Start-up control and DC fault ride-through strategies of a hybrid MMC-HVDC system suitable for overhead line transmission

In this paper, a hybrid MMC-HVDC system which is composed of half-bridge submodules and full-bridge submodules is investigated to enable its capability during startup and severe dc fault conditions. Firstly, a novel two-stage startup control sequence is proposed considering the topological differences compared to the traditional half-bridge MMC-HVDC system. Then the ride-through strategies of the hybrid system under severe dc-side faults including single pole to ground fault and permanent pole to pole short circuit fault are proposed. Using the proposed schemes, under single pole fault, the dc power, dc current and the dc-link voltage are reduced to guarantee the maximum power transfer and protect the semiconductor devices and dc transmission line from over currents. Also, under pole to pole fault, the hybrid system can cut off the dc short circuit current by the dual converters of both sides and is ready for power recovery after fault cleared. The effectiveness of the proposed strategies are validated by a two-terminal reduced-size 21-level hybrid MMC-HVDC model on PSCAD/EMTDC.