Inverter Control Based on Virtual Impedance Under Unbalanced Load

Abstract

Three-phase four-leg inverter combined with appropriate control method has a good performance when facing the unbalanced load. The three-phase four-leg inverter can generate three-phase balanced voltage under unbalanced load and improve the capacity of the inverter. However, as a result of positive sequence, negative sequence and zero sequence currents need to be controlled, the control system of the inverter is more complicated compared with the traditional three-leg inverter. In this paper, the distribution of the sequence currents are analyzed while the three-phase four-leg inverter running in parallel. To make the different sequence currents distributed according to the capacity of the inverters, the droop control and virtual impedance method are used in the positive and negative sequence currents, and the zero sequence current controlled by the virtual impedance method. Through the proposed method, the parallel inverters can output balanced voltage and the output power are distributed according the capacity of the inverters when they supply the unbalanced load. To verify the effectiveness of the proposed method, simulation study is done at the end of this paper.