Different IGBT packaging methods and failure mechanisms of the modular multilevel converter (MMC) are described. The working principle of sub-modules before and after the fault and the closing of the bypass switch is analyzed. The working principle of overvoltage protection for sub-modules in different IGBT packaging modes is introduced. Overvoltage protection strategy of bond wired IGBT modules has the risk of converter tripping due to abnormal voltage sampling. To this end, the optimized design of overvoltage protection strategy for sub-modules is presented. The optimized strategy is implemented in sub-module control board, which is easy to be implemented with less program resources and effective to avoid the risk of misjudgment and malfunction of the high level control equipment. The system overvoltage criterion is proposed to avoid bypassing the sub-module in the case of system overvoltage. The effectiveness and feasibility of the optimization strategy proposed in this paper are verified by simulation, and the proposed optimization strategy has been applied in a practical project.
