Variable structure power stabiliser to supplement static-excitation systems

One of the weaknesses of the small-perturbation theory when applied to power-system stability is that the system differential equations must be linearised with respect to an operating point since they are fundamentally nonlinear. One of the most prominent nonlinear effects is due directly to the loading condition, that is the real and reactive power being transmitted. In the paper, the influence of a generator static-excitation system on the small-perturbation stability of a single-machine infinite system, under widely varying loading conditions, is closely scrutinised by using eigenvalue techniques. The tendency of an unsupplemented static exciter to degrade the system damping for medium and heavy loading is shown in the complex frequency domain. A variable structure power stabiliser (i.e. variable parameters) which is altered to compensate for variations in the system loading is considered. The optimum settings of the stabiliser parameters associated with a selected set of grid points in thePQ domain, are computed offline by minimising a performance criterion. Whereas a stabiliser with fixed parameters is of necessity a compromise, it is shown that one with variable parameters can offer improved dynamic performance under widely varying load conditions.