DISTURBANCEANALYSIS FOR
POWER SYSTEMS
DISTURBANCEANALYSIS FOR
POWER SYSTEMS
Mohamed A. IbrahimNew York Power Authority
Director of Protection and Control (Retired)
Copyright � 2012 by Mohamed A. Ibrahim. All rights reserved
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form
or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as
permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior
written permission of the Publisher, or authorization through payment of the appropriate per-copy
fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400,
fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission
should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken,
NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission.
Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts
in preparing this book, they make no representations or warranties with respect to the accuracy or
completeness of the contents of this book and specifically disclaim any implied warranties of
merchantability or fitness for a particular purpose. No warranty may be created or extended by sales
representatives or written sales materials. The advice and strategies contained herein may not be
suitable for your situation. You should consult with a professional where appropriate. Neither the
publisher nor author shall be liable for any loss of profit or any other commercial damages, including
but not limited to special, incidental, consequential, or other damages.
For general information on our other products and services or for technical support, please contact
our Customer Care Department within the United States at (800) 762-2974, outside the
United States at (317) 572-3993 or fax (317) 572-4002.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print
may not be available in electronic formats. For more information about Wiley products, visit our
web site at www.wiley.com.
Library of Congress Cataloging-in-Publication Data:Ibrahim, Mohamed A., 1943-
Disturbance analysis for power systems / Mohamed A. Ibrahim.
p. cm.
Includes index.
ISBN 978-0-470-91681-0 (cloth)
1. Electric power system stability. 2. Transients (Electricity) 3. Electric power failures.
4. Electric network analysis. I. Title.
TK1010.I27 2011
621.319–dc22
2010048274
Printed in the United States of America
ePDF ISBN: 978-1-118-17211-7
oBook ISBN: 978-1-118-17209-4
ePub ISBN: 978-1-118-17210-0
10 9 8 7 6 5 4 3 2 1
To my mother, who taught me without knowing how to read or write;
my father; my wife; and my family
CONTENTS
Preface xvii
1 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION 1
1.1 Analysis Function of Power System Disturbances 2
1.2 Objective of DFR Disturbance Analysis 4
1.3 Determination of Power System Equipment Health Through
System Disturbance Analysis 5
1.4 Description of DFR Equipment 6
1.5 Information Required for the Analysis of System Disturbances 7
1.6 Signals to be Monitored by a Fault Recorder 8
1.7 DFR Trigger Settings of Monitored Voltages and Currents 10
1.8 DFR and Numerical Relay Sampling Rate
and Frequency Response 11
1.9 Oscillography Fault Records Generated by Numerical Relaying 11
1.10 Integration and Coordination of Data Collected
from Intelligent Electronic Devices 12
1.11 DFR Software Analysis Packages 12
1.12 Verification of DFR Accuracy in Monitoring Substation
Ground Currents 21
1.13 Using DFR Records to Validate Power System Short-Circuit
Study Models 24
1.14 COMTRADE Standard 31
2 PHENOMENA RELATED TO SYSTEM FAULTS AND THE PROCESSOF CLEARING FAULTS FROM A POWER SYSTEM 33
2.1 Shunt Fault Types Occurring in a Power System 33
2.2 Classification of Shunt Faults 34
2.3 Types of Series Unbalance in a Power System 39
2.4 Causes of Disturbance in a Power System 39
vii
2.5 Fault Incident Point 40
2.6 Symmetric and Asymmetric Fault Currents 41
2.7 Arc-Over or Flashover at the Voltage Peak 44
2.8 Evolving Faults 48
2.9 Simultaneous Faults 51
2.10 Solid or Bolted (RF¼ 0) Close-in Phase-to-Ground Faults 52
2.11 Sequential Clearing Leading to a Stub Fault that Shows a
Solid (RF¼ 0) Remote Line-to-Ground Fault 53
2.12 Sequential Clearing Leading to a Stub Fault that Shows a
Resistive Remote Line-to-Ground Fault 54
2.13 High-Resistance Tree Line-to-Ground Faults 56
2.14 High-Resistance Line-to-Ground Fault Confirming
the Resistive Nature of the Fault Impedance When
Fed from One Side Only (Stub) 58
2.15 Phase-to-Ground Faults on an Ungrounded System 59
2.16 Current in Unfaulted Phases During Line-to-Ground Faults 60
2.17 Line-to-Ground Fault on the Grounded-Wye (GY) Side of a
Delta/GY Transformer 63
2.18 Line-to-Line Fault on the Grounded-Wye Side
of a Delta/GY Transformer 65
2.19 Line-to-Line Fault on the Delta Side of a Delta/GY Transformer
with No Source Connected to the Delta Winding 66
2.20 Subcycle Relay Operating Time During an EHV
Double-Phase-to-Ground Fault 68
2.21 Self-Clearing of a C-g Fault Inside an Oil Circuit Breaker Tank 69
2.22 Self-Clearing of aB-g Fault Caused by aLine Insulator Flashover 70
2.23 Delayed Clearing of a Pilot Scheme Due to a Delayed
Communication Signal 71
2.24 Sequential Clearing of a Line-to-Ground Fault 72
2.25 Step-Distance Clearing of an L-g Fault 74
2.26 Ground Fault Clearing in Steps by an Instantaneous Ground
Element at One End and a Ground Time Overcurrent Element
at the Other End 76
2.27 Ground Fault Clearing by Remote Backup Following the
Failures of Both Primary and Local Backup (Breaker Failure)
Protection Systems 78
2.28 Breaker Failure Clearing of a Line-to-Ground Fault 79
2.29 Determination of the Fault Incident Point and Classification of
Faults Using a Comparison Method 81
viii CONTENTS
3 POWER SYSTEM PHENOMENA AND THEIR IMPACTON RELAY SYSTEM PERFORMANCE 85
3.1 Power System Oscillations Leading to Simultaneous
Tripping of Both Ends of a Transmission Line and the
Tripping of One End Only on an Adjacent Line 86
3.2 Generator Oscillations Triggered by a Combination
of L-g Fault, Loss of Generation, and Undesired Tripping
of Three 138-kV Lines 91
3.3 Stable Power Swing Generated During Successful
Synchronization of a 200-MW Unit 95
3.4 Major System Disturbance Leading to Different
Oscillations for Different Transmission Lines Emanating
from the Same Substation 96
3.5 Appearance of 120-Hz Current at a Generator Rotor
During a High-Side Phase-to-Ground Fault 98
3.6 Generator Negative-Sequence Current Flow
During Unbalanced Faults 101
3.7 Inadvertent (Accidental) Energization of a
170-MW Hydro Generating Unit 102
3.8 Appearance of Third-Harmonic Voltage at Generator Neutral 104
3.9 Variations of Generator Neutral Third-Harmonic Voltage
Magnitude During System Faults 106
3.10 Generator Active and Reactive Power Outputs During a
GSU High-Side L-g Fault 107
3.11 Loss of Excitation of a 200-MW Unit 108
3.12 Generator Trapped (Decayed) Energy 110
3.13 Nonzero Current Crossing During Faults
and Mis-Synchronization Events 112
3.14 Generator Neutral Zero-Sequence Voltage Coupling
Through Step-Up Transformer Interwinding Capacitance
During a High-Side Ground Fault 113
3.15 Energizing a Transformer with a Fault on the High Side within
the Differential Zone 115
3.16 Transformer Inrush Currents 118
3.17 Inrush Currents During Energization of the Grounded-Wye
Side of a YG/Delta Transformer 120
3.18 Inrush Currents During Energization of a Transformer Delta
Side 121
CONTENTS ix
3.19 Two-Phase Energization of an Autotransformer
with a Delta Winding Tertiary During a Simultaneous
L-g Fault and an Open Phase 124
3.20 Phase Shift of 30� Across the Delta/Wye Transformer Banks 127
3.21 Zero-Sequence Current Contribution from a Remote
Two-Winding Delta/YG Transformer 128
3.22 Conventional Power-Regulating Transformer
Core Type Acting as a Zero-Sequence Source 129
3.23 Circuit Breaker Re-Strikes 130
3.24 Circuit Breaker Pole Disagreement During a Closing Operation 132
3.25 Circuit Breaker Opening Resistors 133
3.26 Secondary Current Backfeeding to Breaker Failure
Fault Detectors 134
3.27 Magnetic Flux Cancellation 136
3.28 Current Transformer Saturation 138
3.29 Current Transformer Saturation During an Out-of-Step
System Condition Initiated by Mis-Synchronization of a
Generator Breaker 141
3.30 Capacitive Voltage Transformer Transient 143
3.31 Bushing Potential Device Transient During Deenergization
of an EHV Line 144
3.32 Capacitor Bank Breaker Re-Strike Following Interruption of
a Capacitor Normal Current 146
3.33 Capacitor Bank Closing Transient 147
3.34 Shunt Capacitor Bank Outrush into Close-in System Faults 149
3.35 SCADA Closing into a Three-Phase Fault 153
3.36 Automatic Reclosing into a Permanent Line-to-Ground Fault 154
3.37 Successful High-Speed Reclosing Following
a Line-to-Ground Fault 155
3.38 Zero-Sequence Mutual Coupling–Induced Voltage 156
3.39 Mutual Coupling Phenomenon Causing False Tripping
of a High-Impedance Bus Differential Relay During
a Line Phase-to-Ground Fault 159
3.40 Appearance of Nonsinusoidal Neutral Current During the
Clearing of Three-Phase Faults 162
3.41 Current Reversal on Parallel Lines During Faults 164
3.42 Ferranti Voltage Rise 166
3.43 Voltage Oscillation on EHV Lines Having Shunt
Reactors at their Ends 168
x CONTENTS
3.44 Lightning Strike on an Adjacent Line Followed
by a C-g Fault Caused by a Separate Lightning Strike
on the Monitored Line 172
3.45 Spill Over of a 345-kV Surge Arrester Used
to Protect a Cable Connection, Prior to its Failure 173
3.46 Scale Saturation of an A/D Converter Caused
by a Calibration Setting Error 174
3.47 Appearance of Subsidence Current at the Instant
of Fault Interruption 176
3.48 Energizing of a Medium Voltage Motor that has
an Incorrect Formation of the Stator Winding Neutral 177
3.49 Phase Angle Change from Loading Condition
to Fault Condition 179
4 CASE STUDIES RELATED TO GENERATOR SYSTEMDISTURBANCES 183
4.1 Generator Protection Basics 184
Case Studies 186
Case Study 4.1 Appearance of Double-Frequency (120-Hz)
Current in a Hydrogenerator Rotor Due to Stator
Negative-Sequence Current Flow During a 115-kV
Phase-to-Ground Fault 186
Case Study 4.2 Inadvertent (Accidental) Energization of a
170-MW Hydro Unit 193
Case Study 4.3 Loss of Excitation for a 200-MW Generating
Unit Caused by Human Error 204
Case Study 4.4 Loss-of-Excitation Trip in an 1100-MW Unit 212
Case Study 4.5 Mis-synchronization of a 50-MW Steam
Unit for a Combined-Cycle Plant 214
Case Study 4.6 Mis-synchronization of a 200-MW Hydro Unit 222
Case Study 4.7 Undesired Tripping of a Numerical
Differential Relay During Manual Synchronization
of a Hydro Unit 231
Case Study 4.8 Tripping of a 500-MW Combined-Cycle
Plant Triggered by a High-Side 138-kV Phase-to-Ground
Fault 236
Case Study 4.9 Tripping of a 110-MW Combustion Turbine
Unit in a Combined-Cycle Plant During a Power Swing 244
Case Study 4.10 Analysis of an 800-MW Generating
Plant DFR Record for a Normally Cleared
345-kV Phase-to-Ground Fault 247
CONTENTS xi
Case Study 4.11 Tripping of a 150-MW Combined-Cycle
Plant Due to a Failed Lead of One Generator
Terminal Surge Capacitor 250
Case Study 4.12 Generator Stator Ground Fault in an
800-MW Fossil Unit 260
Case Study 4.13 Three-Phase Fault at the Terminal of an
800-MW Generator Unit 265
Case Study 4.14 Three-Phase Fault at the Terminal of a
50-MW Generator Due to a Cable Connection Failure 271
Case Study 4.15 Generator Stator Phase-to-Phase-to-Ground
Fault Caused by Failure of the Rotor Fan Blade 276
Case Study 4.16 Undesired Tripping of a Pump Storage Plant
During a Close-in Phase-to-Ground 345-kV Line Fault 286
Case Study 4.17 Tripping of an 800-MW Plant and the
Associated EHV Lines During a 345-kV Bus Fault 293
Case Study 4.18 Tripping of a 150-MW Combined-Cycle
Plant During an External 138-kV Three-Phase Fault 296
Case Study 4.19 Tripping of a 150-MW Combined-Cycle Plant
During a Disturbance in the 138-kV Transmission System 303
Case Study 4.20 Undesired Tripping of a 150-MW
Combined-Cycle Plant Following Successful
Clearing of a 138-kV Double-Phase-to-Ground Fault 308
Case Study 4.21 Undesired Tripping of an Induction
Generator by a Differential Relay Having a Capacitor
Bank Within the Protection Zone 311
Case Study 4.22 Undesired Tripping of a Steam Unit Upon Its
First Synchronization to the System During the Commissioning
Phase of a Combined-Cycle Plant 314
Case Study 4.23 Sequential Shutdown of a Steam-Driven
Generating Unit as Part of a 500-MWCombined-Cycle Plant 318
Case Study 4.24 Wiring Errors Leading to Undesired Generator
Numerical Differential Relay Operation During the
Commissioning Phase of a New Unit 320
Case Study 4.25 Phasing a New Generator into the System
Prior to Commissioning 324
Case Study 4.26 Third-Harmonic Undervoltage Element Setting
Procedure for 100% Stator Ground Fault Protection 327
Case Study 4.27 Basis for Setting the Generator Relaying
Elements to Provide System Backup Protection 330
5 CASE STUDIES RELATED TO TRANSFORMER SYSTEMDISTURBANCES 335
5.1 Transformer Basics 336
5.2 Transformer Differential Protection Basics 344
xii CONTENTS
5.3 Case Studies 347
Case Study 5.1 Energization of a 5-MVA 13.8/4.16-kV Station
Service Transformer with a 13.8-kV Phase-to-Phase Bus Fault
Within the Transformer Differential Protection Zone 347
Case Study 5.2 Lack of Protection Redundancy for a
Generator Step-up Transformer Leads to Interruption
of a 230-kVArea 353
Case Study 5.3 Undesired Operation of a Numerical
Transformer Differential Relay Due to a Relay Setting
Error in the Winding Configuration 357
Case Study 5.4 Location of a 13.8-kV Switchgear
Phase-to-Phase Fault Using Transformer Differential
Numerical Relay Fault Records 363
Case Study 5.5 Operation of a Unit Step-Up Transformer
with an Open Phase on the 13.8-kV Delta Winding 370
Case Study 5.6 Using a Transformer Phasing Diagram,
Digital Fault Recorder Record, and Relay Targets to Confirm
the Damaged Phase of a Unit Auxiliary Transformer Failure 375
Case Study 5.7 Failure of a 450-MVA 345/138/13.2-kV
Autotransformer 381
Case Study 5.8 Failure of a 750-kVA 13.8/0.480-kV Station Service
Transformer Due to a Possible Ferroresonance Condition 387
Case Study 5.9 Undesired Tripping of a Numerical Transformer
Differential Relay During an External Line-to-Ground Fault 394
Case Study 5.10 Undesired Operation of Numerical
Transformer Differential Relays During Energization
of Two 75-MVA 138/13.8-kV GSU Transformers 407
Case Study 5.11 Undesired Operation of a Numerical
Transformer Differential Relay During Energization of a
5-MVA 13.8/4.16-kV Station Service Transformer 411
Case Study 5.12 Phase-to-Phase Fault Evolving into a
Three-Phase Fault at the High Side of a 5-MVA
13.8/4.16-kV Station Service Transformer 414
Case Study 5.13 Phase-to-Phase Fault Evolving into a
Three-Phase Fault at the 13.8-kV Bus Connection of
a 2-MVA 13.8/0.480-kV Station Service Enclosure 420
Case Study 5.14 Phase-to-Phase Fault in a 13.8-kV Switchgear
Caused by Heavy Rain Evolving into a Three-Phase Fault 426
Case Study 5.15 Undesired Operation of a Numerical Transformer
Differential Relay Due to a Missing CT Cable Connection
as an Input to the Relay Wiring 430
Case Study 5.16 Phase-to-Ground Fault Caused by Flashover of a
Transformer 115-kV Bushing Due to a Bird Droppings 434
CONTENTS xiii
Case Study 5.17 Using a Transformer Numerical Relay
Oscillography Record to Analyze Phase-to-Ground
Faults in a 4.16-kV Low-Resistance Grounding Supply 439
Case Study 5.18 Phase-to-Phase Fault Caused by a Squirrel in
a 13.8-kVCable BusWhich Evolves into a Three-Phase Fault 447
Case Study 5.19 13.8-kV Transformer Lead Phase-to-Phase
Fault Due to Animal Contact, Evolving into a 115-kV
Transformer Bushing Fault 451
Case Study 5.20 Undesired Tripping of a Numerical
Multifunction Transformer Relay by Assertion of a Digital
Input Wired to the Buchholz Relay Trip Output 456
6 CASE STUDIES RELATED TO OVERHEAD TRANSMISSION-LINESYSTEM DISTURBANCES 461
6.1 Line Protection Basics 463
6.2 Case Studies 466
Case Study 6.1 Using a DFR Record From One End Only
to Determine Local and Remote-End Clearing Times
for a Line-to-Ground Fault 466
Case Study 6.2 Analysis of Clearing Times for a Phase-to-Ground
Fault from Both Ends of a 345-kV Transmission Line Using
Oscillograms from One End Only 469
Case Study 6.3 Analysis of a Three-Phase Fault Caused
by Lightning 471
Case Study 6.4 Analysis of a Double-Phase-to-Ground
765-kV Fault Caused by Lightning 473
Case Study 6.5 Assessment of Transmission Tower Footing
Resistance by Analyzing a Three-Phase-to-Ground Fault
Caused by Lightning 476
Case Study 6.6 115-kV Phase-to-Ground Fault Cleared
First from a Solidly Grounded System, Then Connected
and Cleared from an Ungrounded System 478
Case Study 6.7 345-kV Phase-to-Ground Fault (C-g) Caused
by an Act of Vandalism 485
Case Study 6.8 345-kV Phase-to-Ground (A-g) Fault Due to
an Accident Along the Line Right-of-Way 489
Case Study 6.9 False Tripping of a 138-kV Current Differential
Relaying System During an External Phase-to-Ground Fault 495
Case Study 6.10 Undesired Operation of a 13.8-kV Feeder
Ground Relay During a Three-Phase Fault Due to an
Extra CT Circuit Ground 502
xiv CONTENTS
Case Study 6.11 Correction of a System Model Error
from Analysis of a Failure of a Post Insulator Associated
with a 115-kV Disconnect Switch 512
Case Study 6.12 Location of a 345-kV Line Fault Protected
by Electromechanical Distance Relays Using Information
from a DFR Record 519
Case Study 6.13 Location of an Outdoor 13.8-kV Switchgear
Fault at a Cogeneration Facility Using a DFR Fault
Record from a Remote Substation 524
Case Study 6.14 Breakage (Failure) of a 345-kV Subconductor
Bundle During a High-Resistance Tree Fault, Due to the
Heavily Loaded Line Sagging to a Tree 529
Case Study 6.15 115-kV Phase-to-Phase Fault Caused by
Failure of a Circuit Switcher 536
Case Study 6.16 Undesired Tripping of a 115-kV Feeder Due
to a Setting Application Error in the Time Overcurrent
Element for a Numerical Line Protection Relay 539
Case Study 6.17 Mitigation of Mutual Coupling Effects on
the Reach of Ground Distance Relays Protecting High-
and Extrahigh-Voltage Transmission Lines 544
7 CASE STUDIES RELATED TO CABLE TRANSMISSIONFEEDER SYSTEM DISTURBANCES 571
Case Studies 572
Case Study 7.1 Optimum Design of Relaying Protection Zones
Leads to Quick Identification of a Faulted 345-kV Submarine
Cable Section 572
Case Study 7.2 Undesired Operation of a 138-kV Cable Feeder
Differential Relay During the Commissioning Phase of
a 500-MW Plant 578
Case Study 7.3 Phase-to-Ground Fault Caused by Failure
of a 345-kV Cable Connection Between the Generator
and the Switchyard, Accompanied by Mechanical Failure
of One of the Cable Pot Head Phases 588
Case Study 7.4 Troubleshooting a 345-kV Phase-to-Ground
Fault Using Relay Targets Only 595
Case Study 7.5 Failure of a 345-kV Cable Connection
Between a 300-MW Generator and a 345-kV Switchyard,
Causing a Phase-to-Ground Fault 603
Case Study 7.6 138-kV Cable Pot Head Failure Analysis
Using Numerical Current Differential Relay Oscillography
and Event Records 607
CONTENTS xv
8 CASE STUDIES RELATED TO BREAKER FAILUREPROTECTION SYSTEM DISTURBANCES 615
8.1 Breaker Failure Protection Basics 616
Case Studies 626
Case Study 8.1 Tripping of a Combined-Cycle 150-MW Plant
by Undesired Operation of a Solid-State Breaker Failure
Relaying System 626
Case Study 8.2 115-kV Dual Breaker Failures Resulting in the
Loss of a 1000-MW Plant and Associated Substations 634
Case Study 8.3 230-kV Substation Outage Due to Circuit
Breaker Problems During the Clearing of a Close-in
Phase-to-Ground Fault 640
Case Study 8.4 Failure of a 230-kV Circuit Breaker Leading to
Isolation of a 1000-MW Plant and Associated Substations 646
Case Study 8.5 Generator CB Failure During Automatic
Synchronization of the Circuit Breaker 654
Case Study 8.6 Circuit Breaker Re-strikes While Clearing
Simultaneous Phase-to-Ground Faults on a 230-kV
Double-Circuit Tower 660
Case Study 8.7 345-kV Capacitor Bank Breaker Fault
Coupled with an Additional Failure of a Dual SF6
Pressure 345-kV Breaker During the Clearing of the Fault 664
Case Study 8.8 Oil Circuit Breaker Failure Following the
Clearing of a Failed 230-kV Surge Arrester 671
Case Study 8.9 Detection of a Remote Circuit Breaker
Problem from Analysis of a Local Oscillogram
Monitoring Line Currents and Voltages 676
Case Study 8.10 Blackout of a 138-kV Load Area Due to a
Primary Relay System Failure and the Lack of DC Control
Power for the Secondary Relay System Circuit 678
Case Study 8.11 Installation of Two 345-kV Breakers in
Series Within a Ring Substation Configuration to Mitigate the
Loss of Critical Lines During Breaker Failure Events 682
Case Study 8.12 Design of Two 138-kV Circuit Breakers in
Series to Fulfill the Need of Breaker Failure Protection 682
9 PROBLEMS 685
Index 715
xvi CONTENTS
PREFACE
The fault recording equipment used in monitoring power systems evolved from a wet
trace and light beams writing on special photo-sensitive paper or film oscillograms to
digital, microprocessor-based technology. Some of the old records took days to
develop, as in the case of the wet trace and recurring problems with sensitive papers.
As a result, some key records were lost, making the analysis of power system
disturbances extremely difficult. In addition, starting recording equipment was a
hassle, causing unreliable oscillograph operations. A digital fault recorder (DFR) is
considered an intelligent electronic device that can be accessed via communication
links to send fault records automatically to remote operating centers and engineering
offices immediately following a disturbance. This allowed a rapid analysis to make it
possible to restore the system. Accurate root-mean-square measurements as well as a
host of software packages can be executed to verify the systemmodel and to assess the
impact of disturbances on power system equipment.
Analysis of power system disturbances is an important function that monitors the
performance of a protection system. It can also provide a wealth of valuable
information regarding correct behavior of the system. Understanding power system
phenomena can be simplified, and adoption of safe operating limits and protective
relaying practices can be enhanced. Review of DFR and numerical relay fault records
for system operations can help to isolate incipient problems so that corrections can be
implemented before the problems become serious. Understanding power system
oscillations and system relaying response during a power swing condition can be
enhanced, thus avoiding system blackouts. In addition, understanding power system
engineering concepts and the use of symmetrical components in the analysis of power
system faults can be enforced and enhanced through DFR analysis.
A bulk power system is normally protected by two redundant relaying systems.
The performance of these systems can be monitored through an analysis of system
disturbances. Restoration of a power system requires correct analysis of the distur-
bance that caused the outage to confirm that it is safe to reenergize the system. Correct
analysis can contribute to safe restoration without the fear of energizing faulty power
system equipment. In addition, through proper system disturbance analysis confi-
dence can be gained in the philosophy behind relaying application.
To facilitate the reader’s review process, the DFR records are accompanied by
unique functional system diagrams that show the voltages and currents monitored,
using designation labels that match the records. A section is devoted to documenting
power system phenomena as they appear in actual case studies. This will provide
xvii
engineers who have limited experiencewith such problems the necessary background
to perform their own analyses of their systems.
The book serves as a forum to document and present my 40+ years of experience in
the area of power system disturbance analysis. Many colleagues from the American
Electric Power Service Corp., the New York Power Authority, and several utilities
have contributed to the book directly or indirectly, and I am grateful for their input.
It has beenmy intention to simplify the topics presented and provide clear guidance as
well as basic education to relay engineers. In this new format, the theory and basic
fundamentals of relay applications are first briefly explained. This is then followed
by real case studies involving system disturbances, to enforce these basics. The
studies are based on actual occurrences collected through my years of involvement in
the protection of utility systems. The real names of utility plants, substations, and lines
have been replaced by generic labels.
In the old vertical integration environment, training and educationwere essential to
most utilities. In the highly competitive new environment, exchange of experience
and technical information is hampered, as is passing useful experience to young
engineers. At this point in the history of protective relaying, the fundamentals that
have been handed down from generation to generation are in danger of becoming lost.
This has given me the impetus to document my experience in a useful format that can
benefit engineers, since little training is now available for engineers entering the
protection and control field in the area of system disturbance analysis.
In the book I present in detail how power system disturbance analysis is used as an
important tool to judge the performance of protection systems. Actual DFR records,
oscillograms, and numerical relay fault records are analyzed to demonstrate how to
deduce the sequence of events. Topics such as the information needed for analysis,
fault incident angle, and power system phenomena and their impact on relay system
performance are covered. Power system phenomena derived from an analysis of
system disturbances are described. In addition, case studies of actual system
disturbances involving the performance of protection systems for generators, trans-
formers, overhead transmission lines, cable feeders, and breaker failures are included.
Several chapters are devoted to system disturbance analysis as a tool for optimizing
the performance of relaying schemes. In addition, the book can serve as a tool for
validating power systemmodels and provides awealth of technical information about
the behavior of power systems.
The book is intended primarily for engineers and technicians working in the areas
of protection and control, power system operation, and electrical power system
equipment. It is also intended for operators and support staff at energy control centers
to enhance their technical background in the safe restoration of a power system
following a disturbance. The book will provide engineers with a basic background
in most power system phenomena and their impact on the behavior of protection
systems. The book can also be used as a textbook for undergraduate and graduate
students seeking to enhance their power backgrounds. A chapter is devoted to
problems, to enhance understanding of the system disturbance analysis function.
The book can thus provide an incentive to colleges to offer the system disturbance
analysis topic in either an undergraduate or graduate course.
MOHAMED A. IBRAHIM
xviii PREFACE
1
POWER SYSTEMDISTURBANCE ANALYSIS
FUNCTION
An analysis of system disturbances provides a wealth of valuable information
regarding power system phenomena and the behavior of protection systems. Expe-
rience can be enhanced and knowledge can be gained from the analysis function. This
book is organized, first, to cover the analysis function and how it can be implemented.
Then, in the following sections, phenomena related to system faults and the clearing
process of faults from the power system are described. Power system phenomena
derived from an analysis of system disturbances are stated. In addition, case studies of
actual system disturbances involving the performance of protection systems for
generators, transformers, overhead transmission lines, cable feeders, and breaker
failures are provided. A section is devoted to problems that enhance an understanding
of the system disturbance analysis function.
Analysis of system disturbance is based on 60-Hz phenomena associated with
power system faults. Therefore, sampling rates of digital fault recorders (DFRs) are
designed to fulfill this requirement. High-frequency power system transient analysis
requires special devices other than conventional DFRs and numerical relays, with
unique requirements different from those of a traditional power system disturbance
analysis function.
Disturbance Analysis for Power Systems, First Edition. Mohamed A. Ibrahim.� 2012 Mohamed A. Ibrahim. Published 2012 by John Wiley & Sons, Inc.
1
To analyze the performance of protective relaying systems, high-speed digital
fault and disturbance recording devices need to be employed properly. Equipment
can be used for continuous monitoring of the behavior of relaying installed on a
power system during the occurrence of either faults or power swing or switching
operations. The equipment can be used to explain undesired operations and to
assess system performance during correct operation. Analysis of fault records will
help in adapting operating and protection practices and in assuring the reliability
of a bulk power system. The analysis will also help to isolate problems and
incipient failures. In addition, the strategic placement of DFR equipment should
provide adequate coverage of the overall system response to any type of system
fault or wide-area system disturbance. For this reason, DFR applications and
implementation on a bulk power system are mandated by industry standards and
regulations.
A review of DFR records for every operation in a system will help to isolate
incipient difficulties so that corrections can be provided before a serious problem
develops and to provide basic useful information about the performance of the
relaying system. A review of all fault records for disturbances on a system can
enhance the reliability of a relay system. Systematic analysis of disturbances can play
an important role in system blackout avoidance. When they occur during the early
stage of analysis, flagging relay and system problems should be addressed before they
precipitate into wider-area interruption and system blackouts. This can be accom-
plished by analyzing correct operations and finding the causes of incorrect operations.
In addition, it can provide a better assessment of the validity of relay setting
calculations, correct current transformer (CT) and voltage transformer (PT) ratios,
and correct breaker operations. It can also enhance the system restoration process by
providing fault types and locations and a better measure of power quality.
The proposed NERC Reliability Standard PRC-002-02, “Disturbance Monitoring
and Reporting Requirements,” is noted here as a document which ensures that
regional reliability organizations establish requirements for the installation of
disturbance-monitoring equipment and reporting of disturbance data to facilitate
analyses of system events and verification of system models.
1.1 ANALYSIS FUNCTION OF POWER SYSTEM DISTURBANCES
Analysis of power system disturbances can be summarized on the basis of the
following primary functions:
1. The need to view fault data as soon as possible after a fault or disturbance occurs
so as to restore the system safely.
2. The need to design the DFR with a reasonable pre-fault time (5 to 10 cycles) to
capture incipient initiating conditions (e.g., surge arrester spillover).
3. The need to design the DFRwith a long post-fault time, adjustable from 0 to 5 s,
to be able to analyze backup protection clearing times (60 cycles or more) and
2 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION
limited power system swings (several seconds) following the occurrence of
system disturbances.
4. The need to manipulate the data time base on the DFR record to analyze the
effect of faults.
5. The need, finally, to manipulate the DFR data channels and view only those
selected.
Ideally, the analysis function should be carried out for all relay operations in a
system. The normally cleared events can lead to the discovery of equipment problems
and can also be used as a teaching example for power system behavior and
phenomena. From the analysis function, monthly disturbance analysis reports can
be prepared. In addition, other reports can be generated. The analysis function will
focus primarily on providing answers to the following basic questions:
1. What happened?
2. Why did it happen?
3. What is going to be done about it?
In essence, a sequence-of-events report, or time line, needs to be developed.
Traditionally, a DFR monitors power system voltages and currents, whereas a se-
quence-of-events recorder (SER) monitors relay outputs, breaker and disconnect switch
positions, alarms, relay targets, and relay communication channels.ADFR can integrate
both functions bymonitoring events and analog quantities. The followingare someof the
functions that analysis of DFR records, in conjunction with SER records, can provide:
1. Sequence of operation
2. Fault types
3. Clearing times
4. Reclosing times
5. Relay problems such as:
(a) Failure to trip
(b) Failure to target
(c) Failure to reset
(d) Delayed clearing
6. Communication problems such as:
(a) False operation of blocking schemes during carrier transmission holes
(b) Failure to operate for permissive overreaching transfer trip schemes
during signal loss
7. Circuit breaker problems such as:
(a) Contact arcing
(b) Unequal pole closing
ANALYSIS FUNCTION OF POWER SYSTEM DISTURBANCES 3
(c) Unequal pole opening
(d) Re-strike
(e) Reignition
8. Fault current and voltage magnitudes to confirm a short-circuit model
9. CT saturation
10. Asymmetrical current caused by dc (direct current) offset
11. Fault locations, currently provided by numerically based distance relaying,
can also be provided by DFRs when sufficient analog signals per line are
monitored
1.2 OBJECTIVE OF DFR DISTURBANCE ANALYSIS
Data obtained from DFRs and numerical relaying can be used for continuous
monitoring of the behavior of the relay system and assist in setting operating margins
on critical control and protective apparatus in an electric power system during system
disturbance events such as faults, power swings, and switching operations. Analysis
of the data can have the dual role of explaining undesired operations and assessing
system performance during correct operation.
The primary objective of obtaining and analyzing DFR data is for the purpose of
adapting operating and protection practices as well as control strategies to assure the
security and dependability of the bulk power protection system. The secondary
objective is for the purpose of helping to isolate problems and incipient failures. This
requires a review of all DFR data for every operation, to detect and correct incipient
troubles before they become a serious problem. The ability should exist for remote
interrogation for data analysis and manipulation. Data need to be viewed as soon as
possible after a fault or disturbance occurs. The data time base for the DFR record
should be manipulated for analysis. The ability should exist to manipulate data
channels and view only those of importance. This will ensure that other channels will
not obscure vital data.
It is a good idea to analyze all disturbances in a system, but this may require
additional personnel who may not be available within the utility’s environment.
Indeed, it should be realized that the knowledge gained from analyzing mundane
operationsmay prove to be very valuable. Following are some of the benefits that may
be gained from an analysis of system disturbances:
1. Knowledge of the performance of the relaying system and associated inputs,
outputs, communication system and circuit breakers
2. Root-mean-square (RMS) ground current calculations confirming the power
system model
3. Development of statistics summarizing a fault
4 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION
4. Optimization of the performance of the relaying system by optimizing the
design process through analysis feedback
5. Identification of power system phenomena of interest to be used as teaching
tools for engineers to enhance their basic technical backgrounds
6. Review of mundane operations that result in successful fault clearing to reveal
valuable power system phenomena and correction of system design and
modeling errors
1.3 DETERMINATION OF POWER SYSTEM EQUIPMENT HEALTHTHROUGH SYSTEM DISTURBANCE ANALYSIS
As mentioned earlier, an analysis of system disturbances can provide feedback
regarding the integrity of power system equipment and associated protection systems.
The following are examples of some of the feedback of analysis results that can be
used to assess equipment health:
1. Detection of excessive capacitor bank outrush currents into close-in faults
requires assessment of current transformer (CT) secondary-connected bur-
dens to reduce overvoltage stress across CT secondary circuits.
2. Detection of circuit breaker (CB) re-striking current during the CB fault
current interruption process requires CB inspection and examination for
possible testing and maintenance.
3. Detection of unequal CB pole closing or opening requires inspection and
examination for possible testing and maintenance of the circuit breaker.
4. Disappearance of third-harmonic current flow in generator neutrals requires
assessment of generator neutrals for the possibility of either an open neutral or
a stator ground fault near the neutral.
5. Determination of undesired relay operation and follow-up analysis can help in
the detection of misapplications of relay settings.
6. Detection and follow-up analysis of undesired relay operation can lead to the
discovery of certain hidden relay failures before the undesired operation can
precipitate into a serious event that can stress the system.
7. Detection of mutual coupling phenomena can help in fine-tuning ground
distance relay settings.
8. Detection of magnetic flux cancellation for CB tripping functions can help in
identifying single failure criteria that can have a serious impact on clearing
future occurrences of system faults.
9. Detection of excessive capacitative voltage transformer (CVT) transients
upon the occurrence or clearing of close-in faults can lead to fine-tuning of
the zone 1 distance relay setting reach.
DETERMINATION OF POWER SYSTEM EQUIPMENT HEALTH 5
10. Thorough system disturbance analysis can lead to optimization of protective
relaying dc schematics.
11. Thorough system disturbance analysis can lead to optimization of protective
relay settings.
12. Thorough system disturbance analysis can lead to detection of surge arrester
spillover that can lead to mitigation of the spill prior to failure.
13. Thorough system disturbance analysis can lead to detection of power system
oscillation, which may require running stability studies to determine the need
to add either out-of-step blocking or tripping relay schemes.
14. Thorough system disturbance analysis can lead to detection of CT saturation,
which may require either reduction of secondary fault currents (by raising the
CT ratio) or reduction of CT-connected burden.
15. Simulation and running of actual system faults, based on disturbance analysis,
using computer-based test sets with the help of the COMTRADE fault current
format can lead to a determination of failed relays and their associated
auxiliary relays.
16. Analysis of multiphase faults caused by lightning strikes can lead to optimi-
zation of transmission tower footing resistance.
17. Determination offlashover at voltage peaks, leading to insulation failure as the
main cause, can provide feedback to correct any insulation design deficiency.
1.4 DESCRIPTION OF DFR EQUIPMENT
Figure 1.1 illustrates the basic subsystem blocks in a digital fault recorder. The analog
input signals are first interfaced to a surge suppression package and sampling filters.
The input current flows through a shunt and is converted to a voltage that is sampled,
converted to digital form by an analog-to-digital (A/D) converter, and then read and
processed by themicroprocessor. Similarly, the input voltage is scaled down to a range
compatible with the A/D range to be converted and then read and processed by the
microprocessor. The A/D has to be checked periodically with sufficient accuracy and
an acceptable A/D conversion resolution of a true 16 bits. Delta–sigma A/D con-
verters implemented on a commercial single-chip design, with built-in autocalibra-
tion capabilities and built-in linear-phase multistage digital decimation and filtering
capability are used for some commercial DFRs to guarantee no aliasing in analog
input-sampled signals. Binary inputs representing various functions within the
substation are also sampled to give a time resolution of about 1ms. The basic
concepts of a DFR function of sampling and storing data whenever a trigger threshold
is exceeded is executed inside the device memory by instruction steps within specific
firmware. RAM memory is used for data and is normally checked on startup of the
DFR device. ROM and PROM are used in the DFR algorithm and software analysis
package and checked periodically bymemory check sum routines. EPROM is used to
store trigger and parameter settings. A programmable digital signal processing
6 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION
microprocessor is used to perform serial–parallel conversions and extended-precision
adder functions, triggering of recording via various algorithms, and trigger timing
management. The DFR-captured data can be retrieved from a remote location via an
acquisition computer called the master station. The DFR system should be time-
synchronized using an IRIG-B signal from global positioning satellite (GPS) recei-
vers. DFR equipment offers normal communication capability to allow for remote
retrieval of fault and event records, making for immediate disturbance analysis and
reducing the time and cost needed to perform the analysis task.
1.5 INFORMATION REQUIRED FOR THE ANALYSISOF SYSTEM DISTURBANCES
The sequence of events can be derived from an analysis of the fault information that
may be available from several devices. Presently, the problem is that toomanydata are
available from every intelligent electronic device (IED) and the challenge is for relay
and operating engineers to select the most vital data, which need to be analyzed
quickly to restore the affected system safely. A sequence-of-events report may be
developed using some of the following data:
1. Digital fault recorder records and/or oscillograms (if applicable)
2. Sequence-of-event recorder records
I
Microprocessor
RAM
Communication
ROMPowersupply
Digital input
Current &voltage inputs
Contactinputs (DI)V
Shunt
Remote Locations
Parallelport
Serialport
A/D Sample / hold
Signalconditioning
Surge protection& Filters
Samplingclock
IRIG-B
To GPS
receiver
PROMEPROM
HMI
Fig. 1.1 Subsystems of a DFR device.
INFORMATION REQUIRED FOR THE ANALYSIS OF SYSTEM DISTURBANCES 7
3. Relay targets
4. Numerically based protection oscillograph fault records (if applicable)
5. Phasor measurement records
6. System operation logs
7. Event story as created by field personnel
8. SCADA record, indicating system configurations and loading
9. PC-based short-circuit study simulations
10. As-built one-line, ac three-line, elementary, wiring, and logic diagrams
11. Operating procedures
12. Computer logs and customer information
13. Description of system clearances in the event of an operating or technician
error
14. Strip/chart recording or smart IED meters of power system quantities (active
power, reactive power, frequency, voltage, and current)
1.6 SIGNALS TO BE MONITORED BY A FAULT RECORDER
1.6.1 Analog Signals
ADFR will monitor voltages and currents as well as digital inputs from the electrical
power system.Channel assignments to theDFR should considermonitoring sufficient
information to implement the fault location option. This requires the monitoring of
three phase-neutral voltages and three phase currents with an option to either monitor
or calculate the neutral current (In) for each transmission line. In addition, the DFR
should monitor all neutral currents and ground sources at the substation to be able to
validate the short-circuit model for ground faults. Validation of a short-circuit model
for phase faults is difficult to accomplish, due to the effect of loading, which is
normally not factored in a steady-state quasi-short-circuit study simulation.
The analog channels are normally configurable as voltage or current inputs. The
phase-to-neutral voltage inputs may be scaled for about 66.4V, with a range of 0 to
250V RMS, allowing a margin of more than 2 pu (per unit) overvoltage. Current
inputs may be scaled for 5ARMS (nominal load current) and at least 100A full-scale
input using calibrated shunts. The thermal duty can be rated at least 10A RMS
continuous and at least 200A RMS short time for 2s. Monitoring a generator dc field
current can provide valuable educational information about negative-sequence
double-frequency-induced rotor current during unbalanced system faults. In addition,
monitoring a generator dc field will reveal the 60-Hz induced rotor current during
inadvertent energization of generator incidents. Both phenomena are illustrated
herein through applicable generator case studies.
Dedicated sensors with over, under, and rate-of-change value settings were used
for traditional (conventional) oscillographs. DFRs can also be programmed for
8 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION
each analog channel for over, under, or rate-of-change settings. Additional sensors
may include positive-sequence current or voltage, negative-sequence current or
voltage, zero-sequence current or voltage, frequency transducers rate of change of
impedance during a power system swing (long-term rate of change), and total harmonic
distortion.
Following is a list of typical analog channels monitored at the substation level:
. Phase-to-neutral voltages
. Line phase and neutral currents
. Transformer neutral currents
. Transformer tertiary currents
. Transformer polarizing currents (sum of more than one current)
. Capacitor currents (phase and neutral)
. Shunt reactor currents
. Transformer high- and low-side currents
. Zero-sequence voltages
. Bus voltages
. Generator neutral voltages
. Generator fields
. Generator currents
. Generator phase-to-neutral voltages
Monitoring of tertiary (3I0) current by aDFRmayhelp in the classificationof ground
faults. TheCTs for all the phases are paralleled to collect ground current (3I0) and filter
out any loading currents (the sum of balanced positive-sequence currents¼ 0). For
breaker-and-one-half substation configuration, monitoring of the middle breaker
ground current can provide valuable information for circuit breaker maintenance
by showing the last breaker of the two that will interrupt the fault current. In addition,
determinationofwhich of the two line breakers is exhibiting a re-strike during the fault-
clearing process can be accomplished.
1.6.2 Event (Digital or Binary) Inputs and Outputs
Most DFR systems provide means for event recording. This may be status change
(closing or opening) of an auxiliary contact associated with a circuit breaker or a
disconnect switch operation or the presence of voltage at a control circuit node, which
would indicate that a certain control logic function was performed. Examples of
events are positions for circuit breakers; disconnect switches, dc presence for control
circuits, relays, auxiliary relays, lockout relays, and protection communication
signals. Event recording can also be performed by dedicated SERs in the form of
stand-alone packages or as part of other systems, such as remote terminals for SCADA
systems. Most SER systems are designed with a typical 1-ms resolution time.
SIGNALS TO BE MONITORED BY A FAULT RECORDER 9
1.7 DFR TRIGGER SETTINGSOFMONITORED VOLTAGES AND CURRENTS
In older oscillograph equipment, recording was generally begun using dedicated start
sensors to capture fault records. Delta tertiary zero-sequence currents and transformer
neutral currents were commonly used to sense ground faults. Undervoltage sensors
were also used at key voltage points within the substation, together with an operation
limiter, to sense phase faults. Dedicated negative-sequence sensors were also used to
trigger the device for unbalanced faults.
The present state-of-the-art DFR is designed with trigger algorithms that are
capable of detecting over, under, rate-of-change, and swing conditions for each analog
input channel. The trigger algorithm provides concurrent user selectivity for step
change, ramp change, and oscillatory conditions. The DFR is normally triggered to
capture a record by all analog channels and selected binary inputs. The DFRmonitors
for line faults three phase-to-neutral voltages and three phase and neutral currents for
each line connected at the substation. Phase undervoltage and phase overcurrents will
trigger the DFR for phase line faults, and neutral currents will trigger for ground line
faults. One analog trigger is sufficient to capture a DFR record.
In addition, triggering can be initiated using positive-, negative-, and zero-
sequence symmetrical components as a supplement for shunt faults and as a main
trigger for series imbalance, such as open phase. A frequency computation from a bus
voltage can also trigger a frequency deviation. Total harmonic distortion and
individual harmonic distortion for a specific frequency can also be programmed to
trigger a DFR to provide an analysis of power quality. Impedance can also be
calculated and used to trigger a DFR. Power swing amplitude for voltage, current, and
active and reactive power, as well as oscillation frequency and rate of change of
impedance, can also be used to trigger a DFR. In addition, selected digital inputs can
be used to capture a record: for example, emergency shutdown lockout relays, which
can be energized by many abnormal conditions at a generating plant.
Manual triggering is also provided to test the data capture and output function of a
DFR.Themanual triggermaybehardwired or software based,with anoption for remote
acquisition from a master station location. The DFR can also be configured to have a
very slow scan to capture long-term events such as power system oscillations or out-of-
step conditions. Each trigger function is user programmable with an individual dual-
mode limiter function. This function prevents excessive recording both in case a trigger
condition persists for an extended period of time and in case a “chattering” trigger
should occur. The operation limiter feature will restrict data recording to a selectable
length in the event of a continuous long-term trigger condition. An example is the use of
undervoltage to trigger the capture of a record for phase faults on a system. Since all
analog-monitored channels will be used as triggers, this voltage may be associated with
a transmission line. When the line is removed from service during a scheduled outage,
the undervoltage sensorwill trigger theDFR to capture a record. However, ameanmust
be established to limit the length of the record since triggering will continue as long as
the line is out of service. It should be noted that if phase overcurrent is used to trigger a
DFR for faults, the operation limiter feature is not required.
10 POWER SYSTEM DISTURBANCE ANALYSIS FUNCTION