Inverter-Based Local AC Bus Voltage Control Utilizing Two DOF Control

Small- and medium-sized distributed generation (DG) systems in a microgrid normally have inverters, and the inverter outputs are connected to the grid through an LCL filter. In this paper, we are interested in designing a DG inverter that behaves like a uninterruptible power supply (UPS) for its dedicated loads, while enabling power flow control with the microgrid. If the grid power is lost, then the DG system can be transferred smoothly to the island (stand-alone) operation. This islanding capability provides a higher reliability of the local ac bus than the conventional power system. In order to have such a UPS functionality, the inverter is desired to have a high control bandwidth, which is required for regulating the ac bus voltage at the presence of nonlinearities and unpredictable behaviors of the loads. As for a fast controller, a two-degree-of-freedom (DOF) controller is applied to the LC filter plant with the inverse dynamic model. The voltage command levels are selected considering the amount of power share with the grid. The effectiveness of the proposed control scheme is shown by experimental results.