A cascaded high frequency AC link system for large-scale PV-assisted EV fast charging stations

Abstract

The development of high-power electric vehicle (EV) fast charging stations (EVFCSs) that are directly tied to the medium-voltage (MV) grid is a promising solution to shorten the charging time for EVs. The cascaded-high-frequency-link (CHFL) system-based charging station provides an isolated power electronic interface between the low voltage (LV) DC bus (inside the station) and the three-phase MV AC power network. The CHFL system uses a high/medium frequency transformer to provide isolation and high stepping-up ratio. The main disadvantage of this system is the large number of active switches. In this paper, a new design for the CHFL system based on cascaded half-bridge direct matrix converters is proposed. The proposed design reduce the active switch count by more than 40% as compared with topologies recently proposed in the literature. A simplified multilevel hysteresis current controller (MLHCC) for the system is also presented. The proposed architecture for EVFCS is assisted by a photovoltaic (PV) system to decrease the dependency on the power grid. A bi-directional power flow controller is adopted to inject the excess in the generated PV power to the grid, and withdraw power from the grid when the PV power is less than the demand of the station. The results are validated through a number of simulations for a 5MW system.