Small signal impedance measurement in droop controlled AC microgrids

Stable operation of microgrids is highly influenced by characteristics of generators due to their small size and inertia and fast and variable dynamics. Recent work has shown that microgird stability can be determined by analysis of small signal impedances at the point of interconnection. The purpose of small signal impedance measurement is to verify the analytical models of single devices and to provide measured impedances where analytical models are not available, so that stability may be established. Small signal impedance measurement typically involves in-situ injection of currents or voltages superimposed upon the operating system. In droop-controlled microgrids, since the frequency is a function of the power demand, the injection has to be independent of the frequency. In this paper an injection approach is proposed using a three phase buck converter. Analytical models using dynamic phasors are compared to measurement results obtained in a laboratory microgrid. The injection method and the incremental phasor models of passive loads are verified and small signal impedance measurements of an islanded and grid-connected microgrid are obtained including a voltage source PWM converter.