MMC Stationary Operating Regimes in Case of Periodic Power Demand on the DC Side

The operation of modular multilevel converters (MMCs) with a periodic dc-side voltage is considered whose fundamental frequency component is linked to the grid frequency. Such a voltage component arises for instance by operating a pulsed power load, which is analyzed in this article as an example for this problem class. This application entails harmonic voltage components, which in conjunction with the ac current, lead to an imbalance of the available arm voltages. Although purely feedback-based balancing schemes can reduce the imbalance, they may fail to mitigate it sufficiently. Considering the balancing problem already during the stationary regime design, new options appear, which are to be investigated. Recent modeling methods are employed in order to analyze and mitigate this imbalance. In particular, the imbalance is already dealt with during the determination of suitable stationary operating regimes. Thanks to this, four regimes are derived, one of which is analogous to an already published regime for the concerned application. The regimes are compared on the basis of their individual advantages and their impact on the converter rating. Three of them avoid the singularity that is present in the already published regime. The findings are supported by numerical and experimental results.