MVDC Three-Wire Supply Systems for Electric Railways: Design and Test of a Full SiC Multilevel Chopper

Currently, in electric railways, the old dc supply systems are reaching their limits. In several European railway networks, due to the line-voltage drops between substations, there are several sectors where traffic can no longer increase and where locomotives cannot operate at their nominal ratings. To improve the power capacity, a medium voltage dc (MVdc) three-wire supply system with voltage boosters based on imbricated cell multilevel choppers (ICMC) was proposed. In order to increase efficiency, the authors evaluate the potential of new SiC-mosfets in this topology. An opposition method test bench using two ICMCs was built with 1.7-kV/300-A half-bridge modules. The design of the multilevel converters takes into account the constraints related to high switching frequency operation (up to 50 kHz). Thanks to the opposition method, losses in ICMCs can be determined by electrical and thermal measurements with good accuracy. Finally, solutions with SiC-mosfets and Si-insulated gate bipolar transistors are compared in terms of efficiency.