An Improved Active Thermal Balancing Algorithm for Modular Multilevel Converter in HVDC Applications

The modular multilevel converter (MMC) is a typical topology for high power applications, such as HVDC transmission. Thermal loading imbalance of power semiconductor switches in different submodules (SMs) due to SM capacitance mismatch is a problem leading to a negative effect on system reliability. In this paper, the influence of inter-SM thermal imbalance on semiconductor lifetime is investigated for the MMC-HVDC system. Based on the analysis on thermal loading distribution and lifetime estimation results, an improved active thermal balancing algorithm is proposed, with a better performance on SM capacitor voltage balancing and a lighter computational burden than algorithms proposed previously. The validity of the proposed algorithm is verified by simulation results in MATLAB/Simulink with PLECS toolbox.