Generalized Decoupled Control and Capacitor Voltage Balancing for Current Scalable Modular Multilevel Converter

Modular multilevel converters (MMCs) that utilize parallel cells instead of cascaded cells have the potential to provide scalable high-current, medium-voltage solutions without requiring cell redesign to achieve higher current ratings. However, effective control of circulating currents and capacitor voltage imbalances is crucial. This article proposes a decoupled model and control strategy for MMCs with any type of parallel interleaved cells. The modeling relies on linear transformations to decouple the voltage-current and power-capacitor models, allowing the use of conventional control techniques to regulate the MMC. This transformation is applicable to any converter with an arbitrary number of cells. The balance of the floating capacitors is achieved by injecting a circulating current between the parallel cells, which could be dc or ac. An experimental setup of 3 kVA, comprising an MMC double-star bridge-cell (MMC-DSBC) with three arms in parallel, was effectively utilized to validate the proposed modeling and control strategy. This setup demonstrates the accurate balancing of floating capacitor voltages in which an ac circulating current was injected. These results expand the modularity of the MMC from voltage-scalable applications to current-scalable applications.