The operation mechanism of modular multilevel converter（MMC）is described.Different kinds of sub-module faults in MMC are analyzed，which may cause the oscillations of DC voltage and current and finally cause the outage of the converter.Different redundancy protection methods（RPMs）to deal with sub-module faults in cascaded H-bridge converter are compared，and then a new RPM suitable for sub-module faults in MMC is proposed.In this method，one or few redundant sub-modules are put in hot stand-by state in order to replace fault sub-module rapidly，while the others are in cold stand-by blocking state so as to protect the insulated gate bipolar translators（IGBTs）from current flowing in them.Time-domain simulation results show that the proposed RPM enables MMC based voltage source converter based high voltage direct current（VSC-HVDC）system to recover from sub-module faults without interruptions of power delivery.