Mechanism and Suppression Strategy of Low-frequency Ground Circulating Current for Non-isolated Flexible Interconnection Device
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.
flexible interconnection devicetransformerlesssuppression of circulating currentcommon-mode voltagethree-phase four-leg converter
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维普
