Control Strategy Research Of Low Voltage Ride Through For DFIG Based Wind Turbine In Grid Fault

With the lack of energy and environmental pollution,large-scale wind power,as clean,renewable energy,connected grid that will become a development trend.However,due to the time-varying characteristics of wind power output,the frequent occurrence of grid accidents,wind power been off the grid,the safe operation of power systems will be seriously affected.The most challenging requirement is that,under external grid fault,the doubly fed induction generator(DFIG)should remain not off-grid and continue to supply power to the power system,to help the grid recover from the fault in a timely manner,and has a certain reactive power and voltage control capability.Therefore,it is of great theoretical meaning and engineering application value to study the control strategy of low voltage ride through(LVRT)for DFIG under grid fault.The focus of this paper is as follows:(1)For the serious fault of the grid,Crowbar protection circuit is introduced to achieve LVRT.In order to prevent the Crowbar circuit from operating for a long time,the mechanical and electrical transient process is disturbed,and the wind turbine is oversped and off-grid.This paper designs a criterion of Crowbar actively act and exit.The influence of different Crowbar resistance on the LVRT capability of DFIG is analyzed by simulation.The results show that in the reasonable Crowbar resistance range,the greater the Crowbar resistance is,the more conducive to the stability of the system is,the fault through of wind turbine is better to achieve.(2)When a short circuit fault occurred in the grid side connected the DFIG,the LVRT ability of DFIG based on the traditional Crowbar technology is low,the DFIG need to absorb a mass of reactive power from the grid,the terminal voltage is difficult to recover.In reaction to this technical disadvantage,an improved integrated control strategy is proposed.Firstly,through using DC-Chopper in the basis of the Crowbar technology,the DC bus overvoltage and the overcurrent of the rotor side can be limited during the fault,secondly,the rotor side introduces a differential compensation term control of stator exciting current,which enhances mechanical stress of the system unit shaft.When a serious fault occurred in the grid,the improved GSC control strategy can be switched from normal operation mode to supporting mode of the reactive power,which compensates to the needed reactive power of the system,and provides a part of reactive power to grid.The simulation results show that,in different voltage dip,the proposed control strategy can improve the capability of low voltage ride through of DFIG.(3)Concerning on the fault of a wind farm having impact on the reactive power and voltage drop of adjacent wind farms,in this paper,taking into account the output of active and reactive power in the wind farms is large,cascading Crowbar action will absorb more reactive power from the grid.This paper adds a coordinating control of reactive power compensation device of STATCOM,and a coordinating control strategy of reactive power and voltage for adjacent wind farms based on LVRT is proposed.The simulation results show that the proposed control strategy can improve the fault voltage of the adjacent wind farms and compensate output capacity of the reactive power of the wind turbine,and the effectiveness of the coordinating control strategy is verified.