Enabling DC fault blocking capability of hybrid modular multilevel converter HVDC using asymmetrical full-bridge submodule

This research designs a hybrid modular multilevel converter using an asymmetrical full-bridge (AFB) submodule. In the proposed Hybrid-MMC system, the AFB aims to protect the converter from DC fault with minimal steady-state loss. The AFB submodule consists of 4 IGBTs, 4 diodes, and 2 capacitors. A central capacitor generates the submodule voltage and works during normal operation. The other capacitor, named a supplemental capacitor is normally switched off but switched in and thus generates a reverse voltage with the central capacitor when a DC fault occurs. The central capacitor should be sufficiently large for voltage balancing requirement but the supplemental capacitor does not have this requirement. When the DC fault is detected, all IGBTs are turned off and the AFB submodules generate the reverse voltage against the AC line to line peak voltage. Because the AFB submodules can generate higher reverse voltage than the existing full-bridge submodule, the Hybrid MMC with the AFB submodules blocks the DC fault faster. It can be designed to reduce the number of IGBT (switch) conducting and minimize the converter loss. This paper validates the performance of the proposed scheme with simulation and experiment studies.