An enhanced current sharing strategy for islanded ac microgrids based on adaptive virtual impedance regulation

In islanded mocrogrids, the mismatch among the equivalent impedances (sum of output impedance and wire impedance) degrades the current sharing performance. It is an essential mission to eliminate this mismatch to achieve accurate current sharing in different conditions. About this issue, several aspects should be considered. Firstly, for a general microgrid, the inverter equivalent impedances are unknown, both the resistance and reactance should be considered. Secondly, all the balanced, unbalanced and harmonic components of load current should be shared according to the power ratings of distributed generators. Besides, not only the current sharing in steady states but also during transients should be achieved. Herein, a low-bandwidth communication-based distributed current sharing strategy for parallel-connected inverters is proposed in this paper. The virtual resistance and reactance are adaptively regulated according to the magnitude error and phase angle error of the inverter uniform output current respectively. Then they can be used to reshape the inverter equivalent impedance at fundamental positive-sequence frequency, fundamental negative-sequence frequency and harmonic frequencies. With the proposed method, all the three aspects mentioned above can be guaranteed without knowing inverter wire impedances in advance. The simulation and experiments on an islanded microgrid prototype are developed to verify the effectiveness of the proposed strategy.