Zero sequence voltage and current control in four-wire grids-fed by grid-forming inverters

Low voltage three-phase four-wire ac distribution grids experience a high neutral current, which is mainly caused by the asymmetrical distribution of single-phase loads in three-phase and the triple harmonic components of phase currents. High neutral current will not only increase the line losses but also results 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 the several neutral current control approaches and indicates 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 the maximum neutral current suppression and have less impact on the grid voltage balance. The correctness of the theoretical analysis is validated through both simulation and experimental results.