FUNDAMENTALS OF POWER SYSTEM PROTECTION

1 Rs. FUNDAMENTALS OF POWER SYSTEM PROTECTION Paithankar and 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 Paithankar Formerly Professor and Head Electrical Engineering Department Visvesvaraya National Institute of Technolorn Nagpur Bhide Assistant Professor Electrical Engineering Department Visvesvaraya National Institute of Technology Nagpur.

2 I --IL Saeed Book Bank -,f India hc Delhi - 11 2003 Dependence of Modern Society on Electric Supply I Faults and Abnormal Operating Conditions I Shunt Faults (Short Circuits) 1 Causes of Shunt Faults 3 Effects of Shunt Faults 3 classification of Shunt Faults 5 Phase Faults and Ground Faults 5 Phasor Diagram of Voltages and Currents During Various Faults 5 Series Faults 7 Abnormal Operating Conditions 9 Should Protective Relays Trip During Abnormal Operating Conditions? 9 Can Protective Relays Prevent Faults? 9 .3 What are Protective Relays Supposed to Do? 9 volution of POWER Systems 9 Isolated POWER SYSTEM 10 Interconnected POWER SYSTEM 10 Negative Synergy of an Interconnected SYSTEM 10 ates of Operation of a POWER SYSTEM 11.

3 From Natural Monopoly to the Deregulated POWER SYSTEM 12 PROTECTION SYSTEM and Its Attributes 13 . Sensitivity 14 . Selectivity 14 ?- . Reliability and Dependability 14 ystem Transducers 14 . Current Transformer 15 nsforrner 16 iii Circuit Breaker 17 Trip Circuit of a CB 17 1 7 5 Organization of PROTECTION 17 6 Zones of PROTECTION 19 1 Primary and Back-up PROTECTION 20 Maloperations 22 Various POWER SYSTEM Elements That Need PROTECTION 23 Various Principles of POWER SYSTEM PROTECTION 23 Reuiew Questions 24 Problems 25 2 OVER-CURRENT PROTECTION OF TRANSMISSION LINES 26-56 Introduction 26 2 2 Fuse 26 Thermal Relays 27 0.

4 Er-current Relay 28 Instantaneous OC Relay 29 Definite Time Over-current Relay 30 Inverse Time Over-current Relay 30 Implementation of Over-current Relay Using Induction Disk 32 Application of Definite Time OC Relays for PROTECTION of a Distribution Feeder 35 Application of Inverse Definite Minimum Time Relay on a Distnbution Feeder 37 Choice Between IDMT and DTOC Relays 42 PROTECTION of a Three-phase Feeder 42 Directional Over-current Relay 44 1 Other Situations Where Directional OC Relays are Necessary 45 Phasor Diagram for Voltage and Current for Forward and Reverse fault (Single-phase SYSTEM ) 47 1 Application of Directional Relay to a Three-phase Feeder 49 Directional OC PROTECTION of a Three-phase Feeder 52 Directional PROTECTION Under Non- fault Conditions * (Reverse POWER Relay) 53 Drawbacks of Over-current Relays 54 Reuzew Questions 55 Problems 56 3 DIFFERENTIAL PROTECTION 57-73 Introduction 57 j \ Dot Markings 57 Simple Differential PROTECTION 59 Simple Differential PROTECTION : Behaviour During Load 59 I Simple Differential PROTECTION : Behaviour During External fault 60 I Simple Differential PROTECTION .

5 Behaviour During Internal fault 60 Simple Differential PROTECTION , Double-end-fed: Behaviour I During Internal fault 61 i3unr~11rs Y 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 Equivalent Circuit of CT 65 Percentage Differential Relay 67 Block Diagram of Percentage Differential Relay 70 Earth Leakage PROTECTION 71 Earth Leakage PROTECTION for Single-phase Load 71 Earth Leakage PROTECTION for Three-phase Loads 72 Reuzew QuestLons 73 Problems 73 4 TRANSFORMER PROTECTION 74-100 Types of Transformers 74 Phasor Diagram for a Three-phase Transformer 75 Equivalent Circuit of

6 Transformer 77 Types of Faults in Transformers 78 4 5 Over-current PROTECTION 80 Percentage Differential PROTECTION of Transformers 81 1 Development of Connections 81 Phase c-to-Ground (c-g) External fault 82 Phase c-to-Ground (c-g) Internal fault 84 Inrush Phenomenon 86 Percentage Differential Relay with Harmonic Restraint 89 High Resistance Ground Faults in Transformers 91 1 High Resistance Ground Faults on the Delta Side 91 4 High Resistance Ground Faults on the Star Side 92 Inter-turn Faults in Transformers 93 4 10 Incipient Faults in Transformers 93 * Buchholz Relay 93 Analysis of Trapped Gases 95 Phenomenon of Over-fluxing in Transformers 95 PROTECTION Against Over-fluxing 95 Transformer PROTECTION Application Chart 96 An Illustrative Numerical Problem 97 Reuzew Questions 99 Problems 100 5 BUSBAR PROTECTION 101-117.

7 , Introduction 101 Differential PROTECTION of Busbars 102 Selection of CT Ratios in Case of Busbar PROTECTION : Wrong Method 102 Selection of CT Ratios in Case of Busbar PROTECTION : Correct Method 103 5 3 External and Internal fault 104 . Actual Behaviour of a Protective CT 105 Circuit Model of Saturated CT 108 External fault with One CT Saturated: Need for High Impedance Busbar PROTECTION 108 Minimum Internal fault That Can Be Detected by the High Impedance Busbar Differential Scheme 110 Stability Ratio of High Impedance Busbar Differential Scheme 112 Supervisory Relay 112 PROTECTION of Three-phase Busbars 114 Numerical Example on Design of High Impedance Busbar Differential Scheme 115 Review Questions 11 7 6 DISTANCE PROTECTION OF TRANSMISSION LINES 118-152 Drawbacks of Over-current PROTECTION 118 Introduction to Distance PROTECTION 119 Simple Impedance Relay 123 Trip Law for Simple Impedance Relay Using Universal Torque Equation 123

8 Implementation of Simple Impedance Relay Using Balanced Beam Structure 124 Performance of Simple Impedance Relay During Normal Load Flow 126 4 Effect of Arc Resistance on Reach of Simple Impedance Relay 126 Directional Property Exhibited by Simple Impedance Relay 127 Performance of Simple Impedance Relay During POWER Swing 127 Reactance Relay 129 Trip Law for Reactance Relay Using Universal Torque Equation 129 Implementation of Reactance Relay Using the Induction Cup Structure 131 Performance of Reactance Relay During Normal Load Flow 131 Effect of Arc Resistance on Reach of Reactance Relay 132 Directional Property Exhibited by Reactance Relay 133 Performance of Reactance Relay During POWER Swing 134 Mho Relay 134 Trip Law for Mho Relay Using Universal Torque Equation 134 Implementation of Mho Relay Using Induction Cup Structure 135 Performance of Mho Relay During Normal Load Flow 135 Effect of Arc Resistance on Mho Relay Reach 136 Directional Property Exhibited by Mho Relay 137 Performance of Mho Relay During POWER Swing 138 Comparison Between Distance Relays 139 Distance PROTECTION of a Three-phase Line 139 Phase Faults 141 Ground Faults 142 Complete PROTECTION of a Three-phase Line

9 144 .. YIII Coi~rrf~ts 9 INDUCTION MOTOR PROTECTION 184-195 ' Introduction 184 Various Faults and Abnormal Operating Conditions 184 Starting Current 185 Electrical Faults 186 1 fault on Motor Terminals 186 Phase Faults Inside the Motor 186 Ground Faults Inside the Motor 188 Inter-turn Faults 189 Abnormal Operating Conditions from Supply Side 189 Unbalanced Supply Voltage 189 Single Phasing 191 Reduction in Supply Voltage 192 Reversal of Phases 192 Abnormal Operating Conditions from Mechanical Side 192 Failure of Bearing and Rotor Jam 192 Overload 192 Data Required for Designing

10 Motor PROTECTION Schemes 195 Review Questions 195 10 STATIC COMPARATORS AS RELAYS 196-221 Comparison vs Computation 196 Amplitude Comparator 196 Phase comparator 199 The Cosine-type Phase Comparator 199 The Sine-type Phase Comparator 200 Duality Between Amplitude and Phase Comparators 201 Synthesis of Various Distance Relays Using Static Comparators 204 Synthesis of Mho Relay Using Static Phase Comparator 204 Synthesis of Reactance Relay Using Cosine-type Phase Comparator 208 Synthesis of Simple Impedance Relay Using Amplitude Comparator 210 e Development of an Electronic Circuit for Implementing a Cosine-type Phase Comparator 210 An Electronic Circuit for Implementing a Sine-type Phase Comparator 216 Synthesis of Quadrilateral Distance Relay 218 Review Questions 221 11 NUMERICAL PROTECTION Introduction 222 Block Diagram of Numerical Relay 223 Sampling Theorem 225 Correlation with a Reference Wave 228 Fourier Analysis of Analogue Signals 229 I Least Error Squared (LES) Technique 237 --- Conrcnis vii.