Practical Power System Protection by Mark Brown and Ramesh Balakrishnan

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Practical Power System Protection by Mark Brown and Ramesh Balakrishnan

Contents

Preface………………………………………………………………………………………………………………….ix
1 Need for protection ………………………………………………………………………………………. 1
1.1 Need for protective apparatus ……………………………………………………………. 1
1.2 Basic requirements of protection ………………………………………………………… 2
1.3 Basic components of protection………………………………………………………….. 2
1.4 Summary………………………………………………………………………………………… 3
2 Faults, types and effects ……………………………………………………………………………….. 5
2.1 The development of simple distribution systems……………………………………. 5
2.2 Fault types and their effects……………………………………………………………….. 7
3 Simple calculation of short-circuit currents………………………………………………………. 11
3.1 Introduction …………………………………………………………………………………… 11
3.2 Revision of basic formulae ………………………………………………………………. 11
3.3 Calculation of short-circuit MVA………………………………………………………… 15
3.4 Useful formulae ……………………………………………………………………………… 18
3.5 Cable information …………………………………………………………………………… 22
3.6 Copper conductors …………………………………………………………………………. 25
4 System earthing…………………………………………………………………………………………. 26
4.1 Introduction …………………………………………………………………………………… 26
4.2 Earthing devices…………………………………………………………………………….. 27
4.3 Evaluation of earthing methods ………………………………………………………… 30
4.4 Effect of electric shock on human beings……………………………………………. 32
5 Fuses……………………………………………………………………………………………………….. 35
5.1 Historical ………………………………………………………………………………………. 35
5.2 Rewireable type……………………………………………………………………………… 35
5.3 Cartridge type………………………………………………………………………………… 36
5.4 Operating characteristics…………………………………………………………………. 36
5.5 British standard 88:1952………………………………………………………………….. 37
5.6 Energy ‘let through’ ………………………………………………………………………… 38
5.7 Application of selection of fuses ……………………………………………………….. 38
5.8 General ‘rules of thumb’ ………………………………………………………………….. 39
5.9 Special types…………………………………………………………………………………. 40
vi Contents
5.10 General…………………………………………………………………………………………. 40
5.11 IS-limiter …………………………………………………………………………………………42
6 Instrument transformers ……………………………………………………………………………….45
6.1 Purpose…………………………………………………………………………………………45
6.2 Basic theory of operation ………………………………………………………………….45
6.3 Voltage transformers ……………………………………………………………………….46
6.4 Current transformers………………………………………………………………………..54
6.5 Application of current transformers …………………………………………………….65
6.6 Introducing relays …………………………………………………………………………… 66
6.7 Inverse definite minimum time lag (IDMTL) relay ………………………………….67
7 Circuit breakers…………………………………………………………………………………………..70
7.1 Introduction……………………………………………………………………………………. 70
7.2 Protective relay–circuit breaker combination………………………………………..70
7.3 Purpose of circuit breakers (switchgear) ……………………………………………..71
7.4 Behavior under fault conditions…………………………………………………………. 73
7.5 Arc………………………………………………………………………………………………..74
7.6 Types of circuit breakers…………………………………………………………………..74
7.7 Comparison of breaker types…………………………………………………………….81
8 Tripping batteries ………………………………………………………………………………………..83
8.1 Tripping batteries…………………………………………………………………………….83
8.2 Construction of battery chargers………………………………………………………..88
8.3 Maintenance guide …………………………………………………………………………. 89
8.4 Trip circuit supervision……………………………………………………………………..92
8.5 Reasons why breakers and contactors fail to trip………………………………….93
8.6 Capacity storage trip units ……………………………………………………………….. 94
9 Relays……………………………………………………………………………………………………….96
9.1 Introduction……………………………………………………………………………………. 96
9.2 Principle of the construction and operation of the electromechanical
IDMTL relay……………………………………………………………………………………96
9.3 Factors influencing choice of plug setting …………………………………………. 107
9.4 The new era in protection – microprocessor vs electronic
vs traditional ……………………………………………………………………………….. 107
9.5 Universal microprocessor overcurrent relay………………………………………. 114
9.6 Technical features of a modern microprocessor relay…………………………. 116
9.7 Type testing of static relays ……………………………………………………………. 124
9.8 The future of protection for distribution systems…………………………………. 125
9.9 The era of the IED ………………………………………………………………………… 126
9.10 Substation automation…………………………………………………………………… 129
9.11 Communication capability ………………………………………………………………. 132
10 Coordination by time grading ……………………………………………………………………… 133
10.1 Protection design parameters on medium- and
low-voltage networks …………………………………………………………………….. 133
10.2 Sensitive earth fault protection………………………………………………………… 148
Contents vii
11 Low-voltage networks ……………………………………………………………………………….. 150
11.1 Introduction …………………………………………………………………………………. 150
11.2 Air circuit breakers………………………………………………………………………… 150
11.3 Moulded case circuit breakers ………………………………………………………… 151
11.4 Application and selective coordination……………………………………………… 160
11.5 Earth leakage protection………………………………………………………………… 165
12 Mine underground distribution protection ……………………………………………………… 169
12.1 General ………………………………………………………………………………………. 169
12.2 Earth-leakage protection ……………………………………………………………….. 170
12.3 Pilot wire monitor………………………………………………………………………….. 172
12.4 Earth fault lockout…………………………………………………………………………. 173
12.5 Neutral earthing resistor monitor (NERM)…………………………………………. 173
13 Principles of unit protection………………………………………………………………………… 181
13.1 Protective relay systems………………………………………………………………… 181
13.2 Main or unit protection…………………………………………………………………… 181
13.3 Back-up protection ……………………………………………………………………….. 181
13.4 Methods of obtaining selectivity………………………………………………………. 182
13.5 Differential protection…………………………………………………………………….. 182
13.6 Transformer differential protection…………………………………………………… 185
13.7 Switchgear differential protection…………………………………………………….. 185
13.8 Feeder pilot-wire protection ……………………………………………………………. 185
13.9 Time taken to clear faults ………………………………………………………………. 186
13.10 Recommended unit protection systems……………………………………………. 186
13.11 Advantages of unit protection …………………………………………………………. 186
14 Feeder protection cable feeders and overhead lines………………………………………. 188
14.1 Introduction …………………………………………………………………………………. 188
14.2 Translay ……………………………………………………………………………………… 188
14.3 Solkor protection ………………………………………………………………………….. 189
14.4 Distance protection……………………………………………………………………….. 192
15 Transformer protection………………………………………………………………………………. 207
15.1 Winding polarity……………………………………………………………………………. 207
15.2 Transformer connections……………………………………………………………….. 207
15.3 Transformer magnetizing characteristics ………………………………………….. 209
15.4 In-rush current ……………………………………………………………………………… 210
15.5 Neutral earthing……………………………………………………………………………. 211
15.6 On-load tap changers……………………………………………………………………. 212
15.7 Mismatch of current transformers ……………………………………………………. 213
15.8 Types of faults……………………………………………………………………………… 214
15.9 Differential protection…………………………………………………………………….. 216
15.10 Restricted earth fault …………………………………………………………………….. 220
15.11 HV overcurrent …………………………………………………………………………….. 224
15.12 Buchholz protection………………………………………………………………………. 226
15.13 Overloading…………………………………………………………………………………. 227
viii Contents
16 Switchgear (busbar) protection……………………………………………………………………. 233
16.1 Importance of busbars …………………………………………………………………… 233
16.2 Busbar protection …………………………………………………………………………. 234
16.3 The requirements for good protection ………………………………………………. 234
16.4 Busbar protection types …………………………………………………………………. 234
17 Motor protection relays………………………………………………………………………………. 244
17.1 Introduction………………………………………………………………………………….. 244
17.2 Early motor protection relays ………………………………………………………….. 247
17.3 Steady-state temperature rise…………………………………………………………. 248
17.4 Thermal time constant …………………………………………………………………… 249
17.5 Motor current during start and stall conditions……………………………………. 249
17.6 Stalling of motors………………………………………………………………………….. 250
17.7 Unbalanced supply voltages…………………………………………………………… 251
17.8 Determination of sequence currents ………………………………………………… 253
17.9 Derating due to unbalanced currents ……………………………………………….. 253
17.10 Electrical faults in stator windings earth faults phase–phase faults ……….. 254
17.11 General……………………………………………………………………………………….. 256
17.12 Typical protective settings for motors……………………………………………….. 257
18 Generator protection …………………………………………………………………………………. 258
18.1 Introduction………………………………………………………………………………….. 258
18.2 Stator earthing and earth faults……………………………………………………….. 259
18.3 Overload protection ………………………………………………………………………. 261
18.4 Overcurrent protection…………………………………………………………………… 261
18.5 Overvoltage protection…………………………………………………………………… 261
18.6 Unbalanced loading………………………………………………………………………. 261
18.7 Rotor faults ………………………………………………………………………………….. 262
18.8 Reverse power …………………………………………………………………………….. 264
18.9 Loss of excitation………………………………………………………………………….. 264
18.10 Loss of synchronization …………………………………………………………………. 264
18.11 Field suppression …………………………………………………………………………. 264
18.12 Industrial generator protection ………………………………………………………… 264
18.13 Numerical relays…………………………………………………………………………… 265
18.14 Parallel operation with grid……………………………………………………………… 266
19 Management of protection………………………………………………………………………….. 267
19.1 Management of protection ……………………………………………………………… 267
19.2 Schedule A ………………………………………………………………………………….. 267
19.3 Schedule B ………………………………………………………………………………….. 268
19.4 Test sheets………………………………………………………………………………….. 269
Index ………………………………………………………………………………………………………………… 274

Preface

This book has been designed to give plant operators, electricians, field technicians and engineers a
better appreciation of the role played by power system protection systems. An understanding of power
systems along with correct management, will increase your plant efficiency and performance as well
as increasing safety for all concerned. The book is designed to provide an excellent understanding on
both theoretical and practical level. The book starts at a basic level, to ensure that you have a solid
grounding in the fundamental concepts and also to refresh the more experienced readers in the
essentials. The book then moves onto more detailed applications. It is most definitely not an advanced
treatment of the topic and it is hoped the expert will forgive the simplifications that have been made to
the material in order to get the concepts across in a practical useful manner.
The book features an introduction covering the need for protection, fault types and their effects,
simple calculations of short circuit currents and system earthing. The book also refers to some
practical work such as simple fault calculations, relay settings and the checking of a current
transformer magnetisation curve which are performed in the associated training workshop. You should
be able to do these exercises and tasks yourself without too much difficulty based on the material
covered in the book.
This is an intermediate level book – at the end of the book you will have an excellent knowledge of
the principles of protection. You will also have a better understanding of the possible problems likely
to arise and know where to look for answers.
In addition you are introduced to the most interesting and ‘fun’ part of electrical engineering to make
your job more rewarding. Even those who claim to be protection experts have admitted to improving
their knowledge after attending this book but at worst case perhaps this book will perhaps be an easy
refresher on the topic which hopefully you will pass onto your less experienced colleagues.
We would hope that you will gain the following from this book:
• The fundamentals of electrical power protection and applications
• Knowledge of the different fault types
• The ability to perform simple fault and design calculations
• Practical knowledge of protection system components
• Knowledge of how to perform simple relay settings
• Increased job satisfaction through informed decision making
• Know how to improve the safety of your site.
Typical people who will find this book useful include:
• Electrical Engineers
• Project Engineers
• Design Engineers
• Instrumentation Engineers
• Electrical Technicians
• Field Technicians
• Electricians
x Preface
• Plant Operators
• Plant Operators.
You should have a modicum of electrical knowledge and some exposure to electrical protection
systems to derive maximum benefit from this book.
This book was put together by a few authors although initiated by the late Les Hewitson, who must
have been one of the finest instructors on the subject and who presented this course in his own right in
South Africa and throughout Europe/North America and Australia for IDC Technologies. It is to him
that this book is dedicated.