A fuzzy adaptive VSVPWM strategy for fast neutral point potential balance

To address the problems of DC-side neutral point potential imbalance of neutral point clamped (NPC) three-level grid-connected inverters and the poor neutral point potential regulation performance of conventional virtual space vector pulse width modulation (VSVPWM) under high modulation index, this paper proposes a novel VSVPWM strategy to realize fast neutral point potential balance under high modulation index. Based on the imbalance mechanism of neutral point potential in NPC inverters, neutral point current is verified to be effective in regulating the neutral point potential. To enable the neutral point current generated by the action of virtual medium vectors to adjust the neutral point potential timely, the virtual medium vector with adjustable control factors is reconstructed in a 60° coordinate system. Fuzzy adaptive rules are designed to dynamically regulate the control factors according to the deviation and change rate of neutral point potential, so as to realize the optimal reconstruction of virtual medium vectors and achieve fast neutral point potential balance under high modulation index. Meanwhile, when the control factor takes the boundary value, the number of sectors is reduced to further decrease the computational complexity. Simulation and experimental results show that the proposed modulation strategy effectively improves the neutral point potential control performance under high modulation index, possesses satisfactory dynamic performance, and significantly enhances the output power quality.