Introduction

Although this chapter is concerned with the application of a power system stabilizer (PSS) to a single-machine system, the concepts for the most part are applicable to multi-machine systems: such applications will be discussed in Chapters 9 and 10. Various important aspects of the tuning of the PSS can therefore considered in some detail because the analysis involves a simple system only.

The reasons for the wide-spread deployment of PSSs in power systems today are twofold, (i) to stabilize the unstable electro-mechanical modes in the system, (ii) to ensure that there is an adequate margin of stability for these modes over a wide range of operating conditions and contingencies, that is, the electro-mechanical modes are adequately damped. Some systems, such as the Eastern Australian grid, would be unstable without the use of both PSSs and stabilizers installed on certain FACTS devices.

A marginally stable electro-mechanical mode is oscillatory in nature and is very lightly damped. The frequency of rotor oscillations is typically between 1.5 to 15 rad/s, and the 5% settling time may be many tens of seconds. Typically a mode of a lengthy duration would not satisfy the system operator's criterion for modal damping. A stable mode is said to be ‘positively’ damped, whereas an unstable mode is referred to as being ‘negatively’ damped.

With the growth of power systems, and the need to transmit power over long distances by means of high-voltage transmission lines, the problems of instability following a major fault or disturbance have increased. Instability in such events is typically the result of a generator falling out of step due to insufficient synchronizing torques being available to hold generators in synchronism. In order to increase the synchronizing torques between generators, high-gain fast-acting excitation systems were developed with the objective of increasing field flux linkages rapidly during and following the fault. However, such high-gain excitation systems may introduce negative damping on certain electro-mechanical modes.

In linear control systems design, rate feedback is employed not only to stabilize an unstable system but also to enhance the system's damping performance. A PSS that uses generator speed (i.e. the rate of change of rotor angle) as a stabilizing signal is such a rate-feedback controller.