Modular multilevel converter（MMC） is widely used in the field of flexible DC transmission

but the traditional half bridge sub-module（HBSM） does not have the ability of DC side fault self-clearing. In this paper

an improved capacitive sub-module topology with DC side fault self-clearing capability is proposed

and the basic structure and working principle of the new sub-module are analyzed. According to the system equivalent circuit after the sub-module is locked

the fault blocking mechanism of the new sub-module is described. In order to possess the characteristics of fault self-clearing and low loss

the design scheme of MMC bridge arm with the new sub-module and HBSM is given. From the two aspects of device voltage withstand and economy

a detailed comparison and analysis are carried out with the common sub-modules. Finally

a two-terminal MMC system is built in PSCAD/EMTDC to simulate and verify the characteristics of the proposed sub-module. The simulation results show that the fault current after DC side fault can be quickly cleared after the sub-module is locked

which verifies the effectiveness of the proposed sub-module DC side fault self-clearing.