Neutral-Point Voltage Balancing in Three-Level Inverters Using an Optimized Virtual Space Vector PWM With Reduced Commutations

Neutral-point voltage unbalance seriously distorts the output waveform and may result in the shoot-through of switching devices in neutral-point-clamped three-level inverters. Compared with the nearest three vector pulsewidth modulation (PWM), the nearest three virtual space vector PWM ((NTVPWM)-P-2) can balance neutral-point voltage without the limitation of modulation index and power factor angle. However, the switching period of NTV2PWM is increased by one-third at the same switching loss, causing one-fourth lower switching frequency and degrading the acoustic noise performance in some motor drives. To overcome this drawback of NTV2PWM, an optimized NTV2PWM is proposed and verified in experiments. The optimized NTV2PWM reduces the number of switches in a switching period from 4 to 3, and it shortens the switching period by about one-fourth, transferring the switching noise to about one-third higher frequency. The proposed technique is suitable formotor drives where switching noise is expected to be low and the feedback of current as well as neutral-point potential are available.