Fault Tolerant Control for Medium Voltage Hybrid MMC With Cold Reserve Submodules

The hybrid modular multilevel converter (HMMC), which consists of half-and full-bridge sub modules (SMs), offers critical advantages in handling overmodulation conditions and blocking dc fault current compared to half-bridge MMC. The high number of SMs poses a threat to the proper operation of the HMMC after an SM failure. To ensure the reliability of HMMC, the use of redundant SMs is pivotal, particularly in medium-voltage applications where the number of SMs is comparatively lower than in high-voltage ones. The redundant SM can be used as a cold reserve SM, referring to the condition that the redundant SM is not being used in the normal condition and inserted after SM failure, which prevents an increase in MMC losses and components thermal stress. This work proposes a sorting method for the seamless and smooth insertion of the cold reserve SM in the event of SM failure, as a replacement for faulty SM. The main goal is to avoid the inrush current and high stress on the capacitor during the insertion of cold reserve SM and achieve a controlled charging process of the capacitor in the inserted SM. Finally, the feasibility and effectiveness of the suggested method are validated through comprehensive simulation studies.