A Neutral Point Voltage Balancing Method for Multi-Level GTO Inverters

The balancing problem of the DC-link capacitor voltage is inherent in the NPC inverter. It is extremely important to keep the upper and lower DC-link capacitor voltages balanced to guarantee the three level operation of the inverter. Generally, the balancing of the DC-link voltage degrades at very low operating frequencies of the inverter. Most proposed methods of neutral point balancing techniques result in an increase of the switching losses of the inverter. To solve this problem a new energy saving PWM method is proposed. This results in a significant reduction of the fluctuations of the neutral point voltage of NPC inverters. Furthermore, the proposed method incorporates techniques to avoid the inherent minimum ON-OFF pulse width limitation in a GTO, without increasing the switching losses of the devices. Theoretical and practical aspects as well as the experimental results are presented in this paper