非隔离型低压柔性互联装置的低频地线环流产生机理及抑制策略

Flexible interconnection devices are key equipment for optimizing low-voltage distribution network flow and efficiently integrating new energy sources. It is necessary to remove isolation transformers to improve efficiency and power density. However, in actual operation, they face a serious problem of grounding the circulating current, which deteriorates the device's power quality and operational stability. To address this issue, a mathematical model was established to analyze ground circulating current in non-isolated flexible interconnection devices, considering direct grounding and inverter modulation algorithm effects in low-voltage distribution networks. The model revealed the mechanisms behind the injection of common-mode voltage by inverters and the generation of fundamental and third harmonic ground loop currents due to differences in grid operating conditions. On this basis, a low-frequency loop current closed-loop suppression strategy was proposed using a proportional-multiple-mode resonance controller. An improved carrier modulation algorithm was then used to inject additional common-mode voltage signals into each bridge arm on the inverter side to maintain the balance between both sides, thus eliminating low-frequency ground circulating current. The semi-physical simulation results of the controller show that, with the proposed control strategy, low-frequency ground circulating current in non-isolated flexible interconnection devices is effectively suppressed under different grid conditions, significantly improving the device power quality and operational reliability.