The internal AC bus of MMC-HVDC system is directly connected to the converter. Faults on the internal AC bus will lead to more serious harms to the system than those on the AC grid side. To enhance the security and reliability of the DC transmission, it is necessary to analyze the internal AC bus faults in more detail to find out reasonable protection strategies. Firstly the structure and principle of MMC are discussed. Then, the internal bus faults are reviewed, and a protection strategy focusing on single-phase ground fault are proposed. Finally simulation by RTDS is carried out. Results of the simulation show that the proposed protection strategy is valid and effective.