Research On The Control Of NPC Three-level VSI DC-Link Capacitor Voltage Based On SVPWM

The three-level NPC voltage source inverter has the advantages of high voltage,high power and low harmonic.It is widely used in power generation,transmission and electricity use systems.However,the neutral point voltage fluctuation is the key factor affecting the performance of the three-level inverter,which has attracted more and more scholars' attention and research in recent years.In this paper,it is mainly to restrain the fluctuation of the midpoint voltage and the effective control in low modulation of three level inverters.According to the different influence of the space voltage vector sequence on the middle point current of the inverter,the modulation strategy of the inverter is studied.The specific research content is as follows.1.Based on the reliability of the DC-link capacitors and the neutral point voltage offset of the three-level inverter,the function of neutral point voltage about several common modulation strategies is analyzed and compared in this paper.They are SVPWM,SHEPWM,SVPWM and so on.2.The principle and basic implementation steps of three-level SVPWM method are described in detail.The neutral point voltage balancing ability of the traditional three-level SVPWM method and the virtual space voltage vector(VSVPWM)method are compared.The neutral point voltage balancing ability of the traditional SVPWM and the VSVPWM are compared and analyzed in different modulation depth.The study shows that the performance of the traditional SVPWM method has better harmonic characteristics.However,the VSVPWM method has a better ability to change the midpoint current.This paper is aimed at the issue that include the midpoint voltage balance and its validity under the low modulation depth.Based on the SVPWM method,the voltage vector is built accordingly in different small area,then a novel space vector modulation method is given.The method is flexible,accurate and available in wider modulation range.3.The experimental platform of the three-level NPC voltage source inverter is designed and built,finally the the feasibility and validity of the proposed method verified by experimental results.