Design of unified power  flow controller topology based on modular multilevel converter

LIAN Xiao-rang
（College of Electrical Engineering， Zhejiang University， Hangzhou 310027， China）

Abstract： In order to decreasing the switching frequency of modular multilevel converter （MMC），a novel unified power-flow controller （UPFC） topology was presented based on MMC. A reduced switching-frequency phase-shifted pulse-width modulation（PWM） scheme including voltage balancing strategy was described for the operation of the proposed MMC-based UPFC，and the circulating current suppressing method was discussed through analysis of circulating current in MMC. The reported time-domain simulation results show that the proposed circulating current suppressing control strategy can greatly decrease harmonic component in arm current and voltage rippers of capacitor without increasing arm inductance. This topological structure has characteristics of high modularization，good reliability，conrenient maintenance and so on.
Key words： unified power-flow controller （UPFC）；modular multilevel converter （MMC）；circulating current suppressing；pulse-width modulation（PWM）