Stability Analysis for Variable Frequency Operation of Synchronous Machines

A stability study of a synchronous machine is performed by applying the Nyquist stability-criterion to the equations which describe the behavior of the machine during small displacements about a steady-state operating point. This investigation reveals that, in some cases, machine instability can occur at low operating speeds. Regions of machine stability are established from the results of a digital computer study and the maximum resonance peak Mm is used as a criterion for determining relative stability when the response of the electromechanical system is damped. The effect on machine stability for changes in different system parameters is given. Also, the maximum resonance peak is used to establish an equivalent second-order electromechanical system. The response of this system is found to compare favorably to that of an idealized synchronous machine at any operating speed.