Distributed balancing control for modular multilevel series/parallel converter with capability of sensorless operation
Modular multilevel converters (MMC) have many advantages owing to their power-stage modularity. However, this modularity does not extend directly to the control. Particularly, the balancing control usually requires central coordination and increases in complexity with the system size. This paper presents a balancing control method for a more general modular multilevel converter with series and parallel connectivity that 1) is fully distributed and 2) can operate without voltage sensors. The key idea is a phase-shifted carrier modulation scheme to distribute the series and parallel interconnections to all switching sites. As such, the modules are always balanced through switched-capacitor-Type charge equilibration without voltage sensors, for which an analytical proof is given. Beyond this, we explore the redundancies in the carrier order to further optimize the balancing performance and efficiency. Finally, we extend the control scheme with an optional central coordinator, which offers controllability over individual module discharging rates. Nevertheless, the central coordinator only requires moderate computational power and communication speed, thus allowing decentralized implementation as well.
