Aspects on Dynamic Power Flow Controllers and Related Devices for Increased Flexibility in Electric Power Systems Nicklas Johansson Royal Institute of Technology School of Electrical Engineering Division of Electrical Machines and Power Electronics Stockholm 2011 Submitted to the School of Electrical Engineering in partial fulfillment of therequirementsforthedegreeofDoctorofPhilosophy. Stockholm 2011 ISBN 978-91-7501-058-8 ISSN 1653-5146 TRITA-EE 2011:050 A This document was prepared using LTEX. Abstract This thesis studies different aspects of Flexible AC Transmission System (FACTS) devices which are used to improve the power transfer capability and increase the controllability in electric power systems. In the thesis, different aspects on the usage and control of Dynamic Power Flow Controllers (DPFC) and related FACTS devices are studied. The DPFC is a combination of a Phase Shifting Transformer (PST) and a Thyristor Switched Series Capacitor (TSSC)/Thyristor Switched Series Reactor (TSSR). The thesis proposes and studies a new method, the Ideal Phase-Shifter (IPS) method, for selection and rating of Power Flow Controllers (PFC) in a power grid. The IPS method, which is based on steady-state calculations, is proposed as a first step in the design process for a PFC. The method uses the Power controller plane, introduced by Brochu et al in 1999. The IPS method extends the usage of decoupling methods in the Power controller plane to a power system of arbitrary size. The IPS method was in the thesis used to compare the ratings of different PFC:s required to improve the power transfer capability in two test systems. The studied devices were here the PST, the TSSC/TSSR and the DPFC. The thesis treats control of ideal Controlled Series Capacitors (CSC), TCSC, TSSC/TSSR, and DPFC. The goals of the FACTS controllers which are developed are Power Oscillation Damping (POD), fast power flow con- trol, and transient stability improvement in the power system. New adaptive control strategies for POD and power flow control are proposed and studied in different models of power systems by time-domain simulations. A strategy for transient stability improvement is also proposed and studied. i Additionally, different methods for study of Subsynchronous Resonance (SSR), which is associated with series compensation in power systems, are investigated. Here, four of the most common methods for frequency scanning to determine the electrical damping of subsynchronous oscillations in a power grid are studied. The study reveals significant differences of the electrical damping estimates of the studied standard methods when applied to a four- machine test system. Keywords X Adaptive Control X Controlled Series Compensator X Dynamic Power Flow Controller X Frequency Scanning X Phase-Shifting Transformer X Power Controller Plane X Power Flow Control X Power Oscillation Damping X Subsynchronous Resonance X Thyristor Controlled Series Compensator X Thyristor Switched Series Compensator X Transient Stability ii Sammanfattning Denna avhandling studerar olika aspekter av styrbara komponenter (FACTS - Flexible AC Transmission Systems) i kraftsystem. FACTS-komponenter anv¨ands f¨or att ¨oka ¨overf¨oringskapaciteten och kontrollerbarheten i elek- triska kraftsystem. I avhandlingen studeras olika aspekter av en s˚a kallad Dynamic Power Flow Controller (DPFC) samt relaterade FACTS- komponenter. En DPFC ¨ar en kombination av tv˚a tidigare k¨anda komponen- ter, en fas-vridande transformator (PST) och en thyristor-styrd seriereaktans (TSSC/TSSR). En ny metod, IPS-metoden, som anv¨ands f¨or att v¨alja ut och best¨amma ¨onskad karakteristik f¨or en effektfl¨odes-styrande FACTS-komponent (PFC) i ett kraftn¨at, introduceras i avhandlingen. IPS-metoden, som baseras p˚a statiska ber¨akningar, ¨ar t¨ankt att anv¨andas som ett f¨orsta steg vid konstruktionen av en PFC. Methoden ˚ask˚adligg¨or PFC-karakteristiken ge- ometriskt i PFC-planet (The Power Controller Plane), som introducerades av Brochu m. fl. 1999. IPS-metoden g¨or det m¨ojligt att frikoppla PFC- och n¨atkarakteristiken i PFC-planet f¨or ett elkraftsystem av godtycklig storlek. I denna avhandling anv¨ands IPS-metoden till att j¨amf¨ora den n¨odv¨andiga karakteristiken f¨or olika PFC:er f¨or att ut¨oka ¨overf¨oringskapaciteten i tv˚a olika testsystem. PFC-komponenterna PST, TSSC/TSSR samt DPFC j¨amf¨ordes i studien. Avhandlingen behandlar ¨aven styrning av ideala styrbara seriereaktanser, TCSC, TSSC/TSSR och DPFC. M˚alen f¨or de utvecklade reglermetoderna ¨ar d¨ampning av elektro-mekaniska oscillationer (POD), effektfl¨odes-styrning samt f¨orb¨attring av den transienta stabiliteten i kraftn¨atet. Nya adaptiva styrmetoder f¨or POD och effektfl¨odes-styrning f¨oresl˚as och studeras med iii hj¨alp av datorsimuleringar i olika modeller av kraftsystem. En ny strategi f¨or f¨orb¨attring av den transienta stabiliteten i kraftn¨at studeras ocks˚a. Slutligen unders¨oks ocks˚aolikametoderf¨or studier av subsynkron reso- nans (SSR) i kraftsystem. Detta fenomen kan uppkomma i kraftsystem som inneh˚aller seriekompensering t. ex. i form av en TSSC. Fyra av de mest popul¨ara metoderna f¨or s. k. frekvensscanning f¨or att ber¨akna graden av d¨ampning f¨or subsynkrona oscillationer i kraftn¨at studeras. Studien visar p˚a signifikanta skillnader i resultaten f¨or de studerade metodena n¨ar de ap- pliceras p˚a ett testsystem med fyra synkrongeneratorer. iv Preface The work presented in this thesis was carried out at the Laboratory of Electrical Machines and Power Electronics, School of Electrical Engineer- ing, Royal Institute of Technology (KTH). This project was financed from the Elforsk ELEKTRA foundation. This thesis is organized as an extended summary of Conference- and Jour- nal articles which have been published by the author. The included papers are appended to the thesis and they are in the thesis referred to as [Paper 1], [Paper 2], [Paper 3] ..., in contrast to the other references which are denoted [1], [2], [3], .... v vi Acknowledgements Without the funding from Elforsk and the Elektra foundation, this thesis work would not have been possible. Many thanks! Many people have aided me in my research work. The two most impor- tant persons are my outstanding supervisors, Hans-Peter Nee and Lennart Angquist.¨ Hans-Peter has, with his enthusiasm and analytical capabilities, supported me numerous times during the course of my work. Lennart has provided the project with an industrial perspective and he always shared gen- erously of his extensive experience and guided me throughout the project. My sincere gratitude to both of you! Many thanks to the reference group members of my project who guided me with their questions and comments at our meetings: Bertil Berggren (ABB), Peter Lundberg (ABB), Lennart Harnefors (ABB/KTH), Pablo Rey (ABB), Stefan Thorburn (ABB), Therese Fahlberg (SVK), Magnus Daniels- son (SVK), Jonas Persson (Vattenfall), Pernilla Owe (Vattenfall), Mehrdad Ghandhari (KTH), and Lennart S¨oder (KTH). Bertil Berggren at ABB Corporate Research should be especially thanked for answering all of my questions, co-authoring and for giving me access to phase-shifting transformer data and simulation models. Dietrich Bon- mann from ABB in Germany should also be thanked for generously sharing data and answering my questions on design of phase-shifting transformers. Thanks to Mauro Monge at ABB for his excellent work on adaptive con- trollers as a M. Sc. thesis student in my project. Many thanks also to Tomas Jonsson at ABB for getting me involved in the project. vii At KTH, I would like to thank all the current and former people at EME for contributing to the friendly atmosphere at the division. Especially, my current and former room-mates Staffan Norrga, Stephan Meier and Lidong Zhang are thanked for enduring my music. The people at the EPS and ETK divisions as well as my friends at other KTH departments and elsewhere are thanked for the many nice conversations during all of the lunches and coffee-breaks that we spent together. At the EME division, special gratitude goes to Eva Pettersson, Emma Pettersson and Brigitt H¨ogberg for all of that administrative aid and to Peter L¨onn for keeping my computer in order. Finally, my family, Josefin, Julia and Anna are thanked for their love, patience and support. Nicklas Johansson Stockholm, May 2011. viii Contents 1 Introduction 1 1.1Abriefhistoryofelectricalpowersystems........... 1 1.2Challengesintoday’spowergrids................ 3 1.3Scope............................... 5 1.4Scientificcontributions...................... 6 1.5Listofpublications........................ 8 1.6Outline.............................. 10 1.7Terminology............................ 11 2 Background 13 2.1 The concept of power transfer capability ............ 13 2.2 Stability of the electrical power system ............. 15 2.2.1 Small-signal angle instability .............. 15 2.2.2 Transient instability ................... 17 2.2.3 Frequency instability . .................. 17 2.2.4 Voltage instability .................... 18 2.2.5 Subsynchronous resonance ................ 19 2.3Mathematicalmodelingofelectricalpowersystems...... 21 2.3.1 Classicalmodelforasingle-machinesystem...... 22 2.3.2 Classicalmodelforamultiple-machinesystem.... 24 2.4FACTSdevicesandtheircontrol................ 28 2.4.1 Powerelectronicconverters............... 28 2.4.2 Shunt-connectedFACTSdevices............ 29 2.4.3 Series-connectedFACTSdevices............ 30 2.4.4 Combined shunt- and series-connected FACTS devices 34 2.4.5 Basic functionality of the