Circulating Current Analysis and Suppression for Module Grid-Connected Inverters Under Unbalanced Conditions

Parallel photovoltaic inverters system can increase the power rating and efficiency. However, the zero-sequence circulating current (ZSCC) will be generated under unbalanced conditions. Therefore, this paper proposes a ZSCC control scheme for the M-paralleled (M ≥ 3) inverters under unbalanced grid voltage and unbalanced filter parameters. Based on the analysis with the proposed generalized model of parallel inverters, it is observed that the ZSCC of the ith inverter is influenced by the output currents, filter parameters, duty cycles and ZSCCs of the ith and first inverters. To eliminate these adverse effects on the ZSCC control loop, a decoupled controller consisting of modified input type PI-quasi-resonant and the feedforward controller is proposed. The modified input type PI-quasi-resonant controller mitigates the effects of the output currents, filter parameters, and ZSCCs, meanwhile, the feed-forward controller is employed to eliminate the difference of duty cycles. Compared with the conventional PI control method, the ZSCC is effectively suppressed with the proposed scheme, thus improving the quality of the currents. Moreover, the reliability of the paralleled system is improved because such scheme only needs the information of the ith and the first inverters. Simulation and experimental results obtained from three-paralleled inverters validate the effectiveness of theoretical analysis and the proposed method.