Modular Multilevel Converter (MMC) is favorable for high-power grid-connected power electronic applications to achieve efficient operation. The required number of components significantly increases when higher modularity is introduced for the given voltage level, thus reducing the system reliability. This paper suggests a Mixed Redundancy Strategy (MRS) that combines the operational concepts using active and redundant spare sub-modules. It is shown that more than 50 % higher B10 = 10 lifetime is achievable as compared to reliability improvement using Fixed-Level Active (FL-ARS), Load-Sharing Active (LS-ARS), and Standby Redundancy Strategy (SRS) with the same number of redundant submodules. The trade-off between operational efficiency and investment cost is explored to define the boundary for selecting MRS over other redundancy strategies with varying dc-link voltages and average converter loading considering a 10-year payback period and equivalent B10 = 10 lifetime. The results are discussed for two case studies using load profiles adapted from available data from a medium voltage substation. The change in viability boundary for MRS is established with increasing B10 = 10 lifetime and its sensitivity to power electronic component cost as well as assumed failure rate.
