Four-Degree-of-Freedom Hybrid Control Strategy for the Neutral-Point Voltage of Three-Level NPC Multiphase Inverter

The three-level neutral-point-clamped (NPC) multiphase inverter, which combines three-level converter technology with multiphase motor technology, is the main direction to improve the system capacity at present. To control the neutral-point (NP) voltage, this article analyzes the degrees of freedom (DoF) of NP voltage control of a three-level NPC multiphase inverter, which includes zero-sequence voltage, the number of zero-level disassembly phases, zero-level disassembly phase, and the extent of zero-level disassembled. Meanwhile, the influence of four DoF on the NP current is deduced, and a 4-DoF hybrid control strategy is presented. The 4-DoF hybrid control strategy can maximize the control ability of the NP voltage by injecting the zero-sequence voltage and disassembling the zero level of each phase. Therefore, the 4-DoF hybrid control strategy can eliminate the deviation of NP voltage without significantly increasing the system switching frequency and improve the dynamic balance ability of NP voltage. The proposed 4-DoF hybrid control strategy is verified by experiment.