Medium Voltage Dc Systems for Rapid Electric Vehicle Charging

Accelerated electrification of automotive transportation means increased power demand in distribution systems with EV charging stations demanding MW level power. This requires costly distribution transformer and switch-gear reinforcement, which can be costly and disruptive. This paper proposes a 54kV MVDC system to bypass the 33kV/11kV and 11kV/0.4kV AC-transformers. By aggregating all the rectification at the 33kV and using isolated DC/DC converters (with ZVS on all primary switches), a high-power density and high-efficiency EV charging station is realized. MVDC transmission has added benefits of reduced transmission losses, easy integration of PV/battery storage and soft-open-point functionality. In this paper, a 1 MW charging station is considered. Loss comparison is performed between the scenarios of (1) using the existing AC network with upgraded 50 Hz transformers meaning rectification and DC/DC conversion at 400V (2) using a 54 kV MVDC transmission with modular-multilevel-converters. Using clamped inductive switching circuits, experimental measurements of switching energies have been performed on 650 V and 1200 V SiC MOSFETs, IGBTs and Cascode JFETs at different temperatures. The measurements are used to parameterize simulations of Vienna rectifiers and DC/DC converters to quantify converter efficiency. The results show the importance of SiC-MOSFETs in the prospective MVDC systems for ultra-fast EV charging.