Compensation for the neutral-point potential variation in three-level space vector PWM

In performing the three-level space vector PWM, the neutral-point potential of the de-link capacitors has to be controlled at whatever cases. But as it is nearly impossible to control the neutral-point potential exactly to the half of the dc-link voltage at all times, the inverter would produce an erroneous output voltage by this voltage unbalance. So the voltage unbalance has to be compensated in doing PWM, when the voltage unbalance occurs whether it is small or large, to make the inverter output voltage follow the reference voltage exactly the same. In this paper, a new compensating method for the neutral-point potential variation in a three-level inverter space vector PWM(SVPWM) is presented. By using the proposed method, the output voltage of the inverter can be made same as the reference voltage and thus the current and torque ripple of the inverter driven motor can be greatly improved even if the voltage unbalance is quite large. The proposed method is verified experimentally with a 3-level IGBT inverter.