Modular multilevel converter（MMC） is a key device in a flexible dc transmission project. In order to improve its transmission reliability and reduce long-term operating costs

the availability and maintenance policies of MMC are studied. Firstly

based on the Markov model

MMC is used as a repairable system to derive the MMC availability evaluation parameters. Secondly

for regular maintenance

a minimum cost model and a minimum downtime model based on age replacement policy and block replacement policy are established. Finally

the optimal maintenance cycle of MMC under different maintenance policies and the sensitivity of the optimal maintenance cycle with respect to parameters such as failure rate are obtained through case study. The results show that

as the maintenance cycle increases

both the cost and downtime decrease first and then increase. The optimal maintenance cycle can be obtained by a compromise between the cost of preventive maintenance and the loss of downtime. Moreover

choosing an appropriate maintenance cycle can significantly reduce operating costs and failure frequency. Study results of reliability and maintenance policies can provide a reference for the stable operation and regular maintenance of MMC.