A Three Phase Grid Interactive Bidirectional Off-board EV Charger with Weak and Strong Grid Durability

This paper presents a grid interactive off-board three phase EV charger capable for both distorted and healthy grid scenarios. The charging system incorporates three phase VSC, working in active rectification mode during forward power transfer whereas, in grid power fed mode it works as inverter mode. The controlled charging or discharging is made through full bridge (FB) LLC converter. The control for active power factor correction (APFC) stage is designed to not allow any reflection in grid angle with the presence of harmonics, DC offsets and frequency drift in grid voltage due to grid faults, sensing unit anomalies and distributed generation. The intermediate DC link reference is regulated in conjunction with the variation in battery terminal voltage to achieve wide EV charging voltage range and soft switching over complete charging profile with limited range of switching frequency. For this, a coordinated control is designed to cater wide range of EV charging with optimum efficiency point tracking over complete charging profile. The control for DC-DC stage uses both frequency modulation and phase shift over both H-bridges to regulate power in and out from the EV battery. For this, a 6.6 kW system is designed and simulated to validate the effectiveness of the claimed benefits of the designed charging system over both strong and weak grid scenarios.