Research On Control Technology Of Three-Phase Four-Wire Boost Three-Level Rectifier

With the rapid development of the social economy,the demand for power quality of electrical equipment is getting higher and higher,and the demand for uninterrupted power supply is becoming more and more extensive.The uninterruptible power supply has become an important equipment for various industrial power systems.As an important part of the UPS system,its performance not only affects the quality of the input grid,but also affects the inverter of the latter stage.Therefore,the research on the pre-stage rectifier of the UPS is of great significance.The three-phase four-wire Boost three-level rectifier has the advantages of high power density,low switch stress,easy to achieve power factor correction,etc.At the same time,the three-phase four-wire system can achieve three-phase decoupling and simplify the control strategy,so it is widely used.In this paper,the three-phase four-wire Boost three-level rectifier is studied to obtain high power factor and stable DC output voltage,and to promote the application of the rectifier in UPS.In this paper,the three-phase four-wire Boost three-level rectifier is firstly understood,and the topology is equivalent to three single-phase Boost three-level rectifier output parallel structure,and the detailed working principle of the rectifier topology is given.Then the topology is methmatically modeled in CCM and DCM working modes respectively,and the steady-state model and small-signal AC model in CCM and DCM modes are obtained,and lay the theoretical foundation for subsequent design.At the same time,considering the limitation of the use of single-L filter due to the three-level characteristics,the LCL filter is introduced at the input and the modeling and analyzing of the LCL filter is given.The control principle of realizing the power factor correction of the rectifier by current inner loop and voltage outer loop is analyzed in detail.The detailed controller parameters are designed based on system stability,steady state performance and dynamic performance of the system.In order to further reduce the input current THD,it is proposed to introduce the repetitive controller into the inner loop of the rectifier current.By comparing the advantages and the disadvantages of the two combinations of the repetitive controller and the PI regulator in series and parallel,it is finally determined that the repetitive controller and the PI regulator are connected in parallel to solve the disadvantages of the system dynamic performance caused by the repetitive controller.And then the input current waveforms and the dynamic perfowmance of the system before and after adding the repetitive controller are compared by Matlab/Simulink software simuliation.Finally,based on the digital control system,a 10 kW three-phase four-wire Boost three-level rectifier prototypr is designed and built.The exoerimental results show that the introduction of a repetitive controller in the current inner loop can further reduce the input current THD and inprove the input power factor.