Research On Short-circuit Current Characteristics Of Doubly-fed Wind Turbines Considering Crowbar Protection Action

DFIG has become the main type of wind turbine because of its advantages of variable frequency modulation,small capacity of converter and four quadrant operation.But the stator is connected directly with the power grid,it is more sensitive to the voltage change of the power grid.The Crowbar protection is generally used to realize the low voltage crossing.The time of Crowbar protection is short but the transient characteristic is complex.It is different from the fault characteristics of the traditional power network during the fault,and presents the characteristics of complexity and short time.With the increasing capacity of the wind power,the grid can not be disconnected when the power grid fails,but it requires continuous grid operation.The short-circuit current provided by the power grid is directly fed into the system,which affects the safe operation of the power grid.Therefore,it is very important to study the dynamic characteristics of the short circuit current of doubly fed induction generator(DFIG).In this paper,the basic structure of DFIG,the mathematical model and the control strategy are analyzed.The simulation model of DFIG was built in the Matlab/Simulink software,and the simulation of the model's running characteristics under the condition of steady state and failure was carried out to verify the validity of the model.Secondly,starting from the transient DFIG transient process,the physical process of the DFIG magnetic chain characteristics before and after the fault was studied.The change characteristics of the stator and rotor flux and the change mechanism of the frequency content before and after the fault were analyzed,and the characteristics of the DFIG stator and rotor flux chain and its spectrum before and after the fault were simulated and analyzed.On this basis,the short circuit current expression of the symmetrical fault considering the Crowbar protection action time and the expression of short circuit current under the asymmetrical fault of the Crowbar protection action were derived respectively.The frequency content of the fixed rotor current under two kinds of faults is extracted by the Prony algorithm,and the simulation was carried out on the Matlab/Simulink platform.The simulation results verify the correctness of the theory.In view of the many factors affected by the short circuit current characteristics of the doubly fed fan,the influence of the Crowbar resistance on the short circuit current was emphatically analyzed.The reasonable Crowbar resistance can make the stator and rotor flux fast attenuating.The effect of voltage drop depth,Crowbar protection input time and rotor speed on short circuit current was also studied.The simulation results show that the deeper the depth of the voltage drop,the later the input of Crowbar protection,the greater the speed,the greater the impact current of the stator.