As the key equipment of high voltage direct current(HVDC) flexible transmission system, the reliability of modular multilevel converter sub-module operation directly affects the safety and stability of the whole system. A sub-module bypass scheme is fully proposed. An active bypass scheme based on a permanent magnetic dual-drive bypass switch is designed. A passive bypass scheme based on breakover bidirectional thyristor is designed. Considering the economics of active bypass and the reliability of passive bypass, a hierarchical bypass scheme is designed. The test results show that fault sub-module can be reliable bypassed by command from the active bypass scheme. Meanwhile, in the case of the failure of active bypass scheme, the passive bypass scheme can reliably bypass the fault sub-module. The hierarchical bypass solves the problem that a single faulty sub-module causes the transfer system tripping. Design guidance for the implementation of HVDC-flexible valve is provided.