Protective Relaying for Power Generation Systems

564 Pages·2005·29.32 MB·English

by  Donald Reimert

© 2006 by Taylor & Francis Group, LLC
D R
ONALD EIMERT
Boca Raton London New York
A CRC title, part of the Taylor & Francis imprint, a member of the
Taylor & Francis Group, the academic division of T&F Informa plc.
© 2006 by Taylor & Francis Group, LLC
Published in 2006 by
CRC Press
Taylor & Francis Group
6000 Broken Sound Parkway NW, Suite 300
Boca Raton, FL 33487-2742
© 2006 by Taylor & Francis Group, LLC
CRC Press is an imprint of Taylor & Francis Group
No claim to original U.S. Government works
Printed in the United States of America on acid-free paper
10 9 8 7 6 5 4 3 2 1
International Standard Book Number-10: 0-8247-0700-1 (Hardcover)
International Standard Book Number-13: 978-0-8247-0700-2 (Hardcover)
Library of Congress Card Number 2005052952
This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with
permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish
reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials
or for the consequences of their use.
No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or
other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information
storage or retrieval system, without written permission from the publishers.
For permission to photocopy or use material electronically from this work, please access www.copyright.com
(http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA
01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For
organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for
identification and explanation without intent to infringe.
Library of Congress Cataloging-in-Publication Data
Reimert, Donald.
Protective relaying for power generation systems / Donald Reimert.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-0-8247-0700-2 (alk. paper)
ISBN-10: 0-8247-0700-1 (alk. paper)
1. Protective relays. 2. Electric power systems--Protection. I. Title.
TK2861.R36 2005
621.31'7--dc22 2005052952
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
Taylor & Francis Group and the CRC Press Web site at
is the Academic Division of Informa plc. http://www.crcpress.com
© 2006 by Taylor & Francis Group, LLC
Preface
The importance of a generating unit in terms of economics and system reliability cannot be
overstated. This is true for units connected to the nations power grid or for units operating at
industrial facilities. Although there are many books on protective relaying these tend to focus on
transmission and distribution relaying. Many include generator and motor topics. But the
coverage provided does not promote a thorough understanding of the hazards to the generator or
the limitationof protective elements applied.
Settings applied to any protective device represent a balance between adequate sensitivity to
detect a damaging condition and the security required to prevent false tripping during events
thatdonotthreatentheprotectedequipment.Theimportanceofthisbalanceatgenerationfacilities
is highlighted by the intense scrutiny given these protection schemes in the wake of large-scale
system outages. This scrutiny is not limited to the generator but extends to the auxiliary system
aswell.
The aim of this book is to provide in-depth discussions of the major electrical protection
schemesassociatedwithsynchronousgeneratorsandinductionmotors.Theprinciplesandcriteria
presentedareapplicabletobothlargeandsmallmachines.Thediscussionsincludeanalysisofthe
damageanddamagingmechanismsrelatingtoeachprotectivefunction.Anunderstandingofthese
parametersisimportantnotonlyfortheapplicationofprotectionbutalsowhenoperabilityissues
arise during orafter abnormal operating events.
This book includes detailed derivations of complex system interaction. An understanding of
such phenomena is generally not required when applying rule-of-thumb setting criteria.
However, such derivations provide insight into the basis for the rule-of-thumb settings and the
framework to recognize situations when such settings are not appropriate. These derivations are
also often useful for post-incident analysis.
IncludedinthetextareconstructionsforMathcadandExcelworkbooksfortheanalysisofCT
performance in generator differential applications, generator shaft torque transients following
out-of-phase closing, fault-induced impedance swings for two generator systems, steady-state
anddynamicstabilitylimits,undermanualandautomaticvoltagecontrol.Thesefilesareavailable
for download at taylorandfrancis.com.
Inthetextanalyticaltechniquesarepresentedtoassessmotorcapabilityandrelayresponseto
transientssuch asstartingand cyclic loading.
Ihaveattemptedtopresenteachsubjectasastandalonechapterthatallowsquickreferenceon
detailed analysis.Example settings calculations are provided.
© 2006 by Taylor & Francis Group, LLC
Author
Donald Reimert, a registered professional engineer, graduated from Penn State University more
than35yearsago.Hisprofessionalcareerwasfocusedonthedesignandapplicationofprotective
relaysandrelaysystems.Thishasincludeddistribution,transmission,andgenerationsystemfacili-
ties.Inadditiontoprotectiverelaying,hehasdesignexperienceinsubstationandelectricalsystems
associated with fossil, nuclear, and hydro-generation facility. As a system planner, he developed
regional projects with financial justifications to meet future load requirements. He has helped to
developandpresentaseriesofprotectiverelaycoursesfortheUniversityofWisconsin-Milwaukee
including “Introduction to Protective Relaying,” “Protective Relay Principles and Applications,”
and “Advanced Protective Relaying for Transmission Systems.” He has also developed and
presented“AdvancedProtective Relayingfor Generator and Generator Auxiliaries.”
© 2006 by Taylor & Francis Group, LLC
Contents
Chapter1
Generator Normal Operations ............................................................. 1
1.1 The Sample System ................................................................. 1
1.2 Generator Capability ................................................................. 1
1.3 VoltageLimitations ................................................................. 3
1.3.1 Sample System GSU Transformer Limits ...................................... 3
1.4 SystemLimitations .................................................................. 4
1.5 Generator Capability Variationswith Voltage ........................................ 7
1.6 ExcitationSystem ................................................................... 9
Reference ................................................................................ 13
Chapter2
Generator Short Circuit Calculations ..................................................... 15
2.1 Introduction ........................................................................ 15
2.2 Short-Circuit Current Characteristics ................................................ 15
2.3 Generator InternalMagnetics ....................................................... 15
2.4 Generator MagneticStructures ...................................................... 19
2.5 Generator Constants ................................................................ 23
2.6 Fault CurrentCalculations .......................................................... 25
2.6.1 Initial Load ................................................................. 26
2.6.2 Fault CalculationOverview ................................................. 28
2.6.3 DeterminationofX and Fault Currents ...................................... 30
f
2.6.4 Three-Phase Short Circuit ................................................... 31
2.6.5 Phase-to-Phase Short Circuit ................................................ 32
2.6.6 Phase-to-Ground Fault ...................................................... 33
2.6.7 Other Fault Conditions ...................................................... 35
2.6.8 DCComponent ofShort-Circuit Current ..................................... 35
2.6.9 RMS Asymmetrical Current ................................................. 37
2.7 VoltageRegulator .................................................................. 37
2.8 Practical Shortcuts .................................................................. 38
2.9 Short Circuit CalculationExample .................................................. 40
2.9.1 Establish PrefaultConditions ................................................ 40
2.9.2 Three-Phase Fault at Generator Terminals ................................... 41
2.9.3 Phase-to-Ground FaultonHigh-Voltage Terminals ofGSU .................. 42
2.9.4 Effects ofthe Automatic Voltage Regulator .................................. 45
References ............................................................................... 46
Chapter3
Generator Differential Relay: 87G ....................................................... 47
3.1 Introduction ........................................................................ 47
3.2 IdealDifferential Relay ............................................................. 47
3.3 Practical Considerations ............................................................ 47
3.3.1 CT Ratings ................................................................. 49
3.3.2 CT Saturation .............................................................. 50
3.3.3 CTs and FaultCurrentReplication .......................................... 53
© 2006 by Taylor & Francis Group, LLC
3.4 Percentage DifferentialRelay ...................................................... 56
3.5 Relay Characteristics .............................................................. 57
3.5.1 ElectromechanicalRelays .................................................. 57
3.5.2 Solid-State andMicroprocessorRelays ..................................... 57
3.6 Minimum OperatingCurrentSetting ............................................... 59
3.7 Slope Setting ...................................................................... 59
3.7.1 Requirements for Slope Setting ............................................ 59
3.7.2 Advantage ofLow Slope .................................................. 60
3.7.3 Sensitivityand Load Current ............................................... 60
3.7.4 Relay Response toSaturation .............................................. 61
3.7.5 Methods ofChoosing Slope Settings ....................................... 63
3.7.5.1 Manufacturer’s Recommendations ................................. 63
3.7.5.2 Qualitative Determinationof Slope ................................ 63
3.7.5.3 Error CurrentCalculations for Unsaturated CT ..................... 63
3.7.5.4 Mason’sMethod .................................................. 64
3.7.5.5 Example ofMason’sMethod ...................................... 65
3.7.5.6 Fundamental Frequency Analysis .................................. 66
3.8 Sample System DifferentialRelay Settings ......................................... 68
3.8.1 Sample System DifferentialCircuit ......................................... 68
3.8.2 ElectromechanicalRelay ................................................... 68
3.8.2.1 Specifications for Relay Chosen ................................... 68
3.8.3 Choosing Slope Static and Microprocessor Relays .......................... 69
3.9 Stabilizing Resistor ................................................................ 70
3.10 Balancing Burden ................................................................. 73
3.11 TimeDelay ....................................................................... 74
3.12 Frequency Response ............................................................... 74
References ............................................................................... 75
Chapter4
Backup Fault Protection ................................................................. 77
4.1 Purpose andImplementation ........................................................ 77
4.1.1 Standard OvercurrentRelays ................................................ 77
4.1.2 Voltage-Dependent Relays .................................................. 78
4.1.3 Electromechanicalvs. Electronic Relays ..................................... 79
4.2 VoltageSupervisedOvercurrentRelays ............................................. 79
4.2.1 Voltage-Controlledand Voltage-RestrainedRelays .......................... 79
4.2.2 Application Optionsand Fault Sensitivity .................................... 80
4.2.2.1 SchemeSensitivity vs.Potential Transformer (PT) and
CurrentTransformer (CT) Connection .............................. 80
4.2.2.2 Sensitivity Related to Relay Type .................................. 82
4.2.2.3 DeltaRelay Currents ............................................... 82
4.2.3 SettingsConsiderations ..................................................... 83
4.2.3.1 Basic Requirement ................................................. 83
4.2.3.2 Automatic Voltage Regulator inService ............................ 84
4.2.3.3 51VTransmission SystemBackup Limitations ..................... 84
4.2.3.4 Effects of Wye-DeltaTransformer .................................. 86
4.2.3.5 Self-Excitation Generators ......................................... 87
4.2.3.6 Relay Responseto TransientCurrent ............................... 88
4.2.3.7 Equipment Protection .............................................. 90
4.2.4 SettingCriteria ............................................................. 93
© 2006 by Taylor & Francis Group, LLC
4.2.5 Relay Currentand Voltage Calculations ..................................... 93
4.2.5.1 Relay Currentand Voltage Equations ............................... 94
4.2.5.2 SequenceCurrents and Voltages Calculations ....................... 95
4.2.6 Sample System 51V Relay Settings ......................................... 98
4.2.6.1 Fault Calculations .................................................. 99
4.2.6.2 Choosing the Undervoltage Setting ................................ 102
4.2.6.3 Choosing OvercurrentSetting ..................................... 103
4.2.6.4 Choosing Time Delay Setting ..................................... 103
4.2.6.5 AuxiliaryPTs to Correct for Wye-DeltaPhase Shift ................ 104
4.3 Distance Relays ................................................................... 105
4.3.1 Distance Relay Characteristics ............................................. 105
4.3.1.1 Z Measured by Phase Distance Relay .............................. 105
4.3.1.2 Mho Distance Relay .............................................. 107
4.3.1.3 System Impedancevs. Relay Characteristic ........................ 109
4.3.2 Setting Considerations ..................................................... 111
4.3.2.1 Load Limits ...................................................... 111
4.3.2.2 ApparentImpedance .............................................. 112
4.3.2.3 Influenceof anInterposing Wye-Delta Transformer ................ 113
4.3.2.4 AuxiliaryPTs to Correct for Wye-DeltaPhase Shift ................ 115
4.3.3 Other Distance Relay Applications ......................................... 116
References .............................................................................. 116
Chapter5
Generator Ground Fault Protection ...................................................... 117
5.1 Introduction ....................................................................... 117
5.2 Generator Grounding Considerations ............................................... 117
5.2.1 Ground Fault Current Limitation ........................................... 117
5.2.2 OvervoltageConcerns ..................................................... 118
5.2.3 Core Damage Cause byGroundFault ...................................... 120
5.3 Methods of Grounding ............................................................. 121
5.3.1 Ungrounded System ....................................................... 121
5.3.2 Solidly Grounded/Effectively Grounded .................................... 125
5.3.3 High-ImpedanceGrounding ................................................ 128
5.3.3.1 Distribution TransformerGrounding ............................... 128
5.3.3.2 High-ResistanceGrounding on Sample System .................... 130
5.3.3.3 Ground Fault Neutralizers ......................................... 130
5.3.4 Low-Impedance Grounding ................................................ 141
5.3.4.1 Low-Resistance Grounding ........................................ 143
5.3.4.2 Low-ReactanceGrounding ........................................ 144
5.3.4.3 Grounding Transformers .......................................... 144
5.4 Ground Fault Protection ........................................................... 145
5.4.1 Alarm vs.Tripping for High-Impedance
Grounded System .......................................................... 146
5.4.2 Electromechanical and Electronic Relays ................................... 146
5.4.3 High-ImpedanceGround Protection ........................................ 146
5.4.3.1 Neutral OvervoltageScheme ...................................... 146
5.4.3.2 Application of 59GN onSample System ........................... 148
5.4.3.3 Broken Delta Overvoltage Scheme ................................ 150
5.4.3.4 Overcurrent Scheme .............................................. 151
5.4.4 Low-Impedance Ground Protection ......................................... 154
5.4.4.1 Ground Differential ............................................... 154
© 2006 by Taylor & Francis Group, LLC
5.4.5 100% Stator Protection Schemes ........................................... 155
5.4.5.1 Third-HarmonicSchemes ......................................... 155
5.4.5.2 Third-HarmonicUndervoltage Scheme ............................ 157
5.4.5.3 Settingsfor Sample System 27H Scheme .......................... 158
5.4.5.4 Third-HarmonicOvervoltage Scheme .............................. 159
5.4.5.5 Third-HarmonicVoltageRatioScheme ............................ 159
5.4.6 Neutral Injection Scheme .................................................. 161
References .............................................................................. 163
Chapter6
Unbalanced Current Protection .......................................................... 165
6.1 Introduction ....................................................................... 165
6.2 What Is Negative-Sequence Current? ............................................... 165
6.3 Effects of Negative-SequenceCurrent .............................................. 166
6.3.1 Rotor Heating ............................................................. 166
6.3.1.1 Cylindrical Rotor Generators ...................................... 166
6.3.1.2 Salient Pole Generators ........................................... 168
6.3.2 PulsatingTorque .......................................................... 169
6.4 Generator Negative-Sequence Capability ........................................... 169
6.4.1 ContinuousUnbalanced Capabilities ........................................ 170
6.4.2 Short Time Unbalanced Currents ........................................... 170
6.5 Sources ofNegative SequenceCurrent ............................................. 171
6.5.1 Unbalanced Faults ......................................................... 171
6.5.2 Open Phases ............................................................... 172
6.6 In-Service I2tDuty vs. Standards ................................................... 172
2
6.6.1 Calculation ofI2tDuty ..................................................... 174
2
6.6.1.1 IsolatedGenerator ................................................ 174
6.6.1.2 The Interconnected Generator ..................................... 174
6.6.1.3 The’venin’s EquivalentCircuit .................................... 175
6.6.2 Unbalanced Duty on Sample System ....................................... 175
6.7 Unbalanced Current Protection ..................................................... 182
6.8 Negative-Sequence Relay Settings ................................................. 184
6.8.1 Calculation ofOpen-Circuit Current ........................................ 184
6.8.2 Negative SequenceRelay Setting ........................................... 187
References .............................................................................. 189
Chapter7
Motoring Protection ..................................................................... 191
7.1 Introduction ....................................................................... 191
7.2 Effects of Motoring ................................................................ 191
7.2.1 Consequences for aSteamTurbine ......................................... 192
7.2.2 Consequences for Other Prime Movers ..................................... 192
7.3 Protection ......................................................................... 192
7.3.1 Mechanical Protection: Steam Turbines ..................................... 193
7.3.2 Electrical Protection ....................................................... 193
7.4 SequentialTrip Logic .............................................................. 196
7.5 BackupProtection ................................................................. 196
7.6 Setting Device 32 ................................................................. 197
7.7 Applying Reversed PowerRelay onthe SampleSystem ............................ 197
References .............................................................................. 199
© 2006 by Taylor & Francis Group, LLC
Chapter8
Field Winding Protection ................................................................ 201
8.1 Field Ground Protection ........................................................... 201
8.1.1 Field Ground Hazard ...................................................... 201
8.1.2 Field Ground Protection ................................................... 201
8.1.3 Field Ground Detection .................................................... 202
8.1.3.1 Field Ground Relay Selectionand Settings ......................... 205
8.2 Field Overcurrent Protection ....................................................... 206
8.2.1 Field OvercurrentTransients ............................................... 206
8.2.2 OvercurrentProtection Schemes ............................................ 208
8.2.3 ApplicationofAC Relaysto Protect the Field Winding ..................... 209
8.2.3.1 Basic RectifierOperation .......................................... 210
8.2.3.2 Relay Quantities .................................................. 213
8.3 SettingsForField Overcurrent/OvervoltageRelays ................................. 215
8.3.1 Full Load Values .......................................................... 215
8.3.2 MaximumField Current ................................................... 215
8.3.3 MaximumField Current froma BridgeRectifier ............................ 215
8.4 Applying FieldOC Protection on the Sample System ............................... 216
8.4.1 Rated Field Voltage ....................................................... 216
8.4.2 MaximumAvailableFieldCurrent ......................................... 217
8.4.3 Pickup Setting ............................................................. 217
8.4.4 Time Delay ................................................................ 219
References .............................................................................. 220
Chapter9
Overexcitation .......................................................................... 221
9.1 Introduction ....................................................................... 221
9.2 Causes ofOverexcitation .......................................................... 221
9.3 Damage ........................................................................... 223
9.4 V/Hz Limits ...................................................................... 224
9.5 Protection ......................................................................... 225
9.5.1 Field Monitoring Relays ................................................... 225
9.5.2 V/Hz Limiter ............................................................. 226
9.5.3 V/Hz Relay Applications .................................................. 227
9.6 Settings ........................................................................... 228
9.6.1 Generator V/Hz Settings ................................................... 229
9.6.2 Transformer Settings ....................................................... 229
9.6.3 Generator/Transformer Settings ............................................ 230
9.6.4 Setting Limitations ........................................................ 230
9.6.5 Time Delay Settings ....................................................... 231
9.7 Differential Relay Response toOverexcitation ...................................... 233
9.8 Application of V/Hz Protection onthe Sample System ............................. 234
References .............................................................................. 236
Chapter10
Abnormal FrequencyProtection ......................................................... 237
10.1 Introduction ...................................................................... 237
10.2 Effect on Generator .............................................................. 237
10.3 SteamTurbines .................................................................. 237
10.4 Combustion Turbines ............................................................. 241
10.5 Hydro Generators ................................................................ 241
10.6 ExcitationSystem ................................................................ 241
© 2006 by Taylor & Francis Group, LLC