An MMC-based topology using DHB power channels for load balancing in 50 Hz railway applications

This paper proposes a modular multilevel converter (MMC) topology, designed particularly for load balancing in 50 Hz railway applications. The asymmetrical character of the traction load requires higher device rating for a Double-star or Delta-connected MMC compared to a symmetrical static compensator. This is due to the symmetrizing components to rebalance the capacitor voltages in the submodules. In this paper a reduced chain-link MMC topology, consisting of a two phase-leg converter, is proposed in order to avoid overrating the semiconductors in the main current path. In addition, the main circuit is equipped with dc-dc dual half-bridge (DHB) converters that connect the submodule capacitors of the two legs. The DHBs are operating as “Power Channels,” enabling energy exchange to restore the power imbalance among the capacitors. Simulation results show that the proposed converter is capable of achieving load balancing under any operating conditions, while the balance of the capacitor voltage is ensured.