Research On Low Voltage Ride Through Technology For Doubly-Fed Induction Generator

With the impact of wind power on the power system emerging,the grid has put forward more requirements for the grid connection of wind farms.One of the most basic requirements is that the wind turbine has a strong low voltage ride through(LVRT)capability.Due to its many excellent characteristics,doubly-fed induction generators(DFIG)has become the mainstream model of wind farms in recent years.However,the LVRT capability of DFIG is relatively poor,and DFIG units in wind farms often have off grid accidents.Therefore,this paper studies how to improve the LVRT capability of DFIG.(1)Firstly,the structure and working principle of DFIG are analyzed and studied,which provides theoretical support for the research of LVRT technology of DFIG.Through the analysis of DFIG mathematical model,the DFIG system simulation model is built in Matlab/Simulink simulation system;then,the transient process of DFIG during voltage fault is analyzed in detail,and the causes of overcurrent on DFIG rotor side and Overvoltage on DC bus during fault are studied in detail.Finally,the correctness of the theoretical analysis is verified by the simulation experiment of DFIG transient response under the condition of low voltage fault.(2)Research on DFIG low voltage ride through based on STATCOM.First of all,the paper summarizes the problems existing in the grid connected operation of wind farms and the effect of reactive power compensation on the grid connected operation of wind farms.By comparing the characteristics of various reactive power compensation devices,it shows the advantages of STATCOM compensation in wind farms.Then,the basic principle and circuit structure of STATCOM are analyzed,and its mathematical model is established.According to the operation characteristics of STATCOM,a direct current control strategy is proposed.Finally,the influence of STATCOM on DFIG wind turbine is studied when the grid has different voltage sags,and the simulation results are analyzed in detail.The simulation results show that the proposed STATCOM control strategy can effectively help DFIG system maintain voltage stability and assist DFIG to realize low voltage ride through in the case of small voltage drop.(3)Based on the research of DFIG low voltage ride through of crowbar,the control problem of DFIG low voltage ride through of crowbar is studied.Firstly,the principle of DFIG low voltage ride through using crowbar is analyzed,and the best resistance range of crowbar is obtained through theoretical analysis;secondly,the control method of crowbar is proposed based on the transient response characteristics of DFIG and the working characteristics of crowbar;finally,the LVRT problem of DFIG is simulated on MATLAB/Simulink,and the simulation results show that the control method proposed in this paper is in the electric field The LVRT capability of DFIG can be improved effectively under the condition of symmetrical fault and asymmetric fault.