Modified Sorting Algorithm for Fault-Tolerant Operation of Hybrid MMC With Hot Reserve Submodules

The hybrid modular multilevel converter (HMMC) is formed of a combination of half- and full-bridge submodules. HMMC offers advantages in working under overmodulation conditions and effectively blocking the dc fault current with a reduced number of semiconductor switches compared to FB-based MMC, which makes it a fitting option for high and medium-voltage applications. While the modular nature of the MMC is considered a key strength, a high number of power switches raises a potential reliability concern in the event of submodule (SM) failure. Hence, the implementation of fault-tolerant strategies is critical to ensure the uninterrupted operation of the HMMC. This study aims to develop a fault-tolerant technique through hot reserve redundant SMs by using a modified sorting algorithm. The sorting algorithm is adopted to ensure the correct operation of HMMC during fault occurrence. Also, this work introduces a new control on the insertion index to operate in overmodulation operating mode with reduced dc-link voltage. Finally, the feasibility and availability of the modified sorting method for using hot reserve SMs in HMMC are verified through the EMT simulations in MATLAB\Simulink environment.