The topological structure and implementation principle of modular multilevel converter (MMC) based power electronic transformer were introduced. The current path of the MMC with half bridge sub module in the presence of medium voltage DC bus double pole short circuit fault, and the reason why it can not achieve DC fault blocking were analyzed. In order to solve the above problems, the clamp double sub-module was selected to replace the half bridge sub module, and the fault current path and the fault blocking principle were analyzed. However, after the DC fault blocked, the overvoltage of the clamp double sub-module may occur. For this reason, the delay isolation strategy of isolated stage for power electronic transformer was put forward, and the fault current path before isolation and after MMC blocked was analyzed. Finally, the blocking function of clamp double sub-module and isolation level delay blocking strategy were simulated and verified by PSCAD simulation platform.