A direct voltage unbalance compensation strategy for islanded microgrids

In this paper, a control strategy with low bandwidth communications for paralleled three-phase inverters is proposed to achieve satisfactory voltage unbalance compensation. The proposed control algorithm mainly consists of voltage/current inner loop controllers, a droop controller, a selective virtual impedance loop, and an unbalance compensator. The inner loop controllers are based on the stationary reference frame to better mitigate the voltage distortion under nonlinear loads. Droop control and selective virtual impedance loop achieve accurate current-sharing when supplying both linear and nonlinear loads. Moreover, by adjusting voltage references according to the amplitude of the negative sequence voltage, the unbalance factor, which is mainly caused by single phase generators/loads, can be mitigated to an extremely low value. Finally, an AC microgrid which includes three three-phase three-leg inverters was tested in order to validate the proposed control strategy.