Neutral point potential and unity power factor control of NPC boost converters

Neutral-point clamped boost PWM power converters (NPCC) are able to reduce harmonic currents, without requiring the devices to be operated at a high switching frequency, and are suitable for high voltage systems because of their circuit structure. Thus, they are expected to play a greater role in high power applications where device switching is limited below several hundreds hertz. The NPCCs, 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. Accordingly, reducing NPP variations is very important. This paper describes a controlling method for NPCCs. It is shown that the system can be modeled by 3-input 3-output 5th-order state equations. The controller is designed based on the optimal regulator theory in order to achieve the control of DC voltage, power factor (PF) and NPP, simultaneously. The validity or the proposed method is demonstrated by experimental results.