Circulating Current Analysis of Paralleled Grid-connected Inverters Based on the Multi-frequency Model

Paralleled grid-connected inverters are increasingly employed in distributed power generation systems (DPGS). As the penetration of DPGS goes high, the grid exhibits more and more like a weak grid, giving rise to potential resonance problems. Even if the whole injected grid current of the paralleled inverters system is stable, resonant circulating current may arise among the paralleled inverters. In this paper, circulating current resonance of paralleled inverters with asynchronous carriers is analyzed, which is closely related to the sideband effect of sinusoidal pulse-width modulation (SPWM). Thus, taking the sideband effect into consideration, a multi-frequency model is established for the grid-connected inverter, which characterizes the coupling between the perturbed frequency component and the sideband frequency component. So that the impact of asynchronous carriers on the sideband harmonic resonance of paralleled inverters can be analyzed, and the circulating current generating mechanism can be fully revealed. Simulation results of two paralleled inverters verify the correctness of the developed model and the theoretical analysis.