Investigating the best topology for Traction Power Substations (TPSs) in a Medium Voltage DC (MVDC) railway electrification system

Medium or high voltage DC railway electrification systems have been proposed since 1989. Such systems benefit from lowest conduction losses and voltage drops typical of DC systems and can efficiently integrate renewable energy sources in the railway supply system. However, no industrial application has been developed for the limitations connected to DC switchgears and voltage transformation on the trains. The first limitation can be addressed with a suitable power converter topology for medium voltage DC (MVDC) traction power substations (TPSs) with a controlled output current. In this paper, three different AC-DC converter topologies, i.e., bi-directional thyristor converter, cascaded two-level voltage source converter (VSC), and modular multilevel converter with full-bridge submodules (MMC-FB), are analytically compared in terms of their efficiencies and device requirements for application to MVDC TPSs.