Reliability comparison of two types of converters for fractional frequency transmission system

AC-AC converter is a piece of key equipment in the fractional frequency transmission system and both back-to-back MMC and M
³
C have great potential, but to date there are few works focusing on their reliability. A comparison of the reliability of these two types of converter is carried out in this paper, considering the SM redundancy. Firstly, the characteristics of two converters are discussed. Then the reliability model of SM and arm of two converters are presented. Based on it, the reliability models of the whole converters are proposed. The back-to-back MMC and M
³
C are described as the two-state model and three-state model respectively. Employing these models, the comparison of the reliability of the two types of converter and the analysis of the effect of SM redundancy are carried out in the case study.