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 author 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; he explicitly includes the effects of its automatic voltage regulator (AVR) and of the additional stabiliser signal derived from the rotor speed