Control approach of three-phase grid connected PV inverters for voltage unbalance mitigation in low-voltage distribution grids

Low-voltage distribution grids (LV) experience unbalanced voltage condition due to the high penetration levels of rooftop mounted single-phase photovoltaic (PV), the existence of single-phase transformers, unbalanced cable impedances and different load levels on each phase. The voltage unbalance (VU) has a harmful impact on the loads and equipment connected to grid that leads to overheating and fast thermal ageing. There are several solutions proposed to mitigate the VU, that requires the installation of extra hardware components or incorporate complex type of controllers that need special tuning procedures and special arrangements. Therefore, in this study a new and simple control approach of three-phase grid connected PV inverter is proposed to mitigate the unbalanced voltage. The new control approach is able to fully mitigate the unbalanced voltage without the need to extra hardware installation or negative sequence voltage magnitude and phase angle measurements, provided that the ampacity of the converter is high enough. The new system is tested in a LV grid with high PV penetration from Germany. The results show that the VU factor did not exceed 0.8% at the feeder connection point.