Medium voltage railway dc-dc converter design and performance

A more durable solution is to upgrade current rights-of-way with higher-voltage infrastructure, boosting railway transmission capacity without major new land acquisition. However, the transition to higher electrical ratings introduces new challenges e.g. requirement for customised design of power electronic converters. These converters are vital for efficiently meeting higher energy demands and to ensure seamless integration into the existing grid framework.

Accordingly, this document presents design steps of a medium-voltage DC–DC converter intended for railway application, with a purpose of increasing the railway power transfer capacity. The paper motivates the need for medium voltage direct current (MVDC)-based solutions and reviews prior art on DC–DC converter topologies, identifying modular multilevel converter (MMC)–based isolated architecture as a suitable solution for high-power, high-voltage railway applications. Key design considerations are discussed, including number of phases, intermediate AC link frequency, cell capacitor sizing, and suitable modulation schemes. Analytical approach for capacitor sizing and semiconductor loss estimation is provided to support design tradeoffs.

The paper further details the control architecture of the MMC based DC–DC converter, covering start-up, pre-charging, steady-state operation, and emergency shutdown sequences.

Hierarchical control strategies are applied to ensure stable AC link voltage regulation on the medium voltage (MV) side and fast power/current tracking on the low voltage (LV) side.

Extensive simulation results demonstrate robust dynamic performance under step changes in voltage and power references, overload conditions, and bidirectional power flow, while maintaining balanced cell capacitor voltages. The results indicate that the proposed converter can meet stringent dynamic and operational requirements of railway traction systems, offering a scalable and reliable solution for future MVDC railway electrification.