Silicon carbide enabled medium voltage DC transmission systems for rapid electric vehicle charging in the UK

The expected proliferation of rapid EV chargers with more than 100 kW rating will place significant power demand on the UK distribution system. Due to the currently limited headroom in 11kV networks, reinforcement will be costly and disruptive. This paper proposes a medium voltage DC (MVDC) system that bypasses the 33kV/11kV and 11kV/400V AC transformers by transmitting 54 kV DC power directly to the EV charging stations. Additional benefits include the opportunity to reinforce integration of battery storage and photovoltaic sources as well as implement soft-open-points with an MVDC interconnection between asynchronous AC systems at lower voltages. The 33kV AC to 54 kV rectification in this system is proposed to be done by using a 29-level modular multilevel converter (MMC) implemented in 3.3 kV SiC MOSFETs. On the EV side, there will be a 54 kV to 800 V fully isolated DC/DC converter implemented with 3.3 kV SiC MOSFETs on the primary side and 1.2 kV SiC MOSFETs or Schottky diodes on the secondary side. This paper presents converter simulation results demonstrating improved performance in the MVDC system and shows this is only possible with SiC MOSFET technology as the losses using silicon IGBTs make the system less efficient compared to the existing system.