The basic operation principle of modular multilevel converter(MMC)and realization of the third harmonic injection modulation strategy are introduced in this paper. The requirement of MMC grounding mode is analyzed theoretically. The modulation strategy is only suitable for the DC grounding voltage-source-converter based high-voltage direct current(VSC-HVDC)converter. The effects of the modulation strategy on submodule voltage fluctuations and bridge current root mean square(RMS)and peak values are analyzed theoretically. The third and fourth harmonics will be brought into submodule voltage by modulation strategy

and the fundamental component will decrease so that the fluctuation of submodule voltage will decrease. It is favor for improving the voltage safety margin of the converter valves. The bridge current RMS and peak values will decrease

which is favor for improving the current safety margin of the converter valves. Finally

the simulation model based on RTDS is built and the validity of theoretical analyses is verified.