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

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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