Degree Name

Doctor of Philosophy


Department of Electrical and Computer Engineering


This thesis investigates a satisfactory stabiliser to damp out low frequency oscillations which occur in power systems for different operating conditions. A fixed gain power system stabiliser is inadequate to provide acceptable damping characteristics as the operating point changes. Adaptive stabilisers which are equipped by the identification process to recognise these changes can then be used to overcome this problem.

In adaptive control, which is a digital control strategy, the parameters of the discrete-time model of a continuous-time power plant are estimated through continual sampling of the input and output signals. In this regard, different methods for obtaining a discrete-time mathematical model for adaptive control of a single-machine infinite-bus power system are first presented. The usual approach has been to use the shift operator q, or its equivalent z transform, but this gives numerical difficulties with the small sampling periods which are n o w becoming usual with modern control hardware. It is shown, by means of the example of the identification of a generator excitation control system for both singlemachine infinite-bus and multimachine power systems, that these problems can be avoided by the use of the delta operator instead. Calculations show that the delta operator formulation also reflects the frequency and dynamic response of the system more accurately and conveniently.

The adaptive Pole Assignment controller, which has not been applied to power systems as a stabiliser, is developed by fixing the poles of the transfer function Gd(s) = Δδ/ΔPm which reflects the effect of the load disturbances (ΔPm ) on the power angle (Δδ). The development of the control algorithm has been made using the delta operator rather than the shift operator as this removes numerical problems at fast sampling rates. The delta operator also gives transfer functions very similar to those of the continuous system and, therefore allows simplification of the control design by reducing the order of the numerator of the discrete-time transfer function. Comparative results for the adaptive Pole Assignment controller and a fixed parameter stabiliser show the improvement in response obtained with the adaptive algorithm as the operating point changes for a single-machine infinite bus power system.

In the Pole Assignment adaptive controller, the system response somewhat varies although the settling time of the system is fixed since the location of the zeros is not considered. Therefore, this technique is modified such that the new adaptive power system stabiliser is able to locate both the poles and the zeros. The control strategy is based on a new type of model reference adaptive technique in which the transfer function Gd(s) is modified to a standard form based on explicit system identification. This avoids having to compare the actual plant output with a model following the usual model reference adaptive approach. Controller design is simplified by reducing the number of controller parameters to be identified and controller performance is improved by the use of the delta operator rather than the more usual shift operator for discretization. The similarity between continuous-time and discrete-time systems using the delta operator also allows the design of the adaptive controller based on a continuous-time control strategy. Simulation studies performed on a typical excitation control system model are presented. Comparative results of the Model Reference Adaptive controller, the Pole Shifting and the adaptive Pole-Assignment controllers and a fixed parameter stabiliser clearly show the benefits of the proposed adaptive controller for stability enhancement of a single-machine infinitebus power system, especially where there are large changes in operating point. This adaptive controller also provides better damping characteristics than a fixed parameter stabiliser for a multimachine power system.