D-Q impedance specification for balanced three-phase AC distributed power system

In a dc distributed power system, small-signal stability is studied using Nyquist stability criterion. Forbidden region for the polar plot of the ratio of impedances at the interface between source and load subsystems can be defined. For a given source impedance, forbidden region can be transformed to a load impedance specification. System stability can be assured with certain margin while allowing source and load impedance overlap. Although small-signal stability and its margin of balanced three-phase ac system can be addressed using source and load impedances in synchronous reference (d-q) frame, similar load impedance specification is not reported. This paper discusses the challenge of stability analysis in three-phase ac system, namely its coupled multi-input multi-output (MIMO) nature. Then a method to do load impedances specification in d-q frame is proposed. Condition and limitation of the proposed method is discussed. Simulation and measurement verify the analysis.