Switchgear Protection And Power Systems [13 ed.] 8174092323, 9788174092328

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SWITCHGEAR PROTECTION AND

POWER SYSTEMS (Theory, Practice and Solved Problems) Other Related Books of Special Interest “Testing, Commissioning, Operation and Maintenance of Electrical Equipment , * ” by S Rao “Power Transformers and Special Transformers”, by S . Rao * ’’Electrical Substation Engineering and Practice”, by S . Rao m “EHV-A.C . and HVDC Transmission and Distribution ”, by S . Rao m “Energy Technology ( Non-conventional, Renewable & Conventional )” , by Dr . B. B . Parulekar and S . Rao a Utillization Generation & Conservation of Electrical Energy by Sunil S . Rao a “Handbook of Electrical Engineerng” by S . L. Bhatia “Electrical Safety, Fire Safety Engineering and Management” by Prof . H . L. Saluja & S . Rao, New Arrival , Jan. 1999 . B Industrial Safety, Health and Envlornment Management Systems by Sunil S . Rao & Er . R. K. Jain Electrical Power by S .L. Uppal & Sunil S . Rao * m Electrical Engineering Technology by Dr . N . Datta a Electrical Machinery by Dr . P.S . Bimbhra a Electrical Machinery by Dr . S . K. Sen m Electrical Measurement and Measuring Instruments by Dr . R. Prasad Generalised Theory of Electrical Machines by Dr. P.S . Bimbhra B High Voltage Cable Accessories and Cable Installation by T .S . Swaminathan High Voltage Engineering by Dr M . P , Chaursia Industrial and Power Electronics by G.K. Mithal * Linear Control Systems by B.S . Manke * Power Electronics by Dr . P.S . Bimbhra Power System Analysis by Prof . S .S . Vadhera a Utilization of Electrical Power and Traction by G.C. Garg

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A textbook for B . E . , B . Tech., M . E . (Electrical ), Technical Teacher’s Training, Power Engineering Training Courses and a ready reference book for Engineers in Electricity Boards, Projects, Consultants, Switchgear Industry , Power Sector covering EVERY topic on Switchgear Protection , and Power System Operation and Automation.

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SUNIL S RAO M . E . (Electrical ), M . I . E .

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Note : These books are of Topical Interest Tor Students and Professionals . '

KHANNA PUBLISHERS 4575/15 , Onkar House, Opp. Happy School , Daryaganj, Delhi- 110002 Phones : 23243042 Fax : 23243043

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Published

by : Romesh Chander Khanna for KHANNA PUBLISHERS 2-B, Nath Market , Nai Sarak, Delhi-110006.

ISBN No. 81-7409-232-3

All Rights Reserved. Reproductions from this book are stricly prohibited except for Reviews. No written matter and, illustrations shall be reproduced without written consent from the Publishers and the Author. Reproductions in this book are with express permission from the corresponding manufacturers. They have been duly acknowledged by the author .

Dedicated, to Saroj , Sheetal and Chetan !; First Edition Eleventh Edition Twelfth Edition Thirteenth Edition : Fourth Reprint : 2010

1973 1999 (10 Reprints ) 2007, September 2008, October

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Price : Rs 350.00

Computertypetset at: Softserve Computer Systems, Delhi

Printed at: . Mohanlal Printers,

Delhi.

r FOREWORD

PREFACE TO THE THIRTEENTH EDITION

, , There has been a long-felt need of a book giving . e nsiveaa systematic W1 c &ear and Protection information of and Power System Studies , ( " e congratulated for fulfilling 1® ne ® y publishing a book. I am proud , ° ° ^ rlf &U P that Shri Su s u en o mine and I am very happy to write this foreword ° °° °° ’ *S an The author has a brillin < ; 311 k ds a first Engineering of Karnataka B . class E . degree ' in Electrical Uhivnrsiiw an fu ur ) class° degree in ‘Power Systems’ of University . He has good practical . Poona m many of the reputed Electrical Organisations like Hindustan rf erieace Firms and Brown 6 Baroaa > Kirlosker Electrical Co Ltd Bangalore * State Electricity Boards of Knrrmfoi Tl College of Technology, * etaHeisWOrkinZ 'atMaulana .AzadRegional! m ectrica Engineering, and has “Switchgear Protection and Power Systems been teaching -

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The widespread acceptance of the earlier Editions promoted this revised and enlarged edition . The book presents in-depth knowledge about the principles and practices of modern power system engineering. It gives an integrated approach to the complex phenomena related with Switchgear , Protection, Fault-Calculations, Power System Analysis-Operation-Control-Automation , Digital relays, Micro-processor based Relays and Microprocessor based Integrated Control and Protection Systems, Energy Systems . The k k wifi serve as a regular text book for electrical engineering courses to prepare the °° students for the careers in power sector. The book will also serve as a reference book to electrical engineers working in power sector , electrical manufacturing industry , academic and testing

institutions, etc.

Since the publication of the first edition of the book Switchgear and Protection in 1973, many advances have occured in field of the Switchgear, Protection and Power System Automation . While conventional protection and switching devices will continue to serve, entirely new type of devices over 59 chapters. Section the techniques are now available. The development of SF6 and Vacuum circuit- breakers have made and ..... oi various, iuciuamg SI16 circuit breakers ^ wxuwuiicu types nearly obsolete. The static relays have replaced the electro-mechanical relays . EHVother the , vacuum circuit breakers, and discusses about choice, erection , maintenance the : AC and HVDC transmission are now commercially successful . Large interconnected networks are and testing of high voltage apparatus.EHV A C. Transmission and HVDC transmission. /low voltage switchgear and EHV; being automatically controlled from load control centres by means of on-line SCADA, AGC and EMS Systems. The developments in power electronics have resulted in the successful use of static VAR Section II deals with fault current calculations, role of network analysers and digital in the calculation of fault computers Sources (SVS), HVDC Convertors etc. Digital computers and microprocessors are being increasingly current of complicated system networks. used for protection and automation . Fibre- optic cables have been successfully used for data Section III deals with constructional . transmission and operational aspects of electromagnetic protective relays and protective systems for generators, transformers, ; Due energy crisis and increasing capital costs of power projects , there is a world -wide to the motors and transmission lines. Section IV deals with interconnecting adjacent AC Networks by means of EHV-AC or HVDC links. trend towards fundamentals of static relays and static protection schemes. The techniques of testing and maintenance have advanced with an aim of increased reliability Section V deals with advanced topics power in system availability of electrical power supply. Knowledge of specifications, testing, maintenance, and controls, applications of digital computer and microprocessors for load * frequency control commissioning has gained significance. The power system analysis techniques have also advanced and back up protection Load Frequency Control, Voltage , Power System Stability. control and compensation of significantly . Power System Network Automation Reactive Power, Voltage Stability, have been explained. India and other developing countries have ambitions development plans in power sector . Some The matter is presented in a very landmarks in the power sector of India include indigenous capability of design , manufacture and lucid style and simple English. The by neat, clear sketches and book is profusely illustrated commissioning of EHV-AC Sub-stations and apparatus, establishment of 400 kV. AC network , diagrams and graphs. The author is to be congratulated for consulted the leading technical journals having introduction of HVDC Systems, interconnections between Regional Grids, introduction of static in the field and presenting the “Switchgear Protection and Power Systems information relays and static protection systems, increasing use of digital computers and microprocessors, ” up-to-date, in his book. The author has regarding expansion mature art of teaching in the presentation of testing facilities, etc. exhibited Some typical solved problems are given of the subject matter inspite of his short teaching experiencea. technology The of protection and automation have been revolutionised by the introduction of throughout the book. microprocessor based combined protection , control , monitoring systems . Such systems have been With addition of some unsolved problems, summary and provocative questions at chapter, the book may serve as introduced for substation protection, generator protection , HVDC protection . This book covers the the end of a text book in universities for a course in “Switchgear Protectioneach principles and applications of this latest technology and the important topics in Interconnected Power Systems” in the under-graduate and and postgraduate curriculum. The a useful guide and reference to Systems. The new chapters include EHV-AC Transmission , HVDC Transmission Systems, Power book ‘ should also serve as Power Engineers, considering the volume provides . Interconnections, Power System Automation with SCADA Systems, Power System Planning, Latest of practical information it Power map of India , Microprocessor based Protection . Energy Technology- Renewable and I am very proud of the young author and express my sincere Nonconventional and Conventional . The Corelation between Energy Sector and Power sector has of writing the Foreward to thanks to him for giving me the privilege this book of his. been illustrated. Chapters on Power system Calculations and Load Flow Studies, The principles and procedures H.11. KARA KA RADI)] of network calculations and load flow studies have been simplified and explained by a few solved B.Sc. ( Hons.), D.I.I.Sc. i examples. 'Recent Advances’ in Intelligent Circuit Brep kers, Fiber-optic Cable Applicaions , Compact B.Sc. (Tech.) ( Munch.), F. I. E. Intelligent Substations, ISO-9000 and TQMI are covered ip Appendix-A, while Appendix-B highlights Sen. M„ I. E. E . E. ; overall system description of Distribution Management System . Principal The patronage of Academic Institutions and Power System Engineers to this book is hereby Karnataka Regional Engg. College j J \ gratefully acknowledged . ( Suratkal, S .K. ) 1 Karnataka.

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of CONTENTS ACKNOWLEDGEMENTS SECTION I The Author gratefully acknowledges the assistance by various Manufacturers and Organisations :

International Electrotechnical Commission . Indian Bureau of Standards. British Standards Institution , a AEG, West Germany. ABB, Sweden. The Aluminium Industries Ltd. India. Bharat Heavy Electricals Ltd., India. General Electric, U.S.A. GEC Alsthom Ltd., England. * m Hindustan Brown Boveri ltd (ABB), India. Hi Velm Industries Pvt. Ltd. India. Indian Aluminium Company Ltd., India. Jyo\i Ltd ., India. IjCirlosker Electric Co. Ltd., India. Larsen & Toubro Ltd . , India. « MCB (India) Pvt. Ltd., India. 1 Mitsubishi Electrical Corporation , Japan. Reyrolle Parson Ltd., England . Siemens India Ltd. * Universal Electric Ltd.( India. Westinghouse Electric Corporation. , U.S.A.

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1. INTRODUCTION 1.1. Switchgear and Protection 1.2. Sub-station Equipment 1.3. Faults and Abnormal Conditions 1.4. Fault Calculations 1.5. Fault Clearing Process 1.6. Protective Relaying 1.7. Neutral Grounding (Earthing) and Equipment Grounding 1.8. Over-voltages and Insulation Co-ordination 1.9. Some Terms in the Test 1.10 . Standard Specifications 1.11. Electro-mechanical Relays and Static Relays 1.12 . Applications of On-line Digital Computers Microprocessors And Static Protective/control Devices in Power System 1.13. Interconnected Power System 1.14. Load-frequency Control, Load Shedding 1.15. Voltage Levels in Network and Sub-stations 1.16. Voltage Control of AC Network 1.17. Static Var Sources (SVS) 1.18. Power System Stability 1.19. HVDC Obtion 1.20. Power System Analysis 1.21. Power System Network Calculations and Load Flow 1.22. Objective and Tasks 2. HIGH-VOLTAGE A.C, CIRCUIT-BREAKERS 2.1. Introduction 2.2. The Fault Clearing Process 2.3. The Trip-circuit 2.4. Recent Advances 2.5. Classification Based on Arc Quenching Medium 2.6. Technical Particulars of a Circuit-breaker 2.7. Assembly of Outdoor Circuit breakers 2.8. Structural Form of Circuit-breakers 2.9. Operating Mechanisms 2.9.1. Closing Operation 2.9. 2. Opening Operation 2.9.3. Closing Followed by Opening Operating I 2.9.4. Types of Mechanisms 2.10 . Interlocks, Indication and Auxiliary Switch. 2.11. Circuit-breaker Time (Total Break Time) 2.12 . Auto Reclosure 2.13. Auto Reclosure of EHV Circuit Breakers for Transmission Lines

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2.14. Auto Reclosure for Distribution Lines (Upto 33 kv) 2.15. Weight Operated Reclosing, Pole Mounted Circuit breakers

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1 3 4 4 5 5 5 6 6 7 8 8 9 10 11 11 13 13 14

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22 22 23

24 24 24 26

27 27 28 29 29

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11. SHORT CIRCUIT TESTING OF CIRCUIT-BREAKERS 11.1. Introduction 11.2 . Stresses on Circuit-breaker During Short-circuit Tests Part A : Short-Circuit Test Plants 11.3. Short-circuit Testing Plants

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13.6. Maintenance of Circuit Breakers 13.7. Typical Maintenance Record Card 13.8 . Maintenance of Air Break Circuit Breaker , Fusegear for Low And Medium Voltages 13.9. Maintenance of Vacuum Circuit-breaker 13.10. Maintenance of SF@ Circuit-breaker 13.11. Insulation Resistance Measurement 13.12. Insulation Resistance Measurement at Site 13.13. Likely Troubles and Essential Periodic Checks 13.14 . Installation of Drawout Metalclad Switchgear 13.15. Safety Procedures 13.16. Installation of Outdoor Circuit-breakers

164- 189 164 . 164 165

Part B : Direct Testing

11.4. Direct Testing 11.5. Rules for Type Tests 11.6. Short time Current Tests on Circuit-breakers, Isolators, Busbars, CTS Etc. 11.7. Basic Short-circuit Test Duties 11.8. Ci’itical Current Tests 11.9. Short-line Fault Tests 11.10 . Line Charging Breaking Current Tests 11.11. Out-of-phase Switching Tests 11.12 . Capacitive Current Switching Tests 11.12.1. Single Capacitor Bank Current Breaking Test 11.13. Cable:charging Breaking Current Test 11.13.1. Small Inductive Current Breaking Tests 11.13.2 . Recommendations for Small Inductive Current Switching Tests 11.14. Reactor Switching Test

170 173 174 174 175 176 176 178 179 179 180 181

Part C : Indirect Testing 11.15. Unit Testing or Element Testing 11.16. Synthetic Testing 11.17. Substitution Test 11.18. Capacitance Test 11.19. Compensation Test 11.20. Development Testing of Circuit-breakers

183 183 186 187 188 188

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12. INSULATION REQUIREMENT AND HIGH VOLTAGE TESTING OF CIRCUIT BREAKERS 12.1. Introduction 12.2 . Overvoltages 12.3. Design Aspects 12.4. Causes of Failure of Insulation 12.5. Purpose ofH.V. Testing of Circuit breakers 12.6. Tests on a High Voltage Circuit-breakers 12.7. Some Terms and Definitions. 12.8. Impulse Voltage Tests and Standards Impulse Waves 12.9. Impulse Generator 12.10. Test Plant for Power Frequency Tests 12.11 H.V. Testing Transformer 12.12 . Sphere Gaps

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13. INSTALLATION AND MAINTENANCE 13.1. Introduction 13.2 . Break Down Maintenance Versus Preventive Maintenance 13.3. Inspection, Servicing, Overhaul 13.4. Guidelines for Maintenance of Switchgear 13.5. Field Quality Plans ( FQP) /

169 170

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14. HRC FUSES AND THEIR APPLICATIONS 14.1. Introduction 14.2. Types of Devices with Fuse 14.3. Definitions 14.4. Construction 14.4.1. HRC Fuses for Semiconductor Devices and Thyristors 14.5. Fuse Link of HRC Fuse 14.6. Action of HRC Fuse 14.7. Shape of Fuse Element 14.8. Specification of a Fuse Link 14.9. Characteristic of a Fuse 14.10 . Cut-off 14.11. Classification and Categories 14.12. Selection of Fuse Links 14.13. Protection of Motor 14.14. 14.15. 14.16. 14.17. 14.18. 14.19. 14.20.

Discrimination Protection of Radial Lines Protection of Meshed Feeders with Steady Load by HRC Fuses Equipment Incorporating Fuses High Voltage Current Limiting Fuses Expulsion Type High-voltage Fuse Drop-out Fuse i 14.21. Test on Fuse

190-199 190 191 191 191 192 193

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218-232 218 218 218 219 220 222 222 222 223 224 224 224 225 227 228 228 230 230 231 231 231 232

15-A. METAL-ENCLOSED SWITCHGEAR , CONTROLGEAR AND CONTACTOR 233- 248 15.1. Introduction 233 15.2. Types of Switchgear 233 Part A : High Voltage Indoor Metal Enclosed Switchgear I 15.3. General Features of Indoor Metal-enclosed Switchgear 234 15.4. Draw-out Type Metal-enclosed Switchgear 235 15.5. Switchgear with Vacuum Interrupters ! 237 Voltage B Part : Low Metal Switchgear Clad and Voltage Low Circuit Breakers 200- 217 I 15.6. Unit Type Metal Clad Low Voltage Switchgear and Motor 200 Control Centers 237 200 15.7 . Low Voltage Circuit Breakers ' 239 201 15.7.1. Classification. 239 201 15.7. 2. Rated Quantities 239 202 194 195 | 195 196 196 197

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I . SULPHUR HEXAFLUORIDE (SF6) CIRCUIT-BREAKER AND SFe

9. VACUUM INTERRUPTER AND VACCUM

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INSULATED METALCLAD SWITCHGEAR (GIS) Part I : Properties of SFe Gas 7.1. Introduction 7.2. Physical Properties of SIB Gas 7.3. Chemical Properties of SFg Gas 7.4. Dielectric Properties of SF6 Gas 7.5. Arc Extinction in SF6 Circuit-breakers 7.5.1. Single Pressure Puffer Type Circuit-breaker with Single Flow of Quenching Medium 7.5.2. Double Flow of Quenching Medium Part II : Outdoor SF6 Circuit Breakers

7.6. Types Design 7.7. Single Pressure Puffer Type SFg Circuit-breaker 7.7.1. Configuration of a Single Pressure Puffer Type EHV Circuit-breaker . Pressure Dead Tank SFg C.B. ( Now Obsolete) Double 7.8 7.9. Merits of SF6 Circuit-breakers 7.10 . Some Demerits of SF6 Circuit-breaker 7.11. SF@ Filled Load Break Switches 7.12. Gas Monitoring and Gas Handling Systems Part III : SFe Insulated Metalcad Switchgear (Sub-Station) 7.13. Introduction to SFg Switchgear (GIS ) 7.14. Advantages of S 6 Switchgear 7.15 . Demerits of SF6 Insulated Switchgear /, 7.16. General Constructional Features of SFg-Gas Insulated " Switchgear ( GIS) 7.17. Gas Monitoring 7.18. Gas Filling and Monitoring System .for SFg Switchgear 7.19 . Transportation and Handling of SFg Gas 7.20. Gas Transfer Units 7.21. SFg Insulated EHV Transmission Cables (GIC)

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8. MINIMUM OIL CIRCUIT-BREAKER AND BULK OIL CIRCUIT-BREAKER 8.1. Introduction 8.2. Tank Type Bulk Oil Circuit-breaker ( Now Obsolete) 8.3. Minimum Oil Circuit-breaker 8.4. Principle of Arc-extinction on Oil Breakers 8.5. Pre-arcing Phenomenon 8.6. Sensitivity to TRV 8.7. Circuit-breakers with Internal Sources of Extinguishing Energy-

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101 103

103 104 105 106 107 107 107 108 108 109 109

109 114 118 118 118

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9.1. Introduction 9.2. Electrical Breakdown in High Vacuum 9.3. Arc Extinction in Vacuum Interrupters 9.4 . Construction of a Vacuum Interrupter 9.5. Arc Interruption in High Vacuum 9.6. Degree of Vacuum in Interrupters 9.6.1. Construction of a Vacuum Interrupter 9.7 . Interruption of Short-circuit Currents in Vacuum Interrupters 9.8. Design Aspects of Vacuum Interrupters 9.8.1. Length of Interrupter 9.8.2. Contact Travel (Contact (GAP)) 9.8.3. Contact Shape Breaking Current 9.8.4. Contact Size and Shape for Required Short-circuit 9.8.5. Contact Material 9.9. Time/travel Characteristics 9.10. Contact Pressure 9.11. Contact Acceleration During Opening 9.12. Contact Erosion 9.13. Vacuum Level and Shelf Life of Interrupters 9.14. Checking of Vacuum Circuit-breakers 9.15. Range of Vacuum Switchgear, Vacuum Controlgear and Vacuum 9.16. Merits of VCB’s 9.17. Demerits of VCB’s 9.18. Switching Phenomena with VCB 9.18.1. Reignition in Vacuum Circuit-breakers Voltages ' 9.18. 2. Capabilities of Modern Circuit Breakers for Medium Duty, Switching Motor for Vcb with Problem voltage 9.18.3. Switching Over RC Surge Suppressors

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10. TESTING OF HIGH VOLTAGE A.C. CIRCUIT BREAKER 10.1. Classification of the Test nb 10.2. Type Tests 10.2.1. Mechanical Test ( Endurance Tests ) 122 130 10.2.2. Temperature-rise Tests 10.2.3. Measurement of D.C. Resistance 122 10.2.4. Millivolt Drop Tests 128 10.2.5. No-load Operation Tests and Oscillographic and Other Records -r 131 137 10.2.6. Dielectric Tests j 10.2.7. Basic Short circuit Test Duties ; 131 10.3. Routine Tests 131 | 10.4 . Development Tests 133 { 10.5. Reliability Tests 134 \ 10.6. Commissioning Tests 135 | 10.7 . Insulation Resistance Measurement at Site 135 | ) 10.8. High Voltage Power Frequency Withstand Test (Routine Test 10.9. Routine Tests on Circuit-breakers 136 : 10.9.1. Mechanical Operating Tests (Routine Test) 136

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7-A. Routine, Site/Field Testing of GIS 7.22 . Routine Testing of GIS 7.23. Site/field Testing of GIS

Critical Current 8.8. Contact Assembly j

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138 153 138 139 140 140 142 142 142 143

144 144 144 144 145 147 147 148 148 148 149 149 149 151 151 151 151 152

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11. SHORT CIRCUIT TESTING OF CIRCUIT-BREAKERS 11.1. Introduction 11.2 . Stresses on Circuit-breaker During Short-circuit Tests Part A : Short-Circuit Test Plants 11.3. Short-circuit Testing Plants

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13.6. Maintenance of Circuit Breakers 13.7. Typical Maintenance Record Card 13.8 . Maintenance of Air Break Circuit Breaker , Fusegear for Low And Medium Voltages 13.9. Maintenance of Vacuum Circuit-breaker 13.10. Maintenance of SF@ Circuit-breaker 13.11. Insulation Resistance Measurement 13.12. Insulation Resistance Measurement at Site 13.13. Likely Troubles and Essential Periodic Checks 13.14 . Installation of Drawout Metalclad Switchgear 13.15. Safety Procedures 13.16. Installation of Outdoor Circuit-breakers

164- 189 164 . 164 165

Part B : Direct Testing

11.4. Direct Testing 11.5. Rules for Type Tests 11.6. Short time Current Tests on Circuit-breakers, Isolators, Busbars, CTS Etc. 11.7. Basic Short-circuit Test Duties 11.8. Ci’itical Current Tests 11.9. Short-line Fault Tests 11.10 . Line Charging Breaking Current Tests 11.11. Out-of-phase Switching Tests 11.12 . Capacitive Current Switching Tests 11.12.1. Single Capacitor Bank Current Breaking Test 11.13. Cable:charging Breaking Current Test 11.13.1. Small Inductive Current Breaking Tests 11.13.2 . Recommendations for Small Inductive Current Switching Tests 11.14. Reactor Switching Test

170 173 174 174 175 176 176 178 179 179 180 181

Part C : Indirect Testing 11.15. Unit Testing or Element Testing 11.16. Synthetic Testing 11.17. Substitution Test 11.18. Capacitance Test 11.19. Compensation Test 11.20. Development Testing of Circuit-breakers

183 183 186 187 188 188

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12. INSULATION REQUIREMENT AND HIGH VOLTAGE TESTING OF CIRCUIT BREAKERS 12.1. Introduction 12.2 . Overvoltages 12.3. Design Aspects 12.4. Causes of Failure of Insulation 12.5. Purpose ofH.V. Testing of Circuit breakers 12.6. Tests on a High Voltage Circuit-breakers 12.7. Some Terms and Definitions. 12.8. Impulse Voltage Tests and Standards Impulse Waves 12.9. Impulse Generator 12.10. Test Plant for Power Frequency Tests 12.11 H.V. Testing Transformer 12.12 . Sphere Gaps

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13. INSTALLATION AND MAINTENANCE 13.1. Introduction 13.2 . Break Down Maintenance Versus Preventive Maintenance 13.3. Inspection, Servicing, Overhaul 13.4. Guidelines for Maintenance of Switchgear 13.5. Field Quality Plans ( FQP) /

169 170

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14. HRC FUSES AND THEIR APPLICATIONS 14.1. Introduction 14.2. Types of Devices with Fuse 14.3. Definitions 14.4. Construction 14.4.1. HRC Fuses for Semiconductor Devices and Thyristors 14.5. Fuse Link of HRC Fuse 14.6. Action of HRC Fuse 14.7. Shape of Fuse Element 14.8. Specification of a Fuse Link 14.9. Characteristic of a Fuse 14.10 . Cut-off 14.11. Classification and Categories 14.12. Selection of Fuse Links 14.13. Protection of Motor 14.14. 14.15. 14.16. 14.17. 14.18. 14.19. 14.20.

Discrimination Protection of Radial Lines Protection of Meshed Feeders with Steady Load by HRC Fuses Equipment Incorporating Fuses High Voltage Current Limiting Fuses Expulsion Type High-voltage Fuse Drop-out Fuse i 14.21. Test on Fuse

190-199 190 191 191 191 192 193

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203 207 207 208 208 210 210 211 212 214 216

218-232 218 218 218 219 220 222 222 222 223 224 224 224 225 227 228 228 230 230 231 231 231 232

15-A. METAL-ENCLOSED SWITCHGEAR , CONTROLGEAR AND CONTACTOR 233- 248 15.1. Introduction 233 15.2. Types of Switchgear 233 Part A : High Voltage Indoor Metal Enclosed Switchgear I 15.3. General Features of Indoor Metal-enclosed Switchgear 234 15.4. Draw-out Type Metal-enclosed Switchgear 235 15.5. Switchgear with Vacuum Interrupters ! 237 Voltage B Part : Low Metal Switchgear Clad and Voltage Low Circuit Breakers 200- 217 I 15.6. Unit Type Metal Clad Low Voltage Switchgear and Motor 200 Control Centers 237 200 15.7 . Low Voltage Circuit Breakers ' 239 201 15.7.1. Classification. 239 201 15.7. 2. Rated Quantities 239 202 194 195 | 195 196 196 197

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27.9. Frame-leakage Protection 27.10 . Directional Over-current Protection 27.11. Directional Earth fault Protection

521 528 529

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28 DIFFERENTIAL PROTECTIO N 28.1. Differential Protection 28.2 . Applications of Differential Protecti on 28.3. Principle of Circulating Current Differential (merz-prize ) Protection 28.4. Difficulties in Differential Protect ion 28.5. Differential Protection of 3 phase Circuits 28.6. Biased or Per Cent Differe ntial Relay 28.7. Settings of Differential Relays 28.8. Balanced Voltage Differential Protection

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29 DISTANCE PROTECTION 29.1. Introduction to Distance Protect ion 29.2. Principle of R-X Diagram 29.3. Theory of Impedance Measurement 29.3.1. R-X Diagrams of Plain Impedance Relay 29.3.2. Plain Impedance Characteristic s. 29.3.3. Disadvantages of Plain Impedance Relay. 29.3.4. Time Characteristic of High Speed Impedance Relay 29.4. Methods of Analysis ,

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531 531 531 532 533 533 534 534

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536 536 537 538 539 539 540 540

29.5. Directional Impedance Relay 29.6. Torque Equation of Directi onal Impedance Relay

540 541 542

29.7. Modified (Shifted ) Characteristic 29.8. Reactance Type Distance Relay 29.9. Mho Type Distance Relay 29.10. Application of Distance Protect ion 29.10.1. R-X Diagram 29.10 . 2. Line Characteristics 29.10 .3. Condition for Relay Operation 29.10.4. Operating Time 29.10.5./Stages of Relay Time Charac teristics 29.10.6', Co-ordinated Characteristic s of Distance Relays in Three Stations. 29.10 .7. Significance of R -X Diagram and Method of Analysis 29.10.8. Load Impedance 29.10.9. Line Impedance 29.10.10. Power Swings 29.10 .11. Choice of Characteristic Mho /reactance Mho/static

542 543 544 544

544 545 545 545 546 547 547 547 548 548

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30 PROTECTION OF TRAN SMISSION LINES

30.1. Introduction

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Part A : Overcurrent Protection of Transm ission Lines 30.2. Non directional Time Graded System of Feeder (or Line) Protection 30.3. Directional Time and Current graded System 30.4. Setting of Directional Over-current Relays of a Ring Main 30.5. Current Graded Systems 30.6 . Definite Time Overcurrent Protect ion of Lines 30.7. Earth Fault Protection of Lines 30.8. Summary of Overcurrent Protection of

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Part B : Distance Protection of Transmission Lines 30.9. Introduction to Distance Protection of H.V. and E . H.V. Lines 30.9.1. Plain Impedance Protection 30.9.2. Directional Impedance Relay " 30.9.3. Reactance Relay 30.9.4, Mho Relay Admittance Relays 30.9.5. Offset Mho Characteristic 30.10 . Distance Schemes 30.11. Starting Element (Fault Detectors) 30.12. Stepped Characteristic 30.13. Three Step Distance-time Characteristic , 30.14. Power Swings . 30.15. Carrier Assisted Distance Protection 30.15.1. Carrier Transfer (Intertripping) 30.15.2. Carrier Blocking Scheme (Directional Comparison Method) 30.15.3. Carrier Acceleration 30.16. Distance Schemes for Single Pole and Triple pole Auto-Reclosing 30.17. Connections of Distance Relays Part C : Protection of Based on Unit Principle Lines 30.18. Pilot Wire Protection Using Circulating Current Differential Relaying Part D : Carrier Current Protection of Transmission Lines 30.19. Carrier Current Protection 30.20. Phase Comparison Carrier Current Protection 30.21. Applications of Carrier Current Relaying 30.22. Radio Links or Microwave Links

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31 PROTECTION OF INDUCTION MOTORS 31.1. Introduction 31.2. Abnormal Operating Conditions and Causes of Failures in Induction Motors 31.3. Protection Requirements 31.4. Protection of Low Voltage Induction Motor , (below 1000V AC ) 31.4.1. Scheme of Starting Circuit 31.4. 2 . Bimetal Overload Devices 31.4.3. Short Circuit Protection by Hrc Fuses 31.5. Protection of Large Motors 31.6. Overload Protection of Induction Motors 31.7. Protection Against Unbalance 31.8. Protection Against Single-phasing ( Phase Failure ) 31.9. Phase Reversal Relay 31.10. Phase to Phase Fault Protection

31.11. Stator Earth-fault Protection 31.12. Faults in Rotor Winding

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32 PROTECTION OF TRANSFORMERS 32.1. Protection Requirements 32.2. Safety Devices with Power Transformers 32.3. Low Oil Level Fluid Level Gauge 32.4. Gas Actuated Devices 32.4.1. Pressure Relief and Pressure Relay 32.4.2. Rate-of-rise Pressure Relay

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557 559 559 560 560 561 561 562 563 564 564 565

565 566 567 567 567

568 571 574 577 577 579-592 579 580 581 581 581 582 583 584 584 ’ 586 587 588 588 590 591

-

593 613 593 595 595 595 595 596

( xixl (.xviii )

17.14. Switchgear for a Medium Size Industrial Works 17.15. Bus bars 17.16. Some Terms and Definitions 17.17 . Materials for Bus-bars 17.18 . Bus-bar Design 17.19. Electrodynamic Forces on Bus-bars During Short-circuits 17.20 . Important Techno economic Consideration for Construction of Sub-stations/switchyards 17.20.1. Activities in Construction of Sub-station 17.20.2. Cost Effectiveness 17.20.3. Ways and Means of Economizing 17.20.4. Construction Activities 17.20.5. Maintenance of Over -head Transmission Lines 17.20.6 . Maintenance and Repair

-

-

18-A. TRANSIENT OVERVOLTAGE SURGES, SURGE ARRESTERS AND INSULATION CO-ORDINATION 18.1. Introduction 18.2. Terms and Definitions 18.3. Choice of Insulation Levels of Sub station Equipment 18.4. Protective Ratio,' Protective Margin 18.5. Lightning 18.6 . Overhead Shielding Screen ( Earthed ) 18.7. Lightning Stroke on OH Lines ( Overhead Line) 18.8. Protective Devices Against Lightning Surges 18.9. Rod Gaps or Spark Gap 18.10 . Surge Arresters ( Lightning Arresters) 18.11. Surge Arrester Specifications and Terms 18.12 . Tests on Surge Arresters 18.13. Rated Voltage of Surge Arrester 18.14 . Coefficient of Earthing ( Ce) is the Ratio :

-

18-B. NEUTRAL GROUNDING (NEUTRAL EARTHING )

18.15. Introduction to Neutral Grounding 18.16. Terms and Definitions 18.17. Disadvantages of Ungrounded Systems 18.18. Advantages of Neutral Grounding 18.19. Types of Grounding 18.20 . Reactance in Neutral Connection 18.21. Connection of the ARC Suppression Coil 18.22. Neutral Point Earthing of Transformer L.V. Circuits. 18.23. Neutral Grounding Practice 18.24. Earthing Transformer 18.25. Ratings of Neutral Devices 18- C. SUBSTATION EARTHIjNG SYSTEM AND EQUIPMENT EARTHING 18.26. Equipment Earthing ( Grounding) 18.27 . Functions of Substation Earthing System 18.28. Connection of Electrical Equipment to Station-earthing System 18.29. Substation Earthing System

31|

318 328 328 322 322

18.30. Earth Electrodes

Systems for Two or More Installations

18.31. Integrated EarthingTouch Potential Potential and 18.32. Step Earthing System _ Earth -resistance ofMeasurement . 18.33 18.34. Earth Resistance Screens /18.35. Earthed SECTION II - FAULT CALCULATIONS .

,

330 CALCULATIONS 330 19. INTRODUCTION TO FAULT 331 19.1. Introduction 331 19.2 . Procedure of Fault Calculations Systems 334 19.3. Representation of Power 334 19.4. Per Unit Method System 337 19.5. Advantages of Per Unit 19.6. Selection of Bases of Base-impedance 19.7. Single Phase Circuits : Determinations ) (or Resistance or Reactance 340- 359 19.8. Change of Base 340 19.9 . Circuits Connected by Transformer 346 19.10 . Reactances of Circuit Elements 348 19.11. Induction Motors 349 19.12. Synchronous Motor 349 19.13. Thevenin’s Theorem 350 19.14 . Some Terms 351 19.15. Star-delta Transformation 351 352 SIT LIMITING REACTORS AND CURRENT 352 355 356 356 357

360 - 373 360 360 362

364 365 367 368 369 370 371 372 374 - 388 374 375 376 3f 7

20. SYMMETRICAL FAULTS (Steady State) 20.1. Fault Mva and Fault Current 20.2 . Solved Examples by Standards for Short-circuit 20.3. Procedure Recommended . Calculations in Distribution Systems 20.4 . Reactors in Power Systems Reactors 20.5. Principle of Current LimitingLimiting Reactors 20.6. Design Features of Current 20.7 . Dry, Air Cored Series Reactor Shielded Reactor 20.8. Oil Immersed Non magnetically Reactors 20.9. Oil Immersed Shielded 20.10. Terms and Definitions Reactors 20.11. Physical Arrangement of Series Reactors 20.12 . Selection of 20.13. Location of Series Reactors ) by Considering Kvar ( 20.14. Effective Short Circuit Level ESCL Banks Capacitor Contribution of Shunt (ESCR) 20.15. Effective Short Circuit Ratio

-

21. SYMMETRICAL COMPONENTS 21.1. Introduction

21.2 . Symmetrical Components of 21.3. Operator ‘o’ 21.4. Some Trigonometric Relations

3 phase Systems

378 381 381 382 383

385

389-402 389 390 391 391 392 392 393 393 394 395 395 395 396 399

400 403-437 403 403

414 418 418 419 420 420 420 420 421 421 422 432 433

438-447 438 438 439 440

(.xxviii )

{ xxix )

760 761

39.11. Spikes and Block Coincidence Technique in Phase Comparator 39.12. Phase Comparator with Phase Splitting Technique 39.13. Hybrid Comparator 39.14 . Level Detector 39.15. Level Detector by pnp Transistor 39.16. Npn Transistor as Level Detector 39.17 . Schmitt Trigger with Operational Amplifier 39.18. Schmitt Trigger with Two NPN Transistor

.

761 762 762 763 763 764

-

766 778

40 STATIC OVERCURRENT BELAYS 40.1. Introduction to Static Overcurrent Relays 40.2. Single Actuating Quantity Relays 40.3. Double Actuating Quantity Relays 40.4. Basic Principle of Static Overcurrent Relays 40.5. Time Characteristic 40.6. Timing Circuit 40.7. Directional Overcurrent Relay 40.8. Static Instantaneous A.C. Measuring Relays 40.9. Static Time lag Over-current Relays 40.10. Static Directional Relay

.

766 766 767 768 769 770 771 773 774

-

7761

.

41 STATIC DIFFERENTIAL PROTECTION OF POWER TRANSFORMERS 41.1. Introduction 41.2 . Differential Protection of Two winding Transformer 41.3. Differential Protection of Three Winding Transformer 41.4. Inrush- proof Qualities. 41.5. Requirements to be Fulfilled by the Main CT 41.6. Auxiliary C.T .

-

-

779 784

43.3. Protection of Static Relay Circuit Relaying Equipment 43.4. Recommended Protection Practices for Static voltage Transients 43.5. Testing of Static Relays with Regard to Over43.6. Reliability , Dependability, Security 43.7. Static Relay for Motor Protection Comparison 43.8. Static Busbar Protection Based on DirectionalAuxiliary Supply Inplant 43.9. Disconnection of Mains Supply From During System Faults 43.10 . Breaker Back-up Local Back up 43.11. Use of Micro processor for Local Back up up 43.12. Computer Based Centrally Coordinated BackRelaying Me sure Protective for Equipment Programmable 43.13. ) Ments and Control (PPRMC ) ( 43.14. Principle of Centralized Back-up Protection CBP Computers Digital ) by ( 43.15. Post-faulty Control PFC 43.16. Communication Links for Protection Signalling 43.17. Fibre Optic Data Transmission ; 43.18. Local Breaker Back- up Protection : Breaker Fail Protection Stuck-breaker Protection . 43.19 Uninterrupted Power Supply (UPS) Protection of Overhead Lines 43.20. Directional Wave Relays for Fault Detection And

-

-

, 7791 43-B. DIGITAL RELAYS, MICROPROCESSORS BASED RELAYS 1 780 781

7821

783 783|

-

803-827 803 804

43 D, MICROPROCESSOR BASED SUBSTATION PROTECTION AND MONITORING 43.39. Introduction 43.40 . Equipment to Automatic Control Substations

-

824 825 826

828 829 831 834 834 837 838 841

847 847 849 849

851

-

855 864 856 856 857 858 859

-

43 A, IMPORTANT ASSORTED TOPICS AND STATIC PROTECTION SCHEMES 43,1. Combating Electrical Noise and Interferences 43, 2. Transient Overvoltages in Static Relays

820 821 822 823 823

-

/ 43.21. Enter Microprocessors in Protection Technology Relay Digital 43.22. Block Diagram and Components of a 43.23. Basic Principles of Digital Relays 43.24. Microprocessor Based Relays Relay for Motor Protection 43.25. Description of a Microprocessor Based Protective Based Protective Relays Microprocessor of Features Special 43.26. Advantages of and Relay for 43.27. Block Diagram of a Microprocessor Based Distance Line Transmission of Protection 43.28. Architecture of a Microprocessor 43.29 . Programming of Microprocessors Based Relays Relay 43.30 . Self-checking And/or Self Monitoring in Microprocessor based Monitoring 43.31. On Line Microprocessor Based Fault 43.32 . Microprocessor Based Fault Locators 43.33. Principle of Fault Detection in on Line Digital Relays, Fault

-.

-

816 817 818 820

828 854

FAULT RECORDERS AND FAULT LOCATORS

42. STATIC DISTANCE RELAYS AND DISTANCE PROTECTION OF EHV LINES 785 802 785 ! 42.1. Introduction 786 42.2. Voltage Comparator and Current Comparator 790 ; 42.3. Three-input Amplitude Comparator 42.4. Hybrid Comparator 791 ; 42.5. Four Input Phase Comparator with Quadrangular Characteristic 792 : 42.6. Errors in Distance Measurement 792 ; 42.7. Influence of Power Swings on Distance Protection 793 i 42.7.1. Power Swings 793 i 42.7. 2. Effect of Power Swing on the Starting Elements in Distance Schemes. 793 i Locators and Fault Recorders : 42.7.3. Effect of Power Swing on the Measuring Elements in Distance Schemes. 794 794 43 C MODERN PROTECTION SYSTEM 42.7.4. Representation of Power Swing on R X Diagram 796 t 42.8. Protection of Teed Lines by Distance Relays 43.34. Introduction 796 42.9. Back up Protection with Intermediate Infeed 43.35. Numerical Relays 797 i 42.10. Compensation or Compounding in Distance Relays 43.36. Traditionally Separate Networks 798 42.11. Setting of Distance Relays 43.37. Ethernet just a Physical Layer Standard 798 •; 42.12. Solved Examples on Distance Relay Setting 43.38. The IEC’s Initiative

-

806 807 808 809 811 814

CONTROL

-

865 871

865 865

( xxii )

( xxiii )

27.9. Frame-leakage Protection 27.10 . Directional Over-current Protection 27.11. Directional Earth fault Protection

521 528 529

-

.

28 DIFFERENTIAL PROTECTIO N 28.1. Differential Protection 28.2 . Applications of Differential Protecti on 28.3. Principle of Circulating Current Differential (merz-prize ) Protection 28.4. Difficulties in Differential Protect ion 28.5. Differential Protection of 3 phase Circuits 28.6. Biased or Per Cent Differe ntial Relay 28.7. Settings of Differential Relays 28.8. Balanced Voltage Differential Protection

.

29 DISTANCE PROTECTION 29.1. Introduction to Distance Protect ion 29.2. Principle of R-X Diagram 29.3. Theory of Impedance Measurement 29.3.1. R-X Diagrams of Plain Impedance Relay 29.3.2. Plain Impedance Characteristic s. 29.3.3. Disadvantages of Plain Impedance Relay. 29.3.4. Time Characteristic of High Speed Impedance Relay 29.4. Methods of Analysis ,

531-535

531 531 531 532 533 533 534 534

-

536 549

536 536 537 538 539 539 540 540

29.5. Directional Impedance Relay 29.6. Torque Equation of Directi onal Impedance Relay

540 541 542

29.7. Modified (Shifted ) Characteristic 29.8. Reactance Type Distance Relay 29.9. Mho Type Distance Relay 29.10. Application of Distance Protect ion 29.10.1. R-X Diagram 29.10 . 2. Line Characteristics 29.10 .3. Condition for Relay Operation 29.10.4. Operating Time 29.10.5./Stages of Relay Time Charac teristics 29.10.6', Co-ordinated Characteristic s of Distance Relays in Three Stations. 29.10 .7. Significance of R -X Diagram and Method of Analysis 29.10.8. Load Impedance 29.10.9. Line Impedance 29.10.10. Power Swings 29.10 .11. Choice of Characteristic Mho /reactance Mho/static

542 543 544 544

544 545 545 545 546 547 547 547 548 548

'

.

30 PROTECTION OF TRAN SMISSION LINES

30.1. Introduction

550

Part A : Overcurrent Protection of Transm ission Lines 30.2. Non directional Time Graded System of Feeder (or Line) Protection 30.3. Directional Time and Current graded System 30.4. Setting of Directional Over-current Relays of a Ring Main 30.5. Current Graded Systems 30.6 . Definite Time Overcurrent Protect ion of Lines 30.7. Earth Fault Protection of Lines 30.8. Summary of Overcurrent Protection of

550

-

Lines

,

'

551 553 554

554 556 556 557

-

Part B : Distance Protection of Transmission Lines 30.9. Introduction to Distance Protection of H.V. and E . H.V. Lines 30.9.1. Plain Impedance Protection 30.9.2. Directional Impedance Relay " 30.9.3. Reactance Relay 30.9.4, Mho Relay Admittance Relays 30.9.5. Offset Mho Characteristic 30.10 . Distance Schemes 30.11. Starting Element (Fault Detectors) 30.12. Stepped Characteristic 30.13. Three Step Distance-time Characteristic , 30.14. Power Swings . 30.15. Carrier Assisted Distance Protection 30.15.1. Carrier Transfer (Intertripping) 30.15.2. Carrier Blocking Scheme (Directional Comparison Method) 30.15.3. Carrier Acceleration 30.16. Distance Schemes for Single Pole and Triple pole Auto-Reclosing 30.17. Connections of Distance Relays Part C : Protection of Based on Unit Principle Lines 30.18. Pilot Wire Protection Using Circulating Current Differential Relaying Part D : Carrier Current Protection of Transmission Lines 30.19. Carrier Current Protection 30.20. Phase Comparison Carrier Current Protection 30.21. Applications of Carrier Current Relaying 30.22. Radio Links or Microwave Links

-

.

31 PROTECTION OF INDUCTION MOTORS 31.1. Introduction 31.2. Abnormal Operating Conditions and Causes of Failures in Induction Motors 31.3. Protection Requirements 31.4. Protection of Low Voltage Induction Motor , (below 1000V AC ) 31.4.1. Scheme of Starting Circuit 31.4. 2 . Bimetal Overload Devices 31.4.3. Short Circuit Protection by Hrc Fuses 31.5. Protection of Large Motors 31.6. Overload Protection of Induction Motors 31.7. Protection Against Unbalance 31.8. Protection Against Single-phasing ( Phase Failure ) 31.9. Phase Reversal Relay 31.10. Phase to Phase Fault Protection

31.11. Stator Earth-fault Protection 31.12. Faults in Rotor Winding

.

32 PROTECTION OF TRANSFORMERS 32.1. Protection Requirements 32.2. Safety Devices with Power Transformers 32.3. Low Oil Level Fluid Level Gauge 32.4. Gas Actuated Devices 32.4.1. Pressure Relief and Pressure Relay 32.4.2. Rate-of-rise Pressure Relay

—

.

.

.

.

557 559 559 560 560 561 561 562 563 564 564 565

565 566 567 567 567

568 571 574 577 577 579-592 579 580 581 581 581 582 583 584 584 ’ 586 587 588 588 590 591

-

593 613 593 595 595 595 595 596

{ xxv )

( xxiv )

32.4.3. Buchholz Relay ( Gas Actuated Relay) 32.5. Biased Differential Protection, Percentage Differential Protection of Power Transformer 32.6. Problems Arising in Differential Protection Applied to Transformers 32.7 . Harmonic Restraint and Harmonic Blocking 32.8. Differential Protection of Three-winding Transformer 32.9 . Differential Protection of Auto-transformers 32.10. Earth-fault Protection 32.11. Restricted Earth Fault Protection 32.12 . Protection of Transformers in Parallel 32.13. Overcurrent Protection of Power Transformers 32.13.1. Overload Protection 32.14. Thermal Over heating Protection of Large Transformers 32.15. Over-fluxing Protection 32.16. Protection of Arc Furnace Transformers 32.16.1. Power Supply Requirements of Arc Furnace Plants 32.17 . Protection of Rectifier Transformer 32.18. Protection of Grounding Transformer

-

33. PROTECTION OF GENERATORS 33.1. Introduction 33.2. Abnormal Conditions and Protection Systems 33.2.1. External Faults. 33.2.2. Thermal Overloading. 33.2.3. Unbalanced Loading. 33.2.4. Stator Winding Faults. 33.2.5. Field Winding Faults. 33.2.6. Overvoltages 33.2.7. Other Abnormal Condition . 33.3. Percentage Differential Protection of Alternator Stator Windings 33.4. Restricted Earth fault Protection by Differential System 33.5. Overcurrent and Earth fault Protection for Generator Back up 33.6. ( a ) Sensitive Stator Earth -fault Protection 33.7. Protection Against Turn to-turn Fault on Stator Winding 33.8. Rotor Earth Fault Protection 33.9. Rotor Temperature Alarm 33.10. Negative Sequence Protection of Generators Against Unbalanced Loads 33.11. Negative Phase Sequence Circuit 33.12 . Stator-heating Protection 33.13. Loss of Field Protection 33.14. Reverse Power Protection 33.15. Over-speed Protection 33.16. Field Suppression 33.17. Other Protections 33.18. Protection of Small, Standby Generators 33.19. Generator Transformer Unit Protection 33.19.1. Combined Differential Protection for Generator Main Transformer 33.20. Static Protection of Large Turbogenerators And Main Transformer 33.21. Static, Digital, Programmable Protection System For Generator and Generator-transformer Unit

-

^

-

-

-

596

- 34. STATION BUS-ZONE PROTECTION

34.1. Introduction Relays of Connected Circuits 34.2. Bus Protection by Overcurrent of Incoming Line as a Remote Back-up Protection 34.3. Bus Protection by Distance 34.4. Bus-zone Protection by Directional Interlock Principle 34.5. Bus-zone Protection by Differential Protection Differential zone Bus in 34.6. Problems 34.7. Selection of CTS for Bus- zone Protection 34.8 . Biased Differential Bus-zone Protection Differential Bus-zone Protection 34.9. High Impedance Circulating Current Based on Voltage Drop Protection 34.10 . High Impedance Differential System Differential 34.11. High Impedance-voltage 34.12. Check Features in Bus Protection 34.13. LocatiOn\ Qf CTs 34.14. Monitoring of Secondary Circuits Buszone and Generator-unit Zone 34.15. Interlocked Overcuffent Protection for Three-pole Operation 34.16. Non-auto Reclosure and Simultaneous and Industrial Switchgear Switchgear 34.17. Bus Transfer Schemes for Auxiliary

598

6031 604 604 605 60S1

606 608 608 609

610 610 611 611 612 612

APPLICATIONS

614-643 614 616 616 616 617 617 618 618 619 621 623 627

35. CURRENT TRANSFORMERS AND THEIR 35.1. Introduction 35.2. Terms and Definitions 35.3. Accuracy Class 35.4. Burden, on CT 35.5. Vector Diagram of CT 35.6 . Magnetisation Curve of CT 35.7. Open Circuited Secondary of CT 35.8. Polarity of CT and Connections Ratings 35.9. Selection of Current Transformers of Protection Protection Differential Current Circulating 35.10. CT’s for Protection 35.11. CT’s for Other Protection Systems ; CT’s for Distance CT s ’ 35.12. Type of Construction 35.13. Core Shapes for Multiturn Wound Primary Type CT 35.14 . Current Transformer for High Voltage Installations 35.15. Intermediate CT 35.16. Testing of CT’s (Brief ) 35.17 . Transient Behaviour of CT’s

628 629 631

632 632 633 635 635 635 636 637 637 638 639 639 639

36. VOLTAGE TRANSFORMERS AND THEIR APPLICATIONS 36.1 Introduction 36.2 . Theory of Voltage Transformers 36.3. Specifications for Voltage Transformers 36.4. Terms and Definitions 36.5. Accuracy Classes and Uses [B.S. 3914 (1965)] 36.6. Burdens on Voltage Transformer 36.7 . Connections of VT’s ) 36.8. Residually Connected VT (Zero Sequence Voltage Filter 36.9. Electromagnetic Voltage Transformer 36.10. Capacitor Voltage Transformers (CVT) 36.10.1. CVT with Stepped Output

.

641

|

644-655 644 645 646 646 647 648 649 650 650 650 651 652 652 652 653 654 654

,

656-675 656 657 658 659 661 663 664 664 665 666 668 668 669 670 670 672 673 676- 689 676 676 678 678 679 679

680 682 682 683 684

( xxxviii )

i

58.5.5. Switchgear Installations 58.6. High-voltage Switchgear 58.6.1. Definitions and Electrical Characteristics for HV Switchgear Apparatus 58.6. 2. Electrical Characteristics 58.7. Disconnectors and Earth Switches 58.7 .1. Circuit Breakers Function 58.7 . 2. Quenching Medium and Operating Principle for Different Insulating &

1235 123f

1236 1237 123;. 12.1:

Quenching Medium 58.7 .3. Different Types of Operating Mechanisms of HV, CB 58.7 .4. Electrical Control of H V Circuit Breakers 58.7.5 Instrument Transformers for Switchgear Installations 58.7 .6. Current Transformers 58.7 7. Inductive Voltage Transformers 58.7.8. Capacitive Voltage Transformers 58.8. Surge Arresters 58.8.1. Types of Surge Arresters 58.8.2. Application and Selection 58.8.3. Typical Values of Surge Arresters for the Major Voltage Ratings 58.8. 4. Circuit Configurations for High and Medium-voltage Switchgear Installations

1251 1251 1254

. .

.

125;-

1257

.

12«;

1261 1261 126; 126 126

-

1266

-

59. ELECTRICAL SAFETY 1273 m 59.1. Introduction 127 59.2. Requirements for Electrical Safety 127: 59.3. Relevant Indian Standards 127 59.4. Special Precautions in Design, Installation Maintenance of Electrical Equipment in Hazardous Locations 127i 59.4.1. Elements for Ignition L2759.4.2. Classifications of Hazardous Areas & its Sub-groups 127 59.5. Hazardous Areas Classification zones/divisions 127 59.6. Gas/dust/fibre Groups 1277 59.7. Temperature Class 1271 59.8. Weather Protection 127: 59.9 Material of Construction, Design Characteristics and Conformity Type Test Report 1271 59.10. Marking on Ex protected Design Electrical Equipment 128( 59.11. Maintenance of Ex protected Equipment 1281 59.12. Duties and Obligations 1287 59.13. Selection of Right Variety of Ex protected Equipment 128' 59.14. Explosion Protection Techniques 12859.15. Lightning Protection of Structures with Explosive or Highly Flamrriable Contents 128: 59.16. General Principles of Protection 1287 59.17. Types of Lightning Protection System 1287 59.18. Bonding 1286 59.19. Other Considerations 128: Group 59.20. Classification of Inflammable Gas/vapor 1286

-

.

-

-

-

-

Appendix A : Recent Trends and Advances Towards 21st Century Appendix B ; Distribution Management System Bibliography

Index

-

1291 1316 1314 - 133;

133! 1336 T33:

(.xxviii )

{ xxix )

760 761

39.11. Spikes and Block Coincidence Technique in Phase Comparator 39.12. Phase Comparator with Phase Splitting Technique 39.13. Hybrid Comparator 39.14 . Level Detector 39.15. Level Detector by pnp Transistor 39.16. Npn Transistor as Level Detector 39.17 . Schmitt Trigger with Operational Amplifier 39.18. Schmitt Trigger with Two NPN Transistor

.

761 762 762 763 763 764

-

766 778

40 STATIC OVERCURRENT BELAYS 40.1. Introduction to Static Overcurrent Relays 40.2. Single Actuating Quantity Relays 40.3. Double Actuating Quantity Relays 40.4. Basic Principle of Static Overcurrent Relays 40.5. Time Characteristic 40.6. Timing Circuit 40.7. Directional Overcurrent Relay 40.8. Static Instantaneous A.C. Measuring Relays 40.9. Static Time lag Over-current Relays 40.10. Static Directional Relay

.

766 766 767 768 769 770 771 773 774

-

7761

.

41 STATIC DIFFERENTIAL PROTECTION OF POWER TRANSFORMERS 41.1. Introduction 41.2 . Differential Protection of Two winding Transformer 41.3. Differential Protection of Three Winding Transformer 41.4. Inrush- proof Qualities. 41.5. Requirements to be Fulfilled by the Main CT 41.6. Auxiliary C.T .

-

-

779 784

43.3. Protection of Static Relay Circuit Relaying Equipment 43.4. Recommended Protection Practices for Static voltage Transients 43.5. Testing of Static Relays with Regard to Over43.6. Reliability , Dependability, Security 43.7. Static Relay for Motor Protection Comparison 43.8. Static Busbar Protection Based on DirectionalAuxiliary Supply Inplant 43.9. Disconnection of Mains Supply From During System Faults 43.10 . Breaker Back-up Local Back up 43.11. Use of Micro processor for Local Back up up 43.12. Computer Based Centrally Coordinated BackRelaying Me sure Protective for Equipment Programmable 43.13. ) Ments and Control (PPRMC ) ( 43.14. Principle of Centralized Back-up Protection CBP Computers Digital ) by ( 43.15. Post-faulty Control PFC 43.16. Communication Links for Protection Signalling 43.17. Fibre Optic Data Transmission ; 43.18. Local Breaker Back- up Protection : Breaker Fail Protection Stuck-breaker Protection . 43.19 Uninterrupted Power Supply (UPS) Protection of Overhead Lines 43.20. Directional Wave Relays for Fault Detection And

-

-

, 7791 43-B. DIGITAL RELAYS, MICROPROCESSORS BASED RELAYS 1 780 781

7821

783 783|

-

803-827 803 804

43 D, MICROPROCESSOR BASED SUBSTATION PROTECTION AND MONITORING 43.39. Introduction 43.40 . Equipment to Automatic Control Substations

-

824 825 826

828 829 831 834 834 837 838 841

847 847 849 849

851

-

855 864 856 856 857 858 859

-

43 A, IMPORTANT ASSORTED TOPICS AND STATIC PROTECTION SCHEMES 43,1. Combating Electrical Noise and Interferences 43, 2. Transient Overvoltages in Static Relays

820 821 822 823 823

-

/ 43.21. Enter Microprocessors in Protection Technology Relay Digital 43.22. Block Diagram and Components of a 43.23. Basic Principles of Digital Relays 43.24. Microprocessor Based Relays Relay for Motor Protection 43.25. Description of a Microprocessor Based Protective Based Protective Relays Microprocessor of Features Special 43.26. Advantages of and Relay for 43.27. Block Diagram of a Microprocessor Based Distance Line Transmission of Protection 43.28. Architecture of a Microprocessor 43.29 . Programming of Microprocessors Based Relays Relay 43.30 . Self-checking And/or Self Monitoring in Microprocessor based Monitoring 43.31. On Line Microprocessor Based Fault 43.32 . Microprocessor Based Fault Locators 43.33. Principle of Fault Detection in on Line Digital Relays, Fault

-.

-

816 817 818 820

828 854

FAULT RECORDERS AND FAULT LOCATORS

42. STATIC DISTANCE RELAYS AND DISTANCE PROTECTION OF EHV LINES 785 802 785 ! 42.1. Introduction 786 42.2. Voltage Comparator and Current Comparator 790 ; 42.3. Three-input Amplitude Comparator 42.4. Hybrid Comparator 791 ; 42.5. Four Input Phase Comparator with Quadrangular Characteristic 792 : 42.6. Errors in Distance Measurement 792 ; 42.7. Influence of Power Swings on Distance Protection 793 i 42.7.1. Power Swings 793 i 42.7. 2. Effect of Power Swing on the Starting Elements in Distance Schemes. 793 i Locators and Fault Recorders : 42.7.3. Effect of Power Swing on the Measuring Elements in Distance Schemes. 794 794 43 C MODERN PROTECTION SYSTEM 42.7.4. Representation of Power Swing on R X Diagram 796 t 42.8. Protection of Teed Lines by Distance Relays 43.34. Introduction 796 42.9. Back up Protection with Intermediate Infeed 43.35. Numerical Relays 797 i 42.10. Compensation or Compounding in Distance Relays 43.36. Traditionally Separate Networks 798 42.11. Setting of Distance Relays 43.37. Ethernet just a Physical Layer Standard 798 •; 42.12. Solved Examples on Distance Relay Setting 43.38. The IEC’s Initiative

-

806 807 808 809 811 814

CONTROL

-

865 871

865 865

( xxx )

43.41. Two Subsystems in Substations 43.42 . Two Hierarchical Levels in a Substation 43.43. Substation Level ( Upper Level ) 43.43.1. Unit Level 43.43. 2. Inter-level Communication 43.44. Functions Performed by Protection and Control Equipment 43.45. Protection and Control Configuration

SECTION V

( xxxi )

86G 866

867 868 869 870 . 871

SYSTEM ANALYSIS, INTERCONNECTION AND — POWER POWER

SYSTEM CONTROL SQADA SYSTEMS

44. POWER SYSTEM STABILITY, AUTO-RECLOSING SCHEMES, METHODS OF ANALYSIS AND IMPROVEMENT OF TRANSIENT STABILITY

Part A : Concept of Power System 44.1. Power System Stability 44.2. Concept of Power System Stability 44.3. Single Machine Against Infinite Bus Part B : Swing Curves and Swing Equation, Equal Area Criterion 44.4. Dynamics of Synchronous Machines, Kinetic Energy, Inertia Constant and Stored Energy 44.4.1. Kinetic Energy of a Rotating Mass 44.4. 2. Inertia Constant H 44.4.3. Stored Energy in Rotor of a Syn . Machine 44.5. Swing Curve 44.6. Derivation of Swing Equation From Fundamentals 44.7. Equal Area Criterion of Transient Stability 44.8. Critical Clearing Angle 44.9. Method of Improving Transient Stability Limit Part C : High Speed Protection and Circuit Breakers 44.10. High Speed Circuit Breakers and Fast Protective Relaying for Improved Transient Stability 44.11. Auto-reclosure Improves Transient Stability 44.12. Single Pole Reclosing of Circuit-breakers 44.13. Independent Pole Mechanism 44.14. Single Pole Tripping 44.15. Selective Pole Tripping 44.16 . Segregated Phase Comparison Relaying (SPCR ) 44.17. Influence of Power Swings on Transmission Line Protection Part D : Autoreclosing 44.18. Autoreclosing Schemes 44.19. Terms and Definitions Regarding Autoreclosing 44.20. Rapid Autoreclosing Scheme 44.21. Delayed Autoreclosing Scheme 44.22. Synchronism Check 44.23. Control Schemes for Auto-reclosing Part E : Modern Definitions of Power System Disturbance, Stability 44.24. Terms and Definitions in Power System Stability Studies (1980 ) 44.25. Operational Limits with Reference to Steady State Stability Limit and Transient Stability Limit 44.26 . Methods of Improving Transient Stability Limit

875 -91 !)

875 877 88(1

884 884 885 886 888 889 891 894 897

898 900 901 902 902 902 902 903 904 904 905 907 907 908

909 912 914

'

5X LOAD-FREQUENCY CONTROL, LOAD SHEDDING AND STATIC FREQUENCY RELAY 45.1. Introduction to System Frequency Control 45.2. Load-frequency Characteristics of Rotating Machines 45.3. Primary Load -frequency Control 45.4. Secondary Load Frequency Control 45.5. Load frequency Control of a Grid 45.6. Load Shedding 45.7. Usd nf Frequency Relays for Load Shedding 45.8. Static Frequency Relay 45.8.1. Turbine Frequency Capability and Under-frequency Limits 45.9. Network Islanding . ./ Frequency 45.10. Other 'Application of Relay 45.11. Load Dispatching and Network Controller

,|

-

-

CONTROL

920- 930 920

921 921 921 922 923 923 924 925 927 927 927

AND COMPENSATION OF REACTIVE POWER 15 B. VOLTAGE 45.12. Voltage Control in Network ( Power System ) 45.13. Permissible Voltage Variation 45.14. Methods of Voltage Control 45.15. Compensation of Reactive Power 45.16. Effect of Reactive Power Flow on Voltage at Sending-end and Receiving end of Transmission Line 45.17. Series Capacitors 45.18. Applications of Power Capacitors in Electric Power Systems 45.19. Installation of Shunt Capacitors 45.20. Reactive Power Requirements and Voltage Regulation Of Ehv/ uhv A.C. Lines. Surge Impedance Loading 45.21. Reactiye Power Management

931-958

45 - C, VOLTAGE STABILITY OF ELECTRICAL NETWORK

959-966

45.22 . Introduction to Voltage Stability Studies 45.23. Explaining Voltage Instability 45.24. Increasing Voltage Stability Limit by Supply of Reactive Power 45.25. Sequence of Switching-on and Switching-off Shunt Capacitor Banks 45.26. Q V Characteristics 45.27 . Voltage Collapse Occurances, and Their Time-spans 45.28. Preventive Measures Against Voltage Collapse

—

45.29. Definitions

931 932 933 937 938

938 940 947

949 952

959 959 960 961 962 963 965 965

45-D. AUTOMATIC VOLTAGE REGULATORS, VOLTAGE CONTROL AND

STABILITY OF SYNCHRONOUS GENERATORS 45.30. Introduction 45.31. Operation of Synchronous Generator 45.32. EMF and No Load Terminal Voltage, Saturation Curve and Air Line 45.33. Terminal Voltage of an Isolated Generator with Constant Field Current and Without AVR 45.34. Types of Excitation Systems and AVRS 45.35. Synchronous Generator in Parallel with the Grid (Infinite Bus) 45.36. Types of AVR and Excitation Systems 45.37. Terms and Definitions on AVR and Excitation Systems

967 -991

967 971 973 974 975 976 977 980.

4

SWITCHGEAR AND PROTECTION

L3. FAULTS AND ABNORMAL CONDITIONS

5

INTRODUCTION

jg tHE FAULT CLEARING PROCESS defined as a defect in its electrical circuit due to whicj The protective relays are connected in the secondary circuits or current transformers and/or the current is diverted from the intended path . Faults are generally caused by breaking of . , conduc tiai transformers. The relays sense the abnormal conditions and close the trip circuit of the tors or failure of insulation The other causes of faults include mechanical failure, accidents , exces p° nciated circuit-breaker The circuit-breaker opens its contacts. An arc is drawn between the con-

A fault

in

an electrical

equipment is

affected . Voltage becomes unbalanced . The faults can be minimised by improving the system, design , quality of the equipment ant maintenance. However the faults cannot be eliminated completely. For the purpose of analysis, AC faults can be classified as single line to ground fault line to line fault double line to ground fault simultaneous fault three phase fault open circuit . , etc. The other abnormal conditions in AC system include: voltage and current unbalance over-voltages under frequency reversal of power temperature rise power swings instability, etc. Some of the abnormal conditions are not serious enough to call for tripping of the circuit breaker . In such cases the protective relaying is arranged for giving an alarm In more cases, the continuation of the abnormal condition (such as a can be * 11 « « the faulty part should be disconnected the system without any delay . This function is performed by protective relaying and switchgear .

— — —

— — — — — —

—

— — —

faifitl

srnr

The -

£

^

transient variation of the short-circuit currents. transient variation of the voltage after final arc interruption ( transient recovery voltage )

the arc extinguishing phenomenon After final arc extinction and final current zero , a high voltage wave appears across the dr cuit-breaker contacts tending to re-establish the arc. This transient voltage wave is called Transient Recovery Voltage (TRV) The TRV comprises a high frequency transient component superimposed on a power -frequency recovery voltage. These phenomena have a profound influence on the behaviour of the circuit-breakers and the associated equipment ( Ref. Ch. 3, 4).

.

.

1.6 PROTECTIVE RELAYING AC power system is covered by several protective zones . Each protective zone covers one or two components of the system. The neighbouring protective zones overlap so that no part of the system is left unprotected. Each component of the power system is protected by a protective system cornprising protective transformers, protective relays, all-or-nothing relays , auxihanes, tnp-circmt, trip oil etc' Duringthe abnormal conditi°n the Protectb elaying 8611865 ’ triP circuit of the circuit-breaker. Thereby the circuit-breaker opens and the faulty part of the y tem is disconnected from the remaining system.

serioi ? , "

^ ^

- the fault the mrrsnf ana , observed are called ‘transient phenomena’. Thewrd which lasts for a short duration of time. The fault current varies with time

——

tl

^

^

^ ^transient ranSient^

.

^

TheTransient tatSs forleverarcycleT

. ,,

^

^

htTransient - -

—

.

i

l 3

.

.

fc f

is

difference between two or more similar electrical quantities. The protective schemes for large electrical equipment comprise several types of protective systems, . During the first one For low voltage eTuiPment of relatively small ratings, fuses and thermal relays are generally adequat to three cycles, the fault current is very high but regards to designed with due generally equipment are system decreases verv ranidlv This 7 nne in whinh fh « The protective schemes of large power current is very high , but decreases very rapidly is called ) ( the Sub State . After the fimt power swings > power system stabiUty and associated problems. Ref. Sec. Ill and IV . current is called the Transient State. 1 7 NEUTRAL GROUNDING (EARTHING) AND EQUIPMENT GROUNDING Aft state Steady State is reached . During the Steady State the r. m .s value of the short -circuit current The term Grounding or Earthing refers to the connecting of a conductor to earth. The neutral remains almost constant . points of generator and transformer are deliberately connected to the earth . In 3 phase a.c. systems The circuit -breakers operate during the Transient State . the earthing is provided at each voltage level. If a neutral point is not available, a special Earthing Transformer is installed to obtain the neutral point for the purpose of earthing. Neutral points of 1.4 FAULT CALCULATIONS star connected VTs and CTs are earthed. The neutral earthing has several advantages such as : The knowledge of the fault currents is necessary for selecting the circuit-breakers of adequate Freedom from persistent arcing grounds. The capacitance between the line and earth gets rating designing the sub-station equipment, determining the relay settings, etc. The fault calculacharged from supply voltage. During the flash-over the capacitance get discharged to the tions provide the information about the fault currents and the voltages earth. The supply voltage charges it again . Such alternate charging and discharging at various points of the power system under different fault conditions. produces repeated arcs called Arcing Grounds. The neutral grounding eliminates the probThe per- unit system is normally used for fault calculations . The symmetrical faults such as three phase faults are analyzed on per phase basis. For calculations on unsymmetrical faults, the method The6 neutral grounding stabilises the neutral point. The voltages of healthy phases wi of Symmetrical Components is adopted The network analyzer neutral are stabilised by neutral earthing. and digital computers are used for . SP6Ct to , . ° . fault calculations of larger systems. (Ref. Sec. II) f i jn discharging over voltages due to lightning to the earth , The neu ra ear mg During

1

—

-

i

i

li

( xxxiu)

48.14. Insulation Co

( xxxv )

-ordination and Surge Arrester Protection

48.15. Line Insulation, Clearance and Creepage Distances 48.16. Right-of-way (ROW ) 48.17 Corona 48.18. Towers (Supports) 48.19. Bundle Conductors ( Multiple Conductor) 48.20. Switching Phenomena 48.21. Audible Noise (AN) Associated with EHV AC Line Switching 48.22. Biological Effect of Electric Electric Field Strength. Field and Limiting Value of 48.23. Radio Interference and 48.24. Rapid-auto Reclosing Television Interference and Delayed Auto reclosing of Circuit Breakers 48.25. Surge Impedance Loading of AC Transmission 48.26. Sub synchronous Lines Resonance in Series Compensated Ac 48.27. Static Var System ( Lines SVS) 48.28. Applications

.

-

-

-

49.

INTERCONNECTED POWER

SYSTEMS 49.1. Introduction 49.2. System Configuration and of Interconnection 49.2.1. Individual SystemPrinciple (Region or Area). 49.2.2. Total Generation m interconnected in Inte Systems (national Grid) 49.3. Merits ofi' * ted Interconnec Power System 49.4. Limitations of Interconnected Power Systems 49.5. Obligations of Each Interconnected Systems 49.6. Objectives of Automatic Generation on Control a 49.7. Overall Objective and Tie-line Power Flow and oo Control Co-relation ~ v> Betwoo Real reiation Between Power and Reactive Power Control and Tie line Power Flow 49.8. Tie-line Power Flow 49.9. Tie line Power Flow Control in 2 area System 49.10. Alternative Principlesinof3-area System 49.11. Equations of Tie-line Control and the Tie line Bias Control 49.12. Actions by the ControlPower Flow Control Reviewed Room Operators to Change Tie 49.13. Actions by Control line Power Room Operators for Voltage 49.14. Controlling Tie line Control Former (Regulating Power by Means of Phase Shifting Trans

—

-

-

-

-

-

-

-

-

Transformers) 49.15. Phase Shifting r (Regulating Transformer) Transforme 49.16. Types of Interchanges in Interconnected System 49.16.1. Control of 49.17 . National Grid and Power Flow Through Interconnector Growth of Power System in India 50. OPERATION AND AGC AND SCADA CONTROL OF INTERCONNECTED POWER SYSTEMS; 50.1. Introduction 50.2. Main Tasks in Power System Operation 50.2.1. Planning of Operations 50.2.2. Operational Tasks 50.2.3. Operating Accounting and Financial Control 50.3. Automatic Generation Control (AGC) 50.4. Supervisory Control and Data Acquisition (SCADA) System

50.4.1. Division of Tasks Between Various Control Centres 50.4. 2. Functions of Scada Systems 50.4.3. Common Features of All Scada Systems 50.4.4. Alarm Functions 50.4.5. Integration of Measurement Control and Protection Functions

107E 107J 107?

107?

1078 1078 1080 1080

by SCADA Systems 50.5 Automatic Sub-station Control 50.6 Scada Configurations 50.7 Energy Management Systems ( EMS)

. . . 50.8. System Operating States 50.8.1. Normal State (Secure State) 50.8.2. Alert State (Insecure State)

1081 1081

50.8.3. Emergency State 50.8.4. Islanding (In Extermis) State 50.8.5. Restoration State

1082 1082 1082 1083

50.9. System Security 50.9.1. Security Control 50.10. State Estimation 50.11. Expert Systems Using Artificial Intelligence For Power System Operation 50.11.1. What is an Expert System? 50.11.2 . Components of Expert System 50.11.3. Example of an Expert System’s Working 50.11.4. Applications in Power Systems 50.12. Centralised Diagnostic Expert System Using Artificial Intelligence 50.13. Scada Systems for Power System

1089-71 O 4" 1089 1090 1090 1090 1091 1092

1092 1093

51. POWER SYSTEM PLANNING 51.1. Scope of Power System Planning and Design 51.2. Significance of System Planning and Design 1 51.3. Computer Programmes for Planning

1094 1096 1096 1097 1098 1100 1100

1116 1116 1120 1120 1123 1123 1123 1124

1124 1124 1124 1125 112'5 1126 1126 1126 1126 1127 1128 1130 1134- 1137 1134 1134 1135

52. IMPROVING DYNAMIC STABILITY BY FLEXIBLE AC TRANSMISSION 1138-1149 SYSTEM (FACT) AND FVDC SYSTEMS 52.1. Inter-relationship Between Voltage, Active Power, Reactive 1138 Power , Power Angle, Oscillations and Various Types of Stabilities 1138 equations Stability and Basic System 52.1.1. Review of Concepts of Power 1139 Control Dynamic 52.2. Parameters for 1140 52.3. Fundamental Requirements of AC Transmission System 1140 Disturbances and Conditions Ranges Abnormal of . Time 52.4 1140 52.5. Enter Thyristor Control 1141 52.6. First Swing Period and Oscillators Period 1141 52.7. Review of Power System Problems and Methods for Improvement 1144 52.8. Flexible AC Transmission ( FACT) 1145 52.9. Damping of Oscillations in AC Networks by Means of HVDC Damping Control 1146 52.10. Stabilisation of Adjacent AC Lines 52.11. Damping of AC Networks Oscillations with Different 1147 Conditions of DC Control for Synchronous HVDC Link

1100 1101 1102 1103 1103 1105- 1133 1105 1105 1106 1106 1108 1108 1109

1112 1112 1113 1116

-

53. COMPUTER AIDED POWER SYSTEM STUDIES 53.1. Computer Aided Engineering (CAE) for Power System Studies 53.2. Purpose and Need of System Studies

1150-1154 1150 1150

( xxxviii )

i

58.5.5. Switchgear Installations 58.6. High-voltage Switchgear 58.6.1. Definitions and Electrical Characteristics for HV Switchgear Apparatus 58.6. 2. Electrical Characteristics 58.7. Disconnectors and Earth Switches 58.7 .1. Circuit Breakers Function 58.7 . 2. Quenching Medium and Operating Principle for Different Insulating &

1235 123f

1236 1237 123;. 12.1:

Quenching Medium 58.7 .3. Different Types of Operating Mechanisms of HV, CB 58.7 .4. Electrical Control of H V Circuit Breakers 58.7.5 Instrument Transformers for Switchgear Installations 58.7 .6. Current Transformers 58.7 7. Inductive Voltage Transformers 58.7.8. Capacitive Voltage Transformers 58.8. Surge Arresters 58.8.1. Types of Surge Arresters 58.8.2. Application and Selection 58.8.3. Typical Values of Surge Arresters for the Major Voltage Ratings 58.8. 4. Circuit Configurations for High and Medium-voltage Switchgear Installations

1251 1251 1254

. .

.

125;-

1257

.

12«;

1261 1261 126; 126 126

-

1266

-

59. ELECTRICAL SAFETY 1273 m 59.1. Introduction 127 59.2. Requirements for Electrical Safety 127: 59.3. Relevant Indian Standards 127 59.4. Special Precautions in Design, Installation Maintenance of Electrical Equipment in Hazardous Locations 127i 59.4.1. Elements for Ignition L2759.4.2. Classifications of Hazardous Areas & its Sub-groups 127 59.5. Hazardous Areas Classification zones/divisions 127 59.6. Gas/dust/fibre Groups 1277 59.7. Temperature Class 1271 59.8. Weather Protection 127: 59.9 Material of Construction, Design Characteristics and Conformity Type Test Report 1271 59.10. Marking on Ex protected Design Electrical Equipment 128( 59.11. Maintenance of Ex protected Equipment 1281 59.12. Duties and Obligations 1287 59.13. Selection of Right Variety of Ex protected Equipment 128' 59.14. Explosion Protection Techniques 12859.15. Lightning Protection of Structures with Explosive or Highly Flamrriable Contents 128: 59.16. General Principles of Protection 1287 59.17. Types of Lightning Protection System 1287 59.18. Bonding 1286 59.19. Other Considerations 128: Group 59.20. Classification of Inflammable Gas/vapor 1286

-

.

-

-

-

-

Appendix A : Recent Trends and Advances Towards 21st Century Appendix B ; Distribution Management System Bibliography

Index

-

1291 1316 1314 - 133;

133! 1336 T33:

12

SWITCHGEAR AND PROTECT

^

13

INTRODUCTION

Basic Methods of Voltages Control VAr SOURCES (SVS) 1 17 STATIC Voltages Regulators and Excitation Control of Synchronous -stations, load sub-stations for fast stepless VAr sources are installed in receiving sub Generators. _ . In conventional switched schemes the control voltage for Tap-changing transformers at various sub stations. , of readdve power compensation Off-load tap changers are control used , the capacitors/reactors are confif , r seasonal voltage variations. On load tap changers are switched in/out by circuit-breakers. In SVS ed for and magnitude of current daily load duration The . . variation triggering the delay angle of thyristor changing the turns ratio of the transformer v / Nu ,, v y is controlled . Fast the trolled compensation voltages ratio IVV2 is changed . Nl 2 .. . reactor/capacitor is controlled . Thereby amount of buses m EHV AC sub-stat ons . n AC of rie S voltage controlling ^ ° schemes are used for > ° * tong lines . The inductive reactance drop i , by the»rsdrop ue (( By compensated in series capacitors I IK, c Series capacitors „ Dimerly synchronous compensators were used for similar purpose , generally used for long extra high voltage Chapter 45 B. transmission lines. voltage control techniques are described in Shunt Capacitors are used for voltage control in transmission and distribution network,

——

Static £ “

^ ^ "Sc

BJ fS controlling

thTltoB KPTS ‘

,

‘Tcit ^*

.

Compensation

—

r

Class ph. to ph. R . M. S .

“i ph. to ph. R . M . S .

240 V

264 V

MV

216 V

415 V

457 V

M . H . V.

347 V

3.3 kV

3.6 kV

M . H . V.

3 kV

6.6 kV

7.2 kV

6 kV

M . H .V.

11 kV

12 kV

M. H . V .

10 kV

22 kV

24 kV

M . H . V.

20 kV

33 kV

36 kV

H . V.

30 kV

66 kV

72.5 kV

H. V.

60 kV

132 kV

145 kV

120 kV

220 kV

E. H . V .

245 kV

400 kV

200 kV

420 kV

U . H . V.

380 kV

760 kV

800 kV

750 kV

E . H . V.

Note.

—

L. V. Low Voltage M .H.V. = Medium High Voltage E. H. V. = Extra High Voltage

=

M .V. = Medium Voltage H . V. = High Voltage U.H.V. = Ultra High Voltage

Permissible variation is approximately ± 10% Nominal value. Shunt reactors are used with EHV AC lines for compensation of reactive power during low loads. Compensation of Long Lines

During Low Loads and High Receiving Voltage

During High Loads and Low Receiving Voltage Varying Load

The voltage control of each sub-station bus

P=

ph . to ph . R . M . S .

LV( 1 ph)

Switch-off shunt capacitors . Shunt -reactors -unswitched Switch-in shunt capacitors at load end shunt-reactors-unswitched Static VAr Source (SVS) is achieved by appropriate action in that sub-station ,

,

a ofrotat ng masn

sin 5

X

Induced emf , magnitude where |V| = Terminal voltage, magnitude; |E j = reactance. 5 = angle between V and E vectors; X = Synchronous equal to ° is and 90 Steady state stability limit occur at 8 = | V | - | 2S | pss = m - m s i n 9 0° = X

X , the angle delta overshoots beyond 90° and the stability occurs disturbance 1 lowever, if a sudden amount of disturbance AP is defined . may he lost. Hence the limit of loading permitted ( Pts ) for given generator can be loaded safely upto its It is called Transient Stability Limit ( Pts ) A synchronous lesser than steady state stability transient stability limit . The transient stability limit (Pts) is much limit . Assuming safe load angle of 30° electrical, I V | • | E | sin 30° = 1 v i \ E 1 1 Pts 2 X X for critical 5 = 30° i Pt , = 1/2 Pss . Transient state stability limit is half of steady state limit . transmission A similar analysis is applied to power transfer through an AC interconnecting

-

,,

lino

Pst

-

•

| V2 |

I Vi | X

sin 8

whore j Vj j , |V2| = Sending and receiving voltage magnitudes X = Series reactance of line ; 8 = Angle between vectors Vp V2 switchgear and Transient stability lmit can be improved by several methods associated with prut ection. These include the following : Use of faster and superior protection system , -- Use of faster circuit breakers. Use of rapid auto-reclosing of circuit breakers . higher By improving transient stability limit, the installed generating stations can be loaded to ioveis resulting in major economy . Details about transient stability limit are covered in Chapter 44.

—

—

-

-

im 3 *0

8s . 5

3

a

P 5

n

1 Introduction

—

Power — —

Switchgear Protection and Network Automation Significance Energy Management System Normal and Abnormal Conditions Faults-Fault clearing Network Phenomena Network Svetlans Configiirations Switchgear Circuit Breakers Protective Relays Substations EHV AC

—

.

— — — — — — — HVDC Transmission Systems —Interconnected Systems —Load Flow Systems n — Transmissio and Surge Arresters — Static ,Kli ,. _ Grounding of Neutrals —Transient Overvoltages System Calculations—Load Power control and protection integrated — based r relays Microprocesso Studies System Energy in r —Scope of Subject. Microprocesso and Flow Calculations—Computer cjt

s

of Switchgear, Protection and Power Systems to every consumer at all times Electrical Energy Management system ensures supply of energy cost and with minimum enlowest , at form wave at rated voltage, rated frequency and specified are integral part Automation Network and Protection , vironmental degradation. The Switchgear 3 phase, 50 Hz, modern . The Economy National and System of the Modern Energy Management conventional power plants , EHV A ( 1 interconnected power system has several conventional and nonStations, HV Transmission AC and HVDC Transmission Systems , Back-to-back HVDC Coupling Electrical Loads . The enerConnected network , Substations, MV and LV Distribution Systems, and l area , instantly, geographica vast a in located gy in electrical form is supplied to various consumers high-quality continuity and service The . times all at quality required automatically and safely with . important very become have of power supply Con Cenerution Planning, Transmission Planning, System Expansion , Installation , Operation Load , Calculations , Network Calculations , Fault Systems Energy trol mid Maintenance of Electrical Flow Studies have become very essential functions of Modern Power Engineers. Switchgear and Controlgear are also essential with every power consuming devices at Utilization Level. Switchgear and Protection/Control-Panels are installed at each voltage levels at each switching point for (1) normal routine switching, control and monitoring and i '2 ) automatic switching during abnormal and faulty operating conditions such as short circuits, undorvoltage, overloads. The Computer Controlled Network Automation by Load Control Centre, Power Station Control Rooms and Substation Control Rooms and communication channels together ensures the Control of National and Regional Grids and control of Voltage, frequency , Power and waveform under prevailing and ever changing load conditions. This Text-Book covers the principles and practice in Modern Power Systems, Switchgear Protection, Fault Calculation. Load Flow Calculations and Computer Aided Energy Management Systems. This Chapter gives an Overview and the Scope.

-

.

1.1 . SWITCHGEAR AND PROTECTION Everyone is familiar with low voltage switches and rewirable fuses. A switch is used for opening and closing in electric circuit and a fuse is used for over-current protection. Every electric circuit needs a switching device and a protective device. The switching and protective devices have been developed in various forms. Switchgear is a general term covering a wide range of equipment con cerned with switching and protection .-

-

3

SWITCHGEAR AND PROTECTS; INTRODUCTION

2

s of switchgear vary depending upon the location , ratings and switch A circuit-breaker is a switching and current -interrupting device in a switchgear . The circujapplications’ the requirement network, switchgear is necessary in industrial works, industrial supply the j ng duty. Besides | breaker serves two basic purposes : and commercial buildings . A controlgear is used for switching and controlling (1) Switching during normal operating conditions for the purpose of operation and maintenan rojects’ domestic consuming devices. S tchi E normal conditions such as short circuits and interrupting the fault c power" STATION EQUIPMENT 0 ) currents whi, 1 SUBlj simPle il Selves relati > “ , , “ is complex as the fault “ currents are relatively high aif In every electrical sub-station , there are generally various indoor and outdoor switchgear equip „ are ction iSecan 1.1). The equipment are ( iniprrnnf thev should be Wlt n a short time of the order of a few cycles. One cycjjment. Each equipment has a certain functional requirement Ref. Table . Generally indoor , conditions local and mu rating “ voltage the upon depending , in 50 Hz svstem takes 1 / n outdoor or n aei ara several types of faults and abnormal conditions. Tljfeither indoor switchgear ° ? outdoor , , above and kV of 33 voltage fault currents can dnmncro FV e e1ulPment and the supply installation if allowed to flow for a long equipment is preferred for voltages up to 33 kV. For even preferred may be * equipment ] indoor areas polluted heavily to avoir order in , In . duration SUC S every part of the power system is provided with a protejfis generally preferred However ltageS V f C r ieS arge live relaying system and 035 InSUl tei SubS*ati0nS < GIS> are 0ltaSBS' ° ° 3 is generally in clearing process a§ The outdoor equipment is installed under the open sky. The indoor switchgear ) "p0ault system , switchgear converted by the term ‘Switchgear’ clad metal called a units also and assembled th factory | form 0f metal enclosed Afer 1 of ' any electric circuit . In addition to circuit -breaker and nr devices . Basically a circuit -breaker " -breakers are the switching and current interrupting sfCircuit p l J y H b w t s for controlling, regulating and measuring can also be consider ed a separated by means of an operatbe can • owitchgea comprises a set of fixed and movable contacts . The contacts • includes switches, fuses, circuits-breakers isolators relavs mnimi rP,,nni s > v m- nmg , cm jng mechanism. The separation of current carrying contacts produces an arc. The arc is extin arresters ’ rent transformers and various associate d equipments , SF6 gas. The circuit-breakers are . . guished by a suitable medium such as dielectric oil, air( , vacuum Switchgear are necessary at every switching point in AC nower svsW Ref . Fig. 1.1) AC SWiichi ® station and final load point , thore aro several" and H c under no cur * Isolators are disconnecting switches which can be used for disconnecting a circuit ’ , can be isolator An . breaker circuit , , , AUXUMRY the with along installed condition. They are generally rent to closed be can switch earthing AR i i x SWITCHGE opened after the circuit breaker . After opening the isolator , the potential s and transformer current The ground . to the -X - SWITCHGEAR discharge the trapped electrical charges i purpose of transformers are used for transforming the current and voltage to a lower value for the voltages ) over the divert ( surge arresters QD RMER arresters TRANSFO GENERATOR!; AUXILIARY measurement, protection and control. Lightning the about details further The . voltages over from TRANSFORMER equipment station sub the protect to earth and O- GENERATOR MAIN . sub-station equipment are given in Section I of this book TRANSFORMER '

%

.

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^

reJf ”

*

T toT tfernrCuntnTTenti 1 ^

a

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lb„vet rkV

If ^

Swkrtwinirto!

^

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SWITCHING

fHSTATION x

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TRANS MISSION

—

Symbol

i

2.

Isolator

Disconnecting a part of the system from live parts under no load condition.

3.

Earthing-switch

Discharge the voltage on the lines to earth after disconnecting them .

4.

Surge arrester

Diverting the high voltage surges to earth and maintaining continuity during normal voltage.

Current transformer

Stepping down the current for measurement protection and control.

Potential transformer ( Voltage transformer )

Stepping down the voltage for the purpose of protection, measurement and control .

( Disconnecting switch)

X

1

IB

T

5.

STATION

6. (

SUB STATiiON

U

T

A

f f

DISTRIBUTION Fig. 1.1. Location of Switchgear in Typical P'ower System (Single line, simplified diagram ).

Function Switching during normal and abnormal conditions , interrupt the fault currents.

GENERATING

IMHt

Equipment

Circuit-breaker

T

3D^ x X4-X-C1D-X-

XtXK

-

Hi

-

i__

Table 1.1 AC Sub station equipment*

.

SUB STATION

-

-

“

/MAIN SWITCHGEAR 7 -X I l y

TT X

“ ^”

oL , )

9 ft

3w

c 2 I

1 3>

Irf an

s


rjTRTpSQPl^ N

... ( 3.16 )

+ Component. Eq. ( 3.16) is particular solution of Eq. ( 3.8). It is sinusoid called A. C. Complete solution i — ip + ic From Eqs. ( 3.10) and ( 3.16), we get ~

i = AJ

- R/ L)

t

GJ

^

E

3

&

= tan

co L

0

get

FH

Let us see , what happens, when switch S of circuit shown in Fig. 3.1 is suddenly closed. 4

' I 'l N O \ MBNTALS

'I

35

sin (cot + 0 - )

This is a complete solution ofEq . ( 3.8). Let us put

theTnitial condition to evaluate A.

...( 3.17 )

36

SWITCHGEAR AND PROTECTR i \

At t = 0; i

= 0. Because the current in inductive circuit does not change instantaneously .

Assuming R to be too small as compared with coL;

Vi?2 + a 2L2 = coL )

and

co = tan

l

.

Case 1 Switch closed at e = 0 Hence e = 0 at t = 0 Also

0 = 0. i = 0 at t = 0.

From Eqr(3.17 )

0=A+

coL

X = 9°

9

Em

.

' [ ['N

nAMENTALS OF FAULT CLEARING

. ..

37

-

.

of implitude 400 volts Example 3.1 A C transient R L circuit A 50 Hz sinusoidal voltage an expression for Find . H of 0.1 circuit of resistance 10 ohm and inductance is applied to a series at any instant after the voltage is applied , assuming the voltage is zero at current the value of the . Calculate the value of the transient current 0.02 sec after switching on application of the instant rp. Sm . ) Solution. Refer to the derivation in section 3.2 it! = 10 ohm ( liven : L = 0.1 henry f = 50 Hz 2 itf = 314 2 2 2 2 2 fR CO ' + L = VlO + (31, 4) = 33 ohm Angle 4> = tan lo)L / R = tan SI.I/ IO From the mathematical table, we get | < > = 73.35° = 1.26 radians e = Em sin (co t + 0) £ = 0, e = 0 nl zero. voltage at closed is switch since the 0 0= 1 :[ence The equation for R-L circuit current is

’

”

.

A=+

—GOLE

This is maximum value of A, hence the d.c. component is maximum when switch is closed voltage zero. This case is called Doubling Effect. Because peak value is 2 / (nL , at the peak at of Em first current loop. There is as slight drop in the instantaneous value of the current from £ 0 the t

=

2Em / wL.

= to Therefore, the peak value can be considered to be approximately 1.8i? / coL instead| o m

Case II . Switch closed at e = E max e = Emax att = 0. 0 = 7t / 2 i = 0 at t = 0 we get 0 = A + Em / aL sin(7t / 2 - n / 2 ) .

1

^

L

-

..

O C COMPONENT

i

i

ti

O

°

0 = A(e ) +

A~

Vi?

2

8

Em

Vi?2 + U 2L2 ^

72' 35 =

W

(0' 953)

= 12 1 (0- 953) '

~ i = 12.1(0.953e 100i + sin (314£ - 1.26)] . fence ngle in the bracket is given radians . This is the required expression for current . Ans he magnitude of (d .c. component) 11 = 0.02 second is given by ~ 100 x 0 2 { R / L )t = 1.56A Ans = 12.1 x 0.953e idc = Ae Example 3.2 A 50 -cycle alternating voltage is applied to an R - L series circuit by closing a switch. The resistance is 10 ohm. Inductance is 0.1 Henry. The r.m.s. value of applied voltage is 100 volts (a ) Find the value of d c component of current upon closing the switch if instantaneous value of voltage is 50 at that time (b ) What value of instantaneous voltage will produce a maximum d . c . component of current upon closing the switch ? (c ) What is the instantaneous value of voltage which will result in the absence of any d . c . com-

.

~

,

.

.

/i

.

t-

..

ponent upon closing the switch ? (d ) If the switch is closed when instantaneous voltage is zero, find the instantaneous current 0.5,

t Fig . 3.2 Switch closed at voltage zero, d .c. component maximum .

Em

sin (co t + 0 + (|>) 2 + co2L Putting the value of i at t = 0 and other given quantities

-

I

+ Ri = e = Em sin (co £ + 0).

- ( R / L )t + The solution ( Eq . 3.17) is i = Ae

A=0 Hence A is zero , if switch is closed when e = E max Thereby the d .c . component is also zero. From cases I and II we observe that the magnitude of initial value of d.c. component Ae ~ lK / Lt depends upon the moment of closure of switch, or voltage at the instant of occurrence of short circuit. Let us interpret result of the solution. When an R L series circuit is closed with an alternating voltage source, the resulting current: consists of two components, the d.c. component and a.c. component. The . c component . a superis imposed on the d.c. component. The magnitude of d.c. component upon the voltage at the instant of closing the switch . When the switch is closed at voltage depends , the d .c. component is maximum ( Fig. 3.2 ). If the switch is closed at voltage maximum , dzero . c. component is zero and the ! waveform is symmetrical about the normal zero axis as shown in Fig. 3.3.

Fig . 3.3 Switch closed at voltage maximum , no d . c. component .

1.5, 5.5 cycles later

.

f v k

!

40

SWITCHGEAR AND PROTECTION The currents and reactance are given by the following expressions : , OA Ea

...( 3.20)

t ,= O

BK 12 Xd

_ 1" - OC T

^

2

.

~

...( 3.21

Ea

X d„

...(3.22 )

where I = Steady state current, r.m.s. value I' = Transient current r.m.s. value I" = Sub-transient current, r. . m s. value Ea = Induced e.m.f. per phase Xd = Direct axis synchronous reactance

,,, N JAMENTALS OF FAULT CLEARING

, transient and steady-state reactances can be determined experimentally by conductsnbtransient circuit test .

rilioft subtransient, transient and steady R is clear from Eqs. ( 3.20 ) to ( 3.22 ) that while calculatingThe examples of short-circuit current . should be reactances considered respective the ; state currentsSection II of the book . are given in ing

CURRENT INTERRUPTION IN A.C. CIRCUIT-BREAKERS The waveform of the current and the voltage during the arc interruption process will be studied applies to the circuit- breakers employing the principle of zero- point in this section. This description generally adopts the zero-point interruption technique. circuit c . . breaker Every a interruption . to a generator on no load at rated terminal voltage. The connected breaker circuit a Consider the position open other side of circuit-breaker is short circuited ( Fig. 3.6 ) . and in is breaker circuit C.B.

Xcf = Direct axis transient reactance X l" = Direct axis sub-transient reactance OA, OB and OC are intercepts shown in Fig. 3.5.

TI

(

tZZtTSmT \

41

! |

£

3 PHASE

k.

STEAD /

STATE

V"

SHORT CIRCUIT

es o ft

£ Fig. 3.6 Sudden-3 phase-short circuit of an alternator.

Let the circuit-breaker be closed at the instant when voltage of terminal B w .r.t neutral is zero . In such a case the short circuit current in phase B will have maximum d.c. component and the waveform of current If will be unsymmetrical about normal zero axis as shown in Fig. 3.7 . The figure shows the typical waveform of short circuit current in a phase having maximum d . c. com ponent.. The generator is on no load before t = 0. Hence the current is zero before t = 0 . At t = 0 , the short circuit is applied and the current increases to a high value during the first quarter cycle . The peak of the first major current loop (shown hatched ) is OM and this is the maximum instan taneous value of current during the short-circuit the instantaneous peak value of the first major current- loop is called the Making current. . In the figure the making current is OM . It is expressed in kA peak . Let us come to this making current after covering the remaining process ( Sec. 3.19.6 ). The circuit -breaker contacts separate after a few cycles since the relay and the operating mechanism takes atleast a couple of cycles . Let us assume that the circuit-breaker contacts separate at t Ty The r .m.s. value of short circuit at the instant of contact separation is termed as Breaking current After the separation of contacts of the circuit-breaker , an arc is drawn between the contacts. Tlie arc current varies sinusoidally for a few cycles. At t = T 2 a particular current zero, the dielectric strength of arc space builds up sufficiently so as to prevent the continuation of arc. At the current zero, this arc is extinguished and is interrupted. Meanwhile what is happening to the voltage between contacts ? This voltage is recorded in Fig. 3.7. Before t = 0, the contacts are closed and the voltage between them is zero. After the separation iil' tlie contact ( t = Ty ) , the voltage across contact increases. In fact this voltage in the voltage drop acn iss the arc during the arcing period . The voltage across arc is in phase with current since the ore is resistive. The peculiar waveform shape is a result of voltampere characteristic of arc-dis-

,

Fig. 3.5 Oscillogram of current

is the phase having zero

d .c. component. As the short circuit occurs, the short circuit current attains high value. The circuit tacts start separating after the operation -breaker con of the protective relay. separate during ‘trasient state The contacts of the circuit-breaker . ’ The r.m.s. value of the current at the instant of the contact separa tion is called the breaking current of the circuit breaker and is expressed in kA. If a circuit-breaker closes on existing fault , the current would increase to a high the first, half cycle as shown is Figs value during . during the peak of the first current 3.2 and 3.3. The highest peak value of the current is reached . “This peak value is called breaker and is expressed in kA.” Theloop making current of the cirrcuit terms ‘ breaking current’ and ‘making current’ have been cussed in details in section 3.19. disThough the short circuit current varies continuously during the states, the representative values can be sub-transient and trasient calculated from the equations 3.20 , 3.21, and 3.22

-

-

-

. The

.

m 0

(

I

$ f

IE I

*1

42

SWITCHGEAR AND PROTECTION

charge to be studied later . During subsequent half cycles , the voltages across contact increases dm

to increased arc resistance. Finally at t = T % when arc gets extinguished a high frequency voltage transient appears across the contacts which is superimposed on power frequency system voltage. This high frequency transient voltage tries to restrike the arc. Hence it is called Restriking Valtages or Transient Recovery voltage (TRV). The restriking voltage is transient voltage appearing across breaker pole after final current zero. The power frequency system voltage appearing between the poles after arc extinction is called Recovery voltage. The transient recovery voltage or restriking voltage has a profound effect on circuit-breaker behaviour. The current that would flow in the circuit if the circuit-breakers were replaced by solid conductor is called prospective current. The transient recovery voltage (TRV) appearing across the circuit -breaker pole immediately after the final arc interruption causes a high dielectric stress between the circuit-breaker couiucls. If the dielectric strength of the medium between the contacts does not build up faster than the rate of rise of the trasient recovery voltage, the breakdown takes place causing re-establishment of t lie arc. If the dielectric strength of the contact-space builds up very rapidly so that it is more than the rate of rise of transient recovery voltage the circuit-breaker interrupts the current successfully . The rate of rise of TRV, generally depends on the circuit parameters and the type of the switching duty involved . The rate of building up of the dielectric strength depends upon the effective design of the interrupter and the circuit-breaker. While switching capacitive currents, the high voltage appearing across the contact gap can cause reignition of the arc after initial arc extinction. If the contact space breaks down within a period of one-fourth of a cycle (0.02 x 0.25) second from initial arc extinction , the phenomenon is called Reignition. If the breakdown occurs after one-fourth of a cycle, the phenomena is called Restrike. (Ref . sec. 3.20)

Kl ' NDAMENTALS

-

VOLTAGE

recovery

—-

ARC VOLIAGE |« I

tik

0 002 S

—•)

i

%

%

* t

Transient

\

111

1 Ill WM I

POWER FREQUENCY RECOVERY VOCTAGE (50 H | *

III

*

«

l

i T

s

Fig. 3.8 ( b ) Shape of TRV waveform as seen from Cathode-ray oscillographic record .

4

PROSPECTIVE •RENT

“Recovery voltage is the voltage which appears across the terminals of a pole of a circuit-breaker after the breaking of current. It refers to the breaker-pole first to clear .” 'The transient recovery (TRV) or Restriking Voltage is the recovery voltage during the time in which it has a significant transient character. TRV lasts for a few tens or hundreds of microseconds . ( Ref. Fig. 3.85 ) It may be oscillatory or non-oscillatory or a combination , depending upon the characteristics of the circuit and the circuit-breaker. It is the voltage across the first pole to clear , the same is generally higher than across the two poles which clear later. '

ibrr - -El

S \0

I

llr

i

ACROSS ARC

-

t 0 SHORT CURCUtT OCCURS

-

V8 -0

— —

RECOVERY VOLTAGE

/

VOLTAGE

.

- RESTRIK/NG

"Ur

After a current zero, the arc gets ex if the rate of rise of transient voltage between the contacts RECOVERY Ilian the rate of gain of the dielectric VOLTAGE appearing between voltage The . rongth *1 moment of I he breaker contacts at the VOLTAGE ACROSS '' profound ina has zero ARC [ m ; ll current finance on the arc extinction process. The voltage appearing across contacts after \ current zero is a trasient voltage of higher natura l frequency ( restriking voltage). t t superimposed on the power frequency system voltage (recovery voltage). The t rasient component vanishes after a short FINAL CURRENT ZERO time! of the order of less than 0.1 mill-sec ) ( final current zero Voltages after Fig a 3.8 . system voltage mid the normal frequency (TRV) (Simplified ). ( Ref. Fig. 3.86 ) voltage current After . is established zero t ie voltage appearing across the contm ;s is composed of transient restriking voltage and power frequency recovery voltage .

O)

IS t

43

OF FAULT CLEARING

-

RESTRIK VOLTA

CONTACTS SEPARATE t - Tj

,

v

CURRENT ZERO

Fig. 3.7. Oscillogram of current and voltage during fault-clearing.

3.7 TRANSIENT RECOVERY VOLTAGE (TRV) In altenating current circuit-breaker, the current interruption takes place invariably at the natural zero of the current wave .

I

3

Rower Frequency Recovery Voltage is the recovery voltage of power frequency (50 Hz. ) appearing after the transient voltage has been subsided . The transient Recovery Voltage refers to the voltage across the pole immediatley after arc extinction . Such voltage has a power frequency component plus an oscillatory trasient component . The oscillatory trasient component due to the inductance and capacitance in the circuit . The power frequency component is due to the system voltage (Ref. Fig. 3.8). The transient oscillatory component subsides after a few micro-seconds and the power frequency component continuous . The frequency of transient component is given by

-

ARC

INTERRUPTED

l 3 a

fn

yHz

whei e fn = frequency of transient recovery voltage, Hz L = equivalent industance, hency. C = equivalent capacitance, farad .

I

1 l

1 1

43

|— — —

SWITCHGEAR AND PROTECTION

8 jTruwwm

o

R

nnnygyffSTTY

-

line-to line fault

_

VRVY

t

-

B

GnTFOTBUtn. 1 $\ nnxmm

—I— I

o

I

—

VRN

i

-

-to clear factor (VRY / VRN ).. h_ealthy_ ge |

The first pole to clear factor =

-

R N

^^^

-

Normal phase voltage (VIiN ) at the location of the circuit breaker for a phase-to-phase fault (Fig. 3.12).

-

3.8. SINGLE FREQUENCY

/FAULT

^

( b ) Single frequency transient. of TRV. Fig. 3.13. Explaining single frequency transient

is obtained while opening on a terminal fault . In negligible.

.

M . Li ( rt ) Such a transient the circuit-breaker is between the fault and

TRANSIENT

fn =

^ —— —

such cases the reactance

3.9. DOUBLE FREQUENCY TRANSIENTS of the circuit-breaker as shown in Fig, 3.14. Before Tin'. circuit may have L and C on both sides at the same potential. After arc extinction both clearing the fault, both the terminals, 1 and 2 are frequencies and a composite double frequency transient tiie circuits oscillate at their own natural ( b )]. appears across the circuit breaker pole [Fig. 3.14 , L

e

The single frequency restriking transient is produced in the circuit illustrated in 3.13 (6). The frequency of oscillationvoltage Fig. is given by the natural frequency of the circuit.

where L

T

ARC

L

*,&**>(* )

N

faul hase Phase to netural voltage with fault removed at the location of the circuit breaker during a phase-to-phase fault. Ref. Fig. 3.12, first pole-to-clear factor is the ratio of the Voltage between healthy and faulty phase (V y)

Le'

-W

nnrwnnnnr Fig. 3.12 Explaining the first pole

;

..

CB

6

.

R

—

,

B

i

i

JfiRRiTIRRn

47

OF FAULT CLEARING llTNl AMKNTALS )

c, T

LCHz

- Inductance, henry ; C - Capacitance, farads.

(a)

r«

|

i

a VOLTAGE

VOLTAGE

RESTRIKING VOLTAGE

^

/

S ’

t

/

/

j

\

l

t

i

/ (a )

^ VOLTAGE

These frequencies are of the order of The actual power system is composed of 10 to 10,000 Hz depending upon the value of L and C. distributed capacitance and inductance. The circuit configuration is also complex. The TRV for such circuits can have several component ing from a few Hertz to several kilohertz. A typical single frequency transient is frequencies rangillustrated in Fig .

(6) , Fig. 3.14. Double frequency transient of TRV

value of the TRV depends upon In general the frequencies and waveform , rate of rise and peak several aspects such as type of neutral earthing. type of fault net work configuration

—

—

iZ

.1

n PEAK RESTRIKING /

%

—

B

SWITCHGEAR AND PROTECTION

—

The TRV wave can be defined by various methods such as specifying the peak and time to reach the peak . specifying the TRV wave by defining the segment of lines which enclose the TRV The latter method has been now universally adopted and is described in sec. 3.19.9waveform . 3.10. RATE OF RISE OF TRV

.

|' l ' N

i=

,

I

di dt

The rate of rise of restriking voltage volts per micro-second, represents the usually abbreviated by R.R R.V. is a rate expressed in rate of increase in restriking voltage. The rate Trasient Recovery Voltage. (TRV) and the of rise of rate of the rise of TRV depends on natural frequency of TRV are closely associated . Tim the system L parameters. The circuit breaker should be capable of interrupting its rated short-circuit breaking the specified conditions of TRV. Hence the current under j foilwing char- | acteristics of TRV are significant: Peak of TRV, time to reach the peak . Hence the e rate of rise

.

——

-

frequency of TRV Initial rate of rise The term rate of rise of restriking voltage is explained as follows :

!

.

R .R.R .V. = ~ . . . V/ p sec.

t

Fig. 3.15 . Measurement of single frequency transient.

volts tm - time between voltages zero and peak restriking voltage in p sec Ern - peak recovery voltage.

=

—

~

sin

n

a

i

to

...( 3.211

^

c

is maximum when

Em

i. f .

t

LCC0 S TLC n

t 1W

or when

dt 2

)

=0

°

\

t = LC

..

.(3.18)

maximum R .R.R .V. is the value of de / dt at

|

t = VLC

R.R.R .V.max

i .e.

Em

...( 3.23)

lie

Further, peak restriking voltage occurs when e is maximum

i = iL + ic

when

i .e.

when

t

= it , i .e ., t = % LC

^

md peaking restriking voltage is equal to e = Em (1- cos ic) = 2Em

I

1

1

ir

a

2