Research On Bridge Rectifier Of Three-phase Uninterruptible Power Supply

The online Uninterruptible Power Supply(UPS)is widely used on the occasion where strict demand of power quality and reliability is requested,because of the good performance of online UPS that has no switch-time and provides high-quality output voltage.Generally,the online UPS utilizes the two-stage structure,including the preceding rectifier and the backward stage inverter.They are connected to the DC bus.The UPS might be used in many different conditions,where various loads would be connected to the output.In some cases,the output power of inverter is pulsating,the cosequent pulsating voltage is generated on the DC bus.Under this condition,the traditional control strategy of three-phase PWM retcifier is not appropriate because the distortion will be introduced in the input current of rectifier.Therefore,it is important to research the proper control strategy for the preceding stage PWM rectifier of three-phase UPS.This paper studies on the preceding stage PWM rectifier of three-phase UPS,which adopt the three-phase PWM bridge topology.The small signal model of rectifier is established in the rotating coordinate frame.Based on the model,the control parameters of inner current loop and outer voltage loop of rectifier is designed.The improved Phase-Locked Loop(PLL)control based on the SOGI-QSG is proposed,which aims at avoiding the current distortion due to that the conventional method provides distorted phase of grid voltage under the nonideal condition.This paper focuses on the DC bus voltage pulsation caused by pulsating power of inverter output and the corresponding solution.The three-phase loads of inverter are classified into two kinds,including the unbalanced load and the balanced load.As to load in each phase,it is further classified into linear load,nonlinear load and the combination of both.Based on the analysis of power pulsation with different loads,the frequency and amplitude of pulsating power in the inverter output is thus obtained.Then the frequency and amplitude of pulsating voltage on the DC bus can be calculated.In order to eliminate the distortion of the input current of rectifier which is introduced by the pulsating DC bus voltage,the relevant filter is added into voltage control loop which is different from the traditional control method,follwing the voltage sampling.The band trap is used to eliminate the second harmonic voltage of the DC bus,while the low pass filter is used to eliminate the sixth harmonic voltage of the DC bus.Finally,a 26 kW fully digital-controlled three-phase PWM rectifier prototype is designed and constructed.The experimental results indicate that the proposed control method is effective with different loads.