Abstract: Modular multilevel converter（MMC） has become the most promising converter technology for high voltage direct current（HVDC） transmission systems. MMC sub-module（SM） topologies with dc faults ride-through capabilities are emerging which are suitable for overhead line（OHL） applications. The hybrid SM design of each converter arm can get the compromise of higher capability of handling dc faults and lower capital investments and losses. The initial hybrid SM numbers design method for supporting the dc-link voltage and riding-through dc faults, as well as the optimized hybrid SM redundancy configuration strategy for effectively increasing the reliability of MMC were proposed and calculated. In contrast with previously developed redundancy configuration for MMC with single SM topology, this approach solves the first-order differences of the three-dimensional surface to calculate the recommended redundant hybrid SM numbers which takes both the semiconductor device utilization rate and the converter reliability of MMC into consideration.