Topological Overview on Solid-state Transformer Traction Technology in High-speed Trains

The modern trend towards high-speed trains (HST) with distributed propulsion systems, demands high efficiency and high-power density traction systems. Line frequency transformers (LFTs) in railway traction systems are heavy and bulky, quite often necessitating power density to be compromised to achieve maximum efficiency of typically 90-92%. The advancements in power converter topologies, power switching devices and magnetic materials makes it possible to substitute massive LFTs with a new technology called solid-state transformers (SST) (also known as power electronic transformers (PET) or medium frequency transformers (MFT)) traction technology. This technology enables high power density systems with comparatively lower noise emissions which provide essential functionality without compromising efficiency. However, there are still major challenges to overcome associated with power converter connection on the high-voltage (HV) side, architecture modification and the compactness of the transformer design. This paper reviews the existing architectures and also introduces the new research possibilities especially in the power conversion stages, and the power switching devices. Finally, the design guide lines for high-power converters are presented.