Three-phase Four-leg DVR And Its Fault Crossing Technology In Distribution Network

Electric Power is the basic industry related to national economy and people's livelihood,power supply and security are related to national security strategies and the overall economic and social development.With the continuous development of modern society,electrical energy has become one of the most widely used energy sources in production and life.However,as a large number of high-tech electrical equipment and non-linear loads based on power electronics technology are put into use in power systems,they have a serious impact on the power quality of the power grid and reduce the quality of power supply.In additio n,the scale of the power grid is increasing,and the short-circuit level of the system is continuously improving,fault overcurrent has a serious impact on the safe and efficient operation of power users.China's "13th Five-Year Plan for Electric Power Development(2016-2020)" proposes to build a "clean,low-carbon,safe,and efficient" modern electric energy system;Therefore,power quality control and fault over-current suppression are problems that electric power researchers need to solve now.This paper presents a three-phase four-leg dynamic voltage restorer fault traversing control strategy,it can not only improve the power quality,but also suppress the fault overcurrent impact,o vercoming the problem that conventional power quality control devices c annot achieve overcurrent protection in the event of a fault.At the same time,the excitation inrush current control of the load transformer is realized.In this paper,the topology and working principle of the three-phase four-leg dynamic voltage restorer are introduced in detail,and the mathematical model of the three-phase four-leg dynamic voltage restorer is established,based on this,the control strategy of dynamic voltage restorer is analyzed,including parallel side PWM rectifier control strategy and series side PWM inverter control strategy.At the same time,a brief analysis of the system over-current fault mechanism and its impact was carried out,and a detection and judgment method for system over-current generation and elimination was proposed.Through in-depth analysis of the stability of the series inverter control system,a design method for adaptive virtual im pedance parameters was obtained,combined with the advantages of virtual impedance control,a dynamic voltage restorer overcurrent protection strategy was proposed.When the grid voltage dips,the load transformer exposed to the deformation voltage will produce magnetic flux deviation and flux linkage offset.When the dynamic voltage restorer(DVR)compensates the voltage,the load transformer flux may magnetically saturate and generate a magnetizing inrush current.Based on this,this chapter analyzes in detail the excitation inrush current generation and suppression mechanism of the transformer,establishes the equivalent mathematical m odel of the transformer,and proposes a method for estimating the closed loop flux model of the transformer.An integrated DVR control model is established using the flux chain as the state variable,and the flux offset of the load transformer is corrected by the direct flux control.Finally,a control strategy for the excitation inrush current of the load transformer based on the direct flux control is proposed.In order to actually verify the correctness and feasibility of the voltage compensation,detection method,and control method of the three-phase four-leg dynamic voltage restorer proposed in this paper,this paper designs a low voltage experimental prototype of dynamic voltage restorer based on TMS320F28 12 controller.Finally,the work of the full text is summarized and put forward the next research direction.