Improvement of power system stability by crosscoupling generator control systems D.W. Auckland, MSc, PhD, CEng, MlEE M.O. Omoigui, MSc, PhD R. Shuttleworth, PhD zyxwvutsrqpo Indexing terms: Control systems, Power systems and plant, Stability zyxwvutsrqpo Abstract: It is proposed that power system stabil- ity can be improved by controlling the dynamic response of generators so that they act sympa- thetically during a transient disturbance. This is shown to be valid theoretically for a simple system consisting of two identical machines fitted with power system stabilisers, the control signals from one machine being used to stabilise the other and vice versa. The extent of the improvements in system response to be expected from such an arrangement in practice is investigated using a dynamic model consisting of two dissimilar machines linked by transmission lines. It is found that the effectiveness of the machine stabilisers is doubled by crosscoupling control signals. 1 Introduction Trends in power system design are leading to power net- works comprising relatively few machines interlinked by long transmission lines. The large alternators usually have low inertia constants and high transient reactances, whilst long transmission lines are characterised by high impedances. The increased generation and transmission capacities mean that fault levels are higher, posing threats to system stability under transient and steady- state conditions. Recognition of this threat is underlined by the considerable attention which power system stabil- ity has received in recent years zyxwvutsrq [ 1-61. It has been shown that introducing additional control signals into the turbine governor or the automatic voltage regulator of a machine can improve its stability under a wide range of operating conditions. In the USA and some parts of Europe where there are exceptionally long transmission lines, extensive efforts have been made to improve both local and inter-area oscillation modes by introducing power system stabilisers zyxwvutsr (PSSs). The stabili- sers employ one or more signals proportional to power, frequency, rotor speed or acceleration, to suitably modify the characteristics of the governor and/or voltage regu- lating equipment. Efforts to counteract tendencies toward instability have centred mostly on the stiffening of individual machines within a system, but it may well be that improved performance could be achieved by controlling Paper 6795C zyxwvutsrqpo (P9, Pll), first received 31st March 1988 and in revised fonn 14th March 1989 The authors are with the Electrical Engineering Department, The Uni- versity of Manchester, Manchester MI3 9PL, United Kingdom IEE PROCEEDINGS, Vol. 136, Pt. C, No. 5, SEPTEMBER 1989 machines in sympathy. Thus a control signal obtained from one machine would be employed to stabilise a neighbouring machine and vice versa, so that, in control terms, the machines are crosscoupled. In this manner, improved stability may be achieved not only under a small perturbation but also under a large prolonged dis- turbance which may result in separation of part of the network from the main system. The validity of this hypothesis has been tested theoretically and practically for a system comprising two geographically separated large generating units interlinked by a relatively weak transmission line, both machines being fitted with power system stabilisers. The theoretical work employed root locus and multivariable control analysis and the practical work was performed on two micromachines linked by a model transmission system. 2 Theoretical analysis The objective of the analysis presented in this paper is to show that improvements in system stability can theoreti- cally be achieved when power system stabilisers, which normally act independently, are interlinked, and that the stability of one generator is not improved at the expense of another. The system configuration to be considered is depicted in Fig. 1. The arrangement of the system with 10 zyxwvut 26 p U i nfin1 te busbar 1009pu machine E machine A local load local load Fig. 1 Two-machine infinite-busbar system cross connected stabilisers is shown in Fig. 2. For the analysis to be of reasonable proportions it was assumed that both machines and their associated control equipment are identical to the steam unit having the parameters listed in Tables la, c and d, with one excep- tion. This is the zyxwv H constant of machine A which was set at 5.31 s, recognising that inertial imbalances are funda- mental to the phenomenon under investigation. The system of Fig. 2 may be represented by a simple small signal model obtained by linearising the nonlinear algebraic equation describing the operation of each machine and its AVR in the now universally accepted 289