Research On Control Strategy To Improve Power Factor Of Three-Phase Buck Rectifier Under Light Load

Compared with diode rectifier and thyristor phase-controlled rectifier,three-phase PWM rectifier can effectively suppress the input current distortion.Three-phase Buck rectifier is widely used in communication and data center power supply,new energy vehicle charging system,aviation power supply,stationary synchronous compensator and other fields due to its advantages of wide output voltage range,high reliability,direct start-up without pre-charging circuit,suppression of output short-circuit current,and easy parallel operation.In order to improve the power factor of three-phase Buck rectifier under light load,the control strategy of three-phase Buck rectifier is studied.Firstly,this paper analyzes the circuit topology structure and working principle,on this basis,the 12-sector five-segment SVPWM modulation strategy of three-phase Buck rectifier is studied,including 12 sectors,the definition of basic current vector,calculation of action time of each vector in 12 sectors,space vector sequence and duty ratio signal generation.Established the mathematical models of three-phase Buck rectifier in three-phase static coordinate system,two-phase static coordinate system and two-phase rotating coordinate system,analyzed the static characteristics of three-phase Buck rectifier including the output voltage gain of rectifier,active power,reactive power,power factor.At the same time,this paper reveals three-phase Buck rectifier cannot achieve unit power factor in a light load because of the existence of input LC filter.The control strategy of three-phase Buck rectifier is studied emphatically.The influence of input filter capacitor on power factor of three-phase Buck rectifier is analyzed in detail.The analysis shows that the power factor of rectifier will drop significantly when the input frequency is high or the load is light.A double closed-loop indirect current control strategy based on input voltage differential feed-forward is studied,which can realize the compensation of the input filter capacitor current,effectively improve the power factor of rectifier under light load.Thus,the digital realization of the improved control strategy is given.Aiming at the problem of slow response of three-phase Buck rectifier in indirect current control of load transient,an algorithm is developed to compensate the output of controller quickly,which ensured that the rectifier output voltage fluctuation range is small and the adjustment speed is fast during load transients.The control strategy of three-phase Buck rectifier is verified by simulation.Finally,the voltage and current stress of the power device of three-phase Buck rectifier are calculated in detail,which provides a basis for the device selection.The input LC filter,DC side inductance and DC side capacitance are designed,the software implementation process of the system is introduced,and a 1000 W three-phase Buck rectifier experimental prototype is built.The effects of the traditional control strategy and the improved control strategy on the power factor of three-phase Buck rectifier are compared through experiments.It is verified that the improved control strategy in this paper can effectively improve the PF value of threephase Buck rectifier under light load.Thus,it is verified that the fast compensation algorithm can effectively improve the adjustment speed of three-phase Buck rectifier in load transient state.