PROTECTION of electricity distribution networks Juan M. Gers and Edward J. Holmes The Institution of Electrical Engineers
Contents Preface and acknowledgments x 1 Introduction 1 1.1 Basic principles of electrical Systems 1 1.2 Protection requirements 5 1.3 Protection zones 5 1.4 Primary and back-up protection 5 1.4.1 Primary protection 7 1.4.2 Back-up protection 7 1.5 Directional protection 7 2 Calculation of short-circuit currents 11 2.1 Mathematical derivation of fault currents 11 2.2 Methods for calculating short-circuit currents 19 2.2.1 Importance and construction of sequence networks 21 2.2.2 Calculation of asymmetrical faults using symmetrical components 25 2.2.3 Equivalent impedances for a power system 26 2.3 Supplying the current and voltage Signals to protection Systems 26 2.4 Calculation of faults by Computer 28 3 Classification and function of relays 33 3.1 Classification 33 3.1.1 General function 33 3.1.2 Construction 33 3.1.3 Incoming signal 33 3.1.4 Type of protection 34 3.1.5 International identification of electrical devices 34 3.2 Electromagnetic relays 35 3.2.1 Attraction relays 35 3.2.2 Relays with moveable coils 36 3.2.3 Induction relays 37
vi Contents 3.3 Computerised protection 40 3.3.1 Characteristics of computerised relays 41 3.3.2 Architectures 49 3.4 Supplies to the relay circuits 43 Current and voltage transformers 44 4.1 Voltage transformers 44 4.1.1 Equivalent circuit 45 4.1.2 Errors 4g 4.1.3 Bürden 46 4.1.4 Selection of VTs 46 4.1.5 Capacitor voltage transformers 48 4.2 Current transformers 50 4.2.1 Equivalent circuit 52 4.2.2 Errors 4.2.3 AC Saturation 4.2.4 Bürden 4.2.5 Selection of CTs 4.2.6 DC Saturation 4.2.7 Precautions when working with CTs Overcurrent protection gr 5.1 Types of overcurrent relay 55 5.1.1 Definite-current relays 55 5.1.2 Definite-time/current or definite-time relays 67 5.1.3 Inverse-time relays gg 5.2 Setting overcurrent relays gg 5.2.1 Setting instantaneous units 5g 5.2.2 Coverage of instantaneous units protecting lines between substations 71 5.2.3 Setting the time delay units on overcurrent relays 73 5.2.4 Co-ordination with fuses 77 5.3 Co-ordination across Dy transformers 84 5.4 Check of relay co-ordination requirements g6 5.4.1 Minimum short-circuit levels g 5.4.2 Thermal limits g7 5.4.3 Pick-up values g7 5.5 Setting overcurrent relays using Software techniques 99 Fuses, reclosers and sectionalisers 207 6.1 Equipment IQ7 6.1.1 Reclosers 207 6.1.2 Sectionalisers 212 6.1.3 Fuses 213 52 52 53 53 61 63
Contents vii 6.2 Criteria for co-ordination of time/current devices in distribution Systems 116 6.2.1 Fuse-fuse co-ordination 116 6.2.2 Recloser-fuse co-ordination 117 6.2.3 Recloser-recloser co-ordination 119 6.2.4 Recloser-relay co-ordination 122 6.2.5 Recloser-sectionaliser co-ordination 123 6.2.6 Recloser-sectionaliser-fuse co-ordination 123 Directional overcurrent relays 127 7.1 Construction 127 7.2 Principle of Operation 128 7.3 Relay connections 129 7.3.1 30 connection (0 AMT) 129 7.3.2 60 connection (0 AMT) 129 7.3.3 90 connection (30 AMT) 130 7.3.4 90 connection (45 AMT) 130 7.4 Directional earth-fault relays 131 7.5 Co-ordination of instantaneous units 137 7.6 Setting of time-delay directional overcurrent units 142 7.6.1 TAP setting 142 7.6.2 DIAL setting 142 Differential protection 149 8.1 Classification of differential protection 152 8.2 Transformer differential protection 152 8.2.1 Basic considerations 153 8.2.2 Selection and connection of CTs 154 8.2.3 Percentage of winding protected by the differential relay during an earth fault 161 8.2.4 Determination of the slope 162 8.2.5 Distribution of fault current in power transformers 163 8.3 Differential protection for generators and rotating machines 166 8.4 Line-differential protection 166 8.5 Busbar-differential protection 170 8.5.1 Differential system with multiple restraint 170 8.5.2 High impedance differential system 170 Distance protection 176 9.1 Types of distance relay 177 9.1.1 Impedance relay 179 9.1.2 Directional relay 182 9.1.3 Reactance relay 183 9.1.4 Mho relay 184 9.1.5 Completely polarised mho relay 186
viii Contents 9.1.6 Relays with lens characteristics 187 9.1.7 Relays with polygonal characteristics 188 9.1.8 Relays with combined characteristics 188 9.2 Setting the reach and operating time of distance relays 189 9.3 The effect of infeeds on distance relays 193 9.4 The effect of arc resistance on distance protection 197 9.5 Residual compensation 200 9.6 Impedances seen by distance relays 201 9.6.1 Phase units 201 9.6.2 Earth-fault units 201 9.7 Power-system oscillations 202 9.8 The effective cover of distance relays 206 9.9 Maximum load check 207 9.10 Drawing relay settings 209 9.11 Intertripping schemes 219 9.11.1 Under reach with direct tripping 219 9.11.2 Permissive under reach intertripping 220 9.11.3 Permissive over reach intertripping 221 9.12 Distance relays on series-compensated lines 222 9.13 Technical considerations of distance protection in tee circuits 224 9.13.1 Tee connection with infeeds at two terminals 225 9.13.2 Tee connection with infeeds at all three terminals 226 9.14 Use of distance relays for the detection of the loss of excitation in generators 228 10 Protection of industrial Systems 233 10.1 Protection devices 233 10.1.1 Overcurrent relays 233 10.1.2 Direct-acting devices in power and moulded-case circuit breakers 234 10.1.3 Combined thermal relay contactor and fuse 234 10.2 Criteria for setting overcurrent protection devices associated with motors 234 10.2.1 Thermal relays 235 10.2.2 Low voltage breakers 238 11 Industrial plant load shedding 247 11.1 Power-system Operation after loss of generation 247 11.2 Design of an automatic load-shedding system 249 11.2.1 Simple machine model 249 11.2.2 Parameters for implementing a load-shedding system 250 11.3 Criteria for setting frequency relays 251 11.3.1 Operating times 251 11.3.2 Determination of the frequency Variation 252
Contents ix 11.4 Example of calculating and setting frequency relays in an industrial plant 252 11.4.1 Calculation of overload 252 11.4.2 Loadtobeshed 252 11.4.3 Frequency levels 253 11.4.4 Load-shedding stages 254 11.4.5 Determination of the frequency relay settings 254 11.4.6 Verification of Operation 257 12 Protection schemes and substation design diagrams 261 12.1 Protection schemes 261 12.1.1 Generator protection 261 12.1.2 Motor protection 262 12.1.3 Transformer protection 268 12.1.4 Line protection 269 12.2 Substation-design diagrams 273 12.2.1 Single-line diagrams 274 12.2.2 Substation-layout diagrams 274 12.2.3 Diagrams ofac connections 276 12.2.4 Diagrams of DC connections 277 12.2.5 Wiring diagrams 278 12.2.6 Logic diagrams 280 12.2.7 Cabling lists 280 13 Installation, testing and maintenance of protection Systems 281 13.1 Installation of protection equipment 281 13.2 Testing protection schemes 283 13.2.1 Factorytests 283 13.2.2 Precommissioning tests 283 13.2.3 Periodic maintenance 288 13.3 Setting and testing numerical protection 291 13.3.1 Setting the parameters 292 13.3.2 Commissioning tests 292 13.3.3 Operating tests 292 Bibliography 294 Appendix Solutions to exercises 297 Index 338