Avoiding Circulating Current via Current-Limiting Control in AC Microgrids with Parallel Three-Phase Inverters

In this paper, an AC microgrid consisting of paralleled three-phase inverters is investigated and a nonlinear droop controller is proposed. The purpose of the proposed controller is twofold: i) to avoid circulating power among the paralleled inverters and ii) to guarantee a current-limiting property at each inverter in both stand-alone and grid-connected modes, as well as during the transition between them. Contrary to the existing methods that utilize saturation blocks to limit the reference current value, the proposed controller limits the instantaneous value of the current even after extreme faults, i.e., short circuits in both grid-connected and stand-alone cases. Moreover, after incorporating the proposed controller dynamics into the system, the entire microgrid small-signal stability analysis is investigated. In order to validate the effectiveness of proposed controller, a microgrid, which includes three parallel three-phase inverters, is being tested via Matlab/Simulink software and extensive simulation results are provided.