Reliability Analyses for Hybrid Modular Multilevel Converters Considering the Device-Level Uneven Power Losses

The hybrid modular multilevel converter based on full-bridge and half-bridge submodule is a promising candidate for medium- and high-voltage applications. But different from conventional two-level converter, the power loss distribution inside submodules is uneven which will threaten the reliability. In this article, a reliability assessment model considering the device-level uneven power losses is proposed. Then influence of power loss distribution and redundancy schemes are investigated through a case study. The results illustrate that more unbalanced power loss distribution will result in reduced reliability. And improved modulation strategy can reduce the loss imbalance inside submodules. It is also found that passive redundancy scheme works better than active redundancy scheme in improving reliability as the failure rates of bypassed submodules are much lower than operating submodules. And passive scheme with 20% redundancy rate of full-bridge submodules is a good choice for redundancy design which both meets 20-year B10 lifetime requirement and saves cost.