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Modular multilevel converter (MMC) is a new topology of voltage source converters. At present, the research on the flexible DC of this topology mainly focuses on the control strategy. The fault analysis and protection principle research is not complete enough, and it is necessary to analyze it in depth. The bipolar short circuit is the most serious fault type of the DC line. By analyzing the mechanism before and after the closing of the rectifier valve, the voltage and current transient voltages of the rectifier valve and the DC line are obtained, and a simulation model is built on the PSCAD to verify.

This paper designs the control strategies and start procedure of MMC-HVDC. The start procedure consists of three stages, uncontrollable charging stage, controllable charging stage and power-raising stage. The current circuit is analysed and the equivalent circuit is obtained in uncontrolled charging stage. The computational formula of current-limit resistor is obtained in the uncontrollable stage. Two different control strategies to further charge the capacitors to the rated voltage in controllable charging stage are designed. Strategy one makes use of DC voltage control and balancing strategy of capacitor voltage while Strategy two divides the capacitors of one arm into two groups. In power-raising stage, to avoid the impact of power step, slope control is used in power outer loop. Finally, a simulation model is established in the PSCAD/EMTDC. The simulation results show that the effectiveness and rationality of the designed start procedure and control strategies