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Improved Virtual Space Vector Modulation for Three-Level Neutral-Point-Clamped Converter With Feedback of Neutral-Point Voltage

Neutral-point (NP) voltage imbalance is an inherent problem of three-level neutral-point clamped (NPC) converter. To solve this problem, an improved virtual space vector pulse width modulation (VSVPWM) is proposed in this paper. A new repartitioning of the space vector sectors is proposed for three-level NPC converter by considering the amplitude variations of small vectors under NP voltage imbalance. The factor k of the NP voltage imbalance is introduced. The small vector favorable to the NP voltage balance is selected based on the factor  k and phase current direction to achieve precise control and balancing of NP voltage. The proposed method can effectively maintain NP voltage balance and eliminate the low frequency oscillation of the NP voltage over the whole modulation index range. The improved VSVPWM also has advantages of good dynamic performance. The method has been tested and verified by simulations and experiments.