Robust Control Design for Ride-Through/Trip of Transformerless Onboard Bidirectional EV Charger With Variable-Frequency Critical-Soft-Switching

A transformerless electric vehicle (EV) onboard charger is designed for robust grid fault ride-through/trip capabilities and the attenuation of leakage current (
$< $
20 mA) with high efficiency. A zero sequence voltage control method is developed with a modified grid-connected
$LCL$
inverter to stabilize the zero-sequence voltage and bypass the leakage current. The topological modification is configured by connecting the common points of three-phase output capacitors and the positive/negative dc bus terminals. The control strategies include 1) ac grid side:
$dq$
sequence grid side current control,
$q$
sequence reactive power control, and zero-sequence grid voltage/switch side inductor current control; and 2) dc battery side: constant current and constant voltage control. An optimal control parameter design procedure is developed based on active damping method to improve the dynamic performance and attenuate the
$LCL$
filter resonance for a better grid disturbance response. The developed onboard charger is capable of dealing with abnormal grid voltage/frequency conditions to properly ride through and trip based on the standard requirements of IEEE Std. 1547. Also, the onboard charger can be applied to the integrated drivetrain. An ac current mode variable-frequency critical-soft-switching technique is developed for the onboard charger to achieve high efficiency (
$>$
99%). The performance under abnormal grid conditions are tested experimentally to verify the ride-through and trip capabilities of the developed onboard charger in compliance with IEEE Std. 1547. Also, the experiments have validated the low leakage current, stable zero sequence voltage, and full range of variable-frequency soft-switching for high efficiency.