study of dissolved gas analysis under electrical & thermal stresses for oil

8/12/2019 Study of Dissolved Gas Analysis Under Electrical & Thermal Stresses for Oil

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Study of Dissolved Gas Analysis under Electricaland Thermal Stresses for Natural Esters used in

Power Transformers

A thesis submitted to The University of Manchester for the degree of MPhil in the Faculty ofEngineering and Physical Sciences


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Contents

Contents ..................................................................................................................................... 3

List of Figures ........................................................................................................................... 7

List of Tables ........................................................................................................................... 11

Abstract ................................................................................................................................... 13

Declaration .............................................................................................................................. 15

Copyright Statement .............................................................................................................. 17

Acknowledgement .................................................................................................................. 19

Chapter 1 Introduction .......................................................................................................... 21

1.1 Background Study ............................................................................................. 21

1.2 Research Objectives .......................................................................................... 22

1.3 Outline of Thesis ................................................................................................ 22

Chapter 2 Literature Review of Dissolved Gas Analysis on Natural Ester ...................... 25

2.1 Introduction of Transformer Liquid ............................................................... 25

2.1.1 Mineral Oil Nytro Gemini X .................................................................. 25

2.1.2 Natural Ester FR3 ................................................................................... 26

2.1.3 Sample Processing Methodology ............................................................... 27

2.2 Transformer Faults ........................................................................................... 28

2 2 1 P i l Di h F l 28


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2.4.3 PC Data Analysis ......................................................................................... 38

2.5 Previous Work Review ...................................................................................... 39

2.5.1 Electrical Sparking ..................................................................................... 39

2.5.2 Electrical PD Test ........................................................................................ 40

2.5.3 Thermal Test ................................................................................................ 43

2.6 Tests Comparison and Summary ..................................................................... 48

Chapter 3 Experimental Study on DGA under Sparking Faults ....................................... 51

3.1 Introduction ........................................................................................................ 51

3.2 Experiment Setup .............................................................................................. 51

3.2.1 Test Circuit Design ...................................................................................... 51

3.2.2 Test Vessel Design ........................................................................................ 53

3.3 Test Procedure .................................................................................................... 56

3.3.1 Drain Oil out of System .............................................................................. 57

3.3.2 Clean Test System and Fill Processed Oil into the System ...................... 58

3.3.3 Measuring Background DGA level ............................................................ 59

3.3.4 Generating Sparking Faults ....................................................................... 59

3.4 Data Measurement and Analysis ...................................................................... 60

3.4.1 GIG and GIT ............................................................................................... 60

3.4.2 Dissolved Gas Generation Calculation ..................................................... 61

3 4 3 Sparking Energy Calculation 63


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4.1 Introduction ....................................................................................................... 79

4.2 Experiment Setup .............................................................................................. 79

4.3 Test Procedure ................................................................................................... 80

4.3.1 Calibrate the PD Detector ......................................................................... 81

4.3.2 Measuring Background PD Noise ............................................................. 82

4.3.3 Generating PD Faults ................................................................................. 82

4.4 Data Measurement and Process Method ......................................................... 83

4.4.1 Total Gas Generation Calculation ............................................................ 83

4.4.2 PD Energy Calculation .............................................................................. 84

4.5 Test Condition and Observation ...................................................................... 88

4.6 Test Result and Analysis ................................................................................... 89

4.6.1 PD Fault Gas Generation .......................................................................... 89

4.6.2 PD Fault Energy ......................................................................................... 91

4.6.3 Gas generation rate (per J) ........................................................................ 93

4.6.4 Absolute Gas generation rate (per J) ........................................................ 95

4.6.5 Duval Triangle Analysis ............................................................................. 96

4.6.6 Laboratory DGA and Online Monitor Comparison ............................... 98

4.7 Summary ............................................................................................................ 99

Chapter 5 Experimental Study on DGA under Thermal Fault ....................................... 101

5 1 Introduction 101


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5.6.2 Gas Generation Rate Comparison under Different Temperatures ...... 109

5.6.3 Duval Triangle Analysis ............................................................................ 111

5.6.4 Laboratory DGA and Online Monitor Comparison .............................. 113

5.7 Summary ........................................................................................................... 113

Chapter 6 Conclusions and Future Work .......................................................................... 115

6.1 Conclusions ....................................................................................................... 115

6.1.1 Research Areas .......................................................................................... 115

6.1.2 Main Findings ........................................................................................... 116

6.2 Future Work ..................................................................................................... 117

Reference ............................................................................................................................... 119

Appendix I. Matlab Code Used In the Thesis .................................................................... 123

I.1 Sparking Energy Calculation ..................................................................................... 123

I.1.1 High Frequency Energy Calculation .......................................................... 123

I.1.2 Low Frequency Energy Calculation ........................................................... 125

I.2 PD Energy Calculation ............................................................................................... 128

Appendix II. The Results Used in the Thesis ...................................................................... 131

Words count: 34975


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List of Figures

Figure 2.1 Basic Hydrocarbon Structures in Mineral Oil [20] ................................. 25

Figure 2. 2 Molecular Structure of FR3 [23] .............................................................. 27

Figure 2. 3 Diagram of Indicator Gases and Faulty Type and Severity in

Transformers Filled By Mineral Oil [38] ............................................................ 32

Figure 2. 4 Headspace Sampling Method [39] ............................................................ 33

Figure 2. 5 Gas Chromatograph Concept Diagram [41] ........................................... 34

Figure 2. 6 Duval Triangle Diagrams .......................................................................... 35

Figure 2. 7 TM8 Online Transformer Monitor .......................................................... 36

Figure 2. 8 The Working Principle Diagram of TM8 ................................................ 37

Figure 2. 9 Dual- Column GC Analysis Diagram ....................................................... 38

Figure 2. 10 Example of Analysis Diagram of TM8 Viewer [17] .............................. 38 Figure 2. 11 Photo of Lighting Impulse Sparking Test Vessel [12] .......................... 39

Figure 2. 12 Comparision of Fault Gas-in-Oil Generation between Lyra X and FR3

[12] .......................................................................................................................... 40

Figure 2. 13 Electrical PD Test Diagram [10] ............................................................ 40

Figure 2. 14 Test Vessel Diagram of PD Test [10] ...................................................... 41 Figure 2. 15 Thermal Test 1(Heating Element) [11] .................................................. 44

Figure 2. 16 Thermal Test 2 (Heating Element) [12] ................................................. 45


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Figure 3.8 Example Filtered Waveform of Power Frequency Sparking Current ... 66

Figure 3.9 Different Types of Sparking ....................................................................... 67

Figure 3.10 Total Gas Generation in Gemini X /FR3 Tests ....................................... 70

Figure 3.11 GIT Generation rate (per) J in Gemini X and FR3 Sparking Tests ..... 73

Figure 3.12 GIT Generation rate (per J) Comparison between Gemini X and FR3

................................................................................................ ................................. 75

Figure 3.13 Duval Triangle Evaluation (GIO) of Sparking Fault in Gemini X and

FR3 .......................................................................................................................... 77

Figure 4.1 Schematic Diagram of Electrical PD Test Circuit .................................... 80

Figure 4.2 PD Calibration Panel of PD Measuring System Software ....................... 81

Figure 4.3 PD Noise in FR3 under 60 kV .................................................................... 82

Figure 4.4 Example of PD Test DGA Peak Value ....................................................... 84

Figure 4.5 PD Noise Filter ............................................................................................. 85

Figure 4.6 Gas Generation in Gemini X and FR3 PD Test ........................................ 90

Figure 4.7 PD Patterns of Gemini X (60 Minutes PD signals from the 3000 pC Test)

and FR3 (1 Minute PD signals from 3000 pC Test 1) ......................................... 91

Figure 4.8 GIT Generation rate (per J) Comparison between 2000 pC Tests ofGemini X and FR3 ................................................................................................. 93

Figure 4 9 GIT Generation rate (per J) Comparison between 3000 pC Tests of


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List of Tables

Table 2.1 Key Properties of Nytro Gemini X [18] ...................................................... 26

Table 2.2 Key Properties of FR3 [24] .......................................................................... 27

Table 2.3 Water Content and Relative Humidity of Processed Liquid Samples at

Room Temperature [25] ....................................................................................... 28

Table 2.4 Bond Dissociation Energy [33] .................................................................... 31

Table 2.5 GIO DGA Results under PD Fault of Various Amplitudes [10] .............. 42

Table 2.6 GIO DGA Results under PD Fault of Various Energy [10] ..................... 43

Table 2.7 GIO DGA Result of Thermal Test 1 (Heating Element)........................... 45

Table 2.8 GIO DGA Results in both Liquids .............................................................. 46

Table 2.9 Tests Features Comparison ......................................................................... 49

Table 3.1 Example GIO Concentration in Gemini X ................................................. 62

Table 3.3 Sparking Types ............................................................................................. 67

Table 3.4 Example of Group Sparking Energy Calculation ..................................... 68

Table 3.6 Sparking Energy for Each Test Group inside Gemini X/ FR3 ................ 71

Table 3.7 Absolute GIT Generation Rate (t/J) of Sparking Tests .......................... 74 Table 3.8 GIO Generation Rate (ppm/J) .................................................................... 76

Table 3.9 Comparison of GIO Results between TM8 and Laboratory Analysis .... 78


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Table 5.3 Comparison of GIO DGA Results between TM8 and Laboratory

Analysis ................................................................................................................. 113


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Abstract Mineral oil has been traditionally used as an insulating liquid in power transformers for over a

century, and Dissolved Gas Analysis (DGA) technique has been used for decades as one of the most useful

diagnosis tools to assess the conditions of mineral oil filled transformers. However, due to increasing

awareness of environmental protection and fire safety, there is a trend of replacing mineral oil with

environmentally friendly natural esters; DGA data interpretation method should then be studied, if necessary

revised, in order to be applicable for natural ester filled transformers.

This thesis covers experimental studies on performances of a mineral oil (Gemini X) and a naturalester (FR3) in terms of fault gas generation. Laboratory simulated faults include electrical sparks, electrical

partial discharges (PD) and high temperature thermal hotspot types.

The electrical sparking fault was generated by using a sharp needle electrode with a tip radius of

curvature of 5 micrometers, a 2.57 L sealed test vessel was designed and built with the TM8 online DGA

monitoring system, and two CTs were used to measure the high frequency and power frequency components

of the sparking current, respectively. The electrical PD fault was simulated using the same test system but

under lower voltages, and a traditional PD detector was used to record the characteristics of PD signals,

including the repetition rate and amplitude. The hotspot thermal fault was generated by heating up a copper

element locally in a 2.73 L sealed test vessel, and three thermocouples were used to measure the temperatures

of the heating element.

Furthermore, the dissolved fault gases in oil were measured by both the online DGA monitoringsystem and the oil analysis laboratory, and the DGA results were also compared.

The main findings of this thesis are outlined below:


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Declaration

I declare that no part of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other institutes

of learning.


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Copyright Statement

I. The author of this thesis (including any appendices and/or schedules to this thesis) owns

certain copyright or related rights in it (the Copyright) and he has given The University of

Manchester certain rights to use such Copyright, including for administrative purposes.

II. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy,

may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as

amended) and regulations issued under it or, where appropriate, in accordance with

licensing agreements which the University has from time to time. This page must form part of

any such copies made.

III. The ownership of certain Copyright, patents, designs, trade marks and other

intellectual pro perty (the Intellectual Property) and any reproductions of copyright

works in the thesis, for example graphs and tables (Reproductions), which may bedescribed in this thesis, may not be owned by the author and may be owned by third parties.

Such Intellectual Property and Reproductions cannot and must not be made available for

use without the prior written permission of the owner(s) of the relevant Intellectual Property

and/or Reproductions.

IV. Further information on the conditions under which disclosure, publication andcommercialisation of this thesis, the Copyright and any Intellectual Property and/or

Reproductions described in it may take place is available in the University IP Policy (see


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Acknowledgement

Firstly I would like to express my sincerely gratitude to my supervisor Professor Zhondong

Wang for her support and guidance during my MPhil research study at the University of

Manchester. My MPhil research project would not succeed without her hard work and patient

guidance.

I am also truly grateful to all the sponsoring companies, i,e. Serveron and TJH2B who provided

continuous support to this project at the University of Manchester. In particular, John Hinshaw

from Severon and John Noakhes from TJ2HB are extremity helpful. I would also like to thank

Cooper Power System for providing natural ester over the years.

To all my colleagues in the transformer research group , I appreciate for your company

and thank you for offering me an enjoyable working environment. Special thanks to Dr.

Xin Wang who taught me so much on test cell design, experimental setup and thesis writingthrough all the project and Dr. Xiao Yi who offered many patient and wise suggestions.

Last but not least, I would like to take this opportunity to thank my parents for their continuous

support and understanding, to my girlfriend Miss Jinping Huang for her support and selfless

love. They encouraged me to go through all the hard work all the time.


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Chapter 1 Introduction

1.1 Background Study

Mineral oil has been used as a traditional insulating liquid for power transformers for over a

century. However, in face of the increasing awareness of environmental protection recently,

applying environmental friendly transformer liquids such as natural esters or synthetic esters

in transformers of distribution or transmission level is getting more and more popular [1, 2, 3].

Up to now, ester based transformer liquids have been widely used in distribution transformers

and there are more and more development work in the aim of used by esters in power

transformers [4, 5].

DGA, short for dissolved gas analysis, is one of the most useful diagnosis tools for incipient

fault indication of oil-filled transformers [6]. When either thermal or electrical faults are

occurred, transformer oil will decompose and recombine into many kinds of fault gases. In the

past several decades, experience of DGA based fault interpretation of mineral oil-filled

transformers has been accumulated after a wide range of lab research and on-site operation

practices. Many standards were established for assessing conditions of mineral oil-filled

transformers, such as IEC 60599 and IEEE C57.104 [7, 8]. Among all kinds of DGA

interpretation methods listed in the above guide, the most comprehensive one is Duval triangle


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development of TM8 online DGA monitor, the three main types of transformer fault and a

recent experimental study of natural ester DGA.

Chapter 3 Experimental Study on DGA under Sparking Fault

This chapter shows the method to generate the sparking fault and also the method to measure

the sparking current. By using a needle to plate electrode configuration, a test cell is designed.

It has achieved a good sealing state and complete oil circulation. The sealing state of the

electrical test cell is verified by a pressure gauge based sealing test. A proper test procedure is

carefully followed to use the test cell TM8 close loop measuring system in order to obtain

reliable test results. The experiment in this chapter shows the gas generation characteristics of

Gemini X and FR3 under the sparking faults. The simulated faults for both liquids are also

evaluated by using the original and revised Duval triangle. Furthermore, oil samples are

collected after the electrical sparking test and sent out for laboratory DGA analysis.

Chapter 4 Experimental Study on DGA under PD Fault

This chapter describes the method to generate the PD fault using similar configuration to

previous sparking test under lower voltage/ electrical fields and also the method to calculate

the PD energy. The same electrical test cell as Chapter 3 is used and the proper test procedureis carefully followed to reduce gas leakage. The experiments in this chapter study the gas

generation of Gemini X and FR3 under the controlled PD faults up to 2 days


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Chapter 6 Conclusions and Further Work

This chapter summarizes the main conclusions of the thesis and also gives some suggestions

for future studies.


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Chapter 2 Literature Review of Dissolved Gas Analysis onNatural Ester

2.1 Introduction of Transformer Liquid

This MPhil thesis explores the differences of fault gas generation characteristics between

conventional mineral oil which is widely used in large power transformers, and natural ester

which is expected to be an alternative for mineral oil. From now on, Gemini X will stand for

the mineral oil and FR3 will represent natural ester.

2.1.1 Mineral Oil Nytro Gemini X

Nytro Gemini X, a type of inhibited insulating transformer oil, which is produced by Nynas

Oil Company to replace the previous uninhibited Nytro 10GBN, consists of saturated

hydrocarbon molecules, like paraffins and naphthenes and unsaturated aromatics and

polyaromates as shown in Figure 2.1.


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with a high anti-oxidation ability. The dielectric strength of Gemini X is higher than 70 kV

(measurement based on IEC 60156 with a 2.5 mm gap distance) when the liquid is preserved.

However, once it has been contaminated by water or particles, the dielectric strength will

reduce accordingly [19]. The major drawbacks of Gemini X are fire hazards and less

biodegradability. The water saturation level of Gemini X is 55 Parts per Million (ppm) at room

temperature. Table 2.1 shows the key properties of Gemini X.

Table 2.1 Key Properties of Nytro Gemini X [18]

Property Unit Test Method Typical DataPhysical

Density,20 C kg/dm 3 ISO12185 0.882Viscosity,40 C mm 2/s ISO3104 8.7

Flash point C ISO2719 144Pour point C ISO3016 -60

ChemicalAcidity mg KOH/g IEC62021


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Figure 2.2 Molecular Structure of FR3 [23]

FR3 is highly biodegradable but can also oxidize easily due to the structure of triglycerides.

The dielectric strength of FR3 is above 56 kV (measured by ASTM D1816 using a 2 mm gap

distance). FR3 is now mainly applied in distribution transformers in North and South America

[22]. The water saturation level of FR3 is 1100 ppm at room temperature which is 20 times

higher than that of Gemini X. Table 2.2 shows the key properties of FR3.

Table 2.2 Key Properties of FR3 [24]

Property Unit Test Method Typical DataPhysical

Density,20 C kg/dm 3 ASTM D1298 0.92Viscosity,40 C mm 2/s ASTM D445 32

Flash point C ASTM D92 330Pour point C ASTM D97 -20


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under 5 mbar inner pressure and 85 C, a further 24 hours cooling down is also required

afterwards. The qualities of both Gemini X and FR3 are trusted to be the same. The water

content was measured according to the Karl Fisher titration analysis, using Metrohm 684

coulometer and 832 Termoprep ovens [25]. The dissolved gas is measured by the TM8 online

transformer monitor. The result of relative humidity (water content versus saturation level) and

dissolved gas for the processed liquid sample are below 5% and very close to 0 ppm

respectively [10]. Table 2.3 shows the water content and relative humidity of processed

samples.

Table 2.3 Water Content and Relative Hu midity of Processed Liquid Samples at RoomTemperature [25]

2.2 Transformer Faults

The IEC standard 60599 [7] classifies the DGA detectable transformer faults into 2 categories:

the electrical fault and the thermal fault. These two main categories can be further sorted into

6 types of transformer fault, according to the magnitudes of the fault energy: the electrical fault:


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while the others such as the one which occurs in a transformer liquid is commonly named as

streamer [7, 8].

Partial discharges, known as one of the most influencing reasons for insulator degradation,

could lead to electric breakdown when they accumulate and propagate fully between two

conductors. To avoid costly transformer failures, it is critically important to monitor the PD

activities for early detection of the incipient of transformer fault. Dissolved gas analysis (DGA)

is now the most widely used method to determine the condition of transformer insulation liquid

as it is a non-destructive technique [26-30].

2.2.2 Electrical Sparking Fault

After decades of study, it is now generally accepted that the breakdown occurs after the

streamers fully propagate through the gap of the electrodes. When the energy of dielectric

breakdown is limited, it will act as small arcs which are named as sparking faults [7]. In

comparison with PD faults, sparking fault generate much more amount of fault gases under the

same fault time and could be critical for transformer operation.

2.2.3 Thermal Fault


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Dissolved gas analysis (DGA) is known as one of the most widely used diagnosis tools of oil-

filled transformers, it is noted as the non-interrupt test method which has already functioned

for decades. Furthermore, DGA is also famous for the reliable fault forecast tool that is

developed based on a vast amount of faulty oil-filled equipment in service and laboratory

experiment results worldwide [7, 8].

In general, DGA can be divided into 4 steps: collect oil sample, extract dissolved gas, gas

chromatograph measurement and data interpretation. The oil sample collection is based on the

international standard IEC 60567 which gives the recommended procedure for taking an oil

sample from oil filled equipment. The oil sample collection is considered to be the first primary

factor of a good DGA result; therefore, the recommended procedure needs to be followed

carefully.

The extraction of dissolved gas from the oil sample is the second step. The traditional vacuum

method or the alternative vacuum pump method such as headspace and stripper methods are

also available in IEC60567 [31]. The headspace method is used in the TM8 and will be

explained in Section 2.3.2.

The third step is the gas chromatograph (GC) which could separate and analyze different gas

components Detail of the GC will be described in Section 2 3 3


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2.3.1 Gas Formation

The transformer liquid consists of different hydrocarbon atomic groups like CH 3, CH 2 and CH.The molecular bond which is used to link the molecular group together, such as C-H and C-C

bonds, will be broken when electrical or thermal energy is applied. Newly formed unstable

radical or ionic fragments will recombine swiftly into gas molecules like hydrogen (H-H),

methane (CH 3-H), ethane (CH 3-CH 3), ethylene (CH 2=CH 2), acetylene (CH CH), CO (C O)

and CO 2 (O=C=O). Different energy levels are required to break different kind of molecular bonds, as a result, different types and amounts of fault gases will be formed according to the

severity and category of the transformer fault. The energy which is mandatory to crack the

typical molecular bond inside the transformer oil is shown in Table 2.4.

Table 2.4 Bond D issociation Energy [33]

BondC-C (CH 3-

CH 3)

C-H

(average)

C=C

(H2C=CH 2)

CC

(HCCH)

Dissociationenergy

(kJ/mol) 356 410 632 837

Arcing, low energy sparking, PD and overheating are some of the common faults that could

happen in the oil-filled transformers. Once any of these faults occurs, the insulation liquid will


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2.3.2 Headspace Method

Headspace method is a calculation method used to compute gas-in-total or gas-in-oil

concentration using gas-in-gas (GIG) concentration. The case shown in Figure 2.4 is an oil-

filled vial with V L volume of oil and left a V G volume of headspace.

Figure 2.4 Headspace Sampling Method [39]

Some of the dissolved gas will spread to the headspace from the oil until the equilibrium

condition of a certain temperature, agitation and pressure is reached. Afterwards, the headspace

gas will be passed to the gas chromatograph (GC) columns. Then the obtained gas

concentration in headspace, GIG, will be used to calculate gas-in-oil (GIO) or gas-in-total (GIT)

according to Henry s law.


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fault. With the help of software, those monitors will be able to calculate and display some of

the interpretation results like the Duval triangle [42].

2.4 Serveron Online Transformer Monitor TM8

The Online DGA monitor used in this thesis is Serveron TM8 (shown in Figure 2.7). It is able

to provide useful and timely information for oil-filled transformer condition assessment. Withthe help of the built-in sensors and special chromatographic columns, TM8 can provide up to

hourly DGA sampling covering all 8 types of transformer fault gases with 5% accuracy [17].


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Transformer oilTransformer

gas

Liquid blockagemembrane

Gas flow

Selective

columns

Dual-column

GC analysis

Extractor

Oil flow

Carrier gases

Heliumflow

PC basedTM8

system

Data flow

Test cell/transformer

Figure 2.8 The Working Principle Diagram of TM8

In the closed loop system, transformer oil keeps circulating between the test vessel/ transformer

and the oil chamber of the TM8 extractor. The gases dissolved inside the transformer oil will

go through the liquid blockage membrane into the gas chamber of the TM8 extractor. The

carrier gas helium flow (red arrows) will carry the dissolved gases into the extractor gas

chamber and will go to the selective columns. These will separate all 8 kinds of gases and let

them reach the GC analysis part at different times. Lastly, in the GC analysis part, the fault

gases are analyzed by the sequence as shown in next Section.


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Figure 2.9 Dual- Column GC Analysis Diagram

2.4.3 PC Data Analysis

The raw result from the GC analyzer will be further computed based on the built-in partition

coefficient K, the measured oil temperature and the equilibrium pressure in the extractor. The

result plots out timely DGA curves (Figure 2.10 (a)) and can also provide an automatic


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2.5 Previous Work Review

Many researchers made great efforts to understand the FR3 performance under electrical and

thermal fault conditions such as [10-15]. Their research is studied and described below.

2.5.1 Electrical Sparking

Figure 2.11 shows the lighting impulse sparking experiment carried out by Mark. Jovalekic to

investigate the fault gas generation under the lighting impulse sparking fault in mineral oil,

Lyra X and natural ester FR3.

Figure 2.11 Photo of Lighting Impulse Sparking Test Vessel [12]


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Figure 2.12 Comparision of Fault GIO Generation between Lyra X and FR3 [12]

2.5.2 Electrical PD Test

Figure 2.13 shows the electrical PD test that was designed by X. Wang [10]. As we can see

from the circuit diagram, the 50 Hz power transformer is used to provide up to 70 kV test

voltage. A 500 pF discharge free capacitor is connected in parallel with the test vessel. The

measuring impedance of the LDS-6 PD detector is connected in series with the capacitor. The

PD detector is calibrated and used to measure the PD signal with less than 5 pC noise (70 kV

test voltage).

0500

10001500200025003000350040004500

CO2 C2H4 C2H2 C2H6 H2 CH4 CO TDCGLyra X 219 214 2100 0 1775 155 0 4244

FR3 182 229 953 0 605 99 155 2041

L/L


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The test vessel diagram is shown in Figure 2.14. It can be seen from the diagram that the 100

ml glass vial sealed by an aluminum crimp cap is fully filled with test oil. The needle electrode

is penetrated into the rubber sealing whose tip radius of curvature is 6-7 m from front view

and 2-3 m from lateral view.

Figure 2.14 Test Vessel Diagram of PD Test [10]

The assemble of the test vessel and the needle electrode is immersed inside an insulating oil

filled container. A copper base of 100 mm diameter is placed under the bottom of the test vessel

as a plate electrode. The gap distance between the needle and plate electrode is kept as 50 mm

for all tests. A new needle electrode will be replaced after each test. The oil sample is

immediately sealed by the Acrylic-based sealing compound from RS Ltd [43] and is then sent


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Table 2.5 GIO DGA Results under PD Fault of Various Amplitudes [10]

Oil Test PD amplitude (pC)DGA(ppm)

C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TCG

Gemini X

G.Test1 200 0.2 0.2 0.4 12.4 0.9 21.7 35.8

G.Test2 300 0.2 0.1 0.2 7 0.5 12.4 20.4

G.Test3 500 0.2 0.3 0.3 62.4 0.4 13.9 77.5

G.Test4 1000 1.5 3.5 0.9 163 2.9 13.6 185.4

FR3

F.Test1 200 0.2 0 44.7 29.9 1.2 20.1 96.1

F.Test2 300 2.7 5 83.4 63.7 3.9 36.2 194.9

F.Test3 500 5.5 11.5 46 69.1 5.8 30 167.9

F.Test4 1000 9.1 22.4 63.4 140 11.4 49.9 296.2Note: Those unexpected results listed in bold and italic style may be caused by leakage.

The difference is mainly contributed by C 2H6 which makes up to 46.5% (200 pC), 42.8% (300

pC), 80.5% (500 pC), and 21.4% (1000 pC) of the total gas generation for FR3. H 2 is the most

significant hydrocarbon gases except C 2H6. H 2 is making up to 34.6% (200 pC), 34.3% (300

pC), 27.4% (500 pC), and 87.9% (1000 pC) of the total gas generation in Gemini X tests while

that is only 31.1% (200 pC), 32.7% (300 pC), 41.2% (500 pC), and 47.3% (1000 pC) in FR3.

The concentration of CO in FR3 is around twice of that in Gemini X. C 2H2 starts to generate

under the 1000 pC PD fault inside Gemini X while the trace of it could be found inside FR3

under 300 pC PD fault.


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Table 2.6 GIO DGA Results under PD Fault of Various Energy [10]

Oil Test Times(mins)

PDenergy

(mJ)

DGA(ppm)l/J

C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TCG

Gemini X

1 15 7.7 0.5 0.4 0.2 31.3 1.7 10.9 45.0 584.4

2 30 8.1 0.2 0.3 0.3 62.4 0.4 13.9 77.5 956.7

3 45 9.2 0.3 0.5 0.2 70.9 0.9 12.5 85.3 927.2

4 60 15.7 0.6 0.7 0.3 110.0 1.8 40.5 153.9 980.3

FR3

1 15 148.2 0.4 0.6 12.7 46.7 0.7 10.1 71.2 48.0

2 30 161.4 1.5 3.1 18.2 88.4 1.9 17.9 131.0 81.2

3 45 486.6 3.3 7.0 28.0 74.7 3.9 29.7 146.6 30.14 60 1020 6.0 13.6 63.5 138.0 6.6 39.6 267.3 26.2

Note: The unexpected result in bold and italic style may be caused by leakage.

It can be seen from Table 2.6 that the PD fault in Gemini X generates around half of total fault

gases than FR3 under the same test conditions. However, when the PD energy is taken into

consideration, the amount of gas generation rate (per J) in Gemini X is 10 times higher than

that in FR3. The reason is that PD repetition rate in FR3 is much higher than that Gemini X.

For the same type of liquid, the gas generation is increased as the voltage applying time

becomes longer. However, the amount of gas generation rate (per J) in FR3 test is not linearfor different voltage applying times because the needle electrode changed as the test carried on.


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during the test. The test is then redesigned so that it can be carried out inside a sealed closed

loop system in this thesis.

Table 2.7 GIO DGA Result of Thermal Test 1 (Heating Element)

OilTimes(mins)

DGA(ppm/min)C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TCG

Gemini X 35 0.1 0.0 0.3 1.2 4.7 13.8 20.1FR3 50 20.9 0.0 16.9 1.7 6.7 14.4 60.7

2.5.3.2 Thermal Test 2

Mark designed a localized heating element test using a special material which linearly changed

the resistor in a wide range of temperatures up to 550C [12]. Figure 2.16 shows Mark s test

design. As shown in the figure below, the special material Resistherm is used as the heating

element and put inside the oil-filled sealed test vessel. A funnel is set upside down to collect

the generated fault gases; the fault gases will finally go into the top syringe and held there.

Another syringe is used to release the pressure that is caused by the oil expansion during the

test. The voltage across the heating element and the current that passes through it are recorded

for temperature calculation.


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The heating element is maintained at 300C to 600C for 1 to 6 hours. Higher temperatures

cannot be achieved due to the melting of the Resistherm. The DGA results for all tests in bothliquids are shown below in Table 2.8.

Table 2.8 GIO DGA Results in both Liquids(a) GIO DGA Results in FR3

Temperature

(C ) Duration(h)DGA( l/J )

CO 2 C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TCG300 6 1353 27 0 489 92 33 932 1573400 6 2973 209 0 934 278 214 4219 5854500 2 3698 631 0 1005 472 351 3095 5554600 1 3923 1061 0 1307 382 453 5148 8351

(b) GIO DGA Results in Lyra X

Temperature(C ) Duration(h)

DGA( l/J )CO 2 C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TCG

300 1.5 57 8 0 2 11 20 510 551400 1 169 198 38 7 70 149 687 1149

It can be seen from the table that the total generated fault gases in Lyra X is around 5 times

higher than that in FR3 under 400C thermal stress. CO and CO2 are the main generated fault

gases under the thermal fault for both oils. C2H4, CH4 and C2H6 are also significant in FR3


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1. An expansion chamber which is maintained at atmospheric pressure. An insolation valve is

installed between the connection of equipment 1 and 3.

2. A pressure gauge.

3. A gas chamber that can be sealed by the isolation valve.

4. A liquid reservoir.

5. A pump that circulates liquid between 4 and 6. .

6. An oven.

Figure 2.17 Thermal Test 3

The natural ester (the soybean oil, the high oleic sunflower oil) and the mineral oil are all heatedfor 8 hours. The test results are shown below in Figure 2.18. It can be seen from Figure 2.18

there is a 50C temperature difference for main fault gases yielding between the soybean oil


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(a) Gas Generation in Soybean Oil under Various Temperatures

(b) Gas Generation in Oleic Sunflower Oil under various Temperatures


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Chapter 3 Experimental Study on DGA under Sparking Faults

3.1 Introduction

With the purpose of applying the standard diagnosis method for traditional mineral oil to

alternative natural esters, the gas performances of a mineral oil, Gemini X, and a natural ester,

FR3, are studied in this chapter under electrical sparking faults. A specially designed test vesselwith a good sealing capability was tested and used in this study, and the needle to plate

electrode configuration was used to produce electrical sparking faults. It was found that the

amount of fault gases is closely related with the fault energy; therefore the gas generation rate

(per J) was considered as a good parameter to compare the gas performance between FR3 and

Gemini X. The TM8 DGA monitor was used to measure the DGA results. Additionally, some

oil samples were also sent to TJH2B for laboratory analysis in order to compare with online

DGA methods. The results indicated that the two methods agree with each other with an

acceptable deviation.

3.2 Experiment Setup


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R1

R2

Test vesselVoltagedividerRatio

10000:1

TM8

Over Current

Protectionrelay5 A 240V/80kV

Powerfrequency

CTOutput

V/A=1/100

OutputV/A =1/10

CT

500 pF

600 k Water resistor

CT

High frequency CT

PC based TM8 control software

The cage

100 MHzoscilloscope 1

100 MHzoscilloscope 2

Oilinlet

Oiloutlet

Variac0-240 V

Figure 3.1 Schematic View of Electrical Sparking Test Circuit


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60 current clamp, bandwidth from 40 Hz to 40 kHz) with a 1/100 output ratio was used to

measure the power frequency component of the sparking current, and another high frequency

current transformer (Stangenes pulse current transformer, model No. 0.5-0.1, Square Pulse

Rise Time = 20 ns) with a 1/10 ratio was used to measure the high frequency component of the

sparking current. The results of the two current transformers were combined together to get the

total result of current.

3.2.2 Test Vessel Design

To generate a proper amount of fault gases, the gap distance between the needle-to-plate

electrodes is chosen as 35 mm. The plate electrode was made of brass and has a diameter of 20

mm. The needle electrode was a medical needle with a tip radius of curvature in the range from

6-7 m from front view.

3.2.2.1 Main Design Advantages

To obtain a reliable result, the test vessel should be kept in a good sealing state and a complete

oil circulation should be maintained in the test. As the photo of the test vessel that is shown inFigure 3.2, two design factors were tried in this thesis to keep the test working in sealed

condition they are: inner cap and o rings The inner cap is a cap that placed right close to the


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(a) Design Diagram


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and outlet pipe are installed at the top/bottom of the test vessel to make sure that all oil is in

the circulation loop. Finally, the tube between the inlet pipe of TM8 and the syringe adaptor

was as short as possible to reduce the dead volume , since oil in this area is barely circulated

and it represents dead volume .

The syringe of 50 ml connecting to the top of the test cell is also used to remove the gas bubbles

during test setup and also balance the inner system pressure with outside atmosphere pressure

during test operation.

3.2.2.2 Sealing Tests

Two sealing tests are carried out to check whether the sealing state is qualified for both the

electrical sparking and electrical partial discharge (PD) tests.

Sealing test 1 is designed to check how much pressure difference between the inner and outside

of the test vessel is reduced in a period of 23 hours. The setup of sealing test 1 is shown in

Figure 3.3.


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and the test vessel was kept for a further 23 hours. Figure 3.4 plots the pressure difference with

time (the pressure data is not recorded at night).

Figure 3.4 Pressure Versus. Time of Sealing Test 1

Sealing test 1 showed that the test vessel was in a good sealing state, and the pressure difference

between the inside and the outside of the test vessel fell from 98 mbar to 89 mbar after 23 hours.

This means only 10% gas leaked out within 23 hours and equivalent 0.4% in the first hour.

Sealing test 2 aimed at finding out the relationship between pressure, gas volume and sparking

numbers. A test circuit was built up according to Figure 3.1 (the TM8 was not connected in the

circuit) with the same electrode configuration. The test vessel was fully filled with FR3. After

50 sparking tests, a 51.5 mbar pressure difference was detected by the pressure gauge and the

0

2040

60

80

100

0 5 10 15 20 25

Sealing test

Pressure(mbar)

Pressure(mbar)

Time(h)


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Process transformer oil as described in Section 2.1.3. Drain oil out of the system. Clean test system, fill processed oil into the system (eliminate the headspace). Measure background gases. Generate sparking faults. Use syringe to push fault gases to be dissolved back into the oil circulation, and measure

the amount of fault gases.

Process and analyze test data.

3.3.1 Drain Oil out of System

After fresh oil is well processed, it needs to be filled into the TM8 test vessel system. To dothis, transformer oil from the previous test should be drained out first by TM8 which can pump

oil forwards and backwards for several times (normally 2 times). Some of the oil trapped in

TM8 would not be drained out easily if only forward pumping is applied; accordingly, pumping

oil in both directions is helpful to remove the residual oil efficiently. Detail of the steps is

described below.

First of all, the oil inlet pipe of TM8 needs to be disconnected and put into a waste oil barrel.


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pipe of TM8 needs to be disconnected and put into the waste oil barrel while the oil inlet pipe

needs to be taken out from the waste oil barrel and then put on to an empty oil beaker. Next,

the pump oil 35 command needs to be used, making the oil pump rotate forwards at the

maximum pumping speed. Wait around 10 minutes and repeat the pump oil backward and

forward procedures again to make sure most of the oil is drained out from TM8. According to

the test experiment, the previous dissolved gas residue can be reduced to less than 10% after

this procedure.

Sometimes the needle electrode needs to be changed before the processed oil is filled into the

system. In the sparking test, the needle electrode needs to be changed only when the oil is

changed from Gemini X to FR3. To change the needle electrode, the top brass cap nut needs

to be screwed out first and then the needle fixer has to be released to remove the medical needle.

A new medical needle is put into the needle fixer. The needle is carefully measured by ruler,making sure the gap distance is 35 mm.

3.3.2 Clean Test System and Fill Processed Oil into the System

Processed oil can be filled into the system after the previous oil residue was cleaned. The oiloutlet pipe of TM8 needs to be connected back to the bottom of the test vessel while the inlet

i f TM8 d b i h d il l Th il l l f h


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system when the pressure is applied, the leakage place of the vessel or the connection must be

checked and sealed.

Normally the GIO concentration of previous test will reduce to nil after procedure 3.3.1,

therefore the test system didnt require a formal clean procedure. However, the test system

needs to be washed and cleaned by processed oil under two certain circumstances: (1) the GIO

concentration is too high, i.e. several thousand ppm, (2) the next test oil type is different with

previous one.

In this two cases, the Procedure 3.3.1 and 3.3.2 needs to be repeated for a totally clean

background.

3.3.3 Measuring Background DGA level

Before measuring the background dissolved gas value of test oil, the gas extractor chamber

needs to be cleaned. Gas residue inside the gas chamber could be pumped out by using the xtr

resume command and xtr gas.purge command in sequence, resuming TM8 extractor to

normal operation state and then making the oil pump rotate forwards at the maximum speed.The pre command could be used to print out the gas chamber pressure; the gas chamber

ill d d 3 i d h i b k d 15 i (1 h ) i hi


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During the sparking test, the output voltage was increased at a rate of 2 kV/s until a sparking

(an interrupted breakdown) occurred. The reason 5 kV/s is applied is to avoid any sparking

will be formed due to the fast increasing voltage. The sparking voltage and current (high

frequency and power frequency) were recorded for further analysis. This procedure was

repeated 15 times for each liquid sample.

3.4

Data Measurement and Analysis

3.4.1 GIG and GIT

As mentioned in Section 2.3.2, the TM8 on-line DGA monitor measures the amount of gases

using the headspace method. The headspace method actually measures the amount of fault

gases in the gas phase at equilibrium states and then calculates the amount of dissolved fault

gases or the total amount of fault gases. The total amount of fault gases can be calculated by

the Equation (2.1), in which GIT and GIG are the concentrations of total fault gases and fault

gases in gas phase respectively. K, partition coefficient, is a ratio of GIO over GIG at

equilibrium.

The K under different temperatures and pressures can be derived from TM8 monitor. Figure


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FR3 Gemini X

Figure 3.5 Partial Coefficients for FR3 and Gemini X

In Equation (2.1), is the ratio of gas volume and oil volume inside the oil circulation system.

In the sparking test and PD test , =V gas/Voil= 77 ml/ 2570 ml = 0.02996. P 0 is the equilibrium

pressure given by the unit of psi and P is the pressure of one atmosphere that is equal to 14.67

psi. T 0 is the oil temperature and T is the standard temperature that is equal to 25 C which is

298.2 K.

When the test data were plotted in Duval triangle, the GIG value should be converted into GIO

value first. The way to calculate GIO is shown in Equation (3.1) [39].

0.01

0.1

1

10

0 40 80 120 160

H2

N2

CO

O2

CH4

CO2

C2H4

C2H2C2H6

C

0.01

0.1

1

10

0 40 80 120 160

H2

N2

CO

O2

CH4

CO2

C2H4

C2H2C2H6

C


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hour after the sparking test, and then it started to fluctuate and fell due to leakage, consumption

and temperature change. On the other hand, the GIG values of C 2H4, C2H2, CH 4 and CO

reached their peaks at the 3 rd hour after the sparking test. Since all the GIG values will reach

their peaks within 3 hours, the average values around 3 rd hour (result from 2 nd 3rd and 4 th hours)

after the test were reported as the final results in order to minimize the error. The GIT amount

can be obtained as the difference between the background and the final results using the

equation below:

GIT = GIT average - GIT0.

Taking H 2 value as an example, the background GIT value can be calculated as GIT = GIG

(K + ) P/P 0 T 0/T = 48.4 ppm (K+0.02996) 14.3 psi/14.7 psi 298.2 K / 295.5 K.

According to Figure 3.5, K = 0.044 when T is 22.3 C. Substitute K = 0.044 into the aboveEquation, we have GIT = 3.5 ppm.

Following the same calculation step, the GIT1, GIT2 and GIT3 can be obtained as 135.3 ppm,

152.1 ppm, 151.0 ppm. The average GIT is GIT average = (GIT1 + GIT2 + GIT3)/ 3 = (135.3

ppm + 152.1 ppm + 151.0 ppm) / 3 = 146.1 ppm. Therefore, the total amount of H 2 generated

during the test is GIT = GIT average - GIT0 = 146.1 ppm 3.5 ppm = 142.6 ppm.


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Table 3.2 Example GIT Concentration in Gemini X

Mineral oil test 1 GIT (ppm)

No. C 2H 4 C 2H 2 C 2H 6 H 2 O 2 CH 4 COGIT0 0 3 -0.7 3.5 21037.1 0 6.2GIT1 11.9 76 0 135.3 21111.9 11.3 5.8GIT2 15.2 93.7 0 152.1 21126.3 14.4 6GIT3 16.4 94.5 0 151 21030.5 14.5 7.5

Average GIT 14.5 88.1 0 146.1 21089.6 13.4 6.4Generation GIT 14.5 85.1 0 142.6 52.5 9.8 0.3

TDCG 252.2

3.4.3 Sparking Energy Calculation

The sparking energy for each test could be quite different even when the test condition was

well controlled. As shown in Figure 3.1, the fault current was measured by using two current

transformers with one in the power frequency (50 Hz) range and the other in the high frequency

range (5 MHz). The voltage was measured using a voltage divider. Two 100 MHz oscilloscopes

made by Lecroy were used to record low frequency signal and high frequency signal separately.

High frequency sparking current and voltage signals were recorded with 500 k sample points

at a 1 GHz sampling rate while power frequency sparking signals were recorded with a 500 k


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In Section 3.5, it could be found that the oscilloscopes were set to compensate the CT output

ratio and as a result, thus the CT ratios have been taken in account in the recorded readings and

therefore will not affect the calculation equation. On the other hand, as stated in section 3.1,

the voltage divider is used to reduce the voltage to 1/10 k and the probe of the oscilloscope is

also set to 10:1 in compensation, Equation (3.3) needs to be rewritten into Equation (3.4).

W = ( ( ) ( ) )0 10000/10

W = ( ( ) ( ) )0 1000 (3.4)

3.4.3.1 High Frequency Component of Sparking Signal

For the calculation of high frequency energy, the V (n) and I (n) were converted into absolute

value since sparking in both the negative and positive direction will produce fault gases.

Consequently, Equation (3.4) can be rewritten into Equation (3.5).

W h = (| ( )| | ( )| )0 1000 (3.5)


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(a) 200 s time range (b) 2 s time range

Figure 3.6 Example of High Frequency Component of Sparking Current

3.4.3.2 Power Frequency Component of Sparking Signal

For the calculation of power frequency energy, the power frequency current was measured in

the primary winding side of the voltage supply transformer because the current is too small to

be measured in the secondary winding side. Therefore, the measured current should be

converted to the value at the secondary winding side by a factor of 240/ 80k. Equation (3.4)can be rewritten into Equation (3.6) to compute power frequency power.


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can be obtained by using the sparking energy W 1 (as shown in Figure 3.7) minus the

corresponding background energy W 0.

Figure 3.7 Example of Power Frequency Component of Sparking Current

It should be noted that the power frequency current transformer (made by Chauvin Arnoux)

has a frequency range from 40 to 10 kHz. Therefore, the high frequency noises should be

filtered. A Matlab ellipse filter is applied to filter the current signals for two times. As shown

in Figure 3.8, the high frequency noises contained in the original power frequency current (blue

curve) were removed, leaving only the filtered power frequency current (red curve).


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Similar to the high frequency energy, Matlab is used to calculate the power frequency energy.

500 k points are recorded for each sparking test and therefore n in Equation (3.6) is 500,000.

The V[n] and I[n] are stored in two arrays and time step t = 1 ns.

3.4.3.3 Sparking Types

Since sparking (interrupted breakdown) is of the random nature, three different types of

sparking were observed during the tests even under the similar test conditions. The sparking

could be classified as normal sparking, slight sparking and continuous sparking as shown in

Table 3.3.

Table 3.3 Sparking TypesSparking type cut off or not dips before cut off

Normal sparking Yes 1

Slight sparking No 1Continuous sparking Yes 2 or more

A normal sparking is followed by the interruption of the current relay, after which the applied

voltage is cut off. A slight sparking is not followed by the interruption of the current relay, and

the voltage is continuously applied on the sample liquid after having a slight voltage dip.

Therefore, the energy of the slight sparking was not calculated since the amount of fault gases

is small and the sparking energy is also small. A continuous sparking contains two or more


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3.4.3.4 Example of Sparking Energy Calculation

To calculate the energy for each sparking test, firstly, the number of sparking faults should bedetermined. Secondly, the average high frequency power and power frequency power need to

be used for group sparking energy estimation.

For example, Table 3.4 shows the energy of Gemini X sparking test group 2. This group

contains 13 normal sparking and 1 continuous sparking (including two consecutive sparking)

which in total form 15 sparking in this group. When the double sparking occurred, the power

frequency signal is completely recorded as shown in Figure 3.9 (c) while the high frequency

pulse of the second consecutive sparking ( Sparking 10 b) is missed for the sampling period of

the oscilloscope is too short (200 s) to catch the second pulse.

Table 3.4 Example of Group Sparking Energy Calculation

Test 2PF

Energy(J)

HFEnergy(J

)Test 2 PF Energy(J) HF Energy(J)

Sparking 1 1.77 2.02 Sparking 10 a4.93

1.55

Sparking 2 1.37 1.07 Sparking 10 b Missed

Sparking 3 1.64 1.42 Sparking 11 1.75 1.73

Sparking 4 1.63 1.79 Sparking 12 1.81 1.89Sparking 5 1.51 1.42 Sparking 13 2.04 2.3

S ki 6 4 01 2 24 S ki 14 1 92 2 07


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On the other hand, the high frequency energy of sparking 10 only stands for the first

consecutive sparking ( Sparking 10 a) whose energy (1.55 J) is close to the average value (1.75

J). The sum of high frequency power is 15 average energy of high frequency energy (1.75 J)

and such the total energy is 1.75 J 15 = 26.20 J. Group energy is the summary of total power

frequency energy and high frequency energy which is 29.37 J +26.20 J = 55.57 J.

3.5

Test Condition and Observation

Detail of the oscilloscope setting is listed below in Table 3.5. All 13 groups of test including

13 15 normal sparking are controlled in the same conditions for a better comparison. In this

setting, the oil volume of the whole TM8 test vessel system contains 2.57 L oil and 77 ml

headspace.

Table 3.5 Oscilloscope Settings

Oscilloscope SettingPower frequency current High frequency current

Channel 1 Channel 1 Voltage div 50 V Voltage div 50 V

probe 10/1 probe 10/1Voltage divider ratio 1/10 k Voltage divider ratio 1/10 kChannel 2 Channel 3


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Gemini X requires higher energy for the incipient of sparking and will also generate a higher

amount of gas bubbles after each sparking.

3.6 Test Result and Analysis

3.6.1 Gas Generation of Sparking Faults

The amount of total fault gases is summarized in Figure 3.10 for both Gemini X and FR3.

(a) GIT of Gemini X Tests

0.050.0

100.0

150.0

200.0

250.0

C2H4 C2H2 C2H6 H2 CH4 COTest group 1 14.5 85.1 0.0 142.6 9.8 0.3

Test group 2 17.7 100.6 1.4 207.1 14.4 0.9

Test group 3 18.0 103.3 1.8 211.6 14.3 2.0

Test group 4 17.8 101.3 0.0 228.3 14.9 0.7

Test group 5 15.6 90.4 0.5 156.4 12.9 1.2

ppm


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It can be seen that the total amount of fault gases of Gemini X and FR3 are similar at about

200 ppm. However, the fault gases generation of FR3 is relatively stable compared with GeminiX, and the fault gas amount varies in each group probably due to different energies even when

the test condition was well controlled. Therefore, the sparking energy should be taken into

account to compare the gas performance of different oils. Generally speaking, fault gas

generation is relatively similar when the same numbers of sparking faults are applied. However,

when the sparking energy is taken into consideration, the conclusion is varied slightly.

3.6.2 Energy of Sparking Faults

The calculated energy of each test is listed below in Table 3.6, using the energy calculation

method described in Section 3.4.3.

Table 3.6 Sparking Energy for Each Test Group inside Gemini X/ F R3

Gemini X test group Average(J) Total(J) PF average(J) HF average(J)1 2.96 44.47 1.71 1.252 3.7 55.57 1.96 1.75

3 3.52 52.74 1.77 1.754 3.65 54.79 1.76 1.895 3.6 54.05 1.73 1.87


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The sparking energy for each test group is different with the maximum deviation of 20%. FR3

has a 20% lower energy compared with Gemini X. The difference of the energy is mainly

attributed to the high frequency component of the sparking faults, since the difference of high

frequency component energy for Gemini X and FR3 is 48% while that of power frequency

component energy is only 9%.

3.6.3 Gas generation rate (per J)

Figure 3.11 shows the amount of gas generation rate (per J) for Gemini X and FR3. It can be

noticed that the gas generation rate (per J) was different from the total gas amount as shown in

Figure 3.10. Taking H 2 generation of Gemini X test as an example, the H 2 generation of test

group 5 (156.4 ppm) is larger than that of test group 1 (142.6 ppm) in Figure 3.10; however,

the H 2 generation (per J) of test group 5 (3.0 ppm / J) is less than that of test group 1 (3.2 ppm

/ J) in Figure 3.11.


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(a) Gas generation rate (per J) in Gemini X tests

0.0

2.0

4.0

6.0

8.0

C2H4 C2H2 C2H6 H2 CH4 CO TDCGTest group 1

0.3 1.9 0.0 3.2 0.2 0.0 5.6Test group 2 0.3 1.9 0.0 3.9 0.3 0.0 6.5

Test group 3 0.4 2.0 0.0 4.2 0.3 0.0 7.0

Test group 4 0.3 1.9 0.0 4.4 0.3 0.0 6.9

Test group 5 0.3 1.8 0.0 3.0 0.2 0.0 5.4

Average of groups 0.3 1.9 0.0 3.7 0.3 0.0 6.3

ppm/ J

4.00

6.00

8.00

10.00

12.00 ppm/ J


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It can also be seen from Figure 3.11 that the gas generation rate (per J) is repeatable for all

groups. For both liquids, H 2 is the main fault indicator which takes up to 60% of the total fault

gases, followed by C 2H2 which takes up to 25% of the total fault gases. However, CO is only

significant in FR3 which always takes up to 12% of total fault gases, which probably due to

the ester part in the FR3 molecular structure.

3.6.4 Absolute Gas generation rate (per J)

When considering the oil volume of the test system (2.57 L), the gas generation rate in the unit

of ppm/J can be calculated into the absolute gas generation rate in the unit of l/J , as listed in

Table 3.7. It can be seen from Table 3.7 that the gas generation rates of the sparking fault reach

21 l/J for FR3 and 16 l/J for Gemini X, which is comparable with that of Dr. X. Wang s test

conclusion. [10]

Table 3.7 Absolute GIT Generation Rate ( t /J) of Sparking TestsOil Test J/BD ppm/BD ul/BD ppm/J ul/J ml/test

GeminiX

1 3.0 16.8 43.2 5.59 14.4 0.652 3.5 22.8 58.6 6.46 16.6 0.883 3.4 23.4 60.1 6.96 17.9 0.904 3.5 24.2 62.2 6.94 17.8 0.935 3.4 18.5 47.5 5.37 13.8 0.71


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Figure 3.12 GIT Generation rate (per J) Comparison between Gemini X and FR3

It can be seen that the sparking faults in FR3 generates 33% higher amount of total fault gases

than that in Gemini X. The amount of H 2 in FR3 is 27% higher than that in Gemini X, while

the amount of C 2H2 in FR3 is 16% higher. Furthermore, CO takes up to 12% in FR3 while it

is almost 0 for Gemini X.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

C2H4 C2H2 C2H6 H2 CH4 CO TDCGGemini X 0.3 1.9 0.0 3.7 0.3 0.0 6.3

FR3 0.3 2.2 0.0 4.7 0.2 1.0 8.4

ppm/ J


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Table 3.8 GIO Generation Rate (ppm/J)

Mineral oil GIO DGA(ppm)

C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TDCG

Test group 1 14.2 82.7 0 84.9 9 0.2 191.1

Duval ratio 13.40% 78.10% 8.50%

Test group 2 17.4 97.8 1.4 123.4 13.3 0.7 254.1

Duval ratio 13.50% 76.10% 10.40%

Test group 3 17.6 100.4 1.8 125.9 13.2 1.6 260.6

Duval ratio 13.40% 76.50% 10.10%

Test group 4 17.8 101.3 0 228.3 14.9 0.7 363.1

Duval ratio 13.30% 75.60% 11.10%

Test group 5 15.2 87.9 0.5 93.7 1191.20% 1 210.1

Duval ratio 13.20% 76.40% 10.40%

FR3GIO DGA(ppm)

C 2H 4 C 2H 2 C 2H 6 H 2 CH 4 CO TDCG

Test group 1 14.2 91.3 0.6 123.1 5.8 33.2 268.1

Duval ratio 12.80% 82.10% 5.20%

Test group 2 11.4 83.5 0.1 123.4 5.7 30.2 254.3

Duval ratio 11.40% 83.00% 5.70%


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that the Duval triangle plots for different tests of the same oil are quite close to one another,

indicating that the test repeatability is good. It can be seen that the sparking faults in Gemini X

and FR3 were all plotted in D1 area (low energy discharge), indicating that the energy of

sparking faults was not very high because the sparking current was interrupted by the current

protection relay immediately after the fault occurred. Therefore, a continuous arcing path could

not be formed in the oil.

Figure 3.13 Duval Triangle Evaluation (GIO) of Sparking Fault in Gemini X and FR3

3.6.7

Laboratory DGA and Online Monitor Comparison

To make sure that the results from TM8 are reliable some of the oil samples are sent to TJH2B


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Table 3.9 Comparison of GIO Results between TM8 and Laboratory Analysis

Oil type GIO (ppm)

FR3 C 2H 4 C 2H 2 C 2H 6 H 2 O 2 CH 4 COTM8 sample 24 197.3 3 80.4 14190.4 12.1 53.5

Laboratory sample 19 151 3 59 59060 8 34Laboratory / TM8 79.08% 76.52% 99.84% 73.43% 416.20% 65.91% 63.60%

3.7 Summary

In this chapter, the amount of total fault gases in FR3 and Gemini X are measured using a

sealed online DGA test system.

The main summaries are listed as follows:

1. FR3 generates a similar amount of fault gases to Gemini X under sparking faults.

2. Considering the sparking energy, FR3 generates fault gases (per J) 25% higher than

Gemini X.

3. The fault gas generation (per J) might be a more reasonable parameter to evaluate the

gas performances of different liquids.4. The Duval triangle method can recognize these sparking faults as low energy


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Chapter 4 Experimental Study on DGA under PD Faults

4.1 Introduction

In this chapter, the electrical partial discharge (PD) faults is studied using the needle to plate

electrodes and the online DGA monitor and oil circulation system which is similar to the one

described in last chapter. Although in previous publications the PD faults was usually presented

by the PD amplitude [11], it is found in this chapter that the PD energy can be correlated with

the amount of s gases much better. As a result, the gas generation rate (versus energy) is proved

to be a useful parameter to show the gas performances of Gemini X and FR3. In order to

compare the DGA results between online and laboratory methods, some oil samples were also

sent to TJH2B for laboratory analysis.

4.2 Experiment Setup

The experimental setup of PD test is similar to the sparking test, as shown in Figure 4.1. The

same test container was connected with the TM8 online monitor using the same method,

providing a good sealing capability of the oil circulation system. However, the distance

between the needle and plate electrodes was increased to 50 mm. Furthermore, the PD signals


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R1

R2

Test vesselVoltagedividerRatio

10000:1

TM8

Over CurrentProtection relay

6.5 A

240 V/80 kV

500 pF

600 k Water resistor

PC based TM8 control software

The cage

100 MHzoscilloscope

Oilinlet

Oiloutlet

Zm

Measuring

impedence

500 kHz PC based PD

detector

Variac0-240 V


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Clean test system, fill processed oil into the system (eliminate the headspace). (Chapter

3.3)

Calibrate PD detector. Measure the background gases. Generate PD faults. Measure the amounts of fault gases. Data processing and analysis.

4.3.1 Calibrate the PD Detector

A PD experiment system is required to be calibrated and PD background noise needs to be

measured before the start of test.

To calibrate the LEMKE LDS-6 PC based PD detector, both the PD amplitude and voltage

readings need to be calibrated. The PD calibrator was connected in parallel to the test vessel in

order to apply a 50 pC PD signal to the test vessel. The PD detector will then be used to check

and calibrate the measured signal to see if it is 50 pC. The PD calibrator needs to be removed

and a 30 kV voltage will be applied to the test vessel. The measured voltage from the PD

detector was checked and adjusted until the voltage reading matches that of the oscilloscope.


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4.3.2 Measuring Background PD Noise

Before the PD test, the maximum background PD noise signal in air should be determined. Theneedle electrode was firstly removed, and the test circuit was set up as shown in Figure 4.1.

Then, the maximum applied voltage of 60 kV was applied to the test vessel. The PD signal was

recorded for 1 minute and the result are shown in Figure 4.3.

As we can see from Figure 4.3, the maximum PD noise in FR3 under 60 kV is only 30 pCwhich is extremely low in comparison with 4000 pC PD amplitude when the needle electrode

is installed. For this case, the background PD noise could be ignored since the noise is much

lower than the noise cutoff level when the needle electrode is in use. The noise cutoff level was

used to remove the background noise in the PD test, and the detail is described in Section

4.4.2.1.


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is of vital importance for a reliable test result. For this reason, anything could reduce the

dissolved gas concentration such as (1) leakage caused by oil flow or (2) gas consumption

caused by TM8 sampling must be prevented.

To generate a PD fault, the applied voltage is raised at the rate of 2 kV/s until the target voltage

is reached. The voltage is then kept for a certain period of time according to the fault gas

generation rate of each liquid. In FR3 test, because the PD repetition rate is high, the PD signal

was recorded for 1 minute in every 15 minutes; On the other hand, the PD signal in Gemini Xtest was recorded from the beginning to the end due to a much lower repetition rate. The test

voltage was reduced to zero after the test is finished. Then the oil valves were re-opened and

the oil circulation was resumed before the measurement of fault gases by TM8.

4.4 Data Measurement and Process Method

4.4.1 Total Gas Generation Calculation

The calculation method of total fault gases is almost the same as that described in Section 3.4.

The only difference between the total gas generation calculation of the sparking test and the

PD test is that the GIT and GIO are calculated by the peak value instead of the average value.


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Figure 4.4 Example of PD Test DGA Peak Value

The H 2 is the most significant and easy-leaking gas among all generated fault gases. The H 2

peak is therefore chosen as the sign for peak value to obtain a maximum H 2 reading. As we

can see from Figure 4.4, the H 2 (dark blue curve) reaches a peak in 4 hours after the test.

Therefore, the readings of fault gases at the 4th

hour after the test should be used as the results.


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noted that the noise of the PD signal should be filtered out via LDS-6 PD measurement software

before the calculation.

4.4.2.1 Instrument Noise Filtering

During the PD recording, the PD detector was able to remove the small PD noises. This was

achieved by applying a cut-off level manually provided by the operator, and any PDs or noises

with magnitude less than the threshold level was removed. The cut-off level was determined

as a level slightly higher than the PD noise, i.e. a cut-off level of 50 pC based on the noise

result in Figure 4.3. Figure 4.5 shows the effect of noise filtering of a 44 kV test of FR3. As

shown in Figure 4.5, the filtered PD signal (Figure 4.5 (b)) was obtained by removing the noises

less than 130 pC in the recording of all signals (Figure 4.5 (a)).


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W = (4.1)

where the unit of Q is pC and the unit of V is kV. If we convert the pC to C, kV to V, Equation

(4.1) can be rewritten into Equation (4.2) to get the energy in J.

W = (4.2)

In order to judge the PD energy distribution to each band of PD amplitude, the PD energy is

calculated according to 6 PD amplitude bands: 0-1000 pC, 1000-2000 pC, 2000-3000 pC,

3000-4000 pC, 4000-5000 pC, and 5000- 6000 pC (barely used). The Find function of Matlab

will be used here to pick out these PD that are within the proper amplitude band. Equation (4.2)

is still capable for PD energy computation after the qualified PDs are picked out by the Findfunction.

In order to calculate the overall PD energy, the PD power should be obtained by following

Equation (4.3) and linearly extrapolated to the overall period.

P = W/ t (4.3)

S b tit t E ti (4 2) i t E ti (4 3) h


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As stated at the beginning of Chapter 4.4, the PD signal is recorded into several individual PD

files, after the power of each individual file is calculated by Equation (4.5); the average power

needs to be acquired by Equation (4.6):

(4.6)

Lastly, the PD energy can now be computed by Equation (4.7):

(4.7)

Where t total is the full time duration for each PD test. Equation (4.7) is used to compute the total

PD faults energy by Excel, example shown in next Section.

4.4.2.3 Example of PD Energy Calculation

Table 4.1 presents the detail of PD files of the 2000 pC Gemini X PD test which lasts for 1380

minutes. This continuous PD test is separated into 5 PD files. The PD detector recorded 5 PD

files for this continuous PD test with a 60 minutes interval. In this case, according to Equation(4.6), the average power P average of all PD files is equal to (0.02mW*60minutes +


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Table 4.1 Example of PD Test Energy Calculation

PD file of Gemini X test3

Recordingduration

(minutes)

PD power(mW) Energy(J)

1 60 0.02 0.082 60 0.08 0.303 60 0.12 0.44

4 120 0.12 0.865 1020 0.06 3.63

6(not recorded) 60 0.07(notrecorded)0.3(not

recorded)Total 1380 0.07 5.61

4.5 Test Condition and Observation

Table 4.2 shows the list of PD tests. It can be seen that 4 PD tests were carried out in Gemini

X with the PD amplitude of 1500 pC, 2000 pC, 3000 pC and 4000 pC. On the other hand, 8

PD tests were carried out in FR3 with the PD amplitude from 1000 pC to 4000 pC. The PD

faults were applied for different test durations (from 62 minutes to 2880 minutes) until a proper

amount of fault gases was generated Details of the test conditions are listed in Table 4 2


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Table 4.2 List of PD Tests

Oil TestTest

Voltage(kV)

Test duration

(minutes)

PD

amplitude

(pC)

Needle

Gemini X

1 50 2880 1500 New

2 50 2580 3000 After test 1

3 58 1380 2000 New

4 58 1290 4000 After test 3

FR3

1 34 390 1000 New

2 34 360 1000 New

3 44 180 2000 New

4 44 235 2000 After test1

5 57 70 3000 After test 3

6 57 150 3000 After test 5

7 57 70 3000 New

8 61 62 4000 After test 7

All headspace is eliminated from the test vessel before the test started. The oil and headspace

volume of the whole TM8-test vessel system are 2 57 L oil and 77 ml which is the same as the


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Figure 4.6 shows the gas generation rate per hour for Gemini X (Figure 4.6 (a)) and FR3

(Figure 4.6 (b)). The result of FR3 shows in Figure 4.6(b) is the average of two tests with the

same PD magnitude.

(a) Gas generation per hour in Gemini X

0.01.02.0

3.04.05.06.0

C2H4 C2H2 C2H6 H2 CH4 CO TDCG1500pC 0.0 0.1 0.0 0.1 0.0 0.1 0.4

2000pC 0.0 0.5 0.0 0.7 0.1 0.2 1.5

3000pC 0.1 0.3 0.1 0.6 0.1 0.1 1.2

4000pC 0.3 1.2 0.2 3.2 0.6 0.2 5.6

ppm/h

100.0

150.0

200.0

study of dissolved gas analysis under electrical & thermal stresses for oil

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