A novel submodule topology of MMC for blocking DC-fault currents in HVDC transmission systems

This paper proposes a novel topology of submodule (SM) for the modular multilevel converters (MMC) applying to the high-voltage DC (HVDC) transmission systems, where the DC-fault current due to the freewheeling effect of the diode rectifier is blocked during a short-circuit fault in the DC cables. An additional insulated-gate bipolar translator (IGBT) with a low voltage rating is required to be connected in series at the output terminal of a conventional half-bridge SM, where the anti-parallel diode of the additional IGBT is used to block the current from the grid to the DC-link side. Moreover, the additional switching loss of the embedded switch is negligible since the switch is turned on all the time. Therefore, compared with the MMC based on the full-bridge (FB) SM or the hybrid topology of half-bridge (HB) and FBSM, the proposed topology not only provides the DC-fault current blocking capability but also has a lower cost and power loss. The validity of the proposed scheme is verified by the simulation results of 300-MW HVDC transmission system.