Protection of electricity distribution networks 2nd ed

Contents Preface and acknowledgements xi Preface to 2nd edition xiii 1 Introduction 1 1.1 General 1 1.2 Basic principles of electrical systems 3 1.3 Protection requirements 3 1.4 Protection zones 5 1.5 Primary and back-up protection 5 1.5.1 Primary protection 5 1.5.2 Back-up protection 5 1.6 Directional protection 7 1.7 Exercise 1.1 9 2 Calculation of short-circuit currents 11 2.1 Modelling for short-circuit current calculations 11 2.1.1 Effect of the system impedance 11 2.1.2 Effect of rotating machinery 13 2.1.3 Types of fault duty 14 2.1.4 Calculation of fault duty values 16 2.2 Methods for calculating short-circuit currents 19 2.2.1 Importance and construction of sequence networks 22 2.2.2 Calculation of asymmetrical faults using symmetrical components 23 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 31 3.1 Classification 31 3.1.1 Construction 31 vi Contents 3.1.2 Incoming signal 31 3.1.3 Function 32 3.1.4 International identification of electrical devices 32 3.2 Electromechanical relays 33 3.2.1 Attraction relays 33 3.2.2 Relays with moveable coils 34 3.2.3 Induction relays 35 3.3 Evolution of protection relays 38 3.4 Numerical protection 39 3.4.1 General 39 3.4.2 Characteristics of numerical relays 39 3.4.3 Typical architectures of numerical relays 40 3.4.4 Standard functions of numerical relays 41 3.5 Supplies to the relay circuits 43 4 Current and voltage transformers 45 4.1 Voltage transformers 45 4.1.1 Equivalent circuit 45 4.1.2 Errors 46 4.1.3 Burden 47 4.1.4 Selection of VTs 47 4.1.5 Capacitor voltage transformers 47 4.2 Current transformers 51 4.2.1 Equivalent circuit 51 4.2.2 Errors 52 4.2.3 AC saturation 52 4.2.4 Burden 53 4.2.5 Selection of CTs 53 4.2.6 DC saturation 59 4.2.7 Precautions when working with CTs 59 5 Overcurrent protection 63 5.1 General 63 5.2 Types of overcurrent relay 63 5.2.1 Definite-current relays 63 5.2.2 Definite-time/current or definite-time relays 66 5.2.3 Inverse-time relays 66 5.3 Setting overcurrent relays 66 5.3.1 Setting instantaneous units 67 5.3.2 Coverage of instantaneous units protecting lines between substations 68 5.3.3 Setting the parameters of time delay overcurrent relays 70 Contents vii 5.4 Constraints of relay co-ordination 73 5.4.1 Minimum short-circuit levels 73 5.4.2 Thermal limits 73 5.4.3 Pick-up values 75 5.5 Co-ordination across Dy transformers 86 5.6 Co-ordination with fuses 96 5.7 Co-ordination of negative-sequence units 96 5.8 Overcurrent relays with voltage control 97 5.9 Setting overcurrent relays using software techniques 98 5.10 Use of digital logic in numerical relaying 99 5.10.1 General 99 5.10.2 Principles of digital logic 99 5.10.3 Logic schemes 100 5.11 Adaptive protection with group settings change 102 5.12 Exercises 104 6 Fuses, reclosers and sectionalisers 109 6.1 Equipment 109 6.1.1 Reclosers 109 6.1.2 Sectionalisers 113 6.1.3 Fuses 114 6.2 Criteria for co-ordination of time/current devices in distribution systems 117 6.2.1 Fuse-fuse co-ordination 117 6.2.2 Recloser-fuse co-ordination 117 6.2.3 Recloser-recloser co-ordination 120 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 7 Directional overcurrent relays 127 7.1 Construction 127 7.2 Principle of operation 128 7.3 Relay connections 128 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) 131 7.4 Directional earth-fault relays 131 7.5 Co-ordination of instantaneous units 137 7.6 Setting of time-delay directional overcurrent units 141 7.6.1 Pick-up setting 141 7.6.2 Time dial setting 142 7.7 Exercises 146 viii Contents 8 Differential protection 149 8.1 General 149 8.2 Classification of differential protection 152 8.3 Transformer differential protection 152 8.3.1 Basic considerations 153 8.3.2 Selection and connection of CTs 154 8.3.3 Percentage of winding protected by the differential relay during an earth fault 159 8.3.4 Determination of the slope 161 8.3.5 Distribution of fault current in power transformers 162 8.4 Differential protection for generators and rotating machines 164 8.5 Line differential protection 164 8.6 Busbar differential protection 168 8.6.1 Differential system with multiple restraint 168 8.6.2 High impedance differential system 169 8.7 Exercises 170 9 Distance protection 173 9.1 General 173 9.2 Types of distance relays 174 9.2.1 Impedance relay 176 9.2.2 Directional relay 179 9.2.3 Reactance relay 180 9.2.4 Mho relay 181 9.2.5 Completely polarised mho relay 182 9.2.6 Relays with lens characteristics 183 9.2.7 Relays with polygonal characteristics 183 9.2.8 Relays with combined characteristics 185 9.3 Setting the reach and operating time of distance relays 185 9.4 The effect of infeeds on distance relays 188 9.5 The effect of arc resistance on distance protection 193 9.6 Residual compensation 194 9.7 Impedances seen by distance relays 195 9.7.1 Phase units 195 9.7.2 Earth-fault units 196 9.8 Power system oscillations 196 9.9 The effective cover of distance relays 199 9.10 Maximum load check 200 9.11 Drawing relay settings 203 9.12 Intertripping schemes 212 9.12.1 Under reach with direct tripping 213 9.12.2 Permissive under reach intertripping 213 9.12.3 Permissive over reach intertripping 214 9.13 Distance relays on series-compensated lines 214 Contents ix 9.14 Technical considerations of distance protection in tee circuits 215 9.14.1 Tee connection with infeeds at two terminals 215 9.14.2 Tee connection with infeeds at all three terminals 218 9.15 Use of distance relays for the detection of the loss of excitation in generators 219 9.16 Exercises 222 10 Protection of industrial systems 225 10.1 Protection devices 225 10.1.1 Overcurrent relays 225 10.1.2 Direct acting devices in power and moulded-case circuit breakers 225 10.1.3 Combined thermal relay contactor and fuse 226 10.2 Criteria for setting overcurrent protection devices associated with motors 226 10.2.1 Thermal relays 226 10.2.2 Low voltage breakers 227 11 Industrial plant load shedding 239 11.1 Power system operation after loss of generation 239 11.2 Design of an automatic load shedding system 240 11.2.1 Simple machine model 240 11.2.2 Parameters for implementing a load shedding system 241 11.3 Criteria for setting frequency relays 242 11.3.1 Operating times 242 11.3.2 Determination of the frequency variation 243 11.4 Example of calculating and setting frequency relays in an industrial plant 243 11.4.1 Calculation of overload 243 11.4.2 Load to be shed 243 11.4.3 Frequency levels 243 11.4.4 Load shedding stages 243 11.4.5 Determination of the frequency relay settings 244 11.4.6 Verification of operation 247 12 Protection schemes and substation design diagrams 251 12.1 Protection schemes 251 12.1.1 Generator protection 251 12.1.2 Motor protection 252 12.1.3 Transformer protection 258 12.1.4 Line protection 261 12.2 Substation design diagrams 261 12.2.1 Single-line diagrams 262 12.2.2 Substation layout diagrams 263 x Contents 12.2.3 Diagrams of AC connections 264 12.2.4 Diagrams of DC connections 265 12.2.5 Wiring diagrams 266 12.2.6 Logic diagrams 268 12.2.7 Cabling lists 268 13 Processing alarms 269 13.1 General 269 13.2 Alarm processing methods 270 13.3 Expert systems 271 13.4 Equivalent alarms 272 13.5 Rules 273 13.6 Finger printing approach 273 13.7 Hypothesis approach 275 14 Installation, testing and maintenance of protection systems 283 14.1 Installation of protection equipment 283 14.2 Testing protection schemes 285 14.2.1 Factory tests 285 14.2.2 Precommissioning tests 285 14.2.3 Periodic maintenance 290 14.3 Commissioning numerical protection 292 14.3.1 Setting the parameters 292 14.3.2 Performance tests 293 Bibliography 297 Appendix: Solutions to exercises 301 Index 339