Degree Name

Doctor of Philosophy


Department of Electrical and Computer Engineering


A frequency domain approach is used to analyse low frequency oscillations in a power system where the oscillations are viewed as the result of disturbances originating from a section of the network and reflected on the remainder of the network. Available control theory for counteracting exogenous disturbances are thus readily applicable (e.g. the internal model principle). Further, this approach is shown to be capable of revealing risky situations which otherwise could not be detected. The gain characteristics of closed loop transfer functions, which reflect the effect of disturbances on power angle oscillations, are shown to be crucial in identifying frequency ranges where amplified oscillatory disturbances can occur.

The adopted analytical approach also provides additional insight in the development of power system stabiliser tuning rules, which are usually based on eigenvalue analysis. This leads to a proposal for restructuring the transfer function blocks in the excitation control system such that the negative damping effects are decoupled without compromising the benevolent effects of other control loops. Simulation results confirm the validity of this proposal. A special feature of the new structure is the location of a notch filter, acting as an on/off switch, in the forward path of the negative damping signal. This provides the necessary dynamic gain reduction for "switching off the detrimental signals. It is shown that, while a considerable reduction in the tie-line mode oscillations can be achieved, the phenomenon of amplified oscillations can still occur, albeit at a reduced magnitude.

A self-tuning control scheme is therefore proposed which is able to provide both positive damping characteristics and disturbance rejection. A control strategy based on the internal model principle is used to directly cancel tie-line oscillatory disturbances. The thesis describes the internal model principle and its implementation in a power system for disturbance rejection purposes. Since the self-tuner requires on-line estimation of the system parameters, an efficient recursive algorithm for the estimation routine is developed. The algorithm makes use of some properties of the Cholesky matrix decomposition techniques. The numerical stability of the estimation algorithm is improved through the updating of the triangular factors of an augmented information matrix and a fixed width data window, rather than a forgetting factor, is used to cater for parameter drifts.

Simulation studies performed on a typical excitation control system model are presented, demonstrating clearly the benefits of incorporating the internal model principle in an adaptive power stabilising scheme. Results show effective cancellation of tie-line time-varying oscillatory disturbances and enhanced damping behaviour of the power system.