Non-Isolated Battery AC Charging Using CHB Converters - Investigating Leakage Current and Short-Circuit Conditions

Abstract

The demand for efficient and compact electric vehicle battery chargers has sparked interest in non-isolated charging topologies that eliminate the need for a transformer. This thesis investigates the use of a 31-level grid-connected cascaded H-bridge converter for direct AC battery charging, a concept that integrates the converter into the battery itself and enables bidirectional power flow without galvanic isolation. A simulation model was developed to evaluate the system’s behavior with respect to leakage current, touch current, and short-circuit conditions under various grid scenarios. The simulations showed excessive touch current compared to electric vehicle safety standards, which was especially high when using zero-sequence injection. This highlights the importance of minimizing parasitic capacitance. The leakage and touch current were found to be dependent on grid voltages, converter voltages, and modulation scheme. An analytical model was developed to describe the leakage current, yielding results consistent with the simulations. An approximate maximum limit for the total parasitic capacitance of the converter was calculated to be 120 nF. Short-circuit simulations highlighted the importance of grid impedance and R/X-ratio in determining let-through energy and protection requirements.