Enhanced MMC With Mechanical Disconnectors for DC Fault Blocking and Reduced Power Losses

The conventional modular multilevel converter (MMC), which employs half-bridge submodules (HB-SMs), cannot clear dc fault currents. This limitation necessitates either the use of additional dc circuit breakers (DCCBs) or enhancements to the converter topology. Among the various solutions, hybrid DCCBs (HDCCBs) and hybrid MMCs (HMMCs) are two of the most widely recognized options. By incorporating ultrafast mechanical disconnectors into the main branches (MBs), HDCCBs can achieve low power losses. However, their widespread adoption is hindered by high costs, stemming from the hundreds of additional semiconductor power switches required in the current transfer branches (CTBs). Although HMMCs offer rapid dc fault blocking capabilities, they incur substantial additional power losses. This letter proposes a novel topology that integrates the current commutation principle of HDCCBs into MMCs. By directing the dc fault current into a single MMC arm via the MBs, and assigning that arm as the CTB, the proposed topology achieves both fast dc fault blocking and reduced power losses.