Reliability modeling and redundancy design of hybrid MMC considering decoupled sub-module correlation

The conventional reliability analysis models of the modular multilevel converters (MMCs) rarely consider the correlations of all the sub-modules (SMs), which has negative impact on the system redundancy design. This paper proposes a reliability model for hybrid MMC composed of half-bridge SMs (HBSMs) and full-bridge SMs (FBSMs) considering the SM correlations. First the SM correlations included reliability models for both HBSM and FBSM are established. Then a Copula function-based coordinate transformation method is used to decouple the correlations of the SMs in hybrid MMC. Based on the Lagrange multiplier method, an equal reliability incremental principle (ERIP) based optimal SM redundancy design scheme is proposed to obtain a reasonable tradeoff between reliability and capital cost. The proposed approaches are validated to be feasible for practical HVDC system using Matlab.