Control strategy of negative sequence current for the grid-connection converter under the unbalanced grid condition

When a grid voltage is unbalanced, the unbalanced grid voltages act as an external disturbance for the grid-connected PWM inverter systems, resulting in an unbalanced grid current. Firstly, the mathematical model of the grid-connected converter, in view of unbalanced grid voltages, is proposed and analyzed in detail. In order to achieve the decomposition of positive and negative sequence voltage, this paper adopts the software phase-locked loop to decouple based on double synchronous reference frame. This method has good dynamic performance. In order to suppress negative sequence current on the AC side, the positive and negative sequence components of the grid current are controlled in the double synchronous reference frame respectively. Finally, Simulation results verify the correctness of the control strategy.