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Recently, the technology of high-voltage direct-current (HVDC) transmission based on modular multilevel converter (MMC) draws much concerns. Due to its special structure, MMC presents different fault performances compared with conventional voltage source converter (VSC). Considering the serious consequences brought out by the faults on the MMC converter terminal bus and DC side, it is necessary to analyze the fault characteristics deeply to design reasonable protection to improve the reliability of system and reduce the economic cost. In this paper, the performances principally under the single-phase to ground internal AC bus fault and DC line to ground fault with their corresponding protection requirements of MMC are investigated particularly based on the detailed electro-magnetic transient simulation in PSCAD/EMTDC. Simulation results verify fault mechanisms of MMC and show that the MMC-HVDC transmission system has better fault controllability than conventional VSC-HVDC.