Two-dimensional impedance-shaping control with enhanced harmonic power sharing for inverter-based microgrids

In islanded microgrids, the harmonic power of nonlinear loads is distributed among parallel voltage source inverters (VSIs) according to the effective harmonic impedances, i.e., the sums of VSI output impedances and grid impedances. Since grid impedances are unknown and could be mismatched, VSI output impedances are usually reshaped to ensure the harmonic power sharing accuracy. However, as conventional techniques only regulate VSI output impedances in one dimension, only one degree of freedom is provided for the impedance shaping. It is revealed that such maneuvers can hardly fulfill the proper harmonic power sharing requirement under complex grid impedance situations. As a result, circulating harmonic currents will occur and produce additional power losses even if the total harmonic power has been accurately shared. To solve this problem, this paper proposes a two-dimensional impedance-shaping control, which can adaptively regulate VSI output resistances and inductances at the same time. The proposed control strategy requires no prior grid impedance knowledge and can eliminate the circulating harmonic currents for arbitrary grid impedances. Simulation and experimental results from an islanding microgrid prototype with three parallel VSIs are provided to validate the effectiveness of the proposed method.