Abstract

The neutral-point voltage balancing problem is a key issue for three-level three-leg neutral-point clamped (NPC) inverter used in three-phase four-wire system. In this paper, a detailed discussion on this issue and a mathematical model for the neutral-point voltage fluctuation are given. Then a novel neutral-point voltage control strategy based on the disassembly of zero level is proposed. To balance the neutral-point voltage, a variable which can depict the disassembly margin of each phase, named neutral point control margin (NPCM) is defined. The duty ratio of zero level in the phase selected on the basis of NPCM is decomposed to positive and negative level equivalently in order to balance the neutral-point voltage while the average value of the output voltage is preserved unchanged. Furthermore, the relationship between the proposed control strategy and the existing method based on 3-D space vector modulation (3DSVM) in a-b-c coordinates is analyzed. It is showed that the proposed control strategy is superior in simplifying the algorithm and improving the control performance compared with the 3DSVM method. The proposed control strategy is verified by simulation and experiment.

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