Analysis of Multi-Machine Grid-Connected Series Resonance Characteristics of Three-Phase Four-Leg Inverter Under Unbalanced Grid

When the grid voltage is unbalanced, the positive sequence, negative sequence, and zero sequence components coexist in the multi-machine grid-connected system of LCL three-phase four-leg inverter, resulting in more complex resonance mechanism. In this paper, firstly, the series harmonic resonance analysis method based on modal analysis and virtual branch method is used to determine the series resonance frequency of LCL three-phase four-leg inverter multi-machine grid-connected system under unbalanced grid. The influence of the number of inverters on the series resonance characteristics of each sequence network in the grid-connected system is analyzed. Secondly, the series resonance characteristics of each sequence network are discussed with the change of line impedance. Finally, a simulation model is built to verify the correctness of the conclusions.