Distributed Control for Modular Multilevel Converters Operated in Switching-cycle Balancing Mode

Modular multilevel converters (MMC) with conventional control are subjected to large capacitor voltage ripples, especially at low-line-frequencies. It is proved that with appropriate arm current shaping in the timescale of switching period, referred as the switching-cycle control (SCC), such line-frequency dependence can be eliminated and MMCs are enabled to work even in dc-dc mode. Yet the SCC demands precise arm-current shaping in each switching cycle, posing great challenges on digital implementation. In this paper, SCC features are carefully investigated and based on control functions and timing constraints, SCC is separated into multiple control layers. A distributed control architecture and accordingly a custom-build controller system are designed, granting the system with enhanced flexibility and scalability. Detailed timing chart is presented to visualize the temporal cooperation among distributed controller units. Digital delays in the distributed system are identified and minimized to optimize the control performance. The proposed distributed digital implementation is verified in dc-dc operations with a custom-build SiC-based MMC prototype under 6 kV dc-link voltage.