Abstract
Three-level neutral-point-clamped (NPC) converters/inverters have become a popular topology for not only medium-voltage (1k-33k) but also low-voltage level industrial application because of its reduced transistor block voltage and lower output current distortion. The neutral point voltage balancing control is one of the important issues of NPC converter to ensure system operation since the voltage drift in neutral point could lead to low-frequency current distortion and increases the risk of over-voltage in power electronic semiconductor devices. This paper provides a zero-sequence voltage (ZSV) injection to charge and discharge electric potential in the upper or the lower capacitor for unipolar switching. In addition, a novel neutral point potential control by bipolar switching is presented in this paper, which has better control dynamic under pure reactive power operation. The concept of these two neutral point potential control methods will be introduced, and then the laboratory test results are presented for verification.