Effect of excitation regulation on synchronous-machine stability

The paper records a detailed study of the effect of a voltage regulator on the stability of an alternator connected through a reactance to an infinite bus. The stability is analysed by means of Nyquist loci calculated for the transfer functions of alternator and regulator. The accuracy and speed of response of the system are also considered. The first part of the paper considers a simple regulator with proportional feedback, and it is shown that such an ideal regulator can extend the region of steady-state stability to a point corresponding to the maximum of the transient power-angle curve. Practical regulators are classified according to the nature of their transfer functions. The analysis provides a means of predicting their behaviour and explains how they affect the stability, accuracy and response. The effect of delay elements, integrator elements and derivative elements in the regulator is considered particularly; e.g. a buck-boost exciter, which effectively introduces an integrator element, gives good accuracy but less satisfactory response, and has a limited effect on stability, and a derivative regulator which gives rapid response and a large extension of the stability region, but has limited accuracy. Experiments performed on a model machine with various simulated regulators agreed well with the computed results. The computations allowed fully for the system parameters, including alternator resistance and alternator damping.