Voltage Imbalance Compensation for Unbalanced Local Load Under Weak Grid Though Three-Phase Four-Leg Inverter

Under weak grid, three-phase unbalanced local load will lead to serious voltage imbalance at the point of common coupling (PCC). This will result in the fault or damage of local load. Three-phase four-leg inverter can provide a zero-sequence current path for unbalanced load, thus can compensate for load imbalance. In this paper, three-phase four-leg inverter is used in the power condition system working under weak grid, which is controlled to balance the PCC voltage caused by unbalanced local loads and simultaneously to lower the battery discharge. Firstly, through the separate control of the positive and negative sequence components of PCC voltage, balanced PCC voltage is achieved under unbalanced load. Furthermore, zero-sequence current is controlled to not flow through the grid neutral line. Secondly, a droop control method is proposed to eliminate positive sequence active power outputting from the inverter, which can lower the discharge of the battery. Finally, simulation results verified the correctness and validity of the proposed control strategy.