Controlled Insertion of Cold Reserve Submodule in Hybrid Modular Multilevel Converter Considering Inrush Current Mitigation

Hybrid modular multilevel converter (H-MMC), which combines half-bridge (HB) and full-bridge (FB) submodules (SMs), offers significant advantages in handling overmodulation conditions and blocking dc fault currents. In H-MMC, redundant SMs are exploited to enhance the fault-tolerant capability. Cold reserve SMs are considered an appropriate option for fault-tolerant operation as they remain bypassed during normal conditions, avoiding additional power losses compared to hot reserve. However, since they are inactive during normal operation, it is essential to properly manage the charging of their capacitors. Inserting a cold reserve SM during MMC operation can cause a high inrush current in the charging phase, potentially damaging the SM. This work presents an approach that facilitates continuous and controlled insertion of cold reserve SMs, avoiding the associated inrush current and is applicable to both HB and FB SMs. The proposed cold reserve charging method charges SM through a parabolic function by exploiting the capacitor voltage balancing control, achieving the goal of controlled insertion without the need for precharging hardware circuits or additional control systems. Furthermore, the performance of the proposed method is compared with other alternatives. The feasibility and effectiveness of the proposed method for using cold reserve SMs in H-MMCs have been validated through simulation and experimental studies.