The Modular Multilevel Converter (MMC) has become the most promising topology for high-voltage and high-power applications. Sub-Module (SM) fault ride-through capability is one of the most important features of the MMC. To study the approach of controlling SM faults, the differences between hot-backup and cold-backup configuration of redundant SMs are analyzed. After that, an internal asymmetrical dynamic model for the MMC is analyzed, which shows that unexpected fundamental and 3rd harmonic components in the circulating current result in fluctuation of DC current. Based on this model and by integrating the conventional circulating current suppression method and the SM-fault tolerant approach, a non-ideal proportional-resonant (PR) controller is proposed and analyzed. This control scheme can suppress the fluctuation of the DC current of the MMC under SM faults without the knowledge of the number of faulty SMs and extra communication system. Its effectiveness is verified by both PSCADEMTDC simulations and scaled-down prototype experiments.