Analysis of variation of neutral point potential in neutral-point-clamped voltage source PWM inverters

The authors present the analysis of the neutral-point potential variation of the neutral-point-clamped voltage source PWM (pulse-width-modulation) inverter (NPC-VSI) for AC motor drives and static VAr compensators (SVC). The potential variation is analyzed with the focus on the current flowing out of or into the neutral point of the DC link. The theoretical minimum capacity of the DC link capacitors is discussed for its application to both a vector-controlled induction motor system of 2.2 kW and a SVC system of 10 MVA, 6.6 kV, 60 Hz. It is shown that the proposed control scheme makes it possible to suppress the neutral-point potential variation within a few percent, so that the total capacity of the DC link capacitors in the NPC-VSI is almost the same as that in the conventional voltage source inverter