Rs. 195.00 FUNDAMENTALS OF POWER SYSTEM PROTECTION Y.G. Paithankar and S.R. Bhide O 2003 by Prentice-Hall of lndia Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-81-203-2194-4 The export rights of this book are vested solely with the publisher. Published by Asoke K. Ghosh, Prentice-Hall of lndia Private Limited. M-97, Connaught Circus, New Oelhi-110001 and Printed by Meenakshi Printers, Delhi-110006. Fundamentals of Power System Protection Y.G. Paithankar Formerly Professor and Head Electrical Engineering Department Visvesvaraya National Institute of Technolorn Nagpur S.R. Bhide Assistant Professor Electrical Engineering Department Visvesvaraya National Institute of Technology Nagpur . -.. - - I --IL -,f India Saeed Book Bank hc Delhi - 11 2003 Dependence of Modern Society on Electric Supply I Faults and Abnormal Operating Conditions I 1.2.1 Shunt Faults (Short Circuits) 1 1.2.2 Causes of Shunt Faults 3 1.2.3 Effects of Shunt Faults 3 1.3 Classification of Shunt Faults 5 1.3.1 Phase Faults and Ground Faults 5 1.3.2 Phasor Diagram of Voltages and Currents During Various Faults 5 1.3.3 Series Faults 7 Abnormal Operating Conditions 9 1.4.1 Should Protective Relays Trip During Abnormal Operating Conditions? 9 1.4.2 Can Protective Relays Prevent Faults? 9 .3 What are Protective Relays Supposed to Do? 9 volution of Power Systems 9 1.5.1 Isolated Power System 10 1.5.2 Interconnected Power System 10 1.5.3 Negative Synergy of an Interconnected System 10 ates of Operation of a Power System 11 .5.5 From Natural Monopoly to the Deregulated Power System 12 Protection System and Its Attributes 13 .6.1 Sensitivity 14 .6.2 Selectivity 14 ?- .6.4 Reliability and Dependability 14 ystem Transducers 14 .7.1 Current Transformer 15 nsforrner 16 iii 1.7.3 Circuit Breaker 17 1.7.4 Trip Circuit of a CB 17 17 5 Organization of Protection 17 1.7 6 Zones of Protection 19 1 7.7 Primary and Back-up Protection 20 1.7.8 Maloperations 22 1.8 Various Power System Elements That Need Protection 23 1.9 Various Principles of Power System Protection 23 Reuiew Questions 24 Problems 25 2 OVER-CURRENT PROTECTION OF TRANSMISSION LINES 26-56 2.1 Introduction 26 2 2 Fuse 26 2.3 Thermal Relays 27 2.4 0; er-current Relay 28 2.4.1 Instantaneous OC Relay 29 2.4.2 Definite Time Over-current Relay 30 2.4.3 Inverse Time Over-current Relay 30 2.5 Implementation of Over-current Relay Using Induction Disk 32 2.6 Application of Definite Time OC Relays for Protection of a Distribution Feeder 35 2.7 Application of Inverse Definite Minimum Time Relay on a Distnbution Feeder 37 2.7.1 Choice Between IDMT and DTOC Relays 42 2.8 Protection of a Three-phase Feeder 42 2.9 Directional Over-current Relay 44 2.9 1 Other Situations Where Directional OC Relays are Necessary 45 2.9.2 Phasor Diagram for Voltage and Current for Forward and Reverse Fault (Single-phase System) 47 * 1 2.9.3 Application of Directional Relay to a Three-phase Feeder 49 2.9.4 Directional OC Protection of a Three-phase Feeder 52 2.9.5 Directional Protection Under Non-fault Conditions (Reverse Power Relay) 53 2.10 Drawbacks of Over-current Relays 54 Reuzew Questions 55 Problems 56 3 DIFFERENTIAL PROTECTION 57-73 3.1 Introduction 57 j \ 3.2 Dot Markings 57 3.3 Simple Differential Protection 59 3.3.1 Simple Differential Protection: Behaviour During Load 59 I 3.3.2 Simple Differential Protection: Behaviour During External Fault 60 I 3.3.3 Simple Differential Protection: Behaviour During Internal Fault 60 3.3.4 Simple Differential Protection, Double-end-fed: Behaviour I During Internal Fault 61 i3unr~11rs Y 3.4 Zone of Protection of the Differential Relay 62 3 5 Actual Behavlour of a Simple Dlfferentlal Scheme 63 3 5 1 Through Fault Stability and Stabillcy Ratio 64 3.5.2 Equivalent Circuit of CT 65 3.6 Percentage Differential Relay 67 3.6.1 Block Diagram of Percentage Differential Relay 70 3.7 Earth Leakage Protection 71 3.7.1 Earth Leakage Protection for Single-phase Load 71 3.7.2 Earth Leakage Protection for Three-phase Loads 72 Reuzew QuestLons 73 Problems 73 4 TRANSFORMER PROTECTION 74-100 4.1 Types of Transformers 74 4.2 Phasor Diagram for a Three-phase Transformer 75 4.3 Equivalent Circuit of Transformer 77 4.4 Types of Faults in Transformers 78 4 5 Over-current Protection 80 4.6 Percentage Differential Protection of Transformers 81 4.6 1 Development of Connections 81 4.6.2 Phase c-to-Ground (c-g) External Fault 82 4.6.3 Phase c-to-Ground (c-g) Internal Fault 84 4.7 Inrush Phenomenon 86 4.7.1 Percentage Differential Relay with Harmonic Restraint 89 4.8 High Resistance Ground Faults in Transformers 91 4.8 1 High Resistance Ground Faults on the Delta Side 91 4 8.2 High Resistance Ground Faults on the Star Side 92 4.9 Inter-turn Faults in Transformers 93 * 4 10 Incipient Faults in Transformers 93 4.10.1 Buchholz Relay 93 4.10.2 Analysis of Trapped Gases 95 4.11 Phenomenon of Over-fluxing in Transformers 95 4.11.1 Protection Against Over-fluxing 95 4.12 Transformer Protection Application Chart 96 4.13 An Illustrative Numerical Problem 97 Reuzew Questions 99 Problems 100 5 BUSBAR PROTECTION 101-117 ., 5.1 Introduction 101 5.2 Differential Protection of Busbars 102 5.2.1 Selection of CT Ratios in Case of Busbar Protection: Wrong Method 102 5.2.2 Selection of CT Ratios in Case of Busbar Protection: Correct Method 103 5 3 External and Internal Fault 104 .5.4 Actual Behaviour of a Protective CT 105 5.5 Circuit Model of Saturated CT 108 5.6 External Fault with One CT Saturated: Need for High Impedance Busbar Protection 108 5.7 Minimum Internal Fault That Can Be Detected by the High Impedance Busbar Differential Scheme 110 5.8 Stability Ratio of High Impedance Busbar Differential Scheme 112 5.9 Supervisory Relay 112 5.10 Protection of Three-phase Busbars 114 5.11 Numerical Example on Design of High Impedance Busbar Differential Scheme 115 Review Questions 117 6 DISTANCE PROTECTION OF TRANSMISSION LINES 118-152 6.1 Drawbacks of Over-current Protection 118 6.2 Introduction to Distance Protection 119 6.3 Simple Impedance Relay 123 6.3.1 Trip Law for Simple Impedance Relay Using Universal Torque Equation 123 6.3.2 Implementation of Simple Impedance Relay Using Balanced Beam Structure 124 6.3.3 Performance of Simple Impedance Relay During Normal Load Flow 126 6.3 4 Effect of Arc Resistance on Reach of Simple Impedance Relay 126 6.3.5 Directional Property Exhibited by Simple Impedance Relay 127 6.3.6 Performance of Simple Impedance Relay During Power Swing 127 6.4 Reactance Relay 129 6.4.1 Trip Law for Reactance Relay Using Universal Torque Equation 129 6.4.2 Implementation of Reactance Relay Using the Induction Cup Structure 131 6.4.3 Performance of Reactance Relay During Normal Load Flow 131 6.4.4 Effect of Arc Resistance on Reach of Reactance Relay 132 6.4.5 Directional Property Exhibited by Reactance Relay 133 6.4.6 Performance of Reactance Relay During Power Swing 134 6.5 Mho Relay 134 6.5.1 Trip Law for Mho Relay Using Universal Torque Equation 134 6.5.2 Implementation of Mho Relay Using Induction Cup Structure 135 6.5.3 Performance of Mho Relay During Normal Load Flow 135 6.5.4 Effect of Arc Resistance on Mho Relay Reach 136 6.5.5 Directional Property Exhibited by Mho Relay 137 6.5.6 Performance of Mho Relay During Power Swing 138 6.6 Comparison Between Distance Relays 139 6.7 Distance Protection of a Three-phase Line 139 6.7.1 Phase Faults 141 6.7.2 Ground Faults 142 6.7.3 Complete Protection of a Three-phase Line 144 ... YIII Coi~rrf~ts 9 INDUCTION MOTOR PROTECTION 184-195 ' 9.1 Introduction 184 9.2 Various Faults and Abnormal Operating Conditions 184 9.3 Starting Current 185 9.4 Electrical Faults 186 9.4 1 Fault on Motor Terminals 186 9.4.2 Phase Faults Inside the Motor 186 9.4.3 Ground Faults Inside the Motor 188 9.4.4 Inter-turn Faults 189 9.5 Abnormal Operating Conditions from Supply Side 189 9.5.1 Unbalanced Supply Voltage 189 9.5.2 Single Phasing 191 9.5.3 Reduction in Supply Voltage 192 9.5.4 Reversal of Phases 192 9.6 Abnormal Operating Conditions from Mechanical Side 192 9.6.1 Failure of Bearing and Rotor Jam 192 9.6.2 Overload 192 9.7 Data Required for Designing Motor Protection Schemes 195 Review Questions 195 10 STATIC COMPARATORS AS RELAYS 196-221 10.1 Comparison vs Computation 196 10.2 Amplitude Comparator 196 10.3 Phase comparator 199 10.3.1 The Cosine-type Phase Comparator 199 10.3.2 The Sine-type Phase Comparator 200 10.4 Duality Between Amplitude and Phase Comparators 201 10.5 Synthesis of Various Distance Relays Using Static Comparators 204 10.5.1 Synthesis of Mho Relay Using Static Phase Comparator 204 e 10.5.2 Synthesis of Reactance Relay Using Cosine-type Phase Comparator 208 10.5.3 Synthesis of Simple Impedance Relay Using Amplitude Comparator 210 10.6 Development of an Electronic Circuit for Implementing a Cosine-type Phase Comparator 210 10.7 An Electronic Circuit for Implementing a Sine-type Phase Comparator 216 10.8 Synthesis of Quadrilateral Distance Relay 218 Review Questions 221 11 NUMERICAL PROTECTION 11.1 Introduction 222 11.2 Block Diagram of Numerical Relay 223 11.3 Sampling Theorem 225 11.4 Correlation with a Reference Wave 228 11.4.1 Fourier Analysis of Analogue Signals 229 I 11.5 Least Error Squared (LES) Technique 237 . . --- Conrcnis vii 6.8 Reasons for Inaccuracy of Distance Relay Reach 145 6.9 Three-stepped Distance Protechon 146 6.9.1 First Step 146 6.9.2 Second Step 146 6.9.3 Third Step 147 6.10 Trip Contact Configuration for the Three-stepped Distance Protection 149 6.11 Three-stepped Protection of Three-phase Line against All Ten Shunt Faults 150 6.12 Impedance Seen from Relay Side 150 6.13 Three-stepped Protection of Double-end-fed Lines 151 Review Questions 152 e 7 CARRIER-AIDED PROTECTION OF TRANSMISSION LINES 153-167 7.1 Need for Carrier-aided Protection 153 7.2 Various Options for a Carrier 155 7.3 Coupling and Trapping the Carrier into the Desired Line Section 155 7.3.1 Single Line-to-ground Coupllng 157 7.3.2 Line-to-line Coupling 157 7.4 Unit Type Carrier-aided Directional Comparison Relaying 158 7.5 Carrier-aided Distance Schemes for Acceleration of Zone I1 160 7.5.1 Transfer Trip or Inter-trip 160 7.5.2 Permissive Inter-trip 161 7.5.3 Acceleration of Zone I1 161 7.5.4 Pre-acceleration of Zone I1 161 7.6 Phase Comparison Relaylng (Unit Scheme) 163 Review Questions 167 8 GENERATOR PROTECTION 168-183 8.1 Introduction 168 8.2 Electrical Circuit of the Generator 169 8.3 Various Faults and Abnormal Operating Conditions 172 8.3.1 Stator Faults 173 8.3.2 Stator Phase and Ground Faults 173 8.3.3 Transverse Differential Protection 174 8.4 Rotor Faults 175 8.5 Abnormal Operating Conditions 176 8.5.1 Unbalanced Loading 176 8.5.2 Over-speeding 178 8.5.3 Loss of Excltation 179 8.5.4 Protection Against Loss of Excitation Using Offset Mho Relay 180 8.5.5 Loss of Prime Mover 181 Revzew Questions 183 .. Co~zrr~lts jx ~ 11.6 Digital Filtering 239 11.6.1 Simple Low-pass Filter 239 11.6.2 Simple High-pass PLlter 240 11.6.3 Finite Impulse Response (XR) Filters 241 11.6.4 Infinite Impulse Response (IIR) Filter 242 11.6.5 Comparison Between FIR and IIR Filters 243 11.7 Numerical Over-current Protection 243 11.8 Kumerical Transformer Differential Protection 245 11.9 Numerical Distance Protection of Transmission Line 245 -, 11.9.1 Mann and Morrison Method 245 11.9.2 Differential Equation Method 247 11.10 Algorithms and Assumptions 253 Review Questions 254 @ Appendix A-CT AND PT ERRORS 255-267 A.l Introduction 255 A.2 CT Construction 255 A.3 Measurement CT and Protective CT 255 A.4 Steady State Ratio and Phase Angle Errors 256 A.4.1 Current Transformer 256 A.4.2 Potential Transformer 259 A.5 Transient Errors in CT 261 A.6 Transient Errors in CVT 264 A.7 Saturation of CT 265 A.8 CT Accuracy Classification 267 1 Appendix &POWER SWING 268-274 B.l Introduction 268 B.2 Stable and Unstable Power Swing 268 B.3 Impedance Seen by Relay During Power Swing 270 0. B.4 Out-of-step Blocking Scheme 272 B.5 Out-of-step Tripping Scheme 274 Appendix CPROTECTION OF LONGESTAND SHORTEST LINES 275-279 C. 1 Introduction 275 C.2 Longest Line That Can Be Protected 275 C.3 Shortest Line That Can Be Protected 277 REFERENCES 281 -283 INDEX 285-287