Short-Circuit Current Calculation And Adaptive Single-Phase Reclosing Strategy Of Transmission Line Connected To Dfig-based Wind Farm

As an important development direction of new energy,doubly fed induction generators(DFIGs)have been widely used in wind farms with flexible control strategies,active and reactive decoupling control,and other advantages.In a power system with large-scale DFIG-based wind farms,the short-circuit current of the DFIG injecting the system has diffierent characteristics from the traditional synchronous generator.Therefore,this thesis aims at the configuration of relay protection and reclosing of transmission line connected to DFIG-based wind farms.Those work ensure the safe and stable operation of wind farm and system by reducing the off-grid time of wind farms and identifying the faults accurately and quickly.Aiming at the relay protection of transmission line connected to DFIG-based wind farms,this thesis proposes a calculation method for asymmetric short circuit current which considering the crowbar protection and the low voltage ride through control strategy of DFIGs.The calculation method provides an effective basis for protection value setting of transmission line.This thesis also studies the reclosing strategy for single-phase grounding faults on the transmission lines of DFIG-based wind farms.Then the current in the neutral point reactor of the shunt reactor is analyzed and the adaptive reclosing strategy for the transmission lines of DFIG-based wind farms with shunt reactor is proposed.The main contributions of this thesis are as follows:(1)A calculating method for asymmetrical short-circuit current of transmission line connected to DFIG-based wind farms is proposed,which considers the crowbar protection and low voltage ride through control of DFIGs.The method studies the expression of positive and negative sequence components of short circuit current which comes from wind farms and adds those current sequence components into the composite sequence network by dynamic current source.Moreover,this thesis proposes the process of relay protection setting of DFIG-based wind farm transmission line on the basis of the above research.By constructing a power system model with multiple DFIG-based wind farms,the calculation results by proposed method compare with the time-domain results.The error factors of calculation results are analyzed.The proposed method is verified through calculation and simulation analysis of the transmission line of 220 k V wind farm under asymmetric faults.(2)Considering the operation control mode of DFIG and combining with the current in the neutral point reactor of the shunt reactor of the transmission line,the different frequency components of the fault current under different single-phase grounding faults are analyzed.Moreover,the fault current components are verified by relevant simulation.Then the time-frequency characteristic matrix of the output current under the transient and permanent single-phase grounding fault of the transmission line is established by using the current in the neutral point reactor of the shunt reactor.By calculating the similarity of actual measured current with the time-frequency characteristic matrix of the transient fault and the permanent fault,a criterion for determining the type of the fault is proposed.(3)By using the fault criterion based on the time-frequency characteristic matrix of the transient fault and the permanent fault,an adaptive reclosing strategy for transmission line of the DFIG-based wind farm under single-phase grounding fault is proposed.By establishing the transmission line model of DFIG-based wind farm,the current in the neutral point reactor of the shunt reactor is studied.Finally,the reclosing strategy proposed in this thesis is verified by the recognition results of the simulation of transmission line under different fault locations or different transitional resistance for transient single-phase grounding fault or permanent single-phase grounding fault.