Carrier based three-level PWM for improving flying capacitor balancing of Nested Neutral-Point-Clamped (NNPC) converter

Nested Neutral-Point-Clamped (NNPC) converter is a newly proposed 4-level converter. It has advantages similar to other multilevel converters but has disadvantages caused by the difficulties in balancing the voltages on its flying capacitors. This will limit its application in Variable Frequency Drive (VFD) system - an important application area for multilevel converters. In order to improve the flying capacitor voltage balancing, a carrier based three-level PWM is proposed. This method can guarantee enough redundant switching states for voltage balancing during the whole fundamental period without changing the voltage-second product. The output quality under low-frequency operation will be improved significantly due to the improvement in voltage balancing brought by this method. It also features simple and easy implementation compared to space vector based approach.