Condition Assessment of High Voltage

Insulation in Power System Equipment

R.E. James and Q. Su

The Institution of Engineering and Technology

Contents

Preface xi

1 Introduction 1 1.1 Interconnection of HV power system components 2

1.1.1 Alternating voltage systems 2 1.1.2 Direct-voltage systems 8

1.2 Insulation coordination 9 1.3 High-voltage test levels 10

1.3.1 Power-frequency voltages 13 1.3.2 Lightning-impulse voltages 13 1.3.3 Switching surges 14 1.3.4 Very fast transient tests (VFTT) 14 1.3.5 Direct-voltage tests 14

1.4 Power system developments 15 1.4.1 Reliability requirements 15 1.4.2 Condition of present assets 15 1.4.3 Extension of power system life 16 1.4.4 New systems and equipment 16

1.5 Future insulation monitoring requirements 17 1.6 Summary 17 1.7 References 17 1.8 Problems 18

2 Insulating materials utilized in power-system equipment 21 2.1 Review of insulating materials 22

2.1.1 Gases 22 2.1.2 Vacuum 25 2.1.3 Liquids 25 2.1.4 Solids 27

vi Condition Assessment of High- Voltage Insulation

2.2 Characterization of insulation condition 33 2.2.1 Permittivity (e) and capacitance (C) 33 2.2.2 Resistivity (p) and insulation resistance (IR) 33 2.2.3 Time constants 34 2.2.4 Dielectric dissipation factor 34 2.2.5 Partial discharges (PD) 35 2.2.6 Physical and chemical changes 35

2.3 Modes of deterioration and failure of practical insulating materials 36 2.3.1 Dielectric losses 37 2.3.2 Partial discharges - sources, forms and effects 39 2.3.3 Ageing effects 46

2.4 Electrical breakdown and operating stresses 48 2.5 Development of insulation applications 50 2.6 Summary 50 2.7 References 51 2.8 Standards related to insulating materials 53 2.9 Problems 54

Introduction to electrical insulation design concepts 55 3.1 Overview of insulation design requirements 55

3.1.1 Electrical requirements 5 6 3.1.2 Physical limitations 56 3.1.3 Working environment 5 6 3.1.4 Mechanical requirements 57 3.1.5 Thermal conditions 58 3.1.6 Processing 58 3.1.7 Reliability 59

3.2 Electric stress distributions in simple insulation systems 60 3.2.1 Multiple dielectric systems 61 3.2.2 Edge effects 64 3.2.3 Multiple electrode configurations 66

3.3 Electric stress control 68 3.4 Summary 69 3.5 References 69 3.6 Problems 70

Insulation defects in power-system equipment: Part 1 71 4.1 Suspension and post insulators 71

4.1.1 Suspension (string) insulators 71 4.1.2 Post insulators 73

4.2 High-voltage bushings 74 4.3 High-voltage instrument transformers 77

4.3.1 Oil-impregnated current transformers 78 4.3.2 Dry-type current transformers 80 4.3.3 Capacitor-type voltage transformers - CVT 81

4.4 4.5 4.6 4.7 4.8

4.9 4.10 4.11 4.12

List of contents

High-voltage power capacitors High-voltage surge arresters High-voltage circuit breakers Gas-insulated systems (GIS) High-voltage cables 4.8.1 Oil-paper cables 4.8.2 Extruded cables Summary References Standards related to Chapter 4 Problems

Insulation defects in power-system equipment: Part 2 5.1

5.2

5.3 5.4 5.5

Electrical rotating machines 5.1.1 Low-voltage motors 5.1.2 High-voltage machines 5.1.3 Possible insulation failure mechanisms in rotating

machines 5.1.4 OGRE summary of expected machine insulation

degradation 5.1.5 Future of machine insulation Transformers and reactors 5.2.1 Windings 5.2.2 Transformer insulation structures Summary References Problems

vii

82 83 84 86 87 87 90 93 93 95 96

97 97 97 98

100

103 103 104 104 106 118 118 120

Basic methods for insulation assessment 121 6.1 Generation and measurement of test high voltages 122

6.1.1 Power-frequency voltages 122 6.1.2 High-frequency voltages 127 6.1.3 Very-low-frequency voltages (VLF) 128 6.1.4 Direct voltages 128 6.1.5 Hybrid test circuits 129 6.1.6 Lightning impulse voltages 129 6.1.7 Switching surge voltages 133 6.1.8 High-voltage equipment for on-site testing 133

6.2 Non-destructive electrical measurements 135 6.2.1 Insulation resistance (IR) measurements 135 6.2.2 Measurements of the dielectric dissipation factor

(DDF) 137 6.2.3 Measurement of partial discharges by electrical

methods 140 6.2.4 Dielectric response measurements 147

viii Condition Assessment of High- Voltage Insulation

6.3 Physical and chemical diagnostic methods 150 6.3.1 Indicators of in-service condition of oil-paper

systems 150 6.3.2 Analysis of SFö samples from GIS 153 6.3.3 Surface deterioration of composite insulators 153 6.3.4 Water treeing in XLPE cable insulation 15 3 6.3.5 Ultrasonic methods for detection of partial

discharges 154 6.3.6 Miscellaneous techniques 154

6.4 Summary 154 6.5 References 154 6.6 Standards related to basic test methods 157 6.7 Problems 158

Established methods for insulation testing of specific equipment 159 7.1 Overhead line and substation insulators 160

7.1.1 Porcelain and glass insulators (overhead lines) 161 7.1.2 Ceramic and glass insulators (post type - indoor and

outdoor) 161 7.1.3 Composite insulators for overhead lines (string and post

units) 162 7.2 Overhead line and substation hardware 162 7.3 Surge arresters 163 7.4 Switchgear 164

7.4.1 Circuit breakers 164 7.4.2 Self-protected switchgear 166 7.4.3 Disconnectors (isolators) 166 7.4.4 Metal-enclosed switchgear 166 7.4.5 Transformer tap changers 167

7.5 Bushings 167 7.6 High-voltage instrument transformers 168

7.6.1 Current transformers 168 7.6.2 Inductive voltage transformers 169 7.6.3 Capacitor voltage transformers 170

7.7 High-voltage power capacitors 171 7.8 High-voltage rotating machines 171

7.8.1 Stator bars 172 7.8.2 Assembled machine 172

7.9 High-voltage cables 173 7.9.1 Oil-impregnated cables 173 7.9.2 Extruded cables 173

7.10 Distribution and power transformers 175 7.10.1 Power-frequency overvoltage withstand tests 175 7.10.2 Partial-discharge tests 177 7.10.3 Summary of transformer HV test requirements 180

List of contents ix

7.10.4 Additional tests 182 7.11 Dielectric testing of HVDC equipment 182 7.12 Miscellaneous items 184 7.13 Summary 184 7.14 References 184 7.15 Standards related to Chapter 7 185 7.16 Problems 188

Sensors for insulation condition monitoring 189 8.1 Ultra-high-frequency sensors 189 8.2 Optical-fibre sensors 190

8.2.1 Basic physics of optical-fibre sensing 193 8.2.2 Optical-fibre PD sensors 194 8.2.3 Optical-fibre temperature sensors 196 8.2.4 Advantages and disadvantages of optical-fibre

sensors 199 8.3 Directional sensors for PD measurements 200

8.3.1 Directional coupler sensor 200 8.3.2 Directional field sensor 201

8.4 Summary 203 8.5 References 203 8.6 Problems 205

Online insulation condition monitoring techniques 207 9.1 The main problems with offline condition monitoring 207 9.2 Noise-mitigation techniques 208

9.2.1 Noise gating 209 9.2.2 Differential methods 211 9.2.3 Noise identification by signal waveform analysis 214 9.2.4 Multiple terminal PD measurements 215

9.3 Non-electrical online condition monitoring 219 9.3.1 Temperature monitoring of the insulations 219 9.3.2 Online DGA 219 9.3.3 Acoustic-based techniques for PD detection 222

9.4 Online acoustic/electric PD location methods for transformers 224 9.4.1 Acoustic transducers and winding terminal

measurements 224 9.4.2 Application of internal combined acoustic and

VHF/UHF transducers 224 9.5 Electrical online condition monitoring 225

9.5.1 Online dielectric dissipation factor and capacitance measurements 227

9.5.2 Online leakage current measurement 228 9.5.3 Electrical online PD detection 230

9.6 Summary 236

x Condition Assessment of High- Voltage Insulation

9.7 References 236 9.8 Problems 239

10 Artificial-intelligence techniques for incipient fault diagnosis and condition assessment 241 10.1 Database for condition assessment 241

10.1.1 A computer database and diagnostic program 242 10.1.2 A combined method for DGA diagnosis 243

10.2 Fuzzy-logic fault diagnosis 244 10.2.1 The conventional methods 245 10.2.2 A fuzzy-logic method 245

10.3 Asset analysis and condition ranking 255 10.3.1 Equipment ranking according to the insulation

condition 255 10.3.2 Insulation health index 255 10.3.3 Membership functions of fuzzy set 256 10.3.4 Example of fuzzy logic condition ranking 258

10.4 Summary 262 10.5 References 263 10.6 Problems 264

Appendix 1 List of Abbreviations 265

Appendix 2 Major standards organizations 266

Appendix 3 Answers to problems 267

Index 270