//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 1 ± [1±12/12] 17.11.2001 5:49PM N EWNESEWNES P OWEROWER E NGINEERINGNGINEERING S ERIESERIES Power Electronic Control in Electrical Systems //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 2 ± [1±12/12] 17.11.2001 5:49PM N EWNESEWNES P OWEROWER E NGINEERINGNGINEERING S ERIESERIES Series editors Professor TJE Miller, University of Glasgow, UK Associate Professor Duane Hanselman, University of Maine, USA Professor Thomas M Jahns, University of Wisconsin-Madison, USA Professor Jim McDonald, University of Strathclyde, UK Newnes Power Engineering Series is a new series of advanced reference texts covering the core areas of modern electrical power engineering, encompassing transmission and distribution, machines and drives, power electronics, and related areas of electricity generation, distribution and utilization. The series is designed for a wide audience of engineers, academics, and postgraduate students, and its focus is international, which is reflected in the editorial team. The titles in the series offer concise but rigorous coverage of essential topics within power engineering, with a special focus on areas undergoing rapid development. The series complements the long-established range of Newnes titles in power engi- neering, which includes the Electrical Engineer's Reference Book, first published by Newnes in 1945, and the classic J&P Transformer Book, as well as a wide selection of recent titles for professionals, students and engineers at all levels. Further information on the Newnes Power Engineering Series is available from bhmarketing@repp.co.uk www.newnespress.com Please send book proposals to Matthew Deans, Newnes Publisher matthew.deans@repp.co.uk Other titles in the Newnes Power Engineering Series Miller Electronic Control of Switched Reluctance Machines 0-7506-5073-7 Agrawal Industrial Power Engineering and Applications Handbook 0-7506-7351-6 //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 3 ± [1±12/12] 17.11.2001 5:49PM N EWNESEWNES P OWEROWER E NGINEERINGNGINEERING S ERIESERIES Power Electronic Control in Electrical Systems E. Acha V.G. Agelidis O. Anaya-Lara T.J.E. Miller OXFORD . AUCKLAND . BOSTON . JOHANNESBURG . MELBOURNE . NEW DELHI //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 4 ± [1±12/12] 17.11.2001 5:49PM Newnes An imprint of Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group First published 2002 # E. Acha, V.G. Agelidis, O. Anaya-Lara and T.J.E. Miller 2002 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 0LP. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 5126 1 Typeset in India by Integra Software Services Pvt Ltd, Pondicherry, India 605005; www.integra-india.com Printed and bound in Great Britain by MPG Books Ltd, Bodmin, Cornwall Preface 1 Electrical power systems - an overview 2 Power systems engineering - fundamental concepts 3 Transmission system compensation 4 Power flows in compensation and control studies 5 Power semiconductor devices and converter hardware issues 6 Power electronic equipment 7 Harmonic studies of power compensating plant 8 Transient studies of FACTS and Custom Power equipment 9 Examples, problems and exercises Appendix Bibliography Index //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 11 ± [1±12/12] 17.11.2001 5:49PM Preface Although the basic concepts of reactive power control in power systems remain unchanged, state-of-the-art developments associated with power electronics equip- ment are dictating new ways in which such control may be achieved not only in high- voltage transmission systems but also in low-voltage distribution systems. The book addresses, therefore, not only the fundamental concepts associated with the topic of reactive power control but also presents the latest equipment and devices together with new application areas and associated computer-assisted studies. The book offers a solid theoretical foundation for the electronic control of active and reactive power. The material gives an overview of the composition of electrical power networks; a basic description of the most popular power systems studies and indicates, within the context of the power system, where the Flexible Alternating Current Transmission Systems (FACTS) and Custom Power equipment belong. FACTS relies on state-of-the-art power electronic devices and methods applied on the high-voltage side of the power network to make it electronically controllable. From the operational point of view, it is concerned with the ability to control the path of power flows throughout the network in an adaptive fashion. This equipment has the ability to control the line impedance and the nodal voltage magnitudes and angles at both the sending and receiving ends of key transmission corridors while enhancing the security of the system. Custom Power focuses on low-voltage distribution systems. This technology is a response to reports of poor power quality and reliability of supply to factories, offices and homes. Today's automated equipment and production lines require reliable and high quality power, and cannot tolerate voltage sags, swells, harmonic distortions, impulses or interruptions. Chapter 1 gives an overview of electrical power networks. The main plant compo- nents of the power network are described, together with the new generation of power network controllers, which use state-of-the-art power electronics technology to give the power network utmost operational flexibility and an almost instantaneous speed of response. The chapter also describes the main computer assisted studies used by power systems engineers in the planning, management and operation of the network. Chapter 2 provides a broad review of the basic theoretical principles of power engineering, with relevant examples of AC circuit analysis, per-unit systems, three-phase systems, transformer connections, power measurement and other topics. It covers the basic precepts of power and frequency control, voltage control and load balancing, and provides a basic understanding of the reactive compensation of loads. //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D ± 12 ± [1±12/12] 17.11.2001 5:49PM Chapter 3 reviews the principles of transmission system compensation including shunt and series compensation and the behaviour of long transmission lines and cables. Chapter 4 addresses the mathematical modelling of the electrical power network suitable for steady state analysis. Emphasis is placed on the modelling of plant components used to control active and reactive power flows, voltage magnitude and network impedance in high-voltage transmission. The model of the power net- work is the classical non-linear model, based on voltage-dependent nodal power equations and solved by iteration using the Newton±Raphson method. The basic method is then expanded to encompass the models of the new generation of power systems controllers. The new models are simple and yet comprehensive. Chapter 5 introduces the power semiconductor devices and their characteristics as part of a power electronic system. It discusses the desired characteristics to be found in an ideal switch and provides information on components, power semiconductor device protection, hardware issues of converters and future trends. Chapter 6 covers in detail the thyristor-based power electronic equipment used in power systems for reactive power control. It provides essential background theory to understand its principle of operation and basic analytical expression for assessing its switching behaviour. It then presents basic power electronic equipment built with voltage-source converters. These include single-phase and three-phase circuits along with square wave and pulse-width modulation control. It discusses the basic concepts of multilevel converters, which are used in high power electronic equipment. Energy stor- age systems based on superconducting material and uninterruptible power supplies are also presented. Towards the end of the chapter, conventional HVDC systems along with VSC-based HVDC and active filtering equipment are also presented. Chapter 7 deals with the all-important topic of power systems harmonics. To a greater or lesser extent all power electronic controllers generate harmonic currents, but from the operator's perspective, and the end-user, these are parasitic or nuisance effects. The book addresses the issue of power systems harmonics with emphasis on electronic compensation. Chapter 8 provides basic information on how the industry standard software package PSCAD/EMTDC can be used to simulate and study not only the periodic steady state response of power electronic equipment but also their transient response. Specifically, detailed simulation examples are presented of the Static Var compensa- tor, thyristor controlled series compensator, STATCOM, solid-state transfer switch, DVR and shunt-connected active filters based on the VSC concept. Dr Acha would like to acknowledge assistance received from Dr Claudio R. Fuerte-Esquivel and Dr Hugo Ambriz-Perez in Chapter 4. Dr Agelidis wishes to acknowledge the editorial assistance of Ms B.G. Weppler received for Chapters 5 and 6. Mr Anaya-Lara would like to express his gratitude to Mr Manual Madrigal for his assistance in the preparation of thyristor-controlled series compensator simulations and analysis in Chapter 8. Enrique Acha Vassilios G. Agelidis Olimpo Anaya-Lara Tim Miller xii Preface //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 1 ± [1±30/30] 17.11.2001 9:43AM 1 Electrical power systems ± an overview 1.1 Introduction The main elements of an electrical power system are generators, transformers, transmission lines, loads and protection and control equipment. These elements are interconnected so as to enable the generation of electricity in the most suitable locations and in sufficient quantity to satisfy the customers' demand, to transmit it to the load centres and to deliver good-quality electric energy at competitive prices. The quality of the electricity supply may be measured in terms of: . constant voltage magnitude, e.g. no voltage sags . constant frequency . constant power factor . balanced phases . sinusoidal waveforms, e.g. no harmonic content . lack of interruptions . ability to withstand faults and to recover quickly. 1.2 Background The last quarter of the nineteenth century saw the development of the electricity supply industry as a new, promising and fast-growing activity. Since that time electrical power networks have undergone immense transformations (Hingorani and Gyugyi, 2000; Kundur, 1994). Owing to the relative `safety' and `cleanliness' of electricity, it quickly became established as a means of delivering light, heat and motive power. Nowadays it is closely linked to primary activities such as industrial production, transport, communications and agriculture. Population growth, techno- logical innovations and higher capital gains are just a few of the factors that have maintained the momentum of the power industry. //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 2 ± [1±30/30] 17.11.2001 9:43AM Clearly it has not been easy for the power industry to reach its present status. Throughout its development innumerable technical and economic problems have been overcome, enabling the supply industry to meet the ever increasing demand for energy with electricity at competitive prices. The generator, the incandescent lamp and the industrial motor were the basis for the success of the earliest schemes. Soon the transformer provided a means for improved efficiency of distribution so that generation and transmission of alternating current over considerable distances provided a major source of power in industry and also in domestic applications. For many decades the trend in electric power production has been towards an inter- connected network of transmission lines linking generators and loads into large integ- rated systems, some of which span entire continents. The main motivation has been to take advantage of load diversity, enabling a better utilization of primary energy resour- ces. It may be argued that interconnection provides an alternative to a limited amount of generation thus enhancing the security of supply (Anderson and Fouad, 1977). Interconnection was further enhanced, in no small measure, by early break- throughs in high-current, high-power semiconductor valve technology. Thyristor- based high voltage direct current (HVDC) converter installations provided a means for interconnecting power systems with different operating frequencies, e.g. 50/60 Hz, for interconnecting power systems separated by the sea, e.g. the cross-Channel link between England and France, and for interconnecting weak and strong power systems (Hingorani, 1996). The rectifier and inverter may be housed within the same converter station (back-to-back) or they may be located several hundred kilometres apart, for bulk-power, extra-long-distance transmission. The most recent develop- ment in HVDC technology is the HVDC system based on solid state voltage source converters (VSCs), which enables independent, fast control of active and reactive powers (McMurray, 1987). This equipment uses insulated gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) `valves' and pulse width modulation (PWM) control techniques (Mohan et al., 1995). It should be pointed out that this technology was first developed for applications in industrial drive systems for improved motor speed control. In power transmission applications this technology has been termed HVDC Light (Asplund et al., 1998) to differentiate it from the well- established HVDC links based on thyristors and phase control (Arrillaga, 1999). Throughout this book, the terms HVDC Light and HVDC based on VSCs are used interchangeably. Based on current and projected installations, a pattern is emerging as to where this equipment will find widespread application: deregulated market applications in primary distribution networks, e.g. the 138 kV link at Eagle Pass, interconnecting the Mexican and Texas networks (Asplund, 2000). The 180 MVA Directlink in Australia, interconnecting the Queensland and New South Wales networks, is another example. Power electronics technology has affected every aspect of electrical power networks; not just HVDC transmission but also generation, AC transmission, distribution and utilization. At the generation level, thyristor-based automatic voltage regulators (AVRs) have been introduced to enable large synchronous generators to respond quickly and accurately to the demands of interconnected environments. Power system stabilizers (PSSs) have been introduced to prevent power oscillations from building up as a result of sympathetic interactions between generators. For instance, several of the large generators in Scotland are fitted with 2 Electrical power systems ± an overview //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 3 ± [1±30/30] 17.11.2001 9:43AM PSSs to ensure trouble-free operation between the Scottish power system and its larger neighbour, the English power system (Fairnley et al., 1982). Deregulated markets are imposing further demands on generating plant, increasing their wear and tear and the likelihood of generator instabilities of various kinds, e.g. tran- sient, dynamic, sub-synchronous resonance (SSR) and sub-synchronous torsional interactions (SSTI). New power electronic controllers are being developed to help generators operate reliably in the new market place. The thyristor-controlled series compensator (TCSC) is being used to mitigate SSR, SSTI and to damp power systems' oscillations (Larsen et al., 1992). Examples of where TCSCs have been used to mitigate SSR are the TCSCs installed in the 500 kV Boneville Power Administration's Slatt substation and in the 400 kV Swedish power network. However, it should be noted that the primary function of the TCSC, like that of its mechanically controlled counterpart, the series capacitor bank, is to reduce the electrical length of the compensated transmission line. The aim is still to increase power transfers significantly, but with increased transient stability margins. A welcome result of deregulation of the electricity supply industry and open access markets for electricity worldwide, is the opportunity for incorporating all forms of renewable generation into the electrical power network. The signatories of the Kyoto agreement in 1997 set themselves a target to lower emission levels by 20% by 2010. As a result of this, legislation has been enacted and, in many cases, tax incentives have been provided to enable the connection of micro-hydro, wind, photovoltaic, wave, tidal, biomass and fuel cell generators. The power generated by some of these sources of electricity is suitable for direct input, via a step-up transformer, into the AC distribution system. This is the case with micro-hydro and biomass generators. Other sources generate electricity in DC form or in AC form but with large, random variations which prevent direct connection to the grid; for example fuel cells and asynchronous wind generators. In both cases, power electronic converters such as VSCs provide a suitable means for connection to the grid. In theory, the thyristor-based static var compensator (SVC) (Miller, 1982) could be used to perform the functions of the PSS, while providing fast-acting voltage support at the generating substation. In practice, owing to the effectiveness of the PSS and its relative low cost, this has not happened. Instead, the high speed of response of the SVC and its low maintenance cost have made it the preferred choice to provide reactive power support at key points of the transmission system, far away from the generators. For most practical purposes they have made the rotating synchronous compensator redundant, except where an increase in the short-circuit level is required along with fast-acting reactive power support. Even this niche application of rotating synchronous compensators may soon disappear since a thyristor-controlled series reactor (TCSR) could perform the role of providing adaptive short-circuit compen- sation and, alongside, an SVC could provide the necessary reactive power support. Another possibility is the displacement of not just the rotating synchronous com- pensator but also the SVC by a new breed of static compensators (STATCOMs) based on the use of VSCs. The STATCOM provides all the functions that the SVC can provide but at a higher speed and, when the technology reaches full maturity, its cost will be lower. It is more compact and requires only a fraction of the land required by an SVC installation. The VSC is the basic building block of the new gener- ation of power controllers emerging from flexible alternating current transmission Power electronic control in electrical systems 3 [...]... compatibility problems in consumer installations interference in neighbouring communication circuits spurious tripping of protective devices 1.6 The role of computers in the monitoring, control and planning of power networks Computers play a key role in the operation, management and planning of electrical power networks Their use is on the increase due to the complexity of today's interconnected electrical networks... Damper windings consist of bars placed in slots on the pole faces and connected together at both ends In general, steam turbines contain no damper windings but the solid steel of the rotor offers a path for eddy currents, which have similar damping effects For simulation purposes, the currents circulating in the solid steel or in the damping windings can be treated as currents circulating in two closed... Power electronic control in electrical systems 5 Fig 1.1 Power network //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 6 ± [1±30/30] 17.11.2001 9:43AM 6 Electrical power systems ± an overview 1.3.1 Generation The large demand for electrical energy coupled with its continuous varying nature and our inability to store electrical energy in significant quantities calls for a diversity of generating... winding in the stator In the rotor, the direct axis (d-axis) is magnetically centred in the north pole A second axis located 90 electrical degrees behind the direct axis is called the quadrature axis (q-axis) In general, three main control systems directly affect the turbine-generator set: 1 the boiler's firing control 2 the governor control 3 the excitation system control Figure 1.4 shows the interaction... illustrates its electronically controlled counterpart (Kinney et al., 1994) It should be pointed out that the latter has the ability to exert instantaneous active power flow control Several other power electronic controllers have been built to provide adaptive control to key parameters of the power system besides voltage magnitude, reactive power and transmission line impedance For instance, the electronic. .. kV and domestic users with single-phase electricity at 240 V Figure 1.1 also gives examples of power electronics-based plant components and where they might be installed in the electrical power network In high-voltage transmission systems, a TCSC may be used to reduce the electrical length of long transmission lines, increasing power transfers and stability margins An HVDC link may be used for the purpose... draw non-sinusoidal currents and, under certain conditions, they distort the sinusoidal voltage waveform in the power network In general, if a plant component is excited with sinusoidal input and produces non-sinusoidal output, then such a component is termed non-linear, otherwise, it is termed linear (Acha and Madrigal, 2001) Among the non-linear power plant components we have: power electronics... controls and the turbine-generator set The excitation system control consists of an exciter and the AVR The latter regulates the generator terminal voltage by controlling the amount of current supplied to the field winding by the exciter The measured terminal voltage and the //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH001.3D ± 7 ± [1±30/30] 17.11.2001 9:43AM Power electronic control in electrical systems. .. composition of electrical power networks and the computer assisted studies that are used for their planning, operation and management The main plant components used in modern power networks are described, and the growing ascendancy of power electronics-based equipment in power network control is emphasized This equipment is classified into equipment used in high voltage transmission and equipment used in low... ± [1±30/30] 17.11.2001 9:43AM 28 Electrical power systems ± an overview Fig 1.21 Real-time environment The main power systems software used for the real-time control of the network is (Wood and Wollenberg, 1984): state estimation security analysis optimal power flows These applications provide the real-time means of controlling and operating power systems securely In order to achieve such an objective . titles in the Newnes Power Engineering Series Miller Electronic Control of Switched Reluctance Machines 0-7506-5073-7 Agrawal Industrial Power Engineering. 17.11.2001 5:49PM N EWNESEWNES P OWEROWER E NGINEERINGNGINEERING S ERIESERIES Power Electronic Control in Electrical Systems //SYS21/F:/PEC/REVISES_10-11-01/075065126-CH000-PRELIMS.3D