Abstract
Neutral point potential variation is an inherent problem of three-level inverter. When the output point of inverter is connected to the DC input neutral point, there is a neutral point current flowing into or out of the neutral point, so the neutral point potential is varied. The variation will produce a bad influence on the inverter output, and the excessive variation will cause an over-voltage applied to the switching devices and the devices will be destroyed. Therefore, the variation should be suppressed.
In this paper, authors have proposed a new kind of PWM method for three-level inverter based on the voltage space vector with the ability to suppress the neutral point potential variation. By this new method, two kinds of vector output patterns are defined for each sector of vector space. The vector output intervals are calculated firstly to meet the needs of generating the command vector. And then, the calculated vector output intervals are readjusted again to meet the needs of suppressing the neutral point potential variation. The main idea of this method is presented here and the variation suppressible conditions are analyzed. At the end of this paper, it has been confirmed by comparative experiments that the proposed method is an effective and practical one, and it is not necessary to add any extra-equipment for suppressing the variation. Moreover, there is no additional bad influence on the output current waveforms.
