PWM Technique for NPC Inverters by Means of Decoupled Control of Voltage Vector Components.

Abstract

A new control method for Neutral-Point-Clamped (NPC) inverters based on closed-loop regulation of the line-to-line output voltages is proposed. The method uses independent hysteresis comparator controllers to regulate the direct and quadrature axis components of the three phase output voltages. The closed-loop control allows a high performance over the whole range of operation, even when low speed devices such as the GTO are used. A neutral-point potential control is described, which is capable of stabilizing the variations within fixed limits during steady and transient states. Further, a new vector selection technique which used four vectors to generate the sinusoidal output is investigated. The vector selection allows an almost decoupled control of the vector components. The principle of the method is discussed and the vector selection technique is presented. The effectiveness in the output-voltage-waveform generation and the balance of the DC-link capacitor voltages are verified by simulation and experiment.