Peak Evaluation And Three-phase Short Circuit Current Characteristics Of Doubly Fed Induction Generator Integrated Into Distribution Network

Doubly fed induction generator (DFIG) with its flexible control performance iswidely used in wind power generation system. The terminal voltage of DFIG will dropafter the grid short circuit fault, and that produces the inrush current in the statorwinding, so most commercial DFIGs must access the suitable crowbar resistor in therotor side in order to achieve the low voltage ride through running of DFIG. Thecalculation of the short circuit current is an important foundation for the power systemplanning and protection, due to the distribution network structure and the power flowdistribution will be changed by the short circuit fault. Especially, for checking thedynamic stability of the electrical equipment requires and ensuring the reasonableconfiguration of the circuit breaker interrupting capacity, it is necessary to estimate theshort circuit current peak value of DFIG, so an accurately calculation of the short-circuitcurrent is particularly important. The main contents are as follows:According to the DFIG space vector model, the formulas of the stator and rotorcurrents of DFIG are derived before and after the crowbar protection activation. Whenthe crowbar protection deactivation after distribution network fault, using the equationof the rotor voltage and the rotor inverter output voltage instruction to solve the rotorcurrent second-order differential equation, then the formula of the three-phaseshort-circuit current of DFIG is derived under considering the control and regulation ofthe rotor-side converter. When the crowbar protection activation after the gird fault, theformula of the three-phase short circuit current of DFIG is derived by the forcedcomponent and the DC component of the stator flux of DFIG respectively imputed tothe stator and the rotor to form the static equivalent circuits.On the basis of the analytic time domain formula of the three phase short circuitcurrent, an evaluation method to determine peak value of the three-phase short circuitcurrent of DFIG integrated into distribution grid is proposed. According to the rotorcurrent formula under the crowbar protection activation, the impact of rotor currentpeak on the crowbar protection is analyzed with different power controls. And theimpact of the crowbar resistance on the transient DC component, the transient periodiccomponent and the forced component of the short circuit current is analyzed when theshort circuit occurs near to or far from DFIG, and the peak assessment method of thethree-phase short circuit current of DFIG is proposed by the equation of motion of the rotor with taking the change of speed into account, so using it the peak moment of thethree-phase short circuit current of DFIG can be determined.Simulation model is built on MATLAB/Simulink to verify the correctness of theproposed evaluation method. The fault transient of the three-phase short circuit of DFIGis analyzed. The three-phase short circuit current of DFIG is researched with differentunit capacity and failed nodes, and the calculated value of the proposed method iscompared with the simulation results to verify the correctness of the proposedevaluation method of the three-phase short circuit current for DFIG into distributionnetwork.