AN IMPROVED VOLTAGE VECTOR CONTROL METHOD FOR NEUTRAL-POINT-CLAMPED INVERTERS

A new PWM method for Neutral-Point-Clamped (NPC) inverters is introduced. The method is based on closed-loop control of the line-to-line voltage vectors, It uses independent hysteresis comparator controllers to regulate the direct and quadrature aids components of the three phase output voltages, The controllers select the appropriate output voltage-vector through an EPROM table. 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, 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.