A Weighted Average Feedforward Scheme for LCL-Type Grid-Connected Inverter with High LCL Resonance Frequency

To suppress the background harmonics in the point of common coupling (PCC) voltage on the injected current, feedforward schemes of PCC voltage have been widely used for LCL-type grid-connected inverters. Especially, the unit feedforward scheme is attractive due to its simplicity and utility. However, when the unit feedforward scheme is applied to single-loop current control, unstable open-loop poles may appear due to the variation of grid impedance, which will degrade the robustness of the system or even lead to system instability. Therefore, the robust region of LCL resonance frequency against grid impedance variation is limited to be lower than $f_{s}/3 (f_{s}$ is the sampling frequency). Nevertheless, in terms of the cost and volume of the filter, it is preferable to design the LCL resonance frequency higher. Aim at the trouble in the application of the unit feedforward scheme under LCL resonance frequency beyond $f_{s}/3$ , a weighted average feedforward scheme is proposed in this paper, in which PCC voltage and the filter capacitor voltage are fed forward complementarily. The proposed method can guarantee the robustness of the inverter without the constraint of the LCL resonance frequency. Simulations and experimental results validate the effectiveness of the proposed feedforward scheme.