Tài liệu Practical Power Systems Protection pptx

LINK DOWNLOAD MIỄN PHÍ TÀI LIỆU "Tài liệu Practical Power Systems Protection pptx": http://123doc.vn/document/1049025-tai-lieu-practical-power-systems-protection-pptx.htm

For information on all Newnes Publications
visit our website at www.newnespress.com
Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
30 Corporate Drive, Burlington, MA 01803

First published 2004
Copyright © 2004, IDC Technologies. All rights reserved

No part of this publication may be reproduced in any material form (including
photocopying or storing in any medium by electronic means and whether
or not transiently or incidentally to some other use of this publication) without
the written permission of the copyright holder except in accordance with the
provisions of the Copyright, Designs and Patents Act 1988 or under the terms of
a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,
London, England W1T 4LP. Applications for the copyright holder's written
permission to reproduce any part of this publication should be addressed
to the publishers

British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data
A catalogue record for this book is available from the Library of Congress

ISBN 0 7506 6397 9







Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India
www.integra-india.com
Printed and bound in The Netherlands

Working together to grow
libraries in developing countries
www.elsevier.com | www.bookaid.org | www.sabre.org
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 I
S
-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.
Hambani Kahle (Zulu Farewell)
(Sources: Canciones de Nuestra Cabana (1980), Tent and Trail Songs (American Camping
Association), Songs to Sing & Sing Again by Shelley Gordon)
Go well and safely.
Go well and safely.
Go well and safely.
The Lord be ever with you.
Stay well and safely.
Stay well and safely.
Stay well and safely.
The Lord be ever with you.
Hambani kahle.
Hambani kahle.
Hambani kahle.
The Lord be ever with you.
Steve Mackay
1
Need for protection
1.1 Need for protective apparatus
A power system is not only capable to meet the present load but also has the flexibility to
meet the future demands. A power system is designed to generate electric power in
sufficient quantity, to meet the present and estimated future demands of the users in a
particular area, to transmit it to the areas where it will be used and then distribute it within
that area, on a continuous basis.
To ensure the maximum return on the large investment in the equipment, which goes to
make up the power system and to keep the users satisfied with reliable service, the whole
system must be kept in operation continuously without major breakdowns.
This can be achieved in two ways:
• The first way is to implement a system adopting components, which should not
fail and requires the least or nil maintenance to maintain the continuity of
service. By common sense, implementing such a system is neither economical
nor feasible, except for small systems.
• The second option is to foresee any possible effects or failures that may cause
long-term shutdown of a system, which in turn may take longer time to bring
back the system to its normal course. The main idea is to restrict the
disturbances during such failures to a limited area and continue power
distribution in the balance areas. Special equipment is normally installed to
detect such kind of failures (also called ‘faults’) that can possibly happen in
various sections of a system, and to isolate faulty sections so that the
interruption is limited to a localized area in the total system covering various
areas. The special equipment adopted to detect such possible faults is referred
to as ‘protective equipment or protective relay’ and the system that uses such
equipment is termed as ‘protection system’.
A protective relay is the device, which gives instruction to disconnect a faulty part of the
system. This action ensures that the remaining system is still fed with power, and protects
the system from further damage due to the fault. Hence, use of protective apparatus is
very necessary in the electrical systems, which are expected to generate, transmit and
distribute power with least interruptions and restoration time. It can be well recognized
that use of protective equipment are very vital to minimize the effects of faults, which
otherwise can kill the whole system.
2 Practical Power Systems Protection
1.2 Basic requirements of protection
A protection apparatus has three main functions/duties:
1. Safeguard the entire system to maintain continuity of supply
2. Minimize damage and repair costs where it senses fault
3. Ensure safety of personnel.
These requirements are necessary, firstly for early detection and localization of faults, and
secondly for prompt removal of faulty equipment from service.
In order to carry out the above duties, protection must have the following qualities:
• Selectivity: To detect and isolate the faulty item only.
• Stability: To leave all healthy circuits intact to ensure continuity or supply.
• Sensitivity: To detect even the smallest fault, current or system abnormalities
and operate correctly at its setting before the fault causes irreparable damage.
• Speed: To operate speedily when it is called upon to do so, thereby
minimizing damage to the surroundings and ensuring safety to personnel.
To meet all of the above requirements, protection must be reliable which means it
must be:
• Dependable: It must trip when called upon to do so.
• Secure: It must not trip when it is not supposed to.
1.3 Basic components of protection
Protection of any distribution system is a function of many elements and this manual
gives a brief outline of various components that go in protecting a system. Following are
the main components of protection.
• Fuse is the self-destructing one, which carries the currents in a power circuit
continuously and sacrifices itself by blowing under abnormal conditions. These
are normally independent or stand-alone protective components in an electrical
system unlike a circuit breaker, which necessarily requires the support of
external components.
• Accurate protection cannot be achieved without properly measuring the normal
and abnormal conditions of a system. In electrical systems, voltage and current
measurements give feedback on whether a system is healthy or not. Voltage
transformers and current transformers measure these basic parameters and are
capable of providing accurate measurement during fault conditions without
failure.
• The measured values are converted into analog and/or digital signals and are
made to operate the relays, which in turn isolate the circuits by opening the
faulty circuits. In most of the cases, the relays provide two functions viz., alarm
and trip, once the abnormality is noticed. The relays in olden days had very
limited functions and were quite bulky. However, with advancement in digital
technology and use of microprocessors, relays monitor various parameters,
which give complete history of a system during both pre-fault and post-fault
conditions.
• The opening of faulty circuits requires some time, which may be in
milliseconds, which for a common day life could be insignificant. However, the
circuit breakers, which are used to isolate the faulty circuits, are capable of
Need for protection 3
carrying these fault currents until the fault currents are totally cleared. The
circuit breakers are the main isolating devices in a distribution system, which
can be said to directly protect the system.
• The operation of relays and breakers require power sources, which shall not be
affected by faults in the main distribution. Hence, the other component, which
is vital in protective system, is batteries that are used to ensure uninterrupted
power to relays and breaker coils.
The above items are extensively used in any protective system and their design requires
careful study and selection for proper operation.
1.4 Summary
Power System Protection – Main Functions
1. To safeguard the entire system to maintain continuity of supply.
2. To minimize damage and repair costs.
3. To ensure safety of personnel.

Power System Protection – Basic Requirements
1. Selectivity: To detect and isolate the faulty item only.
2. Stability: To leave all healthy circuits intact to ensure continuity of supply.
3. Speed: To operate as fast as possible when called upon, to minimize
damage, production downtime and ensure safety to personnel.
4. Sensitivity: To detect even the smallest fault, current or system
abnormalities and operate correctly at its setting.

Power System Protection – Speed is Vital!!
The protective system should act fast to isolate faulty sections to prevent:
• Increased damage at fault location. Fault energy = I
2
× R
f
× t, where t is time in
seconds.
• Danger to the operating personnel (flashes due to high fault energy sustaining
for a long time).
• Danger of igniting combustible gas in hazardous areas, such as methane in coal
mines which could cause horrendous disaster.
• Increased probability of earth faults spreading to healthy phases.
• Higher mechanical and thermal stressing of all items of plant carrying the fault
current, particularly transformers whose windings suffer progressive and
cumulative deterioration because of the enormous electromechanical forces
caused by multi-phase faults proportional to the square of the fault current.
Sustained voltage dips resulting in motor (and generator) instability leading to
extensive shutdown at the plant concerned and possibly other nearby plants
connected to the system.