A method is provided for measuring D-Q impedance of a component of a polyphase power grid connected to a grid node, and evaluating the margin of stability at a node using Nyquist diagrams generated from the measured D-Q impedance data. A generator, coupled to the polyphase power grid, is controlled to induce suppressed-carrier stimulus current into the grid node. Circuitry measures response signals of suppressed-carrier form existing within a bus voltage at the grid node and a branch current of the component being measured. The method includes measuring complex voltage components of the response signals contained in the bus voltage and complex current components of the response signals contained in the branch current to form simultaneous equations that are resolved to determine the D-Q impedance parameters Zqq, Zqd, Zdq and Zdd of the measured component.