A control strategy for microgrid inverters based on adaptive three-order sliding mode and optimized droop controls

Robust control and seamless formation are the two crucial problems that affect smart microgrids. This paper proposes a new solution for microgrid inverters in terms of circuit topology and control structure. The combined three-phase four-wire inverter, which is composed of three single-phase full-bridge circuits, is adopted in this study. The control structure is based on the inner adaptive three-order sliding-mode closed-loop, the immediate virtual output-impedance loop, and the outer power control loop. Three significant contributions are obtained: (1) the microgrid inverters effectively reject both voltage and load disturbances with the adaptive sliding-mode controllers regardless of whether the inverters are operating in the grid-connected mode, islanding mode, or transition from the grid-connected mode to the islanding mode; (2) the virtual output impedance loop is applied to make a resistive equivalent output impedance of the inverters and to meet the requirements of the inverter parallel operation in the islanding mode; (3) the proposed droop method reduces the line inductive impedance effects and improves the power sharing accuracy by optimizing the droop coefficients. The theoretical analysis and test results validate the effectiveness of the proposed control scheme.