Zero Sequence Voltage and Current Control in Four-Wire Grids-Fed by Grid-Forming Inverters

Low voltage three-phase four-wire AC distribution grids may experience high neutral current, mainly caused by asymmetrical distribution of single-phase loads in three phases. High neutral current will not only increase line losses but also result in neutral potential variations. For the LV AC distribution grid established by a grid-forming inverter (e.g., uninterruptible power supply and solid-state-transformer), it also suffers from the same neutral current issues. Therefore, this paper comparatively studies several neutral current control approaches and their impacts on grid voltage balance, which is required by grid code. Then, this paper proposes an optimal neutral current control approach, which can obtain maximum neutral current suppression with less impact on grid voltage balance. The correctness of the theoretical analysis is validated through both simulation and experimental results.