The flexible DC grid is an effective means to achieve the collection

transmission and grid connection of large-scale renewable energy generation. In view of the power transmission interruption due to DC fault blocking scheme based on hybrid MMC

a kind of fault current blocking method based on hybrid MMC reverse voltage control and ultra-high speed mechanical switch is proposed in this paper. The method can quickly block the fault current and maintain the power transmission of non-fault lines. The control target of converter station is determined by way of analyzing composition of the fault current and the influence of key factor. The control strategy of actively absorbing fault current by using the hybrid MMC output reverse voltage is proposed on the basis of full bridge sub-module voltage regulation of degrees freedom

and its coordination scheme with ultra-high speed mechanical switch is designed so to achieved reliable isolation of fault. Finally

The four terminal flexible DC grid simulation model is set up based on RTLAB real-time digital simulation platform. The simulation results show that the method proposed in this paper can quickly block the DC fault current

shorten the power recovery time of the DC grid and improve safety and stability of the system.