antti keskinen strategy for medium voltage cable …€¦ · computing and electrical engineering...

ANTTI KESKINEN STRATEGY FOR MEDIUM VOLTAGE CABLE CONDITION MANAGEMENT Master Of Science Thesis

Examiner: Professor Pekka Verho The examiner and the subject was approved in the Faculty of Computing and Electrical Engineering council meeting on 9 March 2011

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ABSTRACT TAMPERE UNIVERSITY OF TECHNOLOGY Master of Science Degree Programme in Electrical Engineering KESKINEN, ANTTI: ‘‘Strategy for medium voltage cable condition management” Master of Science Thesis, 83 pages, 4 Appendix pages March 2011 Major: Power Systems and Market Examiner: Professor Pekka Verho Keywords: Medium voltage cable, cable diagnostic, condition management, profitability, operation management In the year 2006 Vattenfall Verkko Ltd. (later VFV) started developing a weatherproof network. For this reason, cabling has become a prevailing construction method. The cabled medium voltage network has formed only a small part of the whole network earlier, but the situation will be changing quickly in the future. This creates the need for systematic condition management of the cable network. The purpose of this thesis has been to analyse and bring forth suitable methods, which can be used in the cable network’s condition management, from inside the Vattenfall concern and elsewhere in the world. The suitable methods have been examined by benefit calculations. The profitability analyses have been implemented through the network business result’s formation, when it has been possible to verify a straight connection between cable diagnostic and result of the network business. In the case of VFV, the distribution network is located within quite a large area in geographical aspect and this causes challenges for the development of the cable network condition management strategy. The geographical factor has been taken into account in the benefit calculations, and the analysis has been made for different type of areas. The division of these areas is based on the reliability criteria which can be regarded as a meter for the customized reliability and which give planning criteria in the future. The construction of the network, construction conditions, customers and their placing depends a lot on the area, as the condition management strategy establishment has to be based on the prevailing conditions of the each area. In rural areas, the construction of the network creates a challenge for the achievement of a long life time for the cable, which is due to the challenging construction conditions. For this reason, quality verification after the installation is emphasized in condition management in rural areas. Other condition management operations have to be tried to eliminate by the structure of the network because the cable diagnostic utilization is unprofitable in rural areas at the moment.

By cable diagnostic pecuniary advantages in urban and city areas can be achieved, whereby the condition management strategy differs in the case of rural areas. The strategy can be implemented in e.g. primary substation level. The operations which will be performed during the life-cycle are based on profitable analysis, structure and the age of the network. The operations management must be integrated in the network information system, so that the indirect costs can be minimized and operations become more profitable.

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TIIVISTELMÄ TAMPEREEN TEKNILLINEN YLIOPISTO Sähkötekniikan koulutusohjelma KESKINEN, ANTTI: “Keskijännite kaapelin kunnonhallinta strategia” Diplomityö, 83 sivua, 4 liitesivua Maaliskuu 2011 Pääaine: Sähköverkot ja -markkinat Tarkastaja: Professori Pekka Verho Avainsanat: Maakaapeli, kaapelidiagnostiikka, kunnonhallinta, kannattavuus, toiminnanohjaus Vuonna 2006 Vattenfall Verkko Oy aloitti säävarman verkon rakentamisen. Tämän myötä verkon vallitsevaksi rakennustavaksi on tullut kaapelointi, eli aina kaapeloidaan kun se on mahdollista. Tällä hetkellä kaapeloitu keskijänniteverkko muodostaa vain pienen osan verkkomassasta, mutta tilanne tullee muuttumaan nopeasti, mikä puolestaan aiheuttaa tarpeen kaapeliverkon systemaattiselle kunnonhallinnalle. Työn tavoitteena on ollut tutkia ja tuoda kaapeliverkon kunnonhallintaan sopivia menetelmiä konsernin sisältä ja muualta maailmasta. Löydettyjä menetelmiä on tarkasteltu kannattavuuslaskennan avulla. Kannattavuustarkastelut on toteutettu verkkoliiketoiminnan tuloksen muodostumisen kautta, jolloin on voitu todentaa kaapelidiagnostiikan yhteys verkkoliiketoiminnan tulokseen. Vattenfall Verkko Oy:n jakeluverkko sijoittuu maantieteellisesti melko laajalle alueelle, jolloin haasteet keskijännitekaapeliverkon kunnonhallinta strategialle kasvavat. Kannattavuustarkasteluissa onkin otettu huomioon tämä maantieteellinen tekijä, jolloin tarkasteluita on tehty erityyppisillä alueilla. Tarkasteluissa käytetty aluejako perustuu toimitusvarmuuskriteerien mukaiseen aluejakoon, jota voidaan pitää asiakaskohtaisen toimitusvarmuuden mittarina ja josta saadaan verkon suunnittelukriteerit tulevaisuudessa. Verkon rakenne, rakentamisolosuhteet, asiakkaat ja asiakkaiden sijainti poikkeavat siis eri alueilla verkossa, jolloin kunnonhallinta strategian luominen täytyy perustua alueella vallitseviin olosuhteisiin. Haja-asutusalueella verkon rakentaminen verrattain vaikeissa olosuhteissa luo haasteen kaapelin pitkän elinkaaren saavuttamiselle. Tällöin kunnonhallinta on painotettava elinkaaren alussa asentamisen laadun varmistukseen. Muu elinkaaren aikainen kunnonhallinta on pyrittävä eliminoimaan verkon rakenteen avulla, koska kaapelidiagnostiikan hyödyntäminen ei ole taloudellisesti järkevää haja-asutusalueella ainakaan tällä hetkellä. Taajama- ja kaupunkialueilla kaapelidiagnostiikalla voidaan saavuttaa taloudellisia etuja, jolloin kunnonhallinta strategia muodostetaan eritavalla kuin haja-asutusalueen tapauksessa. Strategia voidaan toteuttaa esimerkiksi sähköasematasolla. Elinkaaren aikaiset toimenpiteet määräytyvät kannattavuustarkastelun, verkon rakenteen ja iän perusteella. Toiminnanohjaus tulee integroida verkkotietojärjestelmään, jolloin kunnonhallinnan välilliset kustannukset saadaan minimoitua ja toiminta entistä kannattavammaksi.

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PREFACE This work was one part of the bigger Smart Grids and Energy Markets (SGEM) project, where the Vattenfall Verkko Ltd. is involved. SGEM-project consists of different kinds of work packages, which this work belongs packages 2.1 Large scale cabling in distribution network and 4.5 Condition management as real-time process. The supervisor of this work has been worked professor Pekka Verho from Tampere University of Technology. I want to give thanks for him to interesting conversations, which I have to go through with him during this work. By these conversations I got tools to solve the hardest part in my work successfully.

From Vattenfall Verkko Ltd., where I got the possibility to carry out my thesis, I want to give thanks for all team of strategic network planning and “substation brothers”. Thanks belong to you from good working environment and expert comments. Special thanks belongs to Heikki Paananen who was the controller of my work. You gave me great opportunities to represent and to be involved in events, where rarely meet thesis workers. I also want to thanks my superior Sauli Antila for that you gave me possibility to work good team and to learn a lot from operations in the distribution network company.

I am also very grateful for my common-law wife Jenni from good, understanding and encouraging support which from you I got during my studying. Thanks also to my children Ella and Hilla which gave the counterbalance to the school.

Tampere 21.3.2011 Antti Keskinen

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TABLE OF CONTENTS

1. Introduction ............................................................................................................... 1 2. The basic features and needs of the cable condition management ............................ 3

2.1. Construction strategy ........................................................................................ 4 2.2. Medium voltage cable ....................................................................................... 5 2.3. Medium voltage cables accessories .................................................................. 6 2.4. Installation and installation methods ................................................................. 8 2.5. Deterioration and damaging mechanisms ....................................................... 10

2.5.1. Mechanical and installation damage .................................................. 10 2.5.2. Operational damage ........................................................................... 11 2.5.3. Age-related deterioration ................................................................... 11

2.6. Statistics .......................................................................................................... 12 3. MV cable testing ..................................................................................................... 15

3.1. Test methods to check the quality of installation ............................................ 16 3.1.1. Insulation resistance test .................................................................... 16 3.1.2. Sheath integrity test ........................................................................... 16

3.2. Measuring tools to assess condition of MV cable and accessories ................. 18 3.2.1. Dielectric spectroscopy measurements .............................................. 19 3.2.2. Dissipation factor measurements ....................................................... 20 3.2.3. Partial discharge test (PD) ................................................................. 22

3.3. Strategies for field testing medium voltage cables ......................................... 25 3.3.1. Field tests after the installation .......................................................... 26 3.3.2. Field tests after fault .......................................................................... 28 3.3.3. Field tests and diagnostics before reinvestment ................................ 29

4. Life cycle costing .................................................................................................... 31 4.1. Determination of LCC .................................................................................... 31 4.2. Economic evaluation methods for LCC .......................................................... 31

4.2.1. Simple payback method ..................................................................... 32 4.2.2. Discounted payback method .............................................................. 32 4.2.3. Net present value method .................................................................. 32 4.2.4. Internal rate of return method ............................................................ 33

5. Actions of cable condition management in real-time process ................................ 34 5.1. Permanent continuous monitoring system ...................................................... 35 5.2. Portable partial discharge monitoring ............................................................. 36

5.2.1. Online condition monitoring process ................................................. 37 5.2.2. Monitoring process implementation in large distribution network ... 39 5.2.3. Sensors’ positions in distribution network ........................................ 42

6. Strategy of the MV cable life-cycle maintenance ................................................... 44 6.1. Definition of strategy ...................................................................................... 44 6.2. Maintenance .................................................................................................... 44

6.2.1. Run to breakdown maintenance (RTB) ............................................. 46

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6.2.2. Preventive maintenance (PV) ............................................................ 46 6.2.3. Time based maintenance (TBM) ....................................................... 46 6.2.4. Condition-based maintenance (CBM) ............................................... 47 6.2.5. Reliability-centred maintenance (RCM) ............................................ 47 6.2.6. Total productive maintenance (TPM) ................................................ 47

6.3. Optimal maintenance ...................................................................................... 47 7. Key figures and results formation in the network business .................................... 49

7.1. The reliability key figures in delivery ............................................................. 49 7.2. The interruption costs formation ..................................................................... 50 7.3. Cost of cable diagnostic .................................................................................. 51

7.3.1. Cost of investment ............................................................................. 51 7.3.2. Cost of operation ................................................................................ 51 7.3.3. Cost of interruption ............................................................................ 52

7.4. Allowed return on network business ............................................................... 52 8. Benefit calculations ................................................................................................. 55

8.1. Cost and profitability of the portable part time online diagnostic .................. 55 8.1.1. Suitability for Rural areas .................................................................. 57 8.1.2. Suitability for Urban areas ................................................................. 58 8.1.3. Suitability for City areas .................................................................... 60

8.2. Cost and profitability of the permanent online diagnostic .............................. 63 8.2.1. Life cycle cost of the permanent online diagnostic ........................... 63 8.2.2. The profitability of permanent online diagnostic .............................. 65

8.3. Cost and profitability of the off-line diagnostic .............................................. 66 8.3.1. The cost of off-line measurements .................................................... 67 8.3.2. The profitability of off-line measurements ........................................ 68

8.4. Conclusion of the benefit calculations ............................................................ 71 9. Operations management .......................................................................................... 73

9.1.1. Cable database ................................................................................... 74 9.1.2. Cable condition assessment ............................................................... 74 9.1.3. Prioritization with the economical aspect .......................................... 75

9.2. Planning of the maintenance operations in the cable network ........................ 76 9.2.1. Strategy for rural areas ....................................................................... 77 9.2.2. Strategy for urban and city areas ....................................................... 78

10. Conclusions ............................................................................................................. 80 10.1. Conclusions ............................................................................................... 80 10.2. Further study ............................................................................................. 81

References ....................................................................................................................... 84 Appendix 1 - ................................................................................................................... 89 Appendix 2 - ................................................................................................................... 91 Appendix 3 - .................................................................................................................... 92

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ABBREVIATIONS AND NOTATION AXALL-TT Three phase medium voltage cable CAIDI Customer Average Interruption Duration Index CBM Condition-based maintenance

DC Direct current DCO Disadvantage caused by electricity supply outages DEA Data envelopment analysis ERP Ethylene propylene rubber HFCT High frequency current transformers IEC The International Electrotechnical Commission IRR Internal rate of return LC Leakage current LCC Life cycle costing LLLP Low loss linear permittivity MAIFI Momentary Average Interruption Frequency Index MV cable Medium voltage cable NIS Network information system NPV Net present value PD Partial discharge PDIV Partial discharge inception voltage PDEV Partial discharge extinction voltage PE Polyethylene PILC Paper-insulated lead-covered cables PV Preventive maintenance RCM Reliability-centred maintenance RMU Ring main unit RNA Reliability based network analysis RTB Run to breakdown maintenance TBM Time based maintenance TLC Transition leakage current TPM Total productive maintenance SAIDI System Average Interruption Duration Index SAIFI System Average Interruption Frequency Index SFA Stochastic frontier analysis

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VDP Voltage dependent permittivity VFV Vattenfall Verkko Oy VLF Very low frequency XLPE Cross-linked polyethylene ε’ Imaginary part of complex permittivity ε’’ Real part of complex permittivity

Loss angle Probability distribution C Capacitance C flow Cash flow during the year C investment Initial investment cost CLCC The sum of the costs of permanent online diagnostic system CINV The costs of control units and sensors CT The costs of training and installation CSYNC The costs of synchronization CM The cost of monitoring CRV The residual value of the system C savings Annual operating savings Ct The net cash flows in year t C uncovered Unrecovered cost at start of the year I Current IR The Resistive component of the current IC The Capacitive component of the current nj The number of the customers who experience the

interruptions i Ns The total amount of customers R Resistance r The rate of interest tij The time without electricity that customers j have to spend because of the interruptions i t Year T Last year of the period T before Year before full recovery T payback Payback time in years U Voltage U0 Nominal voltage

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1. INTRODUCTION

During the last few years, the reliability of the electricity supply has become a subject of discussions at many different levels of society. The reason for this has been storms and snow loads caused by harsh winters, which have caused problems in the electricity distribution. On the other hand, the customers’ requirements of the quality of the electricity supply have increased and the control of the network business has become tougher. It has caused a need for the development of the operations and the search for new network solutions. In order to meet these challenges, Vattenfall Verkko Ltd. has chosen large scale cabling as its building strategy of the network which aims at weatherproof network and comprehensive long-term improvement of the network. At the moment, a big part of the medium voltage network consists of overhead lines and the share of cables is relatively small. In the case of the overhead lines, the need of the replacement investment will be voluminous in the near future and this will change the structure of the network quite fast. The need of replacements is due to the age structure and the mechanical condition of the overhead line network. Earlier Vattenfall Verkko Ltd. has carried out a master’s thesis, in which was determined the boundary conditions for the medium voltage cables which are now in use. The purpose of this thesis is to continue forward within the subject towards the condition management of the medium voltage cable network, which is the base for the networks’ asset management. It may have a huge influence on the result of the network business in the future, when the total lengths of the cable network increases. Many planning tasks in the network business are based on the technical and economic boundary conditions which under the optimization have to do. The development of the strategy for the medium voltage cable or cable network can also be considered to be included in these tasks. The planned operations forms’ safety, the structure of the network and its components determines the technical boundary conditions for the operations together with available cable diagnostic equipments, and the benefit calculations determines the economic criteria for the operations. The life time of the medium voltage cable is quite long. For this reason, the most important factors which may have an effect on the cable’s life time, and may cause maintenance operations, have been considered in this thesis. The factors which may have influence on the length of the life time of the cable network have been examined by statistical distributions. The examination has focused on the external and internal factors. After the identification the different types of measurement methods have been introduced, how suitable they are for the cable network and how they can be exploited for the conditions management in the cable network. The different types of field

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measurement strategies for different situations have been presented in chapter 3. These strategies are partly based on the methods which are used by Vattenfall Service in Sweden. In addition to these methods, the worldwide knowledge and supply of the cable diagnostic is also attempted to be determined in this thesis. Few service provider solutions suitability has been examined as a part of cable network real time process. The profitability of the cable diagnostic has been examined by benefit calculations. The different types of measurement methods have been considered in present cases of the moment and cases of the future network. The profitability analysis has been implemented by using result formation in the network business. By these calculations it is possible to determine the most important factors which have the biggest effect on the profitability of the cable diagnostic and the risk levels which are willingly accepted if nothing is done or the contrary if the operations fail. The benefit calculations also give good alignment, of what kind of condition management strategy is suitable for different kinds of reliability criteria areas. The creation of the condition management strategy for the cable network and what it requires for a Network Company have been considered at the end of this study. In addition, the condition management strategies have been established for cable networks, which are located in different kinds of reliability criteria areas. The main purpose of these strategies is to give guidelines for the condition management. The model from comprehensive operations management strategy is also presented, which allows conditions management implementation in the case of a large cable network.

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2. THE BASIC FEATURES AND NEEDS OF THE CABLE CONDITION MANAGEMENT

The life-cycle of a medium voltage cable is quite long and this might make medium voltage underground network condition management challenging. The condition management process can be regarded as starting when a strategic decision about the cabling of a medium voltage network is made. There are a lot different facilities which could affect the life-time, reliability and cost of an underground network. For example insulation properties of MV cable, installation methods, soil, loading rate, the structure of the network et cetera. The selection of a cable and its accessories can have great influence on the network’s life-cycle length and cost. Accurate and high-quality components reduce the need for maintenance and faults which are caused by manufacturing defects. In this case, the correct choice of materials can be regarded as one of the most important parts of the cabling process, which may also affect maintenance costs in the future. When the strategic decision of cabling is made, and the components which are used are defined in a higher level, the next steps in the process are design and installation. Designing can affect the cable installation conditions, and thus affect the cable with harmful external factors. The design can also be a harmful influence for internal stresses of the cable, e.g. a situation where cable is under rated and the load is high. The contractor's competence and responsibility is also an important factor that may affect the future maintenance of the cable. Installation errors and errors which cause damage to the cables can shorten their life significantly.

Cabling process and its components have been examined in the following. Also what have been examined are the factors that may cause the need for maintenance in cables and accessories.

2. The basic features and needs of cable condition management 4

2.1. Construction strategy

Vattenfall Verkko Ltd. has chosen large scale cabling as its building strategy of the network which aims at weatherproof network and comprehensive long-term improvement of the network. The new installation methods, cheaper prices of the components and installation work have brought the costs of cabling and overhead lines construction closer to each others. Figure 2.1 presents the cumulative length of the medium voltage cable network from the year 2005 to the year 2010, and in addition shows a prediction of the coming years’ developments in the case of Vattenfal Verkko Ltd.

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500

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2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

Year

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mul

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cab

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Figure 2.1 VFV’s accumulative medium cable length each year and estimation of the future.

The total length of the medium voltage cable network was about 1470 km at the end of year 2009, and it represents about 7 % of total length of the medium voltage network. A big part of that about 7 % was located in urban and city areas, where the density of the customers is quite high. Figure 2.1 shows that the structure of the network will change fast. If the total length of the network will remain almost constant and yearly replacement speed is 450 km per year, then the cabling rate has increased about 21 % in the end of year 2015. The strategic construction decision and the old cables’ location create the biggest input for the condition management strategy development for the medium voltage cable network.


2.2. Medium voltage cable

The one part of a cost effective and reliable distributions network cabling strategy is, to find a good medium voltage cable manufacturer. Manufacturer production methods, materials, construction solution and knowledge of cable testing might have a major impact on the quality of a cable network in a short-time and a long-time period. The cable also has to be easy to handle and highly durable. Other important features, which help cable installation, are low specific weight and high flexibility at low temperatures. Figure 2.2 represents the common MV underground cable construction used in Vattenfall Verkko Ltd.

Figure 2.2 AXAL-TT PRO 12/20(24) kV MV cable from Ericsson. [1]

The conductor is made of aluminium and it is watertight in longitudinal direction. The conductor shield and the insulation shield are semi conducting and extruded. Longitudinal water tightness has been created by using swelling powder and swelling yarn that prevent the spread of water in the whole design as cross sectional water tightness consists of aluminium tape which is glued to the outer sheath. AXAL-TT PRO cable’s insulation is dry cured cross-linked polyethylene, commonly abbreviated XLPE. Outer sheath has been made of black polyethylene composite with a hard outer layer and an impact absorbing inner layer. Outer sheath also includes meter marks and Kevlar tear threads to simplify installation and maintenance work. [1] Nowadays the most common insulate material in MV cables is XLPE, but there are also older cables especially in urban areas, where cable insulation material is paper and oil, known as PILC cables. At the moment only about 6,6 % of VFV’s distribution


network is cable and a very little percent of this is PILC cable. Therefore, it is possible to limit this examination only to XLPE insulated cables. XLPE cable’s benefits are that it is more cost efficient in operation than thermoplastic analogues, because it provides better reliability and higher operating temperature than the thermoplastic materials.[2] If compared to older impregnated paper systems, XLPE cable also has lower environmental and maintenance requirements.

The major problem associated with medium voltage XLPE insulated cables is deterioration by water trees, and it is sometimes the main reason for insulation failure in XLPE cables after a long service period.

2.3. Medium voltage cables accessories

In the aspect of cable networks condition management, joints and terminations have an important role. Joints and terminations are very often the weakest points in a cable network, because the stress of electric field is bigger than elsewhere. Joints and terminations are also likely sources of defects leading to cable system failures. Even if the manufacturing process of joints and terminations are high quality, their failure rate is still high, due to their manual assembling. [3] In figure 2.3 below are presented joints and terminations portion of MV cable network failures.

Figure 2.3 Joints and terminations portion of MV cable network failures. [4]

Figure 2.4 presents electric field distribution without field controlling. Figure 2.5 presents electric field distribution with field controlling in the end of the cable. Figure 2.4 shows, that the stress of electric field hitting a smaller area than figure 2.5. The bad electric field control is one of the main reasons why termination or joint fails.


Figure 2.4 The electric field distribution without field controlling. [5]

Figure 2.5 The electric field distribution with field controlling. [5]

Manufacturers offer wide range of different types of joints and terminations for different construction and maintenance situations. The joint is as reliable as normal cable, if the contractor’s competence to do this is excellent and the working environment is clean. Figure 2.6 illustrates weak places and typical partial discharge areas of termination and joint. As shown, the areas are the right ends of the cable where the stress of the electric field is the biggest.


Figure 2.6 The most common partial discharge sources in terminals and joints. [6] Consequently, improper stress control is the main reason which leads to failures in cable termination. Other risk factors may be tracking, erosion or weathering of external leakage insulation and ingress of moisture. The joints have almost the same factors as termination, but voids in insulation and inadequate insulation may be added to the list. [7]

2.4. Installation and installation methods

A large part of cable damages occur in the cables’ installation situation. Another risk factor is the challenging and variable soil in Finland. In the past there was only one installation method and it was normal digging. The advantage of digging is that backing sand can be used when filling the hole. In this way it is possible to avoid harmful rocks and the effect of earth freezing. It is also easy to control deepness of cable installation by using the digging method. The digging method has its own risks, especially when the soil contains big rocks. It is difficult to obtain smooth trench bottom, which may lead to a situation where water will rinse the back sand away from under the heavy filling earth. If the filling earth contains rocks, it may cause damage when the filling earth presses on the cable. [8] In figure 2.7 non electrical damage portion of MV cable network failures are presented.


Figure 2.7 Non electric damage portion of MV cable network failures. [4]

Trench digging by using an excavator is quite slow and the growth interest of cabling has caused the need to find new alternative installation methods. Faster installation and cost structure of cable networks construction, which are very contractor-oriented, have also influenced the interest in new methods of installation.

At the moment, the most widely used installation method is cable ploughing straight to ground and below the pilots experiment is new earths sawing method. Cable ploughing means that at the same time when the machine ploughs the ground, the cable is being directed in the channel. The ploughing method is a fast but a blind method. It is impossible to see how the cable settles in its channel. In Figure 2.8 cable ploughing and the cable plough are presented.

Figure 2.8 The cable ploughing and cable plough. [9] Today, about 50 % of VFV’s cable installation is made by ploughing. The success of ploughing depends a lot on the contractor’s competence. If the contractor has good ploughing skills, then the ploughing can be even a better installation method than digging. Ploughing requires precise environmental and pre-ploughing assessment from the contractor [8].


Figure 2.9 below shows two solutions which can be used in making the cable trench.

Figure 2.9 Earth saw [10] and earths cutter. [11] New solutions allow the achieving of a better cable trench than before. The earth saw is possible to be used throughout the year in cable trench making without loss of frost.

2.5. Deterioration and damaging mechanisms

The deterioration of dry cured XLPE insulated MV cables can be divided into three different categories, mechanical, chemical and electric deterioration. This kind of cables insulation does not reach the final state immediately after the manufacturing process. It can take years for the insulation structure to be stable and this exposes the XLPE cable to harmful effects which can be mechanical, chemical or electrical. Damaged cable insulation is weaker to resist partial discharge because the electric field stress is bigger than in normal state. Partial discharges are very harmful for XLPE insulation. The chemical deterioration weakens the polymer insulation. In chemical reaction temperature, oxygen and radiation are present. As a result of chemical reaction long polymer chains break which is known as depolymerisation. There can also be new cross-linking bridges formed and it makes isolation more brittle. [12] The electrical deterioration happens as a result of water trees which generate electrical trees and lead to partial discharge. The most common electrical deterioration is a local effect. Low electric field intensity and long development time are common for electrical degradation mechanisms in medium voltage cables [12].

2.5.1. Mechanical and installation damage

Components of MV cable network are very vulnerable during its operation chain, which includes manufacturing process, storing, transporting, handling and installation. Carelessness in cables different operation may lead to cuts, scrapes, too large sidewall


force and water penetration inside the cable. These undesirable damages may affect immediately and make cable energizing impossible or the damages affects long-time period in cable insulation and creates partial discharge which finally leads to failure. Damage to the outer sheath and cross sectional water tightening may allow water to enter the space between the outer sheath and the insulation shield. This may have a corrosive effect to the aluminium screen. If water has entered inside the cable structure and the internal pressure forces to carry it along the cable, the water may create conductive paths and this may lead to short circuit and failures. [3] Damage suffered by digging may also be included in this mechanical and installation damage category.

2.5.2. Operational damage

Overload, short circuit currents and extensive load changing are known as effects, which may cause operational damage. Extensive load changing causes temperatures changing and it leads to both expansion and contraction of the cable. This can cause conductor shields hardening and cracking. Extensive load changing has also harmful effect on cable system terminations and joints. In the interface of insulation and semi conducting conductor shield or insulation shield may arise voids and gaps, which can lead to partial discharges. Overloads and short circuit currents bring high temperatures, which are damaging for the cable systems components. These big temperature changes are more harmful than the changes of the load cycling. High temperatures create a risk that the cable system fails, it can damage the insulation and change its geometry. Insulation geometrical changes lead to a deterioration of dielectric strength. [3]

2.5.3. Age-related deterioration

The cable structure deteriorates over time. The Insulations and semi conductive shields attachment may deteriorate. This can lead to a situation where deterioration creates voids and gaps, which allows water penetration inside the cable. The moisture in the insulation may cause water treeing. The water tree generates when the moisture inside the insulation starts to move electric fields direction. There are two types of water trees which are called bow-tie and vented trees. Bow tie water trees initiate from impurities and voids within the bulk insulation and tend to grow in two directions. Bow tie trees reach a limiting length of some tens of μm, and do not have a significant effect on degradation at the low electric field stresses used in distribution cables [13]. A vented water tree arises at the interface between the semi conductive screens and insulation. It grows in only one direction and has more harmful effects on insulation than bow-tie water trees. If water trees grow enough they can change electric trees, which causes partial discharges and finally cable fails. [3]


2.6. Statistics

This chapter presents some MV cable statistics in Sweden. The medium voltage cables failure statistic has an important role in cable condition management. As has been revealed earlier, cables’ failing is the sum of many different factors. An individual explanatory factor is impossible to find and it makes condition management very difficult. Even determining a sinlge cable condition can be a difficult process, not to mention determining the bigger cable networks condition. However, historical information and cable statistics help to evaluate the condition of the cable network and this way supports decision making. On the other hand, it is to be understood that the materials which are used in cables and the manufacturing processes develop all the time, so direct connection cannot be established between new and old cables which are the basis of the historical data. However, rough estimates of possible behaviour of the cable can be made for certain insulation materials fault trends. Using these estimations, it enables preparing for maintenance, inspection or replacement operations. Following statistics are based on information collected in Vattenfall Distribution Sweden. The distribution networks large scale cabling has started earlier in Sweden than here in Finland. The statistics include construction volume and failure rates from Swedish cable networks. In this case, the main goal is to find some contexts which can help to understand MV cables failing and use it later in chapter five in determining strategy of medium voltage cable life-cycle maintenance. Figure 2.10 presents failure rate sorted by construction year for both PILC and XLPE cable. The red dots represent the total installed cable length for each year.

Figure 2.10 Failure deemed to be due to material or manufacturing defects, construction year1961-2000 Vattenfall Distribution Sweden. [14]


In figure 2.10 can be seen that the older cables failure rate is bigger than the cables which have been installed later than year 1994, but in some cases older cables are as good failure rate than later installed cables. Figure 2.11 below presents the absolute failure rate for XLPE and PILC cables.

Figure 2.11 Failure deemed to be due to material or manufacturing defects, number of failures by age of cable, Vattenfall Distribution Sweden. [14] The high absolute failure rate for XPLE cables with the age 0-3 years old are probably due to the huge amount of new cables laid during the years 2005-2008 [14]. If these youngest cables are left out of consideration, it seems that about 8-25 years old XLPE cables have the highest failure level in Sweden. Figure 2.12 presents different type of cables’ installation lengths and failure frequencies.

Figure 2.12 Failure deemed to be due to material or manufacturing, cable type, Vattenfall Distribution Sweden. [14]


It shows that there are few very critical cable types, e.g. AXKJ and FXKJ. The cables AXAL and AXLJ which are used in Finland have low failure frequencies, but the reason for this can be the cables short in-service time. It is difficult to find a straight context which explains cable failing, but when creating a reliability model for MV cable, the cable age and type are of some importance. Statistics before contained only the XLPE cables statistic from Sweden and failures deemed to be due to material or manufacturing, which contains the installation defects too. Figure 2.13 presents the whole XLPE cables underground systems failure profile.

Figure 2.13 Overall average underground distribution systems failures with XLPE cables. [7]

43 %

20 %

4 %

19 %

14 %

Physical damages Treeing in XLPE insulation Environment Fault cable termination and joints

Other (overload, short circuit effect surgevoltages)

15

3. MV CABLE TESTING

The medium voltage cable network forms a large part of the distribution company’s physical capital. The medium voltage network has a huge influence on interruption which customers suffer, because the biggest part of the interruptions is a result from medium voltage network faults. Defects cause harm to customers, and to the distribution network company. In the overhead lines network faults localization are much easier than in underground network. In some cases the overhead line network faults generation can be detected before the fault occurs. This can be achieved by using visual inspections, which can be done by a helicopter or walking and this way find the critical target e.g. trees over the line. Thus, trees can be removed before the fault happens. In the underground network causes of defects, the structure of network and the physical position are very different than the overhead lines network. It leads to the situation where normal overhead lines fault prevention methods are unusable. Also underground networks disconnector density is lower, and troubleshooting the calculated distance is more difficult to determine than overhead line network. The underground networks condition management is based on electric measurements, which are due to its physical location. The objective of the measurements is to provide information for the networks’ operator on the condition of the network. By means of measurement, it is attempted to determine e.g. the condition of the cables insulation, because the good condition of the insulation is a prerequisite for the functioning of the cable. This emphasizes the case of a three-core cable even more. Measurements are used to verify the condition of cables and aims to predict the remaining life-time. At the moment distribution network operators are very interested in diagnostic tools for cable systems condition management. Knowledge of the network’s condition helps to make the right maintenance and investment decisions.

3. Test methods to assess MV cable deterioration 16

3.1. Test methods to check the quality of installation

After the cable laying but before it is put into service, the quality of the installation has to be checked. At this moment VFV requires its contractor two quality checking tests; insulation resistance and sheath integrity test. Tests are required because the cables production, transportation, installation method or rocks and ground frost may cause damage in the cable structure. Upon installation, unnoticed defect may later become very costly, due to the cost of maintenance operation and supply interruption.

3.1.1. Insulation resistance test

Insulation resistance describes insulations magnitude between two phases or phase and earth. Effective insulation is the base of cable operating. Insulation resistance measurement can be performed only off-line. By measuring insulation resistance it is possible to detect short circuits, accidental earths and incorrectly installed or leaking joints.

The values obtained in the above tests should be recorded so that they are available for comparison purposes in the future. In failure situation insulation resistance test can be used indentifying the faulty conductors and fault classification. Faults can be divided into two categories; high and low resistive faults.

3.1.2. Sheath integrity test

By measuring sheath integrity, sheaths fault can be detected and localized before the bigger drawback. The sheath integrity test is recommended because the cables manufacturing, transportation and laying may cause defects. Sheath integrity measuring can be performed only off-line and it is suitable for polyethylene insulation cables. [15] Cable sheath tests are required for new cable installations to ensure that cable is continuous from end to end, and the cable is laid as planned and route cable joints are sound. Existing cables undergo sheath testing prior to returning to service following cable diversion or repair to ensure circuit integrity. [16] Sheath integrity test can be performed using the same test equipment as with insulation resistance test. Then the following interpretation rules which are presented in figure 3.1 can be used.


Figure 3.1 Cable sheath integrity test process and interpretation by using Megger. [15] Cable sheath testing voltage is 5 kV DC, if the voltage does not rise and stay there one minute there is a fault in outer sheath. If the cable does not pass the test, the fault has to be located and prepared.

Effective tool for sheath fault pre-location is SebaKMT Company’s product MFM 5-1 and fault location ESG-80 with pinpoint sticks. These products also allow performing sheath integrity test, but the difference is that this method measures leakage current. For faultless cable which has outer sheath of polyethylene, the leakage current value is about 1 μA/km and polyvinyl chloride sheath 1,5 μA/km 5 kV DC measuring voltages [17]. The table 3.1 presents interpretation rules for sheath integrity test leakages currents in cases where outer sheath is polyethylene.

Table 3.1. Leakage current values and interpretation rules 5 kV DC measuring voltages.

Measured leakage current x Interpretation x > 1mA/km Sheath fault

10 μA/km < x < 1mA/km Sheath fault is possible x < 10 μA/km Sheath Ok

Performing sheath integrity measurement the following things have to be noticed. The ends of the cable have to be dry and remain dry during the whole measurement, it cannot be performed earlier than seven days after the installation so that the ground gets settled down and the outer sheath over the AXAL-TT cables screen is semi conducting which means that it should not be in contact with earthing or other parts which could breakthrough during the measurement. In addition, some joints which include aluminium foil for cross sectional water tightness have been noticed to cause measurement errors.

Sheath integrity test

using by Megger

Measured insulation resistance

Insulation resistance > 500 MΩ

Insulation resistance < 5 MΩ

Sheath OK

Sheath Fault

Repair


3.2. Measuring tools to assess condition of MV cable and accessories

XLPE cables have a relatively long life-time and the investment costs are high. For that reason different measuring methods have been studied and developed to assess the condition of medium voltage cables and accessories. All the measurement methods have the same goal, to detect weak points and evaluate and schedule future investments. The target of investigation has been how to measure and locate partial discharges and water trees, which generates deterioration of cable insulation, especially XLPE cable network. Partial discharges in cable insulation, joint and termination have bad influence. When partial discharge occurs, it will corrode the insulating material in such a way that carbonized path will be formed and grow, then the electrical treeing will take place and lead to breakdown.[18] This chapter introduces alternative measuring methods, which could support older methods, what information measuring could give and the possibilities of their performance. Figure 3.2 presents the partial discharge process.

Figure 3.2 Partial discharge development processes. [19]

Microscopic spaces may be formed in insulation systems due to water tree growth, aging, installation or manufacturing defects. Continued stress and overvoltage can initiate PD in voids.

Heat and other forms of energy released by PD cause erosion of the internal surface of the voids

Continued erosion forms channels that develop into so-called electrical trees in the insulation

Continued PD produces further erosion until the electrical tree bridges the insulation

Insulation system failure


The alternative methods are very welcome especially later when cable network ages. Then the typical degradation processes have contributed the power cable insulation and its accessories. It is typical for aged and some cases for new joints and terminations that there arises an interface problem which finally leads to partial discharges. In accessories insulation bad hardening and conductor problems with overheating causes cracking which also leads to partial discharges. In figure 3.3 below all the most typical factors which cause partial discharges in MV cable network are presented.

Figure 3.3 Typical factors which cause partial discharges in MV cable network. From figure 3.3 can be seen that to manage cable networks’ condition, the factors which are presented in the figure have to be able to be diagnosed and localized. New supporting measuring methods, which are presented in the next three subchapters, can be divided into two parts on the bases of what is the target of the measurement. Dielectric spectroscopy and dissipation factor measurement are concentrated XLPE cables insulation condition and the partial discharge testing methods focus is in accessories.

3.2.1. Dielectric spectroscopy measurements

The dielectric response can be used to determine the condition of the insulation. It is a useful measurement technique to evaluate insulation deterioration caused by water trees in XLPE cables. This measurement must be performed off-line and it is non-destructive testing method, which means that the electric stress during the test does not reach the level higher than the normal service condition. Consequently, the risk of failure and accelerate of aging remains at a normal level.

Dielectric response can be categorized into four different groups. The first of which are the quite new and water tree free cables, which belong to LLLP group. The low loss linear permittivity response is characterized by an almost frequency independent capacitance. The loss tangent is low and has also very weak frequency

PD

Conductor’s

problems

Interface problems

Bad

hardening

Cracking

Overheating

Local field

concentration

Electrical

trees

Delamination

Insulation

voids

Water trees

XLPE insulation

Accessories


dependence. Both capacitance and loss tangent are independent of applied voltage, i.e. the insulation material is linear [20].

The second dielectric response type is the voltage dependent permittivity (VDP). This response type is the general type for cables, which contains water trees but the trees have not yet formed the whole insulation. The response not dependent frequency in this type and both capacitance, and loss tangent increase with increasing voltage.

The third response type is the transition leakage current (TLC). The leakage current occurs in high voltages levels. Water trees have reached the whole insulation and breakdown strength has decreased significantly. The last response type is leakage current (LC) response. This type has almost the same properties as TLC but the leakage currents already occurs in low voltage levels.[20] Following table 3.2 shows the typical values for different dielectric response types.

Table 3.2. Diagnostic criteria for XLPE cables using dielectric spectroscopy.

Assessment Dielectric response type

Permittivity values

Break down values

Strategy

Good LLLP ε’’ 4*U0 Retest in 5-10 years

Aged VDP ε’≥8*10-4 Δε’’≥1*10-4 Δε’≥2*10-4

2,5*U0


whose surfaces are separated by insulation material. In such a case, the perfect capacitors current is purely capacitive, which means that it is 90 degrees ahead of phase voltage. The impurities in cable insulation increase the currents resistive component when the current is not purely capacitive anymore. [21] For this reason a loss angle is formed in between of the purely capacitive current and the real current of the starting situations. This angle will also often be used for measurement of tan delta measurement name. Below in figure 3.4 are shown schematic models of the insulation, the currents and the angle.

Figure 3.4 Schematic models of the insulation, and currents phase diagram. [22]

So, the more impurities and defects the insulation contains, the more resistive it comes and the loss angle increases. The cable which is in a bad condition has a big loss angle.

Most commonly the measurement is performed at 50 Hz or 0.1 Hz. When 0.1 Hz testing method, also known as Very Low Frequency (VLF), is used much smaller size voltage sources than normal 50 Hz test can be used. Another benefit compared to normal 50 Hz test is, that low frequency is more efficient. Differences in the values are greater at lower frequency, which makes interpretation of results much easier. VLF dissipation factor is measured at nominal voltage U0 and also at 2U0 and the differential loss tangent is calculated as:

(1)

Table 3.3 below presents XLPE cable condition evaluation criteria using dissipation factor diagnostics.

Table 3.3. XLPE cable condition evaluation criteria using dissipation factor diagnostics. [23]

02tan U tan Assessment < 1,2*10^-3 < 0,6*10^-3 Good ≥ 1,2*10^-3 ≥ 0,6*10^-3 Aged ≥ 2,2*10^-3 ≥ 1*10^-3 Highly degraded

00 tan2tantan UU


In turn, this measurement method only tells us which cable is in good or aged condition, and requires testing of all cables. However, it can help a utility prioritize cable replacement. The dissipation factor measurement does not tell the location of the weak spot. It only gives an estimate of insulation condition of the measured part of the cable.

3.2.3. Partial discharge test (PD)

Partial discharge tests are one of the most commonly used test methods for medium voltage cable and its equipment. Partial discharge testing can be performed either on-line or off-line. Measuring tools small size, weight and costs are its advantages. Partial discharge test equipment is commercially available for many companies which produce measuring devices and applications. There are also a few companies which offer on-line partial discharge metering systems e.g. KEMA, HVPD Ltd. and KJ Dynatech Inc.

Partial discharge test is useful for extruded insulation types. XLPE cable insulation is one of the most sensitive to the destructive effects of electrical partial discharge activity, it is imperative that the network operator strives to operate their polymeric cable network discharge-free [24]. Partial discharges test provides a quick and quite a cheap solution for checking the quality of cable installation, because it can detect insulation defects that may have occurred during the cable laying. At the same time it is possible to find defects that may have occurred during the cable manufacturing process, but this is very rare because as all XLPE cable suppliers now must meet the requirements of < 5 pC level as per the IEC 60270 standard factory test. This means that any defects in the cable insulation such as voids or delaminating of the screens are detected during the quality control process in the factory which uses off-line partial discharge testing in a screened room in the factory’s laboratory. Partial discharge test is also used to detect the cable networks deterioration due to normal service operating conditions.

Partial discharge test is a predictive, non-intrusive and non-destructive testing method. Predictive means that the test indicates insulation degradation before the failure. Non-intrusive means, that it is possible to perform the test without interruption of service. The concept non-destructive is due to the fact that the test does not have destructive features such as over voltages or high voltage stress. Partial discharge activity is known to result in deterioration and erosion of the primary insulation in cables and most particularly at cable accessories such as joints and terminations. If partial discharge is not noticed and the cause of the partial discharge activity not repaired, it will result in failures, supply interruption, equipment damage and/or injury to personnel. When partial discharge measurements are performed off-line, the cable has to be disconnected from the grid and this is a huge disadvantage because the supply interruption is expensive for the Network Company. Off-line measurements advantages are that all disturbances which comes corona effect and partial discharge sources


originating outside the cable are eliminated [20]. In other words, the measured data interpretation is easier than on-line situation and also smaller defects can be detected. Another advantage compared to On-line is that off-line measurement enables to determine partial discharge inception and extinction voltages. In turn, determining the extinction voltage, the test voltage can reach as high as two and a half times the normal operating voltages and it increases the risk of failure. Partial discharge measurement off-line method should be used only after the installation or fault, because otherwise it causes supply interruption.

The new on-line technique for insulation condition testing of in-service cables was made possible by the development of inductive PD sensors, clip-on High Frequency Current Transformers (HFCT). These are attached to the earth bar/strap of the cable termination under test, with no outage required [25]. On-line measurement advantages are that during the test, cable or accessories operate under normal condition and the risk of failure does not increase as a result of high test voltages which are used in off-line methods.

A partial discharge does not occur all the time and for that reason the on-line monitoring possibility is a valuable diagnostic tool. Weeks’ or months’ monitoring allows the detection of partial discharge trends, which simplify decision making and its reliability. The biggest drawback of the on-line measuring is complicated interpretation of the results [20], but longer monitoring with partial discharge trends facilitates it. Even though any conclusions can be drawn from the results, they must be able to be interpreted in some way. As mentioned above, XLPE insulated cable has to be almost partial discharge free. It makes the measured data interpretation and partial discharge sources localization easier, because the cable itself is generally very reliable if it is properly produced and tested. Consequently, the vast majority of faults occur in manually installed cable accessories such as joints and terminations along the cable’s length.

As mentioned above, the biggest challenge in partial discharge measurement is the interpretation of measured data. For this reason, all information which the measurement can give has to try to be collected. The results of the partial discharge measurement can be divided into two groups. First group contains directly measured basic data e.g. PD level, PD inception voltage and PD extinction voltage. The results which belong to the second group are derived from results of the first group. Derived values are e.g. PD magnitude, intensity and mapping. By collecting all this information, it is possible to create so called fingerprint for cable section. This fingerprint can act as a reference value for a possible future condition monitoring cases or after the fault situation.

There are generally presented in some values for partial discharge measurement which facilitates interpretation of measured data and the failed component determining. The following table 3.4 presents some interpretation rules for partial discharge diagnostics for the cable.


Table 3.4. Interpretation rules for PD diagnostics on cables. [20]

PDIV < U0 > U0 PDEV < U0 > U0 PD magnitude < Typical values > Typical values PD intensity Low (e.g.5 pulses/period) PD pattern Less Harmful Harmful PD PD location Cables accessories Cable insulation PD mapping Scattered PD along cable PD concentrated at specific

location

In addition, there are some apparent charge values for cables insulation, joints and terminations, which supports interpretation rules and facilitates results’ analysis. Acceptable apparent charge values are presented in table 3.5 below.

Table 3.5. Typical values of PD apparent charge in cables and accessories. [14]

PD Location Type Apparent charge Cable insulation XLPE < 20 pC

Joints

Oil insulation 10 000 pC Oil/ resin insulation 5000 pC Silicone/ ERP insulation 500- 1000 pC

Terminations

Oil termination 6000 pC Dry termination 3500 pC Shrink/ slide-on termination 250 pC

As can be seen in table 3.5, the acceptable apparent charge depends on the type of the cable section component and its insulation material. Using measured results and the interpretation rules, it is possible to evaluate and categorize measured objects into three condition classes. The classes are introduced below in table 3.6.

Table 3.6. Condition class of cable section and the interpretation rules.

Condition class Interpretation First class Not ok, need to replace or repair.

Second class Not ok, need monitoring. Third class Cable section is ok

A decision diagram, which is presented in figure 3.5, can be used for determining condition classes.


Pd locate in accessories?

Pd located in accessories

Critical PD pattern/ intensity?

Critical PD pattern/ intensity?

First class Second class

No

No

No

No

NoNo

No

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

YesYes

PDIV/PDEV < operating

voltage?

Concentrated PD Mapping

Concentrated PD Mapping

Pd level > typical values for both cable

insulation and accessories

Pd level < typical values for both cable

insulation and accessories

Third class

Figure 3.5 Decision diagram for evaluation of cable condition using PD diagnostics. [20, 4]

3.3. Strategies for field testing medium voltage cables

The cable networks faults and quality control creates a need for the distribution Networks Company to have some kind of strategy for field testing. Clear goals and pre-made testing methods improve the efficiency of the operation in various situations. This reduces the time which the operation takes, and this way improves the quality of the service offered to customers. Good testing strategy also reduces the cost of faults repairing. The strategy which can be built depends on a lot of measuring tools and networks’ constructions. However, the main point of the field testing strategies are; the new cable network quality control, improving delivery performance in the existing network and support decision making in a reinvestment situation. The following three subchapters present three different strategies which are based on strategies used in Vattenfall Service Sweden.


3.3.1. Field tests after the installation

After laying the new cable, it should be tested. In this way, quality checking of installations can be made. Different parts of the cable network should be tested, including cable section, joints and terminations. Quality checking and testing ensures that the largest defects can be detected and prepared after the installation. Quality testing after the installation can be applied case by case. It is possible to perform either a strict type quality test or an extensive type quality test. At the moment, only strict type quality test is used here in Vattenfall Verkko Ltd. This strict quality test process ensures only the cable condition at the testing moment, but does not give any information which could be valuable for cable network condition management in future. Strict quality test contains an insulation resistance test and sheath integrity test. As mentioned before, insulation resistance test measures the magnitude of the insulation between two phases or between a phase and earth, and sheath integrity test is a test, which tells only, if the cable sheath is entire or broken. Both of these tests are valuable, but it is important for the life-time of the cable network to be as long as possible and this creates the base for extensive quality checking after the installation. However, there are a lot of commercial cable diagnostic measuring applications available, which can be used to extend the quality checking process at the moment. This new and extensive quality checking could contain at least the following measurements; off-line and on-line partial discharge and dissipation factors. By using these measuring methods, it would be possible to detect the defects, which are caused by installation mistakes even more effectively and determine the baseline values for a cable section. These values may be needed in condition management in the future. In addition, measurements confirm manufacturers’ promises of the quality e.g. almost partial discharge free XLPE cables. By using dissipation factor measurement, it gives an overview of the insulations’ condition. Later, this value could be compared with the new value and changes could be detected. Clear changes of the dissipation factor values can mean that the insulation is in a bad condition and this information could give reason for reinvestment. Another method is partial discharge measurement which allows creating the so called finger print for a cable section. At first, before connecting the new cable section in service, off-line partial discharge measurement could be performed. By using this method it could be possible to find even the smallest defects from the joints and terminations because the off-line methods’ accuracy is better than the on-line methods’. As the joints and the terminations are the most critical points in a cable section, the PD test is very useful in a cable network. Another partial discharge measurement could be performed after the cable is under service condition. On-line measurement would create the base for futures on-line PD measurements, which can be used in cable network condition management in the future. By using on-line partial discharge diagnostic methods, it is possible to find the networks’ weakest point and achieve condition based maintenance strategy. In the


next figure 3.6 the testing process after the installation is presented. Figure 3.6 presents the extensive quality test and the strict quality test which is used at the moment in Finland.

Extensive quality testingStrict quality testing

Insulationresistance test

Sheath integrity

test

Dissipationfactor

(Tan Delta)

Off-linePD

On-linePD

Figure 3.6 Quality testing process after installation. One option is that the extensive quality test is not performed after every installation. It could be performed in a situation where a new contractor, cable, joint or termination has been used. In this way, it is possible to find manufacturing or installation errors immediately. [26] Extensive quality testing should also be recommended to be used in situations where the strict measurement has revealed some problems. As mentioned, field tests after installation gives a lot of valuable information about the cable network and its condition. For this reason, all the information which comes from measurements should be collected. If this is not done, it could almost be said that the diagnostic measurements in the future are useless, because there are no base values. Collected cable network data could be saved into network information system or cable diagnostic data base, where it would be available later if problems arise in the network section. Figure 3.7 presents the information which could be collected from measurements, if both strict and extensive quality tests were performed.

On-linePD

measurement

Cable sections PD magnitude andintensity including

joints and terminations

Partial dischargeinception and

extinction voltages

Ohm valuesSomethingabnormal

revealed during the test )2(tan

)(tantan

0

0

UU

Insulation resistancetest

Sheath integrity test Dissipation factormeasurement(Tan Delta)

Off-linePD

measurement

Cable sections finger print

Cable diagnostic data base

Figure 3.7 Information of the measured cable section.


Collected network data can be valuable, and it provides important information to support decision making in reinvestment situations at the end of the cable’s life-time. On the other hand, comprehensive information of networks condition could be modelled in a network information system. It makes networks condition management easier, because a weak cable section can be seen visually. The possibilities of networks information system and cable diagnostic data base integration are returned to later in chapter five.

3.3.2. Field tests after fault

As mentioned before, fault locating and repairing in the cable network is a much slower process than in the overhead lines network. Indication of the fault becomes harder because the cable network’s physical location is under the ground, so the network’s visual inspection is impossible without digging. Hopefully, a big part of the cable network’s faults are caused by some external factors, e.g. careless ground digging so then the fault location is known. Everything does not go this way and the fault has to be localized by using other methods. In practice, cable network’s fault repairing process starts by the fault’s distance determining. After this, the fault has to be found by using some other method. One possible solution for fault distance determining is the pulse echo meter, which sends a pulse straight to place of the fault where it reflects back and this allows the determining of fault distance by using mathematical methods. As the distance is known, the specific place of the fault can be searched with a cable canon. When the specific fault place has been found the cable is dug up and the damaged section cut off. The next step in the repair process can be interpreted to depend on fault’s instigator and the cable section’s earlier operating condition. If the instigator which caused the fault is some single factor e.g. ground digging and this can be confirmed and cable section operating history is good, then there is no need for any measurements and the fault can be repaired. On the other hand, when the reason which has led to cable failing is not so clear and there is a doubt that there are other faults or faults which are developing, the following shows a way in which to proceed after the cable is cut-off. It is recommended to perform dissipation factor and partial discharge measurements on the cable, starting from the fault position to both directions. The dissipation factor gives an overview of the insulation’s condition and comparing it to earlier results which were measured after the cable’s installation is now possible. In turn, by using partial discharge measurement, especially now when it is possible to be performed off-line, it gives possibilities to achieve better accuracy than with on-line methods and detect even the smallest defects. If these measurements do not reveal anything new, the cable can be repaired. But if the measurement shows that a cable section is in bad condition, it is possible to try and find the weakest point of the cable section using by VLF withstand test, which is known as a


pass or fail test. When the cable does not pass the test, a new fault point was born, which has to be localized and the fault repairing process starts again. This is repeated as long as the cable can be accounted to be in a satisfactory state. On the other hand, even if some single factor would have resulted in the cable failure e.g. excavation and operating history would be good, it is possible to exploit this interruption and perform some diagnostic measurement. Especially in a situation, where an alternative connection could be used and thus the client does not expire any harm. In figure 3.8 field process after the fault is presented.

Dissipation factor and PD test

VLF withstand test

Clear fault mechanism?

Determine fault distance

Cable identifying and fault finding

Found new fault?

START

Yes

Yes

Ok

No

No

Strict quality test

Repair the first fault or faults

Figure 3.8 Field testing process after the fault.

3.3.3. Field tests and diagnostics before reinvestment

The timing of the cable reinvestment is a very difficult case, except in the cases where the cable is totally damaged and unusable. Most commonly a network owner has three different main parameters from which the most economical value is wanted to achieve. Firstly the network owner wants the existing network to achieve as long a life-time as possible. Prolonged life-cycle improves the profitability of the original investment, if this does not cause any additional costs. Secondly, the reinvestment cost is very high and if postponing the reinvestment is possible, it saves money. Thirdly is the cost of interruptions if the reinvestment is postponed. The purposes of the field tests are to optimize these three parameters. The field tests try to predict the cable sections remaining life-time or to prioritize the order of the replacement. For this purpose, appropriate measuring methods are dissipation factor and partial discharge measurements. As mentioned before the dissipation factor organizes


the cable into a right order, which is based on the insulation condition and partial discharge could give more specific information on an individual level. Figure 3.9 shows one possibility of how the diagnostic process could be performing before the reinvestment decision.

On-linePD scanning

On-linePD trending

Dissipation factor measurement

Not ok

Ok

Not ok

Not ok

Possibleinterruption

situationdo extensivequality test

Start

Repeat trendinge.g. after year

Ok

Not ok

Cable diagnostic data

Ok

Ok

OkReinvestment

Figure 3.9 Possible diagnostic process before reinvestment.

As can be seen, the diagnostic process starts with on-line partial discharge scanning. It is a fast method to find weak cable sections. A situation where PD scanning result is clear it may be considered to do the extensive quality test later during the possible interruption situation. If partial discharges which may be deemed to exceed the allowable limits are found, partial discharge trending is recommended. Trending can show the direction of partial discharge development. If there is no critical development, it can be concluded that the cable is in a good condition. In a situation where the development is fast, some off-line tests e.g. dissipation factor measurement are recommended to be done before the reinvestment decision. All data which the measurements offer, should be collected from every case irrespective of the result being acceptable or not, and especially in cases where the decision is to postpone the reinvestment.

31

4. LIFE CYCLE COSTING

4.1. Determination of LCC

In the publication [28] the concept of life cycle costing is defined as follows; “Life cycle costing is the process of collecting, interpreting and analysing data and applying quantitative tools and techniques to predict the future resources that will be required in any life cycle stage of a system of interest.” The life cycle costs which are the results of this analysis contains the costs of the acquisition and also other costs that are caused during the life time of the target product, service or process. The main purpose of the life cycle costing is to be a tool which supports managers’ decision making in analytical processes. However, it should be remembered that an investment decision cannot be solely based on the results of the life cycle costing. In order that LCC can be used, overall picture has to be created of the case which is under investigation. The whole life cycle costing process is presented in figure 4.1 below. Because the LCC is an analytical process it contains a lot of different types of data and assumptions in many cases. As there are different types of data usually used in management accounting, the input data has to be changed into the same format which allows calculation.

Life cycle costing result

Life cycle costingmodel

Input data changing

the same format

Existing data Risks

Assumptions

Figure 4.1 Life cycle costing process. [28]

After the input data formation and the risk levels determination LCC model can be created which gives LCC results. Final result contains cost estimation for object with assumptions and financial implications [28].

4.2. Economic evaluation methods for LCC

The most interesting and most important part of life cycle costing is the calculation phase. There exist many different methods for this purpose, which have their advantages

4. Life cycle costing 32

and disadvantages. All methods are not useful in all situations, because different calculation methods may give the result in a different unit. In the following calculation methods are presented which can be used when evaluating economic value for LCC.

4.2.1. Simple payback method

The simple payback method calculates the time which is required to return the initial investment. It allows comparing which investment has the shortest payback time and also gives rough estimation for investment profitability. The advantages of this method are that it is a quick and an easy calculation, and the results are easy to interpret. Disadvantages are that it does not take into account inflation, interest or cash flows which may come during the life time. If annual cash flows are equal, the payback period is found by dividing the initial investment by the annual savings. The equation of simple payback method is presented as following:

savings

investmentpayback C

CT (2)

Where paybackT is payback time in years, investmentC is initial investment cost and savingsC is

annual operating savings.

4.2.2. Discounted payback method

This method is almost the same as the simple payback method but it takes into account value of the time, which is an advantage of discounted payback method. This method should be used only as a help for the examination but the making of the decision should not be established through this method only. Discounted payback method does not take into account cash flows which come outside the payback period. The equation of discounted payback method can be written as following:

flow

uncoveredbeforepayback C

CTT (3)

Where paybackT is payback time in years, beforeT is year before full recovery, uncoveredC is

unrecovered cost at start of the year and flowC is cash flow during the year.

4.2.3. Net present value method

Net present value NPV is the result of the application of discount factors, based on a required rate of return to each year’s projected cash flow, both inflow and outflow, so

4. Life cycle costing 33

that the cash flows are discounted to present value. In general the positive net present value means a profitable investment. Consequently the best choice between two competing alternatives is the one with the biggest positive net value present. The advantage of this method is that it takes into account the time value of money and it uses all available data. Sometimes the amount of data can make it difficult to interpret. Its disadvantage also is that the method is not usable when the alternatives have different life lengths. The equation of net present value method can be written as following:

01

t

1C

rCNPV

T

tt

(4)

Where t is year, 0C is initial investment cost, tC is the net cash flows in year t, r the rate of interest and T is the last year of the period.

4.2.4. Internal rate of return method

The internal rate of return (IRR) calculates the discount rate value on what net present value is zero. In the [29] internal rate of return is defined as follows: “The internal rate of return is the actual rate of return expected from an investment”. This method is usable only if the investments will generate an income. The equation of internal rate of return method can be written as following:

01 01

t

Cr

CT

tt (5)

From equation (5), r is to be calculated by employing trial and error method, which equals as internal rate of return.

34

5. ACTIONS OF CABLE CONDITION MANAGEMENT IN REAL-TIME PROCESS

To achieve the best possible reliability a real-time condition management should be considered. As known, distribution network already includes a lot of automatics and information technology. Because the cabling rate has been very low so far, there has not been the exceptional need or stimulation to research condition management tools of cable real-time process. On the other hand, the use of cable networks have been considered now in urban areas, where there is almost always an alternative connection that can be used in a fault situation, but the situation changes dramatically when looking at this from the point of view of rural areas. Outages are expensive for distributions Network Company. Preparing for future faults could be done by using real-time process metering application. Faults detecting could be faster and more accurate than without controlling. Especially in area where there are no alternative connection possibilities, accurate fault localizing system is very useful. This chapter presents two different types of methods for two different purposes; what are commercially available now and what can be used as a part of the real-time process.

In principle, medium voltage cable online monitoring means that the cable operates normal voltage during the measurement. On-line monitoring is based on partial discharge measurement and it is only an application for condition assessment of the cable network, which exists and is commercially available at the moment. Its main purpose is to offer the possibility of the cable operations and condition monitoring without the electricity supply being interrupted or disrupted. To make this possible, the measuring equipment must be installed and monitoring carried out without interruption. In this way, the installation does not cause any harm and the measurement is not a disturbance to clients.

Disturbance free operation and additional information of the condition of the cable network improves the reliability and thereby reduces the cost due to interruptions. In the best case, real-time monitoring of the network is thus a tool for condition monitoring, which can be affected by the amount of unplanned interruptions and thus reduces down time of the network which always means the loss of income. By using real-time condition monitoring it is also possible to identify and prioritize the faulty cable sections which have to be repaired or replaced. In this way, it is possib

antti keskinen strategy for medium voltage cable …€¦ · computing and electrical engineering...

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