1.School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, China;2.Research Center for Photovoltaic System Engineering of Ministry of Education (Hefei University of Technology), Hefei 230009, China

The neutral point potential balance is always a hot topic in the research of the neutral point clamped (NPC) three-level inverter. The topology of the NPC inverter is introduced and the working principle and shortcomings of the traditional virtual space vector modulation strategy are analyzed in this paper. On this basis, to improve the balance effect of neutral point potential, an improved variable virtual-space-vector modulation algorithm is proposed. The virtual medium vector is redefined by introducing the variable virtual medium vector balance factor. Then the amount of charge generated by the virtual medium vector in a unit switching cycle is analyzed and the vector order is established accordingly, to improve its dynamic balance speed on the basis of maintaining the steady-state accuracy of the neutral point potential. In addition, in the process of establishing the vector sequence, the output state of the system is analyzed and the phase duty cycle method is adopted for optimization, to reduce the output state switching times of the system in the process of dynamic balance. Finally, the effectiveness of the proposed modulation method is verified by simulation and experiments.