Full SiC multilevel chopper for three-wire supply systems in DC electric railways

Currently, former DC supply systems reach their limits. In several European Railway networks, there are several sectors where it is not possible to operate locomotives at their nominal ratings or to increase traffic. Indeed, the line-voltage drops between substations leads to severe limitations. To improve the power capacity of the line, a three-wire supply system with voltage boosters based on Imbricated Cells Multilevel Chopper (ICMC) was proposed. At first, the principle of the three-wire supply system is recalled. Experimental results, performed on a test platform of the French railways, are then presented in the case of a double 1.5 kV supply system, with a three-level chopper based on 3.3 kV Si-IGBT modules. In order to increase system performance, the authors evaluate the interest of new SiC-MOSFET modules on this ICMC structure. An experimental test-bench is build with 1700 V/300 A SiC-MOSFET Half-Bridge Modules (SiC-MOSFET HBMs). The reactive components forming this converter, such as the flying-capacitors, are dimensioned and a design is selected in order to respond to constraints of high switching speed. A particular control is tested and is used to regulate all flying capacitors, while also operate at high switching frequencies (up to 50 kHz). The characteristics of this DC/DC converter are calculated, plotted, and compared to the same structure with 1700 V / 300 A Si-IGBT Modules.