Introduction

In Chapter 5 a speed-PSS based on the P-Vr design approach is described. The purpose of this chapter is to describe in detail the theoretical basis for some of the widely deployed types of PSSs and the associated practical implications. For some other types of PSSs, including the multi-path, multi-band PSS developed by Hydro-Québec, only a brief overview is provided. Furthermore, the details of a number of other types of PSSs and their development are omitted from this book, for example: delta-omega stabilizers (without and with torsional filters) [1]; the use of notch filters to attenuate the first torsional mode [2]; the application of the coordinated AVR/PSS, called the “Desensitized Four Loops Regulator” [3].

The input to the PSS in Chapter 5 is assumed to be the ‘true’ rotor speed as measured directly by a high-fidelity tacho-generator, a toothed wheel, or some other device mounted on the shaft of the turbine-generator unit. In practice there may be physical difficulties in positioning any such device on the shaft as well as locating it to minimize the introduction of the torsional modes of the shaft into the speed signal. Moreover, other difficulties such as noise, lateral shaft movement (runout or ‘wobble’ [4]) in vertical units, may present themselves. In this chapter, however, synthesized speed perturbations, which are assumed to accurately represent the true rotor speed perturbations, are used as the input to the PSS. This means that the same basis and procedure as that outlined in Chapter 5 can be employed for the design and tuning of the PSS.

The major factor in the selection of a stabilizing signal for input to the PSS is the requirement that the modes of concern, which may be the local-, inter-area, and possibly the intra-station modes, must be observable by the signal over a wide range of operating conditions. Typically, perturbations in rotor speed, the electric power output, and the frequency at the generator terminals are the commonly-used local signals.

Various types of pre-filters are in use which convert one or more signals derived from variable(s) other than speed into a synthesized speed signal. Such variables are electric power, bus-voltage angle, frequency, terminal voltage and current; some manufacturers develop a ‘speed’ signal from such variables using various techniques.