COMPUTER RELAYING
FOR POWER SYSTEMS



COMPUTER
RELAYING FOR
POWER SYSTEMS
Second Edition

Arun G. Phadke
University Distinguished Professor Emeritus
The Bradley Department of Electrical and Computer Engineering
Virginia Tech, Blacksburg, Virginia, USA

James S. Thorp
Hugh P. and Ethel C. Kelley Professor and Department Head
The Bradley Department of Electrical and Computer Engineering
Virginia Tech, Blacksburg, Virginia, USA

A John Wiley and Sons, Ltd., Publication

Research Studies Press Limited


Copyright  2009

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Library of Congress Cataloguing-in-Publication Data:
Phadke, Arun G.
Computer relaying for power systems / Arun G. Phadke. – 2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-470-05713-1 (cloth)

1. Protective relays. 2. Electric power systems – Protection – Data processing. I. Title.
TK2861.P48 2009
621.31 7 – dc22
2009022672
A catalogue record for this book is available from the British Library.
ISBN 978-0-470-05713-1 (Hbk)
Typeset in 11/13 Times by Laserwords Private Limited, Chennai, India.
Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire


CONTENTS
About the Authors
Preface to the First Edition
Preface to the Second Edition
Glossary of Acronyms
1
1.1
1.2
1.3

1.4
1.5

1.6
1.7
1.8

2
2.1
2.2

2.3

xi
xiii
xv
xvii

Introduction to computer relaying
Development of computer relaying
Historical background
Expected benefits of computer relaying
1.3.1 Cost
1.3.2 Self-checking and reliability
1.3.3 System integration and digital environment
1.3.4 Functional flexibility and adaptive relaying
Computer relay architecture
Analog to digital converters
1.5.1 Successive approximation ADC
1.5.2 Delta-sigma ADC
Anti-aliasing filters
Substation computer hierarchy
Summary
Problems
References

1
1
2
3
3

4
4
5
6
12
13
15
16
19
21
21
22

Relaying practices
Introduction to protection systems
Functions of a protection system
Protection of transmission lines
2.3.1 Overcurrent relays
2.3.2 Directional relays

25
25
26
30
30
32


vi


CONTENTS

2.3.3 Distance relays
2.3.4 Phasor diagrams and R-X diagrams
2.3.5 Pilot relaying
Transformer, reactor and generator protection
2.4.1 Transformer protection
2.4.2 Reactor protection
2.4.3 Generator protection
Bus protection
Performance of current and voltage transformers
2.6.1 Current transformers
2.6.2 Voltage transformers
2.6.3 Electronic current and voltage transformers
Summary
Problems
References

35
38
39
40
40
43
43
44
45
45
47
48

51
51
53

Mathematical basis for protective relaying algorithms
Introduction
Fourier series
3.2.1 Exponential fourier series
3.2.2 Sine and cosine fourier series
3.2.3 Phasors
3.3 Other orthogonal expansions
3.3.1 Walsh functions
3.4 Fourier transforms
3.4.1 Properties of fourier transforms
3.5 Use of fourier transforms
3.5.1 Sampling
3.6 Discrete fourier transform
3.7 Introduction to probability and random process
3.7.1 Random variables and probability distributions
3.7.2 Probability distributions and densities
3.7.3 Expectation
3.7.4 Jointly distributed random variables
3.7.5 Independence
3.7.6 Linear estimation
3.7.7 Weighted least squares
3.8 Random processes
3.8.1 Filtering of random processes
3.9 Kalman filtering
3.10 Summary
Problems

References

55
55
55
58
60
62
62
63
63
69
80
81
83
86
86
87
89
90
91
92
93
94
97
98
103
103
108


2.4

2.5
2.6

2.7

3
3.1
3.2


CONTENTS

4
4.1
4.2

vii

Digital filters
Introduction
Discrete time systems
4.2.1 Operations on discrete time sequences
4.2.2 Convolution
4.3 Discrete time systems
4.4 Z Transforms
4.4.1 Power series
4.4.2 Z Transforms
4.4.3 Inverse Z transforms

4.4.4 Properties of Z transforms
4.4.5 Discrete time fourier transform
4.5 Digital filters
4.6 Windows and windowing
4.7 Linear phase
4.8 Approximation – filter synthesis
4.9 Wavelets
4.10 Elements of artificial intelligence
4.10.1 Artificial neural networks
4.10.2 Decision trees
4.10.3 Agents
4.11 Conclusion
Problems
References

109
109
109
110
110
112
113
113
114
115
116
118
119
121
122

124
126
129
129
131
132
133
133
135

5
5.1
5.2
5.3

137
137
142
147
149
149
151
152
154
155
162
163
166

5.4


5.5

Transmission line relaying
Introduction
Sources of error
Relaying as parameter estimation
5.3.1 Curve fitting algorithms
5.3.2 Fourier algorithms
5.3.3 Fourier algorithms with shorter windows
5.3.4 Recursive forms
5.3.5 Walsh function algorithms
5.3.6 Differential-equation algorithms
5.3.7 Kalman filter algorithms
5.3.8 Removal of the DC offset
Beyond parameter estimation
5.4.1 Relay programs based upon fault
classification
Symmetrical component distance relay
5.5.1 SCDFT
5.5.2 Transient monitor

166
170
172
174


viii


5.6

5.7
5.8

6
6.1
6.2

6.3

6.4
6.5
6.6

7
7.1
7.2
7.3
7.4
7.5
7.6

7.7
7.8
7.9

CONTENTS

5.5.3 Speed reach considerations

5.5.4 A relaying program
Newer analytic techniques
5.6.1 Wavelet applications
5.6.2 Agent applications
Protection of series compensated lines
Summary
Problems
References

176
180
182
182
182
183
185
185
186

Protection of transformers, machines and buses
Introduction
Power transformer algorithms
6.2.1 Current derived restraints
6.2.2 Voltage based restraints
6.2.3 Flux restraint
6.2.4 A restraint function based on the gap in inrush current
Generator protection
6.3.1 Differential protection of stator windings
6.3.2 Other generator protection functions
6.3.3 Sampling rates locked to system frequency

Motor protection
Digital bus protection
Summary
Problems
References

189
189
190
191
194
195
199
200
200
202
203
204
204
208
209
210

Hardware organization in integrated systems
The nature of hardware issues
Computers for relaying
The substation environment
Industry environmental standards
Countermeasures against EMI
Supplementary equipment

7.6.1 Power supply
7.6.2 Auxiliary relays
7.6.3 Test switches
7.6.4 Interface panel
Redundancy and backup
Servicing, training and maintenance
Summary
References

213
213
214
216
217
220
222
222
222
222
223
223
225
226
227


CONTENTS

8
8.1

8.2
8.3
8.4

8.5
8.6

8.7
8.8

System relaying and control
Introduction
Measurement of frequency and phase
8.2.1 Least squares estimation of f and df/dt
Sampling clock synchronization
Application of phasor measurements to state
estimation
8.4.1 WLS estimator involving angle measurements
8.4.2 Linear state estimator
8.4.3 Partitioned state estimation
8.4.4 PMU locations
Phasor measurements in dynamic state estimation
8.5.1 State equation
Monitoring
8.6.1 Sequence of events analysis
8.6.2 Incipient fault detection
8.6.3 Breaker health monitoring
Control applications
Summary
Problems

References

ix

229
229
230
232
233
234
237
238
242
244
245
247
248
248
248
249
249
250
250
251

9
9.1
9.2
9.3


Relaying applications of traveling waves
Introduction
Traveling waves on single-phase lines
Traveling waves on three-phase lines
9.3.1 Traveling waves due to faults
9.4 Directional wave relay
9.5 Traveling wave distance relay
9.6 Differential relaying with phasors
9.7 Traveling wave differential relays
9.8 Fault location
9.8.1 Impedance estimation based fault location
9.8.2 Fault location based on traveling waves
9.9 Other recent developments
9.10 Summary
Problems
References

255
255
255
262
265
267
269
272
275
276
276
278
279

280
280
281

10 Wide area measurement applications
10.1 Introduction
10.2 Adaptive relaying

285
285
285


x

CONTENTS

10.3 Examples of adaptive relaying
10.3.1 Transmission line protection
10.3.2 Transformer protection
10.3.3 Reclosing
10.4 Wide area measurement systems (WAMS)
10.5 WAMS architecture
10.6 WAMS based protection concepts
10.6.1 Adaptive dependability and security
10.6.2 Monitoring approach of apparent impedances towards relay
characteristics
10.6.3 WAMS based out-of-step relaying
10.6.4 Supervision of backup zones
10.6.5 Intelligent load shedding

10.6.6 Adaptive loss-of-field
10.6.7 Intelligent islanding
10.6.8 System wide integration of SIPS
10.6.9 Load shedding and restoration
10.7 Summary
Problems
References

286
287
288
289
291
291
292
293
294
295
300
301
302
303
304
304
305
306
306

Appendix A
Representative system data

Transmission lines
Transformers
Generators
Power system
References

309
309
309
311
311
311
312

Appendix B
Standard sampling rates
References

313
313
315

Appendix C
Conversion between different sampling rates
References

317
317
320


Appendix D
Standard for transient data exchange
References

321
321
322

Index

323


About the authors
Dr. Arun G. Phadke worked in the Electric Utility industry for 13 years before
joining Virginia Tech 1982. He became the American Electric Power Professor of
Electrical Engineering in 1985 and held this title until 2000 when he was recognized as a University Distinguished Professor. He became University Distinguished
Professor Emeritus in 2003, and continues as a Research Faculty member of the
Electrical and Computer Engineering Department of Virginia Tech. Dr. Phadke was
elected a Fellow of IEEE in 1980. He was elected to the National Academy of
Engineering in 1993. He was Editor in Chief of Transactions of IEEE on Power
Delivery. He became the Chairman of the Power System Relaying Committee of
IEEE in 1999– 2000. Dr. Phadke received the Herman Halperin award of IEEE in
2000. Dr. Phadke has also been very active in CIGRE. He has been a member of
the Executive Committee of the US National Committee of CIGRE, and was the
Chairman of their Technical Committee. He was previously the Vice President of
USNC-CIGRE and served as Secretary/Treasurer. In 2002 he was elected a ‘Distinguished Member of CIGRE’ by the Governing Board of CIGRE. Dr. Phadke
was active in CIGRE SC34 for several years, and was the Chairman of some
of their working groups. In 1999 Dr. Phadke joined colleagues from Europe and
Far East in founding the International Institute for Critical Infrastructures (CRIS).

He was the first President of CRIS from 1999–2002, and currently serves on its
Governing Board. Dr. Phadke received the ‘Docteur Honoris Causa’ from Institute National Polytechnic de Grenoble (INPG) in 2006. Dr. Phadke received the
‘Karapetoff Award’ from the HKN Society, and the ‘Benjamin Franklin Medal’ for
Electrical Engineering in 2008.
Dr. James S. Thorp is the Hugh P. and Ethel C. Kelley Professor of Electrical and Computer Engineering and Department Head of the Bradley Department
of Electrical and Computer Engineering at Virginia Tech. He was the Charles N.
Mellowes Professor in Engineering at Cornell University from 1994– 2004. He
obtained the B.E.E. in 1959 and the Ph. D. in 1962 from Cornell University and
was the Director of the Cornell School of Electrical and Computer Engineering
from 1994 to 2001, a Faculty Intern, American Electric Power Service Corporation
in 1976–77 and an Overseas Fellow, Churchill College, Cambridge University in
1988. He has consulted for Mehta Tech Inc., Basler Electric, RFL Dowty Industries,


xii

About the authors

American Electric Power Service Corporation, and General Electric. He was an
Alfred P. Sloan Foundation National Scholar and was elected a Fellow of the
IEEE in 1989 and a Member of the National Academy of Engineering in 1996. He
received the 2001 Power Engineering Society Career Service award, the 2006 IEEE
Outstanding Power Engineering Educator Award, and shared the 2007 Benjamin
Franklin Medal with A.G. Phadke.


Preface to the first edition
The concept of using digital computers for relaying originated some 25 years ago.
Since then the field has grown rapidly. Computers have undergone a significant
change – they have become more powerful, cheaper, and sturdier. Today computer

relays are preferred for economic as well as technical reasons. These advances
in computer hardware have been accompanied by analytical developments in the
field of relaying. Through the participation of researchers at Universities and industrial organizations, the theory of power system protection has been placed on a
mathematical basis. It is noted that, in most cases, the mathematical investigations have confirmed the fact that traditional relay designs have been optimum or
near-optimum solutions to the relaying problem. This is reassuring: the theory and
practice of relaying have been reaffirmed simultaneously.
An account of these developments is scattered throughout the technical literature: Proceedings of various conferences, Transactions of Engineering Societies,
and technical publications of various equipment manufacturers. This book is our
attempt to present a coherent account of the field of computer relaying. We have
been doing active research in this area – much of it in close collaboration with each
other – since the mid 1970’s. We have tried to present a balanced view of all the
developments in the field, although it may seem that, at times, we have given a
fuller account of areas in which we ourselves have made contributions. For this
bias – if it is perceived as such by the reader – we seek his indulgence.
The book is intended for graduate students in electric power engineering, for
researchers in the field, or for anyone who wishes to understand this new development in the role of a potential user or manufacturer of computer relays. In teaching
a course from this book, we recommend following the order of the material in the
book. If a course on traditional protection is a pre-requisite to this course, Chapter 2
may be omitted. The mathematical basis for relaying is contained in Chapter 3, and
is intended for those who are not in an academic environment at present. The
material is essential for gaining an understanding of the reason why a relaying
algorithm works as it does, although how an algorithm works – i.e. its procedural
structure – can be understood without a thorough knowledge of the mathematics.
A reader with such a limited objective may skip the mathematical background, and
go directly to the sections of immediate interest to him.


xiv

Preface to the first edition


Our long association with the American Electric Power Service Corporation
(AEP) has been the single most important element in sustaining our interest in
Computer Relaying. The atmosphere in the old Computer Applications Department
in AEP under Tony Gabrielle was particularly well suited for innovative engineering. He was responsible for starting us on this subject, and for giving much needed
support when practical results seemed to be far into the future. Also present at AEP
was Stan Horowitz, our colleague and teacher, without whose help we would have
lost touch with the reality of relaying as a practical engineering enterprise. Stan
Horowitz, Eric Udren, and Peter McLaren read through the manuscript and offered
many constructive comments. We are grateful for their help. The responsibility for
the book, and for any remaining errors, is of course our own.
We continue to derive great pleasure from working in this field. It is our hope
that, with this book, we may share this enjoyment with the reader.
Arun G. Phadke
Blacksburg
James S. Thorp
Ithaca
1988


Preface to the second edition
The first edition of this book was published in 1988. The intervening two decades
have seen wide-spread acceptance of computer relays by power engineers throughout the world. In fact, in many countries computer relays are the protective devices
of choice, and one would be hard pressed to find electromechanical or electronic
relays with comparable capabilities. Clearly economics of relay manufacture have
played a major role in making this possible, and the improved performance,
self-checking capabilities, and access to relay settings over communication lines
have been the principal features of this technology which have brought about their
acceptance on such a wide scale.
It has been recognized by most relay designers – and is also the belief of the

authors – that the principles of protection have essentially remained as established
by experience gained over the last century. Computer relays provide essentially
the same capabilities as traditional relays in a more efficient manner. Having said
this, it is also recognized that changes in protection principles have taken place,
solely because of the capabilities of the computers and the available communication
facilities. Thus adaptive relaying could not be realized without this new technology.
Adaptive relaying, along with the new field of wide area measurements (which
originated in the field of computer relaying) forms a significant part of the present
edition of our book.
A study of published research papers on relaying will show that researchers
continue to investigate the application of newer analytical techniques to the field of
relaying. We have included an account of several such techniques in this edition, but
it must be stated that most of these techniques have not seen their implementation in
practical relay designs. Perhaps this confirms the authors’ belief that the principles
of protection are essentially dictated by power system phenomena, and the long
established techniques of protection system design are very sound and close to
being optimum. The newer analytical techniques which are being investigated offer
very minor improvements at best, and it remains questionable as to when or for
which applications we will see a clear benefit of these newer analytical techniques.
Our book remains a research text and reference work. As such the problem set
at the end of each chapter is often a statement of research idea. Some problems
are quite complex, and each problem leaves room for individual interpretation and


xvi

Preface to the second edition

development. We therefore offer no solutions to these problems and leave their
resolution to the individual initiative of the reader. We are of course interested in

receiving any comments that the users of our book care to make.
The authors have participated with pleasure in project “111”, a Key Research
Project of the North China Electric Power University since its inception in 2008
under the direction of Professor Yang Qixun. In addition to promoting research in
many aspects of computer relaying in which the authors continue to participate, the
facilities provided in Beijing under the auspices of this project for the authors have
facilitated the timely completion of this Second Edition of our book.
We continue to derive great pleasure from working in this field. It is our hope
that, with the second edition of this book, we may share this enjoyment with the
reader.
Arun G. Phadke
Blacksburg
James S. Thorp
Blacksburg
2009


Glossary of acronyms
A/D
ADC
ANN
ANSI
CIGRE
CT
CVT
DFT
EHV
EMI
EMTP
EPRI

EPROM
FFT
GPS
I/O
IEC
IEEE
MOV
MUX
NAVSTAR
PDC
PMU
PROM
PT
RAM
RAS

Analog to Digital
Analog to Digital Converter
Artificial Neural Network
American National Standards Institute
International Council on Large Electric Systems
Current Transformer
Capacitive Voltage Transformer
Discrete Fourier Transform
Extra High Voltage
Electromagnetic Interference
Electromagnetic Transients Program
Electric Power Research Institute
Erasable Programmable Read Only Memory
Fast Fourier Transform

Global Positioning System
Input Output
International Electrotechnical Commission
Institute of Electronic and Electrical Engineers
Metal Oxide Varistors
Multiplexer
NAVSTAR is not an acronym. It represents GPS described above.
Phasor Data Concentrator
Phasor Measurement Unit
Programmable Read Only Memory
Potential Transformer
Random Access Memory
Remedial Action Scheme


xviii

ROM
S/H
SCDFT
SIPS
SWC
WAMS
WAMPACS
WLS

Glossary of acronyms

Read Only Memory
Sample and Hold

Symmetrical Component Discrete Fourier Transform
System Integrity Protection Scheme
Surge Withstand Capability
Wide Area Measurement System
Wide Area Measurement, Protection and Control System
Weighted Least Squares


1
Introduction to computer relaying

1.1

Development of computer relaying

The field of computer relaying started with attempts to investigate whether power
system relaying functions could be performed with a digital computer. These investigations began in the 1960s, a period during which the digital computer was slowly
and systematically replacing many of the traditional tools of analytical electric
power engineering. The short circuit, load flow, and stability problems – whose
solution was the primary preoccupation of power system planners – had already
been converted to computer programs, replacing the DC boards and the Network
Analyzers. Relaying was thought to be the next promising and exciting field for
computerization. It was clear from the outset that digital computers of that period
could not handle the technical needs of high speed relaying functions. Nor was there
any economic incentive to do so. Computers were orders of magnitude too expensive. Yet, the prospect of developing and examining relaying algorithms looked
attractive to several researchers. Through such essentially academic curiosity this
very fertile field was initiated. The evolution of computers over the intervening
years has been so rapid that algorithmic sophistication demanded by the relaying
programs has finally found a correspondence in the speed and economy of the modern microcomputer; so that at present computer relays offer the best economic and
technical solution to the protection problems – in many instances the only workable solution. Indeed, we are at the start of an era in which computer relaying has

become routine, and it has further influenced the development of effective tools for
real-time monitoring and control of power systems.
In this chapter we will briefly review the historical developments in the field of
computer relaying. We will then describe the architecture of a typical computer
based relay. We will also identify the critical hardware components, and discuss the
influence they have on the relaying tasks.
Computer Relaying for Power Systems 2e
 2009 John Wiley & Sons, Ltd

by A. G. Phadke and J. S. Thorp


2

1.2

Introduction to computer relaying

Historical background

One of the earliest published papers on computer relaying explored the somewhat
curious idea that relaying of all the equipment in a substation would be handled
by a single computer.1 No doubt this was motivated by the fact that computers
were very expensive at that time (1960s), and there could be no conceivable way
in which multiple computers would be economically palatable as a substitute for
conventional relays which were at least one order of magnitude less expensive than
a suitable computer. In addition, the computation speed of contemporary computers
was too slow to handle high speed relaying, while the power consumption of the
computers was too high. In spite of these obvious shortcomings – which reflected
the then current state of computer development – the reference cited above explored

several protection algorithmic details thoroughly, and even today provides a good
initiation to the novice in the complexities of modern relaying practices.
Several other papers were published at approximately the same time, and led to
the algorithmic development for protection of high voltage transmission lines.2,3
It was recognized early that transmission line protection function (distance relaying in particular) – more than any other – is of greatest interest to relay engineers
because of its widespread use on power systems, its relatively high cost, and its
functional complexity. These early researchers began a study of distance protection
algorithms which continues unabated to this day. These studies have led to important new insights into the physical nature of protection processes and the limits to
which they can be pushed. It is quite possible that distance relaying implementation on computers has been mastered by most researchers by now, and that any
new advances in this field are likely to come from the use of improved computer
hardware to implement the well-understood distance relaying algorithms.
An entirely different approach to distance relaying has been proposed during
recent years.4,5 It is generally based upon the utilization of traveling waves initiated
by a fault to estimate the fault distance. Traveling wave relays require relatively
high frequencies for sampling voltage and current input signals. Although traveling
wave relays have not offered compelling advantages over other relaying principles
in terms of speed and accuracy of performance, they have been applied in a few
instances around the world with satisfactory performance. This technique will be
covered more fully in Chapter 9; it remains for the present a somewhat infrequently
used relaying application. Fault location algorithms based on traveling waves have
also been developed and there are reports of good experience with these devices.
These too will be covered more fully in Chapter 9.
In addition to the development of distance relaying algorithms, work was begun
early on apparatus protection using the differential relaying principle.6 – 8 These early
references recognize the fact that compared to the line relaying task, differential
relaying algorithms are less demanding of computational power. Harmonic restraint
function adds some complexity to the transformer protection problem, and problems
associated with current transformer saturation or other inaccuracies continue to have



Expected benefits of computer relaying

3

no easy solutions in computer based protection systems just as in conventional
relays. Nevertheless, with the algorithmic development of distance and differential
relaying principles, one could say that the ability of computer based relays to provide
performance at least as good as conventional relays had been established by the
early 1970s.
Very significant advances in computer hardware had taken place since those early
days. The size, power consumption, and cost of computers had gone down by orders
of magnitude, while simultaneously the speed of computation increased by several
orders. The appearance of 16 bit (and more recently of 32 bit) microprocessors
and computers based upon them made high speed computer relaying technically
achievable, while at the same time cost of computer based relays began to become
comparable to that of conventional relays. This trend has continued to the present
day – and is bound to persist in the future – although perhaps at not quite as precipitous a rate. In fact, it appears well established by now that the most economical
and technically superior way to build relay systems of the future (except possibly
for some functionally simple and inexpensive relays) is with digital computers. The
old idea of combining several protection functions in one hardware system1 has
also re-emerged to a certain extent – in the present day multi-function relays.
With reasonable prospects of having affordable computer relays which can be
dedicated to a single protection function, attention soon turned to the opportunities
offered by computer relays to integrate them into a substation-wide, perhaps even a
system-wide, network using high-speed wide-band communication networks. Early
papers on this subject realized several benefits that would flow from this ability of
relays to communicate.9,10 As will be seen in Chapters 8 and 9 integrated computer systems for substations which handle relaying, monitoring, and control tasks
offer novel opportunities for improving overall system performance by exchanging
critical information between different devices.


1.3

Expected benefits of computer relaying

It would be well to summarize the advantages offered by computer relays, and some
of the features of this technology which have required new operational considerations. Among the benefits flowing from computer relays are:

1.3.1 Cost
All other things being equal, the cost of a relay is the main consideration in its
acceptability. In the early stages of computer relaying, computer relay costs were
10 to 20 times greater than the cost of conventional relays. Over the years, the cost of
digital computers has steadily declined; at the same time their computational power
(measured by instruction execution time and word length) has increased substantially. The cost of conventional (analog) relays has steadily increased over the same
period, primarily because of some design improvements, but also because of general


4

Introduction to computer relaying

inflation and a relatively low volume of production and sales. It is estimated that
for equal performance the cost of the most sophisticated digital computer relays
(including software costs) would be about the same as that of conventional relaying systems. Clearly there are some conventional relays – overcurrent relays are an
example – which are so inexpensive that cheaper computer relays to replace them
seem unlikely at present, unless they are a part of a multi-function relay. However,
for major protection systems, the competitive computer relay costs have definitely
become an important consideration.

1.3.2 Self-checking and reliability
A computer relay can be programmed to monitor several of its hardware and software subsystems continuously, thus detecting any malfunctions that may occur. It

can be designed to fail in a safe mode – i.e. take itself out of service if a failure
is detected – and send a service request alarm to the system center. This feature of
computer relays is perhaps the most telling technical argument in favor of computer
relaying. Misoperation of relays is not a frequent occurrence, considering the very
large number of relays in existence on a power system. On the other hand, in most
cases of power system catastrophic failures the immediate cause of the escalation of
events that leads to the failure can be traced to relay misoperation. In some cases, it
is a mis-application of a relay to the given protection task, but in a majority of cases
it is due to a failure of a relay component that leads to its misoperation and the
consequent power system breakdown.11 It is expected that with the self-checking
feature of computer based relays, the relay component failures can be detected soon
after they occur, and could be repaired before they have a chance to misoperate. In
this sense, although computer based relays are more complex than electromechanical or solid state relays (and hence potentially more likely to fail), as a system they
have a higher rate of availability. Of course, a relay cannot detect all component
failures – especially those outside the periphery of the relay system.

1.3.3 System integration and digital environment
Digital computers and digital technology have become the basis of most systems in
substations. Measurements, communication, telemetry and control are all computer
based functions. Many of the power transducers (current and voltage transformers)
are in the process of becoming digital systems. Fiber optic links, because of their
immunity to Electromagnetic Interference (EMI), are likely to become the medium
of signal transmission from one point to another in a substation; it is a technology
particularly suited to the digital environment. In substations of the future, computer relays will fit in very naturally. They can accept digital signals obtained from
newer transducers and fiber optic channels, and become integrated with the computer based control and monitoring systems of a substation. As a matter of fact,
without computer relaying, the digital transducers and fiber optic links for signal
transmission would not be viable systems in the substation.


Expected benefits of computer relaying


5

1.3.4 Functional flexibility and adaptive relaying
Since the digital computer can be programmed to perform several functions as long
as it has the input and output signals needed for those functions, it is a simple matter
to the relay computer to do many other substation tasks. For example, measuring
and monitoring flows and voltages in transformers and transmission lines, controlling the opening and closing of circuit breakers and switches, providing backup for
other devices that have failed, are all functions that can be taken over by the relay
computer. The relaying function calls for intensive computational activity when a
fault occurs on the system. This intense activity at best occupies the relaying computer for a very small fraction of its service life – less than a tenth of a percent. The
relaying computer can thus take over these other tasks at practically no extra cost.
With the programmability and communication capability, the computer based
relay offers yet another possible advantage that is not easily realizable in a conventional system. This is the ability to change relay characteristics (settings) as system
conditions warrant it. More will be said about this aspect (adaptive relaying) in
Chapter 10.
The high expectations for computer relaying have been mostly met in practical
implementations. It is clear that most benefits of computer relaying follow from the
ability of computers to communicate with various levels of a control hierarchy.
The full flowering of computer relaying technology therefore has only been possible
with the arrival of an extensive communication network that reaches into major
substations. Preferably, the medium of communication would be fiber optic links
with their superior immunity to interference, and the ability to handle high-speed
high-volume data. It appears that the benefits of such a communication network
would flow in many fields, and as more such links become available, the computer
relays and their measurement capabilities become valuable in their own right. Where
extensive communication networks are not available, many of the expected benefits
of computer relaying must remain unrealized.
Other issues which are specific to computer relaying technology should also be
mentioned. It has been noted that digital computer technology has advanced at a

very rapid pace over the last twenty years. This implies that computer hardware has
a relatively short lifespan. The hardware changes significantly every few years, and
the question of maintainability of old hardware becomes crucial. The existing relays
have performed well for long periods – some as long as 30 years or more. Such
relays have been maintained over this period. It is difficult to envision a similar
lifespan for computer based equipment. Perhaps a solution lies in the modularity
of computer hardware; computers and peripherals belonging to a single family may
provide a longer service life with replacements of a few modules every few years.
As long as this can be accomplished without extensive changes to the relaying
system, this may be an acceptable compromise for long service life. However,
the implications of rapidly changing computer hardware systems are evident to
manufacturers and users of this technology.


6

Introduction to computer relaying

Software presents problems of its own. Computer programs for relaying applications (or critical parts of them) are usually written in lower level languages, such
as assembly language. The reason for this is the need to utilize the available time
after the occurrence of a fault as efficiently as possible. Relaying programs tend
to be computation and input-output bound. The higher level languages tend to be
inefficient for time-sensitive applications. It is possible that in time, with computer
instruction times becoming faster, the higher level languages could replace much
of the assembly language programming in relaying computers. The problem with
machine level languages is that they are not transportable between computers of different types. Some transportability between different computer models of the same
family may exist, but even here it is generally desirable to develop new software
in order to take advantage of differing capabilities among the different models.
Since software costs are a very significant part of computer relaying development,
non-transferability of software is a significant problem.

In the early period of computer relaying development, there was some concern
about the harsh environment of electric utility substations, in which the relays must
function. Extremes of temperature, humidity, pollution as well as very severe EMI
must be anticipated.
Another concern often raised by users of computer relays can be traced to the
wide range of problems these relays can handle. It is rare to find a computer relay
which does not require very large number of settings before it can be installed and
commissioned. Where the organization using these devices has ample staff dedicated to working with computer relays, handling the complexity of setting these
relays is not a problem. However, where the organization is small and a specialized
staff for these applications cannot be justified, setting of these relays correctly and
maintaining them for future modifications becomes a difficult task. Furthermore,
if relays of different manufacture are in use within a single organization, it may
become necessary to have experts who can deal with devices of different manufacture. Several Working Groups and Technical Committees of the Power Engineering
Society of IEEE have attempted to develop a common user-interface to relays of
different manufacture, but this task seems to be too complex and not much progress
has been made in this direction.

1.4

Computer relay architecture

Computer relays consist of subsystems with well defined functions. Although a
specific relay may be different in some of its details, these subsystems are most
likely to be incorporated in its design in some form. Relay subsystems and their
functions will be described next.
The block diagram in Figure 1.1 shows the principal subsystems of a computer relay. The processor is central to its organization. It is responsible for the
execution of relay programs, maintenance of various timing functions, and communicating with its peripheral equipment. Several types of memories are shown in