A Robust Grid-Voltage Feedforward Scheme to Improve Adaptability of Grid-Connected Inverter to Weak Grid Condition

Abstract

The feedforward schemes of the voltage at point of common coupling (PCC) have been widely used in grid-connected inverters to reject the current harmonics caused by the grid voltage distortion. However, in weak grid, the PCC-voltage feedforward tends to destabilize the grid-connected inverters due to the effect of time delay. In this article, this stability issue is explicitly elaborated by the impedance-based stability criterion and then addressed with an improved feedforward scheme, which uses the grid voltage instead of the PCC voltage as the feedforward variable. Considering that it is hard to sense the real grid voltage directly, a method to extract the grid voltage from the sensed PCC voltage is put forward to implement the proposed feedforward scheme. By carefully arranging the sampling instants according to the duty cycle, a dual sampling mode is adopted to ensure an accurate extraction of the grid voltage. Finally, simulations and experiments are performed on a 6-kW single-phase grid-connected inverter, which confirm that the proposed grid-voltage feedforward achieves superior harmonic rejection ability and strong stability under weak grid condition.