A Modular Multilevel Converter with a Zigzag Transformer for Bipolar MVDC Distribution Systems

This letter presents a modular multilevel converter (MMC) for bipolar medium-voltage direct-current (MVDC)) distribution systems. By employing a zigzag transformer as a grid interface transformer with a dedicated operation method, the operation of two dc poles of an MMC can be fully decoupled. Consequently, the MMC can provide full bipolar-operation capability. No additional component is required, and no penalty of increased power rating is imposed on the grid interface transformer. The scheme based on the proposed topology is competitive and cost-effective for emerging MVDC distribution systems. Feasibility of the proposed scheme is verified by both simulations and down-scale experiments.