Research On The Short-circuit Current Characteristics Of Doubly-fed Wind Farms And The Distance Protection Strategy Of The Transmission Line

As a sustainable and pollution-free green energy,wind energy has been widely used in recent years.As the main type of fan,the biggest characteristic of doubly fed fan is that in addition to the stator side directly connected to the grid,the rotor side is also connected to the grid through the converter.The unique grid connection and excitation mode of doubly fed fans make the transient characteristics of power system change after large-scale wind power is connected to the grid,which makes the relay protection setting difficult.Therefore,it is very important to study the short-circuit current characteristics of doubly fed fan and its influence on the relay protection of wind farm outgoing line,and to improve the reliability measures of the protection action of the outgoing line.Based on the mathematical model of the doubly fed induction generator,this paper analyzes the physical characteristics of the fault process of the doubly fed induction generator.On this basis,the analytical formulas of the stator and rotor currents of the doubly fed induction generator are derived in detail when the symmetrical fault and the asymmetric fault are taken into account,and when the RSC control and crowbar protection action are taken into account.When considering RSC control under symmetrical fault,the mathematical model of DFIG is combined with the control equation of RSC,and the analytical formula of DFIG stator and rotor short-circuit current after fault is solved by solving differential equation method;when considering crowbar protection action under symmetrical fault,the analytical formula of rotor flux is solved by using Laplace and inverse Laplace transformation method,and then the DFIG is solved by the analytical formula of stator and rotor flux Analysis formula of short-circuit current: for asymmetric fault,the fault component method is used to solve the problem,and the positive sequence model and negative sequence model are analyzed and established in the forward rotation coordinate system and the reverse rotation coordinate system under asymmetric fault.Based on this,the stator and rotor asymmetrical short-circuit current of doubly fed induction generator under two operating conditions including RSC control and crowbar protection action are deduced.The correctness of the eight analytical expressions of short-circuit current is verified on the MATLAB / Simulink simulation platform.Through the study of the short-circuit current characteristics of DFIG under various fault conditions,it is found that the short-circuit current characteristics provided by DFIG are quite different from those of traditional motors.Based on this,further study the influence of the short-circuit current characteristics provided by DFIG on the traditional distance protection.By analyzing and deducing the error expression of the short-circuit current provided by the doubly fed induction generator based wind farm calculated by the Fourier algorithm,and then analyzing the influence of the short-circuit current characteristics on the traditional distance protection,and simulating and analyzing the action characteristics of the traditional distance protection of the doubly fed induction generator based wind farm transmission line under the symmetrical fault and asymmetric fault.In view of the influence of the short-circuit current characteristics of the DFIG on the traditional distance protection,this paper proposes a time-domain distance protection suitable for the outgoing line of the DFIG.Based on the principle of time-domain distance protection,the unknown quantity of transition resistance is increased.The equation contains three unknown quantities of line resistance,line inductance and transition resistance.The three unknown quantities are solved by recursive least square method,and then the identified line resistance and inductance are substituted into the impedance characteristics of directional circle to realize the protection criterion.Finally,the simulation analysis of the distance protection action characteristics based on parameter identification is carried out.The simulation results show that the method can accurately measure the line resistance and inductance,and is not affected by the transition resistance.In addition,the applicability of time-domain distance protection under the actual wind farm outgoing line fault is analyzed based on the field measured data.