Discontinuous Modulation Strategy for Voltage and Temperature Balancing of MMCs

Modular multilevel converters are becoming attractive for many high- and medium-voltage (MV) applications due to the serial connection of submodules (SMs). Since these converters contain many power devices, however, it is important to ensure a highly reliable operation of the converter for uninterrupted operation with short maintenance time. Particularly, technical challenges such as the unbalance of SM capacitor voltage and thermal stress on power semiconductors must be addressed because they have a significant impact on system reliability. Despite these reasons, few studies have been conducted to balance SM voltages and thermal stress together for MV MMCs. Therefore, this paper proposes a novel discontinuous modulation strategy that balances both the thermal stress and capacitor voltage in SMs. Indeed, the uniqueness of the proposed approach is that two independent clamping angles are controlled to achieve multi-objective balancing. The effectiveness of the proposed strategy is validated with simulation and experimental results on a five-level prototype.