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.
Medium‐voltage DC (MVDC) electric railway systems have several advantages over conventional DC and AC railway electrification systems. These advantages include higher capacity, possibility of connecting to power networks at lower voltage, removal of neutral sections, smaller line voltage drops, and longer distances between traction power substations. This paper reviews in depth the arrangements for MVDC railway electrification systems proposed in the technical literature and the topologies used for high‐power medium‐voltage AC‐DC converters. With reference to typical requirements of a MVDC railway electrification system, the pros and cons of the topologies are critically analysed. Moreover, this paper reviews the DC‐DC power converter topologies for on‐board power electronic traction transformers, required to interface the MVDC power supply with the traction motors. Finally, the review highlights the existing challenges of MVDC electric railway systems and the potential areas of future research.
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