Quality Confirmation Tests for Power Transformer Insulation Systems

Contents

       Chapter 1: Unused Mineral Insulating Oil

1.1..... Introduction. 2

1.2..... Mineral Oil 2

1.3..... Classification of mineral oil based on application  3

1.3.1...... Transformers oil 4

1.3.2...... Switchgear oil in low temperatures  4

1.4..... Additives. 9

1.5..... Special cases. 11

1.6..... Analysis of potentially corrosive sulphur 11

1.7..... Oil contamination  12

References. 12

      Chapter 2: In-service Mineral Insulating Oil

2.1..... Introduction. 14

2.2..... Oil monitoring and purification  14

2.3..... Oil ageing and degradation  15

2.4..... Oil tests. 15

2.4.1...... Color and appearance  15

2.4.2...... Breakdown voltage  16

2.4.3...... Water content 16

2.4.4...... Water in the insulation system   17

2.4.5...... Acidity. 21

2.4.6...... Dielectric dissipation factor (DDF) and resistivity  21

2.4.7...... Additives and oxidation stability  24

2.4.8...... Sludge and sediment 25

2.4.9...... Interfacial tension  25

2.4.10.... Particle content 25

2.4.11.... Flash point 27

2.4.12.... Compatibility of insulating oil 27

2.4.13.... Pour point 28

2.4.14.... Density. 28

2.4.15.... Viscosity. 28

2.4.16.... PCB.. 28

2.4.17.... Corrosive sulphur 28

2.4.18.... Dibenzyl disulphides (DBDS) 29

2.4.19.... Passivators. 29

2.5..... In-service oil monitoring  30

2.5.1...... Uninhibited oil monitoring  30

2.5.2...... Inhibited oil monitoring  30

2.6..... Time schedule of sampling and testing in-service oil 31

2.7..... Available on-site tests  31

2.8..... Classification of operating oil 32

2.9..... Corrective actions  32

2.10... Purification. 32

2.10.1.... Physical purification  33

2.10.2.... Chemical purification (refinement) 35

2.10.3.... Substitution of additives  36

2.10.4.... Cleaning PCB contaminated oil 36

2.11... Replacing oil in electrical equipment 36

2.12... Adding passivators  37

2.13... Determining water concentration in the oil 38

References. 39

       Chapter 3: Chemical Indicators

3.1..... Introduction. 42

3.2..... Insulation paper life determination  42

3.3..... Cellulose. 43

3.4..... Cellulose molecular structure  44

3.5..... Cellulosic insulation  44

3.6..... Degree of polymerization  46

3.7..... Oil impregnated insulation paper 47

3.8..... Ageing of oil impregnated insulation paper 47

3.9..... Ageing mechanism.. 48

3.9.1...... Pyrolysis. 49

3.9.2...... Hydrolysis. 50

3.9.3...... Oxidation. 51

3.10... Influence from acids  52

3.11... Ageing of oil 52

3.12... Oil oxidation. 53

3.13... Degradation products in oil impregnated insulation systems  53

3.14... Degradation products from cellulosic insulation  54

3.14.1.... Water 54

3.14.2.... Acids. 54

3.14.3.... Furans. 54

3.14.4.... Carbon oxides  55

3.14.5.... Hydrocarbons  55

3.15... Degradation products of oil 56

3.15.1.... Acids. 56

3.15.2.... Sludge. 56

3.16... Chemical indicators  56

3.17... Furan compounds  57

3.17.1.... Furans origin. 57

3.18... The relationship between DP and furans  57

3.19... Stability. 58

3.20... Furans disadvantages  59

3.21... CO₂ and CO.. 59

3.22... The combination of CO₂/CO ratio and 2-furfural 60

3.23... Methanol 61

References. 69

      Chapter 4: Dissolved Gas Analysis (DGA)

4.1..... Introduction. 74

4.2..... Total flammable dissolved gas in the transformer 76

4.3..... Allowable concentration of gases in a transformer 76

4.4..... Gas ratio methods  77

4.4.1...... Dürrenberg method  77

4.4.2...... Rogers ratio. 78

4.4.3...... IEC Ratio method  79

4.5..... Duval triangle method  80

4.6..... Detection of partial discharge using DGA   82

4.7..... Impact of DGA accuracy on fault detection  82

References. 83

        Chapter 5: Other Tests

5.1..... Introduction. 86

5.2..... Partial discharge (PD) 86

5.2.1...... Corona discharge  87

5.2.2...... Surface discharge  87

5.2.3...... Discharge in composite insulation materials  88

5.2.4...... Electric discharge in cavities  88

5.2.5...... Electric treeing  89

5.3..... Partial discharge measurement 90

5.4..... Insulation monitoring by PD measurement 91

5.5..... Comparison of electrical and audio detection methods  94

5.6..... Partial discharge formation in transformers  95

5.7..... Dielectric response analysis  95

5.8..... Polarization. 96

5.9..... Polarization and depolarization currents  98

5.10... Insulation spectroscopy in time domain  99

5.11... FDS test 102

5.12... Returning voltage method  104

5.13... Isothermal relaxation current 106

5.14... Frequency response analysis (FRA) 107

5.15... Frequency response analysis theory  108

5.16... Application of FRA in power transformers  109

5.17... FRA test features  109

5.18... Frequency response measurement methods  110

5.18.1.... Swept frequency method (SFM) 110

5.18.2      Low voltage impulse methods (LVI) 112

5.19... Comparison of LVI and SFM methods  112

5.20... Detectable defects by FRA   113

References. 114

Subject Index. 117

 

Behrooz Vahidi was born in Abadan, Iran. He received the B.S. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, the M.S. degree in electrical engineering from Amirkabir University of Technology, Tehran, Iran, and the Ph.D. degree in electrical engineering from the University of Manchester Institute of Science and Technology, U.K. From 1980 to 1986 he worked in industry as a Chief Engineer in the ?eld of high voltage. Since 1989 he has been with the Department of Electrical Engineering, Amirkabir University of Technology, where he is currently a Professor. He was selected as the Distinguished Researcher of Iran by the Ministry of Higher Education of Iran in 2011, and by the Iranian Association of Electrical and Electronics Engineers in 2012.  He has authored or coauthored more than 450 papers, and one book (in Persian) on high-voltage engineering and power systems. His main ?elds of research are high voltage, electrical insulation, power system transients, lightning protection, and pulsed-power technology.