An Asymmetric Control Strategy of Overmodulated Hybrid Modular Multilevel Converters for Capacitor Voltage Balancing Effect Enhancement

Hybrid modular multilevel converters (HMMCs) are composed of both full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs). Utilizing the negatively inserted state of the FBSM, the HMMC is capable of riding through DC short-circuit faults and attaining overmodulated operation in a steady state. However, the overmodulated operation can impair the capacitor voltage balancing effect. To address this issue, this article first seeks to identify the underlying causes of the deterioration of the capacitor voltage balancing effect in HMMCs operating in overmodulation. The findings uncover that the deterioration can be attributed to the conflict between the asymmetric operations in regions where the number of inserted SMs is negative and the symmetric operations in regions where the number of inserted SMs is positive. To compensate for the asymmetric regions, the operations in symmetric regions are transformed into asymmetric operations. Therefore, an asymmetric control strategy based on the adaptive offset is proposed in order to enhance the capacitor voltage balancing effect. Simulation results in MATLAB/Simulink verify the effectiveness of the proposed control strategy.