Dynamic Characteristics Analysis Of Doubly Fed Induction Generator Low Voltage Ride-through Based On Crowbar Protection

With the development of wind power,wind farms have more and more influence on power system.For the sake of stability of power grid,low voltage ride through(LVRT)capability is required for wind power.The use of Double-fed Induction Generator(DFIG)in large wind turbines has become quite common over the last few years and one of the effective measure to improve LVRT capability is adopting Crowbar protection.It is an effective way to reduce the rotor current and protect rotor side converter,but it can also has impact on dynamic characteristic of DFIG.So it is necessary to study the effect on dynamic characteristic of DFIG which is added Crowbar,in order to improve LVRT capability of DFIG.The dynamic characteristics of the DFIG,which is added Crowbar protection,were analyzed.The main contents are as follows:This thesis proposed the dynamic equivalent circuit of the DFIG,which is based on space vector and deduced dynamic-analytical expressions of the DFIG stator flux under balanced voltage dips.Impedance equivalent method to correct the stator decay time is be proposed,which can reflect the influence of Crowbar on stator flux.The dynamic characteristics of the rotor current with Crowbar were analyzed,which through the dynamic response of the stator flux.According to the steady state analysis,the relationship between the reconstruction topology of Crowbar and the ideal topology is proposed.This thesis analyzed the single-phase dip and phase-to-phase dip and deprived the dynamic-analytical expressions of the DFIG stator flux and rotor current when dips appear at any time.This thesis proposed there is no DC component in the stator flux when the voltage of the fault phase reaches the peak under single-phase dip;and there is also no DC component in the stator flux when the voltage of the non-fault phase crossing zero under phase-to-phase dip.The natural flux of rotor current,which is depending on when dip begins,varies periodically upon the origin of coordinate axis.This thesis pointed the resistance values of Crowbar have a good effect to limit natural flux of rotor current.The relationship between rotor current values and resistance values of Crowbar,different types of voltage dip and dip depth have been analyzed,which indicating that the deeper voltage fall and bigger Crowbar,the bigger over rotor voltage.The Crowbar resistance of DFIG,which considering all types fault,was designed by applying the fuzzy optimization mechanism.The simulation results show that the rotor current can be limited by the Crowbar resistance optimization.Given the influence of Crowbar,this thesis theoretically analyzed the dynamic characteristics of DFIG during LVRT.According to the fuzzy optimization,the Crowbar resistance valves of DFIG was been reset.A series of simulations were conducted in the Matlab/Simulink platform.The simulation results was in good agreement with those obtained theoretically and validate the proposed analysis.