Effect of excitation systems and of power system stabilisers on synchronous generator damping and synchronising torques

The synchronising and damping torque coefficients in the range of the electromechanical oscillation frequencies provide a measure of the generators' contribution towards power-system stability under small deviations, and may be taken as stability indices even for multimachine systems, although their interpretation is rather complex in this case. The paper presents the development of an expression to calculate the electromagnetic torque for small deviations around an operating point, as produced by a synchronous generator connected to an infinite busbar through an impedance; it explicitly includes the effects of its automatic voltage regulator (AVR) and of the additional stabiliser signal derived from the rotor speed. In addition, numerical results from an evaluation of the damping and synchronising torque coefficients obtained from the expression are presented for a given generator. This is done in such a way as to emphasise the effect of a static-excitation system on the deterioration of the damping torque, and the effect of the stabiliser signal leading to an increase of the synchronous generators dynamic stability. The paper also discusses the effects of the presence of the damper windings, and of the system operating conditions and reactance variations.