Abstract The expression of short circuit current through outgoing transformer is derived when the doubly fed wind farm occurs symmetrical and asymmetrical faults. Then, the formula of deviation caused by the calculation of short-circuit current on both sides of the transformer is further deduced by Fourier algorithm, and the Fourier algorithm power frequency phasor calculation expression is verified by simulation. It indicates that the short circuit current frequency offset of doubly fed wind farms will lead to the fact that Fourier algorithm can not accurately extract power frequency phasors. As a result, the performance of traditional transformer protection is affected. In this paper, a new scheme of differential protection applied to double fed wind farm outgoing transformer is put forward. The instantaneous excitation inductance values and variances of each phase in this scheme are calculated directly and a criterion is formed. Consequently, combined the criterion with ratio difference and differential velocity, a differential protection criterion is formed. A double fed wind farm delivery system with Crowbar protection and a protection simulation model are established in Matlab/Simulink. Considering the no-load closing of the transformer, fault location, fault type, speed change and short-circuit capacity ratio, the applicability of the new differential protection scheme is analyzed and verified by capacity ratio of the doubly-fed wind farm, and then the traditional differential protection and new scheme are compared and analyzed. The simulation results show that the new scheme can improve the speed and reliability of the differential protection sent by the double fed wind farm outgoing transformer, and improve the problem that the differential protection cannot operate accurately due to the deviation of short circuit current frequency in wind farm.
Sun Dejuan,Sun Shiyun,Fan Xiaohong等. Doubly-Fed Wind Farm Outgoing Transformer Differential Protection Based on Excitation Inductance Calculation[J]. Transactions of China Electrotechnical Society, 2020, 35(2): 395-407.
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2020, Vol. 35 
