The modular multilevel converter（MMC） is one of the preferred topologies in flexible DC transmission. However

the traditional half-bridge submodule（HBSM） cannot block the DC fault current. Therefore

a clamp dual capacitor submodule（CDCSM） based on bidirectional switch is derived in this paper. The CDCSM contains two capacitors and can output three levels

and possesses the capability of blocking fault current by introducing capacitors into fault circuits. Compared with other submodules that can block DC fault current

CDCSM possesses certain superiorities in investment cost and operation loss. Simulation results based on Matlab/Simulink show that CDCSM-MMC can fast block the DC fault current

and the capacitor voltages of submodules are relatively balanced after the fault

which is beneficial to the rapid restart of MMC.