An improved voltage support strategy of grid-connected inverter under unbalanced grid condition

In order to ensure three-phase grid-connected inverter output voltage symmetry under unbalanced grid voltage, an improved control strategy is proposed in this paper. The proposed control strategy includes power, voltage and current three loops. The complex coefficient-filter (CCF) is used to obtain the positive and negative sequence of unbalance output voltage and current during the grid faults. By calculating output power of the inverter, an improved P+-f and Q+-U droop control strategy is designed to adjust droop curve real-timely. In order to improve the stability of parallel three-phase inverter system, the virtual impedance is further applied. The simulation and experiments results of a 1kVA three-phase grid-connected inverter verify the feasibility of the control strategy.