| Large-scale wind power grid connection has greatly changed the process and regularity of various physical interactions in the power system,which constitutes a major scientific and technological challenge to the safe operation of the power system.The special transient characteristics of wind turbines and wind farms bring huge impact on fault analysis and relay protection.Therefore,it is necessary to carry out a systematic and in-depth study on the short-circuit calculation model of wind turbine and wind farm and its influence mechanism on relay protection,which is an urgent research topic in the field of system protection and control after large-scale wind power access.This thesis,regarding the mainstream doubly-fed wind power generator(Doubly Fed Induction Generator,DFIG)as the research object,conducts the systemic theoretical research in focus on the DFIG transient short-circuit current modeling method,polymerizable equivalent method of doubly-fed wind power and its harmonic characteristic influence on relay protection and other aspects,deepening and improving the wind power system fault analysis theory,so as to provide theoretical support for scientific setting and reasonable configuration of relay protection.The main research achievements and innovations of this paper are as follows:Aiming at the actual problems that under the grid fault DFIG crowbar protect is not instantaneous input but action delay,combining with the circuit dynamic response theory and the frequency domain analytical method,a modeling method analyzing in different time and different stages according to the input time of crowbar is put forward,and a short-circuit current calculation model considering crowbar protection action delay under asymmetric fault is established,based on the analytical model,quantitatively evaluate the influence of crowbar action delay,the operating of DFIG,short circuit time,stator and rotor resistance to short circuit current characteristics,the accuracy of the model is verified by simulation and field experiment data.The method proposed in this paper effectively improves the accuracy of DFIG short circuit current calculation model under the crowbar input scene.In view of the fact that DFIG transient characteristic analysis did not consider the grid side convertor(GSC)transient current impact under the excitation control of converter,which causes the short-circuit current calculation is not accurate enough,based on double closed-loop vector control strategy and grid code requirements for reactive power compensation during the period of low voltage ride through,all transient current refinement analytic expression containing the stator current and GSC current is deduced;The electromagnetic transfer mechanism of the fluctuation characteristics of the electric quantity inside the wind turbine is revealed,and the frequency composition,key influencing parameters and attenuation characteristics of the short-circuit current are clarified.Furthermore,the rotor current peak estimation model is proposed based on the analysis of the transient vector trajectory of rotor current.Considering the influence of current limiting of converter,the nonlinear characteristics of DFIG steady-state short-circuit current are analyzed,and the maximum steady-state short-circuit current estimation formula is proposed.The accuracy of the model is verified by combining simulation and LVRT test data,and the error caused by ignoring GSC current to DFIG short circuit full current is quantitatively analyzed.On the basis of studying the calculation model of single-machine short-circuit current,aiming at the problems brought by the randomness and uncertainty of wind power to the transient equivalence of wind farm,this paper focuses on the transient characteristics of the short-circuit current of wind farm,and proposes a method of the transient equivalence of wind farm based on the two-dimensional cloud model.By improving the selection of clustering index and clustering distance algorithm,the accurate division of wind turbine with similar short circuit current transient characteristics in wind farm is realized.Based on the capacity weighting method,the equivalent wind turbine parameters are calculated,and the equivalent parameters of collector lines are calculated by the method that can adapt to the wind turbine cluster at any location in the wind farm,on this basis,combining with the single machine equivalent model,the method of modeling the short-circuit current calculation model of wind farm is proposed.According to the actual wind data detailed electromagnetic transient simulation model is established,a transient equivalent model of wind farms is established with the method in this paper,the method according to wind speed equivalent and multiplier method,by comparing the voltage and current transient waveforms of equivalent model and detailed model in different fault scenes and error analysis,the effectiveness of the wind farm based on the equivalent method in this paper is verified.In view of the influence of the second harmonic component of DFIG short-circuit current on transformer differential protection,the generation mechanism of second harmonic current in DFIG is fully revealed from two aspects of internal and external cause.The analytical expression of the second harmonic component of the short-circuit current is derived in combination with the DFIG converter control principle under the condition of the step change of the stator voltage and the second harmonic disturbance of the stator voltage.Then,based on the obtained analytical formula,the influence of internal parameters of DFIG generator and converter on the second harmonic component is studied by using the sensitivity analysis method.Combining with the analysis of the generation mechanism of the second harmonic current,the improved strategy of using voltage feedforward compensation control and active power pre-trapper is put forward.Finally,the influence of the second harmonic component of DFIG on the second harmonic braking of transformer differential protection is analyzed based on the actual short circuit experimental data and simulation examples,and the suppression effect of the improved strategy on the second harmonic component is verified.In view of the influence of inter-harmonic component of DFIG short-circuit current on distance protection,starting from the error caused by inter-harmonic component to discrete fast Fourier algorithm,the deviation caused by inter-harmonic component to extract fundamental frequency phasor of discrete Fourier algorithm is derived from an analytical perspective.At the same time,considering the influence of extraction error caused by inter-harmonic component,weak feed characteristic of wind farm,transition resistance and other factors,the influence mechanism of short circuit characteristic of wind farm on the distance protection of outgoing line under different types of faults is analyzed in detail.From the perspectives of analysis and simulation,the law of the change of the measured impedance trajectory with the transition resistance of the distance protection at the side of the wind farm and its influence on the distance protection are revealed.Finally,an example is given to verify the correctness of the theoretical analysis. |
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