Patent ID: 7508224

Claim:
A method for measuring a stability margin at a node of a polyphase power grid, comprising: (a) using a pre-existing polyphase generator permanently residing on said polyphase power grid and electrically coupled to said node to inject into each phase of said polyphase power grid a respective suppressed-carrier stimulus signal that is oscillating at, at least one selected test frequency of a plurality of predetermined test frequencies; (b) extracting from each phase of a first polyphase current branch on said polyphase power grid a respective response signal produced in said polyphase power grid in response to said respective suppressed-carrier stimulus signal to form a set of first response signals; (c) extracting from each phase of a second polyphase current branch on said polyphase power grid a respective response signal produced in said polyphase power grid in response to said respective suppressed-carrier stimulus signal to form a second set of response signals; (d) processing said first set of response signals to generate a first reference value; (e) processing said second set of response signals to generate a second reference value; (f) determining a suppressed-carrier impedance ratio at said selected test frequency based on a division of said first reference value by said second reference value; (g) repeating acts (a) through (f) for each test frequency of said plurality of predetermined test frequencies to form a set of suppressed-carrier impedance ratios; and (h) determining a stability margin at said node of said polyphase power grid based on said set of suppressed-carrier impedance ratios.