An adaptive algorithm for grid-connected inverter to suppress current harmonics and instabilities due to grid impedance and distortion

For grid-connected inverters, the feedforward of the grid voltage at the point of common coupling (PCC) highly suppresses the grid current harmonics caused by the grid distortion; however, when the PCC grid is weak with non-negligible grid impedance, the feedforward of the PCC voltage arouses serious current harmonics or even instability. In order to maintain the stability in the weak grid case, the typical robust design usually had to compromise the dynamic performance or the harmonics rejection performance, while the typical adaptive control had to measure the grid impedance precisely through injecting extra harmonics into the system. Both solutions are not satisfactory enough. Therefore, this study proposes a novel adaptive control algorithm which has overcome the drawbacks of typical solutions. In the propose algorithm, the band-pass filters at harmonics frequencies are used to detect the variation of the grid impedance as well as to facilitate the adaptive PCC voltage feedforward. It is not necessary to inject any extra harmonics to estimate the grid impedance. In this paper, the realization and logic of the proposed control algorithm are briefly explained, and selected waveforms are provided to demonstrate the effectiveness of the proposal.