A theoretical study of neutral point voltage balancing problem in three-level neutral- point-clamped PWM VSI

This paper investigates the neutral point voltage balancing problem in three-level NPC PWM VSI theoretically. Current vector is introduced to model the disturbance of NP voltage due to middle voltage vectors and small voltage vectors. The ripple of NP voltage is consisting of two parts, one is low-frequency and the other is high frequency. The low frequency is three order of frequency of VSI output. The condition of modulation index and power factor angle that NP voltage can balance in one sample cycle is proposed. And in some condition, the NP voltage can balance in one sixth of VSI output cycle.