At present

the research on the operating characteristics of modular multilevel converter（MMC） is mainly focused on the mechanism of the converter circulating current and its suppression

the analysis of con-verter’s output voltage and the harmonic characteristics of the current. There is not much study of the influence of the converter power module capacitance on the converter modulation index. Based on the basic principle of the converter

the influence of the power submodule capacitance on the converter modulation index is analyzed.The analysis shows that the n-th harmonic current in the arms of the converter will produce n-1-th

n-th

and n + 1-th current harmonics on the power submodule capacitance

and then produce n-1-th

n-th

and n+1-th voltage fluctuation on the power submodule capacitance. The output voltage of the converter will include n-2-th

n-1-th

n-th

n+ 1-th and n+2-th voltage harmonics by the coupling function of the switching function. The DC current component

the fundamental current component and the secondary circulating current in the convert-er arms will generate the fundamental voltage component in the output voltage of the converter through the influ-ence of the power module capacitance. A MMC-HVDC simulation of the Luxi back-to-back DC asynchronous in-terconnection project between Yunnan Power Grid and China Southern Power Grid is created based on PSCAD/EMTDC. The results of the simulation experiments demonstrate the correctness of the derivation and analysis.