Virtual Vector Modulation Method Under Unbalanced Midpoint Potential Of NPC Inverter

With the rapid development of industry and the continuous progress of power electronics technology,multilevel inverters are widely used in medium-high voltage high-power applications.Compared with the two-level inverter under the same working condition,the three-level inverter has become a research hotspot in the field of electric power at home and abroad due to its advantages of low voltage stress,high switching efficiency and good output waveform quality.Among them,the most widely used topology in three-level inverters is the Neutral Point Clamped(NPC).Due to its special topological structure,NPC three-level inverter has the inherent problem of unbalanced neutral point voltage,so it has some limitations in practical application.In this paper,a modulation method which can balance the neutral point voltage of NPC three-level inverter is studied.Firstly,the research background and development status of multilevel inverter at home and abroad are introduced by referring to relevant literatures and periodicals at home and abroad.Based on the topological structure,working principle and their advantages and disadvantages,the three level inverters with different structures are described.Secondly,this paper analyzes the causes of the unbalanced neutral point voltage and the related hazards.In order to solve this problem,the hardware method and modulation method are studied in detail.The working principle of Space Vector Pulse Width modulation(SVPWM)method and Virtual Space Vector Pulse Width modulation(VSVPWM)method are analyzed in detail.In the calculation process of solving the problem of unbalanced neutral point voltage,the traditional SVPWM method and VSVPWM method do not consider the effect of the unbalanced neutral point voltage on the basic vector itself,so the error caused by the change of the basic vector and the balance of algorithm will occur.In order to solve this problem,this paper based on VSVPWM method of 60 ° coordinate system as the NPC type three level inverter control algorithm,considered by the unbalanced neutral point voltage,amplitude and direction of the small and middle vector and neutral voltage balance of algorithm are introduced into the calculation error of unbalanced factor of neutral point voltage and neutral point voltage balance coefficient are introduced into the choice of vector and role time calculation,adjust the corresponding virtual vector function time,build a new virtual vector,divide the boundaries of the improved virtual space vector diagram of the sector.Compared with the traditional modulation method,the proposed method can realize the precise balance of the neutral point voltage under the full modulation system,effectively control the unbalanced neutral point voltage,and improve the output performance of NPC threelevel inverter.Finally,in the MATLAB/Simulink simulation environment,according to the improved method proposed the simulation model of NPC three-level inverter's control method of the balance of neutral point voltage is built,and the algorithm module in the simulation is introduced one by one.The output waveform quality and current harmonic distortion rate of the traditional method and the improved virtual vector modulation method under different modulation systems are compared in the case of unbalanced neutral point voltage in the balanced and unbalanced NPC three-level inverter,and the results verify the correctness of the theory proposed in this paper.