The overall performance of modular multilevel converter (MMC)are directly affected by the modulation method. And the nearest level modulation(NLM) has been widely used in high voltage and high power applications today, due to its lower switching losses and more convenience when implemented in MMC. In this paper,the voltage balancing method based on capacitor voltage sequencing is applied,and then a small offset is added to the approximate function during the trigger pulse forming process. The function-rounding process is adjusted so that the time of step changing staggered and the harmonic distortions of output wave can be improved dramatically. Furthermore,the output voltage harmonic level of the combination of different modulation deviation is analyzed. The inventor model of MMC with 10 sub modules is realized by matlab/simulink.Simulation results show a great agreement with the theoretical analysis.
