Low-Cost and Compact Asymmetrical Unidirectional-Current Modular Multilevel Converters

Although the modular multilevel converters (MMCs) that contain submodules (SMs) with negative voltage capability have various advantages, they usually demand many more semiconductors than the conventional half-bridge SM-based MMC (HB-MMC). This study proposes a unidirectional-current clamp-double submodule (UC-CDSM) by combining two unidirectional-current full-bridge SMs (UC-FBSMs) using a shared switching device. The sharing design enables the UC-CDSM-based MMC (UC-CD-MMC) to have 25% fewer switching devices compared with the UC-FBSM-based MMC. The quantity of switching devices is rather similar to that in a conventional HB-MMC while the UC-CD-MMC still retains the advantages, such as low capacitor usage, dc fault clearing capability, and wide-range dc voltage adjustability. Moreover, a unidirectional-current hybrid MMC composed of UC-CDSMs and UC-FBSMs (UC-HYB-MMC) is presented to further enlarge the adjustable range of dc voltage. Detailed comparisons indicate that the UC-CD- and UC-HYB-MMCs can reduce the valve costs by 32% and 25%, respectively, and volumes by 39% and 34%, respectively, compared with HB-MMCs. Simulation and experimental results verify the steady-state and dc-fault clearance of the proposed topologies, and that the capacitor voltages in the UC-FBSMs and UC-CDSMs are well maintained and balanced in the UC-HYB-MMC.