CRITICAL MODES AND VOLTAGE INSTABILITY IN POWER SYSTEMS.

Voltage collapse/instability has been described in the literature in terms of steady-state load flow sensitivities, speculated sequences of events, and lack of control coordination. The dynamic equations for synchronous machines are analyzed here using different models in an attempt to understand voltage collapse/instability from a critical mode point of view. It is shown that for a single machine connected to an infinite bus, a two-axis model is essentially steady-state-stability limited by its field-winding flux linkage. The same result is achieved numerically for a three-machine system. The correlation between this critical mode viewpoint and voltage collapse/instability is discussed.