Analysis of the Neutral Point Potential Variation of Neutral-Point-Clamped Voltage Source PWM Inverters.

A neutral-point-clamped voltage source inverter (NPC-VSI), which is one of double series connected inverters, is able to output 5 level step-shaped line to line voltage without output transformers or reactors, thus reducing harmonic currents. The NPC-VSI is suitable to high voltage applications, because the required voltage rating of the switching devices used in the NPC-VSI is only 1/2 of the DC link voltage. However, the DC link voltage may be applied to the switching devices in the worst case if the potential of neutral point the DC link varies because of the flow-in or flow-out of the neutral point current.In this paper, the authors discuss the variation of the neutral point potential theoretically, and propose a suppression method of the variation. This is based on the control of a zero-sequence voltage that is a voltage difference between the center of the DC link and the neutral point of a load. The analysis is done for the vector control drive system of an induction machine without or with the proposed suppression method. Consequently, it is shown that the variation of the potential can be completely suppressed by the method except for a special operating condition. Moreover, the relation between the potential variation and the capacity of the DC capacitor is clarified to lead to the desing of the DC capacitor.