Characterization and Implementation of Resonant Isolated DC/DC Converters for Future MVdc Railway Electrification Systems

Today, in electric railways, the old dc supply systems are reaching their limits. To improve their efficiency and increase railroad traffic, a new dc electrification has recently been proposed at 9 kV. It is now necessary to prepare the migration of infrastructure and rolling stock, using power electronic transformers (PETs), for adaptation to this voltage level. For this application, high efficiency and reduced volume are essential. This article clearly demonstrates that it is now possible to achieve a compact, high-power, isolated dc-dc converters using 3.3-kV SiC-MOSFET power modules with high efficiency. After a preliminary study based on simulations, this article focuses on the characterization and implementation of elementary isolated dc-dc converters. The proposed topology is a series-resonant converter rated for a nominal power of 300 kW at 1.8 kV. First, laboratory testing using an “opposition method” is used to evaluate the elementary converters up to their nominal power using both electrical and thermal measurements to accurately determine losses and efficiency. At the nominal output power, an efficiency of 98.93% is obtained. This is quite remarkable for an isolated dc-dc converter operating under 1.8 kV with a switching frequency of 15 kHz. Finally, two elementary isolated dc/dc converters are associated with input series/output parallel (ISOP) configuration in order to achieve a 3-kV/1.5-kV PET with a nominal power of 600 kW as a prelude to the final 9-/1.5-kV power conversion.