Optimal-regulator-based control of NPC boost rectifiers for unity power factor and reduced neutral-point-potential variations

Neutral-point-clamped pulsewidth modulation rectifiers (NPCRs) are suitable for high-voltage systems because of their circuit structure. The NPCRs, however, have a problem, in that the neutral point potential (NPP) varies when the current flows into or out of the neutral point. The variations cause voltage deviations in the input waveforms, as well as unbalanced voltage stress on the devices. This paper describes a controlling method for NPCRs based on a state-space model. There are three control objectives: (1) to keep the power factor at unity; (2) to keep the DC-link voltage at a reference value; and (3) to keep the neutral point potential at 0 V. The neutral point current is treated as one of the inputs. The controller is designed based on the optimal regulator theory in order to achieve the three control objectives simultaneously. The validity of the proposed method is demonstrated by experimental results.