Sliding Mode Control Strategy of Grid-Connected Inverter under Unbalanced Grid Voltage

As the actual grid voltage is not always balanced, the control strategy of the photovoltaic inverter will affect the quality of the current injected into the grid. The double closed-loop control strategy of the traditional PI controller is not effective in restraining the second-order harmonic fluctuation of inverter output power and grid current distortion under unbalanced grid voltage. This paper adopts a new type of sliding mode control (SMC) and this method does not need separation of positive- and negative-sequence current and decoupling control. By selecting the proportional-integral sliding surface of current error, the control accuracy of grid-connected current can be improved. The chattering issue can be alleviated by properly selecting the approaching law function of the sliding surface when the system tends to be stable. The control strategy of suppressing the negative sequence current is adopted to reduce the distortion of the grid-connected current. The proposed controller can improve the dynamic performance and insensitivity to external disturbances of the grid-connected inverter. The simulation results verify the effectiveness of the proposed strategy.