Self-blocking modular multilevel converter (SB-MMC) is able to block dc fault current with relatively lower number of power semiconductors and lower valve power loss compared to other dc fault tolerant MMC topologies. Operating principle, design, reliability, and redundancy analysis of SB-MMC have been studied while the equivalent modeling and application of SB-MMC at overhead VSC-HVDC have not been fully investigated. Since operating principle of SB-MMC is different from half-bridge MMC, the existing equivalent modeling method of half-bridge MMC cannot be directly applied to modeling of SB-MMC. As dc fault at overhead VSC-HVDC are typically temporary, how to securely and quickly restart a VSC-HVDC after isolating dc fault also needs detailed studies. To solve the above two challenges, an equivalent model (EM) and a fast recovery control of overhead VSC-HVDC based on SB-MMC are proposed. Accuracy of the proposed model is verified by extensive simulations in PSCAD/EMTDC. The proposed fast recovery control further exploits the control capability of SB-MMC and enables fast and secure recovery of overhead VSC-HVDC based on SB-MMC. The proposed model and controllers also provide guidance for the modeling and application of other MMC topologies that are able to block dc fault current.
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Xiang et al. (2016) studied this question.