A soft switching modular multilevel converter for railway traction systems

The AC/AC single-phase/single-phase Modular Multilevel Converter (MMC) presents very interesting features for the realization of a Medium Frequency Power Electronic Transformer for traction applications, in AC-fed or multisystem railway rolling stock, like locomotives, Electric Multiple Units (EMU), high-speed units, and so on. Aiming to maximize the output frequency of the converter, i.e. the operating frequency of the medium frequency transformer (m.f.t.) and/or to maximize the conversion efficiency, in this paper a proposal is presented, exploiting the opportunities offered by known soft-switching topologies. Simulation results fully validate this idea: in each MMC converter's submodule the H-bridge main semiconductor devices commutate in Zero-Voltage Switching (ZVS) conditions, with so much reduced losses that the switching frequency can be pushed-up toward unimaginable levels respect the classical hard-switching commutation, using usual silicon devices. More, the whole MMC converter operation remains substantially the same, because the soft-switching behaviour inside the single submodules does non disturb the operation of the overall MMC. The negative aspect is that the soft switching solution implies an appreciable increase in the overall amount of active and passive components, with obvious reflections on the reliability and costs issues. This article will discuss this theme with particular regard to the effects of simulation, loss assessment and power semiconductor sizing.