Harmonic Suppression and Stability Enhancement for Parallel Multiple Grid-Connected Inverters Based on Passive Inverter Output Impedance

In this paper, the inverter output impedance is passivized for solving the harmonic and the stability problems in the multiparallel inverters. The harmonics are easily aroused because of the disturbances, and the system stability is challenged by the grid impedance. Based on the simplified equivalent impedance model, the two problems are analyzed in the low-frequency (LF) band and the high-frequency (HF) band, respectively. Aiming at improving the LF performance, the weighted-proportional grid voltage feedforward and the harmonic quasi-resonant controller with phase compensation are proposed. The dynamic performance is enhanced with an additional current reference generation scheme. In order to improve the HF performance, a novel digital phase lead filter which brings the system back to a minimum-phase case is proposed. By the proposed control method, the high modulus of each inverter output impedance Z(o) is guaranteed, while the phase angles of Z(o) over the entire frequency band are avoided to be lower than -90 degrees. The experiments based on four-parallel inverters have been conducted to validate the effectiveness of the proposed control method.