A Study Of PFC Rectifier And Three-level Inverter On UPS System

UPS is widely used in many applications. This dissertation researches on PFC rectifier and three-level inverter of UPS system.PFC adopts the chip LT1248 with mean current controlling method. The working principle is noted. A novel double Boost topology is proposed and the designing method of power and control circuit is illustrated. The thesis induces the small signal mode of current and voltage loop. Based on that, a compensative network is designed in series. The compensated current loop increases the band width and attenuation in high frequency period. The noise of switch is restrained and the dynamic characteristic improves. The compensated voltage loop decreases the band width and increases the attenuation in middle frequency period. The second harmonic wave is restrained and the steady-state characteristic improves. Through simulating and experiment, the power factor reaches at 90% above.The topology and controlling method of three-level inverter is classified in this paper. The designing method of power and control circuit is introduced. The math mode of three-level single phase inverter is induced after comparing of varies of SPWM methods. The control scheme which uses PID instantaneous feedback control with pole assignment is explained. The parametersζ,ωn,n have different effects on system when their values differ. A conclusion comes out, which expresses the bigger values, the better performance below the systemic cut frequency. Then this PID feedback method's shortcoming is announced which is bad for the system steady-state precision. To solve this problem, the system adds another PI mean value loop outside. Furthermore, this thesis focuses on the realization of PID digital control. A comparing of location method and increment method shows the different traits and different applications. Arithmetic flows are made for both methods. Through simulating and experiment, it shows that the inverter has satisfied steady-state and dynamic characteristics.The dissertation illustrates the neutral point balance problem of three-phase inverter. The cause and harm of the neutral point fluctuating is analyzed and a strategy which uses the floating carrier to three-phase SPWM controlling is proposed. In addition, the PFC bus balance control and the invert magnetism compensation is used to control the neutral point balance.