in spite of them having the same contamination severity (see Figure 2-7).

Figure 2-7: Influence of various salts in the contamination layer on the insulator fog withstand voltage41

.

It was also shown that the relationship between the flashover voltage in a steam fog test and the steam input-rate wasdependent on the type of salt on the insulator. A comparison was made between insulators naturally polluted - mainly gypsum- and insulators artificially polluted with NaCl and kaolin 42. The results are presented in Figure 2-8, which show that theflashover strength of insulators polluted with mainly gypsum have a greater dependency on steam input-rate than do insulators

polluted with NaCl.

The decrease in flashover voltage with increasing steam input-rate is ascribed to the greater amount of pollution that isdissolved at the higher wetting-rate. To achieve the same flashover voltage during the test as that applied in-serviceconditions when flashovers occurred, the steam input-rate had to be an order of magnitude higher than that recommended byIEC 507 22.



1999-09-01 16

Figure 2-8: Flashover voltage of naturally and artificially polluted insulators as a function of steam input rate42

.

Flashovers have been reported on insulators polluted by slow dissolving salts - such as gypsum (CaSO 4) - but they generallyoccurred during extended periods of wetting; i.e. dense fog, heavy rain storms lasting longer than three hours or live spraywashing 42 43.

Other factors that complicate the relationship between the type(s) of salt and the flashover voltage are when:

• The solubility of a salt is affected by the existence of other salts; e.g. the solubility of CaSO4 is inhibited by the presenceof NaCl 37.

• The process by which a salt goes into solution can be either exothermic (temperature rises) or endothermic (temperaturelowers). Any temperature change will greatly influence the conductivity of the solution that forms 69.

• The wetting process of the insulator is influenced by the hygroscopic properties of the salt. Therefore, different wetting-rates will occur for different salts - even though the ESDD values may be the same 69.

2.3.2.2 Inert pollution

Inert material deposited on an insulator surface has, until now, been considered to give an indirect and relatively smallinfluence on the withstand voltage. The greater the inert material deposit, the thicker will be the water film retained on theinsulator surface - and so the amount of soluble material dissolved in the water film will be higher.

Recently, significant differences have been found in the d.c. withstand voltage between insulators contaminated artificiallywith Tonoko and kaolin under the same ESDD conditions 44 45. In addition, it has also been reported that there is an influenceof the amount of the inert material on the hydrophobicity and the withstand voltage of polymeric insulators 46 47.

The amount of inert material found in the pollution deposit on an insulator is expressed as the Non-Soluble Deposit Density(NSDD) given in weight of the non-soluble deposit per unit surface area of the insulator 322. NSDD is expressed in mg/cm2.

In this section, the influence of both the type and the amount of inert material on the contamination performance of insulatorswill be discussed.

2.3.2.2.1 The inf luence of inert mater ial type

In the conventional clean-fog procedure for insulator artificial contamination tests, a constant amount of inert material and avariable amount of salt are included in the solution for contaminating a specimen insulator. Kaolin and Tonoko are typicalinert materials for artificial contamination tests and so they will form the basis of this discussion. Although the shape of aspecimen insulator and the contamination method may influence NSDD, 40 g of inert material per 1 litre of water has been

specified in IEC 507. This amount is regarded as giving approximately 0.1 mg/cm2

of NSDD on the insulator surface22

.



1999-09-01 17

However, the experimental results given in Figure 2-9 show that the deposit density on a specimen disc varies with the type of inert material - in this case, Tonoko and Roger’s kaolin - when the specimen is contaminated with a solution having the same‘concentration’ of inert material. The results presented in Table 2-4 are, therefore, given for the same inert material depositdensity.

Figure 2-9: Relationship between NSDD and the quantity of inert materials in the contamination suspension45

.

Comparative test results of d.c. and a.c. contamination withstand voltage with Tonoko and Roger’s kaolin are also shown inTable 2-4 45. Significant differences - 20 to 25% - can be seen in the d.c. withstand voltage between Tonoko and Roger’skaolin although the salt deposit density (SDD) is the same.

Table 2-4: Results form flashover voltage tests45

.

TestVoltage SpecimenInsulator

Quantity of Salt / Non-soluble Contaminantg/l

SDDmg/cm2 NSDDmg/cm2 50% FOVkV/unit Corrected50% FOVkV/unit

Max. LeakagecurrentmA

13/40Tonoko

0.068 16.7[100]

15.8[100]

250

13/60kaolin

0.03 0.079 14.8[89]

14.4[92]

430

250S 133/40Tonoko

0.079 11.0[100]

10.6[100]

850

a.c. 96/60kaolin

0.025 0.135 10.0[91]

10.5[99]

1200

15/40Tonoko

0.076 26.6[100]

25.5[100]

200

320DC 16/60kaolin

0.03 0.113 22.2[83]

22.6[89]

550

13/40Tonoko

0.068 16.3[100]

15.4[100]

230

250S 13/60kaolin

0.03 0.085 12.8[79]

12.5[81]

350

d.c. 13/40Tonoko

0.16 25.0[100]

26.8[100]

80

320DC 13/40kaolin

0.03 0.082 20.9[84]

20.3[76]

160

Note 1: SDD and NSDD values show average values measured on more than 10 insulator units for individual cases. Note 2: Maximum leakage current shows the average maximum value for individual cases. Note 3: Corrected 50% FOV value was the one corrected to NSDD = 0.1 mg/cm2. Note 4: [ ] shows the percentage ratio of 50% FOV for the case of kaolin relative to that of Tonoko.

Note 5: Insulator types are specified in the paper 45.



1999-09-01 18

Table 2-4 shows that a 5-10% difference in the a.c.-contamination withstand voltages was found between Tonoko and Roger’skaolin when the NSDD was adjusted to the same level. The variation of the surface resistance of the contaminated insulator during the tests is shown in Figure 2-10, which illustrates that the surface resistance of an insulator contaminated with Roger’skaolin reduces faster and is much lower than that of an insulator contaminated with Tonoko.

Tonoko

Brazilian kaolinMexican kaolin

Georgia kaolin

Italian kaolin

Roger’s kaolin

10

1.0

0.1

0 10 20 30 40 50 60

Time lapse, min

S u r f a c e R e s i s t a n c e , M o h m / u n i t

Figure 2-10: Time variation of surface resistance during the course of clean fog tests of contaminated insulator units

polluted with a combination of salt and various types of kaolin and Tonoko48

.

The very wide variations in the physical and chemical properties of the various kinds of kaolin used internationally ininsulator contamination tests are shown in Table 2-548.

Table 2-5 : Physical and chemical properties of common inert materials used in insulator contamination tests 48.

kaolinItem Measuring method Tonoko Roger’s Georgia Italy Mexico Brazil

Particle Size, µm(50% value)

Laser Light Scattering 6.2 5.8 6.3 4.5 13.5 25.9

Main Constituentsof material

X-ray Diffraction QuartzMuscovite

QuartzKaolinite

QuartzKaolinite

QuartzKaolinite

QuartzKaolinite

Cristobalite

QuartzKaolinite

Chemical Loss on Ignition 4.8 14 14 12 6 13Composition, X-ray SiO2 67 46 46 48 77 48Percentage by Fluorescence Al2O3 16 37 38 37 16 36

Mass Fe2O3 5.8 0.9 0.7 0.7 0.2 1.0

The surface resistance and the withstand voltage characteristics of an insulator artificially contaminated with these types of kaolin, together with the Tonoko, are shown in Figure 2-10 and Figure 2-11 respectively 48. A large variation is apparent,even among the various types of kaolin 10 49.

The main minerals of Tonoko and kaolin - as determined by the X-ray diffraction method - are Muscovite (Al 2Si2O5(OH)4)and Kaolinite (KAl2Si3Al10(OH)2) respectively, together with Quartz (SiO2) that is common to both.

The different surface resistivities of Tonoko and the various types of kaolin that apply under artificial fog conditions can beexplained by the different crystal structures of these materials. Hydroxyl groups [OH]- are located inside the crystal structurein the case of Muscovite, whereas they are located outside the crystal structure in the case of Kaolinite. Kaolin consisting of Kaolinite is, therefore, much more hydrophilic than Tonoko consisting of Muscovite.

Recently it was confirmed that the type of inert material had a similar influence on the contamination withstand voltage of silicone rubber polymeric insulators 50.



1999-09-01 19

0

20

40

60

80

100

Tonoko Brazilian kaolin Roger's kaolin Mexican kaolin Georgia kaolin Italian kaolin

Type of inert material

C o m p a r i t i v e F l a s h o v e r v o l t a g e , %

( T o n o k o = 1

0 0 % )

Specimen Insulator: 320 DC SDD : 0.03 mg/cm2

NSDD : 0.10 mg/cm2

Figure 2-11: d.c. Withstand voltage test results of artificially contaminated insulators with various kinds of inert material48

.

The type of inert pollution, therefore, influences the formation of a conductive layer. It can be classified as being either:hydrophilic or hydrophobic. A hydrophilic substance will aid the formation of a conductive film on the insulator surface 51

whereas a hydrophobic material will inhibit the formation of such a film. It has been shown that a truly inert material isneither hydrophobic nor hydrophilic - such as is quartz - and so does not significantly influence the flashover voltage of aninsulator 52.

2.3.2.2.2 The in fl uence of the amount of i nert mater ial present

a) Ceramic Insulators

The influence of the amount of inert material on the contamination withstand voltage of the longrod type and the disc typeinsulator is shown in Figure 2-12.

Figure 2-12: The influence of the amount of inert material on the contamination withstand voltage of porcelain longrod and

disc type insulators (Tests performed at NGK).

A substantial reduction is apparent in the withstand voltage with an increase in the amount of Tonoko present, expressed in NSDD. This reduction is in spite of the smaller influence of NSDD compared with that of ESDD. This is due to the thicker

layer of inert material because it retains more water - thereby increasing the amount of soluble contaminant that is dissolved.The result is a lower surface resistance and, therefore, a lower withstand voltage.



1999-09-01 20

b) Polymeri c I nsulators

A similar tendency in the relationship between the NSDD and the contamination withstand voltage exists for polymericinsulators, as shown in Figure 2-1350. A delayed recovery of hydrophobicity with the increase in NSDD on the insulator surface was also reported, as is illustrated in Figure 2-14.

Figure 2-13: The relationship between NSDD and contamination withstand voltage for polymeric insulators50

.

The withstand voltage of hydrophobic polymeric insulators that are contaminated heavily with inert materials may be reduced by the thicker water film and the delayed recovery of hydrophobicity. The latter is due to the inhibited migration of lowmolecular weight silicone from the bulk to the surface of the contaminant layer.

Figure 2-14: The effect of NSDD on hydrophobicity recovery time53

.

(Artificial pollution consisted of kaolin and salt; flashover voltage was determined by the Clean-Fog test).



1999-09-01 21

2.3.3 Mechanisms of contamination accumulation on insulators

The accumulation of contaminants on an insulator’s surface is the net effect of the processes which bring them to that surfaceand those which lead to its self-cleaning54.

2.3.3.1 Contaminating process

The contaminating process is decided by the force that brings the contaminant particles towards the insulator surface and bythe condition of that surface.

The force ‘F p’ which determines the movement of a contaminant particle close to the insulator is the combination of threeforces: wind (Fw), gravitational force (Fg) and the electric field (FE) 54 55 56 57:

v v v v

F F F F p w g E = + + (2-10)

The force of the electric field, E, on a neutral particle is the dielectrophoretic force - sometimes called the ‘grad E’ force - andthat on a charged particle is the electrostatic force. The latter can only have an effect under d.c. voltage.

The results of calculations by Annestrand and Shei55 indicate that wind is the dominant force governing the movement of contaminant particles for wind speeds of about two to three metres per second and above. When the wind speed is low, the

electrostatic force (in case of d.c. voltage) and the gravitational force will dominate. The effect of the dielectrophoretic forceis weaker than that of the other forces. Therefore, for a.c. voltages, wind is the dominant factor. In contrast, under d.c.conditions, the electrostatic force also plays an appreciable part.

The heating effect of leakage current is another mechanism that may contribute to the accumulation of pollution on theinsulator. That is, when salt is deposited on the insulator in the dissolved state - see Section 2.3.2.1.1 - it can be left behindwhen the water evaporates due to the Joule heating of the leakage current. As a consequence:

• In high stress parts of the insulator, the heating effect will hinder its natural cleaning 4.

• Under salt-fog conditions, the repeated drying out of the deposited wet contaminant layer leaves a residue of salt thataccumulates.

It has been shown that under a.c. voltage, the heating effect of leakage current has a larger influence than the dielectrophoreticforce on the pollution accumulation on the insulator surface 4.

2.3.3.2 Pollution deposition by Wind

Wind is due to changes in atmospheric pressure or by differences of temperature between two sites. Speed and direction arethe main characteristics of wind.58 There is a good correlation between the amount of contamination (soluble and insolublematerials) on the insulator surface and the prevailing wind speed, if the wind does not contain large particles. Figure 2-1559

shows an example of the relationship between the Salt Deposit Density (SDD) and the speed of the sea wind for an insulator installed close to the coast and for which the contaminants on its surface are not removed due to wind.

The empirical relationship is:

[ ]S C V t i ii= •∑ 3 (2-11) 59

where: S = Salt deposit density on the insulator surface (mg/cm2)Vi = average wind speed, for each time interval i (m/s)ti = length of time interval i (hour)C = a constant that depends on the location of the testing

station and type of insulators; typical values are between 5.2 x 10-6 and 8.0 x 10-6



1999-09-01 22

Figure 2-15: Accumulation of contaminants by a strong sea-wind on the under surface of a typical insulator

59

.

Wind can transport pollutants over long distances 60. These pollutants can be solids or gasses. Figure 2-16 shows thatalthough the effect of the sea reduces rapidly with distance from the coastline, wind may carry pollutants inland so that theeffect of the coast can still be significant at some distance depending on the topography. A higher than normal pollution-layer can result from the use of fertilisers by spraying or the burning of crop residues, due to the transport of the pollutants by thewind.

Figure 2-16: The relationship between the distance from the coast and measured ESDD on a standard disc insulator under

ordinary salt-pollution conditions76

.

In contrast, the action of wind may mitigate against the pollution flashover process because it could 74 93:

• Remove non-attaching particles.

• Extinguish the arc on a polluted surface.

The processes under which wind brings the contaminants onto the insulator surface is called the “aerodynamic catch”52.Although this process is very complex and can not be fully described, the discussion of this section will highlight theimportant parameters and mechanisms.

When the airflow approaches an insulator, it divides; thereby leaving a stagnation point where the air is at rest. The

suspended particles, having a density greater than that of air, are unable to follow the airflow and so may be deposited on theinsulator surface. Similarly, when the airflow passes the under-rib on an insulator, it generates vortices inside the ribs. As aconsequence, some quite small and low-density particles will be deposited there. Therefore, vertically mounted insulators



1999-09-01 23

with a simple shape - the so called “aerodynamic profile” - will collect less contaminants in wind than do the insulators withan under-rib profile for the same location. A laboratory measurement in a wind tunnel shows the effect of different shed

profiles for vertically mounted insulators 54. The insulators of aerodynamic shed profile are less contaminated when the windis the only dominant force, as indicated in Figure 2-17.

Figure 2-17: Variation of pollution catch with shape54

H: Heavy; M: Medium; L: Light; Z: Zero deposit54

.

For horizontally mounted insulators, the area presented to the wind by the insulator is important. In cases where the pollutionsource has a well-defined direction, horizontal insulators pointing to the source, or away from it, will collect more pollutionthan do corresponding insulators pointing 90o from it125.

A rougher surface and the presence of moisture can also contribute to a higher accumulation of contamination 55.

0

0,5

1

1,5

2

2,5

3

A B C D E F G H I J K L M N

Position on Insulator

E S D D ( m g N a C l / c m

2 )

0

1

2

3

4

5

6

7

8

9

10

N S D D ( m g / c m

2 )

ESDD (Lee side)

ESDD (Wind side)

NSDD (Lee side)

NSDD (Wind side)

Lee Side

Wind Side

AB

CD

EF

GH

IJ

L

M

N

K

Figure 2-18: Pollution distribution on an insulator in a desert area62

.

Investigations conducted in desert areas have shown77 61 that the greatest amount of dust on an insulator surface is collected onits lee side, i.e. opposite to the prevailing wind direction, due to the cleaning effect of the wind. This is contrary to the generalcase, as stated above. Consequently, this produces a non-uniform pollution distribution - as is shown in Figure 2-18.



1999-09-01 24

The pollution deposit on an insulator is also influenced by the cleaning action of wind. This is especially true in desert areaswhere the wind may carry quite large sand particles (>200 µm). These particles ‘sand blast’ the insulator surface, therebyenhancing the natural cleaning of insulators. Also, they erode the metallic parts of the insulator. It is the smaller particlescarried by wind, <100 µm, that adhere to the insulator surface. These particles also polish the metallic parts 62 63.

2.3.3.3 Pollution deposition by rainGases like SO2 in the atmosphere increase continuously due to the influence of industrial waste. These gases dissolve in water and form acids - which lead to acid rain, acid fog and acid ice. Under such acid rain or fog conditions, the surface electricalconductivity of an insulator will increase. Test results presented by Leguan 64 and other workers 32 have suggested that manylocations in the USA and north-west Europe now have rainfall with pH values as low as 3. Gases that are the cause of acid

precipitation can travel (by wind) distances approaching 2000 km.

2.3.3.4 Other mechanisms

Besides the well-known contamination deposition process in coastal areas or regions with solid deposits, there are severalother events that may cause unexpected pollution flashovers. These events occur even if the insulator design seems to becorrect for the pollution conditions at phase-to-earth voltage at that specific site.

Deposition of bird droppings,

This aspect is dealt with in Section 2.3.2.1.1.

Non-uniform axial deposition of contaminants or non-uniform wetting of uniformly polluted insulators

Both of these cases can occur if buildings, roofs or other structures protect a part of an insulator - so that either the build-up of the contamination or the wetting of the deposits will be non-uniform. It has been shown by laboratory tests65 66, that in thesecases, the lowest flashover voltage may be reduced to 70 % of the value obtained with uniform contaminant distribution or with uniform wetting.

2.3.4 Mechanisms of wetting

The mechanisms of wetting are:

• Condensation67 68.

• Precipitation 32.

• Hygroscopic absorption 67 69 70.

• Molecular diffusion.

Precipitation of fog, mist and rain are regarded as the most severe - because they can wet the insulator's underside as well asits top. This effect depends on wind conditions.

Leclerc et al 70 have examined the wetting process of the surface deposit on an insulator. A physical model was built to allowthree processes: collision of water drops (fog) condensation, hygroscopic behaviour (absorptive) and molecular diffusion(vapour pressure). The main conclusion is that the effect of the difference in temperature between the surface deposit and the

fog is very important, as this parameter affects all three wetting processes. If the surface temperature is higher (positive) thanthat of the ambient-air temperature, wetting will only be due to hygroscopic and molecular diffusion. On the other hand, witha surface temperature lower than that of the ambient (negative), all three processes (condensation, hygroscopic behaviour andmolecular diffusion) combine to cause wetting.

The size of the temperature difference, whether positive - for the hygroscopic behaviour and diffusion - or negative - for thethree processes combined - affects the wetting-rate. The more this temperature difference is positive, the lower is the wetting-rate. The greater the negative temperature difference, the higher is the wetting-rate. Also, it was concluded that the absolutehumidity value has a considerable influence on wetting due to hygroscopic behaviour and molecular diffusion but not to anygreat extent on the process due to condensation. It should be noted that the vapour pressure of a solution is always lower thanthat of the liquid in the pure state.

Orbin and Swift71 have examined the physical processes, provided the basic equations and give the results of some sample

calculations for the surface resistivity of a cool polluted insulator. They have also developed a mathematical model for wetting by condensation.



1999-09-01 25

2.3.4.1 Precipitation type and duration

The precipitation characteristics that have the greatest effect on the insulation behaviour are the rain rate and the resistivity of rainwater.

Largely the type of cloud system involved determines the precipitation type (intensity) and duration. This, in turn, is directlyconnected with the cloud-formation processes. In general, cumulus type clouds involve vigorous motions - giving large drops

and intense precipitation for a short period. Their influence is restricted to fairly small geographical areas.

Stratus and Alto Stratus - in contrast - involve more persistent, less vigorous vertical motions over a much wider area. Hence prolonged, steadier, and usually less intense precipitation results. This can be seen from Figure 2-1932, which shows theintensity versus duration rain curves for various return periods (1-100 years). For example, Miami in Florida (USA) is in anarea dominated by cumulus type clouds. Here, short duration rainfalls are likely to be much more intense than in Seattle,Washington, (USA) where precipitation from depressions is predominant. The difference in intensity decreases as theduration increases. The curves indicate that only once in 10 years is Seattle expected to have a rainfall-rate - averaged over 12 h - which will reach or exceed 5.5 mm/h.

0.5 1 2 3 6 12 24Rain duration (hours)

1

2

5

10

20

50

100

200

SEATTLE(Stratus-Alto Stratus)

MIAMI(Cumulus)

1005025105

100502510

5

1

1

R a i n r a t e ( m m / h )

Figure 2-19: Intensity versus duration rain curves for various return period (1-100 years) for Miami, Florida (solid lines)

and Seattle, Washington (dashed lines)32

.

Table 2-6: Major types of precipitation 32.

TYPE OF PRECIPITATION DESCRIPTION

Rain Drops with diameter >0.5 mm, but smaller drops are still called rain if they are widely scattered.

Drizzle Fine drops with diameter <0.5 mm and very close to one another.Freezing rain or Drizzle Rain or drizzle, the drops of which freeze on impact with the ground.Snowflakes Loose aggregates of ice crystals, most of which are branched.Sleet Partly melted snowflakes or rain and snow falling together.Snow pellets also known assoft hail

White opaque grains of ice spherical or sometimes conical with diameter about 2-5 mm.

Snow grains Very small, white, opaque grains of ice - flat or elongated with diameter generally <1 mm.

Ice pellets Transparent or translucent pellets of ice, spherical or irregular, withdiameter < 5 mm.There are two types:a) frozen rain or drizzle drops, or largely melted and then re-frozensnowflakes. b) snow pellets encased in a thin layer of ice (also known as small hail).

Hail Small balls or pieces of ice with diameters 5-50 mm or sometimes more.Ice prisms Un-branched ice crystals in the form of needles, columns or plates.



1999-09-01 26

The major types of precipitation are given in Table 2-6. Resistivity measurements of rainwater taken in Japan 72 are shown inFigure 2-20 as a frequency distribution. The average, of the distribution of resistivity, ranges from 10 to 30 k Ωcm.

Figure 2-20: Distribution of rainwater resistivity72

.

For rain resistivities below 14 k Ωcm, the insulation strength of insulators decreases rapidly - as mainly a function of rain

resistivity but also of precipitation-rate as is shown in Figure 2-21

72

. This figure also shows a marked influence of rain-rateon the insulation strength.

Figure 2-21: The relationship between the resistivity of rainfall and reduction-rate of the flashover voltage72.



1999-09-01 27

2.3.4.1.1 Tropical precipitation

Precipitation in much of the tropics is associated with convective activity. Strong vertical motions occur in a fluctuating bandnear the equator. These release a large amount of water vapour to create a regime of intense, short-lived storms from cumulusclouds. Heavy wetting conditions with rain-rates in excess of 100 mm/h are not uncommon.

More widespread uplift is associated with a monsoon circulation regime. The effects of convective uplift, dynamic uplift and

topographic forcing combine to produce high annual rainfall totals. Locally, rainfall-rates may be very high but - generally -the monsoon condition is characterised by longer lasting, less intense precipitation.

2.3.4.1.2 M id-latitude precipi tation

In mid-latitudes, much of the precipitation production is associated with depressions and fronts. The result is widespreaduplift giving extended periods of gentle rain over a broad area. Rainfall-rates can vary greatly, although 1-2 mm/h can beregarded as a typical value.

The intensity is partly controlled by the amount of vapour available, which - in turn - depends on the source of the air that is being uplifted. Air derived directly from the subtropical oceans, where evaporation rates are high, is likely to lead to higher precipitation-rates. If the source is the tropical deserts, the air is likely to be much drier and it is not uncommon in theseconditions for dust and sand particles to form the condensation nuclei - and hence be deposited in large quantities with therain. Convective activity in the mid-latitudes is primarily a summer phenomenon. In extreme conditions, rainfall of 31 mm in1 minute has been recorded in Maryland, USA.

2.3.4.1.3 Low precipi tation regions

The regions of low precipitation in the subtropics result mainly from a lack of mechanism to create uplift for bringing the air to saturation. Certainly over the oceans and - to a large extent - over deserts as well, there is no lack of moisture in theatmosphere.

In these regions, the available moisture may precipitate on insulators as the result of the formation of fog or dew.

2.3.4.2 Fog

Fog may form when a volume of air is cooled to below its dew point. "Radiation" fog forms when the earth cools throughradiative heat loss. Particularly on calm, clear nights - when the radiation effect is large - the air may be cooled below its dew

point and a fog will result. This will begin to form very close to the ground - around midnight - and will gradually thicken anddeepen as the night progresses 32.

Another mechanism producing fog is associated with the horizontal movement of the air. If a warm air stream starts to blowover a cooler surface, the air rapidly adjusts to the temperature of that surface. Again, given sufficient cooling or sufficientlymoist air, fog will result. This type of fog is known as "advection" fog.

Winter fog conditions can also prove to be severe for pollution-related flashovers180. The presence of fog at 0oC (i.e. calledice-fog in Table 2-7) ensures a high level of relative humidity that promotes effective and complete wetting of the insulator byensuring a conversion of the ice - on the surface of the insulator - to surface wetting, rather than sublimation to water vapour.

If sufficient pollution has been captured in the ice layer on the insulator's surface, the effective wetting produced by the ice-fog conditions will lead to a low surface resistance and an increased likelihood of flashover.

Table 2-7: Fog Characteristics - Typical values.

FOG PARAMETER R ADIATION FOG ADVECTION FOG ICE - FOG

Average droplet size (µm) 10 20 8

Typical range of droplet size (µm) 5 - 35 7 - 65 2 - 30

Fog Density or water content (g/m3) 0.10 7 - 65 0.10

Fog speed (m/s) 0.5 - 4

Horizontal visibility (m) 100 300 200



1999-09-01 28

Natural fog density ranges from 0.01 g/m3 (very light) to 1.0 g/m3 (very heavy sea fog) and about 90 % of all fogs havedensities <0.5 g/m3. Other typical characteristics are given in Table 2-7. Artificial fog for insulator testing has a density of 10to 100 times greater than that of natural fog. One reason for this difference is that most test facilities are not thermallyinsulated well enough to maintain a uniform fog density less than about 2.0 g/m3. Another reason is that artificial tests areintended to encompass the entire spectrum of fog, mist and drizzle4. In Table 2-8 the characteristics of the artificial fog of some laboratories are given.

Table 2-8: Artificial Fog Characteristics - Typical values.

FOG PARAMETER MEXICO MEXICO CRIEPI NGK IREQ

Fog type Ultra Sonic Steam Steam Steam Steam

Average droplet size (µm) 17 10

Typical range of droplet size (µm) 4 - 28 17 - 19 5 - 20

Fog Density or water content (g/m3) 11 4 3 - 7 6 4.5

Fog and rain can, depending on the wind conditions, wet the under surface of the insulator more effectively than the

condensation mechanism.

2.3.4.3 Condensation

Condensation occurs when the surface temperature of the insulator falls below the dew-point temperature.

On clear still nights, the insulator surface - particularly the top one - loses heat through radiation to the night sky faster thatheat can be supplied to it by air currents. If the temperature drops below the dew point, moisture forms on the surface of theinsulator. These conditions are commonly produced in desert environments at night or early mornings 73 74.

Dew-condensation wetting is a major cause of flashover on service insulators. Studies have shown that this often occurs in theearly morning hours when the insulator is at a lower temperature than that of the ambient air - due to thermal lag.

2.3.4.4 Moisture absorption

Wetting of a pollution layer on an insulator can occur through moisture absorption by insoluble and soluble components of that layer. This results in an increase in the surface conductivity and, therefore, the relative humidity of the environment has agreat influence on the flashover voltage 75. Figure 2-22 illustrates this condition.

The absorptive quality of a pollution layer depends on its vapour pressure. If the vapour pressure in the atmosphere is higher than that of the solution vapour pressure, this solution absorbs moisture. The vapour pressure of water is always higher thanthat of aqueous solutions.

Figure 2-22: Influence of ambient humidity on insulator flashover voltage 75.



1999-09-01 29

The main conclusion from the studies of Chizan and Pohl 69 is that the intensity of moisture absorption on an insulator surfacedepends upon the chemical constitution of the pollution. The effect of intense and continuous moisture absorption can causelong-lasting surface discharges at operating voltage or can be a reason for flashover shortly after the voltage has been applied.High air-humidity and the hygroscopic properties of pollution layers are also very important in determining the switching-impulse performance of insulators.

2.3.5 The natural cleaning processes

The natural processes that clean the insulator surface are wind and rain.

While wind can bring contaminants onto the insulator surface, it can also blow them away80. A sandstorm in a desert area canhave a sandblasting effect that cleans the windward side of the insulator, as is illustrated in Figure 2-18.

High-intensity rain is the most effective cleaning process for the insulator. The contaminants are washed away by the high-speed impact of the raindrops. This effect is reduced, however, on the areas of those insulators, that are protected by deepsheds having deep under-ribs. A similar reduction also occurs for sheds having a narrow spacing. Insulators with an open

profile enjoy more effective washing by rain than do those with under-rib profiles. The orientation of an insulator and the sizeof area that it presents for cleaning by rain also play a large part in the effectiveness of natural washing. Horizontallymounted insulators are, generally, more effectively cleaned than are vertically installed ones. Further information on the

effect of insulator diameter is provided in Section 3.3.1.4.Table 2-9 shows the approximate values of natural rain washing performance of insulators under ordinary pollutionconditions.

Table 2-9: Natural rain washing performance76

.

AMOUNT OF

PRECIPITATION

WASHING EFFECT

mm Under surface of disc(%)

Upper surface of disc andcylindrical post (%)

2 15 505 25 80

10 40 9015 45 9020 50 9030 60 90

Note: Natural rain washing-effect values are obtained by the following formula:

Washing EffectSalt deposit before precipitation Salt deposit after precipitation

Salt deposit before precipitation=

−

2.3.6 Cri tical wetting conditions

There is a critical amount of water required on the insulator surface to produce the minimum flashover voltage. The mostsevere conditions require sufficient quantity and time-duration of wetting to dissolve the majority of the conductive

contaminants without removing them from the insulator surface4

.

Depending on the types of pollutant present and the insulator characteristics, the critical wetting conditions most commonlyoccur during fog, dew or light misty rain.

2.3.7 Ef fect of various aspects of the insulator on i ts pollution accumulation

2.3.7.1 Profile

The contamination-collection processes on insulators in-service are very complex. Observations of pollution distribution oninsulators installed in a desert area are illustrated in Figure 2-23 77. In such areas, it has been found that the insulators havingan aerodynamic profile are less contaminated than are those with a more convoluted profile. However, this is not always the

case - as Figure 2-24 shows. Some field observations have shown the opposite situation

80

78

40

. Further, such differences arenot unique to desert environments 79.



1999-09-01 30

A mechanism whereby antifog insulators collect less pollution than do aerodynamically shaped insulators in certain desertareas may be as follows. Near the coast, where the humidity during the night is generally high, the insulators may be wettedso that the bond between its surface and pollution is increased. Due to the relatively larger exposed surface of theaerodynamic insulator - which allows it to cool more effectively than that of other insulator types, this insulator will be wettedmore than the antifog insulator with a more convoluted surface. Hence, the aerodynamic insulators may then collect more

pollution. Another factor that may play a role is the area of the exposed top surface. This is especially so in regions where

pollution fallout may be considerable. Also, there is the difference in the cleaning by the wind of the pollution particles for the different profiles80.

Figure 2-23: Distribution of salt on the surfaces of insulators of two greatly different profiles after field exposure in a desert

area77

.

In areas with regular monthly precipitation, insulators with an aerodynamic profile are less contaminated in both the short-term (monthly) and the long-term (a year or more) exposure 38. Some areas receive rain only for a few months while the restof the year is very dry. In such areas, aerodynamic sheds may collect less contamination during the dry months than do thosewith more complex profiles. After the rainy months, aerodynamic sheds are certainly less contaminated than are those of theconvoluted-shed design81. If maintenance is performed, an open profile is much easier to handle than a profile with aconvoluted underside. The top/bottom ratio of the pollution on the insulator sheds can be different in different areas and for different times of the year. Sometimes, the bottom surface of a shed is more polluted than the top surface and sometimes the

opposite occurs81.



31

20

40

60

80

100

120

140

G h a z l a n

Y a n

b u

D h a h r

a n

R i y a

d h

T a b o

u k

A r a r

B i s h a

A b

h a

E S D D m e a s u r e d ( a s a % o

f t h a t c o l l e c t e d o n a s t a n d a r d p r o f i l e )

Figure 2-24: A comparison of the amount of pollution collected on different shapes of insulator at eight desert-pollution

stations80

.

2.3.7.2 Orientation

Results obtained in Mexico - in 23 insulator testing stations installed under various climatic and pollution conditions - have provided correction factors for chemical composition and uneven distribution of salts for different regions. Also, long-term patterns of pollution-accumulation show that cap-and-pin insulator strings with an inclined orientation tend to collect lesscontaminants than do vertically mounted ones - the ratio being 0.9. Horizontally installed insulators collect even less - theratio being 0.15. However, orientation effects vary depending on the region (rural, marine, industrial or a combination of them)82.

Tension insulators may also be subject to a direction effect if the major source of contamination is from a well-defined source125. In this case, there can be an influence of orientation and direction in determining the insulator performance under natural

pollution for a particular location or type of location. For other locations where contamination can accumulate rapidly, or thefrequency of natural cleaning by rainfall is very low, the influence of orientation may be significantly altered - from that statedabove - for the same insulator type.

2.3.7.3 Diameter

Field experience indicates that - for cylindrical insulators - the larger the diameter of an insulator, the smaller the ESDD levelit accumulates over a given time as compared to that on the bottom surface of a 250 mm suspension insulator 83. The results of the measurement of ESDD on a series of cylindrical insulators with different diameters, which were exposed - under de-energised conditions - to typical coastal contamination, are shown in Figure 2-25 85.

The relationship between the level of relative ESDD and the average diameter, D, of the insulator was found to be:

55.09.13 −•= D ESDDr (2-12)

where ESDDr = 1 for the cylindrical insulator with an average diameter of 115 mm. However, it has been recommended byOzaki et al84 that, for design purposes, it would be more appropriate to use a more conservative relationship - such as:

ESDD Dr = • −2 6 0 21. . (2-13)

Note: this latter function takes into account the rather large scatter of the measured values.



1999-09-01 32

R a t i o o f E S D D ( 1 . 0 a t D = 1 1 5

m m )

1/2

1

ESDD = 13.9D-0.55

ESDD = 0.5+ 6.9D-0.55 = 2.6D-0.21

2.0

1.0

0.5

0.3

0.2

0.7

115 200 400 700 1000

Average Diameter, D, mm

Figure 2-25: Relationship between the diameter of a porcelain insulator and the contaminant-deposit density under de-

energised and natural service conditions85

.

2.3.7.4 Material

Another factor that influences the pollution deposit on insulators is the housing material. Figure 2-26, which is based on thatreported by Imagawa et al 86, shows comparative ESDD measurements taken on silicone rubber and porcelain insulators at

both inland and coastal sites. These results indicate that silicone rubber insulators tend to accumulate more pollution than dothe porcelain ones. Measurements performed in Tunisia61 have indicated that this trend is also true for desert-typeenvironments.

0,0001

0,001

0,01

0,1

0,0001 0,001 0,01 0,1ESDD (Porcelain), mg/cm

2

E S D D ( P o l y m e r ) m g / c m

2

Site "A" - After Typhoon

Site "B" - 3 month

Site "B" - 1 year Site "C" - 3 month

Site "C" - 6 monthSite "D" - 3 month

Figure 2-26: Comparison of ESDD for porcelain and polymer insulators at 4 different sites86

.

2.3.7.5 d.c. Energisation

There are differences in the contamination accumulation between energised insulators (under d.c. voltage) and un-energisedinsulators because of the effect of the electric field. The amount of pollution collected is a function of the magnitude of theapplied d.c. voltage, as well as that of the electric stress at the point of measurement87 88. More information about pollution

accumulation under d.c. voltage is included in Section 7.4.2



1999-09-01 33

2.3.7.6 Conclusion

All of the aforementioned effects culminate in the build-up of contaminants on the insulator surface. In particular, it isdependent on the product of pollution deposit-rate and the time interval between the washing events. An equilibriumcondition may take some years to occur between the deposit-rate and insulator cleaning-rate. This is illustrated in Figure 2-27.

Figure 2-27: Schematic history of polluted ceramic insulator54

.

2.3.8 Physical and mathematical models of polluti on deposit

Cimador and Vitet 89 have recommended that the interaction between the different precipitation-types and the pollutiondeposit be observed, to derive a classification of a particular site. Then such classifications can be generalised to match the

electrical network. Fierro 90 has investigated a dynamic model to predict the surface resistance of insulators by using themeteorological variables - such as wind direction and speed, ambient temperature, atmospheric pressure, the formation of dewand the occurrence of rain. Arabani and Shirani 91 have developed an artificial neural network for the determination of ESDDfor the Iranian environment. Up to now, this approach is applicable only for natural pollution and not the industrial-pollutiontypes.

Other models are discussed in Section 7.2.7.

Additional measurements 59 have involved a laser interferometer system for the precise measurement of the surfacetemperature of a polluted insulator. The method is ideal for the recording of transients as well as for stationary phenomena.Sugawara et al 58 have calculated the temperature and the resistivity of the pollution layer during the different phases of flashover.

A simple measuring procedure to determine fog conductivity is presented by Pilling and Bernd 92. It allows for the direct

measurement of fog conductivity but not for a continuous recording. The effective layer thickness in the measurement variesover a wide range (20-100 µm) and must, therefore, be further investigated under service conditions.

Also, examples exist of extensively recorded meteorological data in combination with insulator pollution measurements 93 58.

2.4 Ice and snow

Ice- and snow-accretion on transmission lines and within substations not only impacts on the mechanical requirements of thesupports - e.g. towers and gantries, insulators and conductors but also greatly influences the electrical strength of the externalinsulation. It also has secondary effects on equipment such as surge arresters, where its presence can sufficiently disturb thegraded electric field to overstress the internal components.

It is, therefore, necessary to take account of both the electrical and the mechanical requirements when considering ice and

snow - especially in areas where it occurs regularly.



1999-09-01 34

2.4.1 F lashover on i nsulators covered with ice.

2.4.1.1 Definitions

The following definitions are taken from published work 94 95 96 97 98.

I cing pr ocesses

Atmospheric icing is a result of three main processes in the atmosphere and are named accordingly:

1. Hoar frost.

2. In-cloud icing.

3. Precipitation icing.

Hoar frost is caused by water vapour condensation on cold surfaces and usually has no adverse influence on the electrical performance of insulators.

In-cloud icing is a process whereby suspended, supercooled droplets freeze immediately upon impact with an object exposedto this airflow; for instance, a power line situated above the cloud base.

Precipitation icing can occur in several ways, including freezing rain and drizzle, as well as by wet and dry snow. Freezing

rain and drizzle consist of super-cooled drops or droplets, which freeze partly - or completely - on impact with exposedsurfaces.

The ice-growth is said to be dry when the ice-deposit temperature, i.e. the equilibrium temperature between the ice surfaceand water, remains below 0OC. The density of the ice accretion is mainly a function of the impact speed, volume of thedroplet and the ice-deposit temperature. The resulting accreted ice is called soft or hard rime, according to its density and

physical appearance.

The ice-growth is said to be wet when the ice-deposit temperature is 0OC. The growth then takes place at the melting point,resulting in a water film on the surface. The accreted ice is called glaze.

When a glaze is grown at a slow rate (i.e. near the transition to the dry-growth regime), no icicles are formed. However, whenthe flux of water impingement is high - mostly in connection with freezing rain - icicles are formed, usually on the windwardside. Icicles may also form due to the heating of thick rime or wet-snow accretion - such as from Joule heating by leakage

current or from a rise in air temperature. The general characteristics of atmospheric ice are shown in Table 2-10 and Table 2-11.

Table 2-10: Characteristics of ice formed on structures.

TYPE OF ICE DENSITY (g/cm3) APPEARANCE SHAPE

glaze 0.8 - 0.9 transparent and clear cylindrical icicles

hard rime 0.6 - 0.9 opaque eccentric pennants into the wind

soft rime <0.6 white and opaque feathery and granular

Table 2-11 : Meteorological parameters controlling icing on structures.

TYPE OF ICING AIR TEMPERATURE

(οοC)

WIND SPEED

(m/s)

DROPLET

DIAMETER

TYPICAL STORM

DURATION

freezing rain -10<T<0 0<V<15 0. 5-5 mm hours

in-cloud icing -20<T<-1 Unlimited 1-50 µm days

wet snow -1<T<2 Unlimited snowflakes hours

2.4.1.2 Characteristics of ice accretion on insulators

The amount and type of ice accretion on insulators is determined by a combination of the following factors:

• Liquid-water content of air containing super-cooled water drops.

• Air temperature.



1999-09-01 35

• Impact velocity.

• Position, shape and type of insulator.

• Presence or absence of voltage, voltage distribution.

• Heat exchange between equipment (power transformers, etc.) and the environment

Supercooled drops and/or droplets can have a meteorological origin (fog, drizzle and rain, or salt spray from the sea) or

anthropogenic (man made) origin (spray from cooling towers, etc.).

Ice accretion on an insulator occurs usually on only one of its sides - i.e. the windward side. In practical cases, some sectionsof insulators may be free of ice. In the case of wet grown-ice, icicles may bridge two or more adjacent insulator sheds or - inthe case of cap and pin insulators - the icicles may bridge two or more adjacent units in the string.

2.4.1.3 Conductivity of melted ice

The flashover of iced insulators is mainly influenced by the conductivity of the water film on the ice-surface.

The conductivity of water dripping from iced insulators is much higher than that of freezing water. The high conductivity of such water is due to the following factors:

1. Freezing-water conductivity.

2. Transfer of the impurities from the liquid to the solid part of water droplets during the freezing process.

3. Corona products.

4. Pre-contamination of the insulator surface.

5. Superimposed pollution on the iced surface.

In the case of wet snow and dry grown ice, the pollution may be trapped inside the accretion.

Laboratory tests have shown that the conductivity of water dripping from the surface of ice accumulated on insulators couldhave a value as high as 10 times that of freezing water. This can explain why some flashovers may occur even when thefreezing water has a low conductivity.

2.4.1.4 Ice flashover mechanism

The mechanisms of flashover of iced insulators are not yet fully understood. However, some explanations have beenadvanced 95 99.

Flashover caused by ice accretions on insulators under operating voltage occurs usually when a water film is present on thesurface of the ice. One frequent situation is when ice has accreted at low temperature (e.g. during the night) and then itssurface starts to melt when the ambient temperature rises above freezing (e.g. due to sunshine or a general rise intemperature).

The water film has a very high conductivity and so causes a large voltage to be impressed across the air gaps (i.e. parts of theinsulator without ice). These air gaps are caused by the melting, or the falling off, of the ice from parts of the insulator. Thedevelopment of arcs over these parts of the insulator sometimes leads to flashover.

The probability of flashover may be significantly enhanced by the presence of rain, drizzle or fog at the critical moment. The

largest decrease in insulator strength is caused by wet-grown ice with icicles bridging the insulator.From the insulator-orientation viewpoint, vertical mounting is the worst - due to the ease by which the insulator can be

bridged by icicles.

Pre-deposited pollution on insulator surfaces further decreases the insulation strength under icing conditions.

2.4.2 F lashover on i nsulators covered with snow

2.4.2.1 Definitions

The general characteristics of atmospheric snow are shown in Table 2-12.

The density of wet snow accretions will vary according to the liquid water content of the snow and the wind speed; and, inexceptional cases, may reach 0.5 - 0.7 g/cm3. The most severe cases in mountains are often a combination of hard rime andwet snow.



1999-09-01 36

Table 2-12: Characteristics of snow formed on structures.

TYPE OF SNOW DENSITY (g/cm3) APPEARANCE SHAPE

dry snow 0.1 white “caps” on horizontal surfaces.

wet snow 0.3 - 0.7 white to opaque eccentric pennants towards the wind.

2.4.2.2 Accumulation of snow on insulator strings

When the ambient temperature is below zero, the quantity of snow accumulated on horizontal insulators depends upon theamount of snowfall. On the other hand, when the ambient temperature is above zero, the snow on such insulators is apt tomelt and slip down. When the ambient temperature increases from being below to above zero, the thickness of the snow onthe insulators decreases and the snow falls away rapidly. However, in the case of multiple-string arrangements (double or triple strings), the snow may bridge the space between the strings - thereby inhibiting the snow from slipping off theinsulators. This is especially so when the increase in temperature is caused by sunshine. The accumulation of snow by this

phenomenon can be significant 100.

2.4.2.3 Characteristics of snow-accretion on insulatorsMany measurements from various sites have been analysed with respect to the relationship of the conductivity of the water melted from the snow versus distance from the coast. It was found that the greater this distance, the lower was theconductivity of the melted snow. In heavy snow areas, the conductivity of such water was found to be 100 and 50 µS/cm for adistance from the coast of 15 and 50 km respectively.

According to the observations made when some electric faults occurred during 1981 and 1985, the snow conditions weredescribed essentially as follows 194:

In most cases, the thickness of snow was between 30 and 70 cm and its coverage along the insulator strings was between 20 to 60%. In a few cases, the snow covering was 100 cm thick and it covered between 80 to 100% of the insulator sting. For the electrical faults that occurred during 1985, the snow on the insulator assembly had avolume density of between 0.31 to 0.39 g/cm3 and the conductivity of the melted snow ranged between 10 to 40

µS/cm at 25o

C.It has also been found that industrial pollution can be transported over very long distances 181.

2.4.2.4 Snow-flashover mechanism

The amount of snow accumulated on tension insulator assemblies, its density and the conductivity of the melted water are themajor parameters that affect the a.c. withstand voltage characteristics of an insulator assembly covered by dry snow. Theleakage current that flows on the insulator surface and through the snow influences these parameters. This current increaseswith increasing density, conductivity and water content of the snow. Some parts of the snow containing a high current densitymay melt more quickly due to Joule heating and this snow may then drop away from the insulator. This produces a non-uniform voltage distribution along the insulator string. Depending on the resistance of the remaining snow and the length of the string not covered with snow, arcs may bridge these parts and - in the worst case - cause flashover 100.



1999-09-01 37

INSULATOR CHARACTERISTICS

Introduction

performance is described - have had widespread application in all types of environment. For overhead line applications, manyfactors other than axial length or creepage path length are known to influence this performance. Shape - such as the number

affect service performance. Differences in the behaviour of insulators in various orientations may be due to the accumulationof pollution, the effect of natural washing by rain and the physical characteristics of discharges on the surfaces.

insulators have had, they have seen increasing application since their first introduction at transmission-class voltages in the1970’s. Many developments and improvements in this technology have taken place to the point that utilities now are

have become available for applications in substations such as support insulators and equipment insulators. The main reasonsfor using polymeric insulators are 101

•

•

•

•

•

•

•

As a general statement, service experience has demonstrated that the performance of polymeric insulators is good if theinsulators have been properly dimensioned and if the housing material and design are appropriate for the intended application.

indicate that service-induced changes in the housing material of these insulators may play a greater role in their long-term

performance than is the case for glass and porcelain insulators.

3- : Summary of properties of insulator dielectric materials54

PROPERTY NITS G

PORCELAIN

OUGHENED

G

POLYMER RBGF**

Density 2.3-3.9 2.5 2.1-2.2Tensile strength 30-100 100-120 1 300-1 600Compressive strength 240-820 210-300 700-750Tensile modulus 50-100 72 43-60Thermal conductivity 1-4 1.0 0.2-1.2Thermal Expansion Coefficient x10-6 3.5-9.1 8.0-9.5 7.5-20

Rel. Permittivity (50-60Hz) 7.3 2.0-5.5Loss tangent (50-60Hz) x10-3 20-40 15-50 0.1-5.0 5-20Puncture strength *** kV/mm 10-45 >25 >12 3-20Volume resistivity at 20°C for a.c. application

Ωm 1013-1015 1013-1015 1015-1019 1013-1016

Volume resistivity at 20°C for d.c. application

Ωm 1015-1017 1015-1017 1014-1019 1013-1016

Notes:* Silicone and carbon based.** Resin-Bonded Glass Fibre used for the core with a polymeric outer housing.*** Varies with sample geometry, standard values used are given.

This report covers only the pollution performance of polymeric insulators, since Cigré Study Committees 15 and 22 aremandated to deal with material and insulator ageing.



1999-09-01 38

3.2 Materials used for outdoor insulators

The classical materials used for outdoor insulation are glazed porcelain and glass. In the 1970’s the use of polymers - either for a complete insulator or as an outer housing in combination with a glass fibre core - became a serious alternative to glassand porcelain. Looms has provided an excellent overview of the properties and manufacturing processes for glass and

porcelain as used in insulators and an overview of the technology of polymeric insulators 54. Table 3-1 is an updated version

of that given in Loom's book and provides a useful comparison of the properties of the different materials used for outdoor insulators.

3.2.1 Porcelain and glass

Provided good quality raw materials and well-established manufacturing practices are used, reliable insulators can be madefor use in HVAC systems. However, insulators for use in HVDC systems need to have particular attention given tocomposition, purity, homogeneity and resistivity if spontaneous failure in service is to be avoided 102.

3.2.2 Polymers

The choice of materials for polymeric insulators will largely determine their pollution flashover performance. The selection

of creepage distance of the insulator may not, therefore, be as an important a factor as it is in the case of glass or porcelaininsulators.

The most common construction for polymeric insulators is the composite longrod . Here a resin-bonded glass fibre core provides the mechanical strength and a polymer outer housing resists degradation from weathering and other environmentalfactors.

There is some evidence to show that the glass fibre core can fail as a result of the ingress - or the internal formation - of acid103 104. Certain manufacturers use special glass formulations that are resistant to this form of attack.

There is a wide variety of materials that can be used for the outer housing. The properties that have been shown to be themost important in service are water repellency (hydrophobicity) and resistance to tracking. In most formulations, a filler -such as alumina trihydrate - is used to impart tracking resistance. Silicone rubber has become very widely used on account of its very low surface energy, which inhibits the formation of a water film on the surface. A further, and considerable,advantage with silicone rubber is that low molecular weight components in the rubber diffuse into contaminant layers on thesurface and impart hydrophobic properties to them105.

Earlier authors (e.g. Looms 54) have given detailed descriptions of the different insulating materials used for polymericinsulators. Values have been quoted for dielectric strength, permittivity, conductivity and other parameters of the materialsalong with value judgements on their advantages and disadvantages for high-voltage insulation applications. However, theever-increasing number of materials and production processes used for such insulators make a simple classification difficult.

Often, polymeric housing materials are divided into simple classes; the most common being, silicone and ethylene propylenediene monomer (EPDM). Such a generalisation is dangerous for there are many different silicone rubber and EPDMformulations used for electrical insulation, each with specific characteristics for the chosen application and manufacturing

process. For example, silicones can first be subdivided into Room Temperature Vulcanised (RTV) and High TemperatureVulcanised (HTV); these are entirely different products whose raw form ranges from a pourable liquid to a dense solid pasteor granules. The production process can be by gravity pouring, extrusion or high pressure/temperature injection to name but a

few.Each manufacturer chooses a formulation adapted to the process and to the characteristics required of the finished product.Hence, the amount of filler, the type of catalyst, the types and proportions of silicone molecules and other elements of the

polymer vary notably from one product to another. Defining specific values for the various mechanical or electrical properties of polymers becomes impossible in such circumstances. Equally the behaviour of the polymer in service isdependent on many parameters, which include not only the material and process but also the form of the housing and thefittings.

The material formulation and production process have an influence on the polymer housing characteristics; the following listgives those which are considered to be among the most important:

• hydrophobicity,

• tracking resistance,

• erosion resistance,

• puncture resistance,



1999-09-01

•

•

•

The appropriate design of polymeric insulators for specific polluted environments must, therefore, take these properties of theinsulator into account and the changes in them that may take place over time. It should also be noted that, as in the case of

pollution level is excessive or has increased appreciably since the initial selection was made.

Current standards provided in IEC publications - such as IEC 1109 - include general tests to evaluate overall performanceof a polymeric insulator (in this case covering materials, manufacturing process and form) taking into account many of the

performance can be determined not from the basic characteristics of the insulating components, but - rather - from the overall behaviour of the finished product.

Insulator performance

Fog, Clean-Fog) determines only the ability of the insulator to cope electrically with a controlled severity of wetted pollution.

In contrast, the natural pollution test fully replicates the service condition in that it shows both the extent to which an insulator

when naturally wetted (e.g. fog, mist, drizzle etc).

Major research programmes - most employing artificial pollution but a substantial number using natural pollution - have

work are found in Section 10the housing materials of the polymeric insulator types were of the ethylene propylene formulations, silicone rubber and epoxy-resin. These various insulator types embrace wide ranges of size - both length and diameter - and shape (i.e. profile); the

10.1. Some tests have been performed

The results have been analysed in terms of both the critical axial stress (i.e. critical voltage divided by the axial distance between metal fittings) and the critical surface stress (i.e. critical voltage divided by the leakage path length along the surface

Table 3 2: Ratio of best to worst insulator performance, for ceramic insulators.

a.c. d.c.* N(E) A N(J) A(C) A(K’) A(K’’’) A(F)

(Axial stress)1.2 2.1 1.4 1.7 1.7 1.4 2.3

(Surface stress)1.5 2.0 1.5 1.5 1.7 1.4 1.9

5** 27 5 17 25 4 6Table

10-25 10-26

Table Table

10-31 Table 10-33

Table Table

10-30 10-29

Table Table

10-29* N(E) marine pollution in England equivalent to about 60 kg/m3

N(S) is natural marine pollution in Sweden.is artificial salt-fog of 80 kg/m (test at CERL) or ESDD of 0.6 mg/cm (at either FGH or NGK).

N(J)A(T) is artificial pollution, Tonoko/NaCl of ESDD = 0.05 2.

is artificial pollution, of cement slurry.A(K’) mg/ cm2

A(K’’) is artificial pollution, Kaolin/NaCl of ESDD = 0.05 2.’’’) is artificial pollution, Kaolin/NaCl of ESDD = 0.05 2.

is artificial salt-fog of 28 kg/m .**

For the same test method and the same severe severity of pollution, Table 3-2 Table 3-3 show - for the ceramic insulators



1999-09-01 40

This variation is expressed as the ratio of the average electrical stress-value for the best insulator to that of the worst one(Section 10.2 provides a comprehensive summary of the corresponding stress-data). In addition, these tables give the number of insulator types in each category and the relevant table number in Section 10.2.

These findings clearly demonstrate that the pollution flashover performance of an insulator can not be related solely to either axial stress or surface stress; i.e. if it were, this ratio would obviously be unity. It is seen that this ratio for axial stress is inthe range of 1.2 to 2.3 and 1.1 to >1.7 for ceramic and polymeric types respectively. For the same two sets of insulator-typesthe corresponding ratio for surface stress is in the range 1.4 to 2.3 and 1.2 to >4.7 respectively. Such a large variation occurseven for insulators of the same generic shape when subjected to both the same test method and pollution severity. For example for cap and pin insulators under a.c. energisation in the Salt-Fog test, this ratio is 1.9 and 1.8 for axial stress andsurface stress respectively.

Table 3-3: Ratio of best to worst insulator performance, for polymeric insulators.

ENERGISATION a.c. d.c.Pollution * N(E) A(S) A(C) A(K) A(K’’) A(F)Performance ratio(Axial stress)

>1.7 1.3 1.2 1.6 1.2 1.1

Performance ratio(Surface stress)

>4.7 1.4 1.2 1.8 1.5 1.3

Number of insulator types 5 4 2 5 6 6Table giving data Table

10-34Table10-35

Table10-37

Table10-36

Table10-38

Table10-38

Notes: * N(E) is natural marine pollution in England, ≈ 60 kg/m3 salt-fog.A(S) is artificial salt-fog of 80 kg/m3.A(C) is artificial pollution, of cement slurry.A(K’) is artificial pollution, Kaolin/NaCl of ESDD = 0.05 mg/ cm2.A(K’’) is artificial pollution, Kaolin/NaCl of ESDD = 0.07 mg/ cm2.A(F) is artificial salt-fog of 28 kg/m3.

Other findings that throw further doubt on the use of solely axial stress or surface stress for insulator dimensioning purposes

are covered in the sections that follow.Although the correlation of pollution flashover performance with the various characteristics of the insulator (e.g. leakage

path, axial length, form factor, pin cavity / core diameter and overall diameter) are generally poor, there are some discernibletrends when examined from the viewpoint of generic shape - e.g. (a) disc design (i.e. cap and pin) (b) cylindrical design (i.e.longrod, post barrel) - waveform of energisation (i.e. a.c. or d.c.); material (i.e. ceramic - meaning glass or porcelain - or

polymeric).

3.3.1 Ceramic insulators

3.3.1.1 Effect of pollution severity

Generally, the critical stress E(kV/m) at flashover and the pollution severity S (e.g. kg/m3 for Salt-Fog) can be expressed as:

E S p∝ − (3-1)

The value of p provided by Lambeth 1, for various types of insulator and numerous types of pollution test, falls within therange 0.08 to 0.6; to which he comments that insulators with plain open shedding tend to have the higher values of p. Further,his analysis indicates that the Salt-Fog test gives higher values of p than does the Solid-Layer-type test.

From the mathematical viewpoint, the value of p can be considered as a weighted average of the one for the electrolyte surface(e.g. p=0.33 for brine) and that for the air breakdown (p=0) between parts of the insulator surface.

The corresponding relationship in terms of specific length SL is:

SL S s∝ (3-2)

where the value of s is obviously related to that of p for each type of insulator and test method.

From the data presented in Section 10.2 and elsewhere, SL can be considered in two ways:



1999-09-01 41

SAL, Specific Axial Length; defined as the axial distance between the metal fittings divided by the voltage across the

2. insulator (i.e. the inverse of surface stress).

Some typical relationships of specific length versus pollution severity are shown in and Figure 3-2insulator types and pollution test methods, the general trend is for s = 0.2 in equation 3-2.

Figure 3-1: Performance of standard types of a.c. cap and pin insulators in the Salt-Fog test and in the Clean-Fog test107

.

Figure 3-1 attempts to correlate the severity scale for the Salt-Fog test with that for the Clean-Fog test when applied tostandard designs of cap and pin insulators. However, this figure clearly shows that there is a substantial spread in the specific

creepage length at any value of pollution severity - thereby reinforcing the point made earlier that specific creepage is not theonly factor that needs to be used when dimensioning such insulators. The data from which this figure has been compiled are

provided in Section 10.3.

Figure 3-2: Dielectric strength of different a.c. insulators in the Salt-Fog test197

.

In contrast, the results for the porcelain longrod - presented in Figure 3-2 as solid dots - are reasonably well ordered and sosuggest that the sole use of specific creepage is probably valid for this design. The likely reason being that the shape of thisinsulator is relatively simple and that the ratio of shed diameter to core diameter is not large. Nonetheless, the general trend

of specific creepage distance with withstand salinity is similar to that for the discs.



1999-09-01 42

Figure 3-2 also provides a comparison of some results for the cap and pin design of insulator with those for some post-typeinsulators. Again these results show a considerable spread of values for these cylindrical types of insulator - therebysupporting the analysis reported earlier.

A complicating feature of comparing the results of different test methods is that the ranking of insulator performance is notalways the same - as amply demonstrated in Figure 3-3 for some d.c. cap and pin insulators, when subjected to: (a) the Salt-Fog test (b) the Clean-Fog test and (c) a dust-spray method. These results indicate the important effect that insulator profilehas on the electrical strength of such insulators.

40

50

60

70

80

90

110

120

E l e c t r i c a l s t r e s

s ( k V / m )

Uw U50/L (kV/m) Uw

(a) Salt-fog method, 28 kg/m3

(b) Solid-layer method, 0.07 mg/cm2 (c) Dust-spray method

Figure 3-3: d.c. Pollution performance of different ceramic insulators under different laboratory pollution test methods, U w :withstand voltage, U 50: 50% flashover voltage, L: the axial spacing between insulator fittings, v-: glass insulator, p-

:porcelain insulator. (Data are from the paper of Pargamin et al315

; the bottom line of each insulator is positioned in respect

to the voltage values).

3.3.1.2 Influence of insulator profile

3.3.1.2.1 a.c. Energisation

With the standard-type of profiles, it has been found that there is no great variation in performance between the differentshapes tested 54 111. It has been, therefore, concluded that unless a large increase in creepage distance per spacing is made no

big improvement in contamination performance will be achieved 111.With the fog-type insulators - that is, insulators with a large creepage distance per spacing ratio - discharges may bridge theunder-ribs causing some profiles to perform worse than others where this bridging does not occur 111.

In general, it is regarded beneficial to have a larger spacing between sheds and that the alternating profiles (i.e. one in whichthe ribs have different lengths) perform better than the box type (i.e. one in which all the ribs have the same length).

3.3.1.2.2 d.c. Energisation

In a study of d.c. station post insulators 108, 12 different shed profiles have been tested using the Solid-Layer method. Thesedifferent profiles were variations on 3 basic ones, where the shed spacing and overhang varies - as is shown in the lower partof Figure 3-4. In the upper part of this figure, the ratio between the creepage distance and the axial length of the tested

insulators is plotted against the ratio between the 50% flashover voltage and the axial length. It is clear from this figure that asignificant improvement in performance cannot be achieved by increasing only the amount of creepage distance in a givenaxial length.



1999-09-01 43

Station post insulators having a deep under-rib profile - very similar to that of type II shown in Figure 3-4, but with differentdimensions - have been tested and compared 109 with one having the alternate long-and-short shed profile and another of the

plain-shed type. The results are provided in Table 3-4. The tests were performed under d.c. voltage with the Fog Withstandmethod. The importance of keeping a large shed spacing while increasing the creepage distance can be seen whencomparison is made with the results for the various deep under-rib profiles.

Figure 3-4: d.c. Laboratory pollution test results with Solid-Layer method for station post insulators108

.

Table 3-5 110 provides results that demonstrate that an insulator of an alternate long-and-short profile with a large shed spacingcan perform as good as an insulator with a deep under-rib profile. The final choice from among the various insulator-shed

profiles should be based on the site conditions, taking account of the different aerodynamic properties that influence the pollution-catch and the natural cleaning ability of the insulator.

Table 3-4: d.c. Laboratory pollution test with Fog Withstand method - SDD: 0.03mg/cm2 - for station post insulators with a

core diameter of 125 mm 109

.

PROFILE SPACING

mm

OVERHANG

mm

EFFECTIVE HEIGHT

mm mm

A STRESS

deep under-rib 70 1006 407085 85 3480 99.2

95 1006 109.395 1008 350595 105 3605 118.0

105 1008 119.0alternate long short 65 1008 104.2normal 70 1008 90.2



1999-09-01 44

Table 3-5: d.c. Laboratory pollution test with Solid-Layer method - SDD, 0.02 mg/cm2 - of an insulator having an effective

height of 1.95 m and a core diameter of 0.22 m 110

.

3.3.1.3 Linearity of insulator performance in relation to string length

3.3.1.3.1 Under a.c. voltage

The question of whether the pollution performance of long strings of ceramic insulators is a linear function of string length has been addressed by making Clean-Fog artificial pollution tests. It is generally agreed that up to EHV levels - i.e. 275 kV to500 kV, the performance is linear 111. In some of the early investigations, there was an indication that a pronounced non-linearity would seriously affect the design of insulation for transmission lines operating at ultra high voltage (UHV), namelyfor a.c. system voltages above 800 kV 111 112. However, an extensive subsequent study exploring these phenomena showedthat, although there was some degree of non-linearity, the required additional string length above a linear extrapolation for such higher voltages was considerably less than originally estimated 113. The concept of long-string efficiency was introducedto take these effects into account.

Long-string efficiency is defined as:

L L V

V UHV

EHV UHV

EHV

= •

• λ(3-3)

Where :LUHV = string length required at a UHV voltage levelLEHV = string length determined at a lower voltage level

VUHV = UHV voltage levelVEHV = lower voltage levelλ = long-string efficiency

Note: the specification is based on EHV data - rather than that from lower voltage levels - to avoid large errors in stringefficiency that could arise from even small errors in the flashover voltage of short strings.

The dependence of long-string efficiency on the line-to-earth voltage is shown in Figure 3-5, which applies to standardvertical insulator strings up to 11.5 m connection length. The equivalent salt deposit density (ESDD) is in the range of 0.01-0.04 mg/cm2.



1999-09-01 45

100

90

80

70

60

0300 400 500 600 700 800 900 1000

kV

L o n g s t r i n g e f f i

c i e n c y ,

λ ,

λ ,

( % )

Figure 3-5: Long-string efficiency for a.c. energisation as a function of line to earth voltage.

Range of ESDD 0.01-0.04 mg/cm2. IEEE insulators (146 mm spacing, 254 mm diameter, and ratio leakage to spacing 2.1).

For antifog insulators, the results for long-string efficiency are shown in Figure 3-6 for string connection lengths up to 8 m. Inthis case, the range of ESDD is 0.02-0.04 mg/cm2.

100

90

80

70

60

0

L o n g s t r i n g e f f i c i e n c y ,

λ ,

λ ,

( % )

300 400 500 600 700 800 900

kV

Figure 3-6: Long-string efficiency for a.c. energisation as a function of line to earth voltage.

Range of ESDD 0.02-0.04 mg/cm2. Antifog insulators (220 mm spacing, 420 mm diameter, and ratio leakage to spacing 3.3).

From a limited number of artificial pollution tests conducted under project UHV, it has been concluded that a proper assessment of the non-linearity aspect can only be determined once all the factors in the wetting process of the insulators areknown 111.

A comparison of the test results for indoor- and outdoor-conditions indicates that the indoor values are probably morecomparable to worst-case conditions111.

In another study 125, carried out at the Brighton Insulator Test station (BITS) in UK, insulators for both EHV (up to 420 kV)and UHV (up to 1560 kV) systems were tested side by side under natural coastal conditions. For overhead line insulators, ithas been concluded that the flashover stress (in kV/m) was unaffected by the voltage level up to at least 1200 kV system; thetest results are given in Figure 7-13 of Section 7.2.8. For the multiple-cone type post insulator tested, on the other hand, asignificant reduction (13 %) was found in the performance at the higher voltage level.



1999-09-01 46

Figure 3-7: Relationship between axial distance and a.c. contamination flashover voltage114

.

For a given type of standard insulator and of an antifog cap and pin design, the withstand voltage (obtained by the Clean-Fogtest method) has been found to be proportional to the creepage distance of the insulator - as is shown in Figure 3-7. Such alinear relationship is also applicable to station post insulators 114.

3.3.1.3.2 Under d.c. voltage

Many laboratory tests have been performed to examine whether or not a linear relationship holds between the pollution performance and the insulator length 115 118 116 117 108 110. For line insulators, such results point to a linear relation between theflashover voltage and the insulator length when the pollution level is high; i.e. SDD ≥ 0.05 mg/cm2. When the pollution levelis low - i.e. SDD< 0.05 mg/cm2, a non-linear relationship has been obtained by some laboratories 118 116 117 110 whilst linearityhas been reported by others 115. An example of some results 117 is provided in Figure 3-8.

Figure 3-8: d.c. Pollution test (Solid-Layer method). Cap and pin insulators. U50 vs. string length117

.



1999-09-01 47

Test results for station post insulators also have the same trend. At a low pollution level, because of the non-linearity reported by several laboratories, the total insulator length for a given type of insulator needs to be increased by 10-15 % for 600 kVsystems and 15-20 % for 800 kV systems with respect to the design made assuming linearity; see Figure 3-9 and Figure 3-10.However, it should be noted that these discussions are based on laboratory results. When performing an artificial pollutiontest the whole insulator string is, in most cases, polluted uniformly. In natural conditions, however, non-uniform pollutiondistribution along the insulator string is often encountered. Furthermore, at a higher voltage level, more pollution may be

attracted to the insulator - as already mentioned in Section 2.3.7.5. All these factors add to the uncertainty for making linear extrapolation of the required insulator lengths from a lower-voltage level to a higher-voltage level.

Figure 3-9: U 50 as function of the length of the suspension string under d.c. (Negative polarity) pollution tests118 116

.

Figure 3-10: The U 50 (d.c. negative polarity) as a function of the total height of the station post insulators

110

.



1999-09-01 48

3.3.1.4 Influence of average diameter

Some of the data indicate that at flashover, or withstand, the relationship between specific length, SL (mm/kV) and averagediameter D (mm) is of the form:

SL D q∝ (3-4)

where q is a constant having a value that is particular for the set of conditions of the insulator’s generic shape, the material, theenergisation waveform and type of pollution. In Figure 3-11 test data are shown for a.c. energisation that reveal thisrelationship. Similar test data exist for the case of d.c. energisation85.

Figure 3-11: Relationship between average diameter and required leakage distance in per unit of a.c. withstand voltage85

(for shed shapes please refer to Table 10-22).

The best support for equation 3-4 occurs when the insulators are of the same profile and only the diameter is varied; Figure 3-12 shows the results for d.c. housings that have been subjected to a Clean-Fog test with an ESDD of 0.12 mg/cm2

Figure 3-12: Specific Axial Length vs. insulator diameter for d.c. ceramic housings under Clean-Fog test 199

; Table 10-33

refers.



1999-09-01 49

When the insulator profiles vary, the spread in the results is greater - even to the extent that they fall within a band rather thanapproximate to a straight line. Figure 3-13 and Figure 3-14 show results for a.c. disc insulators and a.c. cylindrical insulatorsrespectively that are subjected to a Salt-Fog test of 80 kg/m3.

General range level

Upper values

Lower values

Figure 3-13: Specific creepage length vs. insulator diameter for a.c. ceramic disc insulators under salt-fog pollution; Table

10-24 refers (number next to point is ranking in Table 10-24).

Figure 3-14: Specific creepage length vs. insulator diameter for a.c. ceramic cylindrical insulators under artificial pollution;

Table 10-24 refers.

The values of q in equation 3-4 for those test findings that provide a moderate to good support for this relationship are givenin Table 3-6. This table also gives the corresponding table number in Section 10 from which the information was obtained.

All the other findings using the tables given in Section 10.2 give either only weak support for this equation or, in the case of some d.c. results, a negative slope. It is tempting to ascribe this negative-slope finding to the weakness of the d.c. supply; i.e.



1999-09-01 50

for the same pollution conditions, the leakage current increases as the average diameter increases --- thereby, possibly,resulting in voltage-regulation problems. However, at least one such case is known in which the source was strong 118.Therefore, this negative-slope characteristic warrants further investigation. It should be noted that the results presented inTable 3-6 are for ceramic insulators. The findings for the limited number of tests performed on polymeric insulators haveonly a weak agreement with this relationship.

In a similar exercise conducted by CESI for some a.c. insulators, it was shown that the average value of q was 0.35 - i.e. it lieswithin the range (0.14 to 0.65) of the findings shown in Table 3-6.

Table 3-6: Value of q in equation 3-4 for ceramic insulators

DATA SOURCE INSULATOR

SHAPE

ENERGISATION POLLUTION SL PARAMETER VALUE OF q (EQ. 3.1)

Figure 3-11 Cylindrical a.c. Clean-fog SCL 0.43

Table 10-24 Disc a.c. Salt-fog SAL 0.41

Table 10-24 Disc a.c. Salt-fog SCL 0.74

Table 10-24 Cylindrical a.c. Salt-fog SAL 0.24

Table 10-24 Cylindrical a.c. Salt-fog SCL 0.14

Table 10-25 Disc a.c. Marine SCL 0.50

Table 10-27 Cylindrical d.c. Clean-fog SCL 0.37

Table 10-32 Disc d.c. Clean-fog SCL 0.49

Table 10-33 Cylindrical d.c. Clean-fog SAL 0.50

Table 10-33 Cylindrical d.c. Clean-fog SCL 0.51

As to the general fit of the results to equation 3-4 for positive values of q, the trend seems to be that the SCL parameter is a better one to use for disc insulators than the SAL parameter. However, for cylindrical insulators there is no clear advantage of one parameter over the other.

3.3.2 Polymer ic I nsulators

3.3.2.1 Natural pollution

The a.c. flashover performance of a few types of polymeric transmission-line insulators has been determined at the BrightonInsulator Testing Station 126 127 by using the technique developed for ceramic insulators 125. From a summary of the results

provided in Table 10-34, it is seen that both the silicone rubber type and the ethylene propylene rubber ones (i.e. EPR andEPDM) performed better than the corresponding string of porcelain reference insulators of the anti-fog design. As noflashover occurred on the silicone rubber insulators, their ultimate performance could not - unfortunately - be determinedduring this test programme and so this aspect warrants further investigation.

This excellent performance of the silicone rubber type of insulator is now being confirmed in service. For example, SouthAfrica - which probably is currently the largest user of such insulators at transmission-line voltage levels - has had no

pollution flashover on previously troublesome lines after they had been re-insulated with silicone rubber insulators 119 120.

In Tunisian desert conditions, polymeric insulators have performed well over a number of years at various outdoor test-stations in polluted regions comprising: (a) marine/ agricultural, (b) marine/ industrial, (c) marine/ industrial/ chemical, (d)marine/ chemical/ industrial/ desert and (e) desert 61.

In contrast, the pollution flashover performance of epoxy-resin type insulators is - regrettably - inferior to the corresponding porcelain insulators; as quantified in Table 10-34.

3.3.2.2 Artificial pollution

The standard Salt-Fog test performed on the same a.c. insulators that were investigated at BITS has also shown the superior performance of the silicone rubber type and the ethylene propylene types of insulators over that of the porcelain reference

insulator. However, this artificial pollution test failed to produce the same ranking as that found under the correspondingnatural conditions 378; cf. Table 10-34 and Table 10-35. Although the same ranking was achieved in a modified version of



1999-09-01 51

this standard test, the relative performance of EPDM to porcelain was far greater than that occurring under natural pollution121.

A difference of ranking has also been observed with d.c. insulators when the Salt-Fog test and the Clean-Fog test were used81,as amply shown in Table 10-38.

Further, the rankings from the viewpoint of axial stress and surface stress along d.c. insulators have been found to be

substantially different 319 320, as seen in Table 10-36 and Table 10-37.Therefore, these differences of ranking and the need to obtain more valid data on the absolute flashover voltage values for

polymeric insulators when subjected to the various types of artificial pollution indicate an area of research were a substantialamount of work still needs to be done.

3.3.2.3 Effect of insulator profile

The effect of profile on the tracking and erosion performance of various polymer insulators has been studied 122. Six profiles,all of the aerodynamic type, were investigated by exposing insulator sections to long-duration Salt-Fog tests. Insulators with a

positive shed-underside inclination (see IEC 815 5) performed better than did insulators with zero or negative shed-inclination.

In another study123, the flashover performance was determined - under a Clean-Fog test with an artificially applied pollution

layer - of both the alternate long-and-short shed designs and the aerodynamic shapes of regular design. The shed parametersinvestigated were the ratio of leakage path length to shed spacing and the type of shed profile. The results indicated thatalternate long-and-short shed profiles performed better than did the regular types. The results also indicated that the efficacyof the leakage path decreases rapidly for a shed spacing of 300 mm or less. Further, it showed that for each shedconfiguration there exists an optimal leakage path to spacing ratio; typical values range between 4 to 4.6.

As with ceramic insulators, a further complicating feature in the study of the effect of profile is that the ranking of insulator performance is not always the same for all test methods. This is amply demonstrated in Figure 3-15 for some polymericinsulators when subjected to (a) the Salt-Fog test (b) the Clean-Fog test and (c) a dust-spray method. This makes it difficultto give specific guidelines on good proportions for polymeric insulator profiles.

80

100

120

140

160

180

200

E l e c t r i c a l s t r e s s ( k V / m )

Uw/L (kV/m) U50/L (kV/m) Uw/L (kV/m)

(a) Salt-fog method, 28 kg/m3(b) Solid-layer method, 0.07 mg/cm

2 (c) Dust-spray method

Figure 3-15: d.c. Pollution performance of various polymeric insulators under different laboratory pollution test methods.

U w is the withstand voltage; U 50 is the 50% flashover voltage; L is the axial spacing between insulator fittings. (Data are

obtained from Pargamin et al315

; the bottom line of each insulator is positioned in respect to the voltage values).



1999-09-01 52

3.3.3 Ef fect of insulator orientation.

3.3.3.1 Introduction

Wetted pollution on the surface of any high-voltage insulation can produce a substantial reduction in its electric strength 1 54.However, the effect of the orientation and the size of such insulation on its flashover performance is not generally subject tosimple rules. The insulator-type directly affects the performance of the polluted insulation in different orientations. Inaddition, the pollution severity at a site, and the time taken for maximum contamination levels to build up, may determine theeffect of orientation. The nature of the subsequent wetting process and the flashover mechanism (e.g. surface flashover or inter-shed breakdown) are also important factors affecting the influence of orientation and size.

Hence, the flashover strength of different insulator types and orientation is a balance between the various processes thatdirectly influence such performance. The following mechanisms may contribute, or be dominant, for each design andorientation:

1. Improved natural cleaning as the orientation changes from being vertical to being horizontal.

2. Directional effects of pollution deposit for angled/horizontal orientation from a localised (direction-defined) pollutionsource.

3. Inter-shed breakdown due to heavy rain and pollution.

4. Inter-shed breakdown due to pollution and poor profile.

5. Reduced flashover strength due to pollution concentration on the lower surface of horizontal, or near-horizontal,insulation during heavy fog or rain.

In reality, there is no real substitute for testing insulators under the appropriate pollution and wetting conditions to determinehow actual insulator designs will perform in different orientations. Although there is a dearth of published data to quantifythese effects, this section - nevertheless - presents and discusses a few results of investigations into the influence of orientationand size on the flashover strength of polluted insulation of various designs. Experimental results from artificial pollution testsand from outdoor marine testing stations for various insulators and orientations are analysed to investigate if some simplifiedconclusions can be drawn from the data.

3.3.3.2 Insulators

The influence of orientation and size are analysed and discussed for the following types of insulation:

1. Cap-and-pin insulators.

2. Polymeric insulators.

3. Substation post insulators with alternate long-and-short sheds (ALS) and multiple cone type profiles.

4. Tapered bushing porcelains with ALS profiles.

5. d.c. Wall bushings.

6. Interrupter head porcelains with open profiles.

Unfortunately, there seems to be no corresponding data for porcelain longrod insulators.

The results reported herein on the hollow porcelains have been obtained for insulators that were sealed with end-flanges and

pressurised with either dry nitrogen or SF6, to avoid having internal surface discharges. In general, hollow insulators have been tested without their internal grading components because such items complicate the test assembly, but do not affect the pollution flashover process - because it is not an electrostatic-field problem. Salt-Fog test results on a complete SF6 / Air bushing confirm the validity of testing only the porcelains.

3.3.3.3 Cap-and-pin insulators

Salt-Fog tests on several types of cap-&-pin insulator strings mounted in the vertical and for inclined orientations haveestablished that an improved performance for a.c. energisation, corresponding to 10-20% increase in effective length, resultedfrom inclination124. However, inclination in excess of 200 to the vertical gave no further improvement.

A.c. energised insulators subjected to natural marine pollution at the Brighton Insulator Testing station125 have shown thesame trend of good performance of the angled and horizontal orientations for various insulator types relative to that for a

vertical string of the CERL reference insulator - i.e. type A in Table 10-1.



1999-09-01 53

The findings are summarised in Table 3-7. They are given both as a figure of merit (FOM) - i.e. a measure of relative axialstress - and as a leakage path ratio - i.e. a measure of relative surface stress; the definition of these terms is provided beneaththis table. Note: the corresponding value of FOM=1 was obtained for type I insulators when vertically mounted.

Table 3-7: Cap and pin ceramic insulators, angled or near-horizontally mounted - a.c. flashover performance under marine

pollution at BITS*.

R ANKING

NO

INSULATOR TYPE**

ORIENTATION***

FOM****

(AVERAGE)

LPR*****

1 II antifog H 1.35 1.372 I, antifog H 1.31 1.323 I, antifog A 1.27 1.274 VII L.C. antifog H 1.12 1.205 II antifog A 1.11 1.146 VIII Standard disc H 0.79 1.16

Notes:

* Data obtained from reference 125.** L.C. is long creepage; i.e. term used in reference 125.

*** H= approximately horizontal (i.e. 75o to the vertical), A= Angled (45o to the vertical).**** Measure of flashover performance, from the viewpoint of axial length, when compared to

that of a vertical string of reference insulators (i.e. CERL Reference A, Table 10-1); it isan average of all values for the same insulator type.

***** LPR is leakage path ratio; determined as the leakage path of the CERL Reference A insulator divided by that of the Test insulator, for the same pollution flashover performance.

The results from Noto Testing Station, at a coastal location in Japan 59, also showed that tension strings have almost the same- or a little higher - pollution flashover strength compared with that of suspension strings. These tests were similar to thoseconducted at Brighton in that insulator strings were systematically increased in length until approximately equal flashover frequencies were established for all of the tested insulators.

Tests conducted on contaminated (SDD = 0.02 mg/cm2) insulators of the IEEE type have shown that - based on the 50%flashover strength - the long-string efficiency values are 95%, 92% and 90% for quasi-horizontal, V-string and vertical stringrespectively111.

3.3.3.4 Polymeric insulators

The results for some polymeric insulators tested at Brighton, which correspond to those obtained for the cap and pininsulators, are provided in Table 3-8. To facilitate comparison with corresponding data for such insulators when verticallymounted, the relevant values from Table 10-34 are also included in Table 3-8.

Table 3-8: Polymeric insulators: (a) near-horizontally mounted* , (b) vertically mounted - a.c. flashover performance under

marine pollution at BITS**

.

R ANKING NO INSULATOR FOM****

LPR*****

WHEN HORIZONTAL TYPE*** HORIZONTAL VERTICAL HORIZONTAL VERTICAL

1 VII Silicone rubber >1.53 >1.53 >2.5 >2.52 VI EPDM 1.28 1.12 >2.63 2.273 VIII EPR 1.19 1.17 1.62 1.164 V EPDM 1.14 1.21 1.18 1.25

Notes:

* Near-horizontal is about 75o to the vertical.** Data determined from references 126 and 127.*** Descriptions used in References 126 and 127.**** FOM is the Figure of Merit and is the axial length of the Test insulator divided by that

of a vertical string of reference insulators (i.e. CERL Reference A in Table 10-1) for the

same pollution flashover performance.



1999-09-01 54

***** LPR is leakage path ratio, determined as the leakage path of Test insulator divided by that of the Reference insulator, for the same pollution flashover performance.

3.3.3.5 Substation post insulators

Some results for post insulators are presented in Table 3-9 for an ALS design (insulator type P1) and a multiple cone one.

These findings again confirm the orientation effect in the Salt-Fog test. For P1, the corresponding value of s in equation 3-2 is0.11 for both the horizontal and the vertical cases - which indicates that a significant amount of air breakdown is occurring for both orientations. The Withstand Pollution Severity (WPS) values from the heavy wetting test were 160 kg/m3 for both thevertical and the horizontal orientations - probably indicating that the air breakdowns were not directly from shed to shed.

Table 3-9: Specific Creepage length *, (SCL), at a.c. flashover for two types of post insulator when mounted (a) vertical, V,

and (b) horizontal, H.

SCL (mm/kV) FOR SALT-FOG SALINITY (kg/m3) OF

INSULATOR TYPE ORIENTATION 5 14 40 160

Post P1 V 28 31 40 160Post P1 H 45 50 55 66

Multiple cone V 29 33 38 43Multiple cone H 29 35 42 54

* SCL is length of insulating surface divided by the voltage across the insulator.

The multiple cone post had the smallest orientation effect of the post-type insulators tested, but the gradient of the flashover voltage versus salinity relationship was greater for the horizontal case (cf. s=0.17 to s=0.1). WPS values of 160 kg/m 3 wererecorded in both the vertical and the horizontal orientations.

From the above limited data, there is some evidence to indicate that ALS profiles may not represent the most efficientinsulator-shape for horizontal posts.

3.3.3.6 Tapered bushing porcelains

The withstand salinity and the heavy wetting pre-applied salinity - both salinities provided by an artificial salt-fog - are shownin Table 3-10 as a function of orientation for an insulator having alternate long and short sheds (a 65/45 70 profile) with atotal creepage path of 7600 mm and with an applied voltage of 173 kV rms.

Table 3-10: Salt-fog withstand salinity and heavy wetting pre-applied salinity, for an insulator having a 65/45 70 profile and

a total creepage of 7600 mm, when the a.c. test voltage was 173 kV rms and the orientation was varied.

ANGLE TO

VERTICAL

SALT-FOG WITHSTAND SALINITY

(kg/m3)

WITHSTAND PRE-APPLIED SALINITY

(kg/m3)

0 28 160

20 28 56 (front) 80 (rear)

45 14 160

90 5 160

Front means bushing angled towards wetting sprays.Rear means bushing angled away from wetting sprays (usual case)

The corresponding values of SAL at different salinities - as determined using the Quick Flashover test - are shown in Figure 3-16 for both the vertical and the horizontal case.



1999-09-01 55

Figure 3-16: A.c. flashover voltage as a function of salinity of salt-fog for the insulator having a 65/45 70 profile and a total

creepage of 7600 mm for both vertical and horizontal cases128

.

Again it is seen that the flashover performance, in salt-fog, of a large diameter insulator is much inferior when in thehorizontal position as compared with that for the vertical one. The Quick Flashover tests for vertical mounting show a veryflat slope in the SAL versus fog salinity relationship (s=0.06) - thereby indicating significant air-breakdown. In contrast, thecorresponding data for the horizontal case demonstrate a significant increase in gradient (s=0.19) - with increased surfacedischarge activity along the bottom insulator surface.

These heavy wetting test results confirm that this process does not significantly affect horizontal bushings but can reduce theinter-shed breakdown capability of some bushings that have angles close to the vertical. The heavy wetting withstand of 160 kg/m3 when the insulator is vertical suggests that the large component of air breakdowns, indicated by the Quick Flashover tests, were not from shed to shed.

3.3.3.7 d.c. Wall bushings

D.c. wall bushings are discussed under Section 3.4.2.

3.3.3.8 Interrupter head porcelains

The axial electric stress necessary to cause external flashover of the interrupter porcelain of various designs - profile anddimensions are provided in Table 10-23 - is shown in Figure 3-17 as a function of the severity of an artificial salt-fog 129. The

much inferior performance of the horizontal head is reasoned to be associated with the way that the wetted pollution drainsfrom the insulator surface.

Limited data from an interrupter head porcelain, H1, tested at Brighton, showed a Figure of Merit of between 0.7 and 1.0,during a short test period. These results indicate that the concentration of wetted pollution on the lower surface may be asimportant a process under natural conditions as that witnessed in the artificial Salt Fog tests. However, more data are requiredto establish if better washing by rain - on average - negates the pollution concentration effect under natural conditions.



1999-09-01 56

Figure 3-17: A..c. axial stress to cause flashover against fog salinity for various interrupter porcelains129

. V1 was vertically

mounted; H1, H2, H3, and H4 were horizontally mounted. Insulator details are provided in Table 10-23.

3.3.4 I nf luence of a non-uni form poll ution deposit

It was shown in Section 2.3.3 that in-service insulators are rarely, if ever, uniformly polluted. In this section a broad overview

is given of the influence of a non-uniform pollution deposit on the electrical strength of insulators.There is usually a difference in ESDD measured on the top and bottom surfaces of insulators. Often the pollution severity onthe top surface is much lower than that on the bottom surface, usually because of the cleaning action of rain. It was shownthat this non-uniform pollution distribution affects insulator flashover strength under both d.c.118 130 and a.c.111 energisation.Figure 3-18 shows a summary of the reported results for insulators with different top to bottom pollution density ratios, butwith the same average ESDD on the total insulator 280.

Figure 3-18: Withstand voltage characteristics of non-uniformly polluted insulators280

.



1999-09-01 57

For the insulator-string configuration, different pollution deposit densities will also be experienced on the discs making up thestring 131 318 38. Under d.c. energisation, those insulators situated towards the ends of the string will generally collect more

pollution than the ones in the middle131. This non-uniform pollution distribution also seems to affect the d.c. flashover voltage, as is shown in Figure 3-19. This figure provides the d.c. fog-withstand voltage of a non-uniformly polluted string -having the heavier polluted - i.e. 0.08 mg/cm2 - insulators situated towards the ends - as a function of the percentage of suchheavily polluted insulators in the string. It can be seen that for strings containing up to 30% of heavily polluted insulators, the

d.c. withstand voltage remains about the same as that of the insulator string uniformly polluted to the ESDD of its middle part- i.e. 0.03 mg/cm2.

Percentage of heavily polluted insulators in the string

F o g w i t h s t a n d v o l t a g e , k V / u n i t

Figure 3-19: Withstand voltage characteristics of d.c. insulators polluted non-uniformly along a string131

. The insulator

details are provided in the reference.

In the case of gas-insulated bushings of the UHV a.c. class, it was observed at the NGK Laboratory that less pollutioncollected on the earth -side of the bushings than did that on the live-side. This lighter polluted area covered 20-40% of theoverall bushing length. Tests indicated that the withstand voltage was reduced by about 6% for these non-uniformly polluted

bushings as compared with that for the uniformly polluted ones 132.

A study of the effect of non-uniform pollution on longrod insulators 65 has shown that the a.c. electrical strength of the longrodmay be adversely affected by the presence of insulator sections that are polluted to a lesser degree than the rest. For aninsulator with 30% of its length covered by a lighter degree of pollution, a 25% reduction in electrical strength was observedas compared with that for the uniformly polluted insulator. Similar results may, possibly, be expected for d.c. energised

bushings.

3.3.5 Electri c f ield at the sur face of insulators


Discharge activity at the surface of a high-voltage insulator is caused by the local electric field having a value higher than theionisation level of the ambient air. This high electric field is the result of the applied voltage and the environmentalconditions such as rain, pollution and ice. Recent work indicates that surface discharges - such as sparks and corona - caused

by local field enhancement around water drops on the surface of polymeric insulators may lead to severe material degradation.If the surface electric field under the different conditions can be calculated - or measured - it will provide knowledge for applications to discharge models and help to improve the insulator design through proper E-field grading designs. Althoughthe ultimate goal of such research has not yet been achieved, the progress in field calculation techniques and the introductionof new measuring methods have provided a greater possibility.



1999-09-01 58

3.3.5.2 Electric field measurement

Many measuring instruments have been developed for determining either the voltage distribution or the electric field.

Most of the voltage-measurement methods and instruments have a direct electrical connection between the instrument and theinsulator. Therefore, care should be taken not to draw too great a conductive current from the insulator so as to ensure thatthe true voltage distribution is not disturbed. A review has been made133 of several instruments used for measuring the d.c.

voltage distribution.

The electric field can be measured with a probe that has no galvanic contact with any grounded object. The measuring signalis transmitted by an optical link. To minimise the distortion of the field due to the presence of the instrument, a spherical

probe is preferred. The diameter of this probe should be as small as possible. To measure a d.c. electric field, other techniques are needed to prevent charging of the probe. The various principles of d.c. field probes have been reviewed 134.The field probe should be located no closer than a few probe-diameters from the insulator surface to avoid distorting the fielddistribution.

Three instruments, one for potential measurement and two for electric field measurement, have been reviewed for the a.c. case135. Using such instruments, measurements were performed under a.c. voltage along a dry insulator model with no discharges

present. The measured values were in good agreement with the calculated ones.

Field probes for both a.c. and d.c. electric field measurement have been developed, including a computer-controlled

positioning system 134 136. These measurements have provided the total, axial and radial fields. Such probes have been usedin various environmental conditions to study hydrophobic and hydrophilic post insulators and wall bushings 134 136 137 138 139

140. With the probe at a fixed position, the change of the electric field over time under different test conditions can bemonitored. By scanning the probe along a track close to the insulator, the average field distribution along the insulator under different conditions can also be measured.

Field measuring techniques can be used for both laboratory investigations and site-diagnostics of insulators. An example can be found in the literature 141.

3.3.5.3 Electric field calculation

A large number of electric field calculation programmes exist that are based on different calculation methods - such as FDM(Finite Difference Method), FEM (Finite Elements Method), BEM (Boundary Element Method), BIM (Boundary Integration

Method) and CSM (Charge simulation Method)135. To be able to calculate the electric field at the surface of an insulator, it isnecessary to select a suitable calculation method and technique capable of handling the complicated geometric structures of

practical insulators. However, the major obstacle to the obtaining of reliable results is the uncertainty involved in the parameters describing the surface situation (boundary conditions) of an insulator.

The applicability of field calculation programmes for the different conditions of the insulator are reviewed below.

3.3.5.3.1 No Di scharge activity

When an insulator is energised with a relatively low voltage - such that there is no discharge activity - an electric fieldcalculation can be made with good confidence for clean and dry insulators. Then there is good agreement between the resultsof the different calculation methods 135. A calculation has also been carried out for a real insulator under clean and dryconditions. Good correlation has been obtained between the calculated electric field and the field measurements 139.

For an insulator with a wetted and/or polluted surface, it becomes difficult to calculate the electric field reliably. This is because of the uncertainty in the values pertaining to the surface conditions - such as the non-uniformity of the wetting alongthe surface caused by cascading water or the drying effect of leakage current. However, for a totally hydrophobic insulator under uniform rain energised by a.c., the electric field distribution will not differ significantly from that of the dry condition140.

3.3.5.3.2 With Discharge activity

When discharge activity is present, in the form of corona at the metal accessories of the insulator or sparks, glows and arcs atthe dry bands, the situation at the insulator surface is further complicated. Reliable boundary values are not available for either the dry or the wetted conditions.

To perform approximate calculations, attempts have been made to evaluate the boundary conditions by analysing the fieldmeasurements and UV photographs of the discharge activity 136 137 138 139 140.



1999-09-01 59

3.3.6 Cold switch-on and thermal lag

When polluted insulators become well wetted their flashover strength is at a minimum. Such wetting can occur when aninsulator has been de-energised for some time - e.g. during maintenance / repair period or when a line is switched-out for voltage-control purposes - and when its temperature is more than a few degrees lower than that of the surrounding air -thereby resulting in enhanced condensation 71. For energised conditions and thermal equilibrium, the leakage current causes

sufficient evaporation - by Joule heating - to ensure that this high degree of wetting does not happen unless mist, fog or rain is present.

Although service experience shows that flashover can take place when insulators are suddenly energised following an outageor during early morning when the air temperature rises quickly (e.g. in deserts), there is not enough information available toaccurately quantify this problem. Nonetheless, guidance can be provided from the results obtained for semiconducting glazedinsulators. In some cold switch-on tests 142, the flashover voltage of a polluted insulator when soaking wet (0.15 MΩ per suspension insulator) was 40% lower than that of the same insulator when only damp (1000 MΩ per suspension insulator) andfourfold lower than the value for the same insulator when dry (15 000 MΩ per suspension insulator); see Figure 3-20. Suchresults are supported by those from other tests made under normal energisation in clean-fog 210. From this research it can beseen - Figure 3-42 - that the flashover strength of a semiconducting glazed insulator is 2 to 3 times greater than that of astandard glazed insulator when similarly polluted.

Figure 3-20: A.c. flashover voltage versus number of semi-conducting glazed disc insulators in a suspension string, for

various wetting conditions142

.

Using these findings for semiconducting glazed insulators, it is reasonable to conclude that the flashover strength of standardglazed insulators when polluted and highly wetted - as can happen for cold switch-on or due to thermal lag - is at least 40%

less than that of the same polluted insulator under normal service conditions when its temperature is similar to -or greater thanthat - of the surrounding air.

Also during cold switch-on, there may be transient over-stressing - as is discussed in Section 3.3.7.

3.3.7 Contaminated insulators under transient overvoltages


External insulation may be subjected, in service, to various transient stresses - of both internal and external origin whenconsidered from the viewpoint of the power system. These can be represented in the laboratory by lightning impulses, LI,switching impulses, SI, and by short duration a.c. application and transient overvoltages (TO). Depending on the line

condition, the transient overvoltages may be superimposed on the permanent a.c. or d.c. voltage. This condition can berepresented in the laboratory by having composite voltages (e.g. LI, SI, TO, superimposed on a.c. or d.c. stresses).



1999-09-01 60

These transients may occur in various environmental conditions and may affect insulators characterised by various degrees of contamination. In particular, a critical condition may arise when there is the simultaneous presence of pollution and wettingon the insulator surface. The simulation of the above condition may require various pollution test procedures, according tothe peculiarity of the environment considered.

Many tests have been performed to investigate the pollution influence on the withstand characteristics of insulator configurations under transient overvoltages 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157.

The available experimental information indicates that the presence of a wetted pollution layer may appreciably reduce thestrength, not only at operating voltage but also under transient voltages.

In the following discussion, the performance of external insulation under transient overvoltage will be reviewed with the aimof obtaining indications about this reduction in strength.

3.3.7.2 General trends on the performance of contaminated insulators under transient voltages

A summary of published data relevant to the performance of contaminated insulators (suspension- and post-type) under transient overvoltages is presented in the following sections.

Figure 3-21: LI flashover voltage of strings of cap and pin standard and antifog insulators as a function of insulator length.

Solid-Layer method, wet contaminant. Comparison with data for the dry condition147

.

3.3.7.2.1 LI perf ormance

(a) L I alone

The available data generally refer to the standard LI wave shape (1.2/50 µs).

In Figure 3-21, the 50% flashover voltage of cap and pin insulator strings is given as a function of the string length. Theresults were obtained by using the Solid-Layer method (wet contaminant) with a salt deposit density, SDD, ranging from 0.06to 0.25 mg/cm2. For comparison purposes, the strength under the dry condition with positive LI is also provided. Theseresults indicate a substantial strength reduction due to the presence of pollution when compared to that for the dry condition.This reduction tends to increase as the insulator length increases, thereby leading to non-linear characteristics. Therefore, it isdifficult to keep the LI withstand voltage of insulator strings higher than 3000 kV, particularly for standard units. Thestrength of standard-type insulators is reduced for both polarities, resulting in values that are nearly equal. For antifog

insulators, the decrease is larger for negative polarity; which, therefore, represents the critical case.



1999-09-01 61

The influence of the pollution severity SDD on the reduction in strength is shown in Figure 3-22, which provides the specificflashover voltage as a function of SDD of the contaminated layer. The performance of standard and of antifog cap and pininsulators and of a smooth cylinder insulator have been considered. The data indicate that the reduction is practically constantfor SDD greater than 0.1 mg/cm2 for both positive and negative polarities. Further, the results show once again theimportance of the insulator profile. A strength as low as 200 kV/m was found for an insulator shape without sheds (i.e.smooth glass cylinder).

Figure 3-22: LI flashover stress of cap and pin insulator strings and of a post insulator model as a function of pollution

severity. Solid-Layer method (wet contaminant)146

.

(b) L I super imposed on a.c.

Results from Lushnikoff 146 indicate that dry bands on the surface of polluted insulators cause a further appreciable lowering of the impulse strength of heavily polluted insulators. This reduction is about 30-40% with reference to the strength obtainedwith LI only. Without such dry bands, the strength reduction is only 10-20%.

3.3.7.2.2 SI per formance

(a) SI alone

As for air gaps, an influence of the impulse wave shape is to be expected for contaminated insulators. Unfortunately, few dataare available for this case 148 149 - and they refer to rather short insulator lengths (1 to 2 m), thus not allowing accurateindications to be obtained. However, as a general guide, the strength tends to be lower as the impulse-duration increases.

Most of the investigations were carried out with impulse wave shapes close to the standard one (250/2500 µs) and of positive polarity, which is also the critical one under contaminated conditions. Consequently, in the following account, the main

attention will be paid to standard impulse wave shapes of positive polarity.The presence of wetted pollution can cause a large reduction in the flashover voltage with respect to that for the dry condition,as provided by the set of data given in Figure 3-23. This shows the strength of cap and pin insulator strings as a function of thestring length (data derived from Okada et al145, and Hiroshe et al149, obtained with the Solid-Layer method and a SDD withinthe range from 0.05 to 0.23 mg/cm2). Again it is seen that the insulator profile plays a major role; the reduction with standardtype insulators is much larger than that with the antifog one.



1999-09-01 62

Figure 3-23: SI flashover voltage of cap and pin insulators as a function of string length ( d ). Solid-Layer method, wet contaminant

145 149

.

The strength reduction depends largely on the pollution severity, as evident from Figure 3-24 (data derived from Carrara andSforzini143 and Hiroshe et al149, obtained with Salt-Fog and Solid-Layer methods, respectively).

Figure 3-24: SI flashover voltage of cap and pin insulators strings, presented in per unit of the flashover voltage in the dry

condition, as a function of pollution severity. Solid-Layer and Salt-Fog methods143

149

.

The data in Figure 3-24 show that the strength tends to decrease when the pollution severity is increased, even for a high pollution severity.

(b) SI preceded by a.c. energisat ion

Results obtained with standard and antifog cap and pin insulators, standard and antifog longrod insulators and post-typeinsulators are summarised in Figure 3-25. The pollution tests have been performed using the Salt-Fog method with a testseverity ranging from 2.5 to 25 g/1. This figure shows the 50% flashover voltage (U50), normalised to the 50% flashover

voltage for a dry insulator with positive polarity (U50 dry+ ) as a function of severity for both positive and negative polarity.For comparison purposes, corresponding data obtained with SI alone are also given.



1999-09-01 63

Figure 3-25: Pollution tests (Salt-Fog method) with SI preceded by a.c. energisation on cap and pin and longrod insulators.Comparison with data relevant to SI alone

143 144

.

It is evident that the a.c. pre-stress produces a reduction in the SI strength, which is more pronounced with negative polarity.

(c) SI super imposed on a.c. vol tage

Results, relevant to standard and antifog cap and pin insulators and standard longrod insulators, are summarised in Figure 3-26. The data, obtained with the Salt-Fog method, indicate a strength reduction similar to that found for the case of SI

preceded by a.c. energisation.

Figure 3-26: Pollution tests (Salt-Fog method) with SI superimposed on a.c. energisation for cap and pin and longrod

insulators. Comparison with data relevant to SI alone143

153

.

(d) SI super imposed on d.c. vol tage

Figure 3-27 shows the SI flashover voltage as a function of the d.c. pre-stress, in which both the amplitude and the polarity arevaried. The results were obtained by using the Solid-Layer method (wet contaminant) with a fixed test severity (SDD = 0.04mg/cm2).



1999-09-01 64

Figure 3-27: Pollution tests (Solid-Layer method) on insulator columns with SI superimposed on d.c. energisation153

.

Again, the data indicate that the pre-stress may have an appreciable adverse influence on the strength. It is evident that thisstrength reduction is strongly influenced by increasing the amplitude of the pre-stress voltage.

(e) In f luence of dry bands on the SI strength

As suggested by Cortina et al 153, the additional strength reduction found with composite voltages may be attributed to the dry bands formed by the applied pre-stress. Tests made without pre-stress but with a non-uniform distribution of pollution (dry bands simulation) gave results similar to those obtained with pre-stress applied to uniformly contaminated insulators, asshown in the example of Figure 3-28.

3.3.7.3 Performance under transient overvoltage

As the duration of the overvoltage increases, the flashover voltage becomes closer to that obtained for permanent a.c.

stressing.Figure 3-29 shows examples of results obtained with various pollution procedures 157 on a porcelain housing (longitudinalinsulation of a circuit breaker). In this figure, the flashover voltage obtained with TO is given in p.u. of the strength measuredwith permanent voltage - with the same pollution method. It is shown as a function of the TO application-time. From thisfigure it seem that the flashover voltage tends to approach the value of the permanent voltage after a relatively short durationof the overvoltage. However, the curves show different trends - which may be related to the test procedure and thecharacteristics of the test object.



1999-09-01 65

Figure 3-28: Tests on an insulator column152

. Comparison of the results:- in dry condition under positive SI

- contaminated uniformly and applying a positive SI preceded by a.c. stressing

- contaminated non uniformly and applying a positive SI alone (Solid-Layer method, wet contaminant, SDD = 0.04

mg/cm2 ).

Figure 3-29: Pollution tests on longitudinal circuit breaker insulation with TO. Flashover voltage vs. overvoltage-

application duration157

.

Pre-stressing also affects the strength with TO154 157. Examples of results obtained with such a pre-stress are shown in Figure3-30 154. With pre-stressing, the strength becomes very close to that for a permanent voltage when the voltage application-

duration is of a few seconds.



1999-09-01 66

Figure 3-30: Pollution test on suspension type insulators with TO preceded by a.c. energisation. Flashover voltage vs.

overvoltage application-duration154

.

Related information when no pre-stressing is present is shown in Figure 3-31 158 for two types of insulator that were bothsubjected to a salt-fog of 2.5 kg/m3 salinity for 5 minutes and then suddenly energised.

A useful comparison is the ratio of the impulse flashover voltage to that for normal stressing. Such information for the a.c.reference case, i.e. disc insulators and no pre-stressing, is illustrated in Figure 3-32 159 as a function of the duration of theimpulse waveform (stated as the time for which the voltage is greater than 50% of the peak value). Also included in thisdiagram are a few results that apply to the corresponding temporary a.c. overvoltage condition (in this case, the duration is thetime for which the 50Hz voltage is applied before flashover occurs).

Figure 3-31: Short duration a.c. flashover tests in salt-fog (Insulators suddenly energised after 5 minutes of fog)158

.



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1

2

3

4

5

6

1 10 100 1000 10000 100000 1000000Duration of energisation (µµs)

R a t i o o f p e a k : s h o r t d u r a t i o n F / O

v o l t a g

t o p e a k n o r m a l a . c . F / O

v o l t a g e

Lightning wave

Switching surge

a.c. overvoltage

Notes:

(1) Length of horizontal lines represents the duration

that the voltage is greater than 50% of the peak value.

(2) Length of vertical lines represents the spread of th

results

Figure 3-32: Comparison of the short duration stressing strength for positive polarity impulses with the normal a.c. stressing

strength for a 9-unit string of disc insulators without pre-established dry bands on a very heavily polluted surface.

Note: Peak of normal a.c. F/O voltage = 100 kV 159

.

3.3.7.4 Importance of transient overvoltages for insulator design purposes

The analysis of the available information has shown that the strength of insulators under transient overvoltages andcontaminated conditions may be much lower than that in the dry condition. This reduction becomes even greater in the

presence of dry bands - i.e. when an overvoltage occurs on a polluted insulator that is already energised.

A preliminary indication of the effect of transient overvoltages under pollution conditions on the design may be obtained by

comparing the strength with the stress expected under the overvoltages considered. In this respect, it is convenient to comparethe transient overvoltage strength of contaminated insulators with that required by the same insulator under permanent a.c.voltage, when subjected to the same contamination condition.

It is readily seen from Figure 3-32 that the LI strength under contaminated conditions is 4-5 times that under a.c. energisation.

Figure 3-33: Tests on an insulator column: Ratio between the positive SI strength and the a.c. strength as a function of the

insulator length. Solid-Layer method, wet contaminant, SDD = 0.04 mg/cm2 152.



1999-09-01 68

In contrast, the SI strength is closer to that under a.c. energisation. For example, Figure 3-33 shows the ratio between SI anda.c. strength as a function of the insulator length (data from Garbagnati et al 152, obtained with Solid-Layer method).

This latter figure indicates that, at least for the configuration considered, the ratio decreases with insulator length - reachingvalues as low as 1.3 - 1.5 p.u. for very long insulators. When compared to the strength with possible switching overvoltagestresses (usually higher than 1.5 p.u.), the importance of the SI condition for design purpose appears evident.

The strength under TO can be very close to that under a.c. energisation (see Figure 3-29, Figure 3-30 and Figure 3-32).

3.3.8 Ai r density correction factors for polluted insulators

The flashover path of a polluted insulator has two series components:

1. An air gap path.

2. A path across the surface of an electrolyte.

The influence of air density on the air gap is well understood. The gap is very short and breakdown will be dominated by thestreamer mechanism. Thus, the breakdown voltage varies in direct proportion to the change in air density.

An arc is produced along the surface of the electrolyte and the air density dependence can be described by7:

E

E

b

0 0

=

ρ

ρ(3-5)

where: E = Arc voltage at gas density ρ

E 0 = Arc voltage at gas density ρ0

b = Arc index

It is appropriate to note that the above mentioned finding of the decrease in the value of the flashover voltage with reductionin air density has been observed when the temperature was kept constant. Ocampo 160 has found that the effect of a lower temperature can neutralise or even reverse the effect of air density, thereby improving the insulator performance instead of reducing it.

Since the electrolyte path is much longer than the air-gap path on a typical insulator under service conditions, it is usually

assumed that the electrolyte path dominates the flashover process and that the above equation can be used to describe thewhole process with an acceptable degree of accuracy. However, with sheds having deep ribs this approximation may notalways be valid.

Rizk and Rezazada 23 have updated the mathematical model previously established 7 161 19, to include for the effect of reducedair density on the flashover voltage. This was done by introducing the effect of ambient pressure on the physical parameter of the dielectric recovery equation. These authors also referred to the case of sheds with deep ribs.

The performance has been investigated using real insulators in both the laboratory 162 163 164 and under various altitudeconditions 165 166 167 168 169 170 and in simulation experiments involving a thin layer of electrolyte 166 171 172 173. A consistentvalue of the index b - 0.5 and 0.35 for the a.c. and d.c. case respectively - has been found, bearing in mind the accuracy withwhich the 50% flashover value can be determined for polluted insulators.

Relatively little work has been done on actual strings to assess the performance under impulse (both lightning and switching)

for polluted insulators174

. The most consistent work has been done in simulated situations with an electrolyte layer175

176

. Inthis work, an attempt has been made to separate the role played by the air gap and that played by the electrolyte. For Lightning Impulses, the total effect is a direct dependency on air density - giving b = 1.

Under Switching impulse conditions, a more complex situation arises with significant polarity differences resulting in a muchgreater reduction for the positive case. The index for positive polarity is approximately the same as that for a.c. - that is, b

approaches 0.5.

3.3.8.1 Summary of correction factors

Using the binomial expansion, the equations below approximate the reduction in flashover voltage as a result of a decrease inair density, caused by an increase in altitude above sea level, while assuming a constant temperature:

1. d.c. Conditions

( ) E E h= −0 1 0 035. (3-6)



1999-09-01 69

2. a.c. Conditions

( )h E E 05.010 −= (3-7)

3. Lightning Impulse conditions

( ) E E h= −0 1 01. (3-8)

4. Switching Impulse conditions

( )h E E 050.010 −= (3-9)

where: E 0 = flashover voltage at sea levelh = height above sea level in km

E = flashover voltage at an altitude of h km

3.3.9 General trends for i ce covered insulators

a) a.c. Voltage

Ice-flashover is caused by the combination of several elements. These include the decrease in ‘effective’ leakage distance -

due to ice-bridging, the increase is surface conductivity - caused by the formation of a high conductivity water film on thesurface of accreted ice, the presence of a pollution layer on the surface of the insulator and the formation of air gaps caused bythe heating effect of surface arcs during ice-accretion. All of the above mentioned processes are influenced by theenvironmental conditions before, during and after ice-accretion - as well as by the type and configuration of the insulators.

The type and density of the ice are two major factors that influence the flashover voltage of insulators 94 96 177 178. Wet-grownice (glaze) with a density of about 0.87 g/cm3 has been found to be more dangerous than other types of atmospheric ice-accretion 94 96 179. For example, the results of a maximum withstand stress for several insulator types covered with glaze andrime - formed from water with a conductivity of about 80 µS/cm - are presented in Table 3-11 96.

Table 3-11: A.c. Withstand stress E ws of several insulator types when covered with ice grown in wet and dry regimes96

.

TYPE OF ICE ICE DENSITY Ews (kV/m) *

(g/cm3) IEEE Antifog EPDM Post typeGlaze with icicles ≈ 0.87 70 84 96 90

Rime < 0.3 > 148 > 146 > 168 > 197* Ews : Maximum withstand voltage for an insulator of 1 m length.

The amount of ice, including its length and the number of icicles, as well as the thickness of the ice-layer on the insulator surface considerably influence the flashover voltage. Some writers 191 have reported that the withstand voltage of insulatorsthat had their shed-spacing completely bridged by artificial icicles was about 60% of the value for the case without ice. Figure3-34 179 shows the variation in the maximum withstand stress of a 6-unit string of IEEE standard insulators that were coveredwith artificial wet-grown ice. These results are presented as a function of ice thickness on the monitoring cylinder and thecorresponding weight of ice per metre of insulator string. The value of ice-thickness at which the withstand voltage levels off was about 2.5 cm for these insulators. It was also found 177 that this value (between 2.0 and 3.0 cm for the tested insulators)

depends on the shed- or unit-diameter, shed spacing and type of insulator.

When the ice-thickness is much lower than 1 mm and the insulators are pre-contaminated, conductive ions from the pollutiontend to dominate the total electrical conductivity of the ice 180. Consequently, a flashover stress level as low as 20 kV per metre of leakage distance has been obtained.

The presence of the voltage during ice-accretion affects the ice distribution along the insulators. In many cases, some sectionsof the insulator may be free of ice 111. This is due to ice melting and/or falling away - caused by the heating effect of thesurface arcs and/or an increase in air temperature. This situation is especially the case for long insulator strings and it has also

been observed during laboratory tests 359. Accordingly, a non-linear relationship between the withstand voltage and the string-length can be expected.



1999-09-01 70

Figure 3-34: Maximum a.c. withstand stress as a function of the amount of ice179

.

The influence of freezing water conductivity on the flashover of ice-covered insulators has been studied and reported by

several authors 94 177 179 181 182. In general, the higher the conductivity, the lower is the flashover voltage. Figure 3-35 showsan example of the decrease in the maximum withstand stress Ews as a function of freezing-water conductivity - as measured at20oC. These results were obtained using a string of 6 IEEE insulator units, tested at an air temperature of -12oC and an icethickness of 2.0 cm on the rotating monitoring cylinder 179. The decrease in maximum withstand stress in this paper wasexpressed by using the following equation:

Ews = 165.3 σ -0.18 (3-10)for σ≤ 150 µS/cmσ being the conductivity of the freezing water in µS/cm and Ews, the maximum withstand stress in kV/m.

Figure 3-35: Variation of the maximum a.c. withstand stress of the insulators as a function of the freezing-water conductivity179

.

However, in some cases, the conductivity can lead to a reverse effect - i.e. an increase in the conductivity leading to higher flashover values. This phenomenon is associated with the falling away of the ice caused by melting due to surface arcs. Theeffect of freezing rain conductivity on the flashover voltage may also depend on the insulator-type as well as on theexperimental conditions. In some studies 183 358 359 177 182, the flashover voltage reduced even for much higher conductivityvalues.

b) d.c. Voltage

Unfortunately the research work to date193 184 185 186 has provided little information on the effect of ice on the flashover voltageof insulators energised under d.c. voltage. It has been reported that this flashover voltage is, in general, lower under negatived.c. energisation than it is under positive d.c. energisation. In a series of tests carried out on a short string of IEEE standardinsulators that were covered with glaze 187, it was found that the maximum withstand stress was about 17% lower under

negative d.c. than positive d.c. stressing of the insulator.



1999-09-01 71

c) Switching impulse voltage

The value of the Switching impulse flashover voltage depends upon the condition of the ice. The glaze, during the ice-growthstage, has an extremely low flashover voltage. Figure 3-36 shows that the flashover voltage of a post insulator can decrease byas much as 50% of that for dry and clean conditions 188.

Notes:

Hollow marks show results for wet conditions

Marks ⊗, ¤ and ---- show flashover of stacks covered withice for positive switching surge.

¤ Five flashover data

⊗ Five withstand data

Figure 3-36: Positive and negative flashover voltage characteristics of solid core cylindrical post insulator for switching

impulse voltage with a front time of 120 to 140 µ s 188.

3.3.10 General trends for snow covered insulators a) a.c. Voltage

The a.c. withstand (flashover) voltage decreases with increasing water conductivity from the melted snow, and with increasingsnow-density up to 0.5 g/cm3. Thereafter, it remains constant. The minimum withstand (flashover) voltage has been measuredwhen the entire insulator string was covered with snow. Figure 3-37 shows the relation between withstand voltage and thesnow-density 189.

0.1 0.2 0.3 0.4 0.5 0.6

uS/cm203050100

: Naturally Snow -Covered: Artificially Snow -Covered

50

100

150

W i t h s t a n d V o l t a g e , k V / m ( i n s u l a t o r s t r i n g l e n g t h )

Volume Density, g/cm3

Figure 3-37: Relation between a.c. withstand voltage and snow-density 189.



1999-09-01 72

b) Temporary Overvoltage

The time to flashover becomes shorter as the temporary a.c. overvoltage becomes higher - as is shown in Figure 3-38. If thetemporary overvoltage continues for 0.1 second, the withstand voltage per metre of insulator assembly covered with snow isabout 95 kV/m. That is, about 20% higher than the a.c. withstand voltage at the volume density of the snow of 0.3 to 0.4g/cm3 314 190.

Figure 3-38: Temporary a.c. overvoltage and time to flashover 314 190.

(c) d.c. Voltage

The d.c. withstand voltage of an insulator covered with snow is approximately equal to the effective value of its a.c. withstandvoltage when the quantity of snow covering the insulator is the same. Regardless of the contamination severity on theinsulator, the d.c. withstand voltage depends on the snow-density and the conductivity of the water melted from this snowcovering 191 192.

The negative-polarity application usually results in a lower withstand level of snow-covered insulators 191 than does thecorresponding positive-polarity case.

Figure 3-39: Relationship between snow-density and d.c. withstand voltage of insulators artificially covered with snow192

.

Figure 3-39 shows the relationship between snow-density and d.c. maximum withstand voltage of insulators with an artificial

snow covering192

.



1999-09-01 73

d) Switching impulse voltage

Figure 3-40 shows the relationship between the ratio of snow-covered length and the 50% switching impulse flashover stress.The 50% flashover voltage has a so-called U-shape characteristic, where the minimum value occurs at a snow-covered lengthof 60 to 80 % (see Figure 3-40). Arcing horns have no discernible influence on the 50% flashover voltage per unit insulator length 193. There is also no significant difference in the 50% flashover voltage for the positive and negative polarities of theapplied voltage

Symbol

Pos. Neg.Conductivity of water Melted from Snow, uS/cm

Less than 40

Less than 50

Less than 60

FOV WithoutAC Voltage

400

300

200

40 60 80 100

5 0 % F O V , k V / m

Percentage length of insulator covered with snow

Figure 3-40: Relation between switching impulse 50 % flashover voltage and the percentage of snow-covered length of

insulator 193

.

Linearity has been found between the switching-impulse flashover voltage and the length of an insulator covered with snow(up to 6m of string-length).

e) Lightning impulse voltage.The positive-polarity lightning-impulse flashover voltage of an insulator assembly - with a 2 m horn gap-length - that iscovered with snow with a volume density of 0.3 g/cm3, is about 35% lower than that of the assembly without snow. Thenegative-polarity flashover voltage is at its lowest when the whole insulator assembly is covered with snow 194.

3.4 Special insulators

3.4.1 Hollow insulators


Hollow insulators, or shells, can behave differently under pollution conditions compared with other types of insulators. Sucha different behaviour is largely attributed to the following reasons:

• Different axial-voltage distribution.

• Higher surface temperature, due to heat dissipation from internal components.

• Different shape or shed profile.

• Diameter.

Although part of this statement seems - upon cursory consideration - to be at variance with that made in Section 3.3.3.2, it isnot so when examined in more detail from the viewpoint of the internal components. The essential difference is themagnitude of the capacitance and the heat produced by the components.



1999-09-01 74

3.4.1.2 Shed profile

The selection process for the most suitable shed profiles for bushing shells can largely be similar to that for the profiles of capand pin insulators operating in the same environment.

Where the required leakage distance, in application to the conventional shed profiles, would result in an excessive insulator-length, more complex profiles can be adopted83. It should be stated, however, that deep - closely spaced - ribs on bushing

shells have been proven unsuitable in some environments 195 and on some installations 196. Such designs, under certainwetting conditions, may cause a highly concentrated electrolyte in the recesses between the sheds, which then flows off ontothe top surface of the shed below. These designs can also lead to severe inter-shed arcing as a result of uneven wetting of theleakage path between adjacent sheds.

3.4.1.3 Effect of axial voltage distribution

Capacitor-type internal insulation of bushings and current transformers is designed to provide a uniform radial and axialelectrical stress distribution. It is designed such that a combination of the stray capacitance to earth and the capacitance

between the layers of grading foil provide, as close as possible, a linear axial-voltage distribution on the porcelain surface.

Laboratory tests on empty and complete bushing shells have highlighted the effect of the internal components on the level of the flashover voltage of the outside surface. The more uniformly distributed the voltage is along the surface of the shell,

between its energised and earthed ends, the higher is its pollution withstand voltage. This is especially evident for lightly polluted (0.01-0.03 mg/cm2 of ESDD) shells 83. At these levels of pollution, the surface conductivity does not significantlyinfluence a capacitively controlled axial-voltage distribution. For higher levels of pollution, the effect of conductive surfaceleakage current on the axial-voltage distribution becomes more dominant. For these levels of pollution, laboratory tests for

pollution withstand voltages on empty shell and complete bushings produce similar results.

For naturally polluted shells that are subjected to natural wetting in service, the pollution is usually not evenly distributed onthe surface and may not be evenly wetted under some conditions - such as light misty rain. Moreover, the operatingtemperature of the bushing may also significantly contribute to its uneven wetting. This is because the difference intemperatures between the barrel of the shell and the outer sections of sheds leads to a difference in the rate of wetting betweenthese two surfaces - at least during its initial stages. Hence, it is reasonable to assume that the axial-voltage distribution wouldinfluence the pollution withstand voltage of porcelain claded plant such as bushings and instrument transformers in service -even at much higher levels of ESDD than 0.03 mg/cm2 as suggested above.

It is, therefore, imperative - when designing bushings and instrument transformers for operation in polluted environments - toaim at achieving a uniform axial-voltage distribution on the surface of the shell.

One simple and effective method of measuring the axial voltage gradient is described in the literature196.

3.4.1.4 The effect of diameter on pollution accumulation

The discussion in Section 2.3.7.2 concluded that insulators with large diameters collect less pollution than do the smalldiameter insulators in the same situation.

3.4.1.5 The effect of diameter on withstand voltage

The effect of diameter on insulator performance is discussed in Section 3.3.1.4 and shows that in many cases, for both a.c. andd.c. test voltages, the pollution performance of insulators decreases with increasing average diameter. This change has beenfound for both the Salt-Fog method and Solid-Layer method 197 198 199.

3.4.1.6 The effect of a non-uniform pollution distribution

In the state of Victoria, Australia, long-term service experience with several types of 500 kV current transformers and 500 kVtransformer bushings - that operate in an environment of up to 0.25 mg.cm2 ESDD - shows satisfactory performance.Porcelain shells of some of these current transformers have an average diameter greater than 600 mm with a leakage distanceof only 12700 mm.

However, three incidents of external pollution-related flashovers have occurred on one type of a 500 kV current transformer 196. The average level of pollution measured on the surface immediately after the flashovers, in all three cases, was less than

0.03 mg/cm2. Their porcelain shells have a total leakage distance of 13050 mm and an average diameter of less than 550 mm.



1999-09-01 75

The investigation of these flashovers revealed a strongly non-uniform axial electric field with a high concentration of voltagetowards the bottom part of the porcelain shell - due to a non-uniform pollution deposit. The bushing had a complex shed

profile with a double-rib geometry and a deep inter-shed cavity. Also there was a small inter-shed clearance. It wasconcluded that the light pollution level did contribute to the flashovers due to its non-uniform distribution on the bushing.

The discussion on the effect of a non-uniform pollution deposit can be found in Section 3.3.4.

3.4.2 HVDC wall bushings

Service experience shows that the majority of the flashovers in HVDC stations have taken place on wall bushings of untreated porcelain shells 200. Further investigations have revealed that the non-uniform wetting along the bushing surface is one of thereasons leading to the frequent flashover of these bushings 201. This non-uniform wetting may be caused by wind and theshielding provided by the converter hall during rain. Laboratory tests results have confirmed that, under such non-uniformwetting conditions, a HVDC wall bushing with an untreated shell can flashover under the operating voltage 201 66 202 203 204 205.Models of the non-uniform wetting flashover mechanism have also been proposed 203 206.

To avoid such a flashover by increasing the creepage distance may result in a very long wall bushing - as a specific leakagedistance of 60 mm/kV may be needed for moderately polluted site 207. Booster sheds can be installed to improve the bushing

performance. However, damage to the booster sheds or the bushing sheds have been observed in laboratory tests 202.

An efficient counter measure to suppress flashovers across wall bushings is to make their surface hydrophobic 66 203 137. Thisis achieved by coating the porcelain surface with RTV silicone rubber or silicone grease or by having bushings with siliconerubber sheds.

HVDC wall bushings of large diameter can also be avoided by adopting the outdoor-valve configuration 208.

3.4.3 Cir cuit breaker and isolator insulation

When the circuit breaker is closed, the insulator of the interrupter head is unstressed and so when this breaker is opened thisinsulator is subjected to a cold switch-on event - thereby experiencing the problem as discussed in Section 3.3.6. However, inits open position, this insulator is rarely required to withstand steady electrical stress for an appreciable time because a seriesisolator will be open. On the other hand, during a synchronising operation, this insulator may be subjected for several minutesto voltages that may vary by up to twice the normal value. If this interrupter head is horizontally mounted, its flashover

strength will be much lower than that of similarly sized insulators that are vertically mounted - as discussed in Section 3.3.3.8.

0

100

200

0.10 1.00 10.00 100.00∆∆t (s)

U 5 0

2 terminal test without prestressing2 terminal test with prestressing

1 terminal test without prestressing

1 terminal test with prestressing

1 2U p -U p

UL=2U p

1 2U p

UL

UP

Figure 3-41: 50 % flashover voltage as a function of the a.c. overvoltage duration (single cycle)290

.

Using a solid pollution layer of kaolin plus NaCl and steam fog, the 50% flashover voltage of the interrupter heads inclined at45o to the vertical is shown in Figure 3-41 as a function of time for which the stressing is applied. Two conditions have been

investigated. In the first case, one terminal was energised and the other terminal was earthed; in the second case, the terminalswere energised with opposite polarity voltages. These results show the important finding that the flashover voltage is not



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much greater than the permanent a.c. values for an energised duration greater than one second. An intriguing fact is that thetwo-terminal voltage application values were 15% higher than those for the corresponding one-terminal case.

The results on the pollution performance of circuit breakers have quite a spread in values. This can be caused by the different profile and diameter of the hollow insulator, the arrangement of the interrupter units, the influence of the active parts and the presence of various insulators in parallel (e.g. those of grading capacitors). It is, therefore, clear that extrapolation andgeneralisation of the results are rather difficult to make.

As an example, comparative tests 157 aimed at analysing the influence of the active parts have shown that, for a specific breaker, the required creepage distance for the longitudinal insulation of the breaker was about 10% higher than that for thecorresponding hollow insulators without active parts. It is reasoned that larger differences can be expected for certainconstructions that have a highly variable electric field.

As general remarks:

• The influence of various breaker components on the surface withstand is not easy to identify. Tests to obtain informationabout the breaker performance in service should be made - as far as possible - on a complete breaker. That is, oneequipped with active parts and accessories (capacitors, resistors, etc.) and assembled as per in service.

• Additional investigations are needed to obtain a better understanding of the influence of the various breaker parameterson the surface withstand voltage. This is very important, because the spread of the results indicates that there is scope to

achieve improvements in the pollution flashover performance - by properly modifying the breaker design (e.g. varying theinsulator profile, increasing the distance between hollow insulator and active parts, etc).

3.4.4 I nsulators in desert condi tions

In Tunisia, flashover problems with ceramic insulators still occur in some areas in spite of them having a specific creepage path of 52 mm/kV system 61.

Soil analysis has shown that the local desert sand in Tunisia contains calcium and sodium salts. These contaminants are blown onto the insulator’s surface to produce a pollution severity of ESDD as great as 0.65 mg/cm2.

Some researchers in Egypt209 have concluded - from conducting laboratory simulation experiments - that the pollutionflashover performance of silicone rubber insulators is greatly degraded by their exposure to sand storms as well as UVradiation and high temperature. The problem lies with the production of micro-cracks, which collect more pollution than does

the smooth surface. Further, the hydrophobicity of such artificially aged silicone rubber insulators is drastically reduced.

3.4.5 Semiconducting Glaze insulators

Insulators with a semiconducting glaze 111 have been available for some time. The use of resistive coatings has been found to be effective both for suspension- and post-type insulators for EHV applications as a solution for insulation design in heavilycontaminated areas. The basic principle involves the continuous flow of current - of about 1 mA - which provides enoughheat on the insulator surface to keep it dry in dew or fog. This prevents the formation of dry band arcing that may initiateflashover.

The semiconducting glaze contains - as a basis - a metallic oxide, such as tin oxide doped to give its semiconducting properties210. As these insulators continuously conduct current when energised, problems have been experienced in the pastdue to corrosion of both the glaze itself and the glaze to metal joint. However, significant technological improvements have

been made and substantial service experience exists. Consequently, their use should be considered in the contaminationdesign of transmission line and substation insulators. A further problem, that of thermal runaway, may also be experienceddue to the negative temperature coefficient of the glaze. However, this problem can be avoided by having a proper design.For example, the voltage across each disc of such insulators in a string should be kept below the limit recommended by themanufacturer.

Due to electrolytic glaze-corrosion 210, semiconducting glaze insulators may not be suitable for use on d.c. systems.Consequently all of the following information applies only to a.c. systems.

Through making pre-deposited pollution tests, the performance of semiconducting glaze insulators under fog conditions have been shown to withstand a SDD of 0.25 mg/cm2 (i.e. Heavy-pollution class) with a 13 mm/kV specific creepage. It has also been shown that the voltage distribution along a string consisting of semiconducting insulators is very linear and that longstrings will be thermally stable because of the linear-voltage distribution.

Semiconducting glazed insulators also show a withstand voltage of about 3 times that of ordinary glazed insulators and aboutdouble that of glazed fog-type insulators - as is illustrated in Figure 3-42.



1999-09-01 77

Figure 3-42: Contamination a.c. withstand voltage of semiconducting glazed insulators211

.

The minimum string length of semiconducting glaze insulators is determined by its ability to withstand sudden energisationwhen polluted and wet - that is, the condition known as cold switch-on. This situation occurs on lines that have beenunenergised for a period of time long enough to render the heating - which results form the semiconducting glaze while theunits are energised - ineffective in preventing the accumulation of moisture on the insulator surface.

Tests have been carried out to make a direct comparison between the cold switch-on strength of conventional insulators andthat of semiconducting glaze insulators of a similar shape. The results showing 50% flashover strength as a function of stringlength are shown in Figure 3-43.

Figure 3-43: Cold switch-on a.c. flashover voltage as a function of string length111

.

Due to the voltage grading achieved by the semiconducting glaze, the radio interference performance of these insulators issuperior to that of ceramic and polymeric insulators 211.

The issue of constant heat-energy dissipation - and thereby its economic penalty - should be considered in any widespreadapplication of semiconducting glaze insulators.

3.5 Conclusions

• The pollution flashover performance of porcelain and glass insulators is generally good, but problems have occurred in

service in a few places.The pollution flashover strength of some polymeric materials - especially silicone rubber - is superior to that of glass and



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porcelain. In contrast, epoxy resin rapidly degrades from its new - hydrophobic - condition such that its flashover strength can be somewhat inferior to that of the ceramic materials.Service experience has demonstrated that the performance of polymeric insulators is adequate if the insulators have been

properly dimensioned. Such insulators have, therefore, seen increasing application in recent times.

• The classic materials used for outdoor insulators, i.e. glass and glazed porcelain, are well described in the literature.Polymeric materials are, however, much more diverse and manufacturers choose a particular formulation adapted to the

process and the characteristics required of the finished product.• Many factors other than axial length or creepage path length are known to influence insulator performance. Differences

in the behaviour of insulators in various orientations may be due to the accumulation of pollution, the effect of naturalwashing and the physical characteristics of surface discharges.

• The ratio of best to worst insulator performance - as assessed by different research groups and for different types of pollution in terms of average surface stress, at withstand or flashover, for the same pollution conditions and verticalmounting - has been found to vary. For the surface stress, this ratio is as follows:

1. Ceramic insulators, between 1.5 to 2 and 1.4 to 2.3 for a.c. and d.c. energisation respectively,

2. Polymeric insulators, between 1.4 to > 4.7 and 1.2 to 1.8 for a.c. and d.c. energisation respectively.

The corresponding ratios are somewhat lower for average axial stress. The ranking of more than 120 types of ceramicinsulators and nearly 30 types of polymeric ones provides additional information that could be usefully employed for

assessing the likely pollution flashover performance of an insulator of a given material, profile and size.• For all insulators, the specific length needs to be increased as the pollution severity increases. Although there can be a

large spread in the experimental results, there are some clear trends (e.g. for the porcelain longrod) that support the useof a power-law relationship between specific length (SL) - i.e. specific axial length and specific creepage length - and

pollution severity (S); i.e. SL = KS s where K and s are constants. Typically for a.c. energisation, s = 0.2 for the longrod

porcelain insulator and - generally - about the same magnitude for some other shapes of cylindrical-type porcelaininsulators and for standard disc-type insulators. A further complicating feature is that the ranking of insulators canchange from test method to test method, as evidenced by some d.c. tests using (a) the Salt-Fog test, (b) the Clean-Fogtest and (c) a Dust-spray method.

Using a comprehensive set of data for standard disc insulators, it is possible to find a correlation - for a.c. energisation - between the pollution-scale of the Salt-Fog test with that of the Clean-Fog test.

From a detailed study of insulator profile for d.c. applications, it has been clearly shown that a significant improvement

in performance cannot be achieved by increasing only the creepage distance in a given axial length.For a.c. energisation, the pollution flashover performance of disc insulators is essentially linear with string length for avoltage up to 300 kV in a Clean-Fog test and up to 700 kV when subjected to natural pollution. Further, it is onlymoderately non-linear for higher voltages; being about 10% greater than that for the linear extrapolation for a voltage of 800 kV in a Clean-Fog test. Post insulators seem to be more non-linear than is the corresponding case for discinsulators. Although the situation for d.c. insulators is less clear than that of the a.c. ones, there are some indications that- at voltages around 800 kV - the non-linearity effect is more pronounced than the corresponding case for a.c.energisation.

Generally for vertically mounted insulators, there is some experimental support for expressing specific length (SL) -related to either axial stress or surface stress - as a power-law dependence on average diameter (D) for given pollutionand voltage conditions; i.e. SL = κ Dq where κ and q are constants. The best support for this relationship occurs with

porcelain housings in which the profile remains essentially constant and only the diameter is varied. Typical values are

q= 0.4 and q=0.5 for the a.c. and d.c. cases respectively. For more varied changes in profile, the spread in the dataincreases; none-the-less, the data for a.c. disc insulators provide moderately good support for such a power-law withq=0.7. An intriguing and puzzling finding is that q can be negative for porcelain d.c. insulators in which there is asubstantial variation in profile over the range of D studied. For polymeric housing insulators, there is some evidence toshow that the flashover strength decreases as the diameter increases, but the variation is much less than that for the

porcelain case.

• The improved performance of cap-and-pin insulators when inclined compared to that for the vertical orientation has beenconfirmed in both natural and artificial pollution tests.

The artificial pollution performance of horizontal post-type insulators is much inferior to that of the same insulatorswhen vertically mounted.

There is some evidence to show that the alternate long-and-short shed (ALS) profiles may not represent the mostefficient insulator shape for horizontal post and bushing-type designs.

Correction factors for orientation have been identified for some insulator types.



1999-09-01 79

More natural pollution test data are required for inclined post-type insulators to establish relative flashover performancecompared to that of the same insulators when vertically installed.

The pollution flashover performance of large diameter insulators - e.g. an interrupter head - when horizontally mounted,is substantially inferior to the corresponding vertical one. A similar finding is known for tapered bushing porcelainswhen subjected to artificial salt-fog.

• A non-uniform spread of pollution on insulators may have a significant effect on its flashover performance. A higher topto bottom ratio of pollution spread on vertically mounted strings of disc insulators leads to a lower flashover strength.The electrical strength of bushings and longrod insulators may also be adversely affected if some sections of theinsulator are less polluted than the rest.

• Some types of discharge activity (e.g. corona from raindrops) at or near the surface of polymeric insulators may causesevere degradation of the material thereby reducing the flashover performance. Such discharges can be prevented, or minimised, by having the correct design of stress ring.

A large number of electric field calculation programmes are available, based on different calculation methods. For insulators without any discharge activity, good agreement can be achieved between measurements and predictions basedon the different calculation methods. However, when discharges are present the situation is much more complicated andso - unfortunately - an accurate calculation of the electric-field around the insulator is not possible at this stage.

• The flashover strength of insulators that are suddenly energised (i.e. cold switch-on) can be at least 40% less than that of

the same insulators when continuously energised in the same pollution environment.• Contamination can significantly reduce the flashover strength of insulators under transient overvoltages (i.e. Lightning

impulse, Switching surge, system voltage disturbances) when compared with the corresponding situation for dry (clean)conditions.

• The flashover strength of polluted insulators reduces as the altitude of the location increases. The extent of thisreduction depends upon the wave shape of the voltage, but for practical situations (i.e. altitude up to about 3000m) thisdecrease will be less than 20% of the appropriate sea-level value.

• The flashover voltage of an ice- or snow-covered insulator depends on the type of precipitation, the conductivity of thewater when melting occurs, the extent of bridging of the air gaps (e.g. by icicles), the accretion thickness (e.g. snow up to30 mm) and the density of snow and ice. When icicles span most of the insulator, the probability of flashover atoperating voltage during the melting stage is relatively high. There is linearity between the flashover voltage and theinsulator length for axial lengths of up to 1.0 m. For longer insulators, this relationship can be highly non-linear. The

switching surge strength of insulators can decrease by 50% when the insulator becomes covered with ice. Linearity between the SI voltage and the insulator length is maintained for strings up to 6 m long when covered with snow.

A minimum in a U-shaped SI voltage: snow-covering relationship occurs when 60% to 80% of the insulator’s length iscovered with snow.

• Hollow insulators or shells may have a lower flashover performance than that of comparable solid insulators due to theinfluence of both the electric field and heat from internal components. It is thought essential to design the structure toachieve a uniform axial-voltage distribution on the surface of the shell.

• HVDC wall bushings having untreated porcelain shells have suffered a number of flashovers, which is usually due tonon-uniform wetting along the surface. An effective countermeasure is to use a silicone grease or coating to achieve ahydrophobic surface. A bushing with silicone rubber sheds is another promising solution to this problem.

• Tests to obtain information about circuit breaker performance in service should be made, as far as possible, on a

complete breaker that is equipped with its active parts and accessories and assembled as it would be in service.Additional investigations are needed to obtain a deeper understanding of the influence of the various breaker-parameterson the surface withstand voltage.

• In some desert regions, the flashover of ceramic insulators is a problem - even with a specific creepage length of 52mm/kV system voltage. However, it is encouraging to note that polymeric insulators seem to have a reasonably good

pollution flashover performance in these difficult locations.

• By using a semiconducting glaze to achieve a continuous leakage current of about 1 mA, sufficient heating of theinsulator surface is achieved to keep it dry in dew or fog - thereby greatly increasing the pollution flashover performancecompared with that of a normally glazed insulator. Cold switch-on, however, remains a problem with insulators treatedin this way. Although such a glaze has been found to have a long and effective life on post insulators, rapid deteriorationhas taken place around the pin of disc insulators in severe marine pollution. Semiconducting glaze is not recommendedfor d.c. insulators.



1999-09-01 80

4. ENVIRONMENTAL IMPACT

Even without flashover, the presence of pollutants together with wetting on insulators may cause serious side effects on the power system and on the environment. Leakage current flow across the insulator surface can be a source of annoyance to people, or interfere with communication systems, etc. These side effects can be classified as follows:

• Direct nuisances− Visible discharges

− Audible noise (AN)

− Radio interference (RI)

− Television interference (TVI)

• Indirect nuisances

− Corrosion of metal hardware, leading to interference or risk to persons

− Fires arising from leakage-current discharges

In this section a review of each of the above is given.

4.1 Visible discharges

Visible discharges can be a source of severe annoyance to people, especially where a line is passing through a populated area.As shown in Figure 4-1, a pollution severity of only one-tenth of that which is necessary for flashover to happen is sufficientfor audible and/or visual corona discharges 212 to occur.

Audible corona discharge

No audible corona discharge

Withstand voltage characteristics of

320mm ∅ suspension insulator

0.01 0.02 0.03 0.1 0.2 0.25 1.0 1.7

2

4

6

8

10

12

14

16

18

20

SDD, mg/cm2

V O L T A G E , k V p e r i n s u l a t o r u n i t

Figure 4-1: Audible noise/ Visible Corona characteristic of suspension insulators under a.c. energisation212

.

Frequent hot-line washing is an effective measure to minimise the occurrence of visible discharges 213. The use of robottechnology may be advantageous, as very frequent washing is required and the quantity of water used should be kept to aminimum in populated areas214.

The application of hydrocarbon or silicone grease or RTV coatings may also be effective but regular renewing will berequired (see 8.3.3). In the case of porcelain insulators, their manufacture with a semiconducting glaze (see 3.4.5) can alsoreduce discharges.

4.2 Audible noise

It seems that audible noise does not pose a serious interference problem under normal surface discharge conditions for insulators energised by either d.c. or a.c.215 216. However, under d.c. energisation - when a single-unit flashover occurs - theassociated noise can be problematic 217 218. This is especially so in highly populated areas, because the periodic loud bangs -when the insulators flash over - can continue for several hours. In a few isolated cases, the audible noise produced by a.c.



1999-09-01 81

corona or arcing can give rise to complaints when lines cross densely populated areas. The noise level attenuationcharacteristics for the various types of interference are illustrated in Figure 4-2.

RI

TVI

AN

RI*

AN*

TVI*

* Under Partial Discharge

0 20 30 50 100 300 500

20

40

60

80

DISTANCE FROM SOURCES ON TRANSMISSION LINE, m

R I , T V I ; d B A N ; d B A

Figure 4-2: Lateral profile of RI, TVI and AN caused by partial flashover217

.

To avoid single-unit flashovers, the installation of insulator units with high individual flashover voltages has proved to beeffective. Hot line washing is also an effective counter measure, as is the application of silicone grease and RTV coatings219.Once again, regular renewal of grease or coatings will be necessary (see 8.2.2).

Another form of audible noise from insulators is wind-howl, induced aerodynamically54, on certain profiles. This can beavoided or suppressed by using different profiled-insulators or by modifying the profile with the addition of a polymeric part.This measure changes the airflow around the insulator string, thereby preventing the resonance condition.

4.3 Radio interference

Again for insulators energised by either d.c. or a.c. and with ordinary surface discharges, the radio interference will not besevere 215 216 220 221. It is shown in Figure 4-3 that the resultant noise level does not increase much with an increase in

pollution severity 215 216. In the case of porcelain insulators, semiconducting glaze (see 3.4.5) can also reduce discharges. Asthe noise interference is lower at the higher frequencies, as shown in Figure 4-4, radio reception will largely be unaffected 215

216. The signal to noise ratio threshold for such interference is 20 dB 215. More details on the critical conditions and limitsabove which RI can become problematic are given in Section 4.6 hereafter.

No.9 No.1

No.2&3

0.01 0.02 0.04 0.1 0.2 0.460

80

100

120

SALT DEPOSIT DENSITY, mg/cm2

R I V , d B a b o v e 1 µ V

Figure 4-3: Influence of salt deposit density on RIV at a measurement frequency of 1 MHz215 216.

Some small-scale investigations222 conducted in a laboratory found that the RIV level - for a.c. energisation - of a wet and polluted silicone rubber insulator, when aged, was substantially lower that that of two corresponding ceramic ones - a porcelain longrod and a short-string of standard glass discs. The silicone rubber insulator also had a lower RIV level than

polymeric insulators made of EPDM and epoxy resin. Related large-scale tests, conducted using an outdoor facility, have



1999-09-01 82

established the beneficial effect of employing stress grading rings for both silicone rubber insulators and EPDM ones. Thesetest findings are in general agreement with service experience in Eskom's transmission lines in South Africa.

Radio interference may be severe around d.c. lines when the so-called single-unit flashovers occur 216 217 218. The noise leveland attenuation characteristics are shown in Figure 4-2. Measures to prevent the occurrence of such single-unit flashovers arediscussed in the foregoing section.

4.4 Television interference

Television interference does not generally occur with normal leakage current discharges on insulators energised by either d.c.or a.c. This finding is supported by the frequency spectrum characteristics shown in Figure 4-4 215 216 220 221. For clear television reception, however, the signal to noise ratio should be at least 35 dB. That is, considerably higher than that for radioreception - 20 dB 215 217 - because the eye is more sensitive to interference than is the ear. More details on the criticalconditions and limits above which RI can become problematic are given in Section 4.6 hereafter.

Contaminated and fog

Clean and fog

Clean and dry

0.5 1.0 2.0 5.0 10.020

40

60

80

100

R I V , d B a b o v e 1 µ V

Frequency, MHz

Figure 4-4: Frequency spectrum of noise of 500 kV insulator string.

Salt deposit density: 0.06 mg/cm2

. Applied voltage: 303 kV215

216

220

221

.

Single-unit flashovers on d.c. insulators, however, can cause serious television interference 216 217 218. The noise level andattenuation characteristics are shown in Figure 4-2. Some counter measures are explained in Section 4.2.

4.5 Corrosion of metal hardware - television interference

When the surface of metal hardware of insulators becomes oxidised or corroded, the layer so formed will exhibit insulating properties. This light corrosion is not enough to reduce the mechanical strength of the insulator, but in the case of lightlyloaded suspension-type insulator strings, the surface layer will puncture electrically causing a continuous sparking. Thissparking will then produce noise that does not attenuate at the higher frequencies - even up into the television range. The

principle of this discharge and an example of the frequency spectrum are given in Figure 4-5 72.

Hardware

Rust Layer

50 90 130 170 210 25070 110 150 190 230

10

20

30

40

50

60

70

Frequency, MHz

N o i s e v o l t a g e , d B a b o v e 1 µ V

Figure 4-5: Equivalent circuit for an insulator string with corroded hardware and an example of the noise profile caused72

.



1999-09-01 83

Since the insulating properties of the corrosion layer are the cause of the problem, two effective counter measures are theapplication of: (a) added weights that mechanically break the corrosion layer and (b) bridging the corrosion layer byconnecting a bonding wire between the line and the insulator hardware. 223

4.6 Criteria for radio noise limits of insulators

General procedures for setting the limits on radio interference produced by overhead lines and substations are given in CISPR publication 18-2 (1986)224. Further guidance with reference to the effect of polluted insulators is given in CISPR publication18.2 Amendment 1 (1993)225.

The general principle put forward is that the insulator sets be designed such that their noise contribution to the overall noise of the substation, or line, is negligible for any surface condition of the insulator. This design principle is only justified for conductors that produce noise close to the maximum admissible level. This generally arises from conductors that have surfacefield gradients greater than 12 - 14 kVrms/cm.

For the purpose of the CISPR publications, three area types are defined with the following conditions 224 225:

Type A areas: Areas where the insulators remain clean. They are generally characterised by the absence of contaminating phenomena and frequent natural insulator washing due to rain or high and frequent dew condensation.

Type B areas: Areas where the insulators become slightly polluted . They are generally characterised by low-intensitycontaminating phenomena and by cleaning agents such as rain or heavy dew condensation that limit thecontaminant accumulation on the insulator surface so that the formation of discharges across dry bandsappears seldom.

Type C areas: Areas in which the insulators become polluted so that the formation of discharges across dry bands isfrequent.

In type A areas, the radio noise level on insulators decreases with an increase in the relative humidity of the air. In the presence of condensation without water drops, the radio noise behaviour is similar to that of the same insulator under 90-95%relative humidity. With the presence of water drops, the level of radio noise increases. However, it is less than that for conductors under the same conditions.

Insulators installed in type B areas behave similarly to those situated in type A areas. That is, there is a lower radio noise

level for a higher humidity. However, certain types of insulator - designed for low radio noise in clean areas - may exhibithigher noise levels at high levels of relative humidity. In the presence of condensation without water drops, the radio noise behaviour is similar to that of the same insulator under 90-95% relative humidity. As for clean insulators, the radio noisespectrum of slightly polluted insulators is similar to that of the conductor.

Table 4-1: Recommended radio noise limits and appropriate test methods224

225

.

TYPE OF AREA WHERETHE INSULATOR WILL BEINSTALLED (AS ABOVE)

RADIO NOISE VOLTAGELIMITS (DB/1 µV/300 Ω)

TEST METHODS

A

B

Ec+23

Ec+15

According to CISPR 18-2 and IEC 437 (onclean and dry insulators)

C Indication of limits and test procedures applicable to insulators to be installed intype C areas cannot be given at present. Possible remedies, in case of non-acceptable radio noise levels, are: the reduction of the voltage stress by means of longer insulator strings, or leakage paths; the use of polymeric insulators; thegreasing or periodic washing of the insulator sets

Ec = 50 % fair weather radio noise voltage level produced by the conductor at 20 m from the outer phase of the line(dB/1 µV/m)

Notes:1. The limits reported in the table are applicable to lines characterised by a conductor-noise level close to the maximum admissibleone (voltage gradients higher than 12 - 14 kV/cm).For lines of special design (having very low conductor noise), the direct application of the limits indicated in the table could lead touneconomical requirements for the insulators. To avoid this, the formula given in the table could be utilised provided Ec isintended for the conductors of a line of the same category (voltage level, tower geometry, region etc.) but with a normal conductor design.2. The values in the table apply to line insulators. Similar approaches can be applied to substation insulators in respect to the noisein the substation itself and the noise conducted into the outgoing lines.



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For a relative humidity lower than 60-75%, the radio noise behaviour of insulators installed in type C areas is similar to thatof those located in type A or B areas. For higher humidity and for droplet condensation, however, the dry-band activity

produces very high noise levels. These nuisances can be controlled by reducing the electric stress or by using specialinsulators. Alternatively, greasing or regular washing can be the solution.

The frequency spectrum of wet polluted insulators (Type C areas) with dry-band activity extends up to the higher frequencies.The medium frequency reception and that for television viewing can then be disturbed.

Table 4-1 shows the recommendation of CISPR 18-2224 225 for radio noise limits and appropriate test methods for insulator sets installed in the above defined areas.

4.7 Corrosion of metal hardware - mechanical strength reduction

Suspension-type insulators for d.c. transmission lines sometimes suffer from corrosion of the metal hardware due toelectrolysis226. The principle of this electrolytic corrosion and the corrosion of the pin are illustrated in Figure 4-6 and inFigure 4-7. This corrosion can be so severe that it affects the insulator’s mechanical strength. Insulator pins equipped with asacrificial zinc sleeve have been found to be very effective in preserving the long-term mechanical strength of the insulator 226.It has been reported that substation insulators (post type) very rarely suffer from such electrolytic corrosion227.

Metal

Wetted contaminant=

Electrolyte Metal

i Anode Cathode

Fe Fe

Electrolyte

Fe2+

Figure 4-6: Equivalent circuit of a contaminated insulator228

.

The thinning of the pins of suspension insulator has also been experienced on a.c. transmission lines228, notably in areas wherethe relative humidity remains high for long periods. In this case, the d.c. component in the leakage current229 is considered tocause the electrolytic effect.

Corrosion of the pin beneath the mortar surface has lead to the production of radial cracks in the shell of porcelain a.c. capand pin insulators230 231.

Figure 4-7: Damaged insulator subjected to a.c. energisation and outdoor exposure on a 230 kV line (11 years' exposure)232

.



1999-09-01 85

4.8 Fires

Leakage current activity on polluted insulators mounted on wooden tower structures may, in some particular circumstances,cause or exacerbate the following environmental impacts (event ‘a’ usually precedes event ‘b’):

a) Top-pole or whole-pole fires of wooden pole structures.

b) Fires in the nearby environment, vegetation etc.

It has been identified 233 234 235 that the mechanism responsible for the ignition of wooden tower structures may begin at any point of attachment of metal-to-wood and, in some cases, even in the joints of wood-to-wood. The following conditions arenecessary for the start of this fire:

1. Sufficient leakage current magnitude on the wooden surface.

2. Concentration of a voltage drop at a discrete point in the wood, causing local arcing and - therefore - possible ignition.

Condition ‘1’ particularly applies for severe pollution - caused by wet, soluble ion contaminants. A typical contaminant thatcauses the burning of wood poles is a thick layer of sea salt which may build up, not only on the insulators but also on thewhole of one side of the pole, in a very short time during a strong sea-storm236. Then the leakage current may easily exceedtens, and even hundreds, of milliampere. Laboratory tests have shown234, however, that much smaller leakage currents - i.e. inthe range of 10 mA - can cause ignition and fires on wood-pole structures.

Condition ‘2’ is illustrated in Figure 4-8

235

234

, which shows the cross-section through a wood crossarm and the resistancesrepresenting typical leakage currents paths. The wood in the "rain-shadow" zone near the metal bolt can often remain dryduring the moistening of the polluted insulator and the exposed wood surface. Due to the lowering of the resistance of thewood that is exposed to the rain, that part of the wood has a small resistance (R 2 ), whilst the dry wood maintains a highresistance in the narrow localised zones (R 1 and R 3 ). The increased leakage current (I1 ) can cause arcing and so possibly, thestart of a fire if the ignition temperature is reached and a sufficient air supply is available.

Legend:

Darker coloured areas indicate a higher moisture content

Lighter coloured areas indicate a lower moisture content

I Power frequency leakage currentI1 Leakage current close to the surface of crossarm

I2 Leakage current through central crossarm area

R 1, R 3 High resistance current paths

R 2, R 4, Low and medium resistance current paths

A, B Eyebolt and washer

C, Brace bolt

D, Guard electrode

Figure 4-8: Typical leakage paths through the wood crossarm; (a) is without and (b) is with the protective guard electrode234

.

In practice, fires on the wood poles of distribution and sub-transmission lines usually occur at the metal-wood interfaces.These include the insulator pins in the wood, the suspension insulator eyebolts, the cross-arm king-bolts and the arm brace-

bolts. In areas of very severe pollution, the build-up of soluble salts on the insulator and on the wood-pole surface can lead tolarge leakage currents - which produce deep tracks at the metal-wood connections. Tracking may also be associated with the

pollution flashover of an insulator.

On HV and EHV wood-pole lines, crossarm fires may also occur due to large leakage currents across the primary insulationwhen it is polluted. Fires have also been attributed237 to a high electric field that can be normal to the pole surface - as isillustrated in Figure 4-9. If the surface layer of the pole is moist and contains pollution, the conducting wood acts as a

collector-electrode for the capacitive current ic. If this capacitive current is both sufficiently large and concentrated so as toflow into a metal fastener of the earthed downlead, it may cause ignition and a pole fire. Even if there is no downlead, the



1999-09-01 86

capacitive current (ic) collected by the wet polluted surface layer may still cause a pole fire if it is collected by a metal nail,coach screw or pole-step - because such current concentration may lead to arcing.

Figure 4-9: Model for EHV Fires showing capacitive coupling current into a metal fastener237

.

The main measure taken to prevent a wood-pole fire is the installation of a conducting bridge across the high-resistance zonein the wood. Local bonding is usually employed to short-circuit dry zones formed by rain-shadows or poor metal-woodcontacts. An example is the guard electrode that is shown as D in Figure 4-8. This guard electrode is made in different forms,such as:

a) Coachscrew fitted tightly into the crossarm or pole body.

b) Multi-spiked plate ("gang-nails") pressed or nailed into the wood over a critical zone.

c) Galvanised iron, aluminium or copper strips nailed to the wood.

d) Metal bands wrapped tightly around the wood.

e) A 10 cm-wide band of conducting paint applied near the metal-wood connection.

All kinds of such a guard electrode have to be connected to metal.

The original guard-electrode (Figure 4-8 b) is usually replaced by measures b) or c). These surface-type electrodes alsoreduce the possibility of damage due to lightning currents.

With regard to the pollution on the insulators, it is useful to wash them or to apply other maintenance measures - particularlygreasing. But this measure is not effective when pollutant layers are formed quickly - as can occur by strong sea-storms.Generally, the only solution against pole fires is to fit one of the various types of guard electrode so as to provide a low

resistance path over a dry-wood zone.Finally, the fires in the surroundings of HV overhead lines usually arise when the vegetation is in contact with the live parts of line. A pollution flashover rarely leads to the ignition of vegetation fires, but a fire on a wood pole caused by pollution oftencauses the development of a fire in the nearby vegetation and forest. The main remedy is the regular maintenance of the rightof way of HV lines, the use of one of the above described remedies to prevent burning of wood poles and - possibly - theemployment of special chemicals238 to inhibit the ignition process.



1999-09-01 87

5. POLLUTION MONITORING

5.1 Introduction

Cigré has previously reviewed the subject of insulator pollution monitoring in two separate publications; the first one in 19792 and the other in 1994240. These reviews are summarised briefly herein. Some additional informatio, which has been

published recently, is also included.

Insulator-pollution monitoring serves the following main purposes 239 240:

1. Pollution site severity measurement

The results of the pollution monitoring techniques are used to establish the pollution site severity of an area and, if applied extensively, the results can also be used to produce a pollution map. Based on the gathered information, insulator designs and dimensions can be selected to achieve a good pollution performance.The aim of pollution site severity measurements is to provide a severity parameter which can be correlated with the

performance of an insulator, as determined from artificial and/or natural pollution tests.

2. Insulator characterisation

The aim is to establish a comparative study of the performance of various types of insulator installed at the same testing

site. Through such a study, the most appropriate insulator design and dimensions - for the given conditions - can bedetermined.

3. Initiator for insulator maintenance

Some of the monitoring techniques allow for automation that provides continuous monitoring of the condition of aninsulator surface, thereby providing a trigger for insulator maintenance before critical conditions arise.

A wide range of such monitoring devices and techniques has been developed over the years. The most widely used ones are:

• Directional dust deposit gauge239.

• Equivalent Salt (NaCl) Deposit Density (ESDD)4 2.

• Environmental monitoring (Air sampling, Climate measurements)4 240.

• Non-Soluble Deposit Density (NSDD)240.

• Surface Conductance3.

• Insulator Flashover stress3.

• Surge counting3.

• Leakage current measurement3.

Each of the aforementioned can be classified into two main groups of pollution monitoring methods; i.e. pollution performance measurement and environmental severity measurement, as is shown in Figure 5-1. Those methods shown beneath another one indicate a refinement of that method; e.g. Surge Counting is a refinement of the method of Insulator Flashover Stress.

The insulator performance measurements assess the insulator, as installed in service, on the basis that the leakage currentacross the insulator gives a measure of its pollution performance. This measurement includes the effects of both the pollutiondeposit and natural wetting.

On the other hand, the environmental severity measurement relates only to the pollution accumulated on the insulator; in the best case, it includes the effect of natural washing. From this measurement, the pollution performance of insulators is derivedfrom either artificial or natural pollution test results. The results from the environmental severity measurements may also

provide an input to the creation of a dynamic model of the environment to predict instances of high-flashover probability.Short descriptions of these models are given in Section 7.2.7.

In the previous sections, it was demonstrated that the insulator shape affects the amount of pollution collected by the insulator.For this reason, most of the environmental severity measurements employ either real insulators or insulator models as

pollution-accumulation devices. There are, however, purely environmental measurements - as is shown in Figure 5-1 - such asdirectional dust deposit gauges and air pollution sampling.



1999-09-01 88

Leakage Current Measurement

Ihighest

Surge Counting

Insulator Flashover Stress

Insulator Performance

Measurement

Surface Conductance

Equivalent Salt

Deposit Density

Optical Measurement

Non-Soluble

Deposit Density

Measurements onInsulators

Air Pollut ion Sampl ing

Directional Dust

Deposit Gauge

EnvironmentalMeasurement

Environmental

Severity Measurement

Pollution Monitoring

Figure 5-1: Organisation of insulator pollution monitoring methods for site severity estimation, insulator characterisation

and insulator maintenance.

In the following descriptions, each of these methods will be dealt with only briefly because the detailed information isadequately covered in the publications.

5.2 Air pollution measurement

Air pollution measurements are carried out during a given period of time, to determine the amount and characteristics of the pollution of the air at a site. For these methods, the basic assumption is that correlation can be established between theflashover performance of the insulators and the physical, or chemical, analysis of the air at a site.

5.2.1 Directional dust deposit gauge

This technique is probably the simplest of the ones currently available. Four dust gauges, each set to one of the four basic points of the compass, are used to collect air-borne contamination particles. These samples are collected at monthly intervals.A normalised conductivity (pollution index) is then determined by making a solution of the samples collected and using a

conductivity-instrument.To translate the pollution index into an actual site severity, line performance data need to be available. A correlation betweenthe pollution index and the required insulator dimensions can be established through a systematic investigation of both the

pollution index and the performance of insulators installed on actual lines in the vicinity of each test site241. This correlationcan then be used to estimate the required insulator dimensions at new sites without previous knowledge of these sites242.

5.2.1.1 Advantages

1. The equipment is inexpensive.

2. The operator requires no special skills.

3. The equipment can be used at a site without an electrical supply.

4. The technique gives an indication of all types of pollution present at site.5. The results are not dependent on subjective judgement.

5.2.1.2 Disadvantages

1. The response of the insulation to the environment is not assessed; i.e. the effects of washing and insulator wetting.

2. Long periods are necessary to obtain results.

3. The method does not distinguish between slow- and fast-dissolving salts. Critical wetting conditions are, therefore, notdetermined.

4. The amount of rainfall during the measuring cycle influences the obtained severity. A high rainfall during the measuring period will cause the measured pollution level to be higher than the actual level - and vice versa - because the naturalcleaning ability of insulators is not taken into account.



1999-09-01 89

5.3 Equivalent salt deposit density (ESDD)

ESDD is given by the equivalent deposit - in mg NaCl/cm2 - on the surface area of an insulator, that has the same conductivityas that of the actual deposit dissolved in the same volume of water.

The ESDD is determined by removing a pollution sample from the surface of a chosen insulator and dissolving it in a knownquantity of water (the IEC 507 Standard22 recommends the use of 2 to 4 litres of demineralised water per square metre of

insulator surface). The conductivity of the resulting solution, its volume and temperature - together with the insulator surfacearea - are utilised to calculate an equivalent salt deposit density.

Sodium chloride is the reference salt in the ESDD method. It has a linear part in its electrical conductivity-concentrationcurve. Therefore, the ESDD measurements have to be carried out within this linear range. However, insulators in the fieldcan be polluted with a combination of salts of lower solubility. This makes it necessary to add enough water to allow most of the ions to go into solution and to keep the electrical conductivity in the linear part of the curve. Campillo et al 42 have foundgreat variations in the ESDD measurements with the addition of water for different types of natural pollution deposits (e.g.gypsum).

It is often necessary to augment the ESDD measurements with the measurement of NSDD - see Section 5.3.3. This isespecially so when the natural contamination for inland areas are reproduced in artificial tests. In these cases, the best way of selecting the contaminant composition for artificial pollution tests is by taking account of the soluble/ non-soluble ratio and

the chemical composition of the pollution deposit.

5.3.1 Advantages

1. The shed profile of insulators can be assessed in terms of contamination collection.

2. ESDD is the severity parameter of a number of different artificial test methods. This common practice facilitates thecomparison between the different environments and the artificial tests.

3. Many researchers favour this method - and so a free flow of information is, therefore, possible.

4. Unenergised sites can be assessed.

5. The apparatus for this method is relatively inexpensive compared to that of other methods.

5.3.2 Disadvantages 1. It is very time consuming to find the maximum pollution level between the incidences of natural washing. The timing of

monitoring is essential.

2. ESDD is insensitive to volatile chemicals dissolved in rain or mist that do not leave deposits on the insulator surface.Chemicals such as SO2 and H2S would not be detected.

3. To perform the ESDD measurement, a certain amount of skill is necessary

4. The test removes the pollution layer from the insulator surface. Several insulator strings should, therefore, be monitoredto determine the build-up of the pollution.

5. The method does not discriminate between slow- and fast-dissolving deposits.

6. Critical wetting conditions for insulators are not determined by ESDD.

7. There is uncertainty in the applicability of this method to polymeric insulators, due to the transfer of hydrophobic properties to the pollution layer.

5.3.3 Fur ther developments

The 1994 Cigré review240 reported on some automated devices for measuring ESDD. In addition to those devices, another onehas recently been developed in Japan243. This device comprises an insulator-model - which looks very much like a normal cap& pin insulator - that is exposed to the site conditions. The amount of pollution that is collected on the bottom surface is thendetermined from making a special leakage current measurement on sampling plates. To initiate a leakage current, thesesampling plates are cooled by a Peltier module to promote condensation from the natural humidity in the air. After the leakagecurrent measurement has been completed, the Peltier module is also used to dry the sampling plate to prevent the removal of

pollution through a leaching-effect. Calibrations between the measured current and the ESDD level have been established.One limitation of this device is that the condensation does not take place before freezing occurs for the combined conditions

of low temperature and low relative humidity. For example the temperature needs to be above 10o

C if the relative humidity is below 50%. Although field experiments have been performed, little is reported on the comparisons of field measurements and



1999-09-01 90

the corresponding readings by the instrument. Other aspects of employing Peltier coolers for making such measurements havealso been reported 244 245.

5.4 Non-soluble deposit density (NSDD)

The non-soluble deposit density (NSDD) is sometimes measured in conjunction with the ESDD and it characterises the

content of the non-soluble contaminants in a pollution layer. It is normally expressed as mg deposit per cm2 of insulator surface area. NSDD measurements can also be coupled with a chemical / physical analysis of the deposit layer that allows for the identification of the pollution source.

The non-soluble deposit density is an important measurement to take in combination with the ESDD measurement - as theelectrical strength of an insulator is affected by the amount of inert material present. This effect of non-soluble, or inert,

pollution on insulators is discussed in Section 2.3.2.2.

5.4.1 Optical measur ement

Optical measurements are used to determine the thickness of the pollution layer on the surface of an insulator. Some suchdevices also enable the material characteristics to be determined240.

5.5 Surface conductance

The surface conductance is the ratio of the power frequency current flowing over a sample insulator to the applied voltage.The voltage must be of sufficient magnitude for a suitable current reading to be obtained but low enough - and of shortenough duration - to avoid heating and discharge effects.

The conductance indicates the overall state of an insulator surface. It includes the quantity of pollution - i.e. the effect of boththe conductive and inert part of the pollution deposit - and the degree of wetting. By wetting the sample insulator artificially,so that the conductance can be measured, the insulator surface condition can be continuously monitored. If the wettingcondition is such that the pollutants are not leached away, this technique can monitor the build-up of the pollution.

The layer conductivity can be obtained by utilising the insulator “ form factor” to make a conversion for a uniformly pollutedinsulator 2.

Some devices measure the surface layer conductivity directly. A description of such a device can be found in IEC 507 22.

5.5.1 Advantages

1. The shed profile of an insulator can be assessed in terms of contamination collection.

2. The deterioration of the insulator surface, due to the environment, can be monitored.

3. Unenergised sites can be assessed.

4. The test insulator is not continuously energised, thereby reducing the risk of flashover.

5. The results can be used to set up an artificial test.

6. The method lends itself to automation, such that it can monitor the build-up of the pollution on the insulator.

5.5.2 Disadvantages

1. The surface conductance can only be measured under wetting conditions. Application of this method may, therefore, beimpractical in low rainfall or non-fog/mist areas. If artificial wetting is introduced, usually as steam or fog, the resultswill only be applicable for areas with fast dissolving salts.

2. Due to the complexity of the equipment, this method is fairly costly.

3. The method does not discriminate between slow- and fast-dissolving deposits.

4. Critical wetting conditions for insulators are not determined.

5.5.3 Fur ther developments

Various devices that offer an automated conductance measurement have been developed 240. One of these devices, produced by ENEL246, was also listed in the previous review and has now been developed further. It is named AMICO (ArtificialMoistened Insulator for Cleaning Organisation). The monitored insulator is a hollow post-type insulator, filled with a



1999-09-01 91

circulating cooling liquid that lowers the surface temperature to promote wetting. The surface conductance of the entireinsulator is then measured to establish a pollution severity. An interesting detail, is the inclusion of a shield that is raisedduring the cooling and measuring process to reduce turbulence around the insulator. This precaution is taken because such air turbulence may influence the humidification process and so give an erratic result. The available measurements from in-service use are, unfortunately, limited and so a thorough evaluation of this method's performance is not yet possible.

Another example of a recently developed device - that is based on the measurement of surface conductivity - is the LWS,Liquid Water Sensor 247. This device can be used to determine the contamination level when the relative humidity is above65%. It measures the amount of liquid water and the level of contamination on a surrogate insulator. According to theauthors, the LWS is a better indicator of the contamination level than the peak leakage current, which depends on the amountof wetting. However, very little information of the working principles of the LWS has so far been given.

5.6 Insulator flashover stress

The insulator flashover stress is the flashover voltage divided by the overall insulator length. Over a given period, either theminimum flashover stress or the relationship between flashover stress and frequency of flashover is determined. This isusually achieved by bridging out some insulators in a string with explosive fuses so that after a flashover the string isautomatically lengthened. Such an arrangement is shown in Figure 5-2.

Explosive fuses

Figure 5-2: An example of the use of explosive fuses to monitor insulator performance.

5.6.1 Advantages

1. Actual insulators are tested under service conditions, thereby directly giving the required insulation level.

2. Depending on the implementation of this method, the cost involved can be reasonable.

5.6.2 Disadvantages

1. The results are only valid for the type of insulator string under test.

2.

As flashover occurs on the insulator under test, it is generally not acceptable on a service transmission system.3. The source impedance must be low - testing may, therefore, become expensive.

4. No data regarding the mechanism of flashover are obtained.

5.7 Leakage current

The leakage current across an insulator surface depends upon the service voltage and the conductance of the surface layer.Often, the insulator flashover performance is estimated from the leakage current measurement or the surface conductivitymeasurement. One investigation concluded that, when the leakage current peaks are greater than 250 mA, preventativemaintenance - i.e. cleaning or replacement of insulators - is then recommended 248. There are two methods of measuring thecurrent collected at the grounded end of the insulator. These methods are discussed separately as:

• Surge counting.

• Ihighest measurement.



1999-09-01 92

5.7.1 Surge counting

In this method, the number of leakage current pulses above a fixed amplitude conducted on a test insulator - energised at itsservice voltage - are counted over a given period of time.

5.7.1.1 Advantages


2. Depending on the implementation of this method, the cost involved can be reasonable.

3. This technique provides information on all the stages of the pollution flashover mechanism.

4. This method enables information to be determined if an existing line or substation needs to be upgraded.


1. This method only provides comparative data that must be assessed against similar information collected elsewhere.

2. A degree of sophistication is required for the instrumentation.

3. No information regarding the mechanism of flashover is obtained.

4. The results are only valid for the type of insulator under test.

5.7.2 I highest

Ihighest is the highest peak of leakage current that is recorded during a given time period on an insulator continuously energisedat its service voltage. It has been considered as a suitable parameter to indicate how close a glass or porcelain insulator is toflashover.

5.7.2.1 Advantages


2. This technique provides information on all the stages of the pollution flashover mechanism.

3. The information, provided by this measurement, can be easily compared to that obtained from making laboratory tests.4. It provides a continuous record of the insulator performance under various weather conditions.

5. This method enables information to be determined if an existing line or substation needs to be upgraded.


1. A high degree of sophistication is required for the instrumentation.

2. The cost of equipment is high.

3. No data regarding the mechanism of flashover are obtained.

4. The results are valid only for the type of insulator string under test.

5. Due to the complexity of the measuring equipment, this method is not suitable for large-scale surveys.

6. This method may not be applicable for some types of polymeric insulator.

5.8 Conclusions

A wide range of monitoring methods has been developed. It is shown that not all of the monitoring techniques are equallyapplicable for all the environments. It is hoped that Cigré will give some guidance to the selection of applicable methods inthe forthcoming application guidelines that are currently under consideration. For more information, the reader is alsoreferred to the already mentioned reviews 2 240.

The situation concerning the relevance of existing pollution monitoring methods for the dimensioning of the highlyhydrophobic types of polymeric insulators (e.g. silicone rubber ones) warrants a major research investigation to be carried out.



1999-09-01 93

6. TESTING PROCEDURES FOR INSULATORS

6.1 Introduction

The engineer is faced with the problem of insulation strength under natural pollution conditions at service voltage for each

line and substation in a.c. or d.c. systems. To minimise local failures and system outages due to pollution flashovers, thefollowing steps are deemed necessary:

• Determination of the type and the severity of the site pollution (classification).

• Correct choice of the insulator profile and the creepage distance (insulator dimensioning to reach the required pollution performance at service voltage for a given site).

• Planning of additional maintenance measures (washing, cleaning, greasing, coating) if necessary.

To prove the pollution performance of the selected insulator, artificial pollution tests can be performed in the laboratory.These tests are usually short-time ones and are less expensive than testing in outdoor stations. The usual aim of a laboratorytest is the confirmation of a specified withstand degree of pollution or the determination of its maximum value at the phase-to-earth voltage. It may also be used to determine directly the influence of changes in insulator dimensioning on the pollution

performance of an insulator.

The choice of a suitable test procedure is usually made from those that are internationally standardised. Also, other pollutiontest procedures may be used because of their relevance to special climatic or contamination conditions.

No test procedure can simulate all of the important natural conditions and their variations that may lead to a pollutionflashover. Therefore, compromises have to be made to reduce the number of procedures and the cost for testing. The mainrequirements for the acceptance of a test procedure are:

• Validity of the test procedure. The procedure should be representative for those natural pollution conditions that areessential for an insulator flashover. This practise leads to the same ranking of different insulators in laboratory tests andin service.

• Repeatability and reproducibility. The scatter in test results in the same laboratory or between different laboratoriesshould be within the limits of the natural dispersion of pollution test results.

• Cost-effectiveness. The cost for a test shall be reasonable in comparison with the usefulness of the result.

Validity, repeatability and reproducibility are specific to each test procedure, and the acceptance or rejection of a procedurehas to be based on engineering judgement.

Standardised artificial pollution tests use only constant test voltages (usually the phase-to-earth voltage).

6.2 Categories of test methods

6.2.1 Testing under natural pollution conditions

Results closest to the service performance of insulators under natural pollution conditions can be obtained at outdoor testingstations. Information on several facilities have been reported 126 249 250 88 59 251 37 252.

Usually, such stations are located in heavily polluted areas (near the coast, regions with a high degree of industrial pollution).The test specimen, as may be used in HV systems, can be stressed with a fixed or a variable voltage - and in some cases, amechanical load is added.

Current fields of research are:

• Insulation dimensioning of conventional insulators.

• Electrical performance of polymeric insulators and insulators coated with hydrophobic materials - with respect to ageing.

• Long-term performance of Metal-Oxide-Arresters (MOA).

Depending on the research programme the following points may be the terms of actions:

• Determination of the flashover stress by using different lengths of insulators.

• Monitoring of surge counts, leakage current and total charge.

• Determination of the pollution site severity, using methods like Salt Deposit Density (SDD) or Surface Conductivity.

• Visual inspections to detect physical changes of the insulator under test.



1999-09-01 94

The results of testing under natural pollution conditions may be interpreted as a relative ranking within a group of insulators.Also, a comparison with the corresponding results for a well-known reference insulator is possible.

Tests conducted under natural pollution conditions usually require long test periods due to both the natural dynamics of thedeposition of the contaminants and the necessity to collect sufficient data for providing a statistically reliable set of results.

6.2.1.1 Examples of field test stations

It is not within the scope of this section to provide an account of all of the world’s outdoor testing facilities and all of the possible testing philosophies. The following information, therefore, are only examples out of this large amount of information.

Mart igues and St. Remy les Landes, France

Since 1975, ceramic and polymeric insulators have been tested under natural pollution conditions at Martigues (industrial andmarine pollution) and at St. Remy les Landes (solely marine pollution) 253 249 254. The test voltages are given in Table 6-1 249

254. These voltages were kept constant and no mechanical load was applied to the insulators. All of the polymeric specimenswere regularly inspected visually. These results formed the basis for the comparison of the insulator’s damage produced at thesite with that from making corresponding accelerated ageing tests in the laboratory. The site at St. Remy les Landes isregarded as representing most of the pollution situations encountered in France.

Table 6-1: Test voltages at Martigues, St. Remy les Landes, France 121 254 and Brighton, United Kingdom 121.

TESTING STATIONTEST VOLTAGE

(kV)

Martigues 225/√3

St. Remy les Landes 20/√3 (fixed)

Brighton 34,5/√3 (fixed)

132/√3 (fixed)

275/√3 (fixed)

550/√3 - 825/√3(variable)

Br igh ton (closed 1987) and Dungeness, Un ited Kingdom

The Brighton Insulator Testing Station (BITS) was situated on the south coast of England, adjacent to a power station and aharbour. The main pollutants were salt and coal dust 121. For polymeric insulators, it was found that design weaknesses could

be detected in a relatively short testing period. Also insulator designs could be identified that may provide a suitable performance in a severely contaminated area. Therefore, those tests form a natural precursor to service trials. The test voltagesfor different groups of polymeric insulators are given in Table 6-1 249. The groups with fixed test voltage were used todetermine the natural ageing and surface degradation at service conditions, whereas the group with variable test voltage wasused for the determination of flashover statistics. This procedure, as described in reference 126, results in a "normalised Figure

Of Merit" (FOM). This normalised FOM allows the performance of the test insulator, be it ceramic or polymeric, to bequantified relative to that of a reference cap-and-pin insulator. A value of FOM > 1 indicates a better flashover performancethan that of the reference insulator.

The Dungeness Insulator Testing Station, opened in the early nineties, is also situated on the south coast of England close to anuclear power plant. Based on the older results, tests are performed in a way similar to those conducted at Brighton.

Koeberg and Sasolburg I nsulator test stations, South Af ri ca

In 1993, two complementary test stations were set up in South Africa. Koeberg is situated in a coastal environment andSasolburg in an industrial one. At Koeberg255, it was found that design weaknesses in polymeric insulators showed up withina year. In Sasolburg, the ageing processes are slower. The test voltages at Koeberg are 66/√3 kV and 22/√3 kV whereasSasolburg tests are at 88/√3 kV and 22/√3 kV.

Various parameters of the leakage current flowing across the insulator are monitored. They are:

• Maximum positive and negative peak values per time interval.

• Sum of the charge flowing across insulator per time interval.



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• Sum of the square of the leakage current per time interval.

• Statistical spread of peak values.

• Time to flashover.

Tests at these stations are continuing.

6.2.2 Ar tif icial pollution laboratory tests

A pollution flashover requires the presence of some kind of salt and water on the insulator surface. The test procedures mainlyused in laboratories can be classified into two groups, which differ in the insulator-surface conditions before the test.

• The clean insulator, energised at constant test voltage, is subjected to a defined ambient condition (e.g. Salt-Fogmethod).

• The insulator with its surface uniformly coated with a layer of inert material and salt is subjected to a constant testvoltage and specified wetting conditions (e.g. Solid-Layer method).

These two groups cover most situations for pollution flashovers and were taken as the basis for standardising different test procedures.

The choice of one of these test methods should be based on the particular natural conditions found in service to reach relevant

results for the insulator-design under test.

6.3 Test procedures for porcelain and glass insulators to be used in high-voltage a.c.or d.c. systems

The procedures described in the following subsections have been established for ceramic insulators and are not directlyapplicable to polymeric insulators, to greased insulators or to special types of insulator (i.e. insulators with conductive glazeor covered with a polymeric insulating material).

For bushings or other apparatus incorporating hollow insulators with internal equipment, special precautions may be necessaryto avoid over-stressing of the internal insulation since the test voltage may be greater than the nominal design one.

6.3.1 Standardised test procedur es

6.3.1.1 Salt-Fog test

This procedure simulates coastal pollution where a thin conductive layer formed by the salt covers the insulator surface. In practise, this layer contains little - if any- insoluble material.

The degree of pollution in a test is defined by the salinity of the salt-fog, expressed in kg of salt (NaCl) per m³ of water. Thetest conditions (salinity, salt-water flow-rate, and pressure of compressed air) can be controlled easily. Salt-Fog tests are lessexpensive and less time consuming than Solid-Layer tests.

Detailed descriptions of the Salt-Fog procedure can be found in IEC 507, 1991 22 (for a.c. systems) and in IEC 1245, 1993 256

(for d.c. systems).

6.3.1.2 Solid-Layer test

6.3.1.2.1 Procedure A - Wetting befor e and after energisation

This procedure is standardised for a.c. application only.

It simulates pollution conditions with thicker layers of deposits containing binding materials and some kind of salt. Also, thesituation of ‘cold switch-on’ (energising of a line or a station with contaminated insulators that have their surfaces completelywetted) is covered.

The degree of pollution is usually expressed as layer conductivity in µS. The control of the test conditions (surface cleanness before application of the artificial layer, uniformity of the layer, wetting conditions) is difficult and may require additional

testing work. To determine the required layer conductivity is time consuming, leading to a higher cost for testing.



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The wetting process in this test procedure runs under two different conditions: wetting of the dry layer up to the maximumlayer conductivity (severity value for the individual test) in 20 to 40 minutes without applying the test voltage, and continuingthe wetting after immediate application of the constant test voltage for 15 minutes at maximum.

A detailed description of the Solid-Layer test ‘Procedure A’ is given in IEC 507,1991 22.

Note: This procedure is only rarely used today and is not considered to be optimal. For most of the cases, Procedure B

"Wetting after energisation" (see clause 6.3.1.2.2) is to be preferred.

6.3.1.2.2 Procedure B - Wetting af ter energisation

This procedure simulates pollution conditions at service voltage where a layer of binding material and some kind of salt iswetted by condensation. This seems to be the most frequent situation for sites with solid-layer contamination as may occur inrural, industrial and desert regions.

The degree of pollution is usually measured in Salt Deposit Density (SDD), which is expressed in mg salt (NaCl) per cm² of aspecified surface of the test specimen. The control of the test conditions (surface cleanness before application of the artificiallayer, uniformity of the layer, wetting conditions) is difficult and may require additional testing work. To reach the requiredSalt Deposit Density is time consuming, leading to a higher cost for testing.

For this procedure, the wetting process is started after the application of the constant test voltage to the insulator with the layer dry and it lasts with a constant steam input-rate until the end of an individual test.

A detailed description of the Solid-Layer test ‘Procedure B’ is given in IEC 507,1991 22(for a.c. systems) and in IEC 1245,1993 256 (for d.c. systems).

As described in IEC 507, the steam input-rate shall be within the range 0.05 kg/h ± 0.01 kg/h per m3 of the test-chamber volume. This steam input-rate is adequate when the pollution layer is formed only with salt (NaCl) and an amount of kaolin,Tonoko or Kieselguhr as the inert material. However, when laboratory tests are performed on naturally polluted insulators, inwhich a considerable amount of non-soluble material (kaolin or gypsum for example) is deposited on the insulator surface, thesteam input-rate shall be increased to wet adequately the pollution layer and to reproduce field conditions.

Unfortunately, as the steam input-rate increases, the temperature inside the test chamber also increases - thereby reducing thefog density. For this reason, other sources of fog generation shall be considered (cold or ultrasonic fog) to avoid this rise in

temperature. Although various papers dealing with this problem have been published

42

257

, more research is still necessary.

6.3.2 Non-standardised test procedures

6.3.2.1 Quick flashover method

The quick flashover method is based on the Salt-Fog test and uses a variable-voltage application 258. The cost and test-timeare lower than those for the standard test procedure.

Starting with a stabilisation period of 20 minutes at about 90 % of the estimated flashover voltage at the specified salinity, thetest voltage is then raised in 5 % steps, 1 minute at each level, until flashover. The insulator is immediately re-energised at itsinitial voltage and the process repeated until 5 flashovers are obtained. This part of the procedure is a kind of conditioning of the insulator.

For the second part, 90 % of the average of the 5 FOV values is applied to the insulator as a reference voltage. The testvoltage is then raised in steps of 2,5 % - 3,5 % every 5 minutes until flashover. The test is continued with 90 % of the

previous value of the flashover voltage until the required number of flashovers has been obtained. The performance criterionfor the insulator is the mean FOV after the stability of the FOV values has been reached. Lambeth258 has suggested that, for

porcelain insulators, there is an acceptable relation between the withstand salinity determined according to IEC 507 and themean FOV obtained from the quick flashover method.

6.3.2.2 Dust chamber method

This method is intended to simulate solid-layer contamination deposition on the insulator surface by wind 259 260. It can beused without any pre-treatment of the insulator’s surface. The amount of pollution accumulated on the insulator will be

determined by its surface properties, the shed shape, the applied voltage and the number of test cycles. The wetting isachieved by fog and/or rain. Figure 6-1 shows an example of the cycle 260.



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Voltage

Dust

Fog

RainWet

Drying

Time

Figure 6-1: Schematic view of one cycle of the Dust chamber method 260

.

The performance criterion of the insulator is the SDD-value of the artificial pollution layer, the test voltage and the number of cycles required to achieve flashover. To avoid too many cycles, a fixed number can be run to simulate a specificenvironment. The duration of pollution application and the amount of wetting have been calibrated using a standard type of insulator so that the pollution level after the fixed number of cycles corresponds to the specified degree of pollution. If no

flashover occurs during these cycles, the test object is deemed to have withstood the specified degree of pollution for which ithas been tested. A more detailed ranking using the leakage current and the SDD and NSDD-values is possible. Additionalresearch is needed to establish the relation between these results and those determined from tests made according to IEC 507.

6.3.2.3 Dry Salt Layer Method (DSL)

The DSL is intended to simulate dry salt accumulation close to the coast followed by wetting, ‘rain after the storm’, to achievecritical flashover conditions. No special pre-treatment of the insulator surface is required. The profile and adhesion

properties of the insulator surface are allowed to influence the amount of pollution collected. The test is designed to representthe essential features of the pollution accumulation process away from immediate salt spray. Fine humid salt particles from asalt-injection system are blown towards the energised test object by high-speed fans for a predetermined time to give therequired pollution level. Subsequently, it is exposed to a cold fog for wetting and to determine the flashover/withstand

voltage.The equipment needed to apply the salt to the test object are standard Salt-Fog nozzles, as is described in the IEC 507, andlarge high-speed fans. A good control over the relative humidity in the test chamber is necessary. Further tests are needed toestablish the correlation with results of tests conducted according to IEC 507.

6.3.2.4 Heavy wetting conditions

Heavy wetting conditions may occur in service during severe weather situations - like heavy rain, typhoons and strong sea-storms; it can also happen during live-line washing. Large amounts of water descend onto the polluted surface, which maylead to high conductivity values and possibly to an over bridging between the sheds, thereby initiating the final pollutionflashover at phase-to-earth voltage.

To check the ability of a polluted insulator to withstand heavy rain or washing without flashover at service voltage, thefollowing test procedure was developed in the UK with respect to recorded rainfall data for England and Wales 261 262 and theCEGB practise for live washing. This test was an adaptation of a procedure developed to determine the performance of

polymeric sheds fitted as supplements to porcelain barrel insulators. The test investigates whether inter-shed breakdown dueto pollution and heavy rain bridging the sheds is responsible for reducing the flashover strength of insulators. A goodcorrelation was found between types of insulators experiencing failures during this test and those that have a poor servicehistory during heavy rain or live-line washing (e.g. tapered CTs, inclined transformer bushings, inverted-V substationinsulators).

The insulator, prepared and preconditioned as for the usual Salt-Fog test 22, is energised at the specified test voltage andsubjected to the specified salinity for 15 minutes. After this pre-pollution phase, the fog is switched off and the insulator is leftto drain for a further 15 minutes. After that time, the heavy wetting is applied at an angle of approximately 45o to the insulator.The wetting is in the form of an artificial rain of 2 mm/min with a water conductivity of 100 µS/cm. The test is considered awithstand if no flashover occurs during washing off the deposits or if the leakage current activity decreases. The heavy wettingtest is deemed to have been passed if three withstand tests out of four applications could be obtained.



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The withstand salinities obtained from the heavy wetting tests are not equivalent to the withstand values from tests madeaccording to IEC 507. This is due to the decrease of salt-fog deposits during both the drain period and the test period and thelarge difference in the quantity of water impinging on the insulator.

6.3.3 Non-standardised test procedures for laboratory tests on natur all y polluted

insulators To determine the actual strength of insulators from a specific site, laboratory tests on naturally polluted insulators could be

performed. Examples for some of the procedures used are given in various publications 263 264 265 266.

The "Hybrid Test" (artificial wetting of the naturally polluted insulator) basically contains the following parts:

• Wetting of the pollution layer by steam fog or cold fog.

• Application of the constant test voltage before or after the wetting process.

• Increase of the test voltage in steps, or continuously, until flashover occurs.

The test results, in terms of withstand voltage or flashover voltage, can be compared with the phase-to-earth voltage todetermine the safety margin of the insulation to pollution flashover at service voltage. Also, the leakage current measurementduring the test is a helpful indicator for the judgement on the insulation strength 267.

This check of the actual insulation strength can also be used for implementation of remedial measures on line- and station-insulators.

It is also possible to check the pollution performance of an artificially polluted insulator at a specific site 113. In that case(natural wetting of an artificially polluted insulator), the result shows whether or not an insulator could withstand - at the

phase-to-earth voltage - a specified degree of pollution under the wetting conditions at that site.

6.4 Test procedures for polymeric insulators to be used in high-voltage a.c. or d.c.systems

Operational and laboratory experiences show that the pollution performance of new polymeric insulators is superior to that of glass or porcelain insulators. This excellent pollution performance may deteriorate during service time due to the influence of

UV radiation, temperature, humidity and leakage current discharges. Different accelerated ageing test procedures have beendeveloped 268, but as yet no agreed method is available for predicting the pollution performance of a polymeric insulator under given site conditions with time in service. The Cigré Task Force 33.04.07, ‘Testing of polymeric insulators’, is dealing withthis problem. IEC TC 36 also deals with this in its work-programme.

6.5 Test procedures for insulators covered with ice or snow

It is very difficult to do flashover tests on insulator assemblies covered naturally with ice or snow. Such tests would only be possible in field test stations located in areas with regular natural ice accretion or with heavy snowfalls.

To make a statistical evaluation of the test results, it is necessary to perform multiple tests and, so, the use of laboratory testmethods with artificial ice accretion or snow accumulation become necessary.

Separate laboratory test methods have been developed for simulating ice and snow conditions. These tests have been made

applicable to ceramic, polymeric, line-post and station-post insulators. The aim of the laboratory tests is to simulate, as closeas possible, the service conditions the insulators experience under ice or snow.

6.5.1 Laboratory test methods with ice

The development of testing methods for evaluating the flashover voltage of HV insulators under icing conditions is still at anearly stage 94 359 356 357 269. The number of tests carried out in the past 30 years is rather limited when compared to other typesof testing (e.g. Salt-Fog or Clean-Fog tests) and, above all, these tests have only been carried out in a few places. A verylimited amount of information about the different icing-test techniques are currently available, as most researchers havedeveloped test methods of their own - according to their extent of knowledge and to the financial means available. Some of the techniques could be considered closer to an art rather than a science, especially the very first techniques employed.

Nonetheless, some interesting results have been produced. However, due to the differences in testing methodology, it is often

difficult to compare the results of the reported tests.Test methods to determine the flashover voltage of iced insulators involves the following aspects:



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• Mounting arrangement

• Ice accretion

• Voltage application

• Withstand voltage evaluation

6.5.1.1 Mounting arrangement

It is recommended that the test object be mounted in a position similar to that of its service conditions. This is necessary because the distribution of ice on the insulator is influenced by the electrical field distribution around the insulator, as have been observed both in service and during laboratory tests 359 357.

6.5.1.2 Ice accretion methods

Most of the techniques that have been reported use some kind of nozzle for ice accretion on energised or non-energisedinsulators 356 269 270 187 271 177 272. Some of the more sophisticated ice-coating methods also make use of wind generationsystems in combination with the nozzles 94 269 270 271. The voltage (e.g. maximum operating value) should already be appliedduring the ice accretion phase by whatever method.

The reference parameters used up to now, for describing the severity of the icing condition, are:• The time duration of the icing period 269 271.

• The length of the icicles formed 191 194 184 313 273.

• The weight of the ice deposit on the insulator 272.

• The thickness of the accumulated ice on a monitoring pipe or conductor exposed to the same icing conditions as those of the test object 94 187 357.

6.5.1.3 Voltage application methods

In the case of an ice-covered insulator, the surface condition on the test object is particularly sensitive to the presence of voltage. Leakage currents that flow across the insulator surface causes a significant heating effect leading to the melting of

some of the accreted ice. This affects the ice deposit characteristics and can even ‘destroy’ the initial ice deposit. In the casewhere the flashover testing is related to a specific determined icing severity, it becomes obvious that only a single flashover test can be realised for each instance of ice accretion. This is the reason that, in most cases, a constant-voltage method is usedin icing tests.

6.5.1.4 Withstand voltage evaluation

The evaluation of the withstand voltage of the insulator under test usually varies according to the aim of the test and thechosen method of defining the electrical performance of the insulator under icing conditions.

If the purpose of the test is to evaluate and compare the performance of the different types of insulator under a specific voltagelevel, it is suggested that the applied voltage is kept constant and that the time of ice accretion is varied. The test ends if aflashover occurs or if the probability of a flashover is judged to be low. In the latter case, the test-outcome is considered to be

a withstand 359 187 357 182 274.However, in most cases, the aim is to determine the withstand voltage of the insulator under certain icing conditions. In thiscase, the ice accretion is stopped after reaching the specified value and the test voltage is applied in accordance with the

procedures described in the chosen standard (e.g. IEC 60-1, IEC 507) for tests under constant voltage 94 183 194 358 313 271 272 275.

6.5.1.5 Cold fog method

The cold fog test 180, reproduces natural conditions in which there is repeated re-icing of the insulator with a thin ice layer asthe ambient temperature rises from -2oC to +1oC. For the cold fog test, the flashover strength of the insulator is determinedrepeatedly while the ambient temperature is raised slowly. During each cycle, the applied voltage is raised in increments of 3-5% - starting from the service-voltage value - until flashover occurs. At each step, the voltage is held constant for 60 seconds.It is found that roughly half of the flashovers are observed on the voltage-rise and the other half within the 60-second hold

period. The flashover strength of the insulator reduces as the dew-point temperature increases to 0oC. This relation can befound through statistical analysis, to obtain the withstand levels for the insulator at 0oC.



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The cold fog test without icicles does not determine the minimum flashover level of an insulator. However, it does reproducefield conditions that are observed frequently and the test is severe enough to give realistic performance rankings.

6.5.1.6 Outdoor tests

Outdoor tests of ice-covered insulators are mainly performed at test stations located in areas with suitable meteorological

conditions 181.

Also, some laboratory tests on ice-covered insulators have been performed in an outdoor/indoor testing station combination183 358, where the ice was formed under natural frost conditions by spraying water onto the de-energised insulator by means of a hand-held nozzle. The voltage test was then performed after the insulator had been moved inside. It is also possible toallow the insulator to collect ice during the night in an outdoor test facility and then to determine the flashover voltage duringice melting conditions - due to sunshine - in the morning 111.

6.5.2 Laboratory test methods with snow

Various researchers have reported on tests methods with snow covered insulators 189 193 194 190.

6.5.2.1 Snow covering methods

It is necessary to cover the test insulator assembly with snow in a short time to avoid a change in the properties of the snow.Some methods are as follows:

1. A snow-pile “jig” may be used to cover the test-insulator assembly with snow 190. If the test is conducted inside alaboratory, the jig and insulator assembly should first be cooled sufficiently by dry ice. The snow can then be piled intothe jig and so onto the insulator. After the whole insulator assembly is covered with snow, the snow-pile jig is removed.

2. Blocks are cut from naturally accumulated snow on the ground. These blocks are then arranged on top of the insulator assembly under test. The conductivity of the snow may be adjusted by uniformly spraying a salt solution over the snow.Just before the test the volume density, liquid water content, conductivity, and volume resistivity of the snow on theinsulator are measured 189.

6.5.2.2 Voltage application method and test result evaluation

A constant-voltage application method is utilised to determine the a.c. or d.c. withstand voltage during testing. This meansthat a constant voltage is applied to the insulator assembly covered with snow to check the withstand voltage until the snowfalls off the assembly. After each test, the snow covering is renewed. If flashover occurs, the applied voltage is decreased by5 to 10% and - correspondingly if withstand occurs - the voltage is increased by the same extent. This procedure is repeatedabout ten times to obtain the minimum flashover voltage and the maximum withstand voltage. The maximum voltage whichgives 4 withstands and no flashover may be defined as the withstand voltage.

For tests under temporary overvoltages, a continuous a.c. voltage is applied to the insulator assembly for 5 to 20 minutes.Then the voltage is raised steadily for 2 seconds to obtain the temporary a.c. overvoltage characteristics by measuring the timeto flashover. This procedure is repeated every 5 minutes until the snow falls off the insulator assembly.

For switching and lightning flashover tests, the up-and-down method is used to determine the 50% flashover voltage.

6.6 Additional information on particular points of pollution testing

6.6.1 Ambient conditions dur ing testing


The ambient conditions during testing in a pollution chamber are defined as temperature, pressure and fog. These parametersare influenced by atmospheric conditions around the test chamber, which change during the day and throughout the year. Thisis especially so at pollution laboratories located at high altitude or in a warm climate - or in the situation where the fogchamber is not well insulated. Differences in ambient conditions during testing contribute to the variation in the results

obtained at the various laboratories around the world. It is, therefore, necessary either to control the test conditions or toapply correction factors to the test results.



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From a standardisation point of view, the testing methods must meet the requirement of reproducibility 3. Therefore, it has been necessary to investigate the effect of the ambient conditions during testing on the flashover/withstand voltage of contaminated insulators subjected to a.c. and d.c. voltages. Taskforce 07 of Cigré Working Group 33-04 is dealing with theaspects of testing polymeric insulators. The findings of that taskforce will soon be published.

The aim of this section is to review the current knowledge published in the technical literature on the effect of ambientconditions during testing and reproducibility in artificial tests.

6.6.1.2 Effect of Temperature

a) Atmospheric temperature

Arai 276 has shown that the temperature-rise in the chamber is greatly affected by the atmospheric temperature. This effectshould be taken into account during testing, especially if the chamber is poorly insulated and/or tests are conducted in verywarm climates - otherwise, the test results will be different from those obtained at other laboratories. Lambeth et al 277 havesuggested recommendations to overcome this situation.

b) Ambient temperature-difference

Several researchers have investigated the effect of the ambient temperature on the wetting process. For example, Rizk 67,Kawai 112 and Naito 278 have observed that the temperature difference between the insulator’s surface and the ambient is

approximately 2°C in artificial tests, which are similar to that which may occur naturally. Karady 68 has shown that the fog-temperature is not constant but increases with a definite time constant. The influence of the temperature on fog generation isdiscussed in a little more detail in the section dealing with fog.

c) Effect of the temperature on the flashover/withstand voltage

A significant influence of temperature on the contamination flashover voltage has been reported for both the Equivalent-Fogand the Salt-Fog methods under d.c. voltages.

Figure 6-2: Influence of room temperature on 50% FOV281

.

Ishii et al 279 have observed - when using the Equivalent-Fog method - that the d.c. flashover voltage of contaminatedinsulators reduces by about 0.7 - 1.0% /°C rise in temperature and that the d.c. arc-characteristics are temperatureindependent. This effect is attributed exclusively to the change of the resistance of the pollution layer.

Naito 280, using a Salt-Fog procedure for d.c. insulators, has found that the value of the flashover voltage decreased with theincrease in the temperature for the range 5 to about 35 degrees Centigrade. The findings were variable and in, some cases,this decrease in voltage was more than 20% of the lowest-temperature value. An example of the results is shown in Figure 6-2281. It is reasoned that this general trend is caused by the increase in conductivity of the polluted layer as the temperatureincreases 37.

A similar phenomenon has been observed for a.c. voltages by Moreno et al 282 when using the Salt-Fog procedure and byMizuno et al when employing the Clean-Fog procedure 281.

In the standard contamination tests, the influence of temperature has so far been disregarded.



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6.6.1.3 Effect of ambient pressure

The effect of ambient pressure on the flashover voltage is important for pollution laboratories situated at high altitude and has been investigated in both variable-pressure chambers and at special outdoor facilities at high altitude.

This topic is discussed in Section 3.3.8, where a general overview of the air density correction factors is given.

6.6.1.4 Effect of Ambient Fog

It is well-accepted that the flashover voltage and the minimum surface resistance depend on the wetting method and the parameters of the fog.

The artificial fog used to wet polluted insulators in laboratory tests can be produced by different means (i.e. cold fog, steamfog, evaporated fog, etc.). Not every type of fog is, however, suitable for use in Clean-Fog artificial pollution tests. On therealisation that good wetting is achieved on insulators by condensation, only fog produced by boiling water has been acceptedfor conventional tests. The wetting is uniform, the wetting-rate is slow and the washing-effect is small. The fog density can

be controlled by regulating the capacity of the boiler element. However, an increase in chamber-temperature is unavoidableafter some time of fog generation - due to the use of steam. The result is that the flashover voltage tends to be lower as thetemperature increases.283

The comparison of results between different laboratories is difficult to make, because the flashover values differ markedly for the same insulator and pollution level. This may be ascribed to a significant difference in wetting achieved in the variousarrangements. However, to simulate more accurately the natural low-temperature wetting process on polluted insulators - e.g.due to high altitude - an alternative humidifying technique using an ultrasonic clean-fog system, has been proposed for a.c.testing284.

Karady 68 has analysed the wetting process during the testing of artificially contaminated insulators. It was found that theflashover voltage depends on the fog condition. Different fog-generation methods (cold, warm, and steam) were alsoanalysed. It was concluded that the pollution test results for an artificially contaminated insulator depend on the fog

parameters and the fog-generation method. The operation of the fog chamber can be optimised by adjusting the fog parameters to achieve the minimum flashover voltage.

Arai 276 has established the correlation between the steam flow-rate and fog density and has suggested that, for the fogwithstand test using steam fog, the ideal fog condition would be about 3 to 7 g/m3 for the maximum liquid water-content of the

fog. Parameters like the temperature-change after fog generation, the liquid-water content of the fog and the water deposit-density on the insulator surface were measured. The results show that the initial temperature and the steam flow-rate greatlyinfluenced the characteristics of the fog chamber. The results also show that the faster the steam flow-rate, the higher thetemperature rise for a given wetting time. According to the results obtained, a higher initial temperature resulted in a slightlysmaller temperature rise at the same steam flow-rate.

With regard to the effect of the steam flow-rate on the a.c. fog withstand voltage, Arai 276 has shown that the withstand voltagetended to be higher at a lower steam flow-rate of each initial temperature. He concluded that the value of the steam flow-ratecould not be defined as merely the steam-fog conditions.

Arai 276 also reported on the effect of maximum liquid water content of the fog on the fog withstand voltage; the main pointsare:

• When the maximum water content of the fog was smaller than a certain value, of some 3g/m3, the withstand voltage

tended to become higher regardless of the initial temperature.• The withstand voltage was almost unchanged, within 5 to 7% of the dispersion for a certain range of the maximum

liquid-water content of the fog.

• When the liquid-water content of the fog was extremely high, the results tended to show a large degree of dispersion because of the washing effect of the fog.

The influence of the fog parameters on withstand voltage of contaminated insulators has been investigated by Naito et al 278.The optimum fog conditions of the fog withstand method were mainly investigated on the basis of the comparison betweennatural and artificial fog conditions and the influence of fog conditions on the withstand voltage. Their conclusions are asfollows:

a) The fog density of 3 to 7g/m3, under which withstand voltages with a small scatter of values was obtained, was severalorders of a magnitude higher than the density of natural fog. This is because the artificial fog is used to wet the

contaminated insulator in a reasonably short period of time.



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b) The droplet size distribution, which influences the wetting process by collision under the artificial fog, was almost thesame as that of natural fog.

c) By using steam or hot water, the temperature-difference between the insulator and the fog-chamber was 6 to 7°C. Thisvalue was higher than that of natural fog. Therefore, the artificial wetting process is accelerated from the viewpoint of thecondensation.

NGK has reported 44 measurements of fog-density in the range of 2 to 5 g/m3 for steam injection and 0.5 to 1.8 g/m3 for evaporation-fog. Similar measurements with evaporation-fog at HVTRC gave 0.3 to 1.5 g/m3. In their conclusions they havereported that, for fog densities higher than 0.3 - 0.4 g/m 3, both the evaporation-fog and the fog produced by steam injectiongave the same level of flashover voltage. The wetting rates were also quite similar. The authors have suggested the need toinvestigate the performance of insulators under very light fog condition, characterised by fog densities of less than a 0.3 g/m3.This is because the uneven wetting along the string may cause a non-uniform voltage distribution - which, in turn, may affectthe flashover voltage.

6.6.1.5 Reproducibility of artificial tests

Reproducibility 277 is defined as the extent to which a specified test gives the same result when performed in differentlaboratories. In other words, reproducibility describes the degree to which a test can be made in different laboratories and stillachieve the same result.

This requirement for artificial pollution methods is important for all the testing laboratories located in different parts of theworld.

The Salt-Fog and Clean-Fog methods for HVAC have been shown to meet the requirement of reproducibility 277. However,for HVDC, the Salt-Fog method has been unsatisfactory 285 and the variants of the Clean-Fog test-procedure have givendifferent results when performed in different laboratories. To investigate this problem, a comparative programme for theHVDC contamination test was performed by NGK (HV Lab) and EPRI (HVTRC). It was concluded that the flashover voltage obtained in these two different laboratories agree very well when some important test parameters are controlledcarefully 44.

Recently, a worldwide round-robin test of HVDC insulators was carried out in six laboratories 285. It was aimed at thestandardisation of the method for artificial contamination tests on HVDC insulators. The results are summarised as follows:

1. The test results of the Clean-Fog procedure seem to be reproducible because the scatter among different participants isabout that obtained in artificial contamination tests on HVAC insulators, for which the test-procedure has already beenstandardised.

2. The reproducibility of the results obtained by the Salt-Fog procedure is not very satisfactory. Further investigation seemsnecessary, in terms of reproducibility and repeatability, to standardise the procedure of artificial contamination testing onHVDC insulators.

3. It seems that sufficient information is available to allow the preparation of provisional international specifications for artificial contamination testing of HVDC insulators.

6.6.2 Leakage cur rent measur ement

The leakage current that flows during a pollution test gives information on the development of an arc bridging over a certain part of the insulator length 198 286 267 287 153. The highest leakage current that occurs during a laboratory withstand test is,therefore, a characteristic value for a particular insulator at the given severity and the given specific creepage distance. Figure6-3 shows an example of the leakage current characteristic of the longrod insulator L 75/22/150 at the a.c. test voltage of 72kV rms.

If leakage current measurements are performed on an insulator in service at a particular site, these measurements may be usedto judge the insulator’s electrical strength of that site. This is achieved by comparing these site-measurements with theleakage current characteristic determined in the laboratory - provided of course, that the specific leakage path length of theinsulator in service is the same as that used in the laboratory test.

Maintenance measures like cleaning, washing or greasing may also be initiated by measuring the leakage current on site 288.This measurement of current is used only for glass and porcelain insulators and does not apply to hydrophobic polymericinsulators. This is because for the latter, flashover occurs without a clearly pronounced development of the leakage currentwith time.

Another advantage of the leakage current measurement made during laboratory tests has been identified 267. In the case of small clearances between insulator sheds, the bridging over of these air gaps can be indicated by the current measurement.



1999-09-01 104

Such an insulator has a characteristic like that of an insulator with a shorter creepage distance, provided the applied stress onthe latter does not lead to bridging over between the sheds.

2,5 5 10 20 40

0

0,5

1

1,5

2

kg/m³

Salinity

A

L e a k a g e c u r r e n t

Highest leakage current from 1 hour withstand test;

Current in the halfcycle before flashover;

Figure 6-3: Leakage current characteristic of the longrod insulator L 75/22/150 (test voltage 72 kV rms, creepage distance

2480 mm).

6.6.3 Testing of insulators for the UHV range up to 1100 kV

Pollution tests on insulators for the UHV range require a large test-chamber and the corresponding polluting and wettingequipment. To fulfil the requirement of minimum short-circuit current in artificial pollution tests, a test-transformer andregulator with low short-circuit impedance are needed.

The question of full-scale testing depends on whether or not the dielectric strength of polluted insulators is proportional to theinsulator length at the same degree of pollution. If so, the results at lower voltages could be extrapolated to the UHV level.

Verma 289 has conducted a critical analysis of several research projects 290 113 291 251 292 293, taking into account the practicalrange of UHV insulation dimensioning and the influence of the voltage-drop on the test results. Essentially he concluded that:

Considering reasonable high dispersion of pollution test results in general and the relevance of the pollution performance of long insulator chains to practical insulation levels, linearity can be assumed for all practical purposes. Even if a non-linearityof 10% is claimed, it should not be forgotten that inaccuracy in insulation design due to a lack in knowledge of the actual siteseverity is greater than that resulting from assuming linearity for insulation design showing satisfactory performance. Theinsulation design data for 1100 kV can, therefore, be obtained by extrapolating the results already obtained and used for 400kV systems.

6.6.4 Compar ison of test resul ts obtained with dif ferent pollution test methods

Each of the two test methods, the Salt-Fog method and the Solid-Layer method, simulates different pollution conditions thatlead to a pollution flashover. This difference may lead to different rankings for several insulators, using these two testmethods. For different insulators, there is no direct relationship between the severity parameters of the test methods. For oneinsulator type and the same electrical stress, a correlation between the test methods is possible - using either the flashover voltage or the leakage-current characteristic.

6.6.5 Comparison of test resul ts obtained from test stations

In Figure 6-4, the a.c. flashover-voltage data obtained at 3 different natural test stations are compared with those of artificially polluted insulators under a Clean-Fog test. The smaller dispersion of the artificial contamination test results is due mainly to

the more uniform spread of the pollution on the insulator surface. The lower flashover values obtained with the artificial tests,as compared to the insulators polluted naturally, can be ascribed to the use of pure NaCl in the test. Naturally pollutedinsulators may contain low solubility salts that will lead to higher fog withstand values.



1999-09-01 105

Figure 6-4: Results of a.c. natural contamination tests compared with Clean-Fog tests294

.

A similar tendency as reported above for a.c. energisation has been obtained for a comparison of d.c. results - as is shown inFigure 6-5.

Figure 6-5: Results of d.c. natural contamination tests compared with Clean-Fog tests252

.



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7. INSULATOR SELECTION AND DIMENSIONING

7.1 Introduction

External insulation should be properly selected and dimensioned so that the resultant risk of flashover is reasonable. It may

be worthwhile to do a probabilistic, or risk-of failure, assessment. This section starts off with a discussion on how insulator characteristics can best be selected, followed by a comparison of the traditional deterministic design philosophy and the morerecent probabilistic approach. The difficulties in obtaining enough data for the statistical approach will also be highlighted.Finally, a summary of available literature related to the probabilistic approach to insulator design is given.

7.2 Selection of Insulator Characteristics

When insulator characteristics are selected, the complete flashover process should be borne in mind. Both the environmentaland electrical aspects thereof should be incorporated. In Figure 7-1 an approach is given that can be utilised in selecting theinsulator characteristics.

Frequency and

type of wetting

Type of

pollution

Mechanism of

pollution deposit

Precipitation

Wind borneElectrical

Active type Conductive DissolvingInert (effect of)

AmountDensityParticle size

Prediction of critical wettingconditions

Estimation of pollution distributionon the insulator

Identify possibility of self cleaningand required insulator propertiesor need for maintenance

Identify optimal profile & material

for insulator

Profile criteria fromexperience and

supplier’s dataTest andserviceexperience Creepage

Estimation

Insulator selection

and length

Insulator application

Figure 7-1: A structured approach to the selection of insulator characteristics.

Figure 7-1 complements Figure 1-2 in that it shows how the information collected, through the process of insulator selectionas outlined in Figure 1-2, is applied to select the insulator profile, axial length and creepage path length.

Referring to Figure 7-1, the selection process is outlined as follows:



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7.2.1 Selection of prof il e

From a study of the environment in which the insulators must operate, the pollution is characterised together with theidentification of both the most likely mechanism of pollution deposit and the type of wetting conditions.

From the pollution type - e.g. conductive, dissolving etc. - and the different types of wetting that occur, a prediction is made of the critical wetting conditions. That is, the wetting conditions under which flashovers are deemed to be the most likely.

In parallel with this procedure, the likely spread, uniformity and density of the pollution layer on the insulator is determinedfrom the following:

• The identified mechanism of the pollution deposit; whether it is by precipitation, wind or electrical forces.

• The physical characteristics of the pollution; i.e. density, particle size etc.

• The location of pollution sources and prevailing wind-direction.

• The amount of pollution present.

In areas where the mechanism of the pollution-deposit is mostly by precipitation, insulators with large horizontal surfaces maycollect more pollution than do those with small horizontal surfaces - but perhaps with more shed under-ribs. On the other hand, if the deposit mechanism is by wind aerodynamically shaped insulators may again collect less pollution than do the oneswith a more convoluted shape. More information and measurement results are contained in Sections 2.3.3 and 2.3.7.

The severity of the pollution can be estimated by using one of the methods listed in Section 5. The results of suchmeasurements should be compared with the corresponding service experience to obtain an indication of the site severity.

From the type of wetting - as obtained from weather data - and the identified critical wetting conditions (Section 2.3.6), the possibility of self-cleaning is then determined. For instance, if the critical wetting condition is identified as being mist or fog plus the presence of marine salt, self-cleaning can then only occur if the cleaning action of the wetting outweighs its pollution-wetting action. Therefore, significant self-cleaning will only occur under a heavier wetting condition; in this case, rain.Similarly if heavy rain is identified as the critical wetting condition, it can be concluded that self-cleaning by wetting can not

be relied upon. Another self-cleaning mechanism that is worth investigating is that of “wind blasting”. More details are provided in Sections 2.3.3 through 2.3.7.

If the possibility of self-cleaning without the risk of flashover can be ruled out, the need for insulator maintenance should beinvestigated. By taking into account the pollution type, the critical wetting and the distribution of pollution, the appropriate

maintenance procedures can be identified together with the insulator profiles that facilitate such maintenance.Once all the above factors have been investigated, the optimal profile and material can be selected. If self-cleaning isnecessary, insulators with an aerodynamic shape can prove beneficial. If no self-cleaning possibility exists, insulator shapeswith less accessible profiles might be more beneficial.

In selecting profiles, it is necessary to rely on the results of artificial tests and/or service experience. The limitation of profiledesigns for station post insulators are also set out in IEC publication 815. Polymeric insulators may be considered, for reasons given in the introduction to Section 3.

7.2.2 Selection of insulator dimensions

The findings provided in Section 3 can then be applied to estimate the insulator dimensions; i.e. axial length and creepage path length. Correction factors for large diameter insulators, both with regards to pollution deposit (Section 2.3.7.3) andflashover strength (Section 3.3.1.4) can be considered. Insulator diameter and shed spacing may be significant factors indetermining pollution performance in outdoor stations - particularly for equipment such as bushings, circuit breakers, andmeasuring devices.

The application of glass and ceramic insulators for a.c. voltages above 525 kV, as well as for high d.c. voltages, raises thequestion of linearity of insulator flashover voltage with insulator length. Although such data are quite limited at this time,

because of the large test objects and laboratory equipment involved, it appears that the flashover voltage is nearly a linear function of insulator length. However, at system voltages reaching levels of 1200 kV a.c., or 800 kV d.c., even a slight non-linearity of such will require longer insulator assemblies in polluted areas (see also Section 3.3.1.3.).

The possible adverse effect of a non-uniform pollution deposit should also be considered; this has been discussed in Section3.3.4. If constraints on insulator length prohibit sufficient creepage distance with the chosen profile, it becomes necessary tochoose an alternative profile or to consider utilising a different type of insulator material. Again service experience and

artificial test results are of the utmost importance.



1999-09-01 108

In the case of d.c. energisation, the accumulation of pollution is generally higher than that on an insulator for a.c. in the sameenvironment. Consequently, the required creepage distance to withstand pollution for d.c. must be suitably increased over thatrecommended for a.c. to obtain the equivalent performance. For d.c. substation insulators, such as wall bushings, insulator selection must take into account the behaviour of these insulators in relatively clean areas with non-uniform wetting. Section3.4.2 discusses this topic in more detail.

7.2.3 Determini stic method

The deterministic method has generally been used for the design and maintenance of electrical and mechanical components,apparatus, systems etc. The component is then designed according to material selection, dimensioning etc. to achieve awithstand value “W” of the component with a certain acceptable margin of safety between “W” and “S” - where the latter isthe probability relationship associated with the environment.

Environment (S)

Site severity

Probability

Margin

Insulation (W)

Figure 7-2: An example of the deterministic method.

In Figure 7-2, the deterministic approach is illustrated by using an example for obtaining the design withstand pollutionseverity of an insulator with respect to the maximum pollution severity of the environment in which the insulator must operate.In this example, the operating voltage of the insulator, Vs, is known. The maximum withstand pollution severity (ESDD) thatthe insulator must withstand is then calculated by assuming complete wetting of the pollution layer. The design withstand

pollution severity, or the corresponding withstand voltage, Vw, is determined with an acceptable margin; e.g. 10%, betweenVw and Vs.

The following problems exist with this approach:

1. The pollution severity, insulator withstand voltage and the degree of wetting are all probabilistic values.

2. The selected margin depends on the judgement of the design engineer and has, therefore, no statistical significance.3. Only a single insulator string, or stack, is considered in this approach; but, in the actual design, many such insulators are

connected in parallel.

7.2.4 Probabil istic method.

In the probabilistic method, the principal parameters are considered as variables; this is markedly different from thedeterministic method, where the variables are assumed to be constant.

Figure 7-3 shows the principle of the electrical design of insulators against switching overvoltages, which is described in IECPublication 71295 296. The probability density function of the expected switching overvoltage is considered a variable and isdenoted as “ f ” in the figure. Generally, it follows a normal distribution. The flashover probability of the insulator is shown bycurve P in the accompanying figure. The probability of flashover, logically, increases for higher voltages “U”. The risk-of-failure is calculated by integrating the probability density function, f *P; it is shown in the figure as the shaded area.



1999-09-01 109

If the insulation strength is increased, the “P” curve moves to the right of the “f” curve and the risk-of-failure decreases asshown in Figure 7-3b; but such a change can be costly. The optimum design is, therefore, obtained by optimising the costagainst the risk-of-failure.

(a)

P

f

U (b)

P

f

U

Figure 7-3: An example of the probabilistic method; the effect of increasing insulation strength.

The probabilistic approached is considered in a similar fashion for the mechanical design of an overhead line support in IECPublication 826297, where the strength of the support and the load applied to it are considered as variables.

7.2.5 Static and dynamic methods in the probabil istic approach.

Two methods can be used in the probabilistic approach. One is the static method that is described in the previous section, andthe other is a dynamic approach. The former is relatively easy to follow, but the risk-of-failure can only be calculated over along period; i.e. annually, over 50 years, etc. In contrast to this approach, instantaneous risk-of failure can be calculated usingthe dynamic approach; however, the calculation is quite complex and data relevant to that moment must be available. Thedynamic method is, therefore, currently not in general use.

Table 7-2 summarises the two methods of the probabilistic approach in selecting insulators in a polluted environment 298 299.In the static method 298 299 300 301 302 151 303 304 305 306 307, the probabilities of flashover voltage and other factors are combined ina reliability-calculation. In the dynamic method 67 306 308 309, the instantaneous changes in various factors, such as the weather,are taken into account for making a reliability calculation.

Table 7-2: Summary of probabilistic approaches for selecting insulators in a polluted environment.

ITEM STATIC METHOD DYNAMIC METHOD

Description Obtain risk of failure, “R”, by integrating the productof “F(w)”, distribution function of ESDD, and “P(w)”,Flashover probability.

Obtain risk of failure “R(w)” in a certain period, by summing up the product of “w(ti)”,ESDD at a time and “P(w)”, flashover

probability.

Advantage Easier calculation and survey because the requiredinput data are only distributions of ESDD andflashover probability. However, data for thiscalculation are not readily available due to cost andtime constraints.

Possible to obtain risk-of-failure at any timeand, thus, may be utilised as 'an alarm' of

pollution for a system.

Disadvantage Only an overall risk-of-failure in a certain period isavailable.

Necessary to input instantaneous weather data,and the calculation is more complicated.

Possibleimprovement

A method for calculating of risk-of-failure thatis not influenced by the sampling time.



1999-09-01 110

7.2.6 Present status of the probabil istic approach

One of the first practical applications of the probabilistic approach was carried out by Karady et al 302. They showed that thedistribution of Equivalent Salt Deposit Density (ESDD) on insulators at the coast over a period of a year follows a Gammadistribution, as is shown in Figure 7-4. The flashover probability of the insulators was assumed to be a normal distributionand the fifty percent flashover voltage and standard deviation were obtained as a function of ESDD by performing artificial

laboratory tests. The risk-of -failure was then calculated by using the two distribution functions. In Figure 7-5 the resultantrisk-of-failure for 45 parallel insulator strings for a 340 kV transmission line is shown as a function of the voltage per insulator.

1.0 2.0 3.0 4.0 5.0

no pollution

very light

pollution

light pollution

ESDD [mg/cm2 x 10-2]

0

20

40

60

80

100

C u m u l a t i v e p r o b a b i l i t y [ % ]

Figure 7-4: An Example of cumulative probability of ESDD302

.

0 4 8 12 16 20

20

40

60

80

100

R i s k o f f a i l u r e ( % )

Voltage per unit, kV

Figure 7-5: Risk-of-failure as a function of voltage per unit302

.

Lambeth 306 307 deals with statistical factors theoretically to determine the suitable insulator length for polluted conditions. Inhis documents, pollution severity and flashover stress are considered as variables.

Sforzini et al 303 apply the statistical approach for the selection of the type of insulator. The statistical distribution of pollutionseverity is approximated by using a Gaussian distribution, as is shown in Figure 7-6 - this distribution is based onmeasurement of surface conductance made on insulators at three sites. An acceptable value of the risk-of-failure is then



1999-09-01 111

assumed by considering the tolerable number of events per year. For this risk of failure value, the required flashover value atthe equivalent severity of the pollution on the insulator is determined. A suitable insulator is selected from the standpoint of leakage path length. The standardisation of insulators for polluted areas is also discussed.

99.8

99.5

99.0

95.0

90.0

70.050.030.0

10.0

1.0

CAIRO MONTENOTTESurface conductivity

( Aug: 77 - Oct: 81 ) No of critical events: 125

PORTO MARGHERAESDD -( Aug: 77 - Oct: 81 ) No of critical events: 102

PORTO EMPEDOCLESalinity - ( Jan: 79 - Oct: 81 ) No of critical events: 105

Insulator Y

1 2 4 6 8 10 20 40 60 80 100 200

Surface conductivity (µS)

C u m u l a t i v e p r o b a b i l i t y ( % )

Figure 7-6: Examples of cumulative frequency distributions of the maximum values of pollution severity recorded in the

various events at three typical sites (values are expressed in terms of the equivalent severity relevant to the laboratory

method deemed more valid for each site)303

.

Figure 7-7 shows an example design-standard for 132 - to 150 kV lines.

132 kV lines150 kV lines

320

160

80

40

20

10

5

2.59 (10)(11)(12) 9 10 11 12

Standard units Antifog units

W i t h s t a n d S a l i n i t i e s ( k g

/ m 3 )

Figure 7-7: ENEL standardisation; dimensioning of insulator strings for 132 kV and 150 kV lines303

.

Naito et al 299 have extended the approach into three dimensions. They calculated the static risk-of-failure on 800 kVtransmission lines by treating the flashover voltage, pollution severity and degree of wetting as probabilistic values. Aregression curve for relative humidity (RH) was proposed, as shown in Figure 7-8, which is based on hourly observations.The corresponding probability of simultaneous occurrence of ESDD and RH is shown in Figure 7-9.



1999-09-01 112

100 points

8760 points total

99.9

99.

90.

80.70.60.50.40.

30.

20.

1.

0.110 20 30 40 50 60 70 80 90 100

P r o b a b i l i t y e x c e e d i n g a b s c i s s a v a l u e [ % ]

RH [%]

Figure 7-8: Cumulative probability of Relative Humidity299

.

The flashover probability, as a function of RH and ESDD, is shown in Figure 7-10, for 200 parallel insulator strings.

0 20 40 60 80 1000.05

0.01

0.1

1.1.5

0

1

2

3

P r o b a b i l i t y o f o c c u r r e n c e

( % )

R H [ % ] E S D D

[ m g / c

m 2 ]

Figure 7-9: Probability of simultaneous occurrence of ESDD and RH 299.

The risk-of-failure is calculated as the volume indicated in Figure 7-11 and a value of about 0.03 per year was obtained,thereby implying that there are 11 days of flashover per year.



1999-09-01 113

0 20 40 6080 100

0.050.01

0.11.

1.5

0

20

40

60

F l a s h o v e r p r o b a b i l i t y ( % )

R H [ % ] E S

D D [ m

g / c m

2 ]

80

100

Figure 7-10: Flashover probability, P n , as a function of RH and ESDD (N=200) 299.

0 20 40 60 80 1000.05

0.01

0.1

1.1.5

0

20

40

60

R i s k o f f a i l u r e ( % )

R H [ % ] E S D D

[ m g / c m

2 ]

80

Figure 7-11: Risk-of-failure obtained (N=200) 299.

7.2.7 Dynamic method

Rizk et al 67 have described a dynamic statistical method to evaluate transmission line performance. The line was divided intoseveral sections, each of which was assumed to be exposed to uniform conditions of pollution build-up and wetting events.The total number of flashovers expected over a certain period of time were determined from the sum of the flashovers of thedifferent sections.

In any given line-section and exposure-period, the statistical variation of the string flashover voltage for a given pollutionseverity - as well as the statistical variation of the pollution severity itself - were considered. It was shown that the mostimportant parameters to determine line performance are: the ratio of the operating voltage to the 50% flashover voltage of thestring, the resultant coefficient of variation and the number of wetting events 67.

Lambeth 306 has also suggested the need to consider the change with time of the pollutant deposit, the wetting etc. Yamada etal 308 have extended the static model of flashover risk - by Naito et al 299 - into a dynamic model. According to this model, theinstantaneous change in climatic data produces a change in the ESDD value and the wetting-rate. These, in turn, affect the

flashover probability and risk-of-failure. Figure 7-12 shows an example of the results.A similar approach of dynamic-risk prediction under snow/ice conditions is used in Canada 180 310.



1999-09-01 114

Time of day0 6 12 18 0 6 12 18 0 6

0

20

40

60

0.0

0.2

0.4

0.6

0

50

100

0

50

100

100

50

0

15

10

5

0

32

1

0

Actual flashover Flashover probability (%)

Absorption density of moisture (mg/cm2)

Degree of wetting (%)

RH (%)

ESDD (mg/cm2)

Rainfall (mm/10min)

Wind velocity (m/s)

Figure 7-12: A sample simulation of flashover probability311

.

7.2.8 Truncation of the distri bution

In almost all cases, the studies that have been made assume that the flashover voltage, pollution severity etc. follows a normaldistribution. In reality, however, the distribution of relative humidity is truncated - as shown in Figure 7-8. Houlgate et al126

have reported that, in a natural pollution test station, the distribution of flashover voltage is truncated - as shown in Figure 7-13. The values of V50 and sigma derived from this curve are considered different from those obtained from the resultsobtained by artificial pollution tests. A statistical flashover study312, comprising 2800 tests on artificially polluted insulators,also indicates that a truncation of the distribution exists.



1999-09-01 115

0.01 0.1 1.0 10

50

60

70

80

90

100170

160

150

140

130

120

110

100

90

Cumulative Frequency, Flashover/insulator/year

N o r m a l i s e d F l a s h o v e r S t r e s s

k V , s y s t e m m - 1 , o v e

r a l l l e n g t h

k V a c t u a l , m - 1 , o v

e r a l l l e n g t h

EHVUHV

N k E E

E

E

k

n

o

o

n

o

= −

=

==

99

1612 1

..

Figure 7-13: Cumulative frequency distribution of EHV and UHV normalised flashover stresses for the total test period at Brighton insulator testing station126

.

7.2.9 Conclusions

Many probabilistic approaches have been reported for designing insulators under polluted conditions.

From a methodological point of view, a considerable amount of work is still necessary before this type of approach can beinternationally accepted. In addition, for such an approach to be successful, reliable statistical data of both the pollutionseverity and the insulation strength are required. The statistical approach is, therefore, not yet sufficiently advanced to beapplied in the design or maintenance of insulators in a polluted environment. However, such a method can give a clear indication of the critical conditions that will lead to insulator flashover.

7.3 Selection of insulators for application under ice and snow

On the basis of laboratory results, it is possible to employ either a deterministic or a probabilistic approach in selectinginsulators for use under ice or snow conditions.

The following points need to be considered:

a) The maximum electrical stress on insulators for transmission lines and substations should be kept below thecorresponding withstand value determined for ice and snow conditions in the laboratory. Some authors 313 314 indicate astress limit of 75 kV/m for both a.c. (rms value) and d.c. A value of 200 kV/m (peak) is suggested for the switching-surge stress limit where the axial length of the insulator is used to calculate the stress. However, it is still necessary totake account of other effects and conditions on the insulator - such as non-linearity effects on long strings, diameter of the insulator, shed profile and insulator surface condition - to achieve a successful design.

In environments having a moderate pollution level - i.e. 0.1 to 0.2 mg/cm2

- and where cold fog conditions are expected,the design withstand-level should be reduced to about 60 kV/m. This value is based on the results obtained for 1.85 m,230 kV insulators; thereby indicating a 75 kV/m CFO, with a 5% standard deviation.

Under ice and snow conditions, the lightning impulse level of insulators may be reduced by up to 50%. Therefore, thedesigner must consider whether it is necessary to take ice and snow effects into account when considering the lightning

performance. The factors to consider are the probability of simultaneous occurrence of snow or ice and lightning and theamount of snow and ice that is expected to accumulate.

b) The probabilistic method is based on the calculation of the insulator length, or the number of discs in the string towithstand flashovers - as considered from the U50. The latter being obtained from ice and snow test results, assuming anexpected flashover probability of less than 0.2. The calculation should also take into account the presence of other strings - i.e. parallel gaps - on the line subjected to the same icing conditions.



1999-09-01 116

7.4 Selection of insulators for d.c. energisation

7.4.1 Introduction

Several specific characteristics are necessary for effective d.c. insulation:

1. A correct insulator profile is required to enhance the withstand characteristics and to reduce pollution build-up.2. High resistance / high purity dielectrics are necessary to reduce the risk of ion migration / accumulation.

3. Sacrificial electrodes on metal fittings are necessary to avoid the effects of unidirectional current flow, especially inhumid environments.

Points 2 and 3 are covered in detail in IEC 61245 that gives minimum values and test methods to check these parameters.Point 1 is more difficult to specify.

For glass and ceramic cap and pin designs and post and bushing insulators, the optimal profiles are well known 315. However for polymeric insulators, the lack of service experience - especially for d.c. - means that the profiles which are currently usedare based on laboratory artificial pollution tests only and do not take into account pollution deposition mechanisms found inservice. Hopefully the growing use of polymeric insulators for d.c. applications can remedy this lack of knowledge andexperience.

7.4.2 Selection of a site sever ity corr ection factor

When dimensioning outdoor insulators for d.c. lines or stations, the pollution level measured from the nearby a.c. lines or stations provides important indications for the possible pollution levels of the d.c. lines or stations. Since insulators energisedat d.c. voltage may attract more contaminants than occurs at a.c. voltage, a correction factor is sometimes needed 316. Thiscorrection factor, referred to in the following as K p, is the ratio of the pollution level at d.c. voltage, Pdc to the correspondingvalue Pac at a.c. voltage; i.e. K p = Pdc/Pac. Various researchers have reported measurements, from which could be determinedthe values of K p

317 59 318 319 320 81 321 322 323 324.

As has been discussed in Section 2.3.3.1, the main cause of the difference between the contamination accumulation at d.c.voltage and that at a.c. voltage is the electrostatic force. However, the force that can outweigh 323 the effect of thiselectrostatic force is wind. Therefore, in areas where wind is the dominant force that brings contaminants onto the insulator,

the difference between the d.c. and a.c. conditions is small. In areas where wind is not the only major force that bringscontaminants onto the insulator, differences are seen between the contamination accumulation for d.c. and a.c. conditions.The extent of this difference depends on the wind speed, the type of pollution source and the distance to the pollution source.

Contaminants from natural sources - such as sea salt, desert sand or earth particles from open dry land - are mainly generatedand transported by the wind. The amount of contaminants and the transportation distance are the function of the wind-speedand duration. In areas where these types of pollution are the major sources, a lower K p value will be appropriate.

Some other types of pollution sources are: highways, industrial release, residential areas (especially when coal and wood areused for cooking and heating), mining and construction work. These are the man-made pollution sources. Contaminants fromthese sources are “self” generated rather than wind generated. The amount of contaminants produced bears little relation tothe wind-speed. The contaminants can spread from the pollution sources over a distance that ranges from a few hundredmetres to one or two kilometres at low wind-speed. If a d.c. station is located near such pollution sources or a d.c. line is

passing through such areas, there is a larger difference between the pollution levels of d.c. and a.c. insulators, i.e. a higher K p

value may be expected. During high wind, the contaminants from some of the industrial sources may be transported over afew kilometres. However, the reduction in the amount of contaminants with distance from the source is greater for the self-generated pollution than it is for the wind-borne types. In some exceptional weather conditions, industrial pollution - releasedas gases - can be carried over hundreds to thousands of kilometres181.

In areas that are considered as clean from the viewpoint of a.c. voltage, few measurements have been made. However, asignificant difference between d.c. and a.c. pollution levels has been observed in some areas 324 317, whilst no difference has

been found for some other areas. Pollution sources may exist, but their effects may not be discernible because the pollutionlevel is low. In this case, a high K p value is to be adopted. Further investigations are necessary to characterise the cleanareas.

As a rough approximation, the value of this correction factor K p is given Table 7-3325



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Table 7-3: Correction factor, K p , that provides the ratio between pollution levels at d.c. and a.c. voltage325

.

K P SITE CONDITIONS1 - 1.2 areas influenced only by natural pollution sources, such as sea and desert

1.3 - 1.9areas influenced both by natural pollution sources and by industrial pollution sources

but at a few kilometres distance from the industrial pollution sources2 - 3 areas close to (within a few kilometres) industrial pollution sources and are

considered as clean from the viewpoint of a.c. voltage

A further parameter that may intensify the accumulation of contamination at d.c. voltage is the electrical charging of thecontaminants by industrial processes or by corona discharges from high-voltage equipment.

7.5 Insulator pollution design of Phase-to-Phase Spacers

7.5.1 Introduction

Phase-to-phase spacers are mainly used to prevent mid-span flashovers occurring during conditions of galloping, conductor jumping following ice release etc. on transmission lines. These spacers may be either porcelain or polymeric ones. Inaddition, phase spacers may be required for compact line designs, reduced phase spacing to decrease magnetic field levels, or to improve the aesthetics of the line.

The design of phase-to-phase spacers may be different from that of phase-to-ground insulators.

7.5.2 Design Practice

The fundamental procedure for the design, from the pollution viewpoint, of phase-to-phase spacers is the same as that for phase-to-ground insulators except that the withstand voltage is √3 times that for the phase-to-ground voltage 326 327. Not onlyare the design procedures the same for phase-to-phase and phase-to-ground insulators but so are the design parameters such asESDD, specific leakage distance, effect of average diameter etc.

However, considering the consequence of a phase-to-phase flashover on the operation of a transmission line, an additionalsafety margin may need to be included. This is especially so in the case of polymeric phase spacers because of uncertainties -such as ageing deterioration, unknown performance under various conditions etc. In these cases, longer leakage distances arenormally adopted than would be in the case for the corresponding porcelain insulators.



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8. PALLIATIVES AND OTHER MITIGATION MEASURES

8.1 Introduction

In the event that the performance of the insulators selected for a specific application does not meet the design criteria,

remedies may be required to improve theis performance to an acceptable level.Although not usually possible, the most obvious remedy is to change the insulators either by adding additional units or bychanging the type. For example, insulators with higher specific creepage distance may be chosen to replace the originaldesign within the same physical spacing.

Insulators with semiconducting glaze may form a reasonable alternative if replacement is allowed. Such insulators have hadconsiderable application in substations for bus support insulators, but recently new designs of suspension insulators have also

become available.

More than likely, some type of maintenance will be needed if pollution flashovers become unacceptable and the replacementof insulators is not possible. Maintenance procedures can be classified into “periodic” and “semi-permanent”.

The most common type of periodic maintenance consists of insulator washing. Care must be taken to use appropriate procedures, including direction of washing and low conductivity water, to prevent flashovers during this maintenance procedure. The most difficult question to address is: "What is the necessary frequency of washing?" That is, some type of pollution monitoring will be required.

A second type of periodic maintenance is greasing. Although the petroleum version was used in the first introduction of greases - and continues to be chosen in some cases, silicone grease has better characteristics for the higher ambienttemperatures. Greasing must be repeated, with appropriate cleaning, and the intervals are determined by the serviceenvironment. Intervals from one to five years have been found to be acceptable.

If re-greasing is not needed for five years, the maintenance procedure could be considered as “semi-permanent”. Obviously,this is a qualitative judgement and will vary with utility perspectives.

Finally, the use of insulator coatings other than grease may be a semi-permanent or permanent remedy. Such coatings consist,for example, of room temperature vulcanised silicone rubber and have had success in many substation applications.

The options for correcting the performance of polymeric insulators are more restricted than for those made of glass or ceramic. Obviously, if pollution flashovers become unacceptably frequent, replacement should be considered.

Maintenance procedures must take into account the design of the insulator and the recommendations of the manufacturer.

8.2 Maintenance procedures

8.2.1 L ive-insulator washing of ceramic insulators


The growing attention to system reliability implies the necessity of adopting cost-effective measures to reduce outages of service. Among the various options, live-insulator washing - sometimes referred to as “hot-line washing”- is often employed.Herein are reviewed the methods and techniques presently used in live-insulator washing, with special reference to the relatedinsulation aspects. In particular, after a short description of the main washing techniques and equipment, electrical aspectsrelated to washing safety and performance are considered - thereby deriving indications useful for the standardisation of this

practise.

8.2.1.2 Cleaning procedures

8.2.1.2.1 Methods used

The main solutions available for the live-line cleaning of insulators are:

• Use of brushes.

• Projection of solid vegetable particles.



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• Application of water jets.

Cleaning by water projection is nowadays the most widespread solution adopted and the analysis in the following willconcentrate on this solution; it is referred to as live-insulator washing.

Four methods of live-insulator washing are most often used 328. They differ mainly in the type of nozzle arrangement adopted,and namely are:

− Portable Hand-Held Jet Nozzles.

− Helicopter Mounted Nozzles.

− Remote-controlled Jet Nozzles, often automated by using robots.

− Fixed-Spray Nozzles.

Portable Hand-Held Jet Nozzles are operated by qualified workers on the ground or at ground potential at relatively largedistances from the insulator, as required by safety conditions 328 329 330 331. The method is the one most adopted to date.

Fixed Spray Nozzles can be used for special applications and are installed at ground potential in fixed locations at relativelylarge distances from the insulators, as in the previous method 328 332. This technique is, however, not economic for widespreadapplication and requires an excessive amount of water.

Helicopter-Mounted Nozzles are particularly useful when access to insulators is difficult, e.g. in rugged or remote terrain or

when high mobility is required for rapid washing operations over long distances. The system is controlled by a wash-operator or by the pilot. With this self-contained, isolated and ungrounded system, the nozzle can be safely positioned closer to theinsulators than is the situation for the hand-held jet nozzle method.

Remote-Controlled Jet Nozzles. This method, often automated, has been recently proposed 333 334. The equipment generallyconsists of a nozzle fixed to an extendible truck-mounted boom or of nozzles carried by robots that are self-moving systemsafter being placed on the insulator to be washed. Today, many reasons justify the introduction of automated live maintenance;such as, the technological advancement in this field, the increasing requirement for a better quality of work and higher safety.Robotic-devices can allow mobile washing nozzles to be brought relatively close to the surface of the insulator; therebyachieving uniform washing with a small amount of water.

8.2.1.2.2 Water pressur e

In relation to the water pressure 328, the methods for hot-line washing can be subdivided into:• High-pressure water. A high-pressure system is mainly used in connection with hand-held, remote controlled and

helicopter-mounted nozzles. High-pressure washing utilises a narrow stream of water, with a typical pressure rangingfrom about 3000 kPa to 7000 kPa at the nozzle.

• Medium-pressure water. Medium-pressure systems are mainly used in the portable hand-held and remote-controlled jet-nozzle methods. The pressure range is from 2000 kPa to 3000 kPa.

• Low-pressure water. A low-pressure system is mainly used for fixed-spray nozzle methods. The pressures are in therange from 300 kPa to 2000 kPa at the nozzle.

8.2.1.2.3 Water resistivi ty

Water having a resistivity greater than 1500 Ωcm (e.g. from hydrants) is widely used. Demineralised water of 50000 Ωcm, or even greater, resistivity is also used. It is obtainable from steam power plants or from mobile demineralised equipment.

8.2.1.3 Requirements for live-washing operation

8.2.1.3.1 Safety requi rements

When using the Portable Hand-Held Jet Nozzle, where the water jet is directly controlled by the operator, a high degree of safety must be secured by having a relatively large distance between the operator and the insulator; as dictated by the specificstandards 328 and by the general safety requirements related to live maintenance 329 330 331.

In particular, the following requirements need to be met:

• The current that flows in the water stream (‘leakage current’) must be less than a certain value (e.g. 2 mA 328) when anoperator at earth potential uses the equipment.



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• The water stream should withstand the electrical stress under the a.c. system voltage and the corresponding overvoltages;as per the general requirements for live-line maintenance 329 330 331.

The requirements to satisfy these two conditions are analysed in the following section, by making reference to the most criticalcondition of the water stream impinging on the energised part.

The second requirement discussed also applies to the Fixed-Spray Nozzles method.

In the case of Helicopter-Mounted Nozzles and of the Remote-Controlled Jet Nozzles methods, no harm to the personnel mustoccur following capacitive charging of, or arcing along, the water stream. The other aspects that should be considered in thiscase are related to the dielectric strength of the overall configuration with the helicopter, or tool, at “floating” potential. Also,when they are possibly at line potential, discharges from the line-electrodes to the object at floating potential may occur.These aspects are similar to those analysed in the literature 329 330 331 and so will not be considered further herein.

8.2.1.3.2 Performance requi rements

From the performance point of view, the following requirements must be met:

• The insulator should withstand the applied stress under service voltage and overvoltages. This aspect may be of concernfrom the safety point of view, especially when an operator is relatively close to the insulator during the washingoperation.

• The procedure should be highly effective with respect to washing of the insulator.

Figure 8-1: Leakage current I on the water stream in relation to the voltage and the length of the water stream330

.



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Figure 8-2: Leakage current I on the water stream in relation to various parameters330

.

a) Influence of water resistivity.

b) Influence of water pressure.

c) Iinfluence of nozzle orifice diameter.



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8.2.1.3.3 I nf luencing parameters

Safety and performance have been investigated taking into account the influencing parameters, such as voltage applied,nozzle-conductor distance, water resistivity, water pressure and diameter and shape of the nozzle orifice 328 330 335 336 213 337 338

339 340 341.

8.2.1.4 Electrical requirements from the safety point of view

8.2.1.4.1 Leakage cur rent i n the wash water stream

The dependence of leakage current, I, along the water stream on the stream-length is given, as an example, in Figure 8-1 - for different phase-to-ground voltages for a pressure at the nozzle of 3000 kPa, a nozzle diameter of 6.4 mm and a water resistivity of 2.5 Ω.cm 330. For a given applied voltage, I decreases when the stream-length is increased. For a fixed stream-length, the current increases more than linearly when the applied voltage is increased.

The influence of water resistivity, water pressure and diameter of the nozzle orifice, are shown in Figure 8-2 a, b and crespectively - which refer to a stream-length of 4 m and to an applied voltage U of 245 kV 330.

The leakage current decreases when the resistivity is increased. This dependence is, however, rather limited. I reduces by

about 40 % for a variation of the resistivity from 2.5 to 50 k Ω.cm. The leakage current increases greatly when the water pressure is increased. It also rises appreciably when the diameter of the nozzle orifice is increased beyond a certain value.

The above trends conform to the other ones reported in the literature and particularly to those derived from the experimentaldata 328 336 213.

8.2.1.4.2 F lashover voltage along the water stream

For the power frequency case, Figure 8-3 shows the flashover voltage as a function of the clearance between the nozzle andthe energised part - for different water-stream parameters 328 330 213.

Figure 8-3: 50% flashover voltage under a.c. energisation as a function of the stream-length for different water-stream

parameters328

330

213.

As far as impulse voltages are concerned, the dependence of the 50% flashover voltage on the polarity and shape of theimpulse is given in Figure 8-4 330. It shows the flashover voltage in relation to the time-to-crest of the applied voltage for awater-stream length of 4m. In these tests, negative polarity was the more critical. For switching impulse (SI) waveforms, a



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front time of about 1200 µs gave the lowest level of flashover voltage. This value was about 25% smaller than thecorresponding one obtained with a standard SI of positive polarity - which is usually considered the more onerous in other experiments 336 213.

Figure 8-4: 50% flashover voltage along the water stream under impulse voltages in relation to the time-to-crest of the

applied impulse330

.

The 50% flashover voltage under switching impulse and standard lightning impulse wave (LI) is compared to the a.c.energised one in Figure 8-5 330. In this example, all of the stresses are given in peak value to facilitate the comparison. Fromcomparing the results with the corresponding ones for pure air gaps 331, it appears that the reduction in the flashover voltagedue to the water jet is marked with a.c. and SI, while it is minor with LI. Furthermore, the flashover value under SI is close tothat of the peak value under a.c. voltage.

Figure 8-5: 50% flashover voltage along the water stream: comparison of the dielectric strength under different stresses330

.



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Figure 8-6: Average flashover gradient along the water stream in relation to various parameters328

330

213

.

a) Influence of water resistivity.b) Influence of water pressure.

c) Influence of nozzle orifice diameter.



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The flashover voltage of the water jet is almost a linear function of the stream length.

The dependence of the average flashover gradient along the water stream on water resistivity, water pressure and nozzleorifice diameter is shown in Figure 8-6 a), b) and c) respectively 328 330 213.

The flashover gradient increases when the water resistivity is increased. It decreases when the diameter of the nozzle isincreased and has a U-curve relationship to pressure; thereby indicating a critical pressure that causes a minimum in the

flashover strength - which depends on nozzle characteristics.

8.2.1.4.3 M inimum working distances

A case study was considered by Perin et al 330 to obtain indications about the relative severity of the criteria based on leakagecurrent 328 and on dielectric withstand. In this case study, the conditions considered were: a pressure at the nozzle of 3000kPa, an orifice diameter of 6.4 mm and water resistivity of 2.5 k Ω.cm.

The comparison is shown in Figure 8-7, where the safe distances satisfying the limiting current criterion of 2 mA are given inrelation to the system voltage U by taking into consideration the continuous operating voltage and the temporary overvoltagesof 1.3 and 1.5 p.u. These distances are compared to the distances chosen so as to limit the risk of flashover under switchingovervoltages, which are the most critical stresses among the conditions examined from the flashover point of view. Theevaluation was made with reference to the Statistical Overvoltage U2 - which is the overvoltage having a 2% probability of

being exceeded - when the p.u. values are 2, 2.5 and 3, with reference to a defined risk of flashover 330.

This comparison indicates that, in some cases, the SI requirement can be the more critical.

Figure 8-7: Minimum washing distance in relation to the system voltage;

portable hand-held jet nozzle; comparison with distances recommended according to common practice328 330

.

It has to be stressed that the data in Figure 8-7 refer to a particular set of parameters in terms of resistivity, water pressure andnozzle diameter. Larger distances need to be employed when the resistivity is reduced. As an example, with reference to a 420kV system, a decrease of the resistivity from 2.5 to 1.3 k Ω.cm (which corresponds to the minimum value considered in theANSI standard 328) would lead to an increase of 10% to 15% in the minimum required distance. The influences of the nozzleorifice diameter and water pressure also need to be considered.

The distances shown in Figure 8-7 are the values derived from solely the electrical requirement. The minimum approachdistances under SI is evaluated, in a way similar to that employed in the IEEE standard 329 and by Perin et al 330. This isachieved by adding the so-called “ergonomic distance” - i.e. a sort of safety feature - to the above values, to take into accountthe uncertainties in the operation. A typical value for this ergonomic distance is 0.5 m.

The distances evaluated are generally lower than those adopted in common practice328

, as shown in Figure 8-7 (black triangle) - thereby supporting the safety procedures adopted up to now.



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8.2.1.5 Aspects related to washing performance

8.2.1.5.1 Withstand voltage of i nsulator under washing

The dependence of U50 on the water resistivity is shown in Figure 8-8 339. The data indicate that the flashover voltageincreases when the resistivity of the water increases. The influence is greater for low levels of contamination on the insulators.In general, provided water of sufficiently high resistivity is used, the flashover voltage under washing is higher than that under standard pollution tests of the same pollution severity.

Figure 8-8: Flashover voltage in relation to water resistivity339

.

The above conclusions apply when washing is done correctly. When washing is too fast, or when the wash-cycle is not startedfrom the bottom of the insulator, flashover at lower voltages may occur.

Figure 8-9: Flashover voltage along the insulator string in relation to water pressure for various nozzle diameters330

.

As far as SI flashover voltage is concerned, rather low withstand values are obtained during washing - as is shown in Figure 8-9 330. This information refers to standard switching impulses of negative polarity, giving - in this case - results close to the



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critical one (i.e. the minimum on the U-curve). However, the performance under SI is not very critical from the risk viewpoint; because of the low probability of having a high overvoltage during a washing operation. Thus, the correspondingrisk of flashover can essentially be neglected. The dielectric performance during washing is also influenced by water pressureand nozzle-orifice diameter, as may be seen from Figure 8-9. In general, the flashover voltage increases when the pressure isincreased and the orifice diameter is decreased.

Finally, it is worthy of note that the flashover voltages measured in tests simulating washing from a helicopter, were slightlyhigher than those obtained by using the portable hand-held jet nozzle - for these parameters considered by Perin et al 330. Thisfinding can be easily explained if one considers that the orifice diameter, and thus the quantity of water employed, was muchlower in the former case.

8.2.1.6 Washing effectiveness

Indications concerning the efficiency of two of the most common washing methods can be obtained from the tests resultsreported by Perin et al 330. These results are summarised in Figure 8-10.

Figure 8-10: Residual salt deposit density in relation to the washing time;

portable hand-held jet nozzle and helicopter nozzle330

.

These tests were carried out on a vertical insulator string for a 420 kV system, with a total length of 3 m. The followingwashing parameters applied:

• Portable hand-held jet nozzle; orifice diameter of 6.4 mm, pressure of 3000 kPa and a minimum distance to theconductor of 5 m.

• Helicopter simulation; nozzle with an orifice diameter of 1.7 mm, pressures of 4000 to 8000 kPa and minimum distanceto the conductor of 1 m.

The tests were made by contaminating the insulator string with an almost standard suspension and a non-standard one.

The almost standard suspension differed from the standard one by the quantity of kaolin used (100 g per litre). In the non-standard suspension, glue was added (10 g of metylan per litre) to increase the adhesion and the thickness of the layer, withthe aim of simulating conditions typical of industrial areas.

The test results provided in Figure 8-10 show that the washing efficiency improves when the washing time is increased.

Furthermore, the value of the effective washing time depends on the type of contamination. The time needed for an efficient



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wash using portable hand-held jet nozzles was shorter than that with helicopter-mounted jet nozzles, for a similar water pressure. Better agreement could be obtained by increasing the water pressure in the helicopter case.

8.2.1.7 Conclusions

8.2.1.7.1 Safety aspects

• The safe working distances for live-line insulator washing made by an operator at earth potential, with a portable hand-held jet nozzle, must be determined with reference to a limiting value of the leakage current along the water jet. It shouldalso be verified from the point of view of the withstand voltage under SI, as is the situation for any other live-lineworking operation. To this end, the most critical SI is that of negative polarity with a long front time.

• The same requirements may be used conservatively when fixed spray nozzles at ground potential are used.

• Safe working from a helicopter, implying the use of isolated metallic tools, must be determined by considering thedielectric strength of the arrangement with the object at floating potential (or possibly momentarily at live potential, if adischarge from the live electrode to the object at floating potential occurs). For this aspect, reference to the generalrequirements for live-line maintenance can be usefully made.

• When automated procedures are used, without the presence of people in the vicinity, electrical safety requirements are of

concern only to the equipment.

8.2.1.7.2 Performance aspects

• The washing operation does not reduce the system’s reliability, since no flashover is to be expected, provided thewashing operation is performed correctly. The flashover voltage in the helicopter-simulation test was found to be higher than that applying in the hand-held jet nozzle case, for the water-parameters considered.

• To obtain efficient washing, very different washing times may be required that depend on the type of contaminant. Thetime needed for an efficient wash using portable hand-held jet nozzles was shorter than that found with helicopter-mounted jet nozzles, for the same pressure. Better agreement between these two cases can be obtained by increasing thewashing pressure at the helicopter nozzles.

• Washing is influenced by many parameters.High resistivity water is beneficial with regards to safety and reliability, since - by increasing this resistivity - both thedielectric withstand of the water jet and that of the washed insulator are significantly increased. Washing is obviouslyvery much affected by water pressure and nozzle diameter.

8.2.2 L ive-insulator washing of polymer ic i nsulators

Polymeric insulators, generally, have high pollution withstand voltage characteristics when compared with their ceramiccounterparts. This is due to their high surface hydrophobicity, especially when they are new. Nonetheless, the polymericinsulators in the field occasionally flash over due to heavy pollution and wetting 342. It has been reported that the accumulated

pollutant on the polymeric insulators could be more than that on their ceramic counterparts - see Figure 2-26 86 - for the sameatmospheric conditions. Thus, live-line washing using pressurised water is sometimes considered necessary. Live-linewashing of polymeric insulators should only be done after considering the following points:

1. Wash withstand voltage. An effective insulation length shorter than that for ceramic insulators is sometimes used becauseof the higher pollution withstand voltage. Since the withstand voltage under washing is mainly affected by water cascading down the sheds, hydrophobicity is then not the dominant factor. Therefore, the effective insulation length isthe same as that of the corresponding ceramic one. In the case of insulators having a large diameter, such as bushingshells, the amount of cascading water is larger than that of line insulators. In such a case, the fitting of some special sheds- such as booster sheds - is recommended to break up the cascading stream of water.

2. Mechanical damage to material. It is reported that the shed material can suffer damage, such as tearing, or puncture inthe case of high water pressure 343 344. Therefore, the water pressure must be carefully specified; mainly taking thefollowing aspects into consideration,

• Shed material (e.g. silicone rubber, EPDM etc).

• Manufacturing method (e.g. moulded, bonded, un-bonded, etc).

However, it is not wise to clean some types of polymeric insulators by using pressurised water, and so recommendations fromthe manufacturer are to be followed. It is important to note that washing by pressurised water does not always achieve the



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best cleaning. This is especially so when the pollution layer adheres strongly to the insulator surface; e.g. cement or gypsum,and when the water stream can not reach the entire insulator surface 345.

8.3 Use of greases and RTV coatings

8.3.1 Introduction The performance of glazed porcelain insulators can be considerably improved by the application of hydrocarbon (petroleum

jelly) or silicone grease or a RTV silicone rubber coating to its surface346 347. Silicone greases and RTV coatings of differenttypes are widely used today. Both the greases and silicone rubber coatings reduce the surface energy of the insulator andinhibit the formation of a water film. In addition, the greases encapsulate contaminant particles in a thin grease film, therebyisolating them from each other and ensuring that the surface remains hydrophobic. In the case of the silicone rubber coating,low-molecular weight silicone components within the body of the material diffuse to the surface and impart hydrophobic

properties to the contaminant layer 105. The inclusion of arc resistant components, such as alumina trihydrate, in the siliconegrease and RTV coatings stabilise their performance under heavy wetting and contribute to their longer useful life.

The use of these measures with porcelain insulators is well proven. Their use with polymeric insulators is not generallyrecommended and should be discussed in detail with the insulator manufacturer if it is being contemplated. Based on some

experience in North America, a review has been prepared under the auspices of the IEEE347

348

. A further document contains practical information on the preparation of insulators prior to greasing or coating and the techniques for grease or coatingapplication 328.

8.3.2 Hydrocarbon and sil icone greases

Hydrocarbon (petroleum jelly) and silicone greases have been used as protective coatings on insulators for about 40 years andexperience has shown that, as long as they maintain their hydrophobicity, they provide substantially improved protectionagainst flashover when compared with the corresponding bare insulators. Comparisons of the hydrocarbon and siliconegreases have been made by both Lambeth et al 346 and an IEEE committee 347. Some of the practical characteristics are set outin Table 8-1 347.

Table 8-1 Comparison of Hydrocarbon (petroleum Jelly) and silicone greases347

.

PARAMETER HYDROCARBON

(PETROLEUM JELLY)SILICONE

Basic constituents Hydrocarbon oils, petroleum andsynthetic waxes

Dimethyl or phenyl-methyl siloxane fluid, couplingagents, fillers and solvents

Useful temperature 0 to 60°C -50 to 200°C

Melting point 60 to 90°C Does not occur inside useful temperature range

Recommended spraying temperature 90 to 115°C Ambient (-30 to 30°C)

Encapsulation rate, ambient temp. Slow Rapid

Ease of Application Difficult, esp. in cold weather Good

Ease of Removal Labour intensive Labour intensive

Arc resistance (ASTM D 495) Not available 80 to 150s, depending on formulation, fluid & filler

Material cost Low Moderate

Application cost Moderate Moderate

Cleaning cost High High

Water erosion, excessive exposure to corona, UV light and significant contaminant encapsulation reduce water repellency.Once hydrophobicity is lost, leakage currents will commence flowing and, in time, dry band discharges will also commence.These discharges cause the grease to decompose and the filler in the grease adds to the contaminant. Channels begin todevelop resulting in local hot spots and further degradation of the grease and possible damage to the insulator. Once channelshave begun to form, flashover of the insulator is imminent 347. Regreasing should be implemented as soon as dry band arcingis observed.



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The frequency of regreasing depends on the type of grease and the severity of the degrading influences mentioned above.Service experience with both a.c. and d.c. systems, has shown that the useful life of a grease coating can vary from less thanone year to 10 years.

Greases are normally applied by hand, brush or spray. Although application on de-energised systems is simpler, applicationon live systems is also possible. The application of fresh grease over contaminated grease is not recommended.

There are several tests that can be made to assess the suitability of grease as an insulator coating. The most significant are anarc endurance test under wetting and the water repellency tests in a Salt-Fog chamber or using a tracking wheel.Unfortunately, the laboratory tests suffer from a lack of correlation with field experience. Field-testing has proved to be theonly reliable method for evaluating the performance of different greases 347.

The pollution flashover performance of a 132kV epoxy-resin crossarm, which has been used in the UK to achieve aninconspicuous overhead line in areas of outstanding natural beauty, has been assessed using both the artificial salt-fog test and

by exposure to natural marine pollution at the Brighton Insulator Testing Station249. The findings from the salt-fog test areshown in Table 10-39. Although there was a large reduction - up to 50% of the new value - in the flashover voltage of theservice-aged insulator, the performance of such insulators was substantially restored by the application of a hydrophobiccoating; e.g. silicone oil, restored the withstand voltage to 70% of the original value. An even larger improvement wasobtained by using hydrocarbon grease but, because it tends to promote tracking on the insulator surface, it is notrecommended for practical use. The follow-up tests at Brighton showed the benefit of using a silicone oil of as high a

viscosity as possible. In a practical application, a flashover problem was alleviated to a large extent by coating the surfacewith a viscous silicone oil - applied yearly by linesmen with paintbrushes. In this case, the severity of the marine pollution -estimated ESDD of 0.6 mg/cm2 - is even greater than that at Brighton and where flashovers had occurred on such insulatorshaving a specific creepage of 25 mm/kV system.

8.3.3 RTV rubber coatings

The excellent experience with silicone rubber as an outdoor insulating material has prompted the development of thesecoatings. Service experience with RTV coatings has, in general, been very good. They were first applied in the early 1970’sand some utilities have had over 30 years experience with their use in a.c. systems and over 13 years with d.c. systems.

All known commercially available coatings consist of polydimethylsiloxane (PDMS) polymer, alumina trihydrate or alternatefiller for increased tracing and erosion resistance, a catalyst and a cross-linking agent. Several systems also contain a

condensation catalyst, an adhesion promoter, a reinforcing filler or a pigment. These systems are dispersed in either a naphthaor trichlorethylene solvent. The solvent acts as a carrier to transfer the RTV rubber to the insulator surface. It should benoted that the solvent is slightly poisonous. As the solvent evaporates, moisture in the coating triggers a vulcanising actionand the formation of a solid rubber layer. The speed of vulcanisation depends on the type of solvent, the cure-systemchemistry and the relative humidity 349.

Insulators need to be thoroughly cleaned prior to the application of a RTV coating. In some cases, the use of a high-pressurewater jet is sufficient. If cement like material is present, a dry abrasive cleaner - such as crushed corncobs or walnut shellsmixed with limestone - must be used. If the insulators have been previously greased, hand cleaning is necessary to remove the

bulk of the grease and a solvent must be used to remove any residual film347. The silicone coating is applied by brush or byspray. Live application is possible provided a combustible carrier, such as naphtha, is not present.

When a RTV coating looses some of its water repellency it may be washed and the hydrophobicity may be restored. Torecoat, cleaning using a dry abrasive is recommended. A new coat can be applied over an existing coat after some cleaning347

- provided the existing coat is well adhered to the ceramic insulator.

Similarly to that described in Section 8.3.2, surface discharges and corona will cause the coating to degrade 347 350 351 and maythen lead to flashover.

The frequency of re-coating, or washing, depends on the type of RTV and the severity of the degradation. Service experiencewith both a.c. and d.c. systems has shown that the useful life of a RTV coating can vary from less than one year up to ten years347 348 352 353. Experience has also shown that if a coating is applied in situations where the leakage distance is reduced and/or corona occurs, the coating looses its hydrophobic properties and flashover follows 347.

There is no established laboratory test that can predict the performance of a coating in service. Important initiatives areunderway 354, but to date, field-testing is the most reliable assessment procedure 347.

8.3.4 Summary As a summary, a comparison of silicone greases and RTV coatings is given in Table 8-2.



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Table 8-2: Comparison of silicone greases and RTV Coatings.

SILICONE GREASES RTV COATINGS

Effectiveness excellent in lifetime

Market price Low High

Lifetime Environment and quality dependent, a few months to several years

Unsuitable environmental conditions very high concentration of dust in air resulting in fast saturation

very high concentration of dust in air resultingin the loss of hydrophobicity in a short time,

Continuous raining or humid weather

Preparation before application Low demands High demands

Application equipment and technique Simple if applied by handSophisticated if applied by spray

Handling character Dirty and messy, if applied by handSolvent is slightly poisonous but needed if applied by spray

Monitoring needed

maintenance before replacement no washing and cleaning

removal difficult if not done in time, simplified if done timely and with right tools

very difficult if adhesion of old layer is stillgood

disposal varies form country to country

Reapplication direct after a rough cleaning directly, after cleaning, over the old layer if itis still in good adhesion.

8.4 Booster sheds

Booster sheds were invented in the UK for the prevention of the flashover of polluted insulators caused by heavy wetting 262.They are made from a radiation-crosslinked copolymer of silicone rubber and polyethylene; their form and installed positionon the insulator are shown in Figure 8-11. Such sheds have been successfully and widely applied on a.c. systems since 1975.

Figure 8-11: Form and installed position of booster sheds355

.

A variant of the Salt-Fog test was developed to quantify their efficacy, which was measured as a withstood pre-applied salinity(WPS) if flashover did not occur in three out of four identical tests. Some results for 400 kV substation insulators when fittedwith booster sheds are presented in Table 8-3 for various types of wetting that simulate conditions which are known to havecaused flashover in service355. These tests show that, with 7 booster sheds on a multiple cone post insulator and 10 on a barreltype insulator, an improvement of never less than a factor 2 was obtained in the tolerable pollution level. In one case, thisfactor was as much as 128.



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Table 8-3: Improvement in performance of a 400 kV substation insulator from fitting booster sheds355

.

Test Procedure Insulator Angle of tilt,Degrees

No. of Booster sheds

Performance,WPS, kg/m3

Factor of improvementover bare insulator.

20 second wash Multiple cone post

Plain shed cylindricalPlain shed taper Plain shed taper Antifog shed taper

0

00

1010

7

10101010

113

40568040

2.8

2.844

2.82 second wash Multiple cone post

Plain shed cylindricalPlain shed taper Plain shed taper Antifog shed taper

000

1010

710101010

4028402840

24

5.65.68

Side spray Multiple cone postPlain shed cylindrical

00

710

≥240160

≥1616

Impulse wash Multiple cone post

Plain shed cylindricalPlain shed taper Plain shed taper

0

0010

7

101010

40

4040≥56

4

5.65.6≥4

Rain Plain shed cylindricalPlain shed taper Plain shed taper Antifog shed taper

00

1010

10101010

≥240≥24016040

≥2≥4128

8

Investigations for their use under d.c. voltage have shown that by installing booster sheds on a HVDC wall bushing, itsdielectric strength under uneven rain or polluted conditions can be improved by up to 80 % 202. Laboratory tests have also

been performed on vertically installed d.c. station post insulators with booster sheds 110. By fitting 20 booster sheds on astacked station post insulator of 8.8 m overall length, the dielectric strength of this post insulator was increased by 30 % at a

pollution level of 0,02 mg/cm2, as compared to that of the insulator without such booster sheds.

8.5 Methods for increasing insulator reliability under ice and snow conditions

For reliable operation of insulation under ice or snow conditions, it is generally necessary to use insulators with a long dryarc-distance. As ice and snow flashovers are relatively infrequent, it is reasonable to restrict the use of special insulator designs to only selected parts of overhead lines. For example, fit them only on that part of a line that experiences regular icing or that runs close to cooling towers etc. On other parts of the line, more economical measures to improve their operational reliability should be considered.

8.5.1 Some measures to prevent flashovers dur ing i ce conditions

Some measures to prevent flashovers during ice conditions are:1) Prevention of icicle bridging;

• Utilising “V” or horizontal strings.

V-strings offer a substantial improvement over suspension (i.e. vertical) strings with regard to the ice-flashover strengthas water does not easily drip down the string to form an ice bridge 111. This effect is even more pronounced onhorizontal strings 356 357.

• Booster sheds.

The use of 3 booster sheds per metre of insulator can increase the flashover voltage under icing conditions by 20% for asystem voltage of 110 kV and 40% for 400 kV183 358. However, booster sheds tend to restrict natural washing.



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• Special insulator shapes or different types of disc insulators in the same string.

For post insulators, it is possible to use designs with alternate long and short sheds. The difference between the sheddiameters must be sufficient to prevent icicle bridging.

The same effect can be achieved on suspension insulator strings by building up the string with insulators of different

diameters; e.g. an arrangement of alternate normal and aerodynamic discs.

The use of vertical polymeric insulators may prove to be ineffective if the sheds are spaced closer together than is thecase for the discs of the equivalent ceramic insulator string. In this situation, the ice-flashover voltage may actually belower than that for a ceramic string of the same length. An improvement may be achieved by using the polymer insulator in “V” or horizontal configuration or by having an alternate long and short shed-profile with sufficient inter-shed spacing111.

• Semiconducting glaze insulators.

Semiconducting glazed porcelain insulators usually provide a resistive current of approximately 1 mA. This steadycurrent improves the voltage grading and warms the insulator surface slightly. Semiconducting glazed post insulatorshave shown the highest withstand voltage under icing conditions among the various insulators tested - including

conventional ceramic and polymeric insulators180

. However, there is no common agreement on the effectiveness of thismethod.

• Shielding insulators from water melted from ice.

By having shields places between the tower and insulator strings, the water released from the ice during melting will bedrained away from the insulators.

2) Increasing the dry arc-distance of insulators:

Please refer to Section 7.3

3) Lowering the operating voltage:

If provided for in the design of the system, the operating voltage may be lowered sufficiently to reduce the stress on theinsulator below the flashover value during the critical conditions - i.e. ice melting358.

4) Reducing the number of parallel insulators:

In areas with heavy ice accretion (for example close to cooling towers), the number of parallel vertical insulators(insulator posts, equipment etc) should be limited to reduce the probability of flashover 358.

5) Installing stress rings:

The performance of long insulator strings under ice conditions can be improved by using stress rings that even out thegrading of the electric field along the insulator, thereby preventing a high gradient at the live end 359.

8.5.2 Some measur es to prevent f lashovers dur ing snow condi tions

These measures are aimed at preventing the build-up of snow on horizontal or tension insulators.

1) Vertical arrangement of twin or triple tension insulator strings:

A vertical arrangement of strings in lieu of the normal near-horizontal strings presents a smaller collection area for snowaccumulation, thereby preventing the build-up of a large amount.

2) Insertion of a extension rod:

An extension rod of about 1 m length can be inserted between the tower and the insulator string to prevent enhancedsnow accumulation due to snow bridging from the tower cross-arm to the insulator.

3) Semiconducting glaze insulators:

Please refer to the discussion in Section 8.5.1 for more information.

4) Increase the spacing between adjacent insulator strings:

A bigger spacing between parallel tension strings prevents snow from bridging across one insulator string to another.



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9. THERMAL EFFECTS OF CONTAMINATION ON METAL OXIDE

ARRESTERS (MOA)

9.1 Introduction

The effect of contamination on metal oxide surge arresters with porcelain housings has already been the subject of numerousinvestigations, mostly made during the last ten years 360 361 362 363 364 365 366 367 368 369 370 371 372. Work on MOA with polymerichousings is still in progress and will not be referred to herein.

It is generally accepted that contamination of the arrester housing can have three effects:

1. Pollution flashover of the housing when the critical severity is reached.

2. Overheating of the varistor blocks if significant energy is dissipated internally, due to either capacitive coupling to thehousing or redistribution of current at intermediate flanges of multi-unit arresters.

3. Ageing, or even failure, due to internal partial discharges triggered by transient radial fields between the blocks and thearrester housing - particularly during dry band formation and sparkover.

Both the ANSI/IEEE Standard C62.11373 and the Amendment 1 to IEC standard 60099-4374 specifies a pollution test for metal

oxide arresters.The present review deals mostly with the aspect of the temperature rise in metal oxide arrester blocks due to external

pollution.

9.2 Operational Experience and Field Tests

General use of MOAs started on a large scale in the early eighties. Although the experience is generally satisfactory, severalarrester failures due to pollution were reported in Europe for voltage levels from 63 kV to 420 kV 363. It was noted thatarresters could fail at extremely low levels of pollution and without excessive energy stress from the network 363. Thesefailures were mostly attributed to internal discharges resulting from contamination on the arrester housing. Such internaldischarges manifest themselves by:

• Excessive loss of oxygen inside the arrester.

• Substantial degradation of varistors, as evidenced by decreasing reference voltage and increased resistive current and power loss.

• Salt formation on the varistor surface.

The problem appeared to be confined to the first generation of varistor blocks that had inadequate protective coating. The provision of an adequate coating, together with the reduction of radial stress and the filling with a passive medium, were therecommended measures for solving the problem. Nevertheless, several natural-pollution test programmes were launched andsupplemented by extensive laboratory tests, which went beyond the investigation of the problem of internal discharges.

At Brighton Insulator Testing Station, CERL conducted natural pollution tests - for more than five years - on a 1-unit 132 kVarrester, several 3-unit 275 kV arresters and three 3-unit 400 kV arresters. In these tests, as well as in other tests at Martiguesconducted by EdF - referred to below - both external current and internal current at the base unit were measured.Measurement of the mean temperature of the varistor blocks of the 3-unit 275 kV arrester at Brighton yielded 40°C, 107°C

and 124°C in the top, middle and bottom units respectively. In another arrester, these temperature were 57°C, 132°C, and129°C and, in a third arrester, 40°C, 40°C and 143°C. Within the latter arrester, the varistors were punctured near their centreand had been permanently damaged in the base unit 363.

These results showed that the location of maximum temperature rise was random and could occur in any unit : bottom,medium or top.

The field tests at Martigues367 included two 2-unit 245 kV arresters, one 2-unit and one 3-unit 420 kV arrester as well as two2-unit 300 kV arresters. The specific leakage path varied between 18 and 30 mm/kV system voltage and the housing diameter varied between 267 and 386 mm. Here again, the internal and external currents were measured at the base of the bottom unit.Temperature measurements were carried out with fibre optic sensors and thermostrips.

During a 10-month period at Martigues, the monthly-recorded maximum temperature rise - although significant - was muchmore modest than those reported above from Brighton. The following interesting conclusions could be reached:

• For most pollution events, a significant temperature rise occurs in 6 hours or less.



1999-09-01 135

• The external charge/h rose to 11 C/h for a 2-h period and 9 C/h for 6-h period, both were for the 2-unit 420 kV arrester.

• The temperature rise in the bottom unit correlates rather well with the internal charge flow, per 5-min period, throughthat unit.

• To a first approximation, the external charge was found to be proportional to the arrester diameter.

• The external charge, scaled to the housing diameter, appears to be representative of the discharge activity on the arrester.

• The magnitude of the internal current cannot be linked to a block-temperature rise.Similar field tests were reported from a 300 kV switchyard at Lista, Norway. No temperature rise measurement wasundertaken, since the arresters were connected to the network. It was stated that no correlation was found between internaland external charge activity. To avoid confusion, however, it must be underlined that this statement applies only to charges of

just one arrester unit.

9.3 Artificial Pollution Tests of Lightning Arresters

9.3.1 Test Techniques

Several techniques have already been used in the laboratory to simulate conditions leading to varistor block overheating.

These techniques include:• Salt-Fog

• Solid-Layer

• Slurry cycles

• Partial wetting

Salt-Fog tests on surge arresters have been carried out in accordance with IEC Publication 507 22, sometimes with prolongedduration - e.g. 2 h instead of 1 h.

The Solid-Layer test was also carried out with layer preparation and application made according to IEC Publication 507 22,although the steam-input rate was sometimes different from that of the standard - e.g. 0.8 1/h/m3

In the slurry test - which is not standardised - the contaminant was a slurry of water, betonite, non-ionic detergent and sodium

chloride prepared according to ANSI/IEEE C62.11 1987 362. The volume resistivity was between 400 and 500 Ω cm. Thecontaminant was applied to the complete arrester. Several test cycles were performed, each consisting of slurry application, adripping period (3 min.) followed by an energisation period. The latter being, usually, 15 minutes although it can apparently

be shorter without significant effect on the test results 372.

In the partial-wetting test conducted according to ANSI/IEEE C62.11-1987 373, the slurry was prepared as per that mentionedabove but was applied only to the lower half of the surge arrester. The maximum “voltage-off” time for the contaminantapplication was 10 minutes, with a “dripping off” time of less than 3 minutes. The energisation time was 15 minutes. The testcomprised two cycles, followed by a 30-minute interval at MCOV to demonstrate thermal stability. The arrester was deemedto have passed the test if it demonstrated thermal stability, no complete or unit flashovers occurred and no visual physicaldamage of internal parts could be found.

Some variants of the above tests have also been used. For example 365, arresters were contaminated according to the Solid-Layer method but an artificial dry zone was created, having a length equal to approximately 10% of the leakage path. Wettingunder voltage took place in air with relative humidity > 85%.

The control of the test parameters and the calibration needed to fit field conditions will be dealt with, following the section onlaboratory test results.

9.3.2 Laboratory Test Resul ts

In 1984, Lenk reported on pollution tests carried out on 2-unit and 3-unit arresters with MCOV of 140 and 210 kV 360. Thetest techniques comprised salt-fog, 5-h slurry and partial wetting. The 1-h Salt-Fog test was carried out at FGH according toIEC Publication 507. The slurry test comprised 20 cycles, each with a 15-minute voltage application. The partial-wetting testconsisted of applying a slurry made according to ANSI C62.1-1981, with a resistivity of 425-440 Ωm. It was applied to the

bottom unit housing. Usually, there were 3 cycles of tests for each arrester. The voltage was applied for 15 minutes.



1999-09-01 136

Only a moderate temperature rise was recorded for the Salt-Fog test (< 30°C) and the slurry test (< 35°C) 360. The partialwetting test was found to be the most severe, yielding a temperature rise of up to 79°C. This paper served as a basis for thestandardisation of the partial-wetting method referred to in C62.11-1987 373

The results of a Salt-Fog test on the arresters described in Section 9.2 were reported by Vitet et al. 366, for salinities in therange 1.2-80 kg/m3. Typical temperature rise curves - as a function of the test duration for different salinities - are shown in

Figure 9-1

366

. The variation of the varistor temperature of the bottom unit as well as that of the internal current and of theenergy are shown in Figure 9-2 as functions of the salt-fog duration. Figure 9-3 shows the flow of external charge as afunction of the test duration 366.

Figure 9-1: Typical temperature during a Salt-Fog tests of 1.2 to 80 g/l salinity366

.

Figure 9-2: Bottom unit temperature, internal energy and internal current peaks in a Salt-Fog test366

.

From the aforementioned test, the following observations can be made 366:

• No correlation was found between the maximum varistor-temperature and fog salinity.

• A somewhat contradictory finding is that the external charge per hour correlates well with fog salinity and, moreover,

increases almost linearly with the test-duration.• Current peaks cannot be used to determine the thermal stress on the arrester blocks.



1999-09-01 137

• Breaks in the duration of the fog spray have no effect because discharge activity ceases during such breaks.

Solid-Layer tests conducted by Vitet et al. 366 - with ESDD in the range 0.2∼0.7 mg/cm2 - yielded a negligible temperature risein the bottom unit and a maximum temperature rise in the top unit of 26°C; which are much less than the corresponding valueswith the Salt-Fog test. Good correlation was reported between the external charge on the bottom-unit and the temperature riseof the top-unit varistor.

Figure 9-3: External charge build-up during a Salt-Fog test.366

Figure 9-4 shows the varistor temperature variations with test-time of the slurry test 366. In this case, the test comprised 6cycles and the maximum temperature rise occurred with equal probability on the top or bottom unit of a 2-unit arrester. It wasfound that the temperature rise in the slurry test was practically independent of the resistivity of the slurry or of the specificleakage path. The temperature rise in the bottom unit correlated well with its internal charge. Figure 9-5 shows the

temperature variation of the top varistor and the external charge measured on the bottom unit during a partial-wetting test366

. Note, that here, the external charge corresponds also to the internal charge of the top unit and, therefore, correlates quite wellwith the top-varistor temperature rise.

Figure 9-4: Temperature and charge flow during a slurry test with six test cycles366

.



1999-09-01 138

A comparison was made 366 between the flow of external charge and the test-time for the above four test techniques. Theslurry test, after six cycles, resulted in an external charge that was slightly larger than the corresponding charge of a 2-h Salt-Fog test. The charge associated with the Solid-Layer test, or that of a 2-cycle partial-wetting test, was significantly lower thanthat of the slurry test.

Work by ENEL-CESI on pollution testing of metal oxide surge arresters has been reported in an initial paper 361 and in moredetail in two subsequent publications 370 371.

It was concluded 361 that, from the point of view of thermal effects, the Salt-Fog test was more severe than the standard Solid-Layer test. The standard Salt-Fog withstand test did not yield a significant varistor-temperature rise. On the other hand, asignificant temperature rise was obtained after repeated cycles of a salt-fog at salinities much below the withstand level. Witha block temperature up to 130°C, a significant change in the arrester parameter (degradation) can result, particularlymanifested by increased resistive current and additional power loss.

Figure 9-5: Temperature and external charge flow during a partial wetting test 366

.

It was also found that, contrary to the finding 366 discussed above, drying periods under voltage can significantly acceleratethermal stresses in the arrester blocks 361.

Temporary overvoltages led to a significant temperature rise (116°C instead of 38°C in one case).

Finally, it was reported that a better correlation exists between temperature rise and internal current than that with externalcurrent.

Garasim et al 371 have conducted pollution tests on 2-unit arresters that included measurement of internal and external currentsin both top and bottom units. This permits a more directly relevant correlation to be made between the test parameters and thethermal stresses. The tests included the following techniques, as designated in the paper:

a) Partial-wetting test, with 2-cycle application according to ANSI/IEEE C62.11 1987 373.

b) Slurry test, with 6-cycle application.

c) Solid-Layer test according to IEC Publication 507, but with only one arrester unit contaminated with ESDD-0.015mg/cm2.

d) Solid-Layer test as above, but applied to the complete arrester.

e) Salt-Fog test according to IEC 507, but with a 2-h duration and salinities in the range 2.5-40.9 kg/m3.

This paper leads to the following conclusions:

• The test severity is determined by non-uniformity of the pollution rather than by the contamination level.

• Pollution methods with forced non-uniformity have better repeatability. This is particularly so for technique (a) “partialwetting”.

• Block heating is closely related (almost proportionally) to inner charge flow but only loosely correlated to externalcharge, except - of course - when the two are identical.



1999-09-01 139

• Inner charge in the partial-wetting technique is a function of insulator geometry and wetting conditions (quantity of water to be evaporated).

• The proportionality constant between temperature rise and inner charge is generally in the range 7-10°C/Coulomb.

From pollution tests conducted on surge arresters in the UK, reported by Sparrow 364, it was found that:

• The rate of wetting has an important effect on the rate of external charge flow.

• An increase of the applied voltage (thereby decreasing the specific leakage path) led to a significant decrease in the rateof external charge flow.

• Salinity had little effect on the charge flow per hour.

• ESDD measurement does not appear to be a good basis for site severity as far as arrester heating is concerned.

• The aim of an artificial pollution test should be to obtain a value of external charge per hour that is in accordance withthat at natural sites.

Some of the above points are confirmations of previous findings 362.

Verma et al . 369 reported on field experience in Germany and Salt-Fog tests on metal oxide arresters at FGH. The major concern appears to be internal partial discharges caused by external pollution - with their associated varistor degradation and,even, failure as referred to above. To alleviate that concern, German utilities require a 2000-h Salt-Fog test at phase-to-ground voltage with a salinity of 1 kg/m3.

Salt-Fog tests were also reported in Verma’s work 369. It was concluded that, if the ratio of the test voltage Ut (phase-to-ground) to the arrester reference voltage Ur is less than 0.54, pollution will have no significant thermal effect on the varistors.It is noted, however, that such a low ratio may not be practical, owing to the protective-level requirements. This work alsoconfirmed that a high temperature rise can be obtained at salinities much below the withstand level. It also showed that higher temperatures are generally encountered with multi-unit rather than with single-unit arresters and that higher temperaturesoccur on the top rather than on the bottom unit.

Feser et al . 365 found that for both single- and multi-unit arresters, an artificial single dry band - representing approximately10% of the leakage path, particularly in the vicinity of the flange - can lead to a significant temperature rise of the varistor

blocks. A solid-layer contaminant was applied during those tests and wetting took place in air with a relative humidity > 85%.In single units, the temperature rise was attributed to capacitive coupling between the varistor column and the housing andtemperatures as high as 85°C were recorded. In a 2-unit arrester, temperatures as high as 105°C were measured.

In a report on Solid-Layer tests (18-26 µS) 368 of 110 kV and 220 kV ZnO arresters, a temperature rise of up to 46°C occurred.It was found that this temperature rise did not depend on the leakage path or the form factor of the arrester housing but, rather,on the specific capacitance along the resistor stack. The temperature rise proved to be a statistical variable, which can berepresented by an exponential distribution. For clean and dry conditions, the calculations of overheating of the arrester elements, as a function of input power, were provided.

9.4 Standardisation of a Laboratory Test

The purpose of this discussion is to emphasise the factors that could influence the selection of a standard laboratory test. It isnot intended to constitute an endorsement for one technique or another.

It was established in the research referred to above, that a high temperature rise of varistor blocks occurs at a pollution

severity far below the withstand level. It also follows that conventional severity measurements, such as ESDD or leakagecurrent peak-values, can not be used to relate field and laboratory conditions.

A quantity that has served that purpose 366 367 372 is the amount of external charge on a reference insulator, preferably alongrod or an arrester housing, that flows during a 2-h or 6-h duration.

In one investigation 372, the requirements made of a laboratory test method were perceived as follows:

1. It should establish an external charge activity of sufficient intensity and duration - e.g. up to 6 C/h on a small-diameter housing using a current threshold of 2 mA peak.

2. An appreciable temperature rise should appear in any unit of a multi-unit arrester.

3. External charge accumulation should be essentially independent of the specific leakage path.

These investigations 372 concluded that only the Salt-Fog and the slurry methods fulfil the above requirements.



1999-09-01 140

Concerning the first requirement, the effect of selecting a value of threshold current based on the charge flow per hour should be clarified. It would be even better to eliminate that quantity altogether. Instead, the real charge rate should be determined by excluding the capacitive component from the total current.

As for the second point, only the slurry method fulfils that requirement; because in the Salt-Fog test, the top unit is the hottestin most cases 362 367. The third point is not always satisfied by the Salt-Fog test since, as reported by Sparrow 362, the chargeflow-rate increases with the increase of the specific leakage path - particularly at 10 kg/m3 salinity.

Furthermore, Lenk 360 found that - from the thermal point of view - the partial-wetting method was the most severe. However,this condition is, admittedly, infrequent. In practice, some examples are malfunction of the transformer deluge system (fire

protection) and stratified fog. Bargigia et al. 371 have found that this method provides the best repeatability of all the testtechniques investigated.

A comparison between the major laboratory techniques is shown in Table 9-1. Also included are the controlling parameters toachieve the required charge rate, the thermal effect of the test, the representativity (i.e. simulation of field conditions), therepeatability and existing standardisation experience for each method. It should be noted that while the salt-fog technique isknown to have excellent repeatability for insulator pollution tests, Vitet et al 366 have found large variations in the maximumvaristor-temperature under tests with identical salinity. However, these authors provided no satisfactory explanation for sucha large dispersion of the test results. The final column includes some possible modifications to make the method moreversatile, if deemed necessary. As already mentioned, the effect of the drying periods in the Salt-Fog test is somewhat

controversial. Furthermore, the repetition of the partial wetting test - with the wet contaminant applied to the upper half whilstkeeping the lower half clean and dry - would cause a temperature rise in different units. This practise would remove one of the major objections against this test.

Table 9-1: Comparison of pollution test techniques to model pollution stress for varistor block heating.

TEST

TECHNIQUE

CONTAMINANT

APPLICATION

CONTROLLING

PARAMETERS

THERMAL

EFFECTS

R EPRESENT-ATIVE

R EPEAT-ABILITY

STANDARDISATION

EXPERIENCE

POSSIBLE

MODIFICATIONS

Salt-Fog Complete arrester -Nozzle pressure-Liquid flow rate-Test duration

Substantial Good Good IEC Std 507(Polluted insulatorsonly)

Inclusion of drying periods

Solid-Layer Complete arrester -Steam flow rate-Test duration

Mild Good Good IEC Std 507(Polluted insulators

only)Slurry Complete arrester -Cycle duration-Cycles per test- Slurry resistivity

Substantial Fair Good JEC-217

Partial Wetting Lower half -Cycle duration-Cycles per test- Slurry resistivity

Substantial Fair Very Good ANSI/IEEEC62.11-1987

Test repetition with pollution applied tothe upper half

In April 1998, the IEC issued Amendment 1 to IEC standard 60099-4: "Artificial pollution test with respect to the thermalstress on porcelain-housed, multi-unit metal-oxide surge arresters"374. A brief summary of the salient features of thatdocument is given below.

A basic feature of this document is contained in a table that correlates the flow of external charge - qz per hour per metre of arrester housing diameter - to the minimum creepage distance, for the range 16-31 mm/kV - which correspond to the different

pollution zones specified in IEC guide 815. For a 2h-event, qz varies in the range 0.5 to 55 C/h.m, while for a 6h-event itvaries in the range 0.24 to 36 C/h.m. The implicit assumptions here are that the external charge is determined by the specificleakage path for all climatic and pollution conditions - e.g. industrial, marine, desert etc - and that the external charge flow is

proportional to the arrester housing diameter. An estimate of the upper limit of the arrester block temperature rise is firstmade, assuming that all the expected charge will flow internally. If this estimated temperature rise ∆Tzmax is below 40 oC, no

pollution test is required. If ∆Tzmax is equal to or greater than 40 oC, there are two options: either carry out a pollution test or omit that test and carry out the duty-cycle test by preheating the arrester to 20 oC + ∆Tzmax. If carried out, the purpose of this

pollution test will be to determine the ratio of the internal- to the external-charge flow for the different arrester units. Thetemperature of the internal parts may be measured instead of the internal charge. Two options for the pollution-test techniqueare permitted in IEC 60099-4, Amendment 1: the slurry method and the Salt-Fog method.

In effect, the slurry test that is described comprises a wet contaminant - having a volume resistivity in the range 400 to500 Ωcm - that is applied uniformly to the whole arrester housing surface and with no wetting subsequent to the application of

the voltage.



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The Salt-Fog test that is prescribed is performed at two steps below the withstand salinity of the arrester housing. The testcycle comprises 15 minutes of fog application under voltage followed by 15 minutes of energisation without fog (drying

period). As mentioned previously, the drying period under voltage can be an important factor 361.

With the so determined division of the charge flow between the external and the internal paths, and by using the externalcharge severity table referred to above, a new estimate ∆Tz of the block temperature is calculated. If ∆Tz < 40 oC, the arrester

is preheated to 60

o

C to carry out the duty cycle test. Otherwise, preheating will be to 20

o

C + ∆Tz.



1999-09-01 142

10. ADITIONAL INFORMATION AND RESULTS

10.1 Insulator profiles and dimensions

Table 10-1: Details for line insulators; Refer Table 10-24.

TYPE PROFILE PIN CAVITY

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH

(mm)

FORM

FACTOR

R EF.

Cap & Pin Designs

Bullers 54260 34 297 140 426 0.78 376

CEGB 374 kN 381 200 565 124

ENEL 120 kN 280 145 410 124

Doulton 6672 47 380 190 611 0.9 377

IEEE 39 254 146 305 0.62 378

NGK 820 kN 460 290 800 379

NGK 680 kN 440 280 750 379

CERL Reference A 29 394 210 592 0.86 378

Longrod designs CORE

DIAMETER

SHED

SPACING

LEAKAGE

PATH /

SHED

L7524SN/13601.15m length, 24sheds

75 200 46 164 197



1999-09-01 143

Table 10-2: Details for substation insulators (Tapered Barrel/Post); Refer Table 10-24.

TYPE PROFILE CORE DIAMETER

(AVERAGE)

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

LENGTH

(m)

LEAKAGE

PATH

(m)

FORM

FACTOR

R EF

.

min max

Plain Shed 790 810 570 3.5 10.29 0.15 376

3-skirt a.f. shedabcb support

496 890 650 3.2 12.90 0.28 376

3-skirt a.f. shed

sealing end

405 800 490 3.07 10.82 0.37 376

2-skirt a.f. shedabcb support

496 870 630 3.50 11.94 0.30 376

2-skirt a.f. shedoil filled c.t.i.

716 1160 780 2.94 10.16 0.23 376

Notes: a.f. : antifogc.t.i. : current transformer insulator abcb : air blast circuit breaker



1999-09-01 144

Table 10-3: Details for substation insulators (Parallel Barrel / Post); Refer Table 10-24.


(AVERAGE)

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

LENGTH

(m)

LEAKAGE

PATH

(m)

FORM

FACTOR

R EF.

Easy Greaseshed abcbsupport

628 920 3.39 13.60 0.21 376

2-unit plain shed 314 450 3.65 8.69 0.17 376

2-skirt a.f. shedSF6-filled c.t.i.

868 1090 3.73 11.86 0.17 376

2-skirt a.f. shedabcb support 1

498 740 3.46 11.02 0.29 376

2-skirt a.f. shedabcb support 2

623 877 3.40 11.72 0.24 376

Italian 2-unit

plain shed

740 920 3.37 7.95 0.17 376

National GridV1 interrupter head

260 384 1.25 3.52 0.16 129



1999-09-01 145

Table 10-3: Continued.


(AVERAGE)

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

LENGTH

(m)

LEAKAGE

PATH

(m)

FORM

FACTOR

R EF.

CERL

Reference Post

230 484 3.80 12.67 0.32 375

Notes: a.f. : antifogc.t.i. : current transformer insulator abcb : air blast circuit breaker

Table 10-4: Details for substation insulators (Parallel Barrel / Post alternating long and short shed); Refer Table 10-24.


(AVERAGE)

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

LENGTH

(m)

LEAKAGE

PATH

(m)

FORM

FACTOR

R EF.

Long ShortCEGB 70/60 profile 260 400 380 1.85 7.42 9.34 375

CEGB 70/50 profile 260 400 360 1.85 6.70 0.31 375

New CircuitBreaker P1

230 410 350 3.80 14.21 0.40 375

New CircuitBreaker P2

230 356 314 3.80 14.01 0.34 375

National Grid P1 234 376 336 1.30 4.30 0.34 129



1999-09-01 146

Table 10-5: Details for post and cap and pin insulators; Refer Table 10-25. From reference 125.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

FORM

FACTOR

III Multiple cone

Post

317 527 110 352 to 385 0.28

IV Standard discCap and pin

14 254 140 298 0.8

V a.f. Cap and pin 24 381 186 587 1.01

VI Long creepagea.f. Cap and pin

27 415 170 636 1.08

Notes: a.f. : antifog

Table 10-6: Details for cap and pin and pedestal post insulators; Refer Table 10-26. From reference 197.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH (mm)

6a, Cap and pin - 263 - -

6b, Cap and pin - 319 - -

7, Cap and pin - 250 - -

6c, Cap and pin - 250 - -

2a, Pedestal post - 438 - -



1999-09-01 147

Table 10-7: Details for barrel insulators; Refer Table 10-26. From reference 197.

TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH (mm)

1a 113 269 53

1b 69 163 50

2b 100 181 56

2c 113 213 50

2d 125 225 56

2e 113 244 38

4a 138 250 56

4b 75 200 50

5a 75 156 31

5b 88 169 31

8a 125 263 69

9c 300 400 62

8b(Alternating longand short shed)

119 256 31 / 73

9b(Alternating longand short shed)

250 375 25/38



1999-09-01 148

Table 10-8: Details for cap and pin insulators; Refer Table 10-27 and Table 10-28. From reference 380.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

I 4-skirt, a.f. 50 321 165 508

II 5-skirt 40 318 165 508

III Bell shape 42 275 146 356

IV 1 very long skirt - 356 171 566

V 46 381 187 478

VI 5-skirt 49 321 175 502

VII 4-skirt, 2 long - 267 159 483

VIII 5-skirt 49 321 165 508

IX 43 282 149 457

X 6-skirt 40 267 146 406

XI 4-skirt, 1 long - 356 171 566

XIV Aerodynamic profile

39 425 159 356



1999-09-01 149

Table 10-9: Details for post and longrod insulators, Refer Table 10-27. From reference 380.

TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH (mm)

XII Longrod 97 289 64 210

XXI Parallel Post 165 283 84 180

XXII Parallel Post 146 260 57 188

XXIII Parallel Post 125 233 54 149

XXV Parallel Post 165 254 50 161

XXVI 3-shedPedestal Post

- 432 - 864

XXX Multiple conePost

165 337 88 252



1999-09-01 150

Table 10-10: Details for cap and pin insulators; Refer Table 10-29. From reference 315.

TYPE* PROFILE PIN CAVITY

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

V1 Flat profile 37.5 380 130 340

V2 Standard 37.5 280 146 386

V3 Long leakage 44 320 170 534

V4 Very longleakage

47.5 355 171 571

P1 Long leakage 45 292 149 470

P2 Standard 39 254 146 305

* Insulator designation as used in reference



1999-09-01 151

Table 10-11: Details for cap and pin insulators; Refer Table 10-30. From reference 380.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

P3, 4-skirt 321 171 546

A, 4-skirt 254 146 394

B, 4-skirt 320 170 530

D, 4-skirt 400 159 603

E, 5-skirt 380 195 690

G, 4-skirt 254 159 432

H, 5-skirt 330 170 546

J, 5-skirt 267 146 457

K, 4-skirt 321 165 508

M, 5-skirt 290 160 470

N, 5-skirt 260 160 621



1999-09-01 152

Table 10-12: Details for post and longrod insulators; Refer Table 10-30. From reference 381.

INSULATOR

NO *

INSULATOR

TYPE ***

CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)**

0 Long rod 73 200 41 120

1 I, Plain shed, post 233 360 37 1491A I, Plain shed, post 233 360 48 1592 I, Plain shed, post 233 360 66 1743 I, Plain shed, post 235 400 63 2244 I, Plain shed, post 230 420 56 2375 I, Plain shed, post 250 420 84 2536 II, 3 skirt, post 237 370 50 2277 II, 3 skirt, post 237 370 62 2428 II, 3 skirt, post 235 400 62 2739 II, 3 skirt, post 240 430 62 32310 II, 3 skirt, post 240 430 95 35511 III, ALS shed, post 236 420 19/64 215/8712 III, ALS shed, post 236 420 19/64 214/62

Notes: * : Insulator designation as used in reference** : Leakage path per shed; i.e. for quoted shed spacing*** : Profiles of Insulator type I, II, II are given below

I, Plain shed, post II, 3 skirt, post III, ALS shed, post

Table 10-13: Details for cap and pin insulators; Refer Table 10-31 and Table 10-32. From reference 199.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH

(mm)

A, standard - 254 146 280

B1, FogB2, Fog

--

254320

146170

430550

C1, d.c.C2, d.c.C3, d.c., extra creepageC4, d.c., very long creepage

----

280320320400

146165170195

445512545635

D, Longrod - 180 875 2085



1999-09-01 153

Table 10-14: Details for post insulators; Refer Table 10-32. From reference 199.

TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)*

Deep-rib profile 65 236

Under-rib profile 50 157

Table 10-15: Details for polymeric longrod insulators; Refer Table 10-34, Table 10-35 and Table 10-40. From references126 and 127.

TYPE * PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETE

R (mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)***

V EPDM 24 110 66 158

VI EPDM ** 31 134/102 36/90 216

VII Silicone rubber 35 134 49 118

VIII EPR 38 171 61 146

Notes: * : Description as used in reference** : Alternate long and short shed design*** : For quoted shed spacing; i.e. between two large sheds for design VI



1999-09-01 154

Table 10-16: Details for polymeric longrod insulators; Refer Table 10-36. From reference 381.

TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)

R 34 92 32 79

S 42 164 55 139

T 43 127 66 155

V 44 123 60 222

W 40 178 65 156


TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)

XIII 61 222 100 200

XXVII 25 130 40 110


INSULATOR

NO * AND

PROFILE

INSULATOR

TYPE

CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

LEAKAGE

PATH

(mm)**

A

Low slope 35 160 35 134

B

Low slope with rib 35 160 45 158

EMean slope 35 160 55 167

F

Mean slope with rib 35 160 45 169

I

High slope 35 160 45 172

J

High slope with rib 35 160 55 200

Notes: * : Description as used in reference

** : For quoted shed spacing



1999-09-01 155

Table 10-19: Details for cap and pin insulators; Refer Figure 10-1. From reference 111.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

A-11 - 254 146 305

A-12 - 254 130 305

A2 - 290 178 395

B2 - 280 165 370

B3 - 320 198 425

C2 - 280 172 370

C4 - 400 244 535

D5 - 380 220 495

Table 10-20: Details for cap and pin insulators; Refer Figure 10-2. From reference 382.


DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

A - 254 146 280

B - 280 170 370

C - 320 195 425



1999-09-01 156

Table 10-21: Details for cap and pin insulators; Refer Figure 10-3. From references 22, 124 and 143.

TYPE PICTURE PIN CAVITY

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

AXIAL

SPACING

(mm)

LEAKAGE

PATH (mm)

A (A’) - 254 146 305

B (B’) - 254 146 390

C - 254 130 270

D - 279 140 433

1 - 381 200 560

2 - 355 171 530

3 - 280 145 300

4 - 280 145 400

1a - 254 146 290

1b - 254 146 290

2a - 254 146 390

2b - 254 146 390



1999-09-01 157

Table 10-22: Details for cylindrical Insulators (Parallel Barrel); Refer Figure 3-11. From reference 85.

TYPE PROFILE CORE

DIAMETER

(mm)

SHED

PROJECTION

(mm)

SHED

SPACING

(mm)

LEAKAGE PATH PER

SHED (mm)

B Various 70 70 238

C Various 65 65 250

H Various 70 70 190

I Various 70 70 203

J Various 120 92 407

Table 10-23: Details for interrupter head insulators (Parallel Barrel); Refer Figure 3-17. From reference 129.

TYPE PROFILE CORE

DIAMETER

(mm)

OVERALL

DIAMETER

(mm)

SHED

SPACING

(mm)

AXIAL

LENGTH

(mm)

LEAKAGE

PATH

(mm)

V1 260 384 52 1250 3520

H1 340 426 36 1500 4800

H2 380 616 70 1100 3600

H3 380 500 34 1100 4000

H4 380 584 70 1500 4650



1999-09-01 158

10.2 Ranking of insulators

10.2.1 Ceramic Insulators

Table 10-24: Critical a.c. flashover strength of ceramic insulators vertically mounted; performance in artificial pollution test of Very Heavy severity

*.

Ranking No

Insulator Type ** Ref. Axial StresskV/m ***

Surface StresskV/m ****

1 National Grid P1, Post 129 91 282 CEGB 70/60 profile, Post 375 84 213 CEGB 374 kN, Cap & pin 124 76 274 New Circuit Breaker P2, Parallel Post 375 76 215 3-skirt a.f. shed abcb support, Tapered barrel 376 74 186 New Circuit Breaker P1, Parallel Post 375 73 207 L75/24SN/1360, Longrod 197 73 218 ‘Easygrease’ shed abcb support, Parallel barrel 376 71 179 CEGB 70/50 profile, Post 375 70 19

10 ENEL 120 kN, Cap & pin 124 69 2411 Allied 54656, Cap & Pin 376 66 2212 National Grid V1 Interrupter Head, Parallel Barrel 129 68 2413 3-skirt a.f. shed sealing end, Tapered barrel 376 66 1914 2-skirt a.f. shed SF6-filled c.t.i., Parallel Barrel 376 60 1915 2-skirt a.f. shed abcb support 1, Parallel Barrel 376 60 1916 2-skirt a.f. shed Oil c.t.i., Tapered Barrel 376 59 1717 Doulton 6672, Cap & pin 377 58 1818 2-skirt a.f. shed abcb support 2, Parallel Barrel 376 58 1719 2-skirt a.f. shed abcb support, Tapered Barrel 376 58 1720 Italian 2-unit plain shed, Parallel Barrel 376 53 23

21 Plain shed, Tapered Barrel 376 53 1822 IEEE, Cap & Pin 378 51 2423 NGK 820 kN, Cap & Pin 379 47 1724 2-unit plain shed, Parallel Barrel 376 47 2025 NGK 680 kN, Cap & Pin 379 46 1726 CERL Reference, Parallel Post 375 46 1427 CERL Reference A, Cap & Pin 378 44 16

Notes:

* All tests made with a salt-fog of 80 kg/m3 except those reported in reference 197 which were at anESDD of 0.6 mg/cm2

** Name by which the insulator is specified in the relevant reference*** Axial stress is voltage divided by axial distance between metal fittings

**** Surface stress is voltage divided by leakage path lengtha.f. antifogc.t.i. current transformer insulator abcb air blast circuit breaker



1999-09-01 159

Table 10-25: a.c. Ceramic insulators, vertically mounted; flashover performance under marine pollution at BITS*.

Ranking No

Insulator Type ** FOM ***

(Average)LPR ****

1 VI Long creepage a.f. Cap & Pin 1.19 0.892 V a.f. Cap & pin 1.08 0.79

3 III Multiple cone Post 1.07 0.934 IV Standard disc Cap & Pin 1.00 1.325 I a.f. Cap & Pin (CERL Reference A) 1.00 1.00

Notes:

* Data from reference 125** a.f. is antifog*** Measure of flashover performance, from the viewpoint of axial length when compared to

that of a vertical string of reference insulators (i.e. CERL Reference A in Table 10-24); anaverage of all values for same insulator type

**** LPR is leakage path ratio, determined as leakage path of CERL Ref. A insulator divided by that of the test insulator, for the same pollution flashover performance

Table 10-26: a.c. Ceramic insulators, vertically mounted; flashover performance under natural pollution in Sweden *.

Ranking No

Insulator Type Specific Leakage(mm/kV, System) **

Volt/LP(kV/m) ***

1 6a, Cap & Pin 10.4 562 8a, Barrel 10.8 543 1a, Barrel 11.7 504 6b, Cap and Pin 11.7 505 2b, Barrel 11.8 496 7, Cap & Pin 11.8 497 6c, Cap & Pin 11.8 498 1b, Barrel 12.2 489 2e, Barrel 13.1 44

10 9c, Barrel 13.4 4311 8b1, Barrel 13.9 4212 2c, Barrel 14.5 4013 2a, Pedestal post 14.6 4014 4a, Barrel 15.2 3815 4b, Barrel 15.5 3716 5a, Barrel 15.6 3717 2d, Barrel 15.9 3718 8b2, Barrel 15.9 3719 9b, Barrel 16.7 3520 5b, Barrel 19 31

Notes:

* Data from reference 197

** Specific leakage for same number of flashovers at the same location over same time period*** Actual stress along the insulator surface



1999-09-01 160

Table 10-27: Critical d.c. flashover strength of ceramic insulators, vertically mounted; negative polarity; performance in

artificial pollution, using spray fog and Portland cement*.

Ranking Insulator Axial Stress Surface Stress No No ** Type kV/m kV/m

1 XI 4-skirt, 1 long, Cap & Pin 178 54

2 VIII 5-skirt, Cap & Pin 158 513 I 4-skirt, a.f., Cap & Pin 149 484 IV 1 very long skirt, Cap & Pin 149 455 II 5-skirt, Cap & Pin 146 476 XXX Multiple cone, Post 143 577 XXVI 3-shed, Pedestal Post 141 608 XIV Aerodynamic Profile Cap & Pin 139 639 VI 5-skirt, Cap & Pin 139 48

10 X 6-skirt, Cap & Pin 132 4711 VII 4-skirt, (2 long), Cap & Pin 129 4312 XII Longrod 125 5713 XXII Parallel Post 123 44

14 XXIII Parallel Post 113 4515 XXI Parallel Post 111 4116 III Bell shape, Cap & Pin 110 4617 XXV Parallel Post 103 43

Notes:

* Data from reference 380** Numbers as used in reference 380a.f. antifog profile


artificial pollution, using spray fog and kaolin plus salt at ESDD = 0.2 mg/cm2 *.

Ranking Insulator Axial Stress Surface Stress

No No

**

Type kV/m kV/m1 I 4-skirt, a.f., Cap & Pin 236 762 VI 5-skirt, Cap & Pin 209 723 IV 1 very long skirt, Cap & Pin 185 564 VIII 5-skirt, Cap & Pin 171 55

Notes:

* Data from reference 380** Numbers as used in reference 380a.f. Antifog profile

Table 10-29: Critical d.c. flashover stress for ceramic insulators, vertically mounted; positive polarity; performance in

artificial pollution using (a) Salt-Fog test and (b) Clean-Fog test*.

Insulator**

Salt-Fog test Clean-Fog test No *** Type of Cap & Pin unit

Ranking No

AxialStresskV/m

SurfaceStresskV/m

Ranking No

AxialStresskV/m

SurfaceStresskV/m

V3 Long leakage 1 102 32 4 99 32V4 Very long leakage 2 80 24 3 102 31V2 Standard 3 67 26 1 113 45V1 Flat profile 4 63 24 2 103 40P1 Long leakage 5 54 17 5 88 28P2 Standard 6 44 21 6 71 34

Notes:

* Data from reference 315** Insulator number and description are those used in reference 315

*** ‘V’ in number designates glass insulator; ‘P’ in the number designates porcelain insulator



1999-09-01 161


artificial pollution, using clean-fog and kaolin plus salt at ESDD = 0.05 mg/cm2 *.


1 K 4-skirt, Cap & Pin 81 26

2 E 5-skirt, Cap & Pin 80 233 D 4-skirt, Cap & Pin 80 214 B 4-skirt, Cap & Pin 77 255 J 5-skirt, Cap & Pin 77 256 0 Longrod 74 267 P3 4-skirt, Cap & Pin 68 218 N 5-skirt, Cap & Pin 67 179 H 5-skirt, Cap & Pin 63 20

10 A 4-skirt, Cap & Pin 62 2311 M 5-skirt, Cap & Pin 61 2112 10 II, 3-skirt, Post 59 2013 12 III, ALS shed, Post 59 18

14 9 II, 3-Skirt, Post 59 1515 8 II, 3-skirt, Post 57 1716 11 III, ALS shed, Post 55 1517 7 II, 3-skirt, Post 54 1918 G 4-skirt, Cap & Pin 53 2019 3 I, plain shed, Post 53 2020 1A I, plain shed, Post 52 2121 6 II, 3-skirt, Post 51 1622 2 I, plain shed, Post 50 2423 5 I, plain shed, Post 50 2024 1 I, plain shed, Post 50 1725 4 I, plain shed, Post 47 15

Notes:

* Data from reference 381** Insulator designation as used in reference 380*** ALS is alternate long and short shedP3 is a reference insulator, that was tested simultaneously with each other type of line

insulator so as to provide a correction factor

Table 10-31: d.c. Flashover performance of ceramic line insulators; vertically mounted; natural saline pollution*.

Ranking Insulator *** Axial length Leakage path No ** No Type ratio **** ratio *****

1 C2 d.c. Cap & Pin 0.74 1.422 B1 Fog Cap & Pin 0.77 1.183 B2 Fog Cap & Pin, Extra creepage 0.79 1.49

4 A Standard Cap & Pin 1.0 1.05 D Longrod 1.0 1.24

Notes:

* Data determined from reference 199** Based on performance at 3 test stations, for both positive and negative polarity*** Insulator number and description are those used in reference 199**** Mean value of axial length of test insulator divided by axial length of reference insulator

(i.e. type A, standard Cap & Pin) for same flashover performance***** Leakage path of test insulator divided by that of reference insulator for same flashover

performance



1999-09-01 162

Table 10-32: Critical d.c. flashover stress for ceramic insulators, vertically mounted; performance in artificial pollution,

using Clean-Fog test with Tonoko plus NaCl at ESDD = 0.05 mg/cm2 *.


1 C4 d.c. Cap & Pin very long creepage 92 28

2 C3 d.c. Cap & Pin extra creepage 90 283 - Post, Deep-rib profile 90 254 C1 d.c. Cap & Pin 86 295 - Post, Under-rib profile 68 226 A Cap and Pin, Standard Profile 65 34

Notes:

* Data from reference 199** Insulator number and description as used in reference 315

Table 10-33: d.c. Withstand stress for a porcelain housing, vertically mounted, as a function of its average diameter;

performance in artificial pollution using Clean-Fog test with ESDD of 0.12 mg/cm2 *

.

Average diameter, mm 200 270 400 560 680

Axial Stress, kV/m 67 54 48 42 36Surface Stress, kV/m ** 23 19 17 15 13

Notes:

* Data from reference 199** Average values for a normal profile and an under-rib profile

10.2.2 Polymer ic insulators

Table 10-34: a.c. Polymeric insulators, vertically mounted; flashover performance under marine pollution at BITS*.

Ranking No

Insulator Type ** FOM ***

(Average)LPR ****

1 VII Silicone rubber >1.53 <2.52 V EPDM 1.21 1.253 VIII EPR 1.17 1.164 VI EPDM 1.12 2.275 Epoxy resin 0.9 0.56

Notes:

* Data determined from references 126 and 127** Descriptions used in References 126 and 127*** FOM is the Figure of Merit and is axial length of a vertical string of reference insulators

(i.e. CERL Reference A in Table 10-24) divided by the axial length of test insulator for thesame pollution flashover performance

**** LPR is leakage path ratio, determined as leakage path of the reference insulator divided by that of the test insulator, for the same pollution flashover performance

Table 10-35: a.c. Polymeric insulators, vertically mounted; performance in artificial salt-fog of 80 kg/m3 *.

Ranking No

Insulator Type ** Axial StresskV/m

Surface StresskV/m

1 VI EPDM 82 342 VIII EPR 75 313 V EPDM 67 284 VII Silicone rubber 61 25

Notes:

* Data determined from reference 378** Number used in reference 378



1999-09-01 163

Table 10-36: Critical d.c. flashover strength of polymeric insulators, vertically mounted; negative polarity; performance in

artificial pollution, using Clean-Fog test with kaolin plus NaCl at ESDD = 0.05 mg/cm2 *.


1 S ? 124 50

2 V ? 104 283 R ? 100 404 W ? 90 385 T ? 78 33

Notes:

* Data from reference 381** Insulator identification as used in reference 381

Table 10-37: Critical d.c. flashover strength of polymeric insulators, vertically mounted; negative polarity; performance in

artificial pollution, using Spray fog and Portland cement*.


1 XXVII Silicone rubber 159 592 XIII EPDM 138 69

Notes:

* Data from reference 380** Insulator identification as used in reference 380

Table 10-38: Critical d.c. flashover stress for polymeric insulators, vertically mounted; positive polarity; performance in

artificial pollution using (a) Salt-Fog test and (b) Clean-Fog test.*

Insulator ** Salt-Fog test *** Clean-Fog test **** No Type of

Cap & Pin unitRanking

NoAxialStresskV/m

SurfaceStresskV/m

Ranking No

AxialStresskV/m

SurfaceStresskV/m

F Mean slope with rib 1 120 32 4 153 41J High slope with rib 2 114 31 1 183 50E mean slope 3 112 37 3 172 57I high slope 4 111 29 6 174 46B low slope with rib 5 110 32 2 150 43A low slope 6 107 28 5 149 39

Notes:

* Data from reference 315** Insulator identification and shed description are those used in reference 315*** Salt-fog salinity of 28 kg/m3

**** ESDD = 0.07 mg/cm2



1999-09-01 164

10.3 Insulator performance as a function of pollution severity

Figure 10-1: Laboratory a.c. test results, using solid layer method, for cap and pin insulators; showing specific creepage at

50% flashover vs. SDD111

.

Figure 10-2: Laboratory a.c. test results, using solid layer method, for cap and pin insulators; showing specific creepage at 50% flashover vs. SDD

382



1999-09-01 165

Figure 10-3: A.c. test results from various laboratories, using Salt-Fog method, for cap and pin insulators; showing specific

creepage at withstand vs salinity22

143

124

10.4 Ageing of Insulators

Table 10-39: a.c. Flashover voltage of a 132 kV epoxy-resin crossarm determined for various surface conditions; Salt-Fog

salinity of 80 kg/m3.

Insulator NewAfter being inservice Years

Applied coating to ex-service unit

condition 2.5 5 Transformer oil Silicone oil Hydrocarbon greaseMean flashover voltage kV

108 54 51 76 71 >134

Comments

Tested after someconditioningdischarges,

but no surfaceabrasion

Having beenexposed to marine pollution of veryheavy severity

Rapid loss of water repellencyduring test

Non-oilyappearance, but still water repellent at end

of test

Evidence of trackingon insulator surface oncompletion of test

Notes:


Table 10-40: a.c. Flashover stress of new and aged polymeric insulators in artificial salt-fog pollution*.

Flashover stress at 80 kg/m3 salinity, kV/mInsulator type ** New Aged 6 months Aged 4 yearsEPDM VI 89 88 79Silicone rubber, VII 75 - 61

Notes:


** Insulator number as used in reference 378



1999-09-01 167

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Lightning impulse flashover characteristics of long disc insulator strings under polluted conditions, IEEE/PES Winter Meeting, NewYork, N.Y., Conference paper No. 71-CP 114-PWR, 1971.



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148 Macchiaroli, Turner,Switching surge performance of contaminated insulators, IEEE Paper No. 71 TP 141-PWR, 1971.

149 Hiroshe, Seta, Ichiara, Anjo, Okada,Switching surge insulation characteristics of insulators under polluted condition, Cigré Report 33-83 (WG 04).

150 Kizewetter, Lebedev, Merkhalev, Ostapenko,

Characteristics of EHV insulation in contaminated and moist conditions, Cigré 26 session, Paris, Paper no. 33-16, 1976.151 Kucera, Plechanova,

A probabilistic choice of insulators for alternating voltage and electric strength as switching impulses in the areas with pollution, Cigré29th session, Paris, Paper no. 33-04, 1982.

152 Garbagnati, Marrone, Porrino, Perin, Pigini,Switching impulse performance of post insulators in polluted conditions, 5th International Symposium on High Voltage Engineering(ISH), Braunschweig, Germany, Paper 51.09, Aug. 24-28, 1987.

153 Cortina R, Marrone G, Pigini A, Thione L, Petrusch W, Verma MP,Study of the dielectric strength of external insulation of HVDC Systems and application to design and testing, Cigré 30th session, Paris,Paper no. 33-12, Aug. 29 - Sept. 6, 1984.

154 Itabashi, Osada, Seta, Naito,A study of short-time AC flashover voltage of contaminated insulators and a consideration of its application to transmission line

design, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-95, 1976.155 Macchiaroli, Turner,

Comparison of insulator types by the wet contaminant and Clean-Fog test method, IEEE Trans. on Power Apparatus and Systems, Vol.PAS-89, No. 2, Feb., 1970.

156 Kimoto, Fujimura, Naito,Performance of heavy duty UHV disk insulators under polluted condition, IEEE Paper No. 71 TP 649-PWR.

157 Bargigia A, Marrone G, Mazza G, Perin D, Pigini A,Longitudinal insulation performance of HV circuit breakers under the joint effect of pollution and temporary overvoltages, Cigré 33rdsession, Paris, Paper no. 33-205, 1990.

158 Lambeth PJ, Looms JST, Sforzini M, Cortina R, Porcheron Y, Claverie P,The salt fog test and its use in insulator selection for polluted localities, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-92, No. 6, Nov./Dec., 1973, pp. 1876-1887.

159

Hoch DA, Swift DA,Impulse flashover of polluted insulation, Proc. of Southern Africa Universities Power Engineering Conference, Johannesburg, Jan.,1991, pp. 203-210.

160 Ocampo JD,Calculated prediction of flashover levels on polluted insulation at different altitudes, M.Sc. Thesis, University of Salford, UK, 1994.

161 Rizk FAM,A criterion for AC flashover of polluted insulators, IEEE/PES Winter Meeting, New York, N.Y., Conference paper No. 71 CP 135-PWR, 1971.

162 Kawamura T, Ishii M, Akbar M, Nagai K,Pressure dependence of DC breakdown of contaminated insulators, IEEE Trans. on Electrical Insulation, Vol. EI-17, No. 1, 1982, pp.39-45.

163 Ishii M, Shimada K, Matsumoto T,Flashover of contaminated surfaces under low atmospheric pressure, 4th International Symposium on High Voltage Engineering (ISH),Athens, Greece, paper 46.02, Sept. 5-9, 1983.

164 Fryxell J, Shei A,Influence of altitude on the flashover voltage of insulators, Elteknik, Sweden, V9, No. 1, 1966, pp. 1-3.

165 Rudakova VM, Tikodeev NV,Influence of low air pressure on flashover voltages of polluted insulators: Test data, generalisation attempts and somerecommendations, IEEE Trans. on Power Delivery, Vol. 4, No. 1, Jan., 1989, pp. 607-613.

166 Mercure HP, Drouet MG,Dynamic measurements of the current distribution in the foot of an arc propagating along the surface of an electrolyte, IEEE Trans. onPower Apparatus and Systems, Vol. PAS-101, No. 3, March, 1982, pp. 725-736.

167 Zhao T, Zhang R, Xue J,The influence of air pressure on AC flashover characteristics of contaminated insulators, IEEE/CEE joint Conference on High VoltageTransmission Systems, Beijing China, Oct., 1987, p. 219.



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168 Zang R, Xue J, Zhao T,A study of the AC characteristics of contaminated insulators for high altitude regions, CSEE Annual Meeting on High VoltageEngineering, Issue No. 1, Series 39, Beijing, 1986.

169 Serrano D, Ramirez M,High altitude AC standard test on polluted insulators, Tecnolab, Vol. XI, No. 62, July, 1995, (in Spanish).

170 Bergman VI, Kolobova OI,Some results of an investigation of the dielectric strength of polluted line insulation in conditions of reduced atmospheric pressure,Electronika, Vol. 54, No 2, pp. 54-56.

171 Mercure HP,Flashover discharge propagation on polluted insulators, Proc. IEE conference on Electrical Insulation, June, 1984, pp. 106-110.

172 Swift DA, Hoch DA,Influence of air density on the critical DC flashover stress of an electrolyte surface, South African Universities Power EngineeringConference, Durban, 1992.

173 Hoch DA, Swift DA,Flashover of polluted insulation: an assessment of the influence of air density, IEEE Africon Swaziland, 1992.

174 Long Y, Shi Zhi X,Effect of atmospheric pressure on the external insulation of high voltage electrical apparatus, CSEE Annual Meeting on High Voltage

Engineering, Issue No. 1, Series 39, Beijing, 1986.175 Hoch DA, Swift DA,

Impulse flashover of an air-water interface: Influence of air density and water conductivity, 7th International Symposium on HighVoltage Engineering (ISH), Dresden, Germany, Paper 43.23, Aug 26-30, 1991.

176 Hoch DA, Swift DA,Switching surge flashover of an air water interface: influence of an air water interface: influence of water resistance, air density andimpulse wave shapes, 8th International Symposium on High Voltage Engineering (ISH), Yokohama, Japan, Paper 45.11, Aug. 23-27,1993, Vol.4, pp. 117-120.

177 Sugawara N, Takayama T, Hokari K, Yoshida K, Ito S,Withstand voltage and flashover performance of iced insulators depending on the density of accreted ice, Proc. of the 6 th InternationalWorkshop on Atmospheric Icing of structures, Budapest Hungary, pp. 231-235, Sept., 1993.

178 Sugawara N, Takayama T, Hokari K, Ito S, Yoshida K,Effect of icicle growth of hard rime accreted insulators on withstand voltage, 8th International Symposium on High VoltageEngineering (ISH), Yokohama, Japan, Paper 46.04, Aug. 23-27, 1993, Vol.2, pp. 157-160.

179 Farzaneh M, Kiernicki J,Flashover performance of IEEE standard insulators under ice conditions, IEEE Trans. on Power Delivery, Vol. 12, No.4, Oct., 1997, pp. 1602-1613.

180 Chisolm WA, et al,The cold fog test, IEEE Trans. on Power Delivery, Vol. 11, No. 4, Oct., 1996, pp. 1874-1880.

181 Fikke SM, Ohnstad TM, Telstad T, Förster H, Rolfseng L,Effect of long range airborne pollution on outdoor insulation, Nordic Insulating Symposium, Finland, Paper No. 1.6, June, 1994.

182 Kannus K, Verkonnen K, Lakervi E,Effect of ice coating on the dielectric strength of high voltage insulators, Proc. of the 4 th International Workshop on AtmosphericIcing of Structures, Paris, France, pp. 296-300, 1993.

183 Sklenicka V, et al,Influence of conductive ice on electric strength of HV insulators, ISPPISD, Madras, 1983.

184 Renner PE, Hill HL, Ratz O,Effects of icing on DC insulation strength, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-90, 1971, pp. 1201-1206.

185 Farzaneh M,Effect of the thickness of ice and voltage polarity on the flashover voltage of ice-covered high voltage insulators, 7th InternationalSymposium on High Voltage Engineering (ISH), Dresden, Germany, Paper 43.10, Aug 26-30, 1991.

186 Farzaneh M, Zhang J, Brettschneider S, Miri AM,DC flashover performance of ice-covered insulators, 10th International Symposium on High Voltage Engineering (ISH), Montreal,Canada, Proc. Vol. 3, pp. 77-80, Aug 25-29, 1997.

187 Schneider HM,Artificial ice tests on transmission line insulators - A progress report, IEEE/PES Summer Meeting, San Francisco, USA, Paper A75-491-1, July, 1975, pp. 347-353.

188 Udo T, Wanatebe Y, Mayumi K, Ikeda G, Okada T,Switching surge flashover characteristics of long insulator strings and stacks, Cigré 22nd session, Paris, Paper no. 25-04, 1968.



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189 Iwama T, Sumiya Y, Matsuoka T, Ito S, Sakanishi K,Investigation of AC insulation performance of tension insulator assembly covered with snow, Proc. International workshop onAtmospheric Icing of Structures, (IWAIS'90), Tokyo, Japan, Sept., 1990.

190 Yasui M, Naito K, Hasegawa Y,AC withstand voltage characteristics of insulator string covered with snow, IEEE Trans. on Power Delivery, Vol. 3, No. 2, April, 1988.

191 Fujimura T Naito K, Hasegawa Y, Kawaguchi T,Performance of insulators covered with snow or ice, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-98, No. 5, Sept/Oct.,1979, pp. 1621-1631.

192 The Japanese UHV Transmission Demonstration Test Committee,Experimental study of DC UHV power transmission, Report No. Z85802, 1985, (in Japanese).

193 Yasui M, Iwama T, Sumia Y, Naito K, Matsuoka R, Nishikawa M,Investigation of Switching Impulse flashover voltage performance of UHV class tension insulator assembly, Proc. Internationalworkshop on Atmospheric Icing of Structures, (IWAIS'90), Tokyo, Japan, Sept., 1990.

194 Matsuda H, Komuro H, Takasu K,Withstand voltage characteristics of insulator string covered with snow or ice, IEEE/PES Summer Meeting, Paper No. 90 SM 355-8PWRD, 1990.

195 Limbourn GJ, Purdam I, Henderson RT, Edmondson F, Glasson GT,

Australian service experience with pollution of high voltage insulation, AIEng, Electrical Engineering Trans., Sept., 1971, pp. 45-53.196 Melik G, Gascoigne NR,

Performance of 500 kV current transformers under pollution conditions, 8th International Symposium on High Voltage Engineering(ISH), Yokohama, Japan, paper 45.06, Aug. 23-27, 1993, Vol.2, pp. 125-128.

197 Verma MP,Insulator design for 1200 kV lines, IEEE Trans. on Electrical Insulation, Vol. EI-16, No. 3, June, 1981.

198 Verma MP,Insulation performance of d,c, apparatus-housing under pollution, ETZ-Archiv Bd. 5, H.9, 1983.

199 Kawamura T, Seta T, Nagai K, Naito K,DC pollution performance of insulators, Cigré 30th session, Paris, Paper no. 33-10, Aug. 29 - Sept. 6, 1984.

200 Lampe W, Eriksson KA, Peixoto CAO,Operation experience of HVDC stations with regard to natural pollution, Cigré 30th session, Paris, Paper no. 33-01, Aug. 29 - Sept. 6,

1984.201 Lampe W,Pollution and rain flashovers on wall bushings, Proc. of the 2nd International Conference on properties and applications of dielectricmaterials (ICPADM), Beijing China, Vol. 1, pp. 29-32, 1988

202 Lambeth PJ,Laboratory tests to evaluate HVDC wall bushing performance in wet weather, IEEE/PES Winter meeting, Atlanta, Georgia, Paper No.90 WM 167-7 PWRD, Feb. 4-8, 1990.

203 Schneider HM, Lux AE,Mechanism of HVDC wall bushing flashover in non-uniform rain, IEEE Trans. on Power Delivery, Vol. 6, No. 1, Jan., 1991, pp. 448-455.

204 Zhaoying S, Xing C, Xiaokang L, Non-uniform rain on HVDC wall bushings, 7th International Symposium on High Voltage Engineering (ISH), Dresden, Germany, paper 43.04, Aug 26-30, 1991.

205 Schneider HM, Hall JF, Nellis CL, Low SS, Lorden DJ,Rain and contamination tests on HVDC wall bushings with and without RTV coatings, IEEE Trans. on Power Delivery, Vol. 6, No. 3,July, 1991, pp. 1289-1300.

206 Rizk FAM, Kamel SI,Modelling of HVDC wall bushing flashover in non-uniform rain, IEEE Trans. on Power Delivery, Vol. 6, No. 4, Oct, 1991, pp. 1650-1662.

207 Krishnayya PCS, Lambeth PJ, Maruvada PS, Trinh NG, Desilets G, Nilsson SL,An evaluation of the R & D requirements for developing HVDC converter stations for voltages above ± 600 kV, Cigré 32nd session,Paris, Paper no. 14-07, Aug. 28 - Sept. 3.

208 Asplund G, Åström U, Canelhas A, Åberg M, Purra E, Heyman O,A novel approach on HVDC ± 800 kV station and equipment design, Cigré International Colloquium on High Voltage Direct Currentand Flexible AC Power Transmission Systems, Paper 7.3, Wellington, New Zealand, Sept 29 to Oct. 1, 1993.



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209 Rizk MS, Nosseir A, Arafa BA, Elgendy O, Awad M,Effect of desert environment on the electrical performance of silicone rubber insulators, 10th International Symposium on HighVoltage Engineering (ISH), Montreal, Canada, Proc. Vol. 3, pp. 133-136, Aug 25-29, 1997.

210 Fukui H, Naito K, Irie T, Kimoto I,A practical study on the application of semiconducting glaze insulators to transmission lines, IEEE Trans. on Power Apparatus and

Systems, Vol. PAS-93, No. 5, Sept./Oct., 1974.211 Morita K, Matsuoka R, Matsui S, Suzuki Y, Nakashima Y,

Practical application of semiconducting glazed insulators, NGK Insulators.212 Naito K, Hayashi S, Ibi Y,

Insulation co-ordination against insulator contamination, IEEE Mexico Section Power Meeting, Paper No. RVP'93-TRA-02, 1993.213 Naito K, Kikuchi T, Sasaki K, Matsuoka R,

Strength of external insulation under maintenance conditions, Cigré 35th session, Paris, Paper no. 33-302, 1994.214 Kawashima T, Matsuyama A, Suzuki K,

Development of insulator washing robot, Chubu Electric Power Co. Inc. Kohmu Giho, No. 2, 1985, pp. 11-20, (in Japanese).215 Sawada Y, Fukushima M, Yasui M, Kimoto I, Naito K,

A laboratory study on RI, TVI and AN of insulator strings under contaminated condition, IEEE Trans. on Power Apparatus andSystems, Vol. PAS-93, No. 2, March/April, 1974, pp. 712-719.

216 Fujimura T, Naito K, Matsuoka R, Suzuki Y,A laboratory study on RI, TVI and AN characteristics of HVDC insulator assemblies under contaminated condition, IEEE Trans. onPower Apparatus and Systems, Vol. PAS-101, No. 4, April, 1982, pp. 138-144.

217 Yasui M, Takahashi Y, Takenaka A, Naito K, Hasegawa Y,Kato K,RI, TVI and AN characteristics of HVDC insulator assemblies under contaminated condition, IEEE Trans. on Power Delivery, Vol. 3, No. 4, Oct., 1988, pp. 1913-1921.

218 Fukushima M, Sunaga Y, Sasano T, Sawada Y,AN, RI and TVI from single unit flashover of HVDC suspension insulator string, IEEE Trans. on Power Apparatus and Systems, Vol.PAS-96, No. 4, 1977, pp. 1233- 1241.

219 Matsuoka R, Ito S, Sakanishi K,Investigation of single unit flashover in HVDC insulator string, Trans. of IEEJ, Vol. 112-B, No. 1, pp. 36-41, 1992, (in Japanese).

220 Bernardelli PD, Cortina P, Sforzini M,

Laboratory investigation on the radio interference performance of insulators in different ambient conditions, IEEE Trans. on Power Apparatus and Systems, Paper No. T72193-6, 1972.221 Riviere D, Parraud R, Gary C, Moreaue M, Kohoutova D,Vokalek J,

The influence of ambient conditions on the interference level of insulator strings, Cigré 24th session, Paris, Paper no. 36-04, Aug. 28 -Sept. 6, 1972.

222 Swift DA, Britten AC,Electromagnetic interference from high-voltage insulators: a comparison of hydrophobic and hydrophilic cases, Internationalsymposium on electromagnetic compatibility, Rome, Paper T-3, Sept. 1996, pp 846-851.

223 Naito K, Yamada K,Electric and magnetic fields produced by power equipment and related guidelines and standards, IEEE Mexico Section Power Meeting,Paper No. RVP'93-TRA-01, 1993.

224 IEC - CISPR publication 18-2 (1986)Radio interference characteristics of overhead power lines and high-voltage equipment. Part 2: Methods of measurement and procedure for determining limits, Published by IEC, Genève.

225 IEC - CISPR publication 18.2 Amendment 1 (1993)Radio interference characteristics of overhead power lines and high-voltage equipment. Part 2: Methods of measurement and procedurefor determining limits, Published by IEC, Genève.

226 Crabtree, Mckey MJ, Naito K, Watanebe A, Irie T,Studies on electrolytic corrosion of hardware of DC line insulators, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-104, No.3, March, 1985, pp. 645-654.

227 Tanigchi T, Watanabe M, Watanabe Y, Mori S, Watanabe A, Naito K,Electrolytic corrosion of metal hardware of HVDC line and Station insulators, IEEE Trans. on Power Delivery, Vol. 6, No. 5, Dec.,1991, pp. 1224-1233.

228 Naito K, Sakanishi K, Suzuki Y, Ito M,Pin corrosion of suspension insulators, Trans. of IEEJ, Vol. 100e, no. 9/10, pp. 73-79, 1980.



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229 Swift DA, Fitter CN, Li S,The DC component of AC energised outdoor insulation: the effect of dry band discharges, 5th International Symposium on HighVoltage Engineering (ISH), Braunschweig, Germany, Paper 51.14, Aug. 24-28, 1987.

230 Crouch AG, Swift DA, Parraud R, de Decker D,Ageing mechanisms of a.c. energised insulators; Cigré 33rd session, Paris, Paper no. 22-203, 1990.

231 Maddock BJ, Allutt JG, Ferguson JM,Lewis KG, Swift DA, Teare PW, Tunstall MJ,Some investigations of the ageing of overhead lines, Cigré 31st session, Paris, Paper no. 22-09, 1986.

232 Imakoma T, Matsui S, Suzuki Y, Fujii O, Kawamura S,Hardware corrosion of insulators and its countermeasures in harsh environments, 9th Conference on Electric Power supply Industry(CEPSI), Hong Kong, Nov. 23-27, 1992.

233 Ross PM,Burning of wood structures by leakage currents, Trans. of the AIEE, Vol. 66, pp. 270-287, 1947.

234 Darveniza M,Electrical properties of wood and line design, University of Queensland Press, St. Lucia, Queensland, 1980.

235 Darveniza M, Mercer DR, Sekso TA, Krznaric I,Environmental influences on distribution lines and transformers on wooden poles, Proc. of International Conference on WoodenStructures in Distribution, pp 20 - 23 (in Croatian), Zagreb, June, 1988.

236 Sekso TA, Darveniza M,Ten years of experience with protective measures against top pole fires in very severe sea salt pollution region in Dalmatia, Energija No. 4, Zagreb, 1998, pp. 1-6 (in Croatian).

237 Lusk GE, Mak ST,EHV wood pole fires. Their cause and potential cures, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-95, No. 2,March/April, 1976, pp. 621-629.

238 Filter R,The influence of wood pole preservatives on wood fire and electric safety, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-103, No. 10, Oct., 1984, pp 3089-3095.

239 Lambeth PJ, Auxel H, Verma MP,Methods of measuring the severity of natural pollution as it affects HV insulator performance, Electra No. 20, pp. 37-52.

240 Cigré Task Force 04.03 of Study committee 33,

Insulator pollution Monitoring, Electra No. 152, Feb., 1994, pp. 79-89.241 Macey R,The performance of high voltage, outdoor insulation in contaminated environments, Trans. South African Institute of ElectricalEngineers, pp 80-92, April, 1981.

242 Lannes W, Schneider H,Pollution severity performance chart; Key to just in time insulator maintenance, IEEE Trans. on Power Delivery, Vol. 12, No.4, Oct.,1997.

243 Iwai K, Hase Y, Nakamura E, Katsukawa H,Development of a new apparatus for contamination measurement of overhead transmission line insulators, IEEE Trans. on Power Delivery, Vol. 13, No. 4, Oct., 1998, pp.1412-1417.

244 Orbin DRH, Swift DA,Pollution severity mnonitor for relevance to insulator flashover: Some design features of one based on the Pelier effect, Proc. of Southern African Universities Power Engeneering Conference. Cape Town, 1994, pp 167-170.

245 Orbin DRH, Swift DA, Insulator pollution severity instruments employing Peltier coolers, 9th International Symposium on HighVoltage Engineering (ISH), Graz, Austria, Paper 3229, Aug. 28-Sept. 1, 1995.

246 Marrone G, Marinoni F, New apparatus set up at ENEL to monitor pollution deposit and pilot cleaning operations on outdoor insulators, Cigré 36th session,Paris, Paper no. 33-302, 1996.

247 Richards CN, Renowden JD,Development of a remote insulator contamination monitoring system, IEEE Trans. on Power Delivery, Vol. 12, No.1, Jan., 1997, pp.389-397.

248 Fierro-Chavez JL, Ramirez-Vazquez I, Montoya-Tena G,On-line leakage current monitoring of 400 kV insulator strings in polluted areas, IEE Proc. Gener. Transm. Distrib., Vol. 143, No. 6, Nov., 1996.

249 Houlgate RG, Swift DA, Cimador A, Pourbaix F, Marrone G, Nicolini P,

Field experience and laboratory research on composite insulators for overhead lines, Cigré 31st session, Paris, Paper no. 15-12, 1986.



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250 Holtzhausen JP,Leakage current monitoring on synthetic insulators at a severe coastal site, Proc. of the International Workshop on Non-CeramicOutdoor Insulation, Société des Electriciens et des Electroniciens (SEE), Paris, April 15-16, 1993.

251 Lambeth PJ, Looms JST, Roberts WJ, Drinkwater BJ, Natural pollution testing of insulators for UHV Transmission Systems, Cigré 25th session, Paris, Paper no. 33-12, 1974.

252 Low SS, Melvold DJ, Naito K, Hasegawa Y, Fujii O,Laboratory test on naturally contaminated HVDC station post insulators, IEEE/CSEE Joint Conference on HV Transmission Systemsin China, Paper no. #87 JC-76, Oct., 1987, reprinted in NGK Review No. 11, 1987.

253 Riquel G,Accelerated ageing test for non-ceramic insulators, EDF's experience, Proc. of the International Workshop on Non-Ceramic Outdoor Insulation, Société des Electriciens et des Electroniciens (SEE), Paris, April 15-16, 1993.

254 Moreau C, Riquel G,Accelerated ageing test on 24 kV composite surge arresters, 8th International Symposium on High Voltage Engineering (ISH),Yokohama, Japan, Paper 25.26, Aug. 23-27, 1993, Vol.1, pp. 251-254.

255 Vosloo WL, Holtzhausen JP,Koeberg insulator pollution test station (KIPTS), 9th International Symposium on High Voltage Engineering (ISH), Graz, Austria,Paper 3228, Aug. 28-Sept. 1, 1995.

256

IEC Technical Report 1245,Artificial pollution tests on high-voltage insulators to be used on d.c. systems. IEC Technical Report 1245, 1993257 Montesinos J, Campillo MT, Ponce MA, Fierro JL, Ocana L, De La Rosa S,

Characteristicas de flameo de aisladores contaminados naturalmente, IEEE Mexico Section Power Meeting, Acapulco Mexico, Paper IEEE-RVP-94, 1994, (in Spanish).

258 Lambeth PJ,Variable-voltage application for insulator pollution tests, IEEE Trans. on Power Delivery, Vol. 3, No. 4, Oct., 1988.

259 Marrone G, Marinoni F, Galluci F,Set up of a dust chamber providing artificial insulator contamination suitable to reproduce conditions occurring in the field, 5thInternational Symposium on High Voltage Engineering (ISH), Braunschweig, Germany, Paper 52.18, Aug. 24-28, 1987.

260 Eklund A, Gutman I, Hartings R,The dust cycle method: a new pollution test method for ceramic and non-ceramic insulators, International Conference on Power SystemTechnology, Beijing, Oct. 18-24, 1994.

261 Ely CHA, Lambeth PJ,Performance, pollution testing and design improvements to substation insulators under heavy wetting conditions, InternationalConference on the Design and Application of EHV Substations, London, Nov. 22 - 24, 1977.

262 Ely CHA, Lambeth PJ, Looms JST,The Booster Shed: prevention of flashover of polluted substation insulators in heavy wetting, IEEE Trans. on Power Apparatus andSystems, Vol. PAS-97, No. 6, Nov./Dec., 1978, pp. 2187-2197.

263 Reverey G,Die Arbeiten zur internationalen Normung eines Fremdschicht-Prüfverfahrens für Isolatoren, Wissenschaftliche Zeitschrift der Technischen Universität Dresden, Bd. 16 (1976)), H. 3, 1967, pp. 979-988 (in German).

264 FGH,Dauerverhalten von Verbund-Langstabisolatoren aus Kunststoff unter natürlichen Fremdschichtbedingungen, FGH Technischer Bericht Nr. 1-254, 1984, (in German).

265 Lange G,Einflüsse natürlicher Fremdschichten auf die Betriebszuverlässigkeit von Freiluftisolationen - Ergebnisse und Erfahrungen einer 20 jährigen Überwachung der Fremdschichten im 110-kV-Netz der VEW, Elektrizitätswirtschaft, Jg.91, 1992, H. 8, 1992, pp. 436-443(in German).

266 Bonaguro D, Siegert LA, Jerez E,Anticontamination design of 800 kV strings, 4th International Symposium on High Voltage Engineering (ISH), Athens, Greece, Paper No. 145, Sept. 5-9, 1983.

267 Verma MP, Niklasch H, Heise W, Lipken H, Luxa GF, Schreiber H,The criterion for pollution flashover and its application to insulation dimensioning and control, Cigré 27th session, Paris, Paper no. 33-09, 1978.

268 Karady GG, Rizk FAM, Schneider HM,Review of Cigré and IEEE research into pollution performance of non-ceramic insulators: field aging effects and laboratory test

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269 Charneski MD, Gaibrois GL,Whitney BF, Flashover tests on artificially iced insulators, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-101, No. 8, Aug.,1982, pp. 1788-1794.

270 Phan CL, Matsuo H,Minimum flashover voltage of iced insulators, IEEE Trans. on Electrical Insulation, Vol. EI-18, No. 6, 1983, pp. 605- 618.

271 Ervin CC,500 kV insulator flashovers at normal operating voltage, CEA Spring meeting, Montreal, Presentation, March, 1988.

272 Leguan G, et al,AC flashover characteristics of EHV line insulators for high altitude contamination regions, ICPAPM-88, 1988.

273 Sugawara N, Hokari K, Matsuda K, Miyamoto K,Insulation properties of salt contaminated fog type insulators depending on the growth of icicles, Proc. International workshop onAtmospheric Icing of Structures, (IWAIS'90), Tokyo, Japan, Paper No. B4-10, Sept., 1990.

274 Lee LY, Nellis CL, Brown JE,60Hz tests on ice coated 500 kV insulator strings, IEEE/PES Summer Meeting, San Francisco, USA, Paper A75-499-4, July, 1975.

275 Su F, Jia Y,Icing of insulator string on HV transmission line and the harmfullness, Proc. of the 3 rd International Offshore and Polar EngineeringConference, Singapore, 1993, pp. 655-662.

276 Arai N,AC fog withstand test on contaminated insulators by steam fog, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-101, No. 11, Nov., 1982.

277 Lambeth PJ, Schneider HM,Final report on the Clean-Fog test for HVAC Insulators, IEEE/PES Winter Meeting, Feb., 1987.

278 Naito K, Kawaguchi T, Ito M, Katsukawa H,Influence of fog parameters on withstand voltage of contaminated insulators, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-102, No. 3, March, 1983, pp. 729-737.

279 Ishii M, Akabar M, Kawamura T,Effect of ambient temperature on the performance of contaminated DC insulators, IEEE Trans. on Electrical Insulation, Vol. EI-19, No.2, April, 1984.

280 Task Force 04 of Cigré Working group 33-04,

Artificial pollution testing of HVDC insulators: Analysis of factors influencing performance, Electra No. 140, Feb., 1992.281 Mizuno Y, Kusada H, Naito K,Effect of climatic conditions on contamination flashover voltage of insulators, IEEE Trans. on Dielectrics and Electrical Insulation,Vol.4, No.3, June, 1997, pp. 286-289.

282 Moreno M, Encinas J, Ramirez M,Temperature influence on U50 of insulators in the salt fog pollution test applying quick flashover method, IEEE Mexico Section Power Meeting, Mexico, 1996, (in Spanish).

283 Takasu K, Arai N, Imano Y, Shundo T, Seta T,AC flashover characteristics of long air gaps and insulator strings under fog conditions, IEEE Trans. on Power Apparatus and Systems,Vol. PAS-100, No. 2, Feb., 1981, p. 645.

284 Fierro JL,Effects of pollution conditions on insulators. The case of Mexico, International Symposium on Power System Insulation Co-ordination,Mexico, Nov. 8-10, 1994.

285 Naito K, Schneider HM,Round-Robin artificial contamination test on high voltage DC insulators, IEEE/PES Summer Meeting, July, 1994.

286 Verma MP,Höchster Ableitstrom als Kenngröße für das Isolierverhalten verschmutzter Isolatoren, ETZ A, Nr. 94, H. 5, 1973, (in German).

287 Verma MP, Petrusch W,Mechanism of a.c. flashover on polluted insulators, International Symposium on pollution performance of insulators and surgediverters, Madras, 1981.

288 Petrusch W, Lange G, Schmitt W, Kluge W,Experiences on the continuous monitoring of the leakage current on polluted insulators in service in the Federal Republic of Germany,Cigré - Symposium 22-81, Stockholm, Paper 122-03, 1981.

289 Verma MP, Petrusch W,Results of pollution tests on insulators in the 1100 kV range and necessity of testing in the future, IEEE Trans. on Electrical Insulation,

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290 Sforzini M, Cortina R, Marrone G, Bernadelli PD,A laboratory investigation on the pollution performance of insulator strings for UHV transmission systems, IEEE/PES Summer Meeting, Paper No. F 79738-6, July, 1979.

291 FGH,Isoliervermögen von Freiluftisolatoren für Höchstspannungs-Drehstromanlagen unter Fremdschicht-Bedingungen bei

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293 Kawai M,Flashover tests at project UHV on salt-contaminated insulators, Part II, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-89, No. 8, Nov./Dec., 1970.

294 Naito K,Insulator pollution - general aspects, Seminaire sur la pollution des isolements des lignes et postes HT, No.6, Cassa Blanca, March,1989.

295 IEC Publication 71,Insulation Co-ordination.

296

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299 Naito K, Mizuno Y, Naganawa W,A study on probabilistic assessment of contamination flashover of high voltage insulator, IEEE/PES Summer Meeting, Paper No. 94SM 445-7, PWRD, 1994.

300 Fujimura T, Naito K, Iida T, Suzuki Y,A study on statistical evaluation of contamination flashover of insulators, IEEJ Tokai Regional Conference of IEEJ, 1978, (inJapanese).

301 Ad-hoc Committee on fault caused by No. 20 Typhoon,ECR of Japan, Report on fault caused by No. 20 Typhoon and ESDD, 1973, (in Japanese).

302 Karady G, Dallaire D, Mukhedkar D,Statistical method for Transmission line insulation design for polluted areas, IEEE/PES Winter Meeting, Paper No. WM A76 220-4,1976.

303 Sforzini M, Cortina R, Marrone G,Statistical approach for insulator design in polluted areas, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-102, 1983, pp.3157-3166.

304 Kawamura T, Kouno T, et al,Statistical estimation of transmission line performance under polluted condition, IEEE/PES Winter Meeting, New York, N.Y.,Conference paper No. 71 CP 650-PWR, 1971.

305 Nagano Y, Igarashi M, Yagisawa Y, Kitamura S,Estimation of insulator contamination severity by ESDD and Effectiveness of the regular clean, Trans. of IEEJ, Vol. B_98, 1978, (inJapanese).

306 Lambeth PJ,The method of choice of insulators and insulation levels in the UK, private communication, 1978.

307 Lambeth PJ,The importance of statistical factors on operating stresses for insulators, Cigré 33-84(WG04)15IWD, 1984.

308 Yamada K, Mizuno Y, Naito K,Simulation of flashover risk of polluted insulator, IEEJ Tokai Regional Conference, Paper No. 80, 1994 and Second report, No. 1639,IEEJ all Japan Conference, March, 1995, (Both in Japanese).

309 Kawamura T, Ishii M,Study on the temperature difference and flashover probability of polluted insulator under natural condition, Trans. of IEEJ, Vol. 96, No. 1, pp. 7-14, 1976, (in Japanese).



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310 Chisholm WA, North American operating experience: Insulator flashovers in cold conditions, Cigré colloquium SC 33-97, Toronto Ontario Canada, paper 33-4.3, 1997.

311 Mizuno Y, Nakamura H, Naito K,Dynamic simulation of risk of flashover of contaminated ceramic insulators, IEEE/PES Summer Meeting, Paper No. 96 SM 441-6

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313 Watanabe Y,Flashover tests of insulators covered with ice or snow, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-97, No. 5, Sept/Oct.,1987.

314 Kawamura F, Naito K,Draft manuscript of Cigré Guidelines for the assessment of the dielectric strength of external insulation, Cigré WG 33.07 document,April, 1990.

315 Pargamin L, Huc J, Tartier S,Considerations on the choice of the insulators for HVDC overhead lines, Cigré 30th session, Paris, Paper no. 33-11, Aug. 29 - Sept. 6,

1984.316 Hyltén-Cavallius N, Annestrand S, Witt H, Madzarevic V,Insulation Requirements, corona losses, and corona radio interference for high-voltage d-c lines, IEEE Trans. on Power Apparatus andSystems, Vol. PAS-83, 1964, pp. 500-508.

317 Kimoto I, Fujimura T, Naito K,Performance of insulators for direct current transmission line under polluted condition, IEEE Trans. on Power Apparatus and Systems,Vol. PAS-92, No. 3, May/June, 1973, pp. 943-949.

318 Cheng T.C, Wu C.T, Rippey J.N, Zedan F.M,Pollution performance of DC insulators under operating conditions, IEEE Trans. on Electrical Insulation, Vol. EI-16, No. 3, June,1981, pp. 154-164.

319 Shuzi L,Pollution accumulation performance of insulators under negative DC voltage, Proc. of the 2nd International Conference on propertiesand applications of dielectric materials (ICPADM), Xian, China, June 24-29, 1985.

320 Sherif E.M,Performance and ageing of HVAC and HVDC overhead line insulators, Ph.D Thesis, Technical Report No. 169, 1987, School of Electrical and Computer Engineering, Chalmers University of Technology, Gothenburg, Sweden, 1987.

321 Minemura S, Iso T, Naito K, Irie T, Suzuki Y,Insulation design of Hokkaido-Hoshu interconnecting DC transmission line, Paper for Pacific Coast Electrical Association, U.S.A.,March 17-18, 1983.

322 NGK,Technical Guide, Cat. No. 91R, First revision, 1989.

323 Hirsch, F, Rheinbaben H. V, Sorms R,Flashover of insulators under natural pollution and HVDC, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-94, No. 1,Jan./Feb., 1975, pp. 45-50.

324 Takenouchi T, Horie H, Iso T,

Discussion contribution to 'Performance of insulators for direct current transmission line under polluted condition', IEEE Trans. onPower Apparatus and Systems, Vol. PAS-92, No. 3, May/June, 1973, pp. 943-949.

325 Wu D, Su Zhiyi S,The correlation factor between DC and AC pollution levels: Review and Proposal, 10th International Symposium on High VoltageEngineering (ISH), Montreal, Canada, Proc. Vol. 3, pp. 253-256, Aug 25-29, 1997.

326 Kito K, Imakoma T, Shinoda K,Phase-to-phase spacers for transmission line, IEEE/PES Conference paper, No. A75 498-6, 1975.

327 Asai S, Oura H, Matsui S, Torimoto S, Usami D,Design and application of phase-to-phase spacers for overhead transmission lines in snowy areas, Proc. International workshop onAtmospheric Icing of Structures, (IWAIS'90), Tokyo, Japan, Paper No. B4-11, Sept., 1990.

328 ANSI/IEEE Std 957/1987,IEEE Guide for Cleaning Insulators.

329 IEEE Std. 516-1987,IEEE Guide for maintenance methods on energised power lines.



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330 Perin D, Pigini A, Visintainer I, Channakeshava, Ramamoorty M,Live-line insulator washing: experimental investigation to assess safety and efficiency requirements, IEEE/PES Transmission andDistribution Conference, April 10-15, 1994, pp. 480-487.

331 Cigré WG 33.07 and TF 33.07.09,Dielectric strength of external insulation systems under live working, Cigré 35th session, Paris, Paper no. 33.306, 1994.

332 Yamamoto M., Ohashi K,Salt Contamination of External Insulation of High-Voltage Apparatus and its Counter Measures, IEEE Transaction Paper 61-6, 1961.

333 Paris L,Robotised Hot-Line Maintenance: Considerations and first experiences, Cigré 32nd session, Paris, Paper no. 22-14, Aug. 28 - Sept. 3,1988.

334 Frantoni G, Giglioli R, Marrone G,Laboratory Simulation of MV Hot-Line Insulator Washing to Investigate its performance in Relation to the Users' Requirements,Workshop on robotised hot line maintenance, Pisa, 1988.

335 Last FH, Pegg TH, Sellers N, Staleski A,Live Washing of HV Insulators in Polluted Areas, Proc. IEE vol. 113, pp. 847, 1966.

336 Wang R, et al,Study on the Safety Technique of Hot Washing, IEEE/CSEE Joint Conference on High Voltage Transmission Systems in China, Oct.

17-22, 1987.337 El Sayed AH, Aly,

Results of tests on dielectric strength of low pressure water jet for live washing of the 500 kV transmission line in the Arab Republic of Eypt, Cigré 25th session, Paris, Paper no. 33-11, 1974.

338 Calebread RJ, Brown HJ, Dawkins RB,Automatic Insulator Washing System to Prevent Flashover Due to Pollution, Proc. IEE, vol. 125, 1978, pp. 1363.

339 Fujimura T, Naito K, Isozaki T, Kawaguchi T,Hot line Insulator Washing Equipment in Power Plants, NGK Review No 3, 1979.

340 Fujimura, Tetsuo, Okayama, Masami, Isozaki, Takashi,Hot line Washing of Substation Insulator, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-89, No. 5, May/June, 1970.

341 Yoshida A, Kawashima R, Matsuyama A, Suzuki K,Hot-line insulator washing robot for transmission lines, Workshop on robotised hot line maintenance. Pisa, Sept. 5, 1988.

342 Burnham JT, Bush DW, Renowden JD,FPL's Christmas 1991 transmission outages, IEEE Trans. on Power Delivery, Vol. 8, No. 4, Oct, 1993.

343 Burnham JT, Franc J, Eby MR,High-pressure washing tests on polymer insulators, IEEE ESMO-95, Paper CP-11, 1995.

344 Akiyama T, Nakamura T, Shinokubo H, Kondo K,Damages on polymer insulators due to high pressure water washing, IEEJ Tokai-section Conference of IEEJ, Paper No. 75, Sept.,1995.

345 Fierro-C JL, Ramirez-V I, Encinas Rosa J,Aplicacion de aisladores polimericos en lineas de transmision en Mexico. Parte II: Experiencia en campo, IEEE Mexico Section Power Meeting, Acapulco Mexico, Paper RVP'97, Vol.3, pp. 85-90, July, 1997, (in Spanish).

346 Lambeth PJ, Looms JST, Stalewski A, Todd WG,Surface coatings for H.V. insulators in polluted areas, Proc. IEE, Vol. 113, No. 5, May, 1966, pp. 861-869.

347

IEEE Committee S-32-3,Protective coatings for improving contamination performance of outdoor high voltage ceramic insulators, IEEE/PES Winter Meeting,Paper No. 94 WM 096-8 PWRD, 1994.

348 Almgren B,Discussion contribution to Gorur R et al 'Protective coatings for...', IEEE Trans. on Power Delivery, Vol. 10, No. 2, April, 1995, pp.924 -933.

349 Cherney EA, Hackam R, Kim SH,Porcelain insulator maintenance with RTV silicone rubber, IEEE Trans. on Power Delivery, Vol. 6, No. 3, July, 1991, pp. 1177-1181.

350 Bhana DK, Swift DA,An investigation into the temporary loss of hydrophobicity of some polymeric insulation and coatings, Proc. of the 4th InternationalConference on properties and applications of dielectric materials (ICPADM), Brisbane, Australia, Paper 5208, 1994.

351 Dickson AE , Reynders JP,The effects of corona on the surface properties and chemical composition of silicone rubber insulators, 9th International Symposium onHigh Voltage Engineering (ISH), Graz, Austria, Paper 3231, Aug. 28-Sept. 1, 1995.



1999-09-01 184

352 Vosloo WL,End of life failures experienced at a natural ' accelerated' ageing test facility, Cigré colloquium SC 33-95, Harare Zimbabwe, Paper 3.7,May 29-30, 1995.

353 Su ZY,Service experience at Gezhouba and Nanqiao ±500 kV HVDC converter stations and effect of applying RTV coating, ICPST'94,

October 18-21, 1994, Beijing, China, Vol. 1 pp. 471-473.354 Schneider HM, Guidi WW , Burnham JT, Gorur RS, Hall JF,

Accelerated aging and flashover tests on 138 kV non-ceramic line post insulators, IEEE Trans. on Power Delivery, Vol. 8, No. 1, Jan.,1993, pp. 325-336.

355 Ely CHA, Lambeth JP, Looms JST, Swift DA,Discharges over wet, polluted polymers: the booster shed, Cigré 27th session, Paris, Paper no. 15-02, 1978.

356 Kawai M,AC flashover test at project UHV on ice-coated insulators, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-80, No. 8, Aug.,1961.

357 Cherney E,Flashover performance of artificially contaminated and iced longrod transmission line insulators, IEEE Trans. on Power Apparatus andSystems, Vol. PAS-99, No. 1, 1980.

358

Sklenicka V, Vokalek J,Insulators in icing conditions: Selection and measures for reliability increasing, Proc. International workshop on Atmospheric Icing of Structures, (IWAIS'96), Chicoutimi, Canada, June 3-7, 1996, pp. 72-76.

359 Wu D, Halsan KÅ, Fikke SM,Artificial Ice tests for long insulator strings, Proc. International workshop on Atmospheric Icing of Structures, (IWAIS'96), Chicoutimi,Canada, June 3-7, 1996, pp. 67-71.

360 Lenk DW,An examination of the pollution performance of gapped and gapless metal oxide station class surge arresters, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-103, No. 2, Feb., 1984, pp. 337-344.

361 Bargigia A, Giannuzzi L, Inesi A, Porrino A, Pigini A, Thione L,Study of the performance of metal oxide arresters for high voltage systems Cigré 31st session, Paris, Paper no. 33-14, 1986.

362 Sparrow L, Doone RM,UK experience in the investigation of the pollution performance of metal oxide surge arresters, Cigré 32nd session, Paris, Paper no. 33-07, Aug. 28 - Sept. 3, 1988.

363 Bargigia A, Mazza G, LeRoy G, Rousseau A, Sparrow L,Behaviour of metal oxide surge arresters under different environmental conditions, Cigré 32nd session, Paris, Paper no. 33-14, Aug. 28- Sept. 3, 1988.

364 Sparrow LJ,Artificial pollution tests for surge arresters- the underlying characteristics of polluted insulators, NG RDC Report, NoHVT/0066/TAN919, 1991.

365 Feser K, Koehler W, Qiu D, Chrzan K,Behaviour of zinc oxide surge arresters under pollution, IEEE Trans. on Power Delivery, Vol. 6, no. 2, April, 1991, pp. 688-695.

366 Vitet S, Stenstrom L, Lundquist J,Thermal stress on ZnO surge arresters in polluted conditions: Part I: Laboratory test methods, IEEE Trans. on Power Delivery, Vol. 7, No. 4, Oct., 1992, pp. 2012-2022.

367 Vitet S, Schei A, Stenstrom L, Lundquist J,Thermal stresses on ZnO surge arresters in polluted conditions: Part II: Field test results, IEEE Trans. on Power Delivery, Vol. 7, No.4, Oct., 1992, pp. 2023-2036.

368 Garasim SI, Kadnikov SN, Redrugina MN, Usov BB, Shur SS, Vokalek J, Sklenika V,Thermal modes of metal oxide arresters under normal and polluted conditions, calculated and measured, Cigré 34th session, Paris,Paper no. 33-202, 1992.

369 Verma MP, Petrusch W, Weck KH, Brilka R, Gausmann RD, Hudasch M, Schaper D, Schreiber H, Solbach HB, Weinmann T,Long term performance of metal oxide arrester at operating voltage, Cigré 34th session, Paris, Paper no. 33-204, 1992.

370 Bargigia A, DeNigris M, Pigini A, Sironi A,Definition of testing procedures to check the performance on ZnO surge arresters in different environmental conditions, Cigré 34thsession, Paris, Paper no. 33-206, 1992.

371 Bargigia A, DeNigris M, Pigini A, Sironi A,

Comparison of different test methods to assess the thermal stresses of metal oxide surge arresters under pollution conditions, IEEE/PESWinter meeting, Paper No. 92 WM 231-1 PWRD, 1992.



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372 Vitet S, Louis M, Schei A, Stenstrom L, Lundquist J,Thermal behaviour of ZnO surge arresters in polluted conditions, Cigré 34th session, Paris, Paper no. 33-208, 1992.

373 IEEE/ANSI C62.11 1987,IEEE Standard for metal oxide surge arresters for AC power Circuits, 1987.

374 IEC Standard 60099-4, Amendment 1,

Artificial pollution test with respect to the thermal stress on porcelain-housed, multi-unit metal-oxide surge arresters, April 1998.375 Swift DA,

Pollution flashover performance of various types of high voltage AC Insulators, 8th International Symposium on High VoltageEngineering (ISH), Yokohama, Japan, paper 47.15, Aug. 23-27, 1993.

376 Ely CHA, Kingston RG, Lambeth PJ,Artificial and natural-pollution tests on outdoor 400 kV substation insulators (Also same volume, but July a letter from Ely andLambeth entitled 'Further work on pollution characteristics of 400 kV substation insulators'.), Proc. IEE, Vol. 188, No. 1, Jan., 1971.

377 Swift DA, Hoch DUnpublished information.

378 Houlgate RG, Swift DA,Polymeric insulators: AC flashover performance under salt-pollution of new and naturally aged units compared to porcelain, 6thInternational Symposium on High Voltage Engineering (ISH), New Orleans, USA, paper 47.30, Aug. 28-Sept.1, 1989.

379 Naito K,Insulators for UHV Transmission, Int. Seminar on EHV/UHV Power Transmission, New Delhi, 1994.

380 EPRI,Transmission line reference book. HVDC to ±600 kV, Book.

381 EPRI TR - 102764 HVDC,Transmission Line Reference Book, Book, Sept, 1993.

382 Naito K,Strength of polluted insulation, Cigré study committee 33 conference, Brazil, May, 1981.



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