The transformer-less modular multilevel converter (MMC) dc-dc converter can realize a direct dc-dc conversion without the intermediate ac-ac stage. However, the voltage conversion ratio of the existing transformer-less MMC dc-dc is limited. Otherwise, the ac current that circulating inside the MMC arms is too large to affect the efficiency of the converter. A transformer-less hybrid modular multilevel dc-dc converter with minimized circulating current is proposed in this paper. In the proposed topology, the upper and lower arms are composed of half bridge submodules (HBSMs) that are connected through delta configured arms composed by full bridge submodules (FBSMs). The ac circulating current can be minimized by controlling the delta connected FBSMs which can be operated as a STATCOM. Moreover, benefitting from adopting the FBSMs, the proposed topology not only has the capability of blocking the dc fault but also eliminates the large passive filters. Real-time simulation is carried out to validate the performance.

DC transformer; high-voltagedirect current (HVDC); modular multilevel converters (MMCs).