Voltage Unbalancing Reduction in a Stand-Alone AC-DC Hybrid Microgrid Based on Floating Compensation Reference

A hybrid AC-DC microgrid consists of an AC and a DC subgrid which are connected to each other through an interlinking converter (IC). The main function of an IC under islanded conditions is to transfer power between the two subgrids. In this paper, a scheme is presented to reduce the voltage unbalance factor in a hybrid AC-DC microgrid by using the free capacity of the IC. The free capacity of this converter is determined based on the current passing through each leg, and the amount of voltage unbalance compensation on the AC side of the microgrid is then obtained. The reference current of voltage unbalance compensation is calculated by using the positive, negative, and zero sequence components of the voltage of IC terminals. The total reference current is obtained by adding the reference current of voltage unbalance compensation and the current calculated for power transfer. Furthermore, a proportional-resonant (PR) controller is used in the control system of the four-leg inverter. Therefore, the reference current is properly tracked by the power stage of the inverter. Simulation results verify the accuracy of the proposed scheme under different conditions.