In this article, a multi-input multioutput robust proportional–integral (PI) controller is designed for zero-level control of ac microgrids. The study system is a voltage-sourced converter connected to the rest of the microgrid through a distribution line and an ac output filter. The design procedure of the proposed high-gain PI controller is independent from the state matrix of the distributed energy resources. Severe local load changes are considered as the system unknown disturbances. It is proved that the closed-loop system behaves like a first-order system with stable internal dynamics. As the bandwidth of the suggested zero-level control can be systematically adjusted, it can properly operate in both high and medium frequencies without considerable fluctuations and overshoots at the output. It is also shown that circulating currents, which can cause overcurrent conditions in power electronic devices and damage dc-link capacitors, are limited without using any limiters. By considering some realistic scenarios and through time-domain simulations in MATLAB/Simulink and a supporting experimental test, it is shown that the proposed strategy can successfully attenuate adverse effects of unknown disturbances on the load voltage of islanded microgrids.
