Modular multilevel converter（MMC） is a novel topology of voltage source converter（VSC）. Because of the topological differences between MMC and VSC， the fault mechanisms of the two DC systems have many discrepancies; as such it is necessary to have detailed study of the faults characteristics in order to design the corresponding protection schemes. The MMC topology and working principles are introduced. Comparing the fault characteristics of DC transmission system based on MMC with VSC， the impact of different DC faults of MMC-HVDC is analyzed. Furthermore， based on the variations of the electrical parameters in the AC-DC system and the discharge mechanism of submodules， the basic design requirements for protection with the corresponding control and protection strategies are proposed. The simulation platform of DC faults of MMC-HVDC is built in the electromagnetic transient simulation software PSCAD/EMTDC. Simulations including single-line to ground fault， line break fault and line-to-line fault are carried out to validate the analysis on the fault mechanisms and the effectiveness of the proposed control and protection strategies.