Study On Differential Protection Of Large Capacity Double-fed Wind Farm Outgoing Transmission Line

Wind energy has developed rapidly because of its cleanliness,recyclability,abundant reserves and long history.The natural wind as motive power of wind turbine is random and uncontrollable,so it will result in instability of wind power.When a large number of wind turbines with large capacity are integrated into the power grid system,the relay protection of system would be affected.Therefore,it is necessary to research the relay protection of large-capacity doubly-fed wind farms' outgoing system.First of all,for wind machine,doubly-fed induction generator and converter control system mathematical model has carried on the comprehensive research.The characteristic of LVRT and connection of Crowbar circuit are emphatically introduced.Then,the simulation model of DFIG wind farm has established with MATLAB/Simulink sofftware.Besides,short-circuit current characteristics of doubly-fed wind-farm has analyzed,and frequency deviation and weak power characteristics have obtained.Secondly,according to the short circuit current's characteristics of DFIG wind farm,the effect on fault phase selection of outgoing transmission line has analyzed.Then we propose a new method of using the extraction of the fault mode current spectrum to judge the fault phase.The phase-mode transformation avoids the influence of frequency deviation and the morphological spectrum avoids the influence of weak power.The simulation results show that this method is feasible and effective for fault phase selection of DFIG outgoing transmission line.Finally,when the fault occurs in the large-capacity doubly-fed wind farm outgoing transmission line,the effect on current differential protection because of the traditional power frequency Fourier algorithm has analyzed.And the method of using ESPRIT algorithm to extract the power frequency current component has proposed to improve differential protection.The simulation model is established to fulfill the comparison of ESPRIT algorithm and Prony algorithm.We could obtain that the fault current characteristics with different frequencies could be discriminated clearly.The signal extraction accuracy of ESPRIT algorithm is higher than Prony algorithm.The results shows that ESPRIT algorithm can extract the power frequency current component in DFIG short-circuit current and differential protection could act accurately.