After the bipolar short circuit fault of the medium voltage DC bus occurs in the DC distribution system based on MMC-ISOP PET structure, the AC bus will form a three-phase short circuit with the fault point when the conventional half bridge type MMC sub-module is adopted, which will cause a large fault current to damage the power devices. When the input side of the later stage ISOP converter is direct series connection, the short circuit discharge of the input capacitor to ground cannot be avoided as well. In this paper, an MMC sub-module topology with DC fault self-clearance capability is proposed at first, and the blocking ability is guaranteed without increasing the number of devices and voltage withstand level of capacitors. An input side hybrid series connection type ISOP topology of DAB converter is proposed, which can not only effectively block the discharge of the input side capacitor after the fault, but also reduce the low voltage stress of the capacitor and the ripple of the inductance current. Moreover, the system recovery time after fault will be shortened. Then, the simulation models of DC distribution network under steady state and fault state are built by Simulink software, and the simulation results verify the validity of the topology structure and the accuracy of the theoretical analysis.