Abstract:In the research field of multilevel converters, three-level NPC (neutral-point-clamped) converters, which unfortunately may cause the deviation of the neutral-point voltage of DC-link capacitors, are widely discussed. Theoretically, virtual space vector modulation (VSVM) could guarantee the balance control of the neutral-point voltage. However, there still exist some uncontrollable space vector regions. Based on VSVM, this paper proposes a varied virtual space vector modulation (VVSVM) method for three-level NPC converters. Under complete modulation conditions, this method can control the balance of the neutral-point voltage of DC-link capacitors by adjusting the duty cycle of small vectors and regulating the current generated by virtual medium vectors. Compared with commonly used VSVM methods and mixed modulation strategies, this method is simpler and more practical. The effectiveness and validity of this method are verified by simulations and experiments.

Neutral-point (NP)-clamped three-level converter is a widely used topology in the research fields of the multilevel converters, which suffers from the NP voltage drift as its main technical drawback. In theory, it is based on the fact that the virtual space vector modulation approach could completely control the balance of neutral-point potential, so this paper introduces a improved control strategy of neutral-point potential balancing in three-level converter, which combines the space vector modulation approach based on virtual vectors with the algorithm of modifying small vectors pairs distribution time accurately. Relative to the traditional proposed algorithm of setting voltage tolerance range to modify small vectors pairs distribution time,the performance of algorithm of capacitor charge balancing is better and its accuracy of control is more precise. The feasibility of the proposed control approach has been verified via experiment results.