Neutral-point-clamped inverter with improved voltage waveform and control range

A hysteresis controller-based PWM method for neutral point clamped (NPC) inverters is proposed. The closed-loop system uses the two-axis components of the transformed line-to-line voltage vectors as the control variables. In its linear control range, the PWM method includes two operating modes, namely, low and high references. At high references' mode, the output voltage is generated by switching among four vectors instead of three to improve the waveform generation. The proposed system has a linear control range 15.5% wider than conventional phase control methods. Furthermore, the control range can be easily expanded beyond the higher linear controllable range up to the square waveforms without suddenly increasing the waveform distortion. Therefore, the fundamental component of the output voltage can be smoothly and continuously controlled from zero up to square waveforms. The control circuit includes a hysteresis controller that is capable of stabilizing the neutral point potential variations within fixed limits during steady and transient states. The control method also offers high performance and fast response. Computer simulations and experimental results on a laboratory-tested motor drive are given to validate the proposed control scheme.