SUMMARY.....................................................................................................................VII
TABLE OF CONTENTS................................................................................................. XI
1 INTRODUCTION ......................................................................................................... 1
1.1 On-load tap changers in power transformers ............................................... 1
1.2 Failure aspects of on-load tap changers ......................................................... 3
1.3 Tap changer diagnostics.................................................................................... 5
1.4 Objectives of this thesis .................................................................................... 5
1.5 Approach............................................................................................................. 6
1.6 Outline of this thesis ......................................................................................... 7
2 OLTC TECHNOLOGY AND DEGRADATION ........................................................ 9
2.1 OLTC technology overview............................................................................. 9
2.1.1 Selector switch type tap changer ........................................................... 10
2.1.2 Diverter switch type tap changer........................................................... 11
2.2 OLTC degradation mechanisms.................................................................... 12
2.3 Contact degradation ........................................................................................ 12
2.3.1 Clean contacts .......................................................................................... 13
2.3.2 Contacts with a surface film................................................................... 17
2.3.3 Contacts with high local temperatures ................................................. 23
2.3.4 Contact failure due to overheating........................................................ 24
2.3.5 Corrected model of tap changer resistance ......................................... 27
2.4 Conclusions....................................................................................................... 30
3 ON-LOAD TAP CHANGER DIAGNOSIS ................................................................. 31
3.1 Oil and insulation diagnosis ........................................................................... 32
3.1.1 Dissolved gas analysis ............................................................................. 32
3.1.2 Study of the agreement between DGA and DRM results ................ 34
3.2 Mechanical diagnosis ....................................................................................... 37
3.2.1 Acoustic and vibration signature........................................................... 37
3.2.2 Motor power measurement.................................................................... 38
3.2.3 Position measurement............................................................................. 39
3.3 Contact status diagnosis.................................................................................. 39
3.3.1 Temperature difference measurement.................................................. 40
3.3.2 Static resistance measurement ............................................................... 41
3.3.3 Dynamic resistance measurement (DRM)........................................... 43
3.4 Discussion......................................................................................................... 43
3.5 Conclusions....................................................................................................... 48
4 DYNAMIC RESISTANCE OF ON-LOAD TAP CHANGERS...................................... 49
4.1 Measurement setup.......................................................................................... 50
4.1.1 Indirect measurement of the OLTC..................................................... 50
4.1.2 Direct measurement of the OLTC ....................................................... 51
4.2 Measurement current ...................................................................................... 55
4.2.1 Winding configuration ............................................................................ 55
4.2.2 Amplitude of the measurement current............................................... 56
4.2.3 Sensitivity .................................................................................................. 61
4.3 Circuit resistance .............................................................................................. 62
4.4 Secondary short circuit.................................................................................... 66
4.5 Determination of the contact resistance ...................................................... 69
4.6 Conclusions....................................................................................................... 69
5 POWER TRANSFORMER DIAGNOSTIC SYSTEM.................................................... 71
5.1 International standards for power transformer testing.............................. 73
5.2 Transformer diagnostic system...................................................................... 75
5.3 Measurement procedure ................................................................................. 77
5.3.1 Dynamic resistance of the OLTC......................................................... 77
5.4 OLTC drive system condition ....................................................................... 81
5.5 Hardware and data processing....................................................................... 83
5.6 Conclusions....................................................................................................... 84
6 EVALUATION OF TYPICAL DRM DIAGNOSTIC DATA ....................................... 85
6.1 Regular DRM plots.......................................................................................... 86
6.2 Irregular DRM plots........................................................................................ 93
6.3 Irregularities in the change-over selector ..................................................... 95
6.3.1 Long-term aging in selector switch type OLTCs ............................... 95
6.3.2 Current interruptions due to the change-over selector ...................104
6.4 Irregularities in the arcing switch ................................................................106
6.4.1 Long-term aging in the arcing switch .................................................107
6.4.2 Contact wear of the arcing switch.......................................................109
6.4.3 Timing difference between phases......................................................110
6.4.4 Maintenance errors................................................................................110
6.4.5 Current interruptions due to the arcing switch.................................112
6.4.6 OLTC transition times..........................................................................113
6.5 Defects in both parts of the OLTC............................................................115
6.6 Conclusions.....................................................................................................116
7 LABORATORY VERIFICATION OF SELECTED FAILURE MECHANISMS ..........119
7.1 Measurability of the surface film using DRM ...........................................119
7.1.1 Correlation between DRM and contact degradation.......................119
7.1.2 Effect of the surface film on DRM ....................................................123
7.1.3 Accelerated degradation tests...............................................................124
7.2 Arcing contact wear.......................................................................................128
7.2.1 Arcing contact dimensions...................................................................128
7.2.2 Arcing contact timing............................................................................131
7.3 Conclusions.....................................................................................................134
8 DRM KNOWLEDGE RULES FOR DECISION SUPPORT ......................................137
8.1 Condition assessment....................................................................................137
8.2 DRM knowledge rules...................................................................................138
8.3 Quantities reflecting the OLTC condition.................................................141
8.4 Condition indexing ........................................................................................143
8.5 Application example......................................................................................146
8.5.1 Open contacts ........................................................................................147
8.5.2 Contact resistance ..................................................................................148
8.5.3 Flowchart verification ...........................................................................152
8.6 Conclusions.....................................................................................................155
9 CONCLUSIONS & RECOMMENDATIONS.............................................................157
9.1 Conclusions.....................................................................................................157
9.2 Suggestions for future work .........................................................................160
REFERENCES..............................................................................................................163
APPENDIX A - OLTC TECHNOLOGY, DEGRADATION AND PROTECTION....173
APPENDIX B - INDIRECT MEASUREMENT OF THE OLTC
DYNAMIC RESISTANCE .............................................................................................189
APPENDIX C - EXAMPLES OF STATIC RESISTANCE MEASUREMENTS
WITH INCREASING CURRENT ON SERVICE-AGED OLTC CONTACTS..............195
APPENDIX D - DESIGN CONSIDERATIONS OF A TRANSFORMER
DIAGNOSTIC SYSTEM................................................................................................203
LISTS ............................................................................................................................217
ACKNOWLEDGEMENTS ...........................................................................................225
SAMENVATTING........................................................................................................227
CURRICULUM VITAE.................................................................................................231
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