Device-level loss balancing control for modular multilevel converters

Abstract

Modular multilevel converter (MMC) is a promising topology in medium/high-voltage applications for its merits, e.g., scalability, low output current total harmonic distortion, and high efficiency. But the reliability of MMCs is questionable due to more semiconductor devices used than other topologies. Another reliability problem of MMC is the significant uneven loss distribution between the top and bottom semiconductor devices within one submodule (SM) owing to the dc component in the arm currents. The stress difference caused by the uneven loss distribution between the top and bottom semiconductor modules will lead to the lifetime difference of the semiconductor devices, which shortens the lifetime of the whole MMC system. To extend the lifetime of MMCs without the modification of system hardware, this article proposes a device-level loss balancing control method. In this method, the accurate device loss models are established to feedback real-Time device losses into the loss balancing controller. The controller (by regulating the SM capacitor voltages) adjusts the semiconductor conducting duty cycles to even the loss distribution without affecting the performance of the ac output and capacitor-voltage balancing. The effectiveness of the proposed control method is validated through experimental results.