PROTECTION of electricity distribution networks

Transcription

1 PROTECTION of electricity distribution networks Juan M. Gers and Edward J. Holmes The Institution of Electrical Engineers

2 Contents Preface and acknowledgments x 1 Introduction Basic principles of electrical Systems Protection requirements Protection zones Primary and back-up protection Primary protection Back-up protection Directional protection 7 2 Calculation of short-circuit currents Mathematical derivation of fault currents Methods for calculating short-circuit currents Importance and construction of sequence networks Calculation of asymmetrical faults using symmetrical components Equivalent impedances for a power system Supplying the current and voltage Signals to protection Systems Calculation of faults by Computer 28 3 Classification and function of relays Classification General function Construction Incoming signal Type of protection International identification of electrical devices Electromagnetic relays Attraction relays Relays with moveable coils Induction relays 37

3 vi Contents 3.3 Computerised protection Characteristics of computerised relays Architectures Supplies to the relay circuits 43 Current and voltage transformers Voltage transformers Equivalent circuit Errors 4g Bürden Selection of VTs Capacitor voltage transformers Current transformers Equivalent circuit Errors AC Saturation Bürden Selection of CTs DC Saturation Precautions when working with CTs Overcurrent protection gr 5.1 Types of overcurrent relay Definite-current relays Definite-time/current or definite-time relays Inverse-time relays gg 5.2 Setting overcurrent relays gg Setting instantaneous units 5g Coverage of instantaneous units protecting lines between substations Setting the time delay units on overcurrent relays Co-ordination with fuses Co-ordination across Dy transformers Check of relay co-ordination requirements g Minimum short-circuit levels g Thermal limits g Pick-up values g7 5.5 Setting overcurrent relays using Software techniques 99 Fuses, reclosers and sectionalisers Equipment IQ Reclosers Sectionalisers Fuses

4 Contents vii 6.2 Criteria for co-ordination of time/current devices in distribution Systems Fuse-fuse co-ordination Recloser-fuse co-ordination Recloser-recloser co-ordination Recloser-relay co-ordination Recloser-sectionaliser co-ordination Recloser-sectionaliser-fuse co-ordination 123 Directional overcurrent relays Construction Principle of Operation Relay connections connection (0 AMT) connection (0 AMT) connection (30 AMT) connection (45 AMT) Directional earth-fault relays Co-ordination of instantaneous units Setting of time-delay directional overcurrent units TAP setting DIAL setting 142 Differential protection Classification of differential protection Transformer differential protection Basic considerations Selection and connection of CTs Percentage of winding protected by the differential relay during an earth fault Determination of the slope Distribution of fault current in power transformers Differential protection for generators and rotating machines Line-differential protection Busbar-differential protection Differential system with multiple restraint High impedance differential system 170 Distance protection Types of distance relay Impedance relay Directional relay Reactance relay Mho relay Completely polarised mho relay 186

5 viii Contents Relays with lens characteristics Relays with polygonal characteristics Relays with combined characteristics Setting the reach and operating time of distance relays The effect of infeeds on distance relays The effect of arc resistance on distance protection Residual compensation Impedances seen by distance relays Phase units Earth-fault units Power-system oscillations The effective cover of distance relays Maximum load check Drawing relay settings Intertripping schemes Under reach with direct tripping Permissive under reach intertripping Permissive over reach intertripping Distance relays on series-compensated lines Technical considerations of distance protection in tee circuits Tee connection with infeeds at two terminals Tee connection with infeeds at all three terminals Use of distance relays for the detection of the loss of excitation in generators Protection of industrial Systems Protection devices Overcurrent relays Direct-acting devices in power and moulded-case circuit breakers Combined thermal relay contactor and fuse Criteria for setting overcurrent protection devices associated with motors Thermal relays Low voltage breakers Industrial plant load shedding Power-system Operation after loss of generation Design of an automatic load-shedding system Simple machine model Parameters for implementing a load-shedding system Criteria for setting frequency relays Operating times Determination of the frequency Variation 252

6 Contents ix 11.4 Example of calculating and setting frequency relays in an industrial plant Calculation of overload Loadtobeshed Frequency levels Load-shedding stages Determination of the frequency relay settings Verification of Operation Protection schemes and substation design diagrams Protection schemes Generator protection Motor protection Transformer protection Line protection Substation-design diagrams Single-line diagrams Substation-layout diagrams Diagrams ofac connections Diagrams of DC connections Wiring diagrams Logic diagrams Cabling lists Installation, testing and maintenance of protection Systems Installation of protection equipment Testing protection schemes Factorytests Precommissioning tests Periodic maintenance Setting and testing numerical protection Setting the parameters Commissioning tests Operating tests 292 Bibliography 294 Appendix Solutions to exercises 297 Index 338