The modular multilever converter (MMC) based unified power flow controller (UPFC) is the newest flexible AC transmission system (FACTS) technology at present. The output voltage of the series MMC in UPFC can be arbitrarily regulated from 0 to a rated value

which is different from the MMC based high voltage direct current (HVDC)-light with the AC voltage of MMC near the rated value in operation. As a minor modulating ratio of MMC

there may be a higher harmonic level of AC voltage. With respect to the actual UPFC project in the Western Grid of Nanjing

to study the impact of UPFC on the harmonic of power system

and by proceeding from the working principles of UPFC

an analysis is made on the harmonic calculation method and distribution characteristic of MMC. The formulas for calculating the harmonic of power network with UPFC interconnected are given for use in evaluating the influence of UPFC at different voltage levels and modular ratios

on power network harmonic in various operation modes to provide a basis for UPFC project design and operation. Finally

the effectiveness of the harmonic analyzing and calculating methods proposed are verified by theoretical calculation and simulation results. This work is supported by State Grid Corporation of China (No. SGCC-2015-011).