AGEING ASSESSMENT OF INSULATION PAPER WITH …

AGEING ASSESSMENT OF INSULATION PAPER WITH CONSIDERATION

OF IN-SERVICE AGEING AND NATURAL ESTER APPLICATION

A thesis submitted to The University of Manchester for the degree of

PhD

in the Faculty of Engineering and Physical Sciences

2012

NORHAFIZ AZIS

School of Electrical and Electronic Engineering

____________________________________________________________________________

List of contents

3

LIST OF CONTENTS

LIST OF CONTENTS .......................................................................................................... 3

LIST OF FIGURES ............................................................................................................... 6

LIST OF TABLES .............................................................................................................. 12

ABSTRACT .............................................................................................................. 15

DECLARATION .............................................................................................................. 16

COPYRIGHT STATEMENT ........................................................................................... 17

ACKNOWLEDGEMENT .................................................................................................. 18

CHAPTER 1. INTRODUCTION ............................................................................ 19

1.1. Background ..................................................................................................... 19

1.2. Research objectives ........................................................................................ 20

1.3. Insulating oil used in transformers ................................................................. 21

1.3.1. Background .................................................................................. 22

1.3.2. Mineral oil .................................................................................... 22

1.3.3. Natural ester ................................................................................. 24

1.3.4. Effect of oil ageing to the paper degradation ............................... 26

1.4. Thesis outlines ................................................................................................ 26

CHAPTER 2. LITERATURE REVIEW .............................................................. 28

2.1. Introduction .................................................................................................... 28

2.2. Ageing of cellulose ......................................................................................... 29

2.2.1. Background .................................................................................. 29

2.2.2. Cellulose insulation used in transformers .................................... 30

2.2.3. Paper ageing mechanisms ............................................................ 35

2.2.4. Paper ageing accelerators ............................................................. 46

2.2.5. Paper ageing products .................................................................. 48

2.2.6. Paper ageing assessment techniques ............................................ 53

2.3. Summary ......................................................................................................... 61

CHAPTER 3. METHODOLOGY .......................................................................... 62

3.1. Introduction .................................................................................................... 62

3.2. Paper sampling procedure from scrapped transformers ................................. 62

3.2.1. Phase and winding profile ............................................................ 62

3.2.2. Layer profile ................................................................................. 64

3.3. TS measurements ............................................................................................ 65

3.3.1. TS or TI measurements ................................................................ 65

3.3.2. Factors affecting TI measurements .............................................. 66

3.4. TAN, LMA and HMA measurements ............................................................ 73

List of contents

4

3.4.1. TAN measurements ..................................................................... 73

3.4.2. LMA and HMA measurements .................................................... 74

3.5. Pre-processing of materials for ageing experiment ........................................ 79

3.5.1. Pre-processing of oil .................................................................... 79

3.5.2. Pre-processing of paper ................................................................ 80

3.6. Summary ........................................................................................................ 80

CHAPTER 4. AGEING ASSESSMENT OF PAPER THROUGH TENSILE

STRENGTH MEASUREMENTS ................................................. 81

4.1. Introduction .................................................................................................... 81

4.2. Case study....................................................................................................... 81

4.3. TI measurements of scrapped transformers.................................................... 83

4.3.1. TI profiles of scrapped transformers ............................................ 83

4.3.2. Relationship between TI and age ................................................. 89

4.4. Modelling relationship between TI and transformer age ............................... 93

4.4.1. Modelling procedure for transformer population ........................ 93

4.4.2. Life assessment of transformer population through TI

measurements ............................................................................... 97

4.5. Effect of ageing on dielectric strength of paper ........................................... 104

4.5.1. Experiment description .............................................................. 104

4.5.2. Dielectric strength of paper ........................................................ 106

4.5.3. Oil influence on breakdown voltage of paper ............................ 107

4.5.4. Statistical analysis on breakdown voltage of paper ................... 110

4.5.5. Breakdown mechanism of insulation paper ............................... 113

4.6. Summary ...................................................................................................... 115

CHAPTER 5. AGEING ASSESSMENT OF PAPER THROUGH LOW

MOLECULAR WEIGHT ACID MEASUREMENTS .......... 116

5.1. Introduction .................................................................................................. 116

5.2. Effect of LMA and HMA on breakdown voltage of oil and ageing of paper

116

5.2.1. Effect of LMA and HMA on breakdown voltage of oil ............ 117

5.2.2. Effect of LMA and HMA on ageing of paper ............................ 120

5.3. LMA measurements of scrapped transformers paper................................... 124

5.3.1. LMA profile of scrapped transformers ...................................... 124

5.3.2. Relationship between LMA in paper, TI and transformer age .. 129

5.4. LMA and HMA ratio between paper and oil ............................................... 135

5.4.1. LMA ratio between paper and oil .............................................. 135

5.4.2. LMA and HMA ratio of oil ........................................................ 137

5.5. Life assessment of transformers through LMA measurement ..................... 146

List of contents

5

5.5.1. Calculation procedure ................................................................ 146

5.5.2. Prediction of TI based on LMA in oil ........................................ 148

5.5.3. Prediction of transformers life consumption based on LMA ..... 154

5.6. Summary ....................................................................................................... 155

CHAPTER 6. AGEING ASSESSMENT OF NATURAL ESTER ................ 156

6.1. Introduction .................................................................................................. 156

6.2. Oxidation of natural ester ............................................................................. 156

6.2.1. Ageing of natural ester only ....................................................... 157

6.2.2. Ageing of natural ester with presence of paper, copper and steel

.................................................................................................... 158

6.3. Application of natural ester to sealed transformers ...................................... 164

6.3.1. Ageing of natural ester only ....................................................... 164


.................................................................................................... 165

6.3.3. LMA and HMA of natural ester ................................................. 169

6.4. Effect of LMA and HMA on breakdown voltage of natural ester and ageing

of paper................................................................................................................. 174

6.4.1. Effect of LMA and HMA on breakdown voltage of natural ester

.................................................................................................... 174

6.4.2. Effect of LMA and HMA on ageing of paper ............................ 176

6.5. Summary ....................................................................................................... 179

CHAPTER 7. CONCLUSIONS AND FUTURE WORK ................................ 180

7.1. Conclusions .................................................................................................. 180

7.1.1. General ....................................................................................... 180

7.1.2. Summary of main findings ......................................................... 181

7.2. Future work .................................................................................................. 182

APPENDIX I: Tensile Index (TI) of New and Service Aged Paper ............... 184

APPENDIX II: Low Molecular Weight Acid (LMA) of Service Aged Paper .....

............................................................................................................ 192

APPENDIX III: Effect of Ageing on Breakdown Voltage of Paper .................. 196

APPENDIX IV: Effect of Low Molecular Weight Acid (LMA) and High

Molecular Weight Acid (HMA) on Breakdown Voltage of Oil

and Ageing of Paper ...................................................................... 199

APPENDIX V: List of Publications ........................................................................ 206

REFERENCES ............................................................................................................ 207

Word Count: 50 788

List of figures

6

LIST OF FIGURES

Figure ‎1.1 Hydrocarbons structure of mineral oil [10]. .......................................... 23

Figure ‎1.2 Oxidation of mineral oil [9]. .................................................................. 24

Figure ‎1.3 Structure of trglyceride [12]. ................................................................. 24

Figure ‎1.4 Oxidation of natural ester [13]. ............................................................. 25

Figure ‎1.5 Hydrolysis of natural ester [15]. ............................................................ 26

Figure ‎2.1 Anhydro β-D-glucopyranose monomer units [29]. ............................... 29

Figure ‎2.2 Fibre: macro and micro structure [29]. .................................................. 30

Figure ‎2.3 Cyanoethylation of cellulose [32]. ........................................................ 32

Figure 2.4 Manufacturing process of pressboard [30]. ........................................... 34

Figure 2.5 Factors that could influence oil and paper ageing in the transformer

[26]. ....................................................................................................... 36

Figure 2.6 Procedures to obtain a) ageing rate, k and b) activation energy, E and

pre exponential factor, A. Calculation made based on Lundgaard’s data,

Kraft paper with 3 % in weight water content (Thermo 70) and mineral

oil (Nynas Nytro 10X) are sealed without air and aged at 70 °C, 90 °C,

110 °C and 130 °C. ................................................................................ 41

Figure 2.7 Arrhenius plot under different ageing mechanisms [54]. ...................... 42

Figure 2.8 Oxidation mechanism on cellulose [51]. ............................................... 43

Figure 2.9 Carboxyl and carbonyl groups [55]. ...................................................... 43

Figure 2.10 Hydrolysis mechanism on cellulose [51]. ............................................. 43

Figure 2.11 Mechanism of furanic compounds formation through hydrolysis [56]. 44

Figure 2.12 Pyrolysis mechanism on cellulose [51]. ................................................ 44

Figure ‎2.13 Ageing mechanism of paper in transformers. ....................................... 45

Figure 2.14 Effect of water on paper ageing. Kraft paper (Clupak HD75) and

mineral oil (Nynas Nytro 10CX) are sealed under air and aged from 60

°C to 120 °C [60]. .................................................................................. 46

Figure 2.15 Effect of oxygen on paper ageing. Kraft paper (Thermo 70), mineral oil

(Nynas Nytro 10XN) and copper are sealed with different oxygen

pressures and aged at 90 °C, 110 °C and 130 °C [53]. .......................... 47

Figure 2.16 Effect of LMA on paper ageing under wet condition. Kraft paper

(Thermo 70) and mineral oil (Nynas Nytro 10X) containing LMA and

HMA are sealed without air and aged at 70 °C, 90 °C, 110 °C and 130

°C [17]. .................................................................................................. 48

Figure 2.17 Relationship between water generation and chain scission of paper [22,

61, 62]. ................................................................................................... 49

Figure 2.18 Type of furanic compounds [64]. ......................................................... 49

Figure 2.19 Formation of 2FAL through hydrolysis/oxidation mechanisms. .......... 50

Figure 2.20 Generation of CO and CO2 at different temperatures. Paper, mineral oil,

steel, and aluminium are sealed without air and aged at 100 °C, 110 °C,

120 °C and 140 °C [36]. ........................................................................ 51

Figure 2.21 Relationship between tensile strength/degree of polymerization and

CO/CO2. Paper, mineral oil, steel, and aluminium are sealed without air

and aged at 100 °C, 110 °C, 120 °C and 140 °C [36]. .......................... 51

Figure 2.22 Acids generation for Kraft and thermally upgraded paper. Kraft paper,

thermally upgraded paper, mineral oil and copper are sealed without air

and aged at 90 °C, 110 °C and 130 °C [2]. ............................................ 52

List of figures

7

Figure 2.23 Relationship between a) methanol (CH3OH) and b) furanic compounds

(2FAL) and chain scission (NS). Kraft paper (Clupak HD75) and

mineral oil (Nynas Nytro 10CX) are sealed under air and aged from 60

°C to 120 °C [68]. .................................................................................. 53

Figure ‎2.24 Ubbelohde viscometer tube. .................................................................. 54

Figure 2.25 Stress and strain behaviour in two directions: MD (Machine Direction)

and CD (Cross Machine Direction) [71]. .............................................. 55

Figure 2.26 Long span TS setup. .............................................................................. 56

Figure ‎2.27 Zero span tensile tester. ......................................................................... 57

Figure ‎2.28 Basic HPLC system. .............................................................................. 58

Figure ‎2.29 Basic GC system. ................................................................................... 60

Figure 3.1 Sampling procedure for a) Phase and b) Winding profiles. .................. 63

Figure 3.2 Sampling procedure for layer profile..................................................... 64

Figure 3.3 Paired t-test calculation procedure. ....................................................... 67

Figure 3.4 TI of new conductor paper versus length. Data are mean of 10 samples

and error bars represent the standard deviation. .................................... 68

Figure 3.5 Paper samples arrangement for the effect of folding stress. .................. 69

Figure 3.6 TI of new conductor paper with and without folding stress. Data are

mean of 10 samples and error bars represent the standard deviation. ... 70

Figure 3.7 Preparations of paper samples for the effect of oil impregnation. ......... 71

Figure 3.8 Influence of oil on TI. Data are mean of 10 samples and error bars

represent the standard deviation. ........................................................... 71

Figure 3.9 Paper samples arrangement for the effect reducing paper length in the

clamp. .................................................................................................... 72

Figure ‎3.10 Effect of reducing paper length in the clamp area of TI. Data are mean

of 10 samples and error bars represent the standard deviation. ............. 73

Figure ‎3.11 Procedure to measure TAN. .................................................................. 74

Figure ‎3.12 Procedure to measure LMA and HMA in oil. ....................................... 75

Figure ‎3.13 Procedure to measure LMA in paper. .................................................... 76

Figure ‎3.14 Low molecular weight acid extraction efficiency using water. Data are

mean of 2 measurements. ...................................................................... 78

Figure ‎4.1 TI profiles among different phases a) Transformer 4 (SW) and b)

Transformer 6 (CW). Data are mean of 5 to 10 samples and error bars

represent the standard deviation. ........................................................... 85

Figure ‎4.2 TI profiles among different windings of Transformer 8. Paper samples

were taken from B phase. Data are mean of 4 to 6 samples and error

bars represent the standard deviation. .................................................... 85

Figure ‎4.3 TI profiles among different layers of Transformer 10. Paper samples

were taken from B phase, HV bottom. Data are mean of 9 to 10 samples


Figure ‎4.4 TI profiles among different layers of Transformer 6. Paper samples

were taken from B phase, CW top. Data are mean of 9 to 10 samples


Figure ‎4.5 Example of paper taken from Transformer 10 a) 1st layer b) 2

nd layer c)

3rd

layer d) 4th

layer e) 5th

layer f) 6th

layer g) 7th layer h) 8th

layer.

Paper samples were taken from B phase, HV bottom of Transformer 10.

............................................................................................................... 88

Figure ‎4.6 TI versus age of 9 transformers. Paper samples were taken from

different locations. Data are mean of 4 to 10 samples. .......................... 90

Figure ‎4.7 Daily loading of typical summer (19/06/2009). .................................... 91

Figure ‎4.8 Daily loading of typical winter (04/12/2009). ....................................... 91

List of figures

8

Figure ‎4.9 Average TI and average daily loading of typical summer. ................... 92

Figure ‎4.10 Average TI and average daily loading of typical winter. ...................... 92

Figure ‎4.11 Definition of initial and measured TI. ................................................... 94

Figure ‎4.12 Fitting for the average of the measured TI according to Weidmann

model. The initial TI was not set as fitting parameter. .......................... 95

Figure ‎4.13 Fitting for the average of the measured TI according to Weidmann

model. .................................................................................................... 96

Figure ‎4.14 Fitting for the average of the measured TI according to Ding and Wang

model. .................................................................................................... 97

Figure ‎4.15 Fitting for the average of the measured TI at the top winding. Each point

represents the average of the measured TI at the top winding of each

transformer. ........................................................................................... 98

Figure ‎4.16 Fitting for the average of the measured TI at the bottom winding. Each

point represents the average of the measured TI at the bottom winding

of each transformer. ............................................................................... 99

Figure ‎4.17 Fitting for the average of the measured TI. The square point represents

the average of the measured TI from different locations of each

transformer. ......................................................................................... 101

Figure ‎4.18 Fitting for the lowest of the measured TI. The round point represents the

lowest of the measured TI from different locations of each transformer.

............................................................................................................. 102

Figure ‎4.19 Breakdown voltages of different types of paper impregnated in new oil.

............................................................................................................. 106

Figure ‎4.20 Relationship between breakdown voltages of paper impregnated in new

oil a) Density and b) Grammage. ........................................................ 107

Figure ‎4.21 Breakdown voltages of new and aged oils. ......................................... 108

Figure ‎4.22 Breakdown voltages of layer type paper impregnated in new and aged

oils. ...................................................................................................... 108

Figure ‎4.23 Breakdown voltages of conductor type paper impregnated in new and

aged oils. .............................................................................................. 109

Figure ‎4.24 Breakdown voltages of service aged paper impregnated in new and aged

oils. ...................................................................................................... 109

Figure ‎4.25 Average breakdown strengths of new and aged paper. Data are mean of

50 tests and error bars represent the standard deviation. ..................... 110

Figure ‎4.26 Weibull probability plots of new and aged paper impregnated in new

oil. ........................................................................................................ 110

Figure ‎4.27 Weibull probability plots of new and aged paper impregnated in aged

oil. ........................................................................................................ 111

Figure ‎4.28 Normal probability plots of new and aged paper impregnated in new oil.

............................................................................................................. 111

Figure ‎4.29 Normal probability plots of new and aged paper impregnated in aged

oil. ........................................................................................................ 112

Figure ‎4.30 Multiple breakdowns at single point of paper impregnated in new oil.

............................................................................................................. 114

Figure ‎4.31 Multiple breakdowns at single point of paper impregnated in aged oil.

............................................................................................................. 114

Figure ‎4.32 Paper condition after breakdown tests a) New paper and b) Aged paper.

............................................................................................................. 114

Figure ‎5.1 Influence of LMA on the breakdown voltage of mineral oil. Breakdown

voltage data are mean of 50 tests and error bars represent the standard

deviation. LMA in oil data is 1 measurement. .................................... 118

List of figures

9

Figure ‎5.2 Influence of HMA on breakdown voltage of mineral oil. Breakdown

voltage data are mean of 50 tests and error bars represent the standard

deviation. HMA in oil data is 1 measurement. .................................... 119

Figure ‎5.3 Influence of TAN on the breakdown voltage of in-service oil.

Breakdown voltage data are mean of 50 tests. TAN data is 1

measurement. ....................................................................................... 120

Figure ‎5.4 Influence of LMA and HMA on the paper aged under dry condition.

Data are mean of 8 samples and error bars represent the standard

deviation. ............................................................................................. 121

Figure ‎5.5 Influence of LMA and HMA on the paper aged under wet condition.


deviation. ............................................................................................. 122

Figure ‎5.6 LMA in paper profiles among different phases a) Transformer 4 (SW)

and b) Transformer 6 (CW). Data are mean of 2 measurements. ........ 125

Figure ‎5.7 LMA in paper profiles among different windings of Transformer 8.

Paper samples were taken from B phase. Data are mean of 2

measurements. ..................................................................................... 125

Figure ‎5.8 LMA in paper profiles among different layers of Transformer 10. Paper

samples were taken from B phase, HV bottom. Data is 1 measurement.

............................................................................................................. 127


samples were taken from B phase, CW top. Data are mean of 2

measurements. ..................................................................................... 127

Figure ‎5.10 TI and LMA in paper profiles among different layers of Transformer 10.

Paper samples were taken from B phase, HV bottom. LMA in paper

data is 1 measurement. TI data are mean of 9 to 10 samples. ............. 128


Paper samples were taken from B phase, CW top. LMA in paper data

are mean of 2 measurements. TI data are mean of 9 to 10 samples. ... 129

Figure ‎5.12 LMA in paper versus TI of 9 transformers. Paper samples were taken

from different locations. LMA in paper data are mean of 2

measurements. TI data are mean of 4 to 10 samples. .......................... 130

Figure ‎5.13 LN (LMA in paper) versus TI of 9 transformers. Paper samples were

taken from different locations. LMA in paper data are mean of 2

measurements. TI data are mean of 4 to 10 samples. .......................... 131

Figure ‎5.14 LMA in paper versus transformer age of 9 transformers. Data are mean

of 2 measurements. .............................................................................. 132

Figure ‎5.15 Average LMA in paper and average daily loading of typical summer.

............................................................................................................. 133

Figure ‎5.16 Average LMA in paper and average daily loading of typical winter. . 133

Figure ‎5.17 Fitting of LMA in paper versus transformer age of 9 transformers. Data

are mean of 2 LMA in paper measurements. ....................................... 134

Figure ‎5.18 LMA ratio of paper and oil versus weight ratio of paper and oil. LMA in

paper data are mean of 2 measurements. LMA in oil data is 1

measurement. ....................................................................................... 137

Figure ‎5.19 Daily loading of typical summer (19/06/2009). .................................. 138

Figure ‎5.20 Daily loading of typical winter (04/12/2009). ..................................... 138

Figure ‎5.21 TAN, LMA and HMA of in-service oil. LMA, HMA and TAN of oil

data is 1 measurement. ......................................................................... 139

Figure ‎5.22 LMA and HMA ratio of mineral oil aged under sealed condition without

copper. LMA, HMA and TAN of oil data is 1 measurement. ............. 141

List of figures

10

Figure ‎5.23 LMA and HMA ratio for mineral oil aged under open condition without


Figure ‎5.24 LMA and HMA ratio for mineral oil aged under sealed condition with


Figure ‎5.25 LMA and HMA ratio for mineral oil aged under open condition with


Figure ‎5.26 Calculation procedure for predicting TI and transformer life

consumption. ....................................................................................... 147

Figure ‎5.27 Procedure of modelling the relationship between LMA in oil and

transformer age. ................................................................................... 148

Figure ‎5.28 TAN versus transformer age. Data is 1 measurement. ........................ 150

Figure ‎5.29 LMA in in-service oil versus transformer age. .................................... 151

Figure ‎5.30 Modelling procedure for the relationship between LMA in paper and

LMA in oil ........................................................................................... 152

Figure ‎5.31 Ratio of LMA in paper and oil versus transformer age ....................... 154

Figure ‎6.1 TAN of natural ester and mineral oil aged at 115 °C under open

condition without any presence of paper, copper and steel. Data is 1

measurement. ....................................................................................... 157

Figure ‎6.2 TAN of natural ester aged at 170 °C under open condition with presence

of paper, copper and steel. Data is 1 measurement. ............................ 159

Figure ‎6.3 Viscosity of natural ester aged under open condition with presence of

paper, copper and steel. Data are mean of 5 measurements. Mineral oil

was aged under open condition with paper, copper, zinc, iron and

aluminium [107]. ................................................................................. 161

Figure ‎6.4 Viscosity versus TAN of natural ester aged under open condition with

presence of paper, copper and steel. Viscosity data are mean of 5

measurements. TAN data is 1 measurement. ...................................... 162

Figure ‎6.5 Natural ester aged under open condition with presence of paper, copper,

and steel for single ratio samples. a) Natural ester condition after 50

days of ageing and b) Mixture of aged natural ester and water. ......... 164

Figure ‎6.6 TAN of natural ester and mineral oil aged under sealed condition

without any presence of paper, copper and steel. Data is 1 measurement.

............................................................................................................. 165

Figure ‎6.7 TAN of natural ester and mineral oil aged under sealed condition with

presence of paper, copper and steel. Data is 1 measurement. For S.

Tenbohlen, mineral oil was aged under sealed condition with paper,

copper, zinc, iron and aluminium [107]. For R. Liao, mineral oil was

aged under sealed condition with paper and copper [2]. ..................... 166

Figure ‎6.8 Viscosity of natural ester and mineral oil aged under sealed condition

with presence of paper, copper and steel. Data are mean of 5

measurements. For S. Tenbohlen, mineral oil was aged under sealed

condition with paper, copper, zinc, iron and aluminium [107]. .......... 167

Figure ‎6.9 Viscosity versus TAN of natural ester aged under sealed condition with

presence of paper, copper and steel. Viscosity data are mean of 5

measurements. TAN data is 1 measurement. ...................................... 169

Figure ‎6.10 LMA and HMA of natural ester and mineral oil aged under sealed

condition without any presence of paper, copper and steel. LMA and

HMA of natural ester data is 1 measurement. ..................................... 170


condition with presence of paper, copper and steel. LMA and HMA of

natural ester data is 1 measurement. .................................................... 172

List of figures

11

Figure ‎6.12 LMA in paper of natural ester and mineral oil aged under sealed

condition with presence of paper, copper and steel. Data are mean of 2

measurements. ..................................................................................... 174

Figure ‎6.13 Influence of LMA on the breakdown voltage of natural ester. Data are

mean of 50 tests and error bars represent the standard deviation. LMA

in natural ester data is 1 measurement. ................................................ 175

Figure ‎6.14 Influence of HMA on the breakdown voltage of natural ester. Data are

mean of 50 tests and error bars represent the standard deviation. HMA

in natural ester data is 1 measurement. ................................................ 176

Figure ‎6.15 Influence of LMA and HMA on the paper aged under dry condition.


deviation. ............................................................................................. 177

Figure ‎6.16 Influence of LMA and HMA on the paper aged under wet condition.


deviation. ............................................................................................. 178

List of tables

12

LIST OF TABLES

Table ‎2.1 Previous ageing experiments. ................................................................ 36

Table ‎2.2 Activation energy for oxidation and hydrolysis [27, 53]. ..................... 42

Table ‎2.3 Causes for specific furanic compounds [80]. ........................................ 59

Table ‎3.1 Properties of new conductor and layer types paper. .............................. 66

Table ‎3.2 TI and TS comparison between conductor and layer types paper. ........ 66

Table ‎3.3 Means comparison between each length ............................................... 69

Table ‎3.4 Percentage difference of TI between baseline and samples. ................ 72

Table ‎3.5 Means comparison between baseline and samples. ............................... 72

Table 3.6 Age and paper properties of each transformer. ..................................... 77

Table ‎3.7 LMA measurement on service aged paper. ........................................... 78

Table ‎3.8 Basic properties of Gemini X and FR3. ................................................ 79

Table 4.1 Case study of transformers. ................................................................... 82

Table 4.2 Paper properties and TAN of each transformer for phase and winding

profiles. .................................................................................................. 83

Table 4.3 Age, paper properties and TAN of each transformer for layer profile. . 86

Table 4.4 Classification of initial TI. ..................................................................... 95

Table 4.5 End of life criterion based on TI. .......................................................... 97

Table 4.6 Ageing rate for the average of the measured TI at the top winding. ..... 99

Table 4.7 Ageing rate for the average of the measured TI at the bottom winding.

............................................................................................................... 99

Table 4.8 Estimated life for the average of the measured TI at the top winding. 100

Table 4.9 Estimated life for the average of the measured TI at the bottom winding.

............................................................................................................. 100

Table 4.10 Ageing rate for the average of the measured TI. ................................. 102

Table 4.11 Ageing rate for the lowest of the measured TI. ................................... 102

Table 4.12 Estimated life for the average of the measured TI. ............................. 103

Table 4.13 Estimated life for the lowest of the measured TI. ............................... 103

Table 4.14 Average estimated life for the transformer population. ...................... 104

Table ‎4.15 Properties of paper used for breakdown test. ...................................... 105

Table ‎4.16 Number of paper layers used in breakdown tests. ............................... 105

Table ‎4.17 Average breakdown voltage of new and aged paper impregnated in new

and aged oil based on Weibull fitting. ................................................. 112


and aged oil based on normal fitting. .................................................. 112

Table 5.1 Properties of LMA and HMA. ............................................................ 117

Table 5.2 Ageing rate of dry condition. .............................................................. 122

Table 5.3 Ageing rate of wet condition. .............................................................. 123

Table 5.4 Increment factors of dry and wet ageing conditions. .......................... 123

Table ‎5.5 Increment rate of LMA in paper and constant of the fitting. .............. 135

Table 5.6 LMA ratio of paper and mineral oil. Data is 1 measurement. ............. 135

Table 5.7 LMA and HMA ratio of paper and mineral oil. LMA in paper data are

mean of 2 measurements and LMA in oil data is 1 measurement. ..... 136

Table ‎5.8 Case study of transformers. ................................................................. 137

Table ‎5.9 LMA and HMA percentages of in-service oil. .................................... 140

Table 5.10 LMA and HMA percentages of mineral oil aged under sealed condition

without copper. .................................................................................... 141

Table 5.11 LMA and HMA percentage of mineral oil aged under open condition

without copper. .................................................................................... 143

List of tables

13

Table ‎5.12 LMA and HMA percentage of mineral oil aged under sealed condition

with copper. ......................................................................................... 144

Table ‎5.13 LMA and HMA percentage of mineral oil aged under open condition

with copper. ......................................................................................... 145

Table ‎5.14 Increment rate of TAN and constant of the fitting. ............................. 151

Table ‎5.15 Increment rate of LMA in oil and constant of the fitting. ................... 151

Table ‎5.16 Life consumptions based on average initial TI of 107 Nm/g. ............. 155

Table 6.1 Amount of oil, paper, copper and steel ratio for single ratio. .............. 158

Table 6.2 Amount of oil, paper, copper and steel ratio for double ratio. ............ 158

Table ‎6.3 LMA and HMA percentages in natural ester and mineral oil aged under

sealed condition without any presence of paper, copper and steel. ..... 170


sealed condition with presence of paper, copper and steel. ................. 173

Table 6.5 Ageing rate of dry condition. ............................................................... 177

Table 6.6 Ageing rate of wet condition. .............................................................. 178

Table 6.7 Comparison of increment factors between natural ester and mineral oil

under dry ageing conditions. ............................................................... 179

Table I.1 TI of new conductor type paper without oil impregnation at different

lengths. ................................................................................................. 184

Table I.2 TI of new layer type paper without oil impregnation. ......................... 184

Table I.3 TI of new conductor type paper with oil impregnation. ...................... 184

Table I.4 TI of new conductor type paper under folding stress. ......................... 185

Table I.5 TI of new conductor type paper under less paper length in the clamp

area. ...................................................................................................... 185

Table I.6 TI of Transformer 1. ............................................................................ 185









Table I.15 TI of different layers for Transformer 10. Paper samples were taken

from B phase, HV bottom.................................................................... 190

Table I.16 TI of different layers for Transformer 6. Paper samples were taken from

B phase, CW top. ................................................................................. 191

Table II.1 LMA in paper of Transformer 1. ......................................................... 192



Table II.4 LMA in paper of Transformer 4. ........................................................ 192






Table II.10 LMA in paper of different layers for Transformer 10. Paper samples

were taken from B phase, HV bottom. ................................................ 194

Table II.11 LMA in paper of different layers for Transformer 6. Paper samples were

taken from B phase, CW top. .............................................................. 195

List of tables

14

Table III.1 AC breakdown voltages of different types of paper impregnated in new

oil. The gap distance between two sphere electrodes was set at 0.258

mm. ...................................................................................................... 196

Table III.2 AC breakdown voltages of different types of paper impregnated in aged

oil. The gap distance between two sphere electrodes was set at 0.258

mm. ...................................................................................................... 197

Table IV.1 AC breakdown voltages of new mineral oil with different amount of

LMA. The gap distance between two VDE electrodes was set at 1 mm.

............................................................................................................. 199

Table IV.2 AC breakdown voltages of new mineral oil with different amount of

HMA. The gap distance between two VDE electrodes was set at 1 mm.

............................................................................................................. 200

Table IV.3 AC breakdown voltages of new natural ester with different amount of

LMA. The gap distance between two VDE electrodes was set at 1 mm.

............................................................................................................. 201

Table IV.4 AC breakdown voltages of new natural ester with different amount of

HMA. The gap distance between two VDE electrodes was set at 1 mm.

............................................................................................................. 203

Table IV.5 TI of conductor type paper ageing in mineral oil under dry condition.

Data are mean of 8 samples. ................................................................ 204

Table IV.6 TI of conductor type paper ageing in mineral oil under wet condition.


Table IV.7 TI of conductor type paper ageing in natural ester under dry condition.


Table IV.8 TI of conductor type paper ageing in natural ester under wet condition.


Abstract

15

ABSTRACT

One of the cellulose insulation in high voltage power transformers is paper, and it is

widely used in transformers due to low cost and excellent physical/electrical

properties. However, the performance of paper could be affected by ageing. Ageing of

paper is a complex phenomenon and can be influenced by many factors. This PhD

thesis aimed to examine the ageing of paper in transformers based on Tensile Index

(TI), dielectric strength and Low Molecular Weight Acid (LMA) measurements.

The effect of ageing on the dielectric strength of paper was examined, through

studying the influence of paper and oil ageing by-products such as LMA and HMA on

the AC breakdown voltage. Meanwhile, the end of life for transmission transformer

population in the UK was examined by studying the profile of LMA and TI of paper in

scrapped transformers, with phase, winding, and layer locations taken into

consideration. The relationship between TI, LMA and transformer age was modelled

using previously published formulae in literature. The partitioning of LMA and HMA

between mineral oil and paper was also investigated. Since TI is not directly

measureable when a transformer is in-service, a TI prediction model was developed

based on LMA measured in oil.

The results indicated that there is no reduction effect of ageing (moderate towards

severe) on the dielectric strength of paper, whereas the dielectric strength of paper is

mainly influenced by the oil condition. LMA significantly reduces the breakdown

voltage and accelerates the ageing of paper in mineral oil. The end of life based on TI

is best represented by the 20 % retention of TI. Based on the case studies, the

estimated end of life of transmission transformer population in the UK is around 43

years. The vertical distribution profile of LMA in paper and TI of paper along a

winding is mainly influenced by vertical temperature profile. As paper ages, the

amount of LMA in paper tends to increase as TI reduces. The percentage of LMA in

service aged oil is about one fourth of Total Acid Number (TAN). The partition of

LMA in mineral oil and paper favours its stay in paper and is heavily influenced by the

weight ratio of paper and oil.

The performance of the natural ester under high temperature ageing in open or sealed

condition was evaluated based on TAN and viscosity measurements. The ratio of LMA

and HMA in aged natural ester was examined. The effect of LMA and HMA on the

AC breakdown voltage of natural ester and ageing of paper was also investigated. It

was suggested that natural ester is suitable for application in hermetically sealed

transformers. High temperature oxidation has significant effect on the physical and

chemical properties of natural ester. Due to the polar nature of natural ester, the stay of

LMA in natural ester is favoured over in paper which could be the reason why the

breakdown voltage of natural ester and the ageing of paper are not significantly

affected by LMA as in mineral oil.

Declaration

16

DECLARATION

I declare that no portion 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 institute of learning.

Copyright Statement

17

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 s/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 property (the “Intellectual Property”) and any reproductions of copyright

works in the thesis, for example graphs and tables (“Reproductions”), which may be

described 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 and

commercialisation 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 http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-

property.pdf), in any relevant Thesis restriction declarations deposited in the

University Library, The University Library’s regulations (see

http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s

policy on presentation of Theses.

http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-property.pdf

http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-property.pdf

http://www.manchester.ac.uk/library/aboutus/regulations

Acknowledgement

18

ACKNOWLEDGEMENT

I would like to express my sincere gratitude to my supervisor Professor Zhongdong

Wang for her constant guidance to my research and her great effort for the research. I

thank her for her trust in my ability to conduct research. Many thanks are also given to

my advisor Dr William Sampson for his technical support for the research.

I am greatly indebted to Malaysian Ministry of Higher Education for the financial

sponsorship which covered my tuition fees and living maintenance for three years. I

would also like to thank Dr. Hongzhi Ding, Mr. Simon Ryder and Dr. Richard

Heywood of Doble Engineering, Mr. Paul Jarman and Dr. Gordon Wilson of National

Grid for their technical support throughout the project. Great thanks are also given to

Mr. Adrian Handley and Mr. Frank Hogan for their laboratory support and Dr. Keith

Cornick for some advices during talks with him.

To all my colleagues in the transformer research group and others in Ferranti building,

School of Electrical and Electronic, I appreciate your company and thank you for

providing an enjoyable working environment.

Last but not least, I wish to give my thanks to my parents, my wife Mrs. Siti Nur Sarah

and my daughters Aimi Balqis and Auni Balqis for their patience and selfless support

during my research.

Chapter 1 Introduction

19

CHAPTER 1. INTRODUCTION

1.1. Background

The majority of power transformers in the power system network are oil-filled and

contain cellulose as an insulator. Apart from cooling transformers, oil also provides

electrical insulation together with cellulose. One of the major cellulose based materials

used in transformers is paper.

As transformers get older, both oil and paper will experience ageing and generate by-

products. These products could affect the performance of transformers and lead to the

risk of failure. Thus, it is important to determine the ageing status of transformers by

monitoring the condition of oil and paper. Oil condition can be determined by different

types of test such as acidity, interfacial tension and oxidation stability. On the other

hand, paper condition is best represented by mechanical strength which can only be

measured directly after transformers are scrapped. There are two types of the test that

can be used to determine the mechanical strength of paper which are Degree of

Polymerization (DP) and Tensile Strength (TS). DP is known as representative

strength measurement of paper while TS is regarded as the true strength measurement

of paper. Both methods can be used as a criterion to determine the end of life of

transformer.

According to [1], it was suggested that the definition of end of life of a transformer

should be made based on measureable mechanical, dielectric and chemical

characteristics. TS or DP is not directly measureable when a transformer is in

operation. Dielectric strength is the characteristic that would relates closely to the

functionality of a transformer, however, it deteriorates extremely slowly as the ageing

progresses, thus, the other alternative to describe the end of life of a transformer is the

chemical characteristic. Through latest knowledge on the paper ageing, the chemical

characteristic could be determined by means of Low Molecular Weight Acid (LMA).

Moreover, LMA could also acts as an ageing indicator provided the partitioning

between LMA in oil and paper could be obtained.


20

Natural ester is considered as the alternative for conventional mineral oil due to the

high fire safety and environmental friendliness. There are several aspects such as

dielectric properties and ageing performance that requires further investigation at

laboratory level before natural ester could be applied in transformers since failure in

the field could be costly. Whilst the ageing performance of natural ester is well

investigated by previous researchers [2-7], the performance of high temperature

oxidation on the natural ester still remains unclear. Moreover, there is less information

regarding the impact of oxidation toward its physical and chemical properties. Apart

from oxidation, there has been no study carried out on the LMA and High Molecular

Weight Acid (HMA) partitioning in natural ester and the effect of these acids on paper

ageing and the dielectric strength.

1.2. Research objectives

The aim of this research is to examine the in-service ageing of paper through physical,

electrical and chemical analyses by means of scrapping transformers. Ageing

assessment of natural ester is also covered in this PhD thesis. Topics covered in this

thesis are as follows:

Ageing assessment of transformers through physical and electrical analysis

It is regarded that the life of a transformer is represented by the paper condition.

Several methods including intrusive and non-intrusive were developed in order to

represent the paper ageing condition. However, Intrusive methods such as Tensile

Index (TI) and DP measurements are known as most accurate representation. It is

impossible to obtain paper samples for TI or DP measurement while transformers are

still in-service and the only way is through scrapping transformers. Most of previous

investigations on the scrapped transformers used DP, and the approach used in this

research uses TI to represent the strength of the paper. Conventional winding and

phase profiles are analyzed with the addition of layer profile which exists in the

multilayer paper insulation. The major attempt is to determine the end of life of

transformer population based on TI measurement. Apart from mechanical strength, the

dielectric strength of the scrapped transformer’s paper is also investigated in order to

verify the effect of ageing on the dielectric strength of the paper obtained.


21

Ageing assessment of transformers through chemical analysis

Ageing of paper generates different by-products such as water, furanic compounds,

gases and acids. It was found that acids generated by oil and paper ageing can be

differentiated as either LMA or HMA. It is necessary to study the effect of these acids

on the dielectric strength of mineral oil, since this property is most closely related to

transformer functionality. These acids could also act as paper ageing accelerator.

Further investigation is required to verify the effect of LMA and HMA on the paper

ageing in mineral oil. The knowledge on the tendency of LMA to stay in paper

provides an opportunity to measure the chemical property of the scrapped

transformer’s paper. The partitioning of LMA at different phase and winding can be

determined based on this knowledge. By combining TI information, an additional

criterion of the paper end of life based on LMA can be made. Most importantly, it is

proposed that the TI could be predicted through LMA measurement.

Ageing assessment of natural ester

The research on the application of natural ester as the insulation liquid for transformers

is still currently on-going. One of the crucial areas is the oxidation of natural ester

under high temperature. There is not much knowledge of the effect of severe oxidation

on physical and chemical properties of natural ester. Viscosity and Total Acid Number

(TAN) measurements are carried out on the natural ester subjected to high temperature

ageing under either open or sealed condition. It is necessary to examine the ratio of

LMA and HMA in natural ester. Further investigation is also required to determine the

effect of LMA and HMA on the dielectric strength and paper ageing in natural ester.

Based on this study, it is proposed that the reason for the good ageing performance of

paper ageing in natural ester is due to the fact that less LMA stays in the paper whereas

most of them are absorbed by the natural ester.

1.3. Insulating oil used in transformers

This thesis focuses on the ageing of paper in transformers. However, knowledge on the

ageing of oil is required since it co-exists together with paper in transformers. An


22

overview on the oil ageing mechanisms and how its by-products affect the paper

ageing is given in this section.

1.3.1. Background

Oil is known as an integral part of insulation in a transformer apart from cellulose. The

reliability of a transformer is mainly determined by both oil and paper. The main

function of oil is to dissipate the heat generated by the transformer winding and core,

to insulate between components at different potentials including being able to

withstand system transients due to switching or lightning surges [8].

Mineral oil is the common insulating liquid used in a transformer. It is derived from

crude petroleum formed from buried and decayed vegetable matter or by the action of

water on metal carbides. It is a complex mixture of carbon and hydrogen with a small

proportion of sulphur and nitrogen [8].

Another type of insulating liquid available on the market nowadays is natural ester. It

is derived from vegetable seed oil which is biodegradable and non toxic. Another

advantage of natural ester is the high fire and flash point. However, it has poor

oxidation stability and not suitable to be used in a free breathing transformer.

1.3.2. Mineral oil

Mineral oil is made from different types of hydrocarbons such as paraffins,

naphthenes, aromates and polyaromates as shown in Figure ‎1.1. Hydrocarbons of

paraffinic type display a straight chain or branched configuration, unlike the cyclic

form of the naphthenic compound, resulting in a considerable variation in chemical

properties [9]. Paraffinic structure is susceptible towards water and oxidation products

which could lead to the generation of sludge. However, it has a good viscosity index.

On the other hand, the naphthenic structure known as cycloalkanes has a good low

temperature property. The ring structure could be in 5, 6 or 7 carbons. Aromatic

molecules are ring structures with alternating double bonds. They are totally different

from paraffinic and naphthenic molecules, chemically and physically. Nearly all the

sulphur and nitrogen in oil are present in its aromatic structures. Aromates can be


23

present as both monoaromates and polyaromates. Polyaromates have several aromatic

rings directly adjacent to each other [10]. Most mineral oils are saturated hydrocarbons

where carbon atoms are connected by single C-C valence bonds. The saturated

compounds display better chemical stability than the corresponding unsaturated

compounds [9].

Figure ‎1.1 Hydrocarbons structure of mineral oil [10].

1.3.2.1. Mineral oil ageing mechanisms

Ageing of mineral oils are mainly subjected to oxidation. The result of oxidation could

lead to formation of different chemicals species, including aldehydes, ketones,

hydroperoxides and carboxylic acids.

Oxidation

Oxidation of mineral oil starts with the induction period where the hydrocarbon

molecule reacts with oxygen to produces the radical as shown in Figure 1.2. The

radical will react with oxygen to produce peroxide. The next stage is the propagation

period where the peroxide decomposes into hydroperoxide. The instantaneous products

of oxidation include alcohols and ketones, which are subjected to further degradation,

could form carboxylic acids, keto- and hydroxy-carboxylic acids. Other product from

the decomposition of ketones is aldehydes which eventually form resinous deposits by

complex condensation reactions. Generally, the final products of oxidation could be

classified into 2 types. The first type is soluble compounds which are acids and gases.

The second compounds include sludge and x waxes which are insoluble in mineral oil


24

[11]. The oxidation of mineral oil is influenced by fluid composition, temperature,

oxygen and catalyst metals such as copper and iron [9].

Figure ‎1.2 Oxidation of mineral oil [9].

1.3.3. Natural ester

Natural ester originated from glycerol and fatty acids which are known as triglycerides.

The fatty acid segments are composed of straight chains having an even number of

carbon atoms. This is the natural result of the biosynthesis of fats, where molecules are

built up to two carbons at one time. Figure 1.3 shows the typical structure of the

triglycerides molecule where the (R, R′, R″) groups consist of C8-C22 chains. Natural

ester used for transformer application is fatty acid ester triglycerides type. The fatty

acid components are linear chains 14-22 carbons long containing zero to three double

bonds [12].

Figure ‎1.3 Structure of trglyceride [12].


25

1.3.3.1. Natural ester ageing mechanisms

Natural ester could be subjected to either oxidation or hydrolysis due to the structure of

its molecule. In natural ester, the double C-C valence bonds could promote oxidation

while the single COOH valence bonds could introduce hydrolysis.

Oxidation

According to the previous findings [7, 13], oxidation starts with the induction period

where the carbon atom in natural ester reacts with oxygen to create free radical by the

loss of hydrogen atom [14]. The free radical reacts with oxygen and creates peroxide

radicals as shown in Figure 1.4. The propagation period starts once the peroxide

radicals decompose to create hydroperoxide. The final by-products of this reaction

include aldehydes and ketones. The reaction between radicals needs catalysts such as

iron or copper.

Figure ‎1.4 Oxidation of natural ester [13].

Hydrolysis

Hydrolysis of natural ester can take place in three stages where all of them are

reversible which can be seen in Figure 1.5. The first stage is the creation of diglyceride

formed by trglyceride interaction with water [15]. Once the diglyceride reacts with

water, it will create monoglyceride which will form into glycerol as final reaction

product. During all stages, fatty acid will be created as a by-product [7].


26

Figure ‎1.5 Hydrolysis of natural ester [15].

1.3.4. Effect of oil ageing to the paper degradation

Oxidation of mineral oil could generate water and acids such as LMA which could

accelerate the paper ageing [16-22]. The insoluble compound such as sludge could

accumulate and might block the flow of the oil which eventually could promote

hotspot and increase the rate of ageing of the paper [11].

On the other hand, it is shown that the ageing of paper in natural ester could be delayed

due to the water scavenging and hydrolytic protection [4]. However, natural ester has

low oxidation stability where, under extreme condition, the physical and chemical

properties could change and affect its performance as an insulating liquid [8].

1.4. Thesis outlines

The summary of each chapter in this PhD thesis can be seen as follows:


This chapter provides the general background of the work and the motivation of the

research. A general overview on the ageing of oil is summarized in this chapter.

Chapter 2 Literature review

This chapter gives detail overview on the ageing of paper. The latest knowledge on

paper ageing mechanisms, ageing by-products and ageing accelerators is summarized

together with techniques that can be used to assess the paper ageing condition.


27

Chapter 3 Methodology

This chapter describes the paper sampling procedure from transformers and detail

procedures for TI, LMA and HMA measurements. Several issues regarding the effect

of paper sample condition on TI measurement are also discussed in this chapter.

Chapter 4 Ageing Assessment of Paper through Tensile Strength Measurements

This chapter investigates the TI distribution profile in a transformer with phase,

winding and layer profiles taken into consideration. Analysis was carried out on a

transformer population and modelled using formulae obtained from previous literature.

Through this assessment, the end of life of transformers population is determined and

compared with the existing criterion in the standard. The relationship between

mechanical strength and dielectric strength is also determined under different condition

of oil.

Chapter 5 Ageing Assessment of Paper through Low Molecular Weight Acid

Measurements

This chapter discusses the effect of LMA and HMA on the dielectric strength of oil

and paper ageing. The LMA distribution profile among different phases, windings and

layers is described. The correlation between LMA and TI is examined based on

transformer population analysis. Through this finding, the TI prediction model based

on LMA in oil is proposed, using the knowledge obtained from the LMA/HMA in oil

and partitioning ratio of LMA in oil/paper.

Chapter 6 Ageing Assessment of Natural Ester

This chapter describes the performance of natural ester under high temperature

oxidation. The suitability of natural ester under sealed condition is also examined.

TAN, viscosity, LMA and HMA measurement are used for the ageing assessment of

natural ester. The effect of LMA and HMA on the paper ageing is examined as well as

its effect toward the breakdown voltage.

Chapter 7 Conclusions and Future Work

This chapter summarizes all the main findings in the PhD thesis and gives further

suggestions on the future work of ageing assessment of the mineral oil and natural

ester filled transformers.


28

CHAPTER 2. LITERATURE REVIEW

2.1. Introduction

Paper and pressboard are one of the basic insulation materials used in the oil filled

transformers. The good electrical and thermal properties are one of the advantages of

these materials. Paper is usually used to wrap the winding conductors. It is known that

under large oil gap, the breakdown could occur at a lower voltage due to the fact that

the probability of finding weakest spot will be high. This effect is also known as

volume effect [23]. Pressboard is used as spacer and barrier to divide the oil volumes

into smaller oil gaps in order to enhance the dielectric strength of oil gaps. The total

breakdown can be avoided if flashover occurs in an oil gap since the voltage is taken

by the rest of oil gaps [24].

Both paper and pressboard are made from cellulose. One of the drawbacks of cellulose

insulation is that the ageing is irreversible. As the paper ages, the transformer

performance could be affected. For example, local carbonising of the paper increases

the conductivity to cause overheating which leads to conductor fault and brittle paper

can break away from the winding and block the ducts [25, 26]. Furthermore, the by-

products of paper ageing such as water and acid also could accelerate the degradation

of the paper.

Another type of insulation material in the transformer is oil. Oil can provide dual

functions as cooling and electrical insulation medium within the transformers.

Conventionally, most of transformers are filled with mineral oil. Mineral oil is not

biodegradable and has low fire and flash points. Due to the increase of high fire safety

demand, alternative liquid such as natural esters are becoming available for

transformers.

It is important to monitor the condition of the paper to avoid catastrophic failure of a

transformer. Mechanical strength measurement is the best technique to determine the

ageing state of the paper since only this property is affected by ageing and not

electrical and other properties. However, it is impossible to obtain paper samples from

in-service transformers. Therefore, non-intrusive techniques are used to determine the


29

condition of the paper by measuring by-products of paper ageing from the oil such

Furanic Compound Analysis (FCA) and Dissolved Gas Analysis (DGA).

A review on the mechanisms and factors that could affect paper ageing is given in this

chapter. A study was conducted in order to examine techniques that can be used to

monitor the condition of paper ageing based on ageing by-products. The mechanisms

of oil ageing and its effect to the paper degradation are discussed in the last part of this

chapter.

2.2. Ageing of cellulose

2.2.1. Background

Cellulose consists of anhydro β-D-glucopyranose units linked to each other as shown

in Figure 2.1. The repeating unit of glucose ring in the structure is joined by the

glycosidic oxygen linkage which is one of the parameters that contributed to the

mechanical strength of the paper [27, 28]. This glucose ring is also connected together

through the hydroxyl groups to form a cellulose fibre.

Figure ‎2.1 Anhydro β-D-glucopyranose monomer units [29].

Insulation paper consists of 90% cellulose, 6-7% hemi-cellulose, lignin and pentosans

[26]. Alpha cellulose is normally used as the main cellulose content because it has a

high polymerization level of 2000. The strength of the paper will be affected if the

hemi-cellulose content is higher than 10 %. Lignin provides more rigid fibre bonding

for extra strength and flexibility [30].

Most of the insulation paper is made from softwood because it is more flexible and

stronger than hardwood [29]. Apart from that, softwood has much longer fibre than

hardwood. The structure of the fibre can be seen in Figure 2.2. Fibre is derived from


30

the multiple units of fibrils consisted of cellulose associated with amorphous and

crystalline region [18].

Figure ‎2.2 Fibre: macro and micro structure [29].

2.2.2. Cellulose insulation used in transformers

2.2.2.1. Insulation paper for normal purpose

Kraft paper

Kraft paper is made from unbleached softwood pulp by the sulphate process [8]. The

result of process will produce a paper that is slightly alkaline due to the existence of

the sulphate content. The process to produce paper differs according to the

manufacturer. The quality and reliability of the paper depend on the type of the

manufacturing process [8].

The general procedure starts by creating wood pulp mat from dried fibre. Next, large

amount of water was mixed with the wood pulp mat in order to remove any chemical


31

residuals and this procedure is known as repulping process. Refining is done by

crushing the fibre in the wet state which will result in ‘hydrogen bonding’ between

cellulose molecules [8]. In cellulose materials, hydrogen bonds usually form by

bonding of hydroxyl group (OH) bonds to an electronegative atom. In principle,

hydrogen bonds between fibers or between fibrils are not different with hydrogen

bonds between cellulose [31].Extensive cleaning of the refined fibre will remove any

chemical or physical residue that might exist during the refining process. The last step

is to direct the cellulose and water mixture to the rotating cylindrical machine to form a

paper. The density, thickness and moisture of the paper can be controlled by means of

heat and pressure as it progress to the rolls [8].

Cotton paper

Cotton paper is another alternative to Kraft paper. The manufacturing of cotton paper

for electrical purpose starts in 1930s and the source of cotton was obtained from waste

and offcuts from cotton cloths. However, this source is not used anymore since most of

cloths in recent years contain synthetic fibres and other materials. Cotton linters

obtained from cotton plant are now used as an alternative source for cotton paper

manufacturing. Cotton linters are obtained from the cotton plant fibre [8].

Cotton paper has longer fibre than Kraft paper but the intrinsic bond is not good. The

production cost of cotton paper is quite high due to the extra work for refining and

crushing processes. The performance of cotton paper could also be improved by

combining with Kraft fibre which has excellent electrical property. The result of this

process creates a paper which has good electrical and mechanical properties as well as

maximum oil absorption capability. [8].

2.2.2.2. Insulation paper for special purpose

Creped paper

One of the earliest special purpose papers is Creped paper. It is made by irregular close

crimp, which significantly increases its thickness and extensibility in machine

direction. It is usually used at the connection lead or on the electrostatic stress control


32

rings. Its extensibility enables the paper to be used on the irregular contour. The

disadvantage of the Creped paper is that it will lose its extensibility with time and may

cause some of the connection or joint wrapped by this paper becoming loose and not as

tight as when it was first applied [8].

Highly extensible paper

Highly extensible paper is one of the available alternatives for Creped paper. The

paper is made by adding elastic property during manufacturing process which gives

improved burst, stretch, and cross machines tear properties while retaining its electrical

and mechanical capability. This paper also could withstand rough condition and

wrinkling, which are perfect for winding subjected to high stress [8].

Thermally upgraded paper

Another type of special purpose paper is thermally upgraded paper. It is made by

adding stabilizers which improves its temperature stability and reduces thermal

degradation. Through this process, the OH groups in cellulose will link to the

cyanoethyl-ether groups which are shown in Figure 2.3. Another method is to add

weak organic bases such as dicyandiamide, urea, or melamine during manufacturing

processes to consume water and neutralize acids produce by oxidation of oil and paper

[18]. The main function of thermally upgraded paper is to slow down the ageing

process of the paper and increases the maximum operating temperature of

transformers [8].

Figure ‎2.3 Cyanoethylation of cellulose [32].


33

Diamond dotted paper

Diamond dotted paper is used for application where high mechanical strength is

needed. The paper is made by coating the normal Kraft paper with thermosetting resin

and curing the resin later in the process. The diamond dotted shape is obtained by pre-

coating the paper with two stages resin in a diamond pattern. The resin diamond dotted

creates a large bonding surface whilst ensuring that paper can be efficiently dried and

impregnated with oil [8].

2.2.2.3. Pressboard

Pressboard is another type of solid insulation, which is made by laying number of

papers together during the wet stage. The typical manufacturing process can be seen in

Figure 2.4. At the beginning, the manufacturing process is similar to paper

manufacture. The difference is at the end process where multiple layers of paper web

form are conveyed to the forming roll, instead on single layer, for manufacturing

pressboard [30]. Wet stage is a process where water is removed from cellulose/water

mixture by rotating cylindrical screen and creates paper web form. Dry stage is the

final process of pressboard manufacturing where the paper sheet is fed to a heavily

loaded steel rollers followed by drying means of heat [8]. Pressboard can be divided

into two categories:

Pressboard and presspaper without any adhesive [33].

Pressboard with adhesive and high thickness due to the number of individual

boards bonded together and known as laminated paper [34].


34

Figure 2.4 Manufacturing process of pressboard [30].

Pressboard and presspaper for electrical purpose is available in thickness up to 8 mm

and is generally used at thickness around 2 to 3 mm for inter-winding wraps and 4.5 to

6 mm for strips. On the other hand, laminated pressboard is available in thicknesses up

to 50 mm and is generally used for support. The material of both types of pressboard

are produced in three different methods [8]:

Calendered pressboard

This type of pressboard initially undergoes pressing at about 55 % water content. The

next step is the dry stage without pressure until it reaches 5 % of moisture level. The

final density of this pressboard is around 0.9 kg/dm³ to 1.0 kg/dm³ and further

compression will increase the density up to around 1.15 kg/dm³ to 1.3 kg/dm³ [8].

Mouldable pressboard

Mouldable pressboard does not undergo initial pressing such as calendered pressboard.

Instead of pressing, it goes directly for drying until the moisture reaches 5 % and gives

a soft pressboard which is good for oil impregnation and flexibility to be shaped if

necessary. The final density is much lower than calendered pressboard which is around

0.90 kg/dm³ [8].


35

Precompressed pressboard

Precompressed pressboard is made by combination of pressing and drying during the

wet stage. The process will create a pressboard that is strong and stable with a density

of 1.25 kg/dm³. This type of pressboard is preferred by most of the transformer

manufacturers nowadays because of its stability against thermal and mechanical stress

[8].

2.2.3. Paper ageing mechanisms

Ageing of oil and paper in transformers is a complex phenomenon and can be

influenced by many factors as described by Figure 2.5. It is shown that the ageing of

oil and paper could be accelerated by the presence of oxygen and water from external

surrounding. Thermal fault also could enhance the ageing of oil and paper. Other

source of ageing accelerator for paper could come from water, acids and oxygen

produced by ageing of the oil.

As the paper reaches the end of its life, fibres might break from the paper and are

dissolved in the oil. Fibres combined with metallic particles originated from the tank,

could initiate discharge in the oil which eventually led to arcing and sparking. The risk

of electromagnetic force, transient overvoltage, mechanical vibration and short circuit

force to the transformer will also be high once the paper of a transformer is degraded.

Water could reduce the resistivity and increase the dielectric loss of paper while

fibrous materials could align in the oil and provide pathways for short circuit between

conductors.

The aim of obtaining the knowledge of ageing of oil and paper is to prevent the

transformer from failing in-service by determining the current condition state. The

common approach is through analysis of ageing by-products which can be achieved

through DGA and FCA. DGA is mainly used for analyzing gases produced by faults

on oil and paper. On the other hand, FCA is a common method to analyze the ageing

state of paper since it is impossible to obtain paper while the transformer is still in-

service.


36

Figure 2.5 Factors that could influence oil and paper ageing in the transformer [26].

Laboratory ageing experiments have been conducted by previous researchers under

various conditions in order to study and understand the mechanisms of paper ageing.

The summary of the work can be seen in Table 2.1.

Table ‎2.1 Previous ageing experiments.

Author Temperature

and type of

oil/paper

Type

of

test

Main findings and

conclusions

Comments

Shroff

[35]

110 °C,

120 °C,

130 °C and

140 °C

(MO, KP and

TUP)

DP,

TS,

FCA,

DS

and

MC

Proposed the paper end of

life based on the criteria:

o DP equal to 250

o TS equal to 50 % of

its strength.

Demonstrated the

relationship between DP

and water/furanic

compounds.

Demonstrated that ageing

has small effect on the

paper dielectric strength.

DP is the

common

method for

determining

the ageing of

paper.

Only few used

TS and it is

commonly

used for

analyzing

laboratory


37

Yoshida

[36]

100 °C,

110 °C,

120 °C and

160 °C

(MO, KP and

PB)

DGA,

TS,

DP

and

DS

Demonstrated the

relationship between CO)/

CO2 and DP/TI.


has no effect on the

breakdown voltage of

paper.

aged paper.

The approach

of this research

is to analyze

the ageing of

paper obtained

from scrapped

transformers

by TS.

In addition, the

end of life of

transformer

population is

determined

based on TS.

FCA and DGA

are the

common non-

intrusive

method used

for analyzing

the ageing of

paper.

In this

research, it is

propose to use

LMA as an

additional

paper ageing

assessment

tool.

Moreover, a TI

prediction

model based

on LMA is

proposed.

Morais

[37]

125 °C,

140 °C and

155 °C

(MO and KP)

DP,

DGA,

FCA

and

MC

Demonstrated the

relationship between DP

and CO/furanic

compounds/water

Hill [38-

40]

129 °C,

138 °C,

153 °C and

166 °C

(MO and KP)

MW,

TS,

DP

and

FCA


of paper could be

represented by MW

measurement.

Introduced the paper

degradation model based

on TS where the

activation energy for TS

is almost similar to MW

(76 kJ/mole and 79

kJ/mole).

Described the relationship

between furanic

compounds and DP/ TI.

Lessard

[41]

150 °C,

200 °C and

250 °C

(MO, KP and

TUP)

Sugar

and

FCA

Suggested that sugar

(cellobiose) can be used

to monitor the ageing of

paper and is more stable

than furanic compounds.

Levchik

[42]

70 °C to

160 °C

(MO and KP)

FCA,

DGA,

MC

and

DP

Demonstrated that

significant amount of

furanic compounds,

water, CO and CO2

occurs once there is a

large reduction of DP.


38

Gasser

[43]

120 °C,

135 °C and

150 °C

(MO, KP and

TUP)

DP

and

TS

Suggested that Weidmann

paper degradation model

[22], requires required

less data for fitting of

experiment data [28, 44].

Emsley

[20, 25,

44-46]

120 °C,

140 °C,

160 °C and

180 °C

(MO, KP and

CP)

DP,

TS

and

FCA

Suggested that ageing of

paper in transformers is

mainly dominated by

hydrolysis with a

common activation

energy value of 111

kJ/mole.

Demonstrated that fast

increment of furanic

compounds occurs at DP

around 400 and the

concentration starts to

reduce at DP around 200.

Introduced the paper


on TS and its relationship

with DP.

McShane

[3-5, 47]

130 °C,

150 °C and

170 °C

(MO, NE, KP

and TUP)

DP,

TS

and

MC

Suggested that the ageing

rate for paper aged in

natural ester is lower than

in mineral oil.

Proposed mechanisms of

protection provided by

natural ester to the paper:

o Water scavenging –

Water removal from

paper by natural ester.

Water has significant

effect on the paper

ageing.

o Hydrolytic protection

– Water is consumed


39

by hydrolysis of

natural ester.

Thomas

[48]

85 °C,

100 °C and

115 °C

(MO and KP)

DP,

TS,

DS

and

FCA

Verify previous findings

on furanic compounds as

a non-intrusive technique

to monitor ageing of

paper.

Mirzae

[49]

140 °C,

150 °C and

160 °C

(MO and KP)

TS

and

DGA

Introduced the


on TI according to power

exponential law.

Verma

[50]

120 °C,

140 °C and

160 °C

(MO and KP)

TS Demonstrated that for

multilayer insulation

(several layers of paper

wrapped around same

conductor), outer layer

paper is more affected by

ageing than inner layer

paper.

Demonstrated that

thermal stress has greater

effect than electrical

stress on accelerating the

ageing of paper.

Luundgard

[16-18]

70 °C,

90 °C,

110 °C and

130 °C

(MO, KP and

TUP)

DP,

TS,

LMA

and

HMA

Demonstrated that

hydrolysis of paper is

catalyzed by dissociation

of acids.

Introduced acids

generated by oil and paper

ageing can be LMA and

HMA.

Demonstrated that LMA

has more significant

effect on paper ageing

than HMA.


40

Liao [2] 90 °C,

110 °C and

130 °C

(MO, NE, KP

and TUP)

DP,

MC

and

FCA


of paper in natural ester

follows the paper

degradation model

proposed by Emsley [44].

Suggested that most of

acids produced by ageing

of natural ester are HMA.

MO= Mineral Oil, NE= Natural ester, KP= Kraft Paper, TUP= Thermally Upgraded Paper, PB=

Pressboard, CP= Cotton Paper, DP= Degree of Polymerization, TS= Tensile Strength, TI= Tensile

Index, MC= Moisture Content, MW= Molecular Weight, LMA= Low Molecular Weight Acids, HMA=

High Molecular Weight Acids, DGA= Dissolved Gas Analysis, FCA= Furanic Compound Analysis,

DS= Dielectric Strength, CO= Carbon Monoxide, CO2= Carbon Dioxide

According to several studies [18, 27, 29, 35, 51], paper degradation mechanisms can

be oxidation, hydrolysis and pyrolysis. Oxidation mechanism dominates at

temperatures less than 60 °C while hydrolysis initiates at a temperature region from 60

°C to 150 °C and pyrolysis dominates at high temperature usually over 150 °C [52].

Oxidation, hydrolysis and pyrolysis can be differentiated by looking into the activation

energies. Activation energy can be calculated by Equation (2.1) known as the

Arrhenius equation.

-E

RT

t 0

1 1- = k×t = Aexp ×t

DP DP (2.1)

Where DPt = Current Degree of Polymerization (DP)

DP0 = Initial DP

k = Ageing rate

A = Pre exponential factor

E = Activation energy (kJ/mole)

R = Molar gas constant (8.314 J/mole/K)

T = Temperature (K)

t = Ageing duration (hours)


41

The value of the pre exponential factor is dependent on the environment condition.

Activation energy is used to describe how fast the ageing rate changes with

temperatures [35]. The steps to obtain activation energy and pre exponential factor are

described below:

First, calculate the ageing rate, k for different ageing temperatures based on DP

as shown in Figure 2.6 a).

Next, plot ln k against 1/T. Through regression analysis, the activation energy

and pre exponential factor can be calculated as shown in Figure 2.6 b).

a) b)

Figure 2.6 Procedures to obtain a) ageing rate, k and b) activation energy, E and pre

exponential factor, A. Calculation made based on Lundgaard’s data, Kraft paper with 3

% in weight water content (Thermo 70) and mineral oil (Nynas Nytro 10X) are sealed

without air and aged at 70 °C, 90 °C, 110 °C and 130 °C.

The typical activation energy obtained from laboratory accelerated ageing experiments

for both oxidation and hydrolysis can be seen in Table 2.2 [27, 53], it was found that

oxidation is promoted by oxygen and copper while hydrolysis is catalyzed by acids

and water. On the other hand, pyrolysis is promoted by high temperature. Based on

Lessard’s data [41], it was found that the activation energy for pyrolysis is around 82

kJ/mole which is lower than hydrolysis. It is suggested that the ageing temperature is

not high enough to increase the activation energy. According to survey study by

Emsley [27], for cellulose aged under vacuum or air, the pyrolysis activation energies


42

range from 110 kJ/mole to 170 kJ/mole at temperature between 200 °C and 380 °C.

Above 380 °C, the activation energy could increase to 225 kJ/mole.

Table ‎2.2 Activation energy for oxidation and hydrolysis [27, 53].

Ageing

mechanism

Activation energy

(kJ/mole)

Oxidation 74

Hydrolysis 111

Figure 2.7 shows the sketch of ageing rates to different ageing mechanisms. It is

observed that the ageing rate is influenced by both activation energy and

environmental factor for each given temperature.

Figure 2.7 Arrhenius plot under different ageing mechanisms [54].

2.2.3.1. Oxidation

Oxidation will be started once the hydroxyl groups in cellulose structure are attacked

by oxygen which weakened the glycosidic linkage. Through oxidation, carbonyl and

carboxyl groups will be created as secondary ageing products while moisture as

primary ageing products and these groups will promote hydrolysis in paper and oil as

seen in Figure 2.8 [27, 35, 51]. Carbonyl group (CO) consists of carbon atom (C)

double bonded with oxygen atom (O) while carboxyl group composes of combination


43

of carbonyl and hydroxyl groups (OH) as shown in Figure 2.9. Carbonyl is also known

as an aldehyde group and carboxyl as a carboxylic acid group.

Figure 2.8 Oxidation mechanism on cellulose [51].

Figure 2.9 Carboxyl and carbonyl groups [55].

2.2.3.2. Hydrolysis

Hydrolysis is known as the main paper ageing mechanism in transformers. The process

will promote glycosidic scission of cellulose chains and creates free glucose rings as

shown in Figure 2.10. The several stage of hydrolysis of free glucose rings will result

in formation of furanic compounds such as 2-furaldehyde and 5-

hydroxymethylfuraldehyde as shown in Figure 2.11 [18, 51, 56].

Figure 2.10 Hydrolysis mechanism on cellulose [51].


44

Figure 2.11 Mechanism of furanic compounds formation through hydrolysis [56].

2.2.3.3. Pyrolysis

Pyrolysis is initiated if a transformer’s operating temperature is high. It is proposed

that 140 °C as the minimum temperature limit for the pyrolysis to take place for pure

cellulose [57]. According to other literature, it is suggested that pyrolysis takes place

at temperature higher than 150 °C [52]. Glycosidic scission will start to take place

through pyrolysis and at the same time opening of the glucose rings as shown in Figure

2.12 [27, 35, 51]. The result of this process will create moisture and gases such as

carbon monoxide/carbon dioxide.

Figure 2.12 Pyrolysis mechanism on cellulose [51].

2.2.3.4. Ageing mechanisms in transformers

Several studies have suggested acid catalyzed hydrolysis as a major mechanism of

paper ageing in operating transformers [18, 57, 58]. According to Figure 2.13, the

overall degradation process started with the generation of hydrogen peroxide created

from the interaction of metal cations such as Cu+/Cu

2+ or Fe

+/Fe

2+, water, oxygen and


45

heat. Decomposition of this compound will create Hydroxy (HO) radicals which will

catalyze the oxidation of oil and paper.

Figure ‎2.13 Ageing mechanism of paper in transformers.

Through oxidation, water, carbonyl group and carboxyl group known as carboxylic

acids known as will be generated [16, 17, 19]. There are two types of carboxylic acids

which are Higher Molecular Weight Acid (HMA) and Lower Molecular Weight Acid

(LMA). Dissociation of specific acids such as LMA will generate H+ ions and catalyze

the hydrolysis of the paper. Water does help in dissociation process of LMA by

increasing the number of H+ ions available in the system. It was found that hydrolysis

will become the main mechanism in the system once the environment becomes acidic

due to the reduction in oxidation [18]. The process will be repeated since it is an auto

catalytic process [58].

Oxidation

in oil and

paper

Oxygen

Metal

cations

(Cu+/Cu

2+

or

Fe+/Fe

2+)

Water

Acid

High molecular

weight acids

Low molecular

weight acids

Hydrolysis in paper

Dissociated carboxylic acids

H+ ions

HO

radicals

Decomposition

of hydrogen

peroxide

Water

Hydrogen

peroxide

Heat

+

Auto catalytic

process where

hydrolysis will

continue with

dissociation of

LMA

Dominant


46

2.2.4. Paper ageing accelerators

According to Figure 2.13, degradation process of paper is mainly influenced by the

presence of water, oxygen, and acids.

2.2.4.1. Water

The rate of degradation for paper with 4 % water content in weight is 20 times higher

than that of dry one [20, 21]. An example of the effect of water on ageing rate can be

seen in Figure 2.14. The ageing experiment was conducted at different temperatures

ranging from 60 °C to 130 °C and the initial paper water content was controlled to 0.47

%, 1.11 % and 1.92 % by weight [59]. It is shown that the ageing rate, k increases as

the initial paper water content increases from 0.47 % to 1.92 %.

Figure 2.14 Effect of water on paper ageing. Kraft paper (Clupak HD75) and mineral

oil (Nynas Nytro 10CX) are sealed under air and aged from 60 °C to 120 °C [60].

2.2.4.2. Oxygen

The rate of ageing will be reduced by a factor of 16 if oxygen in the system can be

reduced from 2000 ppm to 300 ppm [20]. Other studies show that the presence of

oxygen can increase the rate of ageing by a factor of 2.5 for paper having 0.3 to 5 %

water content [20, 61]. The rate of ageing will further increase with the existence of

copper which could accelerate the oxidation of the paper [20, 22]. An example of the


47

effect of oxygen on the ageing rate is shown in Figure 2.15. In order to monitor the

effect of different amount of oxygen present during ageing, different oxygen pressures

were applied above the oil surface in the bottles. The bottles were re-pressurized at

certain interval (initially once per day and later less frequent) in order to maintain the

pressure throughout the ageing experiment. It can be seen that the rate of ageing, k

increases with the increasing oxygen.

Figure 2.15 Effect of oxygen on paper ageing. Kraft paper (Thermo 70), mineral oil

(Nynas Nytro 10XN) and copper are sealed with different oxygen pressures and aged

at 90 °C, 110 °C and 130 °C [53].

2.2.4.3. Combination of water and oxygen

At low temperature, the combination of water and oxygen could actually reduce the

rate of paper ageing. It was proposed that the hydrogen bonding between water and

oxygen could create ‘shielding effect’ which reduces the reaction between water and

cellulose [20]. At the working temperatures of a transformer, the inhibiting effect of

the oxygen could be stronger and increasing the oxygen content in the oil could

actually reduce the rate of paper ageing by preventing the water in oil from migrating

to the paper [20].


48

2.2.4.4. Acids

The rate of paper degradation depends on the dissociation rate of the acids [18]. The

dissociation rate depends on the ability of specific acid such as LMA to dissociate

[17]. The dissociated acid increases the H+ concentration in the system and accelerates

the depolymerization process of the paper. An example of the effect of acids on the

ageing rate is shown in Figure 2.16. Water could help to increase the concentration of

H+ by causing the carboxylic acids to dissociate [18].

Figure 2.16 Effect of LMA on paper ageing under wet condition. Kraft paper (Thermo

70) and mineral oil (Nynas Nytro 10X) containing LMA and HMA are sealed without

air and aged at 70 °C, 90 °C, 110 °C and 130 °C [17].

2.2.5. Paper ageing products

2.2.5.1. Water

One of the main products of paper ageing is water. According to Figure 2.13, water

could be generated by oxidation and hydrolysis and the production is an auto-catalytic

process. According to the previous finding, it is shown that the main source of water

generation is originated from paper and not from oil [59]. Water is the result of chain

scission of cellulose molecule as shown in y-axis of Figure 2.17 [54]. It was found that

DP will be reduced to half each time water content in paper is increased by 0.5 % due

to the chain scission of cellulose molecule [20-22].


49

Figure 2.17 Relationship between water generation and chain scission of paper [22,

61, 62].

2.2.5.2. Furanic compounds

Other products of paper ageing are furanic compounds. Furanic compounds are

generated as a result of cellulose chain scission through either pyrolysis or

hydrolysis/oxidation [63]. Furanic compounds are one of the main products of paper

ageing and it can be monitored through oil. Figure 2.18 shows the main furanic

compounds from paper ageing which are 2-Furaldehyde (2FAL), 5-

HydroxyMethylFuraldehyde (5HMF), 5-MethylFuraldehyde (5MF), 2-AcetylFuran

(2AF), 2- FurfurylAlcohol (2FA) and 2-Furoic acid (2F) [45, 51, 64].

Figure 2.18 Type of furanic compounds [64].

The origin of furanic compounds is still unclear, but it was suggested that it is mainly

originated from hemicellulose. Others suggested that the main source of furanic


50

compounds is from levoglucosans and glucose [56]. Through pyrolysis, levoglucosans

will be created as a result from opening of the glucose rings and decomposed into

5HMF which give 2FAL as a final product [56]. On the other hand, the formation of

furanic compounds through hydrolysis/oxidation starts with the formation of glucose

through hydrolysis of cellulose as shown in Figure 2.19. The glucose will oxidize to

become pentose and gives 2FAL as the end product.

Figure 2.19 Formation of 2FAL through hydrolysis/oxidation mechanisms.

2.2.5.3. Gases

Oil and paper ageing could create different type of gases and two of them are Carbon

Monoxide (CO) and Carbon Dioxide (CO2) [59, 65, 66]. These gases could be formed

through oxidation and pyrolysis [51, 67]. Figure 2.20 shows an example of CO and

CO2 generations with ageing duration at different temperatures. It is shown that except

for 140°C, the amount of CO and CO2 generated from the ageing of oil and paper

increase slowly until 6 months and saturate at the later stage of ageing. Yoshida [36]

explained that at the initial stage of ageing, a part of cellulose is easily dissociated and

contributes to the increment of CO and CO2 as shown in Figure 2.21. At the later

stage of ageing, most part of the cellulose left is rigid and dissociates slowly. However

at 140 °C, the rigid part of cellulose is also consumed at the initial stage of ageing and

contributes to the high amount of CO and CO2. The less rigid part of cellulose is

known as amorphous and the rigid region is known as crystalline [18].

Cellulose Hexose

(Glucose)

Pentose

(Arabinose) 2-FAL

Hydrolysis Oxidation

-CO

-CO2


51

Figure 2.20 Generation of CO and CO2 at different temperatures. Paper, mineral oil,

steel, and aluminium are sealed without air and aged at 100 °C, 110 °C, 120 °C and

140 °C [36].

Figure 2.21 Relationship between tensile strength/degree of polymerization and

CO/CO2. Paper, mineral oil, steel, and aluminium are sealed without air and aged at

100 °C, 110 °C, 120 °C and 140 °C [36].

2.2.5.4. Acids

Acids are known as a common product for oil and paper ageing in a transformer. Acids

could be generated from both oil and paper through oxidation/hydrolysis mechanisms

[16, 17]. An example of acids generated from oil and paper ageing can be seen in

Figure 2.22. Recently it is shown that ageing of oil and paper could generate two types

of acids known as LMA and HMA [16, 17, 19]. These acids can be differentiated


52

through its molecular weight where the value ranging from 46 g/mol to 116 g/mol for

LMA and 240 g/mol to 285 g/mol for HMA. Most of LMA tends to stay in paper while

most of HMA tends to stay in the oil.

Figure 2.22 Acids generation for Kraft and thermally upgraded paper. Kraft paper,

thermally upgraded paper, mineral oil and copper are sealed without air and aged at 90

°C, 110 °C and 130 °C [2].

2.2.5.5. Methanol

Methanol is another product of paper ageing. It was proposed that methanol originated

from the glycosidic scission of cellulose chain [68]. The result of methanol production

from the cellulose chain scission can be seen in y-axis of Figure 2.23 a). The ageing

experiment was conducted at different temperatures ranging from 60 °C to 130 °C and

the initial paper water content was controlled to 0.47 %, 1.11 % and 1.92 % in weight

[68]. It was found that the relationship between methanol and chain scissions is almost

linear. The calculation of activation energy shows that the value obtained for methanol

production (106.9 kJ/mole) is in agreement with reduction of DP (103.5 kJ/mole) [60].

Methanol is more sensitive than furanic compounds. Figure 2.23 b) shows that the

furanic compounds increase slowly up to chain scissions equal to 2 (DP equals to 400)

and increases exponentially up to 100 mg/kg [68]. The result indicates that furanic

compounds are less sensitive at a DP range from 1200 to 400 while methanol does not

have this limitation.


53

The production of methanol is not affected by the initial paper water content and the

concentration is stable up to 130 °C [68]. There is a partitioning of methanol between

oil and paper where most of the portion tend to stay in paper similar to furanic

compounds [60].

a) b)

Figure 2.23 Relationship between a) methanol (CH3OH) and b) furanic compounds

(2FAL) and chain scission (NS). Kraft paper (Clupak HD75) and mineral oil (Nynas

Nytro 10CX) are sealed under air and aged from 60 °C to 120 °C [68].

2.2.6. Paper ageing assessment techniques

2.2.6.1. Degree of polymerization

DP of paper can be measured using Ubbelohde viscometer tube as shown in Figure

2.24. The first step of the measurement is to measure the viscosity of paper and

Cupriethylene-diamine (Cuen) mixture and the next step is to calculate the specific

viscosity, vs based on the measured viscosity using Equation (2.2).

s

viscosity of paper and Cuen mixture - viscosity of Cuenv =

viscosity of Cuen (2.2)


54

Figure ‎2.24 Ubbelohde viscometer tube.

Intrinsic viscosity, v can be obtained based on the specific viscosity and concentration

of the solution based on Equation (2.3).

s

c 0

vv = lim

c

(2.3)

where c = Concentration of paper and Cuen.

The last step is to calculate the DP based on intrinsic viscosity using Equation (2.4)

[69].

α

v[v] = KDP (2.4)

Where K and α = Mark Houwink constant ( 0075.0K and 1 ).

vDP = Average degree of polymerization.


55

The measurement of DP can be affected by several factors [25, 70].

The choice of Mark Houwink constant to convert intrinsic viscosity to DP.

Oxidation of the sample could affect the DP measurement. This can be avoided

by storing the solution under nitrogen.

Sample degradation in the viscometer tube which can be avoided by using

slower efflux time.

2.2.6.2. Tensile strength

Paper derived its strength from the fibre strength and inter-fibre bonding strength [71].

Tensile Strength (TS) can be described by stress and strain curve as shown in Figure

2.25. Machine Direction (MD) is defined as the direction in which the paper moves

during manufacture while Cross Machine Direction (CD) is the direction at right

angles to the machine direction. Paper in MD is stronger but less flexible than in CD.

Paper obtained from scrapped transformers usually is measured in MD. TS can be

measured in two ways which are known as long and zero span measurement. Long

span measurement is also known as “wide span” measurement in electrical insulation

research paper.

Figure 2.25 Stress and strain behaviour in two directions: MD (Machine Direction)

and CD (Cross Machine Direction) [71].


56

a) Long span tensile strength

The first method is called long span TS which is known as measurement of paper inter

fibre bonding strength [46]. The test setup can be seen in Figure 2.26 where paper is

placed between two clamps. The upper clamp will move to break the paper and the

force is automatically recorded by the measuring unit.

Figure 2.26 Long span TS setup.

According to standards, BS 4415 and BS 1924 [72, 73], the gap distance and width of

sample required for tests are about 180 mm ± 1 mm and 15 mm ± 0.1 mm. Tensile

strength, TS can be obtained using Equation (2.5).

F

TS = w

(2.5)

where F = Maximum load (N).

w = Width (m).

Long span Tensile Index (TI) is preferred for comparing different types of paper since

it is independent of the grammage of paper. Grammage can be calculated by dividing

the weight with the cross area of the paper. Long span tensile index, TI can be

calculated using Equation (2.6).

Gap between

two clamps

Upper clamp

(moveable)

Lower clamp

(stationary)

Paper

samples

Force


57

TSTI =

g (2.6)

Where g = Grammage (g/m²).

b) Zero span tensile strength

The second method of TS measurement is zero span TS. It is known as a measure of

paper fibre strength itself since no gap is present between the clamps [46]. The test

setup can be seen in Figure 2.27 and the width of the tested paper can be varied from

15 mm to 25 mm ± 0.1 mm [74].

Figure ‎2.27 Zero span tensile tester.

The calculation of zero span TI is based on failure load, FL which can be obtained

from Equation (2.7).

0

FL = K(P-P ) (2.7)

P = Observed pressure at failure (psi).

P0 = Equipment ‘Zero’ pressure (psi).

K = 0.422 (kg/psi).

No gap between

two clamps

Left clamp

(moveable)

Right clamp

(stationary)

Paper samples in the

middle of both clamps

Force


58

The observed psi can be read manually from the equipment once the paper has broken.

Once the FL has been obtained from Equation (2.7), the breaking length, BL can be

calculated using Equation (2.8).

FL

BL = w×g

(2.8)

Finally, zero span tensile index, TI can be calculated using Equation (2.9) below where

9.81 is known as the specific gravity.

TI = 9.81×BL (2.9)

2.2.6.3. Furanic compound analysis

One of the non-intrusive techniques that could be used to monitor the condition of the

paper is furanic compound analysis. According to previous finding, it is shown that

there is a relationship between furanic compounds and DP which is described by

several models [75-77]. Generally, furanic compounds in transformer oil are

determined through High Performance Liquid Chromatography (HPLC). Furanic

compounds could also be measured from paper through extraction with methanol [42,

78]. The basic operation of HPLC system can be seen in Figure 2.28.

Figure ‎2.28 Basic HPLC system.

Solvent

UV

detector

Waste

HPLC Column

Injector

Pump

Sample will be

injected here

Furan compound

will be detected here

Sample will be

eluted here

PC


59

The first step of the measurement is to extract the furanic compounds from the oil

sample. Solid-liquid and liquid-liquid extraction is known as the common method for

extraction [79]. After the extraction process, it will be analyzed by HPLC where it will

be eluted into specified column and detected via a UV detector.

The condition of transformers could be determined based on the information obtained

from furanic compounds. According to previous in-service transformer experience, it

was suggested that specific furanic compounds could be generated based on individual

events in the transformer as shown in Table ‎2.3.

Table ‎2.3 Causes for specific furanic compounds [80].

Compound Observed event

5HMF Oxidation

2FA High moisture

2FAL General overheating or normal ageing

2AF Rare, cause not fully defined

5MF High temperature

Monitoring furanic compounds in a transformer presents a great challenge since it

could be affected by many factors such as [65, 81-85]:

Design of transformer

Type of transformer

Type of paper

Moisture content of the paper

Type and ageing state of oil

Operating condition (overloading)

Oil change and reclamation

Instability at high temperature

The final concentration of furanic compounds is an equilibrium state from

formation to decomposition


60

2.2.6.4. Dissolved gas analysis

Dissolved Gas Analysis (DGA) is known as a common technique to identify faults in

the transformers. Based on previous research, DGA could also be used to describe the

ageing of paper based on dissolved CO and CO2 in oil. It is found that these gases

could be correlated with DP and TS of paper [36]. Conventionally, gases are detected

from oil using a technique called Gas Chromatography (GC) and the working principle

is similar to HPLC. The basic configuration of GC system can be seen in Figure 2.29.

Figure ‎2.29 Basic GC system.

Firstly, gases are extracted from the oil sample using methods suggested in the

standard, BS 60567 [86]. There are four methods that can be used for gases extraction

which are:

Multi-cycle vacuum extraction using Toepler pump apparatus.

Vacuum extraction by partial degassing method.

Stripping extraction method.

Head space method.

Once gases are extracted from the oil sample, it will be eluted by the GC column and

detected by Flame Ionization Detector (FID) and Thermal Conductivity Detector

(TCD). FID is used to detect compounds with high carbon concentration while TCD is

used for all other compounds. There are some difficulties on interpreting the CO and

Carrier gas

FID and TCD

detector

Waste

GC Column

Injector

Sample will be

injected here

Gases compound

will be detected here

Sample will be

eluted here

PC


61

CO2 from in-service transformers since it can be produced from oil, layer of paint,

varnish and phenolic resins [54]. CO2 also could be introduced from atmosphere

especially for a free breathing transformer. CO2/CO ratio is proposed as a method to

identify the ageing of paper. According to standard, BS 60599 [87], it was proposed

that ratio higher than 3 is defined as normal ageing and less than 3 as fault involving

excessive paper degradation.

2.3. Summary

Ageing of cellulose insulation such as paper is mainly influenced by water, acids and

oxygen. In transformers, these products are originally formed from oxidation of

mineral oil and will be produced even further from paper ageing through autocatalytic

process. It is found that the main ageing mechanism of paper in a transformer is

dominated by acid hydrolysis. Through dissociation of specific acid such as LMA, H+

ions are generated and accelerate the hydrolysis of the paper. The rate of LMA

dissociation could be controlled by water.

As paper ages, the risk of transformer failure will increase. Thus, it is important to

monitor the condition of the paper. One of the techniques is by monitoring the paper

ageing by-products. Water, acids, furanic compounds, CO, CO2 and methanol are

among by-products generated by ageing of paper. The common compounds usually

monitored are Furanic compounds and CO/CO2. Recently, it is shown that methanol

also could be a good candidate for paper ageing indicator.

Ideally, the best way to analyze the condition of the paper is through DP or TS.

However, it is impossible to obtain paper samples from in-service transformers and the

only opportunity is through scrapping transformers. In this research, paper samples

were obtained from scrapped transformers. Ageing assessment of scrapped transformer

paper based on tests such as TS, LMA and AC breakdown voltage is discussed in this

thesis. Laboratory experiments are also conducted in order to support assessment of

scrapped transformer paper.


62

CHAPTER 3. METHODOLOGY

3.1. Introduction

Mechanical strength is a common parameter used for assessing the ageing status of

paper. Tensile Strength (TS) can be measured either in long or zero span although in

electrical insulation research papers, long span TS is often written also as TS “wide

span” measurement. It was reported from previous work [46] that the main factor for

tensile strength reduction of paper is due to the loss of inter-fibre bond strength and

long span TS is known as the best way to represent this parameter. There are several

issues such as paper size and condition that need to be addressed when measuring long

span TS of scrapped transformers paper samples.

Based on previous findings, acids in transformers can be split to Low Molecular

Weight Acid (LMA) or High Molecular Weight Acid (HMA) [16, 17, 19]. It was also

shown that paper could contain acids which are mainly LMA. Due to the polar nature

of LMA, a technique based on water extraction was proposed to measure LMA and

HMA in paper and oil [19].

The first section of this chapter explains the procedure for sampling paper from

scrapped transformers. A study was conducted to examine the factors that could affect

the TS measurements of scrapped transformers paper samples. The process to measure

LMA and HMA content in paper and oil is presented in the next section. The last

section describes the pre-processing of oil and paper samples for ageing experiments.

3.2. Paper sampling procedure from scrapped transformers

3.2.1. Phase and winding profile

Most of the transformers in the UK are of the core type design, and they can be further

categorized as three limbs or five limbs. Core type transformers have their core limbs

concentrically wound by winding as shown in Figure 3.1 a). The arrangement of

windings in a transformer is shown in Figure 3.1 b). The closest winding to the core is

normally a tertiary winding (TV) followed by a low voltage winding (LV) and a high


63

voltage winding (HV). For economical reasons, an autotransformer is mainly used for

the high voltage transmission system. Generally, the connection is made by letting the

LV winding to be the common mainly shared with the HV winding. Through this

connection, the LV winding is also called the common winding (CW) and the rest of

the HV winding becomes the series winding (SW). Almost all transformers used for

ageing analysis in this thesis are autotransformers.

Figure 3.1 Sampling procedure for a) Phase and b) Winding profiles.

The first step of the procedure, once a transformer is taken out from service for

scrapping, is to measure the oil properties such as acids, interfacial tension, furanic

compounds and dielectric properties. The next step is to remove the oil from

SIDE VIEW

Phase A Phase B Phase C

TOP VIEW

FRONT VIEW

CORE

WINDING

HV (SW)

LV (CW)

TV

a)

Core TV HV

(SW)

Inner Outer Top

Centre

Bottom LV

(CW)

b)

5 LIMBS (CORE

TYPES)

INSIDE VIEW – FROM FRONT VIEW


64

transformers; the main tank of transformers is cut to reveal the core and windings. At

this stage, usually an engineer will inspect transformers for any irregularities or source

of fault. Finally the windings are transported to the scrapping yard. The paper

sampling is done in the scrapping yard.

For phase profile, paper samples were taken from phase A, phase B and phase C as

shown in Figure 3.1 a). The winding profile was derived from the phase as shown in

Figure 3.1 b) where paper samples were taken from tertiary windings, common

windings and series windings. For both phase and winding profiles, paper samples

were taken from top, centre and bottom positions so that a vertical comparison ageing

status can be made. The scrapped transformer was analyzed vertically because of the

difference in the temperature distribution from bottom to top windings [88]. In general,

the operating temperature at the bottom windings is lower than top windings.

Horizontal comparison is also made among windings in the same phase and among

phases to check the thermal design.

3.2.2. Layer profile

The layer profile was obtained from a single conductor and the sampling procedure is

shown schematically in Figure 3.2. Normally, a number of paper layers were wrapped

around a single conductor varying from 6 layers to 12 layers depending on the design

of a transformer and the thickness of the paper. Each layer is unwrapped carefully and

paper that is in contact with oil was assigned as outer layer paper and inner layer paper

was the paper close to the conductor.

Figure 3.2 Sampling procedure for layer profile.

~~

Outer Inner

Insulation

Paper

Conductor Oil

Layer profile

Core TV LV HV

Inner Outer

(CW) (SW)

Winding profile


65

3.3. TS measurements

All measurements of paper strength in this PhD thesis were carried out based on long

span TS. The gap distance and width of sample required for tests are about 180 mm ±

1 mm. For each location, at least 10 tests were done on paper samples. However, the

amount of paper from a scrapped transformer usually is limited and thus the number of

tests was sometimes reduced to a minimum of 4 tests for each sample in several cases.

The crosshead speed and full scale load range were set to 20 mm/min and 0.5 kN

according to [72, 73]. Tests were carried out in the paper science laboratory with a

condition of 23 °C and 50% relative humidity. During the refurbishment of the

laboratory, the tests were conducted in a building having the temperature ranging from

21 °C to 23 °C and the humidity ranging from 35 % to 50 %.

3.3.1. TS or TI measurements

Paper from scrapped transformers usually comes in different types and grammages.

Tensile Index (TI) is preferred to TS for since it considers grammage in the

calculation.

Conductor and layer types of paper were tested in this section. Table 3.1 shows the

properties of both types of paper. Twenty thickness measurements were carried out for

each type of paper by a digital micrometer. The grammage for the conductor type

paper tested in this thesis range from 46 g/m² to 51 g/m² with an average value of 48

g/m². On the other hand, the grammage of the layer type paper range from 168 g/m² to

171 g/m² with an average value of 170 g/m². All grammage measurements were

carried out without any present of oil. The calculated density does match with the

value given by the manufacturer for both types of paper. For each type of paper, 10

tests of long span TS were carried out without any present of the oil. The TS and TI of

both types of paper were calculated by Equation (2.5) and Equation (2.6) in Chapter 2.


66

Table ‎3.1 Properties of new conductor and layer types paper.

Type of

paper

Thickness

(mm)

Grammage

(g/m²)

Apparent density

(Manufacturer)

(g/cm³)

Apparent density

(Calculated)

(g/cm³)

Conductor 0.053 48 0.93 0.92

Layer 0.237 170 0.76 0.72

It is observed that the conductor type paper has a lower strength than the layer type

paper if comparison is made based on TS as shown in Table 3.2; however the reason

for the high strength of layer type paper is due to the grammage since thickness does

not affect the fibre bonding at a given density and load carrying capability.

The effect of grammage is considered in the TI calculation and it is shown that the

specific strength of the conductor type paper is higher than that of the layer type paper.

Furthermore, TI also could show that the specific strength of the paper is a function of

density where high density paper has higher strength than low density paper depending

on the type of fibre.

Table ‎3.2 TI and TS comparison between conductor and layer types paper.

Type of paper TS (kN/m) TI (Nm/g)

Conductor 5.25 ± 0.21 109 ± 4.58

Layer 15.6 ± 0.53 91.6 ± 3.14

± represents standard deviation

Thus, in this thesis, TI will be used to describe the strength of the paper.

3.3.2. Factors affecting TI measurements

3.3.2.1. Paper samples condition

All samples tested for TI in this section were new, conductor paper. The properties of

this type of paper can be seen in Table 3.1. All samples tested were not impregnated in

oil except for the influence of oil impregnation section. All measurements of paper

strength in this section were carried out based on long span TI.


67

3.3.2.2. Paired t-test

A statistical method known as paired t-test was applied to all data in this section in

order to check whether there is any significant difference between means of samples.

The calculation procedure can be seen in Figure 3.3. The first step is to split the data

into two groups. Each group contains at least 10 TI measurements. The next step is to

subtract data in group 1, x1 with data in group 2, x2. The mean, d and standard

deviation, Sd of the difference is calculated and standard error, Se can be obtained by

dividing Sd with the square root of sample size, n. The T can be calculated based on d

and Se while tcrit at 95 % confidence level can be obtained from T table based on

specified degree of freedom, df. If T (calculated from mean and standard deviation of 2

groups) is higher than tcrit (from T table), it indicates that the mean for 2 groups are

different from each other.

Figure 3.3 Paired t-test calculation procedure.

Arrange the data into two groups, x1 and x2

Calculate the mean difference, d and Se

1 2d = x -x , d

e

SS =

n

Calculate T

e

dT =

S

If T > tcrit Mean

difference

No mean difference

Determine tcrit at 95 % confidence based on degree of freedom, df

df = n-1


68

3.3.2.3. Effect of length

The study was carried out since it is difficult to obtain the standard size, 180 mm long,

samples of the service aged paper according to the standard [72, 73]. It can be seen

from Figure 3.4 that the percentage difference between 30 mm and 180 mm is less than

5 % which indicate the lengths within the measurement range has small influence on

the TI measurements. There is a decreasing trend of TI as the length is increased. This

is called ‘scale effect’ where the probability of finding weakest link will be high if the

size of the sample is increased [89].

Figure 3.4 TI of new conductor paper versus length. Data are mean of 10 samples and

error bars represent the standard deviation.

According to the paired t-test, it is observed that the TI difference between 15 mm and

30 mm is not statistically significant as shown in Table 3.3. Similar observation was

also found for the comparison between 60 mm and 180 mm. On the other hand, it is

shown that there is some significant difference of TI statistically for other length

comparisons.


69

Table ‎3.3 Means comparison between each length

Mean difference (Based on T test at 95% confidence interval)

15 mm 30 mm 60 mm 180 mm

15 mm - No Yes Yes

30 mm No - Yes Yes

60 mm Yes Yes - No

180 mm Yes Yes No -

3.3.2.4. Effect of folding stress

Service aged paper can be subjected to folding stress due to the wrapping of the paper

around the conductor. A study was carried out on the influence of the folding stress on

TI measurement. In order to study this effect, new paper samples were folded with

moderate force at 2 different locations at an angle of 90 ° in order to imitate the folding

stress in paper from a service aged transformers as shown in Figure 3.5. It is

anticipated that the folding stress will create a weak point on the fibre. The gap length

for TI measurement was set as 30 mm.

Figure 3.5 Paper samples arrangement for the effect of folding stress.

It can be seen that paper under folding stress undergoes 14 % reduction of TI in Figure

3.6. There is a significant difference of TI at the 95 % confidence interval according to

Gap between

two clamps was

set at 30 mm

Upper clamp

(moveable)

Lower clamp

(stationary)

Paper

samples

Force

Arrangement

of TI

measurement

For clamp Folding stress is introduced

at these locations


70

the paired t-test. This effect might be more severe for service aged paper since the

folding area condition could be more severe compared to new paper.

Figure 3.6 TI of new conductor paper with and without folding stress. Data are mean

of 10 samples and error bars represent the standard deviation.

3.3.2.5. Effect of oil impregnation

The influence of oil impregnation on TI measurements was also examined. Three types

of paper conditions were prepared according to Figure 3.7. The gap length for TI

measurement was set at 30 mm and 180 mm.

Results are shown in Figure 3.8. It is observed that paper impregnated fully with oil,

with or without temperature, experiences a small TI reduction between 3.7 % and 6.1

% as seen in Table 3.4. Statistically, it is found that there is a significant difference of

TI between dry and full impregnation with/without temperature for both 30 mm and

180 mm as shown in Table 3.5. There is no influence of impregnation temperature on

the TI measurement based on paired t-test which suggests that there is no premature

ageing occurring during impregnation process.

A test was also conducted in order to see whether the small reduction is caused by the

slippage of the paper on the clamp by having only the gap between two clamps


71

impregnated with oil. It is found that paper still undergoes a small TI reduction

between 2.6 % to 2.8 %.

Figure 3.7 Preparations of paper samples for the effect of oil impregnation.

Figure 3.8 Influence of oil on TI. Data are mean of 10 samples and error bars

represent the standard deviation.

For clamp

Gap between

two clamps

Baseline- Dry

Sample 1

Only gap is impregnated with oil

No vacuum and temperature

Oil

Gap between

two clamps was

set at 30 mm

and 180 mm

Upper clamp

(moveable)

Lower clamp

(stationary)

Paper

samples

Force

Arrangement

of TI

measurement

Sample 2 and 3

For Sample 2, full impregnation

with oil in vacuum oven without

temperature

For Sample 3, full impregnation

with oil in vacuum oven at 85 °C


72

Table ‎3.4 Percentage difference of TI between baseline and samples.

Sample Difference at

30 mm (%)

Difference at

180 mm (%)

Between Baseline and Sample 1 2.6 % 2.8 %



Table ‎3.5 Means comparison between baseline and samples.

Mean difference at 30 mm (Based on T test at 95% confidence interval)

Baseline Sample 1 Sample 2 Sample 3

Baseline - No Yes Yes

Mean difference at 180 mm (Based on T test at 95% confidence interval)

Baseline Sample 1 Sample 2 Sample 3

Baseline - No Yes Yes

3.3.2.6. Effect of reducing paper length in the clamp

Another study was carried out in order to study the influence of reducing paper length

under the clamp i.e. clamp area is fully filled with paper or only 50 %. The gap length

for the TI measurement was set at 15 mm. The arrangement of the test can be seen in

Figure 3.9.

Figure 3.9 Paper samples arrangement for the effect reducing paper length in the

clamp.

17.5 mm

Gap between

two clamps was

set at 15 mm

Upper clamp

(moveable)

Lower clamp

(stationary)

Paper

samples

Force

Arrangement

of TI

measurement

17.5 mm

Sample 1

The paper length in

both clamps is 35 mm.

35 mm

35 mm

Sample 2

The paper length in both

clamps is 17.5 mm.

For clamp


73

There is about 35 % reduction of TI for the case where paper has shorter length under

the clamp which can be seen in Figure 3.10. Statistically, it is found that reducing the

length of paper in the clamp can caused significant reduction of TI. The reduction

might be caused by paper slippage on the clamp. It is essential to keep the paper in the

full clamp area with a shorter but fixed gap length to avoid large error in TI

measurement in the case of paper shortage from a service aged scrapped transformer.

Figure ‎3.10 Effect of reducing paper length in the clamp area of TI. Data are mean of

10 samples and error bars represent the standard deviation.

3.4. TAN, LMA and HMA measurements

3.4.1. TAN measurements

The common practise to measure the Total Acid Number in (TAN) oil is based on

automatic potentiometric measurement [90]. The measurement was done by dissolving

the sample in the solvent (500 ml of toluene, 495 ml of iso-propanol and 5 ml of water)

and titrated potentiometrically with the reagent (0.1 M (Molar Concentration)

Potassium hydroxide in iso-propanol) using a glass indicating electrode and a reference

electrode (Metrohm Solvotrode 6.0229.100). The end point of the measurement was

taken once the volume of reagent corresponds to a PH of 11.5 which was recorded by

Metrohm oil titrino plus. The detail procedure to measure TAN can be seen in Figure

3.11.


74

Figure ‎3.11 Procedure to measure TAN.

The calculation of Total Acid Number (TAN) of oil can be seen in Equation (3.1).

1 v v(V -CV)×m ×C

TAN = m

(3.1)

where V1 = Volume titrated for oil + solvent (ml).

CV = Volume titrated for solvent (ml).

mv = Molar mass of reagent (g/mol).

Cv = Concentration of reagent (mol/l).

m = Weight of oil (g).

3.4.2. LMA and HMA measurements

The principle of LMA and HMA measurements in oil and paper is based on the

chemical nature of the acid itself. LMA is known as polar compound and HMA is

known as less-polar compound. Thus, by using another polar compound such as water,

the LMA could be extracted from the oil and paper. The measurement of LMA and

HMA was done according to the method developed by Ingebrigtsen [19].

a) Measurement of LMA and HMA in oil

25ml of oil was mixed with an equal volume of distilled water and mixed overnight

using a magnetic stirrer as seen in Figure 3.12. The next step was to leave the mixture

sample still for at least half an hour in order to separate into two layers. Water and oil

5.0 g of oil 70 ml of solvent

+

1st STEP

Titrate the solvent

2nd

STEP

Titrate the oil and solvent

3rd

STEP

Calculate TAN by Equation (3.1)

70 ml of solvent


75

layer were then extracted and titrated according to BS 62021 [19, 90]. The acidity

result obtained from water layer is defined as LMA while the oil layer as HMA.

Acidity of clean water was measured and subtracted from the measurement of LMA in

oil. The oil layer defined as HMA was measured in the same manner as TAN. The

acidity of the sample was calculated according to the weight of oil used for extraction.

Figure ‎3.12 Procedure to measure LMA and HMA in oil.

The calculation of LMA and HMA in oil can be seen in Equation (3.2).

o w v v

o

(V -CV )×m ×CLMA/HMA in oil =

m (3.2)

where Vo = Volume titrated for water extract or oil extract+ solvent (ml).

CVw = Volume titrated for water + solvent (ml).



mo = Weight of oil (g).

Mixture of

oil and water

25ml of oil

(Measure the weight)

25ml of water

+ = 1

st STEP

Mix oil and water

2nd

STEP

Stir the mixture overnight

Magnetic stirrer

4th

STEP

Titrate the oil and water for LMA and HMA using Metrohm Oil

Titrino Plus. LMA and HMA are calculated by Equation (3.2)

3rd

STEP

Extract water from the mixture

Oil extracts

(HMA)

Water extracts

(LMA)

+ =

Mixture of

oil and water


76

b) Measurement of LMA in paper

A known weight of paper, ranging from 0.5 g to 1.0 g, was prepared and immersed in

25 ml of water for 3 days without stirring as shown in Figure 3.13. After the extraction

period, 10 ml of the extracting water was titrated according to BS 62021 [19, 90]. The

leftover paper was dried in air circulating oven at 70 °C for 2 day and acidity was

calculated by dividing with the 40 % of the dry weight of paper since only 10 ml was

used for titration.

Figure ‎3.13 Procedure to measure LMA in paper.

The calculation of LMA in paper can be seen in Equation (3.3).

w v v

p

(Vp-CV )×m ×CLMA in paper =

m (3.3)

1st STEP

Mix paper and water

Mixture of

paper and

water

0.5 to 1.0 g of paper 25ml of water

+ =

3rd

STEP

Extract water from the mixture

10 ml of water

(LMA)

=

Mixture of

paper and

water

2nd

STEP

Leave the mixture without any stirring

for 3 days Mixture of

paper and

water

4th

STEP

Titrate the water for LMA using Metrohm Oil Titrino Plus. LMA is

calculated by Equation (3.3)


77

where Vp = Volume titrated for water extract + solvent (ml).

CVw = Volume titrated for water + solvent (ml).



mp = Dry weight of paper (g).

3.4.2.1. Efficiency of LMA extraction from paper

A study was conducted to check whether 3 days is sufficient to extract the LMA from

the paper. The test was conducted by having the same paper sample titrated for 3

times and after each titration, new water was added again to the paper for the next

extraction. The age and properties of paper samples tested can be seen in Table 3.6.

Table 3.6 Age and paper properties of each transformer.

Transformers Age

(years)

Paper

thickness

(mm)

Grammage

(g/cm³)

Apparent paper

density

(g/cm³)

Transformer a 37 0.13 137 1.08

Transformer b 45 0.09 101 1.17

Transformer c 35 0.11 89 0.79

Transformer d 42 0.04 42 1.07

Two paper samples from scrapped transformers with different LMA in paper values

are shown in Figure 3.14. It is shown that 72.2 % of LMA was extracted on the first,

23.8 % on the second and 4 % on the third extraction for Transformer a. For

Transformer b, 75.5 % of LMA was extracted on the first, 19.7 % on the second, and

4.8 % on the third extraction.


78

Figure ‎3.14 Low molecular weight acid extraction efficiency using water. Data are

mean of 2 measurements.

The results show that the majority of LMA would be extracted on the first extraction

and only a small amount of LMA was left in the paper on the second and third

extraction. There is no significant difference in the percentage of extraction for paper

having either low or high amount of LMA, therefore extraction once is appropriate and

the value of LMA in paper can be used to conduct cross comparison.

The repeatability of LMA measurements was also checked by measuring twice the

water extract. It can be seen from Table 3.7 that the repeatability of the measurements

is good. The difference between first and second measurements for Transformer b, c

and d are around 18 %, 0.4 % and 6 %. The slight deviation between first and second

measurements for Transformer b might be due to some of LMA is not distributed well

in the water since no stirring is involved during the extraction process. Since the

repeatability of the measurements is good, almost all LMA measurements in this thesis

are measured twice and the average value is taken.

Table ‎3.7 LMA measurement on service aged paper.

Measurement Transformer b

(mg KOH/g)

Transformer c

(mg KOH/g)

Transformer d

(mg KOH/g)

1st 1.30 2.81 3.76

2nd

1.06 2.80 3.98


79

3.5. Pre-processing of materials for ageing experiment

3.5.1. Pre-processing of oil

Mineral oil Gemini X and natural ester FR3 were first filtered using a nylon membrane

filter with a pore size of 0.2 µm. The basic properties of these oils based on the

Product Datasheets can be seen in Table 3.8. The important properties are acidity,

viscosity and AC breakdown voltage since it could influence the performance of

transformers after subjected to ageing. The next step is to dry mineral oil in air

circulating oven for 24 hours at 105 °C in order to remove the water. On the other

hand, for natural ester, the drying is done in vacuum oven (less than 10 mbar) at 85 °C

for 72 hours. The drying of natural ester is done in vacuum oven and at a long duration

because the water saturation is higher than mineral oil. The final water content for

mineral oil and natural ester are 10 ppm and 20 ppm respectively.

Table ‎3.8 Basic properties of Gemini X and FR3.

Type of paper Unit Gemini X FR3

Density @ 20 °C kg/dm3

0.88 0.92 (25 °C)

Viscosity @ 40 °C mm2/s 8.7 34

Flash point °C 144 316

Pour point °C -60 -21

Acidity mg KOH/g <0.01 0.04

Aromatic content % 10 -

Water content mg/kg <20 30

Antioxidant content wt% 0.38 -

AC breakdown voltage

-Before treatment kV 40- 60 (d=2.5 mm) -

- After treatment kV >70 (d=2.5 mm) 56 (d= 2.0 mm)

Dielectric dissipation factor

@ 90 °C - 0.03 (100 °C) <0.001


80

3.5.2. Pre-processing of paper

Conductor paper was used for all ageing experiment. The property of this paper can be

seen in Table 3.2. In order to prepare dry paper sample, the paper was dried in air

circulating oven for 24 hours at 105 °C. The next step was to impregnate the paper

with either dry mineral oil or natural ester in vacuum oven for 24 hours at 85 °C. The

final water content for paper is less than 0.5 %.

For wet paper sample, the dried paper impregnated with either mineral oil or natural

ester was left in desiccators with a relative humidity of 90 % for 7 days. The final

water content paper is around 5 %.

3.6. Summary

There are several aspects that need to be taken care when measuring TI of service aged

paper sample. Reducing the length of the paper sample and oil impregnation only has

minor influence on the TI measurements. It is important to eliminate paper samples

which are damaged and mechanically stressed during scrapping since folding effect

show that it could influence the TI measurements. The most crucial practice is to make

sure that the size of the paper sample is sufficient to cover the clamp area in order to

avoid significant errors on the TI measurements of service aged paper.

Three days of extraction period is sufficient to extract most of the LMA from the

paper. Furthermore the repeatability of the LMA measurements of the water extract is

good.


81

CHAPTER 4. AGEING ASSESSMENT OF PAPER THROUGH TENSILE

STRENGTH MEASUREMENTS

4.1. Introduction

Up until now, there are several studies on the ageing of paper in transformers were

carried out through laboratory ageing experiments. Although much knowledge on the

ageing of paper was obtained through these studies, it is difficult to apply to in-service

transformers. One of the reasons is the ageing mechanisms in transformers is complex

and involves many factors at one time.

Transformers scrapping provide a good opportunity to study the ageing of in-service

transformers. The study could give an overview on the ageing profile in a transformer

when it was in-service. Only a few studies were carried out on scrapped transformers

using techniques such as Degree of Polymerization (DP) and water content of paper

[91, 92]. Apart from DP and water content, Tensile Index (TI) is another method that

could be used to analyze the ageing of paper.

This chapter discusses the TI phase, winding and layer profiles of scrapped

transformers. The relationship between TI and a transformer age is modelled using the

Weidmann’s equation. Life assessment of transformer population was carried out

based on this relationship. Finally, the effect of ageing on the dielectric strength of

paper is discussed in this chapter.

4.2. Case study

A total of 10 transformers were measured for TI as seen in Table 4.1. These

transformers were used at transmission voltage levels where power ratings vary from

150 MVA to 1000 MVA. Only Transformer 10 was retired due to ageing. It is

observed that most of the transformers failed due to the core overheating problem [93].

Other typical problems found on transformers are loose clamping bolts, dielectric

faults and gassing at high load. Only one failure is related to the paper which is caused

by hotspot near to the top location. The hotspot might be caused by the design

problem. This case study will be used for analysis in this chapter and Chapter 5.


82

Table 4.1 Case study of transformers.

Transformer Rating Age

(years) Reason for scrapping

1 240MVA

400/132kV 11

- Core and frame circulating current

problem

2 240MVA

400/132/13kV 11

- Localised conductor erosion causing

circulating current within the winding

through a poor and possibly

intermittent contact

3 1000MVA

400/275kV 35 - Gassing at high load

4 240MVA

275/132kV 41

- Loose clamping and core

overheating

5 180MVA

275/132kV 42

- Loose clamping and core

overheating

6 120MVA

275/132kV 45

- Closure of substation

- Vulnerable to dielectric fault

7 240MVA

275/132kV 45 - Corrosive sulphur

8 180MVA

400/132/13kV 48

- Core and frame circulating current

problem

9 150MVA

275/132/11kV 50

- Winding hotspot causing severe

paper degradation

10 180MVA

275/66kV 37

- Short circuit in C phase HV winding

caused by severe solid insulation

ageing

- Over wrapping on the HV winding

causes localised heating on certain

areas


83

4.3. TI measurements of scrapped transformers

The sampling of paper for TI analysis was carried out according to the procedure

mentioned in Figure 3.1 and Figure 3.2 of Chapter 3. The oil residue on the paper

samples was wiped using a tissue before being tested for TI. Based on all

measurements, it was found that the TI of paper for scrapped transformers ranges from

84.6 Nm/g to 20.3 Nm/g. The first part of this section describes the profile of TI along

phase, winding and layer.

4.3.1. TI profiles of scrapped transformers

4.3.1.1. Phase and winding profiles

Three transformers were analyzed for phase and winding profiles which are A, B and

C phase Common Windings (CW) of Transformer 6, A, B and C windings Series

Windings (SW) of Transformer 4 and the CW and SW of B phase of Transformer 8.

These transformers are chosen for analysis because there are enough paper samples

available to give a good representation of the vertical profile of top, centre and bottom

locations. It is known that these locations could show the temperature profile. Paper

properties and Total Acid Number (TAN) of oil before scrapping for each transformer

can be seen in Table 4.2.

Table 4.2 Paper properties and TAN of each transformer for phase and winding

profiles.

Transformers

Paper

thickness

(mm)

Apparent

paper density

(g/cm³)

Highest TAN of oil

before scrapped

(mg KOH/g)

Transformer 4 0.15 0.93 0.21

Transformer 6 0.04 1.07 0.16

Transformer 8 0.11 0.79 0.16

The vertical profile of TI of each phase can be seen in Figure 4.1. Significant

difference of TI at 95 % confidence interval is observed between A phase, B phase and

C phase at the centre and bottom windings of Transformer 4 according to the paired t-


84

test. On the other hand, there is no significant difference of TI at 95 % confidence

interval according to the paired t-test for all phases of Transformer 6 except between A

phase and C phase at the centre winding of Transformer 6.

It is suspected that the difference of TI at the centre and bottom windings of

Transformer 4 might be caused by the difference of moisture and acid distribution

since it is generally known that temperatures at these locations are relatively low.

Moisture and acids are known as the main accelerator of paper ageing [18, 27]. On the

other hand, small difference of TI between each phase is observed at the top winding.

The observation might indicate that paper at the top winding experienced almost

similar degradation. One of the possible reasons is that the paper degradation at the top

winding is mainly dominated by temperature.

Figure 4.2 shows the vertical profile of TI between windings of phase B of

Transformer 8. According to the paired t-test, there is significant difference of TI at 95

% confidence interval between windings of phase B for Transformer 8. The finding

might indicate that the temperature and moisture/acids distribution between windings

for this transformer might be different.

It is noticed that the TI for a young transformer could be similar to an old transformer

by comparing B phase CW of Transformer 8 and Transformer 4. One of the possible

reasons is the loading of Transformer 8 is higher than Transformer 4. TAN in oil

indicates ageing, and it is shown that the TAN in oil of Transformer 4 is higher than

Transformer 8. It is believed that the “chemical age” of Transformer 4 is older than

Transformer 8although the “usage age” is showing oppositely. However, no loading

history could be obtained for both transformers, so the hypothesis cannot be

confirmed. Another reason might be due to the paper thickness of Transformer 4 is

higher than Transformer 8. According to previous literature [43], thick paper degrades

faster than thin paper due to the higher temperature and slower heat conduction.

Based on the phase and winding studies, it is found the TI at the top winding is slightly

lower than bottom winding. The observation is in line with previous findings [91, 92].

One of the main factors that could affect the paper degradation is heat. Since

temperature is usually high at the top winding and hotspots are normally located near


85

to this location, it is reasonable to expect the TI at this position is lower [91, 92].

However, in some cases, the TI at the bottom winding could be lower than top winding

as shown by B phase of Transformer 4. It is suspected that there might be the

accumulation of water at the bottom winding of Transformer 4 when it was in-service.

It is known that that water has significant impact on the ageing of paper in

transformers [35].

a) b)

Figure ‎4.1 TI profiles among different phases a) Transformer 4 (SW) and b)

Transformer 6 (CW). Data are mean of 5 to 10 samples and error bars represent the

standard deviation.

Figure ‎4.2 TI profiles among different windings of Transformer 8. Paper samples were

taken from B phase. Data are mean of 4 to 6 samples and error bars represent the

standard deviation.


86

4.3.1.2. Layer profile

Two transformers were analyzed for layer profile. The paper samples were taken from

B phase, High Voltage (HV) bottom winding of Transformer 10 and B phase, CW top

winding of Transformer 6. These transformers are chosen in order to show the

difference of layer profile between severely aged paper (shown by Transformer 10)

and moderately aged paper (shown by Transformer 6). As mentioned in Table 4.1,

Transformer 10 failed due to the short circuit in turn conductors as a result from severe

solid insulation ageing while Transformer 6 is decommissioned due to the closure of

the substation. Paper properties and TAN of oil of these two transformers can be seen

Table 4.3.

Table 4.3 Age, paper properties and TAN of each transformer for layer profile.

Transformers

Paper

thickness

(mm)

Paper

density

(g/cm³)

Highest TAN of oil

before scrapped

(mg KOH/g)

Transformer 10 0.086 1.17 0.42

Transformer 6 0.040 1.07 0.16

The profile of TI along layer for Transformer 10 can be seen in Figure 4.3. It is

observed that the 1st layer paper has lower TI than the rest of layers with a value of

22.4 Nm/g. TI remains unchanged from the 2nd

layer to 8th

layer paper with an average

value of 62.5 Nm/g.

Another example of layer profile for Transformer 6 can be seen in Figure 4.4. It is

observed that the 1st and 11

th layers of paper have lower TI than the other layers. TI for

both 1st and 11

th layers paper are almost the same, with values of 33.2 Nm/g and 34.4

Nm/g. TI remains unchanged from the 2nd

layer to 10th

layer paper with an average

value of 44.3 Nm/g


87

Figure ‎4.3 TI profiles among different layers of Transformer 10. Paper samples were

taken from B phase, HV bottom. Data are mean of 9 to 10 samples and error bars


Figure ‎4.4 TI profiles among different layers of Transformer 6. Paper samples were

taken from B phase, CW top. Data are mean of 9 to 10 samples and error bars


Previous studies have shown similar pattern for multilayer insulation where the outer

layer paper usually degraded more than others [94, 95]. It was proposed that multilayer

insulation in transformers is dominated by two main degradation mechanisms [94].


88

It is noticed that the TI for a young transformer could be lower than an old transformer

by comparing first layer paper of Transformer 10 and 6. It believed that the fast

degradation of Transformer 10 is dominated by chemical degradation which mainly

involves the paper layers in direct contact with oil. The chemical degradation mainly

depends on the condition of the oil. From the scrapping report, it is reported that the

highest TAN of Transformer 10 is 0.42 mg KOH/g while that of Transformer 6 is 0.16

mg KOH/g. The high TAN of Transformer 10 might indicate that it has experienced

high loading when it was in-service. However, no loading history could be obtained

for both transformers. It is suspected that the difference in the TAN could be the

reason that the 1st layer paper of Transformer 10 has lower TI than that of Transformer

6. Acids, especially Low Molecular Weight Acid (LMA) could accelerate the ageing

of paper. Moreover, according to previous studies, it was found that LMA tends to

increase with the increment of total acidity [19]. Figure 4.5 shows the visual inspection

for each layer of the paper sample Transformer 10. It is observed that the paper

becomes darker as the number of layer progresses from inner layer to outer layer.

Outer layer paper usually is the darkest since it is in direct contact with the oil.

Figure ‎4.5 Example of paper taken from Transformer 10 a) 1st layer b) 2

nd layer c) 3

rd

layer d) 4th

layer e) 5th

layer f) 6th

layer g) 7th layer h) 8th

layer. Paper samples were

taken from B phase, HV bottom of Transformer 10.

Another reason probably is that the initial TI of 1st layer paper for Transformer 10 is

already lower than that of Transformer 6 since no information on the initial TI could be

obtained for both transformers. However, based on tests carried out on new conductor

type paper obtained from a transformer manufacturer, it was found that TI is high

ranging from 109 Nm/g to 115 Nm/g as shown in section 3.3.2.3 of Chapter 3. Paper

used in transformers usually has a high initial strength in order to withstand

mechanical stresses such as short circuit [18]. Thus, it is believed that the initial TI of

h)

a) b) c) d) e) f) g) h)


89

1st layer paper for both Transformer 10 and 6 should be high when it was first placed

in-service.

The low TI of 1st layer paper could lead to the failure of the transformer. As the TI of

paper reduces, there is a higher probability that the fibres might break from paper and

combines with metallic particles to initiate discharge and finally led to arcing and

sparking. The fibres could also align in the oil to provide a channel for short circuit

between conductors.

The second mechanism is thermal degradation which concerns the layer of paper close

to the conductor. Thermal degradation generally relies on the conductor temperature. It

can be seen that the effect of thermal degradation is apparent to Transformer 6.

4.3.2. Relationship between TI and age

The relationship between the TI of paper and a transformer age for, 9 transformers, can

be seen in Figure 4.6. Transformer 10 was not included in Figure 4.6 since it will be

used as the end of life criterion for next section. The scattering of data for each age is

due to the TI at different locations in a transformer.

It is observed that the TI for transformer population decreases as the age increases. A

similar trend was also observed with the reduction of DP of paper with a transformer

age [96]. However, the trend does not indicate ageing is a function of transformer age,

but a result of accumulative effect from temperature, moisture and acids. Loading is a

major source of heat while moisture might be introduced from the surrounding

atmospheric condition and as a product of ageing generated together with acids.


90

Figure ‎4.6 TI versus age of 9 transformers. Paper samples were taken from different

locations. Data are mean of 4 to 10 samples.

Figure ‎4.7 and Figure ‎4.8 show the daily typical summer and winter loading for some

of the transformers in Figure ‎4.6. The date for the daily loading of both seasons were

chosen based on the suggestion given by the utility report [97]. The average daily

loading at both seasons for Transformer 1 is high compared to other transformers as

seen in Figure ‎4.9 and Figure ‎4.10 which explains why it has the low TI at a young

age. Even though the average daily loading during summer for Transformer 2 is lower

than Transformer 1, it is observed that its TI is almost the same as Transformer 1 and

this might indicate the intrinsic thermal design of Transformer 2 is not as good as

Transformer 1.

On the other hand, it is noticed that TI is low even though the average daily loading of

both season for Transformer 4 and 5 is relatively low. This could be again, a thermal

design issue, however, at this point of research, we assume that all transformers under

study are having normalized thermal design and loading, generic to all transmission

transformers in the UK, so the ageing of paper in a transformer is mainly influenced by

age. As a transformer age increases, paper will degrade which results in loss of

mechanical strength.


91

Figure ‎4.7 Daily loading of typical summer (19/06/2009).

Figure ‎4.8 Daily loading of typical winter (04/12/2009).


92

Figure ‎4.9 Average TI and average daily loading of typical summer.

Figure ‎4.10 Average TI and average daily loading of typical winter.

The prominent factor that could affect the reduction trend of transformers under study

is thermal design. An example of bad thermal design is insufficient size of oil duct

which could cause disruption in oil flow and lead to the temperature increase and

accelerate the ageing of paper. However, since all transformers in Table 4.1 are from

different designs, this is also one of the reasons to assume a generic design exists for

this study.


93

4.4. Modelling relationship between TI and transformer age

4.4.1. Modelling procedure for transformer population

4.4.1.1. Comparison between Weidmann and Ding & Wang model

Initially, two models were considered to represent the relationship between TI and

transformer age. The model developed by Emsley and Heywood [46] is not considered

since it is difficult to use their model to make a practical estimation from limited

ageing data [43, 98].

The first model developed by researchers in Weidmann is as seen in Equation (4.1)

[43]. The model was derived from first order reactions. It has the advantages of

requiring only a few number of data points and provide a quick empirical solution to

the fitting of the measured TI data [22].

TI-C t

0TI = TI exp

(4.1)

where TI = TI at time (t) (Nm/g).

TI0 = Initial TI (Nm/g).

CTI = Ageing rate (years-1

).

t = Time (years).

The second model developed by Ding and Wang is shown in Equation (4.2) [98]. The

degradation of paper can be characterized by TI

ω where TI

ω = 0 represents the new

paper with 100 % retention of TI while TI

ω = 1 corresponds to the failure of paper.

TI-k t*

TI TIω = ω (1-exp )

TI

0

TIω =1-

TI (4.2)


94

where TIω = Percentage of TI loss.

*TIω = Capacity of TI degradation reservoir.

TIk = Ageing rate (years-1

).

Equation (4.3) can be obtained by re-arranging of Equation (4.2). This equation has a

similar form to Equation (4.1) and can be used to describe the relationship between TI

and transformer age.

TI-k t*

0 TITI = TI (1-ω (1-exp ))

(4.3)

The main criterion for choosing the suitable model for the relationship between TI and

transformer age is that it can estimate the life when TI reaches the lowest TI of

Transformer 10. The fitting of both models was carried out based on the average of the

measured TI of each transformer. The definition of initial and measured TI is

illustrated in Figure 4.11.

Figure ‎4.11 Definition of initial and measured TI.

The initial TI was set as a parameter for fitting for both models. It is difficult to obtain

information on the initial TI of paper used in each transformer. Thus, a range of initial

TI values were chosen from both literatures [18, 46] and the measured TI of new

papers obtained from manufacturers. Once the range has been obtained, it is split into


95

the average, lowest and highest values as seen in Table 4.4. The values of initial TI is

reasonable since papers used in transformers usually have high initial strength in order

to withstand mechanical stresses such as short circuit [18].

Table 4.4 Classification of initial TI.

Initial TI TI (Nm/g)

Average 107

Lowest 104

Highest 112

An example of not setting initial TI as a fitting parameter can be seen in Figure 4.12.

At first instance it is observed that there are 2 stages of relationship between TI of

paper and a transformer age. The first stage is the fast reduction of TI of paper from

start until the age reaches 10 years. The second stage is the slow reduction of TI of

paper between age of 10 and 50 years. However, no modelling was carried out to

represent the 2 stage relationship since the paper with age of 10 years could be

subjected to higher loading which in turn caused the low TI. The initial TI based on

fitting is around 96 Nm/g which is lower than any values given in Table 4.4. Thus, the

initial TI needs to be set, in advance, when the fitting is carried out for both models.

Figure ‎4.12 Fitting for the average of the measured TI according to Weidmann model.

The initial TI was not set as fitting parameter.


96

It is observed that for the Weidmann model, the fitting line slightly deviates from the

actual average of the measured TI of the 11 years old transformers (Transformer 1 and

2) as shown in Figure 4.13. One of the possible reasons is that the paper obtained from

these transformers might be subjected to advanced ageing as discussed in section 4.3.2.

It is also noticed that the Weidmann model meets the criterion of end of life at the

same age intersect with the Transformer 10 criterion when all three different values of

initial TI are used.

On the other hand, it is found that the average measured TI of 11 years old

transformers (Transformer 1 and 2) could be modelled by Ding and Wang model as

shown Figure 4.14. As mentioned earlier, the paper samples for both transformers

might be subjected to advanced ageing due to the high loading. Furthermore, Ding and

Wang model cannot reach the criterion of end of life i.e. the final estimated TI remains

at a value around 40 Nm/g, and this indicate that its suitability is more towards the

initial and mid-age trend estimation

Thus, the Weidmann model is selected to represent the relationship between TI and

transformer age since it could meet the criterion defined earlier. Life assessment of

transformer population will be carried out based on this model.

Figure ‎4.13 Fitting for the average of the measured TI according to Weidmann model.


97

Figure ‎4.14 Fitting for the average of the measured TI according to Ding and Wang

model.

4.4.1.2. End of life criterion

The end of life criterion of transformer population is determined based on three criteria

as shown in Table 4.5. The first and second criteria are obtained from the IEEE

standard, C57.91-1995 [1] where the first criterion is based on 50 % retention of TI

while the second criterion is based on 25 % retention of TI. The third criterion is based

on the lowest TI of Transformer 10 with a value of 22.44 Nm/g. This criterion was

chosen because Transformer 10 failed due to the reason of ageing.

Table 4.5 End of life criterion based on TI.

Criteria End of life criteria

Criterion 1 50 % retention of TI

Criterion 2 25 % retention of TI

Criterion 3 Lowest TI of Transformer 10

4.4.2. Life assessment of transformer population through TI measurements

In this section, the relationship between TI and transformer age is represented by the

Weidmann model. The initial TI was set as a fixed parameter where its values are


98

given in Table 4.4. The lives at three ends of life criteria are determined based on the

fitting line where the life criteria are shown in Table 4.5.

4.4.2.1. Life assessment between top and bottom windings

The first step is to assess the life of paper sample taken from top and bottom windings.

Figure 4.15 and Figure 4.16 show the fitting results of the average of the measured TI

at the top and bottom windings.

It is noticed that the fitting lines for top winding reach the end of life criterion faster

than those of bottom winding. For example, the estimated life from the top winding

based on Criterion 3 is around 58 years while that from bottom winding is around 81

years. As mentioned earlier in section 4.3.1.1, high temperature and hotspot usually is

located at the top winding. Since heat could accelerate the ageing of paper, it is

expected that the average measured TI at the top winding is low which in turn lead to

the reduction of paper life.

Figure ‎4.15 Fitting for the average of the measured TI at the top winding. Each point

represents the average of the measured TI at the top winding of each transformer.


99

Figure ‎4.16 Fitting for the average of the measured TI at the bottom winding. Each

point represents the average of the measured TI at the bottom winding of each

transformer.

As expected, the ageing rate based on the highest initial TI is the highest for both top

and bottom windings as shown in Table 4.6 and Table 4.7. The r² value is high for both

fittings, ≥ 0.855. The average ageing rate at the top winding is around 2.70×10-2

while

at bottom winding is around 1.94×10-2

. The ageing rate at the top winding is 1.4 times

higher than that of bottom winding.

Table 4.6 Ageing rate for the average of the measured TI at the top winding.

Fitting

parameters

Based on average

of the initial TI

Based on lowest

of the initial TI

Based on highest

of the initial TI

CTI (years-1

) 2.68×10-2

2.59×10-2

2.82×10-2

r² 0.924 0.928 0.905

Table 4.7 Ageing rate for the average of the measured TI at the bottom winding.

Fitting

parameters

Based on average

of the initial TI

Based on lowest

of the initial TI

Based on highest

of the initial TI

CTI (years-1

) 1.93×10-2

1.85×10-2

2.06×10-2

r² 0.888 0.899 0.855


100

Life assessment of paper at the top and bottom windings is given in Table 4.8 and

Table 4.9. It is observed that the difference between the life estimated based on lowest

and highest initial TI is smaller at the top winding compared to the bottom winding.

The difference at the top winding ranges from 2 years to 3 years while at the bottom

winding ranges from 3 to 7 years.

Generally, the estimated life at the top winding is lower than bottom winding. It is also

observed that the estimated life based on Criterion 1 is around 50 % of estimated life

based on Criterion 2. The finding is expected since the Weidmann model is similar to

the exponential decay equation which is often used to express the half life of

radioactive materials. If Criterion 3 is chosen as the end of life criterion, the estimated

life at the top winding is 58 years with 48 years and 70 years as the lower and upper

limit of 95 % confidence band. On the other hand, the estimated life at the bottom

winding is 81 years with 66 years and 96 years as the lower and upper limit of 95 %

confidence band.

Table 4.8 Estimated life for the average of the measured TI at the top winding.

End of life

criteria

Based on average of

the initial TI (years)

Based on lowest of


Based on highest of


F LCL UCL F LCL UCL F LCL UCL

Criterion 1 26 22 31 27 23 32 25 21 30

Criterion 2 51 43 61 53 44 63 50 40 59

Criterion 3 58 48 70 59 49 71 57 47 69

F= Based on fitting, LCL and UCL= Based on lower and upper limit of 95 % confidence band

Table 4.9 Estimated life for the average of the measured TI at the bottom winding.

End of life

criteria

Based on average of


Based on lowest of


Based on highest of



Criterion 1 36 30 43 37 31 45 34 28 41

Criterion 2 72 59 86 75 62 89 68 55 82

Criterion 3 81 67 96 83 69 99 78 63 94



101

Based on this result, it is found that paper at the top winding degrades at a higher rate

than bottom winding. Due to this reason, the life of transformer should be based on

those TI obtained from top.

4.4.2.2. Life assessment of transformer population

A TI reduction trend can be observed for transformer population by considering all

transformers are from the same design and experienced the same loading as shown in

Figure 4.17 and Figure 4.18.

It is shown that if Criterion 3 is taken as the end of life criterion, the life estimated

based on the average of the measured TI is around 68 years while based on the lowest

of the measured TI is around 42 years. It is found that the life estimated based on the

lowest of the measured TI agrees with the current utility end of life definition of 40

years.

Figure ‎4.17 Fitting for the average of the measured TI. The square point represents

the average of the measured TI from different locations of each transformer.


102

Figure ‎4.18 Fitting for the lowest of the measured TI. The round point represents the

lowest of the measured TI from different locations of each transformer.

The ageing rates for both the average and lowest of the measured TI are given in Table

4.10 and Table 4.11. The r² value is high for both fittings, ≥ 0.8. As expected, the

ageing rate for the lowest of the measured TI is higher than the average of the

measured TI. The average ageing rate for the average of the measured TI is around

2.31×10-2

while the lowest of the measured TI is around 3.72×10-2

.

Table 4.10 Ageing rate for the average of the measured TI.

Fitting

parameters

Based on average

of the initial TI

Based on lowest

of the initial TI

Based on highest

of the initial TI

CTI (years-1

) 2.30×10-2

2.21×10-2

2.43×10-2

r² 0.831 0.836 0.8

Table 4.11 Ageing rate for the lowest of the measured TI.

Fitting

parameters

Based on average

of the initial TI

Based on lowest

of the initial TI

Based on highest

of the initial TI

CTI (years-1

) 3.71×10-2

3.60×10-2

3.86×10-2

r² 0.912 0.918 0.897


103

The estimated life of the transformer population can be seen in Table 4.12 and Table

4.13. It is observed that the difference between estimated life based on the lowest and

highest initial TI is smaller for the lowest TI than the average of the measured TI.

If Criterion 3 is taken as the end of life criterion, the estimated life from the average of

the measured TI is around 68 years with 55 years and 82 years as the lower and upper

limit of 95 % confidence band. On the other hand, the estimated life from the lowest of

the measured TI is around 42 years with 33 years and 52 years as the lower and upper

limit of 95 % confidence band.

Table 4.12 Estimated life for the average of the measured TI.

End of life

Criteria

Based on average of


Based on lowest of


Based on highest of



Criterion 1 30 25 37 31 26 38 28 24 36

Criterion 2 60 49 73 62 51 75 57 46 70

Criterion 3 68 55 82 70 57 83 65 53 81


Table 4.13 Estimated life for the lowest of the measured TI.

End of life

Criteria

Based on average of


Based on lowest of


Based on highest of



Criterion 1 19 15 24 20 16 25 18 14 23

Criterion 2 37 29 46 39 30 48 36 28 45

Criterion 3 42 33 52 43 34 53 42 32 52


The estimated life based on Criterion 2 is slightly lower than Criterion 3 and if

Criterion 1 is chosen as the end of life criterion, the estimated life would be much

lower than Criterion 2 and 3. Thus, it is proposed to re-introduce the 20 % retention of

TI criterion proposed previously by Dakin [99]. Table 4.14 shows the comparison of

the estimated life between Criterion 3 and Criterion 4 (20 % retention of TI). It is

observed that the estimated life is almost similar where the difference between

Criterion 3 and 4 is less than 5 %.


104

Table 4.14 Average estimated life for the transformer population.

Parameters

Based on average of the

measured TI (years)

Based on lowest of the

measured TI (years)

Criterion 3 Criterion 4 Criterion 3 Criterion 4

F 68 70 42 43

LC 55 57 33 34

UCL 82 83 52 53


The end of life for transformers is usually defined based on the lowest of the measured

TI or DP. It is found that Criterion 4 is sufficient to represent the end of life of the

transformer population and the estimated life agrees with the current end of life of 40

years.

4.5. Effect of ageing on dielectric strength of paper

In this section, different experiments were conducted for examining the dielectric

performance of the paper subjected to in-service ageing.

4.5.1. Experiment description

Service aged paper was obtained from Transformer 10 and the TI of the paper is 24.13

Nm/g. New paper, conductor and layer types were used to compare with the service

aged paper. The properties of each paper can be seen in Table 4.15. The density of

conductor and layer types paper was obtained from the manufacturer’s datasheet. It can

be seen that service aged paper has the highest density followed by conductor and

layer types paper. Aged mineral oil was obtained from previous ageing experiments.

The oil was aged without copper and paper under open condition for 6 months at 105

°C.


105

Table ‎4.15 Properties of paper used for breakdown test.

Type of paper Grammage

(g/m²)

Apparent density

(g/cm³)

Thickness

(mm)

Layer paper 170 0.76 0.237

Conductor paper 48 0.93 0.053

Service aged paper 101 1.17 0.086

New and aged mineral oils were first filtered and dried according to the procedure in

section 3.5.1 of Chapter 3. The pre-processing procedure of paper is described in

section 3.5.2 of Chapter 3. The acidity of new and aged mineral oil was measured after

the pre-processing procedure. The acidity of new oil is 0.01 mg KOH/g and the acidity

of aged oil is 0.22 mg KOH/g.

AC breakdown test was carried out using sphere electrodes with a diameter of 12.5

mm. All oil and paper samples were tested for breakdown voltage at a same gap

distance. In order to achieve this purpose, different number of paper layers were used

since paper comes in difference thickness as seen in Table 4.16. The gap between

electrodes was determined by the thickness of service aged paper (highest) and the

same gap distance was maintained for conductor and layer type papers. The gap

distance was also maintained when testing breakdown strength of the oil. A 50 Hz AC

voltage with increasing rate of 0.5 kV/s was applied to the paper impregnated with

mineral oil. The breakdown can be detected by the current relay set to 4 mA which

isolates the sample tested from the source in order to avoid the oil and paper

deterioration. A total of 40 measurements of breakdown voltage were carried out on

each sample.

Table ‎4.16 Number of paper layers used in breakdown tests.

Type of paper Number of layers Thickness (mm)

Conductor paper (New) 4 0.212

Layer paper (New) 1 0.237

Service aged paper 3 0.258


106

4.5.2. Dielectric strength of paper

All samples were first degassed in a vacuum oven at room temperature before being

tested for breakdown voltage. All paper samples were subjected to 40 breakdowns and

the position of the paper between the electrodes was moved after each breakdown. The

gap distance for all samples is set based on the thickness of service aged paper with a

value of 0.258 mm.

It is observed that the breakdown voltage for service aged paper is higher than both

new conductor and layer types papers as seen Figure 4.19. The results show that the

mechanical strength of paper has no effect on the breakdown voltage of the paper,

however, the density does. Similar observation was noticed for laboratory accelerated

aged paper where the breakdown strength remain almost unchanged even though the

DP of the paper has already reaches 270 after 6 months of ageing at 120 °C [100].

Another study for paper aged at different temperatures also shows the same pattern

where the reduction of the mechanical strength will not affect the paper’s AC dielectric

strength [101].

Figure ‎4.19 Breakdown voltages of different types of paper impregnated in new oil.

It is observed that density has a significant effect on the breakdown voltage of the

paper as shown in Figure 4.20 a). Service aged paper has the highest breakdown

voltage followed by the conductor and layer types paper. The result is in line with


107

previous finding where the breakdown voltage is high for paper with a high density

[100]. It is also noticed that from Figure 4.20 b) there is no effect of grammage on the

breakdown voltage of paper.

a) b)

Figure ‎4.20 Relationship between breakdown voltages of paper impregnated in new

oil a) Density and b) Grammage.

4.5.3. Oil influence on breakdown voltage of paper

According to previous work, a theory was proposed that the discharge happens in the

oil channel since it is the weak link in oil impregnated paper insulation and takes more

electrical stress than cellulose [102]. Therefore, if the quality of the oil is

compromised, the breakdown voltage of the paper might be affected as well. A study

was conducted in order to look into the effect of oil quality on the breakdown voltage

of the paper. First, a comparison of breakdown voltage is made between new and aged

oil as seen in Figure 4.21. It is shown that the aged oil suffers a 3.8 % reduction of

breakdown voltage and it is believed to be caused by the acids in the oil.


108

Figure ‎4.21 Breakdown voltages of new and aged oils.

For paper impregnated in aged oil, it is observed that the reduction of breakdown

voltage increases ranging from 7 % to 9 % as seen in Figure 4.22, Figure 4.23 and

Figure 4.24. The reduction for paper impregnated in aged oil is almost 2 times higher

than that of the aged oil itself. These results show that the oil quality could affect the

overall performance of breakdown voltage of oil/paper system.

Figure ‎4.22 Breakdown voltages of layer type paper impregnated in new and aged

oils.


109

Figure ‎4.23 Breakdown voltages of conductor type paper impregnated in new and

aged oils.

Figure ‎4.24 Breakdown voltages of service aged paper impregnated in new and aged

oils.

Figure 4.25 shows the comparison between breakdown strengths of paper impregnated

in new and aged oil. Similar trend is observed for all types of paper, that paper

impregnated in aged oil suffers around 7 % to 9 % reduction of breakdown strength.


110

Figure ‎4.25 Average breakdown strengths of new and aged paper. Data are mean of

50 tests and error bars represent the standard deviation.

4.5.4. Statistical analysis on breakdown voltage of paper

Statistical analysis was performed in order to determine the suitable distribution for the

breakdown voltage data. Normal and Weibull distribution were used to fit all

breakdown voltage data as seen in Figure 4.26, Figure 4.27, Figure 4.28 and Figure

4.29. A distribution is considered a good fit if the data points follow a straight line. It is

found that normal distribution is the best fitting for all breakdown voltage data.

101

0.01

0.05

0.1

0.25

0.5

0.75

0.9 0.95

0.99

0.999

Breakdown voltage (kV)

Pro

ba

bility

Layer paper

Weibull fit of layer paper

Conductor paper

Weibull fit of conductor paper

Aged paper

Weibull fit of aged paper

New oil

Weibull fit of new oil

Figure ‎4.26 Weibull probability plots of new and aged paper impregnated in new oil.


111

101

0.01

0.05

0.1

0.25

0.5

0.75

0.9 0.95

0.99

0.999


Pro

ba

bili

ty

Layer paper

Weibull fit of layer paper

Conductor paper

Weibull fit of conductor paper

Aged paper


Aged oil


Figure ‎4.27 Weibull probability plots of new and aged paper impregnated in aged oil.

0 5 10 15 20 25

0.01

0.05

0.1

0.25

0.5

0.75

0.9

0.95

0.99 0.995

0.999


Pro

ba

bili

ty

Layer paper

Normal fit of layer paper

Conductor paper

Normal fit of conductor paper

Aged paper

Normal fit of aged paper

New oil

Normal fit of new oil

Figure ‎4.28 Normal probability plots of new and aged paper impregnated in new oil.


112

0 5 10 15 20 25

0.01

0.05

0.1

0.25

0.5

0.75

0.9

0.95

0.99 0.995

0.999


Pro

ba

bili

ty

Layer paper

Normal fit of layer paper

Conductor paper

Normal fit of conductor paper

Aged paper

Normal fit of aged paper

Aged oil

Normal fit of aged oil

Figure ‎4.29 Normal probability plots of new and aged paper impregnated in aged oil.

Table 4.17 and Table 4.18 show the average breakdown voltage comparison between

normal and Weibull fitting. It is found that the average breakdown voltage obtained

from Weibull fitting is higher than normal fitting. According to the normal fitting, the

oil has the highest value of standard deviation followed by the service aged paper,

conductor and layer types paper. This indicates that oil has a higher variation of

breakdown voltage than paper.


and aged oil based on Weibull fitting.

Test condition Oil (kV) Layer paper

(kV)

Conductor

paper (kV)

Service aged

paper (kV)

Average (new oil) 14.1 15.3 17.9 20.1

Average (aged oil) 13.6 14.2 16.3 18.8


and aged oil based on normal fitting.

Test condition Oil

(kV)

Layer paper

(kV)

Conductor

paper (kV)

Service aged

paper (kV)

Average (new oil) 13.9±1.57 15.2±1.33 17.7±1.16 20.0±1.43

Average (aged oil) 13.4±1.87 14.0±1.37 16.2±0.91 18.6±1.58

± represents standard deviation


113

4.5.5. Breakdown mechanism of insulation paper

Multiple breakdowns were applied to a single point on paper impregnated in new and

aged oil. Two types of paper were used for the experiment, which are layer type paper

and service aged paper. The properties for both types of paper can be seen in Table

4.15. The gap distance between electrodes was determined by the individual thickness

of each paper. After each breakdown, the oil and paper sample is degassed for 5

minutes in order to reduce the influence of bubbles trap inside the paper pores. Due to

the 4 mA protection relay setting, the energy release during the breakdown test is

small, thus the fibre will not be seriously damaged.

The test results in Figure 4.30 and Figure 4.31 show that the breakdown voltage of

new paper impregnated in either new or aged oil remains almost unchanged even after

100 breakdowns. The degassing process helps to remove bubbles and thus prevents the

reduction of breakdown voltage of the new paper.

However, the situation is different for service aged paper where there is a reduction of

breakdown voltage after a certain number of tests. Point 1 and point 2 represent two

different paper samples. It is observed that for service aged paper impregnated in new

and aged oil, the breakdown voltage suddenly reduces to 0 kV after 32nd

, 42nd

, 83rd

and

90th

breakdown all varied but shorter than 100 breakdowns. A 0 kV breakdown voltage

represent the two electrodes is short circuited. The reduction of breakdown voltage for

service aged paper probably due to the carbonization of the fibre. As the paper aged,

the fibre is easily carbonized which could creates a conductive channel between the

two electrodes [100].

According to the visual inspection, service aged paper was easily destroyed when

subjected to breakdown voltage tests as compared to new paper. New paper creates

only carbonized dot while service aged paper create carbonized hole after being

subjected to breakdown tests as shown in Figure 4.32. It is believed that the carbonized

channel in the new paper is not as conductive as that of service aged paper to cause

sudden reduction on the breakdown voltage. In our case study, it is believed that the

energy released by the test equipment is not high and thus requires a numbers of tests

before sudden reduction on the breakdown voltage can be observed.


114

Figure ‎4.30 Multiple breakdowns at single point of paper impregnated in new oil.

Figure ‎4.31 Multiple breakdowns at single point of paper impregnated in aged oil.

a) b)

Figure ‎4.32 Paper condition after breakdown tests a) New paper and b) Aged paper.


115

4.6. Summary

There are only small differences in TI profiles among different windings and phases.

This is reasonable since most transformers are designed symmetrically among phases

and oil flow distributions considered among windings. A study on multi layer

insulation shows that layer profile does exists and it is influenced by thermal and

chemical degradations.

A relationship between TI and transformer age was found and represented by the

Weidmann model. It is shown that the end of life of transformer population based on

paper TI is best represented by the 20 % retention of TI. Based on this criterion the

estimated life based on the lowest of the measured TI is around 43 years with 34 years

and 53 years as the lower and upper limit of 95 % confidence band.

It is observed that reduction of mechanical strength due to ageing does not bear any

relationship with the breakdown strength of the paper. However, paper density and oil

condition could influence the dielectric strength of the paper. Further study on the

breakdown mechanism shows that new paper could maintain its dielectric strength

even after being subjected to multiple breakdowns, provided that bubbles and other

impurities such as carbonized cellulose particles are avoided. However, as the paper

aged, the fibre strength is damaged and it is easy to carbonize. So, a conductive

channel along the carbonized fibre will be created by this process lead to the reduction

of the dielectric strength of the aged paper.

As a summary, it is shown that ageing of a transformer could be represented by the

mechanical property of the paper. It is difficult to determine the ageing status of the

paper based on electrical property since paper can maintain its electrical performance

under ageing conditions.


Measurements

116

CHAPTER 5. AGEING ASSESSMENT OF PAPER THROUGH LOW

MOLECULAR WEIGHT ACID MEASUREMENTS

5.1. Introduction

Ageing of paper and oil could generate different types of by-product such as water,

furans, gases and acids. Some of these by-products were used to determine the ageing

condition of transformers.

Recently it is found that there are two types of acids existing in paper and oil which are

Low Molecular Weight Acid (LMA) and High Molecular Weight Acid (HMA),

whereas most of LMA tend to stay in paper and most of HMA tend to stay in oil [16,

17]. Since a transformer’s functionality is closely related to its dielectric strength, it is

essential to study the effect of LMA and HMA on this property. It is also important to

examine the effect of these acids on the paper ageing.

So far, there is no publication describing study being conducted on the LMA contents

in scrapped transformer’s paper samples. LMA in paper samples from scrapped

transformers was measured in this PhD research study. Analysis of the relationship

between LMA and Tensile Index (TI) was performed in order to determine if LMA can

be used to indicate ageing profiles of transformer population.

LMA and HMA tests were carried out on in-service aged oil samples in order to

determine the partitioning between these acids for in-service transformers. Similar tests

were conducted on laboratory aged oil samples in order to study factors that could

affect the generation of LMA in oil. A paper TI prediction model was proposed based

on the measurement of LMA in oil.

5.2. Effect of LMA and HMA on breakdown voltage of oil and ageing of paper

The effect of LMA and HMA on the breakdown voltage of oil was first examined in

this section. A study was also carried out on the relationship between breakdown


Measurements

117

voltage and TAN of in-service oils. The effect of LMA and HMA on the paper aged

under dry and wet condition was studied in the second part of this section.

5.2.1. Effect of LMA and HMA on breakdown voltage of oil

5.2.1.1. Experiment description

Different amounts of either LMA ranging from 0.2 g to 0.4 g or HMA ranging from 1

g to 19 g were mixed with 500 ml dry and filtered mineral oil by magnetic stirrer for 3

hours at 50 °C. The oil was dried and filtered according to the procedure described in

section 3.5.1 of Chapter 3. The properties of LMA and HMA can be seen in Table 5.1.

The pKa value describes the extent of an acid can dissociate in solution where smaller

number means the acid has high dissociation rate. After the mixing procedure, the oil

sample was titrated for acidity measurement and then tested for breakdown voltage.

Table 5.1 Properties of LMA and HMA.

Types of acid

Molecular

weight

rM (g/mol)

Acid

dissociation

constant (pKa)

Melting

temperature

mT (°C)

Boiling

temperature

bT (°C)

LMA (Formic) 46 3.8 8.3 101

HMA (Stearic) 285 4.9 69.3 232

The AC breakdown test was carried out by mushroom VDE electrodes with a diameter

of 36 mm according to the ASTM standard, ASTM D1816 [103]. A 50 Hz AC voltage

with increasing rate of 0.5 kV/s was applied to the natural ester. The gap distance was

set at 1 mm and a total of 50 measurements of breakdown voltage were carried out on

all samples.

5.2.1.2. Test results

It is found that LMA has a significant effect on the breakdown voltage of the oil as

seen in Figure 5.1. The oil suffers a rapid reduction of breakdown voltage as LMA

increases up to 0.1 mg KOH/g. As the LMA increases further, the low breakdown


Measurements

118

voltage value remains unchanged around 6 kV. It is also observed that the breakdown

voltage reduces to 50 % of its initial value when LMA is around 0.06 mg KOH/g.

When LMA around 0.14 mg KOH/g, the mineral oil suffers a 70 % of reduction. It is

known that polar compounds such as water could affect the breakdown voltage of

materials. When the water saturation limit is exceeded, breakdown voltage of oil is

reduced significantly. Thus, by borrowing a similar principle to water, it is suspected

that the saturation limit for LMA in oil is quite low, which in turn causes the acid to

stay as free molecules in the oil and lead to the significant reduction of breakdown

voltage.

Figure ‎5.1 Influence of LMA on the breakdown voltage of mineral oil. Breakdown

voltage data are mean of 50 tests and error bars represent the standard deviation. LMA

in oil data is 1 measurement.

On the other hand, there is no significant effect of HMA on the breakdown voltage of

the oil when HMA is within the range from 0.1 mg KOH/g to 4 mg KOH/g as shown

in Figure 5.2. Significant reduction of breakdown voltage occurs after HMA reaches 4

mg KOH/g. At HMA around 9 mg KOH/g, the oil suffers 64.2 % reduction of

breakdown voltage. However, for in-service transformers, the acidity of mineral oil

will never reach 4 mg KOH/g and rarely reach 1.0 mg KOH/g. Thus, it is believed that

if there is a reduction of breakdown voltage of an acid oil, it might mainly be

contributed by LMA.


Measurements

119

Figure ‎5.2 Influence of HMA on breakdown voltage of mineral oil. Breakdown

voltage data are mean of 50 tests and error bars represent the standard deviation. HMA

in oil data is 1 measurement.

5.2.1.3. Relationship between breakdown voltage and Total Acid Number

(TAN) of in-service transformers

A total of 147 transformers excluding scrapped and oil regenerated transformers were

analyzed from a distribution transformer database. Yearly measurements of AC

breakdown tests and TAN were carried out on each transformer and corresponding

relationship is shown in Figure 5.3. The breakdown voltage test was carried out

according to IEC standard, IEC 156 [104] while TAN according to BSI standard, BS

62021[90].

It is observed that there is no clear correlation between TAN and the breakdown

voltage of in-service oil. The scattering of the data is high where the breakdown

voltage can be as low as 7 kV for TAN less than 0.1 mg KOH/g or as high as 60 kV

for TAN around 1.11 mg KOH/g.

Previous finding shows that under laboratory controlled experiment, the breakdown

voltage of oil could be affected by many factors such as water, particles and acidity

[105]. However, for in-service transformers, these ageing by-products co-exist


Measurements

120

together and it is difficult to determine any individual effect of these factors on the

breakdown voltage of in-service oil.

Figure ‎5.3 Influence of TAN on the breakdown voltage of in-service oil. Breakdown

voltage data are mean of 50 tests. TAN data is 1 measurement.

5.2.2. Effect of LMA and HMA on ageing of paper


In order to prepare oil samples with acids, 0.2 g of either LMA or HMA was mixed

with 200 ml dry and filtered mineral oil by magnetic stirrer for 3 hours at 50 °C. The

oil was dried and filtered according to the procedure described in section 3.5.1 of

Chapter 3. After the mixing procedure, the oil sample was titrated for acidity. It is

observed that the acidity of samples with LMA has a high deviation ranging from 0.22

mg KOH/g to 0.45 mg KOH/g. On the other hand, the acidity for samples with HMA

is stable ranging from 0.22 mg KOH/g to 0.24 mg KOH/g.

All samples were aged in an air circulating oven at 120 °C. Each sample contains 1.75

g of conductor paper and 200 ml of mineral oil containing either LMA or HMA. The

paper was pre-processed according to the procedure described in section 3.5.2 of

Chapter 3. For dry paper sample, the water content is less than 0.5 % while for wet


Measurements

121

paper sample, the water content is around 5 %. The properties of paper and mineral oil

can be seen in Table 3.1 and Table 3.8 of Chapter 3. All samples were sealed by PTFE

screw cap with a gasket ring in order to reduce the migration of oxygen from outside

and aged in an air circulating oven at 120 °C for up to 64 days. A total of 8

measurements of TI were carried out on each sample since the amount of paper used in

the ageing experiment is quite small. The ageing experiment was conducted under

sealed condition.

5.2.2.2. Ageing under dry condition

Figure 5.4 shows the affect of LMA and HMA on the paper aged under initial dry

condition. It is observed that the TI of samples with no acid reaches 50 % of its

strength after 57 days of ageing, with HMA after 49 days of ageing and with LMA

after 24 days of ageing.

Figure ‎5.4 Influence of LMA and HMA on the paper aged under dry condition. Data

are mean of 8 samples and error bars represent the standard deviation.

The relationship between TI and ageing duration was fitted by Equation (4.1). Table

‎5.2 shows that the ageing rate for samples with LMA is 2.1 and 2.4 times higher than

with no acid and with HMA.


Measurements

122

Table 5.2 Ageing rate of dry condition.

Condition Ageing rate, CTI (days-1

) r²

With no acid 1.5×10-2

0.986

With LMA 3.1×10-2

0.940

With HMA 1.3×10-2

0.956

5.2.2.3. Ageing under wet condition

For wet ageing condition, the effect of LMA on the paper ageing is greatly enhanced

as seen in Figure 5.5. The TI of samples with LMA reduces to 50 % of its strength

after 7 days of ageing while with no acid and with HMA after 33 days of ageing.

Figure ‎5.5 Influence of LMA and HMA on the paper aged under wet condition. Data


The ageing rate is further increased for wet ageing condition as seen in Table ‎5.3. The

ageing rate of samples with LMA under wet ageing condition is around 3 times higher

than under dry ageing condition. It is also observed that the ageing rate of samples

with LMA is 4 times higher than without no acid and with HMA. The r² fitting of

samples with LMA is low because the variation of TI measurement will be high, as

paper reaches the brittle state.


Measurements

123

Table 5.3 Ageing rate of wet condition.


) r²


0.793

With LMA 9.7×10-2

0.700

With HMA 2.2×10-2

0.911

The ageing rate of samples with no acid is almost similar to with HMA, for both dry

and wet condition, this indicates that there is no effect of HMA on the paper ageing.

The increment factor was calculated by Equation (5.1).

Ageing rate of samples with LMA or HMA

Increment factor = Ageing rate of samples with no acid

(5.1)

Table ‎5.4 shows the increment factors for the combination of acids and water, when

taking dry and new oil and paper condition as reference; the ageing rate for samples

with LMA increases 3.1 times from dry to wet ageing condition. On the other hand, a

low increment of ageing rate is observed for HMA added samples where it increases

1.7 times from dry to wet ageing condition.

Table 5.4 Increment factors of dry and wet ageing conditions.

Condition Increment factor

(dry ageing condition)

Increment factor

(wet ageing condition)

With LMA 2.1 6.5

With HMA 0.9 1.5

It is clear that LMA and water have multiplicative effect on the paper ageing. Similar

observation was also found with DP in previous finding [16]. According to previous

theory, the hydrolysis of paper is catalyzed by H+

originated from dissociated acids

[18]. LMA can be easily dissociated since the pKa value is lower than HMA. By

adding LMA, the rate of acid dissociation will increase and the resultant H+ catalyzes

the hydrolysis of wet paper and thus accelerates the paper degradation.


Measurements

124

5.3. LMA measurements of scrapped transformers paper

In total, 9 transformers were examined for LMA analysis. Paper samples for LMA

measurement were taken from the same location as in section 4.3 of Chapter 4. The oil

residue on the paper samples were wiped using a tissue before being tested for LMA.

Based on all measurements, it was found that the LMA in paper for scrapped

transformers ranges from 0.2 mg KOH/g to 6.4 mg KOH/g. LMA in paper was first

analyzed by comparing phases, windings and layers where the sampling of paper is

carried out according to the procedure described in Figure 3.1 and Figure 3.2 of

Chapter 3. The second part of this section describes the relationship between LMA in

paper, TI and transformer age.

5.3.1. LMA profile of scrapped transformers

5.3.1.1. Phase and winding profiles

The same transformers as in section 4.3.1.1 of Chapter 4 were analyzed for the profile

of LMA along windings and phases. The vertical profile of LMA in paper for A, B and

C phases can be seen in Figure 5.6. The age, paper properties and TAN of oil of each

transformer can be seen in Table 4.2 of Chapter 4. It is observed that the LMA in paper

has a big difference between phases at the top and centre winding. For example, the

LMA in paper at the top winding of Transformer 6 for A and C phase are much lower

than B phase.


Measurements

125

a) b)

Figure ‎5.6 LMA in paper profiles among different phases a) Transformer 4 (SW) and

b) Transformer 6 (CW). Data are mean of 2 measurements.

Figure ‎5.7 LMA in paper profiles among different windings of Transformer 8. Paper

samples were taken from B phase. Data are mean of 2 measurements.

It is suspected that the LMA in paper at the top winding for some of the phases of

Transformer 4 and Transformer 6 might be evaporated due to the high temperature at

this location. Top winding usually has high temperature and hotspot is normally

located near to this location. On the other hand, it is reasonable to expect the LMA in

paper at the centre location between phases has a big difference since the TI for both

transformers also shows similar profiles. Paper ageing will result in the reduction of TI

and at the same time lead to the formation of LMA which mainly stays in paper. Small


Measurements

126

difference of LMA in paper between phases is noticed at the bottom winding. Since

temperature is usually low at the bottom winding, the LMA should not evaporate and

should remain in the paper.

Figure 5.7 shows the vertical profile of LMA in paper for windings. It is difficult to

draw any conclusion for the winding profile since only one transformer is available for

the analysis.

It can be seen that paper at the top winding yields slightly higher LMA than bottom

winding. The paper at the top winding is usually more degraded than bottom winding

because the temperature is higher at the top. Since most of LMA originates from paper

ageing, it is reasonable to expect that the LMA in paper is slightly higher at the top

winding than bottom winding. However in some cases, LMA in paper at the bottom

winding can be slightly higher than top winding which can be seen from A phase of

Transformer 6. It is suspected that there might be an evaporation of LMA in paper at

the top winding for this transformer.

5.3.1.2. Layer profile

Same transformers as in section 4.3.1.2 of Chapter 4 were analyzed for the profile of

LMA along layer. The age, paper properties and TAN of oil of each transformer can be

seen in Table 4.3 of Chapter 4. It is observed that between the 8th

to 4th

layers paper,

the LMA for Transformer 10 ranges from 1.88 mg KOH/g to 3.98 mg KOH/g with an

average value of 2.90 mg KOH/g as shown in Figure 5.8. The highest LMA is

observed at the 2nd

layer paper with a value of 7.24 mg KOH/g. The 1st layer paper has

a lower LMA than the 2nd

layer paper probably due to the evaporation of LMA.

Similar pattern of LMA in paper for multilayer insulation is observed for Transformer

6, as shown in Figure 5.9. The lowest LMA is observed at the 11th

layer paper with a

value of 1.86 mg KOH/g. The LMA slightly increase at the 10th

layer paper and remain

almost unchanged up to the 2nd

layer paper with an average value of 3.31 mg KOH/g.

The 1st layer paper has the highest LMA with a value of 5.5 mg KOH/g.


Measurements

127


samples were taken from B phase, HV bottom. Data is 1 measurement.


samples were taken from B phase, CW top. Data are mean of 2 measurements.

Both results suggested that the outer layer paper could be subjected more to chemical

degradations. The paper in direct contact with oil has a higher LMA than the inner

layer. This observation supports the previous assumption that the outer layer paper is

affected by the chemical degradation process [94]. The main mechanism behind this

phenomenon is related to the condition of oil. The ageing of oil and paper produce


Measurements

128

different by-products and some of them could accelerate the ageing of paper such as

water and LMA. Even though most of water and LMA stay in the paper, there is still a

small amount left in oil. Thus, paper in close contact with oil could be subjected to a

higher rate of degradation due to the existence of these by-products. As paper

undergoes further degradation, more water and LMA will be produced and stay in the

outer layer paper. Furthermore, as the condition of oil becomes worse, it will also

contribute to the increment of LMA in the outer layer paper.

It can be seen that the LMA in paper of the first three layers of Transformer 10 is

higher than Transformer 6. The high LMA in paper in Transformer 10 is expected

since the outer layer paper suffers more severe degradation than Transformer 6 based

on the mechanical strength reduction. Furthermore, it is noticed that a high amount of

LMA is expected for paper with a low TI as shown in Figure 5.10 and Figure 5.11.

Thus, it is suggested that there is a relationship between TI and LMA in paper.


Paper samples were taken from B phase, HV bottom. LMA in paper data is 1

measurement. TI data are mean of 9 to 10 samples.


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129


Paper samples were taken from B phase, CW top. LMA in paper data are mean of 2

measurements. TI data are mean of 9 to 10 samples.

The study on the 1st layer paper of both transformers shows that the LMA in paper for

a young transformer could be higher than an old transformer. On the other hand, the TI

for a young transformer can lower than an old transformer. It is suspected that

Transformer 10 might be subjected to either a higher loading or worse thermal design

than Transformer 6. This is supported by the fact that TAN of the oil for Transformer

10 is higher than Transformer 6. The scrapping report of Transformer 10 briefly

mentioned that the severe degradation of solid insulation is due to the high loading.

However, no loading history could be obtained for both transformers.

5.3.2. Relationship between LMA in paper, TI and transformer age

5.3.2.1. Relationship between LMA in paper and TI

Figure 5.12 shows the relationship between LMA in paper and TI based on the

measurements of 9 scrapped transformers. It is shown that there is a link between TI

and LMA; i.e. LMA in paper increases as TI of paper decreases. The correlation

between TI of paper and LMA in paper was calculated based on Pearson correlation


Measurements

130

factor as shown in Equation (5.2). The Pearson correlation describes that the closer the

value to either -1 or 1, the stronger the correlation between variables.

xy

2 2

(x-x) (y-y)r =

(x-x) (y-y)

(5.2)

where rxy

= Correlation factor.

x and y

= Variables.

x and y

= Mean of variables.

The correlation factor between TI and LMA in paper is –0.71 which indicates that

there is strong correlation between these parameters. As the paper degrades, LMA will

be generated and it accumulates in the paper which in turn contributes to the further

increment of LMA. The relationship between LMA in paper and TI of paper also

might indicate that the generation of LMA is higher than the consumption of LMA.

The measured LMA is a result from the subtraction of LMA generation with LMA

consumption.

Figure ‎5.12 LMA in paper versus TI of 9 transformers. Paper samples were taken

from different locations. LMA in paper data are mean of 2 measurements. TI data are

mean of 4 to 10 samples.


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131

The relationship between TI of paper and LMA in paper can be represented by

Equation (5.3). The r² of the fitting is 0.516 which indicates significant scattering of

the data. One of the possible reasons is due to the variation of the thickness of the

paper; it was suggested that thick paper degrades faster than thin paper [43]. The paper

thickness in this study ranges from 0.04 mm to 0.15 mm.

(2.41-0.03TI)LMA in paper = exp (5.3)

By expanding Equation (5.3), it will become a linear equation as seen in Equation (5.4)

and the plotting can be seen in Figure 5.13. The form of this equation is quite similar

to the equation representing the relationship between Furanic Compounds (2FAL) and

DP [75]. It can be seen clearly that the scattering of the data is high at the initial stage

of ageing which indicates the LMA in paper varies when TI is still in a good condition.

ln(LMA in paper) = 2.41-0.03TI (5.4)

Figure ‎5.13 LN (LMA in paper) versus TI of 9 transformers. Paper samples were

taken from different locations. LMA in paper data are mean of 2 measurements. TI

data are mean of 4 to 10 samples.

If the relationship between LMA in paper and LMA in oil could be obtained, the

condition of paper can be known based on the measurement of LMA in oil. In order to


Measurements

132

achieve this goal, a study on the partitioning of LMA in paper and oil under different

conditions is needed.

5.3.2.2. Relationship between LMA in paper and transformer age

The relationship between LMA in paper and transformer age can be seen in Figure

5.14. The LMA in paper data was obtained from the same location as TI in Figure 4.6.

It can be seen that the LMA in paper can be as high as 3.2 mg KOH/g for a young

transformer - Transformer 1. As shown in Figure 5.15 and Figure 5.16, the average

daily loading of typical summer and winter for Transformer 1 is relatively high as

compared to the other transformers, and this could be the reason for the high LMA

observed in the paper at a young age. The effect of loading on the amount of LMA in

paper is also shown by Transformer 5. It is shown that the low amount of LMA in

paper corresponds well with the low average daily loading at both seasons. However, it

is observed that the LMA in paper could be high even though the average daily loading

of both seasons is, low as shown by Transformer 4. Thus, it is believed that age could

also influence the amount of LMA in paper. As transformer age increases, the paper

will degrade with the accelerated help from ageing by-products even at low load which

in turn increase the amount of LMA in paper.

Figure ‎5.14 LMA in paper versus transformer age of 9 transformers. Data are mean of

2 measurements.


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133

Figure ‎5.15 Average LMA in paper and average daily loading of typical summer.

Figure ‎5.16 Average LMA in paper and average daily loading of typical winter.

Another possible factor that could influence the amount of LMA left in paper is

design. However, it is difficult to see the effect of design on the amount of LMA in

paper since all transformers under study come from different designs.

An increment trend can be observed between LMA in paper and transformer age by

considering all transformers are from the same design and experienced the same

loading as shown in Figure 5.17. The trend suggested that the LMA tend to accumulate

in the paper with the increment of time. However, there is no evidence to prove the


Measurements

134

accumulation of LMA in paper since it is impossible to measure LMA in paper of

individual transformer while it is in-service. It is found that the significant increment

of LMA occurs after 40 years in service.

Figure ‎5.17 Fitting of LMA in paper versus transformer age of 9 transformers. Data

are mean of 2 LMA in paper measurements.

An exponential relationship was used to represent the increment of LMA in paper with

transformer age as shown in Equation (5.5).

Ct

0A = A +Bexp (5.5)

where A and A0 = Current LMA in paper and initial LMA in paper (mg KOH/g).

B = Constant (mg KOH/g).

C = Increment rate of LMA in paper (years-1

).

t = Transformer age (years).

Table 5.5 shows the fitting parameter of the relationship between LMA in paper and

transformer age. A0 was set to 0.12. The constant, B was adjusted in order to make sure

the initial LMA in paper obtained from the fitting as close as possible to (A0+B= 0.12

mg KOH/g) since LMA measured from the new conductor paper is around this value.

The r² of the fitting is low for both average and highest LMA at a value of 0.38 and

0.294 due to high deviation of LMA in paper for Transformer 1 and 2.


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Table ‎5.5 Increment rate of LMA in paper and constant of the fitting.

Fitting parameters Based on average LMA Based on highest LMA

B (mg KOH/g) 2105.6 2108.0

C ( 1years ) 2108.9 2109.0

r² 0.380 0.294

5.4. LMA and HMA ratio between paper and oil

5.4.1. LMA ratio between paper and oil


An experiment was conducted in order to study the partitioning of LMA in paper and

oil. Three different ratios of paper and oil were used as shown in Table 5.6. Around

0.08 g of LMA was mixed with the 75 g of dry and filtered mineral oil by magnetic

stirrer for 3 hours at 50 °C in a 250 ml beaker. The oil was dried and filtered according

to procedure described in section 3.5.1 of Chapter 3. After the mixing procedure, the

oil and LMA mixture is titrated for acidity and the value is recorded as initial amount

of LMA in oil. The dry paper is added into oil containing LMA and it is left to stand in

a 250 ml bottle for at least 10 days. The paper was dried according to the procedure

described in section 3.5.2 of Chapter 3. It is observed that there is about 13 %

difference between the highest and lowest value of initial LMA in oil. The difference

might be due to the incomplete mixture of LMA with the oil especially for Sample 2.

Table 5.6 LMA ratio of paper and mineral oil. Data is 1 measurement.

Samples Ratio

(paper (g): oil (g))

LMA added

(g)

Initial LMA in oil

(mg KOH/g)

Sample 1 4.67: 75 0.084 0.68

Sample 2 7: 75 0.083 0.62

Sample 3 14: 75 0.082 0.70


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136

5.4.1.2. Influence of paper and oil ratio on the partitioning of LMA in

paper and oil

Table 5.7 shows that as the paper weight increases, the LMA in oil decreases. On the

other hand, the value of LMA in paper ranges from 12.89 mg KOH/g to 21.17 mg

KOH/g. The results show that most of the LMA tends to stay in the paper. The ratio of

LMA in paper and oil is heavily influenced by the weight ratio of paper and oil. By

increasing the amount of paper in the paper/oil system, more LMA could migrate from

oil to paper and stay in the paper. Furthermore, the percentage ratio of LMA and HMA

in oil might change if the weight ratio of paper and oil changes.

Table 5.7 LMA and HMA ratio of paper and mineral oil. LMA in paper data are mean

of 2 measurements and LMA in oil data is 1 measurement.

Ratio

(paper (g): oil (g))

LMA in paper

(mg KOH/g)

LMA in oil

(mg KOH/g)

Absolute weight

ratio of LMA

(paper: oil)

4.67: 70 18.09 0.27 4.47: 1

7: 70 21.17 0.14 15.1:1

14: 70 12.89 0.01 257.8: 1

The relationship between weight ratio of paper and oil and LMA ratio of paper and oil

can be seen in Figure 5.18. It can be seen that the LMA ratio doubled once the paper

weight is increased by 2.33 g. As the paper weight is increased by 7 g, the LMA ratio

increased almost 8 times.

It is observed that the weight ratio of paper and oil has significant effect on the LMA

ratio of paper and oil. The information could be used for predicting LMA in paper in

transformers based on LMA in oil provided the design information (weight of paper

and oil) is known.


Measurements

137

Figure ‎5.18 LMA ratio of paper and oil versus weight ratio of paper and oil. LMA in

paper data are mean of 2 measurements. LMA in oil data is 1 measurement.

5.4.2. LMA and HMA ratio of oil

5.4.2.1. LMA and HMA ratio of in-service oil

Table 5.8 shows the list of transformers used for analysis of LMA and HMA ratio of

oil. All transformers are used at transmission level and still in-service. The oil

sampling was done by the oil maintenance company. All transformers come from

different designs.

Table ‎5.8 Case study of transformers.

Transformer Rating Age (years)

11 240MVA 400/132kV 1

12 40MVA 275/20.5kV 9

13 180MVA 275/66kV 16

14 1000MVA 400/275kV 20

15 240MVA 400/132kV 26

16 240MVA 400/132kV 37

17 120MVA 275/66kV 42

Only two loading history could be obtained for this case study. The daily loading of

typical summer and winter of Transformer 15 and 16 can be seen in Figure 5.19 and


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138

Figure 5.20. The dates for the daily loading of both seasons were chosen based on the

suggestion given by the utility [97]. It is noticed that the average daily loading of both

seasons for Transformer 15 are slightly higher than Transformer 16.

Figure ‎5.19 Daily loading of typical summer (19/06/2009).

Figure ‎5.20 Daily loading of typical winter (04/12/2009).


Measurements

139

a) LMA and HMA analysis of in-service oil

Ideally, the amount of TAN should equal to the total amount of LMA and HMA.

However, in the measurement process, the total amount LMA and HMA could be

slightly lower or higher than the amount of TAN, since it is difficult to precisely

extract all the water that contains LMA from the oil.

It is observed that there is an increment trend of LMA by assuming all transformers

from the same design and experiencing the same loading as shown in Figure 5.21.

However, there is an abnormality in the amount LMA and HMA in oil observed for

Transformer 16. The LMA and HMA for Transformer 16 should be higher or similar

to Transformer 15.

Figure ‎5.21 TAN, LMA and HMA of in-service oil. LMA, HMA and TAN of oil data

is 1 measurement.

The percentage of LMA and HMA is calculated based on Equation (5.6).

KOH/g) (mg HMA] [LMA

KOH/g) (mgLMA percentageLMA

KOH/g) (mg HMA] [LMA

KOH/g) (mgHMA percentageHMA

(5.6)


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140

No clear increment pattern of LMA percentage is observed in oil as seen in Table 5.9.

Most of acids observed in the in-service aged oil are HMA and the percentage ranges

from 64.9 % to 81.6 %. On the other hand, the LMA percentage ranges from 18.4 % to

33.6 % with an average percentage of 26.1 %.

Table ‎5.9 LMA and HMA percentages of in-service oil.

Age (years) LMA percentage (%) HMA percentage (%)

1 26.6 73.4

9 33.6 64.9

16 23.4 76.6

20 33.0 67.0

26 24.9 75.1

37 18.4 81.6

42 22.9 77.1

5.4.2.2. LMA and HMA ratio for ageing of oil without any presence of

paper

A study was conducted on laboratory aged oil sample in order to determine the amount

of LMA generated by oil and factors that could affect the generation. A series of LMA

and HMA measurements were performed on previous laboratory aged mineral oil.

a) Experiment description

Aged mineral oil was obtained from previous ageing experiments. The oil was aged

without paper at 115°C under open and sealed condition with or without the presence

of copper. TAN, LMA and HMA were measured for each sample.

b) Ageing under sealed and open condition without copper

It is observed for sealed samples that there is a small increment of LMA in oil and it

reaches to 0.013 mg KOH/g after 45 days of ageing as seen in Figure 5.22. Similar as


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141

LMA, the TAN increases slowly and saturates at 0.06 mg KOH/g. The result indicates

that the oil suffers moderate ageing without any presence of oxygen and copper.

Figure ‎5.22 LMA and HMA ratio of mineral oil aged under sealed condition without

copper. LMA, HMA and TAN of oil data is 1 measurement.

It can be seen that the percentage of LMA ranges from 17.5 % to 21.2 % with an

average percentage of 16.8 % and for HMA ranges from 78.8 % to 86.8 % as seen in

Table ‎5.10.

Table 5.10 LMA and HMA percentages of mineral oil aged under sealed condition

without copper.

Ageing duration (days) LMA percentage (%) HMA percentage (%)

1 17.5 82.5

2 17.9 82.1

3 14.3 85.7

14 13.2 86.8

56 21.2 78.8

Similar to sealed samples, LMA in oil for open samples show increment as the ageing

duration increases and can reach up to 0.074 mg KOH/g as seen in Figure 5.23. It is

believed the actual amount of LMA in oil could be higher than the measured value.


Measurements

142

According to the previous literature, there are three types of LMA that could exist in

transformers which are formic acid, acetic acid and levulinic acid. The boiling

temperatures of these acids range from 101 °C to 243 °C. Some of the LMA might be

evaporated to the air since the ageing is conducted under open condition and the

temperature is set to 115 °C. It is suspected that after 14 days, the sample has

experienced higher evaporation compared to other samples since the amount measured

is quite low. It is also observed that the TAN of open samples is higher than sealed

samples due to the presence of oxygen that accelerates the oxidation of the oil.

Figure ‎5.23 LMA and HMA ratio for mineral oil aged under open condition without


The percentage ratio of LMA and HMA can be seen in Table 5.11. The variation of

LMA percentage is higher for open samples ranging from 3.5 % to 33.7 % with an

average percentage of 23.4 % ignoring the 3.5 % LMA of 14 days sample, while HMA

ranges from 66.3 % to 96.5 %. The percentage of LMA is low for 14 days sample is

low because the amount of LMA is too low compared to HMA which in turn affect the

calculation of LMA percentage in Equation (5.6).


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Table 5.11 LMA and HMA percentage of mineral oil aged under open condition

without copper.


1 33.7 66.3

2 28.6 71.4

3 15.8 84.2

7 14.2 85.8

14 3.5 96.5

56 24.6 75.4

According to both sealed and open samples, it is shown that oil could also generate

LMA with a percentage among TAN from 16.8 % to 23.4 %.

c) Ageing under sealed and open condition with copper

It can be seen in Figure 5.24 that the LMA in oil is almost at the same level as HMA

in oil for sealed samples aged with copper. There is a slight increment pattern of LMA

in oil observed and the highest recorded is 0.052 mg KOH/g. There is no significant

increment of TAN observed for all ageing durations.

According to previous literature, oxidation of oil is promoted by Hydroxy (HO)

radicals originated from Hydrogen Peroxide (H2O2) [18]. H2O2 can be produced from

oxygen and water. The existence of metal cations such as copper helps to dissociate the

H2O2 into HO radicals. Since oxygen is not present in the sealed test, less H2O2 is

produced which in turn reduces the presence HO radicals and slows down the

oxidation of the oil.


Measurements

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Figure ‎5.24 LMA and HMA ratio for mineral oil aged under sealed condition with


It is observed that the LMA percentage of sealed samples aged with copper is higher

than without copper as shown in Table 5.12. The percentage of LMA ranges from 44.2

% to 46.3 % with an average percentage of 45.4 % and HMA ranges from 53.7 % to

54.1 %. It is found that copper could affect the percentage ratio of LMA and HMA

ratio.

Table ‎5.12 LMA and HMA percentage of mineral oil aged under sealed condition with

copper.


14 45.9 54.1

21 44.2 55.8

36 46.3 53.7

Only 2 aged oil samples could be obtained for the case study as seen in Figure 5.25. As

expected, the presence of both copper and oxygen increases the TAN of the oil

significantly and the highest LMA in oil observed is 0.098 mg KOH/g. It is also

noticed that the LMA in oil for both samples is almost similar. It is believed that the

actual amount of LMA in oil for 56 days sample should be higher than the 12 days

sample. Thus, it is suspected that the LMA in oil for 56 days sample might be


Measurements

145

evaporated since the temperature of ageing is slightly higher than the lowest boiling

temperature of LMA. Table 5.13 shows the calculated percentage of LMA and HMA.

Figure ‎5.25 LMA and HMA ratio for mineral oil aged under open condition with


Table ‎5.13 LMA and HMA percentage of mineral oil aged under open condition with

copper.


12 43.7 56.3

56 15.1 84.9

d) Summary on LMA and HMA ratio of oil ageing without any presence of paper

It is found that LMA could also be generated by oil and its amount could be increased

under oxidation and presence of copper. The result supports the assumption that the

ageing mechanisms in transformers starts from the oxidation of oil which generates

LMA and initiates the hydrolysis of paper as shown Figure 2.13 of Chapter 2.

Once paper is introduced in the oil, the percentage ratio of LMA in oil might change

since this acid prefers to stay in the paper. Thus, the LMA measured in paper might be

a combination of LMA obtained from oil oxidation and paper ageing.


Measurements

146

5.5. Life assessment of transformers through LMA measurement

It is proposed that LMA could be used as an ageing marker to determine the ageing

condition of the paper. Ideally, the LMA in paper could be predicted if the relationship

between LMA in paper and LMA in oil is known. Through LMA in paper, the TI can

be calculated based on Equation (5.4). Furthermore, life assessment of transformers

could be carried out based on the predicted TI.

5.5.1. Calculation procedure

The calculation procedure of life assessment of a transformer can be seen in Figure

5.26. Two inputs are required for the calculation which is:

a) Equation to represent LMA in paper versus transformer age

b) Equation to represent LMA in oil versus transformer age

The calculation can be divided into two stages. The first stage calculates the TI and the

second stage computes the transformers life consumption. LMA in paper and TI are

predicted in the first stage of the calculation. The second stage calculates the life

consumption of a transformer. TI and a transformer’s life consumption will be

computed as the outputs for the calculation.


Measurements

147

Figure ‎5.26 Calculation procedure for predicting TI and transformer life consumption.

Equation to represent LMA in oil versus

transformer age

Equation to represent LMA in paper

versus transformer age

INPUT

Modelling the relationship between LMA in paper and LMA in oil

Prediction of LMA in paper based on LMA in oil

Calculate TI based on Equation (5.4)

Prediction of TI based on LMA in paper

PREDICTION OF TI BASED ON LMA IN OIL

OUTPUT 1

TI

OUTPUT 2

Transformer life

consumption

Equation (5.4) = LMA in paper versus TI

Equation (4.1) = TI versus transformer age

PREDICTION‎OF‎TRANSFORMER’S‎LIFE‎

CONSUMPTION BASED ON LMA Prediction of transformer’s life consumption

Calculate transformer life consumption based on Equation (4.1)


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148

5.5.2. Prediction of TI based on LMA in oil

5.5.2.1. Prediction of LMA in paper based on LMA in oil

The first step is to determine the relationship between LMA in paper/LMA in oil and

transformer age. Two inputs are required for the model as follows:

Input 1 Relationship between LMA in paper and transformer age

LMA in paper versus transformer age can be represented by Equation (5.5).

Input 2 Relationship between LMA in oil and transformer age

The procedures to determine the equation that can represent LMA in oil versus

transformer age are shown in Figure 5.27.

Figure ‎5.27 Procedure of modelling the relationship between LMA in oil and

transformer age.

PRO 1 Assumptions

There are two limitations in determination of the equation that can represent the

relationship between LMA in oil and transformer age, and they are:

Assumptions

PRO 1

Fitting of TAN versus transformer age (based on distribution

transformers)

PRO 2

Apply the same equation of TAN versus transformer age

to LMA in oil versus transformer age

PRO 3


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149

Lim 1 LMA in oil of scrapped transformers could not be measured since oil

samples are not available.

Lim 2 The number of LMA in oil of in-service transformers data in

Figure 5.21 is too small to have confidence on the fitting process.

Thus, three assumptions are made in order to simplify the fitting process and they are

listed as follows:

Asm 1 The amounts of LMA in oil of scrapped transformers are the same as

in-service transformers.

Based on the fact that both scrapped and in-service transformers operate

at transmission level where the loading profiles of all transformers do

not vary significantly.

Asm 2 The yearly LMA in oil follow the same increment pattern of yearly

TAN.

Based on the fact that the LMA percentage for transformers with age

ranging from 1 year to 42 years varies slightly with the average value of

26.1 % as shown in Table 5.9.

Asm 3 The yearly TAN of transmission transformers (400/132 kV, 400/275 kV

and 275/132 kV) is similar to distribution transformers (132/33 kV).

Based on fact that both 400/132 kV and 275/132 kV transformers are

interconnected at the 132 kV bus with 132/33 kV transformers and

should have similar pattern of loading.

PRO 2 Fitting of TAN versus transformer age

Due to Lim 2, it is difficult to model the relationship between LMA in oil versus

transformer age. Thus, an approach based on Asm 2 was employed which required

fitting of TAN versus transformer age. Further assumption is made based on Asm 3

since the available TAN data are from distribution transformers.

The same case study as in section 5.2.1.3 was used to determine the relationship

between TAN and transformer age as shown in Figure 5.28. It is observed that there is

a scattering of the data in the early age which is believed to be caused by abnormal


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150

ageing experienced by some of transformers. The scattered data were excluded from

analysis to avoid their influence on the fitting process. The filtering of the data was

made for transformers with age less than 30 years and TAN larger than 0.1 mg KOH/g.

Figure ‎5.28 TAN versus transformer age. Data is 1 measurement.

Through filtering the data, it was found that there is a general pattern of TAN

increment with age. Thus, it is assumed that the LMA in oil would also follows the

same pattern since the LMA percentage varies slightly at a value around 26.1 % and

the percentage is similar for both distribution and transmission transformers based on

Asm 2 and Asm 3. It is found that the relationship between TAN and transformer age

could be fit by Equation (5.5). The fitting was done according to the average, highest

and lowest TAN of each transformer age. The fitting parameters are shown in Table

5.14. The initial TAN was set to be 0.01 mg KOH/g. The r² for fitting based on highest

TAN is low, 0.446, due to the fact that less data is available when the transformer age

is exceed 55 years. However, the r² for fitting based on the average and lowest TAN is

quite high, 0.886 and 0.886, which indicates that the equation is suitable to represent

the relationship between TAN and transformer age.


Measurements

151

Table ‎5.14 Increment rate of TAN and constant of the fitting.

Fitting

parameters

Based on

average TAN

Based on

highest TAN

Based on lowest

TAN

B (mg KOH/g) 4.3×10-3

9.3×10-3

0.14×10-3

C( 1years ) 7.7×10-2

7.1×10-2

1.3×10-2

r² 0.886 0.446 0.886

PRO 3 Apply the same equation of TAN versus transformer age to LMA in oil

versus transformer age

To model the LMA in oil versus transformer age, by applying the same equation as

TAN versus transformer age, is shown in Figure 5.29. The result of the fitting can be

seen in Table 5.15. The initial LMA in oil was set to 0.001 mg KOH/g. The low r²,

0.364, is due to less data available for the study.

Figure ‎5.29 LMA in in-service oil versus transformer age.

Table ‎5.15 Increment rate of LMA in oil and constant of the fitting.

Fitting parameters Value

B (mg KOH/g) 2.0×10-3

C (years-1

) 6.3×10-2

r² 0.364


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152

The final step is to combine Equation (5.7) and Equation (5.8) in order to develop

Equation (5.9) as shown in Figure 5.30. The constant, B and increment rate of LMA in

paper, C in Equation (5.7) was obtained based on the average LMA as shown in Table

5.5.

Figure ‎5.30 Modelling procedure for the relationship between LMA in paper and

LMA in oil

5.5.2.2. Prediction of TI based on LMA in paper

Once the model for LMA in paper versus LMA in oil is obtained, the TI could be

predicted based on Equation (5.4). An example of the TI calculation based on LMA in

oil is shown as follows:

Step 1 Measure LMA in oil from in-service transformers

For examples, the LMA in oil measured from two transformers are 0.01 mg

KOH/g and 0.03 mg KOH/g.

0.09AgeLMA in paper = 0.12+0.056exp (5.7)

LMA IN PAPER VERSUS TRANSFORMER AGE

0.06AgeLMA in oil = 0.001+0.002exp

(5.8)

LMA IN OIL VERSUS TRANSFORMER AGE

+

=

1.41LMA in oil-0.001LMA in paper = 0.12+0.056( )

0.002 (5.9)

LMA IN PAPER VERSUS LMA IN OIL


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153

Step 2 Calculate LMA in paper based on Equation (5.9)

Based on Equation (5.9), the LMA in paper for both transformers are 0.59 mg

KOH/g and 2.55 mg KOH/g.

Step 3 Calculate TI based on Equation (5.4)

Through Equation (5.4), the predicted TI for both transformers are 97.9 Nm/g

and 49.1 Nm/g.

Since the prediction is made based on a few reasonable assumptions, an error on the

prediction of TI of the paper could be expected. The model could be improved if the

oil of a transformer which is planned to be scrapped can be obtained for the

measurement of LMA. Through this procedure, the relationship between LMA in

paper and oil could be refined and thus improve the prediction of TI.

5.5.2.3. Ratio of LMA in paper in oil

A study was conducted to examine how the LMA in paper and oil ratio changes over a

transformer’s life. The ratio shown in Equation (5.10) was calculated based on the data

of the same age obtained from both Equation (5.7) and Equation (5.8).

LMA in paper versus transformer age

Ratio of LMA in paper and oil = LMA in oil versus transformer age

(5.10)

The relationship between LMA in paper/oil ratio and transformer age can be seen in

Figure 5.31. It is observed that initially, the ratio reduces slightly and starts to increase

after 10 years in-service. It is believed that at the initial stage of ageing, less LMA is

generated in transformers and most of them are consumed to accelerate the ageing of

the paper. As the paper aged, more LMA will be generated in the system and stay in

the paper which contributes to the increment of the ratio. It is suspected that at this

point, the generation of LMA is higher than the consumption of LMA since the

measured LMA is a result from the subtraction of LMA generation with LMA

consumption.


Measurements

154

Figure ‎5.31 Ratio of LMA in paper and oil versus transformer age

5.5.3. Prediction of transformers life consumption based on LMA

5.5.3.1. Prediction of transformer life consumption

Transformer life consumption could be calculated based on the predicted TI obtained

from the LMA in oil. The first step is to re-arrange Equation (4.1) into Equation (5.11).

0

TI

lnTI -lnTIt =

C (5.11)

where TI = Predicted TI (Nm/g).

TI0 = Initial TI (Nm/g).

CTI = Ageing rate (years-1

).

t = Life (years).

Ageing rate based on average initial TI and lowest measured TI are chosen in order to

illustrate the life consumption calculation. The ageing rate based on these criteria is

3.71×10-2

as given in Table 4.11 of Chapter 4 while according to Table 4.4 of Chapter

4, the average initial TI is 107 Nm/g. By using the predicted TI from the previous


Measurements

155

example in section 5.5.2.2, the life consumption can be calculated based on Equation

(5.11) and the result can be seen in Table 5.16.

Table ‎5.16 Life consumptions based on average initial TI of 107 Nm/g.

Predicted TI (Nm/g) 97.9 Nm/g 49.1 Nm/g

Life consumed (years) 2 21

5.6. Summary

LMA could affect the performance of the breakdown voltage of oil. It is shown that the

breakdown voltage of oil could be reduced to half when LMA in oil is as low as 0.06

mg KOH/g. There is no effect of HMA on this property. LMA has a significant impact

to the ageing of paper. The impact is greatly enhanced with moisture as the ageing rate

can be increased by a factor of 6. On the other hand, it is observed that there is little

effect of HMA on ageing of paper.

There is no significant difference of LMA in paper profile along phases and windings

of scrapped transformers. The profile of LMA in paper for multilayer insulation shows

that there is an existence of chemical degradation which mainly involves the paper

layer in contact with oil. Two relationships were found between LMA in paper, TI and

transformer age. The first relationship shows that as LMA in paper increases, TI

decreases. The shape of Chendong’s model between furanic compounds and degree of

polymerization [75] was found suitable to represent the relationship between LMA in

paper and TI. It is also found that LMA in paper increases as transformer age

increases.

The LMA ratio in paper and oil is strongly influenced by the weight ratio of paper and

oil. A study on the in-service oil shows that there is a slight increment of LMA in oil

as age increases. The average percentage of LMA in in-service aged oil is around 26.1

%. Further study on laboratory aged oils shows that LMA could also be generated

from the oil. Using information obtained from scrapped and in-service transformers, a

general TI prediction model based on LMA in oil was developed with a few reasonable

assumptions.


156

CHAPTER 6. AGEING ASSESSMENT OF NATURAL ESTER

6.1. Introduction

In recent years, application of alternative insulating liquids for transformers is

becoming an important topic since the demand for the high fire safety is increased.

Natural ester is viewed as one of the possible replacement for mineral oil.

Acidity and viscosity are common methods used to assess the oxidation level of

natural ester. Acidity is usually expressed as Total Acid Number (TAN) and viscosity

is defined as fluid’s internal resistance to flow, also known as a measure of fluid

friction. Acids accelerate the ageing process of oil and paper. Recently it was

suggested that the current measurement technique of TAN is unsatisfactory since it

could not distinguish the types of acid in oil [16, 19]. According to these studies, it

was found that acids generated from ageing of paper and mineral oil consist of Low

Molecular Weight Acid (LMA) and High Molecular Weight Acid (HMA), where

LMA was found to be more dangerous since it could accelerate the paper degradation

[16]. However, there is no study being conducted on the LMA and HMA of natural

ester and its impact toward paper ageing.

The performance of natural ester under oxidation is examined through TAN and

viscosity measurements. Next, the behaviour of natural ester aged under sealed

condition is examined. TAN, viscosity, LMA and HMA measurements were carried

out throughout the ageing duration. Finally, the effect of LMA and HMA on the paper

ageing and the breakdown voltage of natural ester are discussed in this chapter.

6.2. Oxidation of natural ester

The first part of this section investigates the oxidation process of natural ester. The

second part discusses the behaviour of natural ester under oxidation and pyrolysis with

the presence of paper, copper and steel.


157

6.2.1. Ageing of natural ester only


Aged natural ester and mineral oil were obtained from previous ageing experiments.

The natural ester and mineral oil were aged without paper and copper at 115 °C under

open and sealed conditions. TAN was measured for each sample.


The comparison of TAN between natural ester and mineral oil aged under open

condition without any presence of paper, copper and steel can be seen in Figure 6.1. It

is observed that the TAN of natural ester is higher than mineral for all ageing

durations. The TAN of natural ester starts to increase rapidly after 7 days of ageing. On

the other hand, the increment of TAN for mineral oil is almost linear. The results show

that under oxidation, the generation of TAN for natural ester is much higher than

mineral oil.

Figure ‎6.1 TAN of natural ester and mineral oil aged at 115 °C under open condition

without any presence of paper, copper and steel. Data is 1 measurement.


158



High temperature laboratory accelerated ageing experiments were conducted based on

two ratios ageing materials. The first ratio, define as single ratio for material

weight/surface area of oil, paper, copper and steel can be seen in Table 6.1. The ratio

was calculated according to a 6.6 kV/415 V, 1 MVA free breathing natural ester filled

distribution transformer.

Table 6.1 Amount of oil, paper, copper and steel ratio for single ratio.

Natural ester (ml) Kraft paper (g) Copper (cm2) Steel (cm

2)

900 7.86 0.10 3.31

The second ratio can be seen in Table 6.2 and it is defined as double ratio. Natural

ester and mineral oil were used for the ageing experiments of double ratio for

comparative purpose.

Table 6.2 Amount of oil, paper, copper and steel ratio for double ratio.

Natural ester/mineral oil (ml) Kraft Paper (g) Copper (cm²) Steel (cm²)

900 7.86 0.20 6.62

Kraft paper with a density of 0.93 g/cm³ was used for all ageing experiments. The oil

and paper were dried and filtered according to the procedure described in section 3.5.1

and 3.5.2 of Chapter 3. On the other hand, copper and steel were de-coated by a sand

paper in order to remove the surface oxidation layer.

Open and sealed ageing experiments were conducted for both ratios of natural ester

and only sealed ageing experiment was carried out for mineral oil. For sealed samples,

PTFE screw cap with a gasket ring was used and the sealing tape was wrapped around

the bottle neck in order to reduce the migration of oxygen from outside.

The ageing temperature was set at 170 °C and only 6 samples were placed in the air

circulating oven at any given time in order to obtain a good and even temperature


159

distribution for all the samples. The longest ageing duration for single ratio samples

was set to 50 days. Only natural ester was used for the single ratio ageing experiment.

All double ratio samples were aged for 10, 20, 23 and 27 days. However for natural

ester, 23 days sealed sample, was not used for analysis since there was probably a

sealing problem.


a) TAN

Figure 6.2 shows the comparison of TAN for natural ester and mineral oil aged under

open condition with presence of paper, copper and steel between single and double

ratios.

Figure ‎6.2 TAN of natural ester aged at 170 °C under open condition with presence of

paper, copper and steel. Data is 1 measurement.

It is observed that for natural ester, the initial increment of TAN is slow for both single

and double ratio samples. After 10 days of ageing, the TAN increases rapidly and

saturates after 30 days of ageing for single ratio samples and 23 days of ageing for

double ratio samples. The high amount of TAN for natural ester might be caused by

severe oxidation and pyrolysis. The oxidation is contributed by the free oxygen of

open samples while pyrolysis from the high temperature of the ageing experiment.


160

It was found that there is a small effect of doubling the surface area of steel and copper

on the TAN of natural ester. The TAN of double ratio samples are slightly higher and

saturate faster than those of single ratio samples. The highest TAN for single and

double ratio samples is 9.27 mg KOH/g and 8.77 mg KOH/g.

b) Viscosity

The viscosity of natural ester aged under open condition with the presence of paper,

copper and steel can be seen in Figure 6.3. It is observed that the increment of

viscosity for mineral oil is not as rapid as natural ester. The viscosity increases from 10

cP to 23 cP after 60 days of ageing.

It was found that the viscosity of natural ester starts to increase rapidly after 10 days of

ageing and reaches the plateau after 30 days of ageing. The high viscosity of natural

ester indicates that both oxidation and pyrolysis could significantly change the

molecular structure of natural ester. According to the previous literature [106], the

mechanism of viscosity change can be described by the molecular weight change.

Under the presence of air, the triglycerides molecules of natural ester are exposed to

thermal polymerization. Through this process, the molecules will react with oxygen

and lead to chain scission or cross linking. Chain scission and cross linking will result

in the production of large molecules which can change the molecular weight and

increase the viscosity.


161

Figure ‎6.3 Viscosity of natural ester aged under open condition with presence of

paper, copper and steel. Data are mean of 5 measurements. Mineral oil was aged under

open condition with paper, copper, zinc, iron and aluminium [107].

It is shown that there is no clear effect of doubling the surface area of copper and steel

on the viscosity of natural ester. The only difference of viscosity between single and

double ratio samples is observed at 10 days of ageing where the viscosity of double

ratio samples is twice higher than that of single ratio samples. The highest viscosity for

single and double ratio samples is 417.6 cP and 518.6 cP.

It is generally known that even severely aged mineral oil in an in-service transformer

can seldom have a significant increase of viscosity, therefore viscosity is not normally

used as the indicator of ageing for mineral oil. To show the comparison, some

viscosity results of mineral oil are also plotted in Figure ‎6.3. The viscosity data of

mineral oil was obtained from previous literature [107]. The materials ratio of the

ageing experiment is 30 g of paper/pressboard, 300 ml of mineral oil, 0.15 g of

aluminium, 0.15 g of zinc, 0.75 g of iron and 0.75 g of copper. The sample was aged at

130 °C under open condition for 60 days [107].


162

6.2.2.3. Relationship between viscosity and TAN

Figure 6.4 shows that there are 2 stages which can be observed from the relationship

between viscosity and TAN of natural ester aged under open condition with the

presence of paper, copper and steel. In the first stage, the viscosity increases linearly as

TAN increases to 8 mg KOH/g which indicates that the molecular weight change

occurs at the same time as the generation of acids. The molecular weight change of

natural ester occurs as a result of chain scission and cross linking of triglycerides

molecules [106]. The second stage starts once the chain scission and cross linking of

triglycerides molecules reduces and TAN continues to increase. It is shown that the

viscosity remains unchanged as TAN increases beyond 8 mg KOH/g.

Figure ‎6.4 Viscosity versus TAN of natural ester aged under open condition with

presence of paper, copper and steel. Viscosity data are mean of 5 measurements. TAN

data is 1 measurement.

6.2.2.4. Oxidation inhibitor depletion

Oxidation inhibitor is quite important to be present in order to prevent severe oxidation

of natural ester. Oxidation of natural ester can be promoted by free radicals produced

by interaction of triglycerides molecules with oxygen. The oxidation inhibitor helps to

interrupt and terminate the process of the free radicals generation [108]. Once the


163

oxidation inhibitor is depleted, natural ester will suffer rapid oxidation, which led to

the rapid change of its physical and chemical properties.

It is believed that the ageing of natural ester under severe oxidation and pyrolysis can

be described by 3 stages. The first stage is the consumption of oxidation inhibitor. At

this period, the TAN and viscosity increases slowly with time because the oxidation

inhibitor is still available to protect the natural ester from oxidation as shown in Figure

6.1, Figure 6.2 and Figure 6.3.

The second stage starts once the oxidation inhibitor is depleted. It is shown by the

rapid increment of TAN and viscosity, as shown in Figure 6.2 and Figure 6.3 after 10

days of ageing for natural ester. Since the ageing temperature is high in these case

studies, it is believed that pyrolysis might exist together with oxidation and helped to

accelerate the degradation of natural ester.

The third stage initiates once the TAN saturates at a high value and it is observed that

at this stage the natural ester changes to a dark coloured and gelling substance which

first appears on the top part of the bottle as shown in Figure 6.5 a). The bulk natural

ester is not gelled yet because it has a small surface area to access oxygen i.e. large

volume/surface ratio. Exposure of natural ester with a large surface area to access

oxygen i.e. small volume/surface ratio, will cause it to oxidize more rapidly.

Moreover, it was also found that the LMA and HMA content of natural ester at this

stage could not be determined since the mixture of water and oil could not be separated

and a yellowish solution was created as shown in Figure 6.5 b). It is believed that the

chemical structure of the sample has changed significantly after subjected to oxidation

and pyrolysis.


164

a) b)

Figure ‎6.5 Natural ester aged under open condition with presence of paper, copper,

and steel for single ratio samples. a) Natural ester condition after 50 days of ageing and

b) Mixture of aged natural ester and water.

6.3. Application of natural ester to sealed transformers

In order to ensure and maintain optimum performance of natural esters, exposure to

oxygen must be minimized. Thus, sealing against air is the best way to gain benefits

from natural ester. This section investigates the performance of natural ester aged

under sealed condition. The LMA and HMA ratio of natural ester is also examined in

the final part of this section.

6.3.1. Ageing of natural ester only


The description of the ageing experiment can be seen in section 6.2.1.1. TAN, LMA

and HMA were measured for each sample.


Figure 6.6 shows the TAN of natural ester and mineral oil aged without any presence

of paper, copper and steel. It was found that the TAN generated by natural ester aged

under sealed condition is much lower than open condition as seen in Figure 6.1. The

highest TAN for natural ester aged under sealed condition is 0.21 mg KOH/g while

under open condition is 0.72 mg KOH/g. There is no rapid increment of TAN and it is

Gelling

substance


165

observed that the pattern of TAN increment for natural ester is almost similar as

mineral oil.

Figure ‎6.6 TAN of natural ester and mineral oil aged under sealed condition without

any presence of paper, copper and steel. Data is 1 measurement.



The description of the ageing experiment can be seen in section 6.2.2.1.


a) TAN

The comparison of TAN between natural ester and mineral oil aged under sealed

condition with the presence of paper, copper and steel can be seen in Figure 6.7. For

double ratio samples, the TAN of mineral oil is available for comparison with natural

ester.

It was found that the TAN of natural ester for both ratios is much lower than in open

ageing condition as seen in Figure 6.2 and fluctuate at a value between 1 mg KOH/g to


166

2 mg KOH/g. On the other hand, for mineral oil, there is a linear increment of TAN at

a much lower value than natural ester. The double ratio samples have a highest value

of 0.09 mg KOH/g after aged for 27 days. It is believed that the generation of TAN in

sealed samples is dominated by pyrolysis since there is little oxygen and water existing

in a sample initially. As the ageing progresses, water is produced from degradation of

paper and natural ester which will initiates the hydrolysis of natural ester.

Figure ‎6.7 TAN of natural ester and mineral oil aged under sealed condition with

presence of paper, copper and steel. Data is 1 measurement. For S. Tenbohlen, mineral

oil was aged under sealed condition with paper, copper, zinc, iron and aluminium

[107]. For R. Liao, mineral oil was aged under sealed condition with paper and copper

[2].

It is observed that the effect of doubling the surface area of steel and copper on the

TAN of natural ester is slightly different in sealed condition from open condition. The

TAN of double ratio samples are lower than those of single ratio samples. The highest

TAN of single ratio samples is 2.06 mg KOH/g while that of double ratio samples is

1.37 mg KOH/g.

Additional TAN of mineral oil was also obtained from literatures [2, 107] and plotted

together in Figure 6.7. The ageing condition for S. Tenbohlen’s experiment is

described in section 6.2.2.2 b). The ageing condition for R. Liao’s experiment is

described in [2]. The materials ratio of this ageing experiment is 5 g of paper, 50 g of


167

mineral oil and 100 cm² of sheet copper [2]. The sample was aged at 130 °C under the

sealed condition for 65 days. The highest TAN of mineral oil is from S. Tenbohlen

with a value of 0.22 mg KOH/g and R. Liao with a value of 0.08 mg KOH/g.

b) Viscosity

The comparison of viscosity between natural ester and mineral oil aged under sealed

condition with paper, copper and steel can be seen in Figure 6.8. For double ratio

samples, the viscosity of mineral oil is available for comparison with natural ester.

The viscosity of natural ester for both ratios remains unchanged at a value around 32

cP until the end of ageing period. Based on double ratio samples, it was found that the

pattern of viscosity for mineral oil is similar to natural ester, it remains unchanged at a

value around 8 cP. Since little oxygen and water exist in the sample initially, the

polymerization of tryiglycerides molecules is slow which results in the little change of

molecular weight. Thus, since the change in the molecular weight is less, the viscosity

of natural ester remains unchanged during the ageing period.

Figure ‎6.8 Viscosity of natural ester and mineral oil aged under sealed condition with

presence of paper, copper and steel. Data are mean of 5 measurements. For S.

Tenbohlen, mineral oil was aged under sealed condition with paper, copper, zinc, iron

and aluminium [107].


168

There is no effect of doubling the surface area ratio of copper and steel on the viscosity

of natural ester. The average viscosity for both single and double ratio at 40 °C is

around 30 cP.

On the contrary, there is a slight change in the viscosity of mineral oil observed by S.

Tenbohlen. The ageing condition for S. Tenbohlen is described in section 6.2.2.2 b)

[107]. Since the viscosity measurements were only done at the beginning and the end

of S. Tenbohlen’ experiment, his results are inconclusive.

6.3.2.3. Relationship between viscosity and TAN

The relationship between viscosity and TAN of natural ester aged under sealed

condition with presence of paper, copper and steel can be seen in Figure 6.9. It is

observed that the pattern is different from open samples as seen in Figure 6.4. The

viscosity increases only slightly and remains unchanged when acidity is increased

from 1.03 mg KOH/g to 2.06 mg KOH/g. The result indicates that by minimising the

contact of natural ester with oxygen, the chain scission and cross linking of natural

ester triglycerides molecules could be reduced significantly.

In sealed condition, it is envisaged that reaching stage 2 as shown in Figure 6.4 will

take considerably long time hence it is absolutely right for the natural ester

manufacturers to recommend natural ester to be used in sealed transformers.


169

Figure ‎6.9 Viscosity versus TAN of natural ester aged under sealed condition with

presence of paper, copper and steel. Viscosity data are mean of 5 measurements. TAN


6.3.3. LMA and HMA of natural ester

6.3.3.1. LMA and HMA ratio for ageing of natural ester only



b) Test results

Figure 6.10 shows the comparison of LMA and HMA for natural ester and mineral oil

aged under sealed ageing condition without any presence of paper, copper and steel. It

was found that there is a small increment of LMA in natural ester. The highest LMA

and HMA in natural ester are 0.048 mg KOH/g and 0.19 mg KOH/g. On the other

hand, the increment trend of LMA in mineral oil is not significant as in natural ester.

The highest LMA and HMA in mineral oil are 0.0048 mg KOH/g and 0.032 mg

KOH/g.


170


condition without any presence of paper, copper and steel. LMA and HMA of natural

ester data is 1 measurement.

The LMA/HMA percentages versus ageing duration for both natural ester and mineral

oil without any presence of paper, copper and steel are given in Table 6.3. The

calculation of LMA/HMA percentage was carried out according to Equation (5.5) of

Chapter 5. For natural ester, it is observed the LMA percentage ranges from 7.5 % and

26.8 % with an average percentage of 16.9 %. For mineral oil, the LMA percentage is

slightly lower than natural ester and ranges from 13.2 % to 17.9 % with an average

percentage of 15.8 %.


sealed condition without any presence of paper, copper and steel.

Ageing time

(days)

Natural ester

(%)

Mineral oil

(%)

LMA HMA LMA HMA

1 17.1 82.9 17.5 82.5

2 7.5 92.5 17.9 82.1

3 26.8 73.2 14.3 85.7

7 20.2 79.8 - -

14 12.8 87.2 13.2 86.8


171

It is observed that the average percentage of LMA generated by natural ester is 16.9 %.

However, the percentage might be changed if the natural ester is aged with paper.

Thus, the study of LMA and HMA ratio was carried out on natural ester aged under

sealed condition with the presence of paper, copper and steel.

6.3.3.2. LMA and HMA ratio of natural ester aged with presence of paper,

copper and steel



b) Test results

The comparison of LMA and HMA in natural ester and mineral oil aged under sealed

condition with the presence of paper, copper and steel can be seen in Figure 6.11.

There is no clear increment pattern of LMA in natural ester observed for single ratio

samples. On the contrary, it is observed that there is an increment trend of LMA in

natural ester for double ratio samples.

It was found the amount of LMA for ageing of natural ester with presence of paper,

copper and steel is higher than ageing of natural ester only shown in Figure 6.10. One

of the possible reasons is due to the polar nature of natural ester. As paper aged, more

LMA will be produced in the system. Since natural ester is much more polar, therefore

the partition between paper and natural ester favours the stay of LMA in natural ester.

It is observed that the LMA in natural ester for single and double ratio samples is

higher than mineral oil. From section 6.3.2.2 a), it was shown that the TAN is much

higher in natural ester and so is the amount of LMA in natural ester is higher. It is

noticed that there is no clear increment pattern of LMA in mineral oil. The highest

LMA and HMA in mineral oil are 0.038 mg KOH/g and 0.015 mg KOH/g.

It was found that there is an effect of doubling the surface area of steel and copper on

the LMA of natural ester. The LMA of double ratio samples is slightly higher than that


172

of single ratio samples. The highest LMA in natural ester for single ratio samples is

0.094 mg KOH/g while for double ratio samples 0.135 mg KOH/g.


condition with presence of paper, copper and steel. LMA and HMA of natural ester


The LMA and HMA percentages of natural ester and mineral oil aged under sealed

condition with the presence of paper, copper and steel can be seen in Table 6.4. It was

found that mineral oil has a higher LMA percentage than natural ester. Most of acids in

mineral oil are LMA which percentages are ranging from 51.1 % to 71.3 %. On the

other hand, natural ester has a much lower LMA percentage than mineral oil ranging

from 1.5 % to 8.6 % for single ratio and from 4.6 % to 11.6 % for double ratio

samples. Most of acids in natural ester are HMA ranging from 91.4 % to 98.5 % for

single ratio samples and 88.3 % to 95.4 % for double ratio samples.

It was found that the average percentage of LMA for ageing of natural ester with the

presence of paper, copper and steel is lower than ageing of natural ester only as seen in

Table 6.3.


173


sealed condition with presence of paper, copper and steel.

Ageing time

(days)

Natural ester -

single ratio

(%)

Natural ester -

double ratio

(%)

Mineral oil -

double ratio

(%)

LMA HMA LMA HMA LMA HMA

10 1.5 98.5 4.6 95.4 68.4 31.6

20 4.3 95.7 7.5 92.5 70.4 29.6

23 - - - - 71.3 28.7

27 - - 11.7 88.3 51.1 48.9

30 8.6 91.4 - - - -

40 3.4 96.6 - - - -

50 2.7 97.3 - - - -

A comparison was made for LMA in paper between natural ester and mineral ester as

seen in Figure 6.12. It is observed that LMA in paper for mineral oil is higher than

natural ester. The increment of LMA in paper for mineral oil is almost linear where the

concentration starts to slightly declined after 23 days of ageing. On the other hand,

LMA in paper for natural ester for both ratios remains almost unchanged after 10 days

of ageing at much lower value than mineral oil.

It was found that there is a significant effect of doubling the surface area of steel and

copper on the LMA in paper of natural ester. The LMA in paper of double ratio

samples saturates at much higher value than single ratio samples. The highest LMA in

paper of double ratio and single ratio samples are 4.1 mg KOH/g and1.7 mg KOH/g.


174

Figure ‎6.12 LMA in paper of natural ester and mineral oil aged under sealed condition

with presence of paper, copper and steel. Data are mean of 2 measurements.

6.4. Effect of LMA and HMA on breakdown voltage of natural ester and

ageing of paper

The first part of this section examined the effect of LMA and HMA on the breakdown

voltage of natural ester. The effect of LMA and HMA on the paper aged under dry and

wet condition was studied in the second part of this section.

6.4.1. Effect of LMA and HMA on breakdown voltage of natural ester


Different amounts of either LMA ranging from 0.2 g to 1.6 g or HMA ranging from 1

g to 19 g were mixed with 500 ml dry and filtered natural ester by magnetic stirrer for

3 hours at 50 °C. The properties of these acids can be seen in Table 5.1 of Chapter 5.

The natural ester was dried and filtered according to the procedure described in section

3.5.1 of Chapter 3. After the mixing procedure, the natural ester sample was titrated for

acidity and tested for breakdown voltage. The breakdown voltage test was carried out

using mushroom VDE electrodes with a diameter of 36 mm according to ASTM

standard, ASTM D1816 [103]. A 50 Hz AC voltage with increasing rate of 0.5 kV/s


175

was applied to the natural ester. The gap distance was set at 1 mm and a total of 50

measurements of breakdown voltage were carried out on each sample.


It was found that the breakdown voltage remains almost unchanged even after the

LMA reaches to 4 mg KOH/g as shown in Figure 6.13. The reason behind this

phenomenon might be due to the polar nature of natural ester. It is known that the

breakdown voltage of oils could be influenced by polar compounds such as water.

Once the saturation limit of water in oils is exceeded, the breakdown voltage will be

significantly reduced. Since LMA is a polar compound, it is suspected that its

saturation limit in natural ester is high, which might explain the reason why the

breakdown voltage is not affected.

Figure ‎6.13 Influence of LMA on the breakdown voltage of natural ester. Data are

mean of 50 tests and error bars represent the standard deviation. LMA in natural ester


It is observed that there is no effect of HMA on the breakdown of natural ester. The

breakdown voltage remains unchanged even after the HMA increases up to 9 mg

KOH/g as seen in Figure 6.14.


176

Figure ‎6.14 Influence of HMA on the breakdown voltage of natural ester. Data are

mean of 50 tests and error bars represent the standard deviation. HMA in natural ester


6.4.2. Effect of LMA and HMA on ageing of paper


In order to prepare natural ester sample with a different type of acids, 0.2 g of either

LMA or HMA was mixed with 200 ml dried and filtered natural ester by magnetic

stirrer for at least 3 hours at 50 °C. The natural ester was dried and filtered according

to the procedure described in section 3.5.1 of Chapter 3. After the mixing procedure,

the oil sample was titrated for acidity. For samples with LMA, the acidity ranges from

1.17 mg KOH/g to 1.26 mg KOH/g while with HMA the acidity around 0.26 mg

KOH/g. Each sample contains 1.75 g of dry or wet conductor paper and 200 ml of

mineral oil containing either LMA or HMA. The paper was pre-processed according to

the procedure described in section 3.5.2 of Chapter 3. The properties of paper and

mineral oil can be seen in Table 3.1 and Table 3.8 of Chapter 3. All samples were

sealed by PTFE screw cap with a gasket ring in order to reduce the migration of

oxygen from outside and aged in an air circulating oven at 120 °C for up to 64 days. A

total of 8 measurements of TI were carried out on each sample since the amount of

paper used in the ageing experiment is rather limited.


177

6.4.2.2. Ageing under dry condition

It was found that LMA has a significant effect on the paper aged under dry condition

in natural ester as seen Figure 6.15. There is no effect of HMA on the paper ageing and

it is observed that the effect is almost similar to with no acid. It is observed that the TI

of samples with LMA reach 50 % of its strength after 37 days of ageing while with

HMA and no acid after 64 days of ageing.

Figure ‎6.15 Influence of LMA and HMA on the paper aged under dry condition. Data


The fitting of relationship between TI and ageing duration was carried out by Equation

(4.1). It was found that the rate of ageing for samples with HMA is similar to with no

acid as seen in Table 6.5. The rate of ageing of samples with LMA is 2 times higher

than with HMA and no acid.

Table 6.5 Ageing rate of dry condition.


) r²


0.928

With LMA 1.9×10-2

0.909

With HMA 0.9×10-2

0.984


178

6.4.2.3. Ageing under wet condition

Figure 6.16 shows the effect of LMA and HMA on the paper aged under wet condition

in natural ester. For all ageing conditions, the TI of 14 days samples are lower than

those of 28 days and 63 days samples and this could be caused by the systematic flaw

of sample preparation due to time limit, no confirmation test have been done so far.

Figure ‎6.16 Influence of LMA and HMA on the paper aged under wet condition. Data


The fitting parameters for paper aged under wet condition in natural ester can be seen

in Table 6.6. It is observed that the fitting of samples with LMA and HMA is not good

due to the high scattering of the data. The r² for both samples are quite low compared

to with no acid.

Table 6.6 Ageing rate of wet condition.


) r²


0.809

With LMA 2.4×10-2

0.388

With HMA 1.2×10-2

0.295

Table 6.7 shows the increments factors for natural ester and mineral oil under dry

ageing condition. The increment factor is calculated by Equation (5.1) of Chapter 5.

The increment factor of samples with LMA for natural ester aged under wet condition


179

is not calculated because the fitting is not good. It is observed the increment factor for

natural ester is lower than mineral oil. It is suspected that some of the LMA remains in

natural ester because it is more polar which explain the increment factor of natural

ester is slightly lower than mineral oil.

Table 6.7 Comparison of increment factors between natural ester and mineral oil under

dry ageing conditions.

Condition Natural ester Mineral oil

With LMA 1.9

2.1

With HMA 0.9

0.9

6.5. Summary

Oxidation can significantly change the chemical and physical properties of natural

ester. The mechanism of the natural ester degradation under severe oxidation could be

divided into 3 stages. The first stage is the consumption of oxidation inhibitor. The

second stage is the fast oxidation phase of natural ester where the oxidation inhibitor

has already been depleted. The third stage is the saturation stage where natural ester

starts to experience severe physical and chemical changes. Thus, the cooling

performance of natural ester might be affected if it is used in free breathing

transformers.

On the other hand, it was found that natural ester is suitable for hermetically sealed

transformers. The TAN and viscosity remain stable even under high temperature of

ageing for a considerably long period. It is also noticed that more LMA stays in natural

ester which could be the direct reason why there is less severe degradation to the paper

in natural ester than in mineral oil, apart from water scavenging and hydrolytic

protections.

There is no effect of LMA and HMA on the breakdown voltage of natural ester within

the case study range. It is suspected that the saturation limit of LMA in natural ester is

high which prevents any reduction on the breakdown voltage.

Conclusions and future work

180

CHAPTER 7. CONCLUSIONS AND FUTURE WORK

7.1. Conclusions

7.1.1. General

This thesis described an extensive study on ageing assessment of the scrapped

transformers through Tensile Index (TI) and Low Molecular Weight Acid (LMA)

measurements as well as high temperature ageing assessment of paper/natural ester

insulation system. Through experimental examinations and data analyses, the

objectives of this thesis are accomplished and thus useful findings and conclusions are

made.

The topics covered in this thesis are as follows:

Ageing assessment of scrapped transformers though TI of paper measurements

o Examination on the TI profile among different phases, windings and layers,

o Modelling the relationship between TI and transformer age for the

transformer population,

o The relationship between TI and dielectric strength of the paper.

Ageing assessment of scrapped transformers through LMA measurements

o Examination on the LMA profile among different phases, windings and

layers,

o Modelling the relationship between LMA, transformer age and TI for the

transformer population,

o Development of the TI prediction model based on LMA in oil,

o The effect of LMA and HMA on the breakdown voltage of oil and on paper

ageing in mineral oil.

Ageing assessment of paper/natural ester system

o Examination on the performance of natural ester under high temperature

oxidation,

o Evaluate the suitability of natural ester application for sealed transformers,

o The effect of LMA and HMA on the breakdown voltage of natural ester and

paper ageing in natural ester.


181

7.1.2. Summary of main findings

The TI profile of scrapped transformers was examined with consideration of phases,

windings, and layer locations. Based on the case studies, it was found that there is no

significant difference on the TI profiles among different phases and windings, and this

indicates that basic symmetry of thermal design and uniform ageing are achieved. The

vertical TI profile is mainly influenced by the temperature distribution and might

change if a large amount of water is present at the bottom location. It is known that the

ageing of paper could be accelerated by water. Investigation reveals that chemical and

thermal degradation are the main mechanisms affecting the layer profile. Based on the

population analysis, it was found that there is a relationship between TI and

transformer age and is well represented by the Weidmann model. According to this

model, the end of life of the transformer population is best represented by 20 %

retention of TIs and the estimated life for the population is around 43 years with 34

years and 53 years as lower and upper limit of 95 % confidence band. There is no end

of life criterion which can be made based on the dielectric strength of the paper since

this property is not influenced by ageing. However, aged paper might fail if it is

subjected to repeated discharges due to the weakness of the fibre and is susceptible

towards carbonization. The dielectric strength of the paper could also be affected if the

oil quality is heavily compromised.

It was found that LMA has a significant reduction effect on the breakdown voltage of

mineral oil. Investigation on the effect of LMA on paper ageing reveals that the ageing

rate is greatly enhanced if LMA is dissociated by water. The LMA in paper profile of

scrapped transformers was examined according to phases, windings and layer

locations. Similar as TI profile, the LMA in paper profile among different windings

and phases shows no major difference. The examination shows that the chemical

degradation mechanism does exist in multilayer insulation and it mainly affects the

outer layer paper. Based on the population analysis, it was found that LMA in paper

increases exponentially as the transformer age increases. The end of life criterion based

on LMA in paper could be determined if the analogy of the relationship between LMA

in paper and TI of paper is described the same as Chendong’s equation of furanic

compounds and degree of polymerization. The study on the LMA partitioning in paper

and oil shows that it is strongly influenced by the weight ratio of paper and oil. The


182

average percentage of LMA in in-service aged oil is around 26.1 %. TI prediction

model based on LMA in oil was developed by using information obtained from

scrapped and in-service transformers based on a few reasonable assumptions. The life

of transformers could also be determined by this model.

The effect of high temperature oxidation on natural ester was studied and compared

with mineral oil. It was found that the oxidation of natural ester under high temperature

can be classified into three stages. The first stage is the fast consumption of oxidation

inhibitor. The second stage initiates once the oxidation inhibitor is depleted and the

third stage is the gelling and solidifying stage. It was found that natural ester is suitable

for application in sealed transformers since the ageing performance is good and

comparable to mineral oil. It is observed that most LMA tend to stay in natural ester

and this could be the reason for the good ageing property of paper in natural ester apart

from water scavenging and hydrolytic protection. Furthermore, it is shown that the

effect of LMA on the paper aged in natural ester is not as severe as in mineral oil. It is

suspected that the LMA migrates into natural ester more since it is more polar and thus

prevents severe degradation to the paper. It is also suspected that the saturation of

LMA in natural ester is high which prevents the natural ester from suffering the

reduction of AC breakdown voltage.

7.2. Future work

Although the end of life of transformer population is modelled by 10 transformers, the

end of life model can be further refined by obtaining more data from the early stage of

ageing (i.e. 5 years to 35 years).

The LMA in paper profiles along phases, windings and layers can be further examined

by studying the migration of LMA through oil circulation. Taking layer profile as an

example, the layer profile can be further studied by determining the extent of LMA

migration under different layers. The chemical and thermal degradation mechanisms of

layer profile could also be verified by laboratory experiments. Apart from migration of

LMA, the TI prediction model can be improved by obtaining LMA in oil before the

transformer is scrapped.


183

Further examination can be carried out on the depletion of oxidation inhibitor by

GC/MS spectroscopy. The analysis can provide the detail view of the structural change

suffered by natural ester under severe oxidation. The polar nature of natural ester can

be further investigated by looking into partitioning of LMA in paper and natural ester

with the consideration of water.

Appendix I: Tensile Index (TI) of New and Service Aged Paper

184

APPENDIX I: Tensile Index (TI) of New and Service Aged Paper

Table I.1 TI of new conductor type paper without oil impregnation at different

lengths.

Test No. 15 mm (Nm/g) 30 mm (Nm/g) 60 mm (Nm/g) 180 mm

(Nm/g)

1 117.3 117.1 105.6 112.8

2 108.7 113.2 104.3 104.7

3 108.8 119.7 102.2 109.5

4 115.5 106.9 105.4 105.9

5 108.1 108.8 111.3 104.7

6 112.8 115.6 107.2 114.0

7 117.7 111.7 113.1 114.6

8 110.3 122.8 111.1 112.7

9 116.3 119.4 104.4 112.6

10 114.3 112.0 117.6 102.3

Average (Nm/g) 113.0 114.7 108.2 109.4

Standard

deviation (Nm/g) 3.8 5.1 4.9 4.6

Table I.2 TI of new layer type paper without oil impregnation.

Test No. 180 mm

(Nm/g)

1 89.5

2 92.5

3 94.9

4 93.3

5 96.5

6 89.1

7 87.9

8 92.1

9 87.1

10 93.7

Average (Nm/g) 91.6

Standard

deviation (Nm/g) 3.1

Table I.3 TI of new conductor type paper with oil impregnation.

Test No. S1 (Nm/g) S2 (Nm/g) S3 (Nm/g)

30 mm 180mm 30 mm 180mm 30 mm 180mm

1 109.7 107.5 117.2 105.8 105.8 102.7

2 113.8 97.6 99.9 99.9 110.1 111.3

3 109.7 108.0 108.4 97.1 112.3 105.8

4 108.6 112.9 105.3 105.2 104.9 106.6

5 113.0 105.9 108.0 104.8 110.8 105.4

6 117.3 113.4 107.7 106.5 105.0 107.9

7 101.9 105.4 101.9 107.5 109.8 98.9

8 117.3 101.6 112.1 106.0 110.1 100.9


185

9 118.7 105.4 102.0 101.6 109.1 104.2

10 109.3 103.1 114.8 111.2 109.2 96.6

Average (Nm/g) 111.9 106.1 107.7 104.6 108.7 104.0

Standard

deviation (Nm/g) 5.1 4.8 5.7 4.0 2.6 4.4

S1= Only gap between two clamps is impregnated with oil.

S2= Paper impregnated with oil without temperature.

S3= Paper impregnated with oil with temperature.

Table I.4 TI of new conductor type paper under folding stress.

Test No. 30 mm (Nm/g)

1 98.0

2 96.3

3 103.5

4 100.8

5 101.1

6 95.3

7 99.5

8 96.7

9 105.1

10 94.1

Average (Nm/g) 99.0

Standard


Table I.5 TI of new conductor type paper under less paper length in the clamp area.

Test No. 15 mm (Nm/g)

1 70.2

2 86.1

3 64.3

4 76.6

5 71.5

6 67.8

7 75.1

8 76.1

9 68.5

10 71.5

Average (Nm/g) 72.8

Standard


Table I.6 TI of Transformer 1.

Parameter

B phase

CW SW

L1 L2 L3 L4 L1 L2 L3

1 64.0 61.1 81.7 76.2 56.8 81.4 81.3

2 67.6 81.1 81.0 65.1 54.7 63.4 71.8

3 59.9 70.4 86.7 74.6 42.7 61.4 63.9


186

4 52.6 65.5 79.8 77.1 29.6 63.2 51.1

5 61.5 72.2 82.1 69.7 61.9 74.3 43.3

6 67.4 74.3 80.7 73.7 59.9 74.0 61.5

7 57.7 82.3 86.3 70.8 33.1 87.7 47.6

8 54.9 63.6 76.3 67.6 60.0 65.1 -

9 53.8 81.9 73.5 72.4 - 77.0 -

10 - 84.6 84.4 73.4 - - -

Average (Nm/g) 59.9 73.7 81.3 72.1 49.8 72.0 60.1

Standard

deviation

(Nm/g)

5.7 8.5 4.1 3.8 12.9 9.2 13.7

L1= Top outside

L2= 72nd

discs from top outside.

L3= 144th discs from top outside.

L4= Bottom outside.


Parameter

A phase

CW SW

L1 L3 L1 L4 L2 L3

1 66.5 74.1 53.1 60.8 63.1 68.4

2 58.1 63.3 45.0 65.2 82.0 72.7

3 75.2 61.8 57.5 60.0 60.3 63.3

4 59.6 72.7 56.3 64.2 75.0 67.6

5 69.8 62.8 45.3 62.1 70.9 63.3

6 69.7 61.5 51.5 64.5 73.7 69.0

7 67.3 64.2 43.7 62.3 62.7 -

8 - - 54.4 52.6 77.0 -

Average (Nm/g) 66.6 65.8 50.9 61.4 70.6 67.4

Standard

deviation

(Nm/g)

6.0 5.3 5.4 4.0 7.8 3.6

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

L4= Top inside


Parameter

A phase B phase C phase

HV HV LV TV HV top

L1 L2 L1 L3 L1 L4 L1 L1 L4

1 60.0 81.6 28.1 56.3 24.0 25.1 32.6 28.1 39.5

2 65.9 85.7 37.0 94.1 65.9 37.3 46.6 20.5 28.7

3 53.5 94.5 34.5 60.7 47.8 59.4 51.3 46.8 54.3

4 78.0 64.1 42.1 59.5 73.0 32.1 25.3 28.4 77.0

5 43.7 87.4 24.4 61.0 52.8 47.4 61.6 29.7 60.9

6 62.3 89.4 36.3 74.1 18.1 44.1 29.5 9.5 66.7

7 59.3 78.8 47.6 53.3 68.7 41.7 46.2 30.3 67.3

8 56.7 92.7 45.3 41.5 54.9 32.5 29.4 25.5 56.7


187

9 68.1 78.1 38.9 85.9 60.0 47.6 23.4 28.5 60.8

10 - 93.5 43.6 57.4 21.1 29.5 21.8 16.5 60.8

Average

(Nm/g) 60.8 84.6 37.8 64.4 48.6 39.7 36.8 26.4 57.3

Standard

deviation

(Nm/g)

9.6 9.3 7.4 15.8 20.5 10.4 13.6 9.8 14.0

L1= Top outside.

L2= Top outside (2nd

layer paper)

L3= Bottom outside.

L4= Top outside (near to conductor).


Parameter

B phase

CW SW Tap

L1 L2 L3 L1 L2 L3 L3

1 21.9 32.8 31.2 30.3 49.9 24.1 34.1

2 25.0 41.8 23.9 29.3 51.2 19.3 41.3

3 42.6 40.1 28.7 36.7 30.3 55.5 35.3

4 40.6 47.6 33.5 23.6 42.1 59.8 26.5

5 57.6 30.2 29.7 42.3 32.0 50.6 30.1

6 38.6 34.8 31.4 43.3 35.2 48.7 42.3

7 35.1 30.5 - 41.2 39.0 30.2 29.1

8 34.0 28.9 - 26.1 19.2 40.3 -

9 27.3 28.5 - 33.7 49.6 25.7 -

10 47.9 - - 37.8 30.9 40.6 -

Average (Nm/g) 37.1 35.0 29.7 34.4 37.9 39.5 34.1

Standard

deviation

(Nm/g)

10.9 6.7 3.3 7.0 10.4 14.1 6.0

Parameter

A phase C phase

SW SW

L1 L2 L3 L1 L1 L2 L3

1 52.5 70.8 55.4 46.1 34.7 48.6 42.1

2 30.8 51.2 38.6 41.2 35.9 54.0 38.6

3 58.4 46.2 55.2 48.1 33.4 69.0 49.1

4 39.6 76.8 52.3 52.2 30.8 42.8 34.7

5 50.0 65.1 40.9 40.7 41.7 55.8 36.4

6 39.2 53.7 45.2 45.2 - 43.4 54.0

7 - - 57.4 43.4 - 51.7 -

8 - - 47.7 56.1 - 34.1 -

9 - - 52.0 58.1 - 43.1 -

10 - - 65.4 56.1 - 50.4 -

Average (Nm/g) 45.1 60.6 51.0 48.7 35.3 49.3 42.5

Standard

deviation

(Nm/g)

10.3 12.1 8.1 6.5 4.0 9.5 7.6

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.


188


Parameter

A phase

Not stated

L3 L4 L5 L6 L7 L8 L9 L10 L11 L12

1 57.3 70.5 61.9 58.0 44.2 57.9 51.9 50.9 31.7 40.0

2 43.0 68.0 31.5 60.4 50.3 62.1 48.1 56.3 50.0 26.0

3 49.9 62.8 65.0 61.2 52.8 64.0 57.9 29.5 44.7 46.5

4 44.1 63.0 61.8 61.7 53.9 57.4 61.0 45.7 51.5 35.7

5 56.8 59.7 52.9 44.5 59.9 61.6 56.3 48.8 54.8 44.5

6 50.4 53.3 50.2 59.0 39.7 57.2 59.6 60.4 57.3 29.4

7 73.4 37.2 62.5 55.9 54.0 56.0 44.9 37.7 39.0 49.7

8 - 39.3 59.7 57.3 41.8 53.4 57.8 67.2 35.6 46.8

9 - 61.4 53.2 49.9 59.3 50.3 63.6 46.0 - 39.4

10 - 53.8 51.5 57.4 60.3 66.1 58.9 - - 48.8

Average (Nm/g) 53.5 56.9 55.0 56.5 51.6 58.6 56.0 49.2 45.6 40.7

Standard

deviation

(Nm/g)

10.3 11.2 9.8 5.4 7.5 4.9 5.9 11.4 9.4 8.2

Parameter

B phase C phase

Not stated HV

L1 L2 L1 L2

1 44.9 55.4 9.9 67.9

2 57.0 44.2 33.3 66.3

3 52.4 29.2 31.0 57.3

4 57.7 53.3 38.9 51.5

5 50.5 44.9 36.8 56.2

6 52.1 37.0 46.5 57.3

7 52.6 45.4 24.9 62.8

8 59.6 24.0 37.9 60.3

9 51.7 50.6 35.4 57.2

10 - 51.6 - 46.7

Average (Nm/g) 53.2 43.6 32.7 58.4

Standard

deviation

(Nm/g)

4.4 10.5 10.4 6.4

L1= Top outside.

L2= Bottom outside.

L3= 3rd


L4= 3rd

discs from bottom outside.


L6= 4th discs from bottom outside.








189


Parameter


CW CW CW

L1 L2 L3 L1 L2 L3 L1 L2 L3

1 32.3 24.8 33.4 16.7 25.3 22.1 15.6 32.0 37.3

2 41.4 34.1 46.4 40.2 47.3 61.2 18.9 33.0 49.7

3 38.5 27.7 42.3 31.8 38.3 46.0 30.5 49.7 27.4

4 29.4 30.6 43.5 15.8 57.0 71.7 15.6 38.6 27.6

5 23.8 20.5 27.0 24.4 15.7 47.7 32.5 27.6 52.8

6 15.8 34.5 49.8 27.9 32.6 39.0 23.9 39.6 69.4

7 27.4 18.4 62.7 33.1 29.1 52.2 24.3 42.5 56.4

8 32.3 31.2 43.3 62.9 20.6 45.7 52.7 57.0 69.7

9 30.8 30.2 30.1 46.5 27.6 61.0 13.6 33.2 55.5

10 21.4 36.9 - 32.8 - 43.8 - 30.6 38.9

Average (Nm/g) 29.3 28.9 42.1 33.2 32.6 49.0 25.3 38.4 48.5

Standard

deviation

(Nm/g)

7.7 6.1 10.9 14.1 13.1 13.7 12.2 9.2 15.3

L1= Top outside.

L2= Middle outside

L3= Bottom outside.


Parameter

A phase

CW SW

L1 L2 L1 L2

1 48.2 44.1 25.1 28.7

2 48.4 41.4 16.5 40.4

3 46.8 57.1 18.7 41.0

4 58.2 33.7 28.9 45.3

5 55.6 46.8 23.6 45.1

6 58.2 59.0 46.1 46.7

7 37.7 54.3 37.5 60.8

8 46.7 58.0 41.5 68.6

9 60.2 - 34.1 64.0

10 59.5 - - -

Average (Nm/g) 51.9 49.3 30.2 48.9

Standard

deviation

(Nm/g)

7.5 9.2 10.2 12.9

L1= Top outside.

L2= Bottom outside.


Parameter

B phase

CW TW SW

L1 L2 L3 L4 L5 L1 L3 L1 L2 L3

1 19.7 35.4 53.2 15.3 23.3 30.2 46.0 31.0 40.9 35.4

2 26.8 43.2 53.7 35.2 24.9 19.8 35.6 24.5 39.7 52.1


190

3 22.5 45.5 36.3 36.3 20.7 11.0 32.4 24.5 36.8 51.4

4 27.5 47.6 38.7 15.6 17.7 20.1 45.0 23.9 38.3 36.9

5 - 37.8 - - 20.8 - - 33.1 31.7 -

6 - - - - 22.1 - - 33.5 - -

7 - - - - 21.8 - - - - -

Average (Nm/g) 24.1 41.9 45.5 25.6 21.6 20.3 39.8 28.4 37.5 43.9

Standard

deviation

(Nm/g)

3.7 5.1 9.3 11.8 2.3 7.8 6.8 4.6 3.6 9.0

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

L4= Top inside

L5= Top 2nd

strand from inside


Parameter


HV LV HV LV HV

L1 L1 L3 L1 L2 L1 L1 L2

1 9.7 42.1 30.9 42.2 37.0 20.2 37.1 45.7

2 12.1 23.3 56.7 49.5 59.4 23.8 41.1 58.5

3 21.5 40.7 47.5 38.1 55.1 20.2 26.4 65.9

4 36.1 43.2 49.0 46.3 55.3 25.0 37.7 51.7

5 28.3 35.2 - 41.1 58.1 - 35.7 52.0

6 32.0 47.8 - 47.2 52.0 - 41.0 47.3

7 40.3 44.4 - - 47.5 - 45.3 50.6

8 28.0 40.9 - - 54.1 - 40.5 -

9 31.3 38.5 - - - - - -

10 29.5 - - - - - - -

Average

(Nm/g) 26.9 39.6 46.0 44.1 52.3 22.3 38.1 53.1

Standard

deviation

(Nm/g)

9.8 7.1 10.9 4.3 7.2 2.5 5.6 7.0

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

Table I.15 TI of different layers for Transformer 10. Paper samples were taken from B phase,

HV bottom.

Parameter Number of layer

1 2 3 4 5 6 7 8

1 11.3 52.3 65.9 50.0 76.5 75.1 76.1 61.9

2 29.0 44.6 61.5 71.6 74.1 80.3 39.3 78.9

3 16.9 56.7 70.4 63.4 71.8 54.1 76.3 68.6

4 16.4 56.7 57.4 40.9 51.0 77.6 71.9 81.9

5 54.6 42.4 70.8 52.9 84.9 55.9 39.9 53.7

6 10.4 59.8 60.6 40.4 55.4 70.1 73.1 49.5

7 36.8 32.9 72.0 71.1 45.9 83.4 59.1 83.4

8 5.0 83.6 62.7 80.6 67.3 75.2 37.6 56.9


191

9 28.3 59.6 45.1 72.9 77.2 33.3 70.3 64.8

10 15.8 58.0 - 69.7 86.6 85.9 24.4 38.9

Average (Nm/g) 22.4 54.6 62.9 61.3 69.1 69.1 56.8 63.9

Standard

deviation

(Nm/g)

14.9 13.5 8.4 14.3 14.0 16.5 19.5 14.7

Table I.16 TI of different layers for Transformer 6. Paper samples were taken from B phase,

CW top.


1 2 3 4 5 6 7 8

1 16.7 60.2 25.9 46.7 42.4 40.7 45.4 50.7

2 40.2 42.7 54.9 50.0 35.0 39.1 51.8 35.5

3 31.8 36.2 40.5 54.9 61.3 45.9 53.2 56.8

4 15.8 40.1 38.3 33.6 51.4 36.5 46.1 53.1

5 24.4 62.5 42.4 35.3 48.8 40.9 38.7 41.4

6 27.9 52.1 48.7 57.8 49.8 47.9 54.6 26.1

7 33.1 50.1 52.0 52.4 43.4 59.5 52.0 23.6

8 62.9 62.0 43.0 32.1 35.0 53.8 42.7 46.5

9 46.5 32.6 46.6 18.4 45.3 38.0 24.5 33.9

10 32.8 21.3 45.5 - 49.2 - 54.2 38.9

Average (Nm/g) 33.2 46.0 43.8 42.4 46.2 44.7 46.3 40.6

Standard

deviation

(Nm/g)

14.1 13.8 8.1 13.1 7.9 7.8 9.4 11.2

Parameter 9 10 11

1 47.8 46.2 14.0

2 45.5 47.9 24.0

3 20.8 55.2 31.0

4 32.0 38.9 27.3

5 45.2 52.4 41.6

6 39.5 40.3 39.7

7 17.9 50.8 35.1

8 56.7 69.5 50.6

9 24.4 48.8 46.2

10 47.1 56.9 -

Average (Nm/g) 37.7 50.7 34.4

Standard

deviation

(Nm/g)

13.2 8.8 11.6

Appendix II: Low Molecular Weight Acid (LMA) in Service Aged Paper

192

APPENDIX II: Low Molecular Weight Acid (LMA) of Service Aged Paper

Table II.1 LMA in paper of Transformer 1.

Parameter

B phase

CW SW

L1 L2 L3 L4 L1 L2 L3

1 2.08 1.88 0.38 1.77 3.23 1.67 0.43

2 2.24 3.19 0.11 2.32 3.19 1.55 0.57

Average (mg

KOH/g) 2.16 2.53 0.24 2.05 3.21 1.61 0.50

L1= Top outside

L2= 72nd



L4= Bottom outside.


Parameter

A phase

CW SW

L1 L3 L1 L4 L2 L3

1 2.07 2.32 1.98 0.55 1.53 0.40

2 1.50 1.51 1.96 1.39 1.14 1.24

Average (mg

KOH/g) 1.79 1.91 1.97 0.97 1.33 0.82

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

L4= Top inside


Parameter


HV HV LV TV HV top

L1 L2 L1 L3 L1 L4 L1 L1 L4

1 2.81 2.11 4.02 2.59 3.79 2.84 2.38 2.44 1.92

2 2.79 1.51 3.58 3.13 - - - 2.06 1.30

Average (mg

KOH/g) 2.80 1.81 3.80 2.86 3.79 2.84 2.38 2.25 1.61

L1= Top outside.

L2= Top outside (2nd

layer paper)

L3= Bottom outside.

L4= Top outside (near to conductor).


Parameter

B phase

CW SW Tap

L1 L2 L3 L1 L2 L3 L3

1 3.34 2.44 2.70 3.23 2.34 1.92 2.24

2 3.26 - 3.47 2.41 3.39 3.63 2.64

Average (mg

KOH/g) 3.30 2.44 3.08 2.82 2.87 2.77 2.44


193

Parameter

A phase C phase

SW SW

L1 L2 L3 L1 L1 L2 L3

1 3.19 1.92 2.73 1.99 3.85 2.03 1.80

2 3.48 1.82 2.88 - 4.19 3.38 3.81

Average (mg

KOH/g) 3.33 1.87 2.81 1.99 4.02 2.70 2.80

L1= Top outside.

L2= Middle outside

L3= Bottom outside tap.


Parameter

A phase

Not stated

L3 L4 L5 L6 L7 L8 L9 L10 L11 L12

1 0.78 0.42 2.98 1.53 2.96 0.70 1.30 1.80 3.04 2.29

2 2.26 0.36 1.86 1.15 1.43 1.32 1.06 2.14 1.88 2.33

Average (mg

KOH/g) 1.52 0.39 2.42 1.34 2.19 1.01 1.18 1.97 2.46 2.31

Parameter

B phase C phase

Not stated HV

L1 L2 L1 L2

1 2.55 1.53 2.84 1.42

2 3.14 1.18 4.48 0.10

Average (mg

KOH/g) 2.85 1.35 3.66 0.76

L1= Top outside.

L2= Bottom outside.

L3= 3rd


L4= 3rd

discs from bottom outside.










Parameter


CW CW CW

L1 L2 L3 L1 L2 L3 L1 L2 L3

1 3.35 4.26 3.72 4.27 4.67 2.80 4.44 3.76 4.10

2 3.83 4.79 3.70 6.80 4.83 4.04 3.63 3.97 3.97

Average (mg

KOH/g) 3.59 4.52 3.71 5.53 4.75 3.42 4.03 3.87 4.04

L1= Top outside.

L2= Middle outside

L3= Bottom outside.


194


Parameter

A phase

CW SW

L1 L2 L1 L2

1 1.69 1.93 3.01 2.67

2 1.10 1.12 - 3.31

Average (mg

KOH/g) 1.40 1.53 3.01 2.99

L1= Top outside.

L2= Bottom outside.


Parameter

B phase

CW TW SW

L1 L2 L3 L4 L5 L1 L3 L1 L2 L3

1 5.93 4.87 3.61 6.35 5.79 4.96 3.10 4.23 3.72 3.54

2 5.66 5.66 3.78 6.44 6.15 5.06 2.79 4.42 4.14 3.54

Average (mg

KOH/g) 5.79 5.26 3.70 6.39 5.97 5.01 2.95 4.33 3.93 3.54

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

L4= Top inside

L5= Top 2nd

strand from inside


Parameter


HV LV HV LV HV

L1 L1 L3 L1 L2 L1 L1 L2

1 2.30 6.40 4.18 2.63 1.48 5.29 6.84 1.57

2 2.88 5.26 3.29 2.67 2.38 5.94 5.86 2.09

Average (mg

KOH/g) 2.59 5.83 3.74 2.65 1.93 5.62 6.35 1.83

L1= Top outside.

L2= Middle outside.

L3= Bottom outside.

Table II.10 LMA in paper of different layers for Transformer 10. Paper samples were taken

from B phase, HV bottom.


1 2 3 4 5 6 7 8

1 5.51 7.24 4.67 2.89 3.83 1.88 3.98 1.94

2 - - - - - - - -

Average (mg

KOH/g) 5.51 7.24 4.67 2.89 3.83 1.88 3.98 1.94


195

Table II.11 LMA in paper of different layers for Transformer 6. Paper samples were taken

from B phase, CW top.


1 2 3 4 5 6 7 8

1 4.27 2.92 2.13 3.89 2.72 3.34 3.22 2.94

2 6.80 2.86 3.74 - 3.76 4.34 - 3.30

Average (mg

KOH/g) 5.53 2.89 2.94 3.89 3.24 3.84 3.22 3.12

Parameter 9 10 11

1 3.26 2.81 1.92

2 - 4.01 1.80

Average (mg

KOH/g) 3.26 3.41 1.86

Appendix III: Effect of Ageing on Breakdown Voltage of Paper

196

APPENDIX III: Effect of Ageing on Breakdown Voltage of Paper

Table III.1 AC breakdown voltages of different types of paper impregnated in new oil. The

gap distance between two sphere electrodes was set at 0.258 mm.

Test No.

Breakdown voltages of different types of paper and new

oil (kV)

Layer type Conductor

type

Service

aged paper New oil

1 14.3 18.6 20.2 16.7

2 14.3 17.2 18.1 17.1

3 16.9 20.4 19.0 14.6

4 15.3 20.4 17.8 14.1

5 15.8 16.7 21.5 14.2

6 16.7 18.6 21.7 14.9

7 15.2 18.6 21.1 16.8

8 13.8 18.9 24.2 15.0

9 14.3 17.9 20.5 17.1

10 14.8 19.3 20.4 11.8

11 12.2 16.7 19.7 15.9

12 13.7 17.1 20.4 12.7

13 13.5 16.8 19.7 14.4

14 16.4 16.9 19.4 13.9

15 17.1 18.0 18.4 14.8

16 17.3 18.4 22.8 15.0

17 15.7 17.5 18.3 13.1

18 17.1 19.3 20.2 14.1

19 15.7 18.9 18.6 14.3

20 13.8 17.5 21.4 15.6

21 16.2 17.1 20.8 16.3

22 14.5 17.6 18.4 12.7

23 16.5 16.7 18.4 14.8

24 14.8 16.7 20.7 12.9

25 16.1 16.5 20.1 13.5

26 16.3 16.2 20.2 11.8

27 16.9 16.1 19.0 13.6

28 15.7 18.4 18.6 12.7

29 13.1 19.0 21.3 12.6

30 14.3 17.0 21.1 13.5

31 13.5 16.2 20.7 11.5

32 15.2 18.6 18.3 12.7

33 17.2 17.0 18.9 12.2

34 14.2 19.5 18.8 11.5

35 15.7 18.3 19.6 13.7

36 15.3 18.1 20.2 12.1

37 14.4 16.0 18.0 14.2

38 12.8 17.1 21.0 12.4

39 14.4 17.2 20.2 12.8


197

40 15.6 16.7 22.2 12.9

Average (kV) 15.2 17.7 20.0 13.9

Standard deviation (kV) 1.3 1.2 1.4 1.6

Table III.2 AC breakdown voltages of different types of paper impregnated in aged oil. The

gap distance between two sphere electrodes was set at 0.258 mm.

Test No.

Breakdown voltages of different types of paper and aged

oil (kV)

Layer type Conductor

type

Service

aged paper Aged oil

1 14.9 17.7 18.0 16.4

2 15.7 16.6 17.7 14.4

3 15.0 15.2 16.6 15.8

4 14.9 15.5 19.8 16.1

5 14.6 16.1 18.7 10.5

6 15.5 16.7 20.1 12.7

7 13.5 18.3 19.0 11.8

8 16.0 17.3 19.4 9.4

9 14.9 16.0 16.0 9.4

10 14.9 17.7 17.7 13.4

11 12.4 15.5 20.2 13.2

12 13.1 16.8 15.5 14.3

13 12.7 16.1 18.9 14.0

14 15.1 15.3 19.0 14.5

15 15.0 17.4 20.7 14.0

16 13.8 15.9 19.7 14.4

17 12.6 16.7 19.8 11.8

18 16.0 17.1 20.4 11.7

19 14.9 16.2 16.6 15.5

20 11.8 16.7 17.8 14.3

21 12.7 17.5 19.8 14.4

22 14.9 16.7 17.9 11.8

23 16.9 16.5 17.5 11.4

24 13.1 15.3 18.9 12.4

25 13.3 17.3 20.2 14.6

26 13.5 15.9 15.5 13.0

27 15.0 15.6 15.2 12.1

28 13.5 15.3 20.2 15.3

29 13.5 14.7 19.6 14.3

30 14.8 15.5 17.5 15.0

31 12.1 15.8 19.2 16.8

32 15.8 15.1 18.4 14.1

33 15.7 15.9 20.3 12.3

34 12.6 15.3 18.4 9.4

35 13.1 14.8 16.9 14.1

36 13.3 17.3 21.6 14.0

37 11.6 15.8 18.5 11.3


198

38 11.4 15.7 18.8 14.2

39 13.4 15.0 17.0 12.9

40 13.5 16.7 20.2 14.5

Average (kV) 14.0 16.2 18.6 13.4


Appendix IV: Effect of Low Molecular Weight Acid (LMA) and High Molecular Weight

Acid (HMA) on Breakdown Voltage of Oil and Ageing of Paper

199

APPENDIX IV: Effect of Low Molecular Weight Acid (LMA) and High

Molecular Weight Acid (HMA) on Breakdown Voltage of Oil and Ageing of

Paper

Table IV.1 AC breakdown voltages of new mineral oil with different amount of LMA. The

gap distance between two VDE electrodes was set at 1 mm.

Test No.

Breakdown voltage of new mineral oil with different

amount of LMA (kV)

(Clean) 0.02

mg KOH/g

0.05mg

KOH/g

0.4 mg

KOH/g

0.6 mg

KOH/g

1 49.4 25.5 13.1 7.2

2 47.5 21.5 12.2 9.4

3 49.0 21.2 11.6 8.7

4 46.0 27.1 11.8 7.0

5 49.4 25.9 12.8 7.8

6 54.6 25.8 13.1 9.0

7 51.9 17.0 13.1 8.7

8 44.9 26.6 11.3 8.2

9 52.2 25.1 10.0 8.7

10 49.2 22.4 12.5 10.0

11 45.7 26.6 11.8 8.7

12 53.0 18.4 10.7 10.0

13 51.5 29.5 11.6 8.7

14 37.3 26.7 11.9 10.0

15 46.0 22.8 12.8 8.7

16 41.7 18.9 10.8 10.0

17 56.9 21.2 12.0 8.7

18 49.5 21.9 9.4 8.2

19 54.1 26.0 11.9 9.2

20 53.5 28.2 11.1 9.4

21 39.9 27.2 12.5 8.7

22 56.5 19.0 12.7 6.4

23 42.9 19.3 11.7 8.1

24 50.5 17.2 11.0 7.1

25 44.2 27.0 11.8 8.6

26 51.8 20.3 12.6 6.9

27 55.0 18.2 9.3 7.3

28 44.6 18.9 10.8 7.3

29 53.1 14.4 11.5 8.3

30 50.0 25.4 10.7 10.4

31 48.5 19.6 12.7 8.4

32 47.2 26.9 12.1 8.5

33 51.0 26.2 10.9 8.7

34 39.7 19.5 11.4 9.1

35 55.6 24.7 11.9 7.8

36 54.3 21.5 9.9 8.5



200

37 47.8 16.8 11.5 7.3

38 45.7 23.8 2.5 6.8

39 47.9 22.9 11.0 7.9

40 51.2 20.8 9.3 7.5

41 50.3 24.4 9.5 7.5

42 45.8 23.6 12.7 7.6

43 53.6 26.4 10.3 6.7

44 53.9 24.3 11.3 6.5

45 50.1 20.7 10.4 8.1

46 56.3 21.1 10.0 6.5

47 54.9 23.8 11.4 6.9

48 52.7 18.1 11.3 5.6

49 45.7 17.5 10.2 6.4

50 51.1 19.8 11.4 8.0

Average (kV) 49.5 22.6 11.2 8.1


Table IV.2 AC breakdown voltages of new mineral oil with different amount of HMA. The


Test No.

Breakdown voltage of new mineral oil with different

amount of HMA (kV)

0.6 mg

KOH/g

1.6 mg

KOH/g

3.1 mg

KOH/g

9.0 mg

KOH/g

1 45.0 59.5 50.3 19.4

2 45.2 58.0 48.8 15.7

3 45.3 55.3 52.3 23.3

4 47.5 53.2 50.2 21.4

5 38.1 53.8 50.3 23.0

6 42.2 53.6 49.6 22.3

7 45.6 53.8 43.2 7.1

8 46.9 47.0 50.5 9.1

9 39.7 50.4 47.8 18.5

10 38.3 48.2 44.2 19.5

11 39.6 51.0 47.3 17.9

12 41.2 51.5 42.9 18.4

13 44.2 47.9 40.6 14.7

14 37.7 57.4 42.6 22.3

15 42.0 46.4 45.6 29.1

16 41.5 52.6 49.2 27.2

17 43.3 48.0 41.1 20.1

18 39.5 52.5 43.2 17.7

19 41.5 50.0 40.2 18.7

20 39.1 55.4 41.5 27.4

21 46.8 53.2 40.3 9.5

22 44.3 44.5 37.5 13.2



201

23 43.8 51.5 45.1 25.1

24 43.3 47.5 43.5 18.5

25 37.1 49.4 44.6 12.0

26 43.8 46.3 43.8 9.3

27 37.5 42.2 37.0 9.7

28 44.9 47.9 48.5 5.8

29 41.4 36.6 47.8 15.1

30 36.3 42.7 49.1 11.4

31 40.2 42.9 45.9 11.9

32 39.0 43.3 38.1 26.2

33 41.8 45.8 35.2 40.4

34 40.2 46.8 36.2 6.5

35 41.5 46.8 44.0 16.0

36 41.9 45.3 46.3 17.1

37 41.1 47.3 43.5 7.2

38 39.8 44.1 38.5 5.7

39 41.5 35.7 52.5 15.1

40 40.9 43.7 47.8 13.4

41 40.0 42.4 42.8 19.1

42 45.9 41.5 45.5 15.9

43 39.5 47.8 47.5 17.1

44 43.7 45.5 39.1 24.1

45 46.4 43.7 56.3 15.1

46 32.6 40.9 49.2 15.5

47 39.2 48.6 46.4 22.6

48 44.5 44.7 39.3 28.8

49 47.0 41.7 51.8 32.2

50 42.4 39.3 51.1 11.9

Average (kV) 41.8 47.7 45.1 17.7


Table IV.3 AC breakdown voltages of new natural ester with different amount of LMA. The


Test No.

Breakdown voltage of new natural ester with different

amount of LMA (kV)

(Clean)

0.04 mg

KOH/g

0.6 mg

KOH/g

1.2 mg

KOH/g

2.0 mg

KOH/g

4.0 mg

KOH/g

1 52.6 47.7 42.1 48.8 47.3

2 51.9 48.4 40.6 46.9 38.3

3 49.9 46.3 46.4 49.5 44.2

4 45.4 46.8 42.7 49.0 43.5

5 32.7 48.7 40.8 57.5 42.0

6 54.4 53.5 46.3 51.3 41.5

7 39.9 47.1 28.2 53.0 45.1

8 48.6 45.9 44.0 50.3 27.1



202

9 48.2 48.1 44.2 58.7 37.4

10 46.9 48.5 45.5 53.7 38.3

11 50.8 49.0 30.8 53.1 34.3

12 51.5 45.5 45.1 55.2 39.4

13 42.6 47.5 39.1 50.0 34.2

14 49.1 48.8 46.6 54.2 38.5

15 48.4 50.9 44.2 58.2 34.8

16 50.1 45.6 41.9 54.4 42.8

17 44.6 49.1 38.5 50.4 39.2

18 36.4 46.1 43.8 52.1 30.8

19 56.3 40.8 44.4 47.6 37.2

20 50.3 50.0 46.3 50.9 29.1

21 41.0 27.9 44.2 59.0 29.2

22 50.5 48.6 42.8 58.7 40.5

23 33.7 51.6 40.2 47.1 41.0

24 49.5 50.3 43.6 52.1 35.6

25 48.7 47.7 43.3 54.0 41.5

26 42.5 45.2 43.2 47.3 42.0

27 48.0 53.1 47.8 46.6 39.6

28 44.2 49.4 39.0 49.7 38.3

29 40.2 46.2 44.8 58.0 32.2

30 50.9 40.0 36.9 51.0 38.3

31 44.9 42.9 35.8 58.9 32.2

32 49.1 51.3 43.3 58.3 36.9

33 49.5 53.1 37.9 50.4 39.0

34 43.1 50.9 33.5 50.4 37.1

35 47.6 38.7 30.0 53.7 37.9

36 40.7 48.1 46.4 48.7 38.5

37 42.0 45.3 44.3 55.3 37.6

38 36.5 45.3 44.8 60.9 36.9

39 45.9 51.3 33.2 50.7 40.3

40 46.9 51.9 30.0 36.2 37.1

41 47.4 48.5 32.0 43.3 36.0

42 47.6 45.8 32.1 45.6 38.0

43 47.2 51.4 35.5 46.4 28.5

44 24.2 49.7 39.8 56.6 32.6

45 38.6 50.1 35.2 47.8 34.7

46 35.8 48.6 38.1 46.4 36.5

47 44.5 49.5 31.8 46.3 33.5

48 34.4 43.3 35.3 51.3 37.4

49 49.0 51.7 35.3 50.0 34.0

50 46.4 48.1 31.4 49.0 31.5

Average (kV) 45.2 47.6 39.9 51.5 37.2

Standard deviation (kV) 6.3 4.3 5.5 4.8 4.4



203

Table IV.4 AC breakdown voltages of new natural ester with different amount of HMA. The


Test No.

Breakdown voltage of new natural ester with different

amount of HMA (kV)

9.0 mg KOH/g

1 51.0

2 53.1

3 48.4

4 49.4

5 54.2

6 45.8

7 49.3

8 45.8

9 48.4

10 42.9

11 54.9

12 43.8

13 51.3

14 51.9

15 49.3

16 44.0

17 44.2

18 45.0

19 52.6

20 41.5

21 50.4

22 45.0

23 45.3

24 30.8

25 55.6

26 46.2

27 43.8

28 46.1

29 51.3

30 53.9

31 41.5

32 36.2

33 53.1

34 53.8

35 39.5

36 39.6

37 50.4

38 48.4

39 45.1

40 51.8



204

41 45.9

42 38.4

43 53.3

44 38.2

45 54.1

46 54.1

47 45.3

48 47.5

49 45.1

50 47.9

Average (kV) 47.3

Standard deviation (kV) 5.4

Table IV.5 TI of conductor type paper ageing in mineral oil under dry condition. Data are

mean of 8 samples.

Parameter

Ageing under dry condition

Start With no acid With LMA

Ageing duration 0 14 28 63 14 28 63

Average (Nm/g) 117.4 94.3 73.6 51.7 58.9 64.1 13.5

Standard deviation (Nm/g) 8.8 11.3 10.3 16.5 10.5 8.2 5.7

Parameter

Ageing under dry

condition

With HMA

Ageing duration 14 28 63

Average (Nm/g) 103.9 74.7 52.1

Standard deviation (Nm/g) 8.7 10.1 14.0

Table IV.6 TI of conductor type paper ageing in mineral oil under wet condition. Data are

mean of 8 samples.

Parameter

Ageing under wet condition



Average (Nm/g) 117.4 66.9 60.5 54.3 12.6 18.7 14.6


Parameter

Ageing under wet

condition

With HMA


Average (Nm/g) 78.4 74.0 39.5




205

Table IV.7 TI of conductor type paper ageing in natural ester under dry condition. Data are

mean of 8 samples.

Parameter

Ageing under dry condition



Average (Nm/g) 120.0 94.4 83.7 69.5 72.4 78.1 32.2


Parameter

Ageing under dry

condition

With HMA


Average (Nm/g) 104.0 94.3 61.7


Table IV.8 TI of conductor type paper ageing in natural ester under wet condition. Data are

mean of 8 samples.

Parameter

Ageing under wet condition



Average (Nm/g) 120.0 79.5 79.2 66.6 40.5 58.0 52.3


Parameter

Ageing under wet

condition

With HMA


Average (Nm/g) 77.6 83.2 76.0


APPENDIX V: List of Publications

206

APPENDIX V: List of Publications

[1] N. Azis and Z.D. Wang, "Acid generation study of natural ester," in

International Symposium of High Voltage (ISH), Hannover, Germany, pp.

E-048, 22-26 August, 2011.

[2] N. Azis and Z.D. Wang, "Examining paper ageing in service," in Euro

TechCon 2010, Chester, United Kingdom, 8 - 10 November, 2010.

[3] Q. Liu, Z.D. Wang, N. Azis, W.W. Sampson, P. Jarman, S. Ryder, and H. Z.

Ding, "Assessment of ageing conditions through paper tensile strength analysis

of scrapped transformers," in International Symposium of High Voltage (ISH),

Cape Town, South Africa, pp. 264-269, 24-28 August, 2009.

References

207

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