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, including overvoltage protection, triggering underneath IGBT overvoltage protection and overvoltage tripping protection which are aimed at bond wired IGBT modules, and overvoltage protection aimed at press-pack IGBTs for average voltage of capacitor on bridges. 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. The system overvoltage criterion is proposed to avoid bypassing the sub-module in the case of system overvoltage.
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