Ziegler Numerical Differential Protection Numerical Differential Protection Principles and Applications by Gerhard Ziegler Second edition, 2012 Publicis Publishing Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de. Author and publisher have taken great care with all texts and illustrations in this book. Nevertheless, errors can never be completely avoided. The publisher and author accept no liability, regardless of legal basis. Designations used in this book may be trademarks whose use by third parties for their own purposes could violate the rights of the owners. www.publicis-books.de ISBN 978-3-89578-670-9 Second edition, 2012 Complete eBook edition of Gerhard Ziegler „Numercial Differential Protection“, ISBN 978-3-89578-351-7 (print edition) Editor: Siemens Aktiengesellschaft, Berlin and Munich Publisher: Publicis Publishing, Erlangen © 2012 by Publicis Erlangen, Zweigniederlassung der PWW GmbH This publication and all parts thereof are protected by copyright. Any use of it outside the strict provisions of the copyright law without the consent of the publisher is forbidden and will incur penalties. This applies particularly toreproduction, translation, microfilming or other processing‚ and to storage orprocessing in electronic systems. It also applies to the use of individual illustrations or extracts from the text. Printed in Germany Foreword to the First Edition Differential protection provides absolute selectivity and fast operation, and is applied in numerous variations for the protection of electrical machines, trans- formers, busbars and feeders at all voltage levels. Substantial progress has been made with numerical technology which has made this measuring principle even more attractive for the user, such as for example the integrated CT ratio adaptation and the large degree of CT saturation tolerance. The application of digital data exchange over interference free fiber optic cables has simplified the protection of cables and overhead lines in urban and industrial net- works substantially, while also improving the security. Digital communication net- works are finding increasing application for the transfer of protection data in over- head line networks. Thereby, the differential protection may also be applied on lon- ger lines well exceeding 100km as well as complex system configurations with multiple line terminals. The book at hand initially conveys the basic principles of differential protection with analogy and digital technology. Special note will be taken of current trans- formers, data transfer and digital communication. Subsequently the various types of differential protection and the practical applications will be covered, using the Siemens SIPROTEC product range. In principle, the explanations however also apply to the devices of other manufacturers. Practical examples are calculated for illustration purposes. This book is aimed at students and young engineers, who require an introduction to the topic of differential protection. However, users with practical experience, seeking an entry to digital technology of differential protection may also find this book to be a useful addition to their library. Furthermore, it may also be used as a reference for special application problems. Nuremberg, March 2005 Gerhard Ziegler 5 Foreword to the Second Edition Differential protection is a fast and selective method of protection against short- circuits. It is applied in many variants for electrical machines, transformers, bus- bars, and electric lines. Initially this book covers the theory and fundamentals of analog and digital differ- ential protection. Current transformers are treated in detail including transient behaviour, impact on protection performance, and practical dimensioning. An extended chapter is dedicated to signal transmission for line protection in particu- lar to modern digital communication and GPS timing. The emphasis is then placed on the different variants of differential protection and their practical application illustrated by concrete examples. This is completed by recommendations for commissioning, testing and maintenance. Finally the design and management of modern differential protection is explained by means of the latest Siemens SIPROTEC relay series. A textbook and standard work in one, this book covers all topics which have to be paid attention to for planning, designing, configuring and applying differential protection systems. The book is aimed at students and engineers who wish to familiarise themselves with the subject of differential power protection, as well as the experienced user, entering the area of digital differential protection. Further- more it serves as a reference guide for solving application problems. For this second edition all contents have been revised, extended and updated to the latest state of protective relaying. Nuremberg, August 2011 Gerhard Ziegler 6 Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.1 Protection principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2 Numerical differential protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3 Mode of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2 Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2.1 Current differential protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2.2 Biased (stabilized) differential protection . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2.3 Differential Protection with two pilot wire cores . . . . . . . . . . . . . . . . . . . . 27 3.2.4 Operating characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.3 Measuring circuit for three phase systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3.1 Measurement per phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.3.2 Composite current version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.4 High Impedance (HI) Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.5 Partial Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4 Measuring Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.1 Classical analog systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.2 Numerical measuring technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2.1 Acquisition of measured values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2.2 Differential protection with instantaneous value comparison . . . . . . . . . 57 4.2.3 Differential protection with phasors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2.4 Additional stabilisation with CT saturation . . . . . . . . . . . . . . . . . . . . . . . . 66 5 Current Transformers (CTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5.1 Current Transformer equivalent circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5.2 Specifications for the steady state response ofcurrent transformers . . . . . 72 5.3 Transient response of the CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.4 TP Current transformer classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.5 Polarity of the CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.6 CT errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.7 Dimensioning of the CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.8 Interposing CTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 7 Contents 6 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.1 Transmission channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.1.1 Pilot wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.1.2 Fiber Optic Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 6.1.3 Line of sight radio links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 6.2 Digital protection communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.3 Digital communication networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 7 Generator/Motor Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 7.1 Generator differential protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 7.2 Motor Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 8 Transformer Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 8.1 Basic physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 8.2 Numerical measured value processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 8.3 High impedance differential protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 8.4 Relays for transformer differential protection . . . . . . . . . . . . . . . . . . . . . . . . 188 8.5 Application examples for transformer protection . . . . . . . . . . . . . . . . . . . . . 190 9 Line Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 9.1 Three core pilot wire (triplet) differential protection . . . . . . . . . . . . . . . . . . 203 9.2 Two core (twisted pair) pilot wire differential protection . . . . . . . . . . . . . . . 204 9.3 Line differential protection with digital communication . . . . . . . . . . . . . . . 215 9.3.1 Devices and system configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 9.3.2 Measuring technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 9.3.3 Communication variants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 9.3.4 Additional functions and application notes . . . . . . . . . . . . . . . . . . . . . . . 223 9.4 Phase comparison protection with digitalcommunication . . . . . . . . . . . . . 226 9.5 Differential protection of feeders includingtransformers . . . . . . . . . . . . . . 232 9.5.1 Protection of transformer feeders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 9.5.2 Differential protection for feeders with tee-offs (tapped lines) . . . . . . . 233 10 Busbar Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 10.1 Low impedance busbar differential protection . . . . . . . . . . . . . . . . . . . . . . 239 10.1.1 Partially numerical busbar differential protection 7SS600 . . . . . . . . . 241 10.1.2 Fully-numerical busbar protection 7SS52 . . . . . . . . . . . . . . . . . . . . . . . 245 10.2 Response of the numerical busbar protection in the case of CTsaturationandthe demands placed on CT dimension . . . . . . . . . . . . . 254 10.3 High-impedance busbar protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 8 Contents 11 Relay Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 12 Commissioning and Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 12.1 Commissioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 12.2 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Addendum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 9