Control of Neutral-Point Voltage in Three-Phase Four-Wire Three-Level NPC Inverter Based on the Disassembly of Zero Level

It is important to maintain the neutral-point (NP) voltage balanced for the three-phase four-wire three-level neutral-point clamped (NPC) inverter. In this paper, after detailed discussion, a mathematical model of the neutral-point voltage are derived. Then a novel control strategy is proposed based on the disassembly of zero level (O Level) to maintain the neutral-point potential. A variable named neutral-point control margin (NPCM) is defined to represent the disassembly margin of each phase. The zero level of one of the three phase is disassembled quantitatively after calculation. Furthermore, the neutral-point voltage can keep balanced while the average output voltage remains unchanged. The proposed control strategy is verified by simulation and experiments.