A novel neutral point voltage automatic balancing carrier-based modulation strategy of three-level NPC converter

A novel carrier-based modulation strategy of NPC three-level converter which achieves neutral point voltage balance under the whole modulation index and power factor is proposed in this paper. It divides the traditional single modulation wave of each phase in SPWM strategy into two modulation waves, which are compared respectively with the carrier to decide the output switch state of the converter. It analyzes the internal relations of the modulation waves between the new strategy and traditional SPWM strategy, and the 64 solutions of the modulation waves of the new strategy are deduced. On the basis of analysis and derivation, the new strategy is simple and easy to be implemented in digital system. Through the analysis of the DC voltage utilization ratio, the device switching losses and the output phase voltage THD characteristics, the new strategy has the disadvantage of larger switch losses and higher harmonic content. Finally, the analysis of this paper is verified by experiment.