Control Design of a 99% Efficiency Transformerless EV Charger Providing Standardized Grid Services

A non-isolated fast charger is proposed for electric vehicle (EV) battery system. The leakage current can be reduced by the novel charging system without bulky transformer. A zero sequence voltage control method is developed to stabilize the common mode voltage thus reduce the leakage current. The proposed fast charger includes two energy conversion stages: DC/DC converter for battery side control; DC/AC converter for grid interface and common mode voltage control. The parameters of the system with LCL filter are analyzed and optimized for a better performance of grid interface. Grid service functions are designed for the fast charger to provide grid voltage/frequency compensations. High power (22kW) and high efficiency 99% are achieved with low leakage current 20mA. The experiments are implemented to verify the proposed fast charger system.