A study of voltage feedforward under weak grid conditions

The voltage feedforward (VF) is widely used in grid-tied inverter to improve the inverter's starting performance, disturbance rejection performance and suppress the effect of grid voltage harmonics. However, under weak grid conditions, VF has negative effect on system stability and disturbance-resistant performance. In this paper, taking the L-filtered grid-tied inverter as an object, the influence mechanism of the VF on the inverter in weak grid is clarified by the equivalent transformations of the control block diagram. It is pointed out that root cause of the VF's negative effect is the control delay in VF path. Therefore, the method that incorporates a lead-lag compensator in the VF path is proposed. The proposed method improves not only the system stability but also the ability of suppressing the low order current harmonics. Simulation and experiment result verify the effectiveness of the proposed method.