1.State Key Laboratory for Alternate Electrical Power System with Renewable Energy Sources，North China Electric Power University，Beijing102206，China;2.State Grid Smart Grid Research Institute，Beijing102200，China

Since the modular multilevel converter（MMC）based multi-terminal DC（MTDC）transmission system has the advantage of MMC technology combined with that of the voltage source converter（VSC）based MTDC transmission system（VSC-MTDC），it will have broad application prospects.The topological structure and working principle of MMC are presented.A multi-point DC voltage control strategy based on the DC voltage margin method is then proposed and the controller is designed.Furthermore，the impact of different DC faults of the system is analyzed and the corresponding control and protection strategies are given.This approach can realize automatic switching between active power and DC voltage control modes，thus improving system stability.The DC faults include single-line to ground fault，line to line fault and line break fault.Finally，the feasibility and effectiveness of the proposed control strategies and the designed controller are verified by simulation.