Table Of ContentNon-destructive Diagnostic of High
Voltage Electrical Systems:
Theoretical Analysis and
Practical Applications
RIVER PUBLISHERS SERIES IN ENERGY ENGINEERING
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Non-destructive Diagnostic of High
Voltage Electrical Systems:
Theoretical Analysis and
Practical Applications
Josef Vedral
Czech Technical University in Prague, Czech Republic
River Publishers
Published 2023 by River Publishers
River Publishers
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www.riverpublishers.com
Distributed exclusively by Routledge
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Non-destructive Diagnostic of High Voltage Electrical Systems: Theoretical
Analysis and Practical Applications / Josef Vedral.
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Contents
Preface xiii
List of Figures xv
List of Tables xxv
List of Abbreviations xxvii
1 Introduction 1
2 General View on Diagnostics of Electrical Machines 5
2.1 Diagnosed Object . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Online and Offline Diagnostic . . . . . . . . . . . . . . . . . 7
2.2.1 Offline diagnostic . . . . . . . . . . . . . . . . . . . 8
2.2.2 Online diagnostic . . . . . . . . . . . . . . . . . . . 9
2.3 Forecast of the Condition of the Diagnosed Device . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3 Diagnostics of Mechanical Parts of HV Machines 13
3.1 Measuring Machine Vibration . . . . . . . . . . . . . . . . . 13
3.1.1 Types of vibrations and vibrodiagnostic quantities . . 13
3.1.2 Vibrometer sensors . . . . . . . . . . . . . . . . . . 15
3.1.2.1 Deflection sensors . . . . . . . . . . . . . . 17
3.1.2.2 Speed sensors . . . . . . . . . . . . . . . . 18
3.1.2.3 Acceleration sensors . . . . . . . . . . . . . 19
3.1.3 Analysis of vibrometric signals . . . . . . . . . . . . 24
3.1.3.1 Analysis of vibrometric signals in the time
domain . . . . . . . . . . . . . . . . . . . . 24
3.1.3.2 Analysis of vibrometric signals in the
frequency domain . . . . . . . . . . . . . . 25
3.2 Torsional Oscillation of Shafts . . . . . . . . . . . . . . . . 33
3.2.1 Causes of shaft torsional oscillation . . . . . . . . . . 33
3.2.2 Measurement of shaft torsional oscillation . . . . . . 35
v
vi Contents
3.3 Measurement of Temperature Images of Electrical Machines 37
3.3.1 Contact temperature sensors . . . . . . . . . . . . . . 39
3.3.2 Non-contact temperature sensors . . . . . . . . . . . 45
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Author Queries . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4 Insulation Systems of HV Electric Machines 49
4.1 Electrical Properties of Insulators . . . . . . . . . . . . . . . 49
4.1.1 Electrical strength of insulators . . . . . . . . . . . . 49
4.1.1.1 Electrical strength of gaseous insulators . . 49
4.1.1.2 Electrical strength of liquid insulators . . . . 54
4.1.1.3 Electrical strength of solid insulators . . . . 57
4.1.2 Electrical conductivity of insulators . . . . . . . . . . 59
4.1.2.1 Electrical conductivity of gaseous
insulators. . . . . . . . . . . . . . . . . . . 60
4.1.2.2 Electrical conductivity of liquid
insulators. . . . . . . . . . . . . . . . . . . 60
4.1.2.3 Electrical conductivity of solid
insulators. . . . . . . . . . . . . . . . . . . 63
4.1.3 Polarization of insulators . . . . . . . . . . . . . . . 64
4.1.4 Dielectric losses in insulators . . . . . . . . . . . . . 66
4.1.4.1 Dielectric losses in gaseous insulators . . . 67
4.1.4.2 Dielectric losses in liquid insulators . . . . . 68
4.1.4.3 Dielectric losses in solid insulators . . . . . 68
4.2 Types of Insulators . . . . . . . . . . . . . . . . . . . . . . 69
4.2.1 Gaseous insulators . . . . . . . . . . . . . . . . . . . 70
4.2.2 Liquid insulators . . . . . . . . . . . . . . . . . . . . 71
4.2.3 Solid insulators . . . . . . . . . . . . . . . . . . . . 74
4.3 Electrical Stress on Insulators . . . . . . . . . . . . . . . . . 76
4.3.1 Stress on plate insulators . . . . . . . . . . . . . . . 77
4.3.2 Stress on cylindrical insulators . . . . . . . . . . . . 78
4.4 Degradation of Isolators . . . . . . . . . . . . . . . . . . . . 80
4.4.1 Mechanical aging . . . . . . . . . . . . . . . . . . . 81
4.4.2 Thermal aging . . . . . . . . . . . . . . . . . . . . . 81
4.4.3 Electrical aging . . . . . . . . . . . . . . . . . . . . 84
4.4.4 Combined aging . . . . . . . . . . . . . . . . . . . . 86
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Contents vii
5 Measurement of Properties of Insulation Systems of
HV Electrical Machines 91
5.1 Measurement of Insulation Current and Resistance . . . . . . 91
5.2 Measurement of Capacity and Loss Factor . . . . . . . . . . 93
5.2.1 Measurement of capacity and loss factor by the
HV Schering bridge . . . . . . . . . . . . . . . . . . 95
5.2.2 Measurement of capacitance and loss factor of
HV impedance meters . . . . . . . . . . . . . . . . . 97
5.2.3 Measurement of frequency dependence of
capacity and loss factor . . . . . . . . . . . . . . . . 102
5.3 Measurement of Dielectric Absorption of Insulation
Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6 Partial Discharges in Insulating Systems of
HV Electric Machines 109
6.1 Formation and Types of Partial Discharges . . . . . . . . . . 109
6.2 Partial Discharge Parameters . . . . . . . . . . . . . . . . . 112
6.3 Partial Discharge Models . . . . . . . . . . . . . . . . . . . 117
6.3.1 Model of external and surface partial discharges . . . 117
6.3.2 Gemant–Philippoff model of internal partial
discharges . . . . . . . . . . . . . . . . . . . . . . . 118
6.3.3 Bönings model of internal partial discharges . . . . . 121
6.4 Effect of Partial Discharges on the Insulation Systems of
HV Machines . . . . . . . . . . . . . . . . . . . . . . . . . 123
6.4.1 Electro-erosive effects of partial discharges . . . . . . 124
6.4.2 Chemical effects of partial discharges . . . . . . . . . 124
6.4.3 Thermal effects of partial discharges . . . . . . . . . 125
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7 Measurement and Detection of Partial Discharges 129
7.1 Electrical Methods Measurement and Detection of Partial
Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
7.1.1 Properties of partial discharge current pulses . . . . . 129
7.1.2 Galvanic methods for measuring partial
discharges . . . . . . . . . . . . . . . . . . . . . . . 131
7.1.2.1 Coupling capacitors . . . . . . . . . . . . . 134
7.1.2.2 Coupling impedance . . . . . . . . . . . . . 134
viii Contents
7.1.3 Measurement of partial discharges . . . . . . . . . . 138
7.1.3.1 Measurement of partial discharges with
oscilloscope . . . . . . . . . . . . . . . . . 139
7.1.3.2 Measurement of partial discharges by a
peak detector . . . . . . . . . . . . . . . . . 139
7.1.3.3 Partial discharge meters with direct signal
digitization . . . . . . . . . . . . . . . . . . 142
7.1.4 Charge calibration . . . . . . . . . . . . . . . . . . . 149
7.1.5 Measurement of partial discharges by
current transformers . . . . . . . . . . . . . . . . . . 151
7.1.6 Measurement of partial discharges by inductive
probes . . . . . . . . . . . . . . . . . . . . . . . . . 153
7.1.7 Measurement of partial discharges by capacitive
probes . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.2 Non-electrical Methods for the Detection of Partial
Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.2.1 Ultrasonic detection of partial discharges . . . . . . . 159
7.2.2 High-frequency detection of partial discharges . . . . 162
7.2.3 Optical partial discharges detection . . . . . . . . . . 166
7.2.4 Chemical detection of partial discharges . . . . . . . 167
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
8 Voltage Tests of HV Electric Machines 173
8.1 DC Voltage Tests . . . . . . . . . . . . . . . . . . . . . . . 173
8.2 AC Voltage Tests . . . . . . . . . . . . . . . . . . . . . . . . 173
8.3 Voltage Tests by Long-term Induced Voltage . . . . . . . . . 174
8.4 Impulse Voltage Tests . . . . . . . . . . . . . . . . . . . . . 174
8.4.1 Course of impulse voltage tests . . . . . . . . . . . . 175
8.4.2 Test pulses . . . . . . . . . . . . . . . . . . . . . . . 175
8.4.3 Generation of test voltage pulses . . . . . . . . . . . 178
8.4.3.1 Single-stage voltage shock generators . . . . 178
8.4.3.2 Multi-stage voltage shock generators . . . . 179
8.4.4 Voltage level impulse tests . . . . . . . . . . . . . . 181
8.4.4.1 Tests with a constant number of voltage
levels . . . . . . . . . . . . . . . . . . . . 182
8.4.4.2 Up–down tests . . . . . . . . . . . . . . . 182
8.4.4.3 Phased stress tests . . . . . . . . . . . . . . 184
8.4.4.4 Statistical methods for the evaluation of
level tests . . . . . . . . . . . . . . . . . . 186
8.5 Shock Wave Tests . . . . . . . . . . . . . . . . . . . . . . . 189
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
Contents ix
9 Diagnostics of Power Transformers 193
9.1 Types of Power Transformers . . . . . . . . . . . . . . . . . 193
9.2 Properties of Transformers . . . . . . . . . . . . . . . . . . 194
9.3 Diagnostics of Electrical Properties of Power
Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . 197
9.3.1 Measurement of active winding resistance . . . . . . 198
9.3.2 Checking the sequence and rotation of the phases . . 200
9.3.3 Measurement of transmission, voltage, and
no-load losses . . . . . . . . . . . . . . . . . . . . . 202
9.3.4 Measurement of short-circuit voltages and losses . . . 204
9.3.5 Measuring the efficiency of transformers . . . . . . . 205
9.3.6 Measurement of insulation resistance, polarization
index, and transformer time constant . . . . . . . . . 210
9.3.7 Measurement of loss factor and winding capacity . . 211
9.3.8 Winding fault detection . . . . . . . . . . . . . . . . 215
9.4 Diagnostics of Transformer Oil Properties . . . . . . . . . . 220
9.4.1 Measurement of electrical properties of
transformer oils . . . . . . . . . . . . . . . . . . . . 221
9.4.1.1 Measurement of breakdown voltage at
mains frequency . . . . . . . . . . . . . . . 221
9.4.1.2 Measurement of relative permittivity and
loss factor . . . . . . . . . . . . . . . . . . 222
9.4.1.3 Measurement of internal resistivity of
transformer oils . . . . . . . . . . . . . . . 224
9.4.2 Measurement of physical properties of
transformer oils . . . . . . . . . . . . . . . . . . . . 225
9.4.2.1 Transformer oil color measurement . . . . . 225
9.4.2.2 Transformer oil density measurement . . . . 225
9.4.2.3 Measuring the viscosity of transformer oils . . 227
9.4.2.4 Measurement of flash and combustion
temperatures . . . . . . . . . . . . . . . . . 228
9.4.2.5 Freezing temperature measurement . . . . . 228
9.4.3 Measurement of chemical properties of
transformer oils . . . . . . . . . . . . . . . . . . . . 229
9.4.3.1 Water content measurement . . . . . . . . . 229
9.4.3.2 Oil acid measurement . . . . . . . . . . . . 231
9.4.3.3 Measurement of the presence of sulfur . . . 232
9.4.3.4 Content of polycyclic aromatics and
polychlorinated biphenyls . . . . . . . . . . 232
9.4.3.5 Gas content measurement . . . . . . . . . . 232
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
