A Unified Power Flow Controller With Nine-Arm Modular Multilevel Converter

This paper proposes an improved topology of unified power flow controller (UPFC) based on nine-arm modular multilevel converter which could realize the same function as conventional UPFC composed of two back-to-back modular multilevel converters (MMCs). The proposed improved UPFC topology based on nine-arm MMC (9AMMC-UPFC) could offer significant advantages, that the number of required submodules and arm inductors could be reduced by 25% and 50%, respectively over the traditional UPFC based on MMCs. Therefore, the system volume and cost could be drastically reduced. The structure and operation principle of proposed 9AMMC-UPFC are firstly illustrated in this paper. Next, phase-shifted carrier-based PWM technique including capacitor voltage balancing control is briefly described for the operation of proposed 9AMMC-UPFC. Then, in order to suppress the exclusive odd-order harmonics in circulating currents of proposed 9AMMC-UPFC, circulating current suppression control method is put forward. Finally, the performance of the proposed 9AMMC-UPFC are evaluated by the simulation and experiment results.