Optimal Sizing on a Mission Profile of Isolated NPC DC-DC Converters using 3.3 kV SiC MOSFETs for Power Electronic Traction Transformers

Abstract:
Multilevel converters, known as power electronic traction converters (PETT), are foreseen to replace bulky traditional on-board low frequency transformers (LFT) and two-level rectifiers in railway rolling stock. In addition to volume and weight reduction, a gain in the efficiency can be expected. This paper focuses on the isolated DC/DC conversion stage within the PETT, realised by a resonant converter, with 3-level NPC half-bridge primary and 2-level full-bridge secondary. Design is optimised both at nominal power and on a typical mission profile for a regional application. The comparison is carried out for 15 kV/16.7 Hz infrastructure and 2 MW PETTs using 3.3 kV SiC MOSFETs and nanocrystalline core transformers. The best MFT designs are achieved at 4 kHz when optimised on a mission profile and at 6 kHz when sized at rated power. A reduction of 13 % of the total energy loss on a typical journey can be obtained by optimising the design of the DC/DC converter on a mission profile rather than at nominal power.