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

A nonisolated DC 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 the LCL filter are analyzed and optimized for a better performance of grid interface. Grid service functions are designed for the EV charger to provide grid-voltage/frequency compensations. High power (22 kW) and high efficiency ($>$99%) are achieved with low-leakage current ($<$ 20 mA). The experiments are implemented to verify the proposed EV charger system.