The Short-circuit Current Characteristics Of The Doubly-fed Wind Turbine And Its Influence On The Protection Performance Of The Output Transformer

The doubly-fed induction generator(DFIG)has gradually become one of the mainstream models of wind turbines because of its wide speed range and small capacity requirements for excitation inverters.However,as an important measure for DFIG to achieve low voltage ride through,the input of Crowbar protection will cause the frequency of the short circuit current to shift,at present,the transmission of the doubly-fed wind farm is still protected by conventional transformers,the performance of traditional transformer protection based on power frequency is affected by the frequency offset of short circuit current in wind farm.Therefore,it is especially important to study the influence of the short-circuit current of the doubly-fed wind farm on the action characteristics of the traditional protection and the improvement of the reliability of the wind farm.In this paper,the basic structure,working principle,mathematical model and control strategy of DFIG are introduced,secondly,the short-circuit current characteristics under the condition of symmetrical fault and asymmetrical fault are analyzed when considering Crowbar protection,on the basis of this,the expression of short circuit current through transformer transformation is deduced when symmetrical and asymmetric faults occur in the output transformer of doubly fed wind farm,the analytic expression of the short-circuit current sent to the two sides by the Fourier algorithm is further deduced,and the correctness of the analytical expression is verified by simulation,the results show that the frequency offset of the short-circuit current of the doubly-fed wind farm will lead to the deviation of the phasor calculation of the Fourier algorithm,substituting it into the protection criteria will cause the performance of traditional transformer protection to be affected.Full current differential protection,fault component current differential protection and composite voltage start-up overcurrent protection are selected as the research objects,the output system and protection simulation model of doubly-fed wind farm with Crowbar are established in Matlab/Simulink.In the case of three-phase and two-phase short-circuit faults,the effects of short-circuit current frequency offset of wind farms on the performance of three protection actions are analyzed separately,the simulation results show that the differential components of full current differential protection and fault component current differential protection and second harmonic brake components are affected by frequency offset,which results in delay of operation time,while the composite voltage starting overcurrent protection can not operate accurately.Aiming at the problem that short-circuit current frequency offset of doubly-fed wind farm will lead to inaccurate operation of transformer traditional protection,a new differential protection scheme for doubly-fed wind farm transmission transformer is proposed,the instantaneous excitation inductance values and variances of each phase of the scheme are calculated directly to form a criterion,which is combined with the ratio differential and differential current velocity interruption to form a differential protection criterion.A double fed wind farm delivery system with Crowbar protection and a protection simulation model are established on Matlab/Simulink platform,the applicability of the new differential protection scheme is analyzed and verified by considering the no-load closing of the transformer,different fault locations and fault types,and the changes of the speed and shortcircuit capacity ratio of the doubly-fed wind farm,the performance of traditional differential protection and new scheme is compared and analyzed,the simulation results show that the new scheme can improve the speed and reliability of the differential protection of the double fed wind farm outgoing transformer.