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A Novel Series-Connected Asymmetric Hybrid Modular Multilevel Converter for HVDC Tapping Application

In this paper, a three-phase series-connected asymmetric hybrid modular multilevel converter (SCAH-MMC) tap is proposed. The proposed SCAH-MMC reduces the requirement of switches by connecting phases in series to withstand the dc-bus voltage and possesses the dc fault ride-through capability by partially employing full bridge sub-modules. The system structure and operation principle of SCAH-MMC are presented in detail, which reveals the immunity of the SCAH-MMC from dc faults. In order to solve the third harmonic problem of SCAH-MMC, the interactions of harmonic components in arms are investigated. Based on that, two feed-forward third harmonic suppression methods are proposed. Furthermore, a control strategy for the SCAH-MMC is introduced to maintain dc voltage balancing in each phase under unbalanced ac fault conditions. The effectiveness of third harmonic suppression methods and the proposed control strategies of SCAH-MMC are validated by simulations using PSCAD/EMTDC. The proposed SCAH-MMC is compared with other MMC topologies in terms of cost and control to show the merits of it.