Research On Digital Control Technology Of Three-phase On-line UPS

With the continuous development of society and technology,people have higher and higher requirements for power supply stability and power quality.It is difficult for a single power grid to meet the requirements of important equipment for power quality and stability.Uninterruptible Power Supply(UPS)is used more and more widely in many important power applications because it can guarantee the normal supply of power to the load when the power failure occurs.Compared with other types of UPS,the three-phase online UPS has the advantages of zero switching time,high output power quality and low pollution to the power grid.This paper mainly studies the control strategy of rectification and inverter of three-phase online UPS.Firstly,the three-phase PWM rectifier is modeled and analyzed.The indirect current control and hysteresis current control are introduced.The simulation results show that the hysteresis current control strategy has better output performance.Secondly,the three-phase voltage inverter is modeled and analyzed,and the double closed-loop decoupling control strategy is designed.Combined with the repeated control strategy,the output voltage dynamic and steady-state performance of the inverter are further improved,and the output voltage harmonics are reduced.This paper proposes a fast SVPWM algorithm,which is very simple and can realize inverter modulation quickly.In order to improve the power supply capacity and reliability of the UPS,it is very necessary to operate the UPS in parallel.For this reason,the phase-locked loop and the droop control strategy applied to the UPS parallel system are designed.The principle of synchronous reference frame phase-locked loop(SRF-PLL)and dual second-order generalized integrator loop(DSOGI-PLL)is expounded and analyzed.It is verified by simulation that DSOGI-PLL has better phase lock performance than SRF-PLL.The basic principle of traditional droop control is introduced,and a robust droop control is designed to improve the robustness of the droop control strategy.Finally,the overall circuit,working mode,key circuit and power device selection of the three-phase online UPS are designed,and the joint simulation of the UPS is carried out to verify that the designed UPS can maintain the power supply to the load in the case of power failure.The UPS performance meets the design requirements.In order to further verify the feasibility of the design control strategy,a three-phase online UPS experimental platform was designed and built.In the experiment,two UPSs were connected in parallel to test the dynamic and steady-state performance of the UPS parallel system with resistive load,resistive and inductive load and rectifying load.Experiments show that the control strategy of the paper can effectively achieve the power sharing between the UPS and reduce the circulation.The three-phase online UPS has excellent output performance and meets the design requirements.