Reliability Analysis of Modular Multilevel Converters in MVDC Applications

Abstract

In modular multilevel converters (MMCs) designed for medium-voltage DC (MVDC) systems, since the required sub-modules (SMs) in one arm are fewer, the converters cannot afford the same level of redundancy as observed in high-voltage DC (HVDC) applications. In order to ensure high availability of converters in MVDC systems, reliability becomes a challenge and requires different design approaches. Considering the redundancy design, operating modes, and correlation across failures of SMs in one arm, this paper analyses and compares reliability in MMCs for MVDC systems with multiple SM topologies. Under different scenarios in MVDC systems, SM topologies with better performance of reliability have been identified in this research. It is demonstrated that, for a given output voltage in converters, three-level SMs generally provide higher reliability than two-level SMs. By implementing passive and load-sharing operation schemes in converters, the MMC reliability can be improved compared with the conventional active mode. In terms of the correlation between lifetime of SMs in the same arm, the smaller disturbance introduced by one faulty SM promotes the improvement of converter reliability. Also, the correlation between failures of SMs in one arm has greater effects on the reliability of an MMC with higher level of redundancy. The outcomes of this work contribute to the optimisation of converter design and operation with high reliability and low investment for an MMC in MVDC systems.