Reliability Modeling and Redundant Design of MMC-HVDC Converters Considering Direct-Coupled Sub-Module Correlations

Recently, the half-bridge (HB) and full-bridge (FB) sub-modules (SMs) based hybrid modular multilevel converter (MMC) become a preferred topology for high voltage direct current (HVDC) system. In order to incorporate SM correlations into the optimal redundant configuration of both HBSMs and FBSMs, this paper proposes a reliability modeling approach of hybrid MMC considering direct-coupled SM correlations. Firstly, the detailed structure and reliability model of each SM are provided with component failure rates as input. Secondly, the Copula theory is introduced so that the correlations among the requisite and redundant SMs are considered. Thirdly, the sensitivity analysis of the hybrid MMC reliability with varying correlation coefficients, service years or time and numbers of redundant SMs are respectively discussed. Finally, the optimal redundant configuration of HBSM and FBSM segments considering the IGBT utilization rate and overall converter reliability is obtained. It is calculated by solving curvatures of the three-dimensional (3-D) reliability surfaces. Calculations have validated the feasibility of all the proposed approaches.