An Optimal Sequence Small-AC-Signal Injection Based Reactive Power Sharing Method for Parallel Grid-Forming Inverters in Islanded Microgrids with Harmonic Loads

The small-AC-signal (SACS) injection based method is attractive for solving the reactive power sharing issue among parallel grid-forming inverters in islanded microgrids. However, if the SACS frequency is closed to the one of the harmonic load currents, the SACS current extraction will be disturbed, and then causes the system oscillation. In this paper, the harmonic currents of three-phase bridge rectifiers and nonlinear inductive loads are analyzed. Based on the sequence of harmonic currents, the SACS whose sequence is opposite to the harmonic currents is injected into the output voltage of each parallel inverter to mitigate the influence of harmonic currents, while the frequency of SACS droops with reactive power. Therefore, the reactive power will be accurately sharing and the effect of harmonic loads can be mitigated. The effectiveness of the proposed method is verified by the simulation and experimental results.