Study On The Influence Of Wind Speed Variation On Protection Scheme For Collector Lines In Doubly Fed Wind Farms

In recent years,wind energy has developed rapidly in China.According to the existing voltage level,the collector lines of doubly-fed wind farms are generally equipped with coordination current protection.Under wind speed variation the fault characteristic of doubly-fed induction generator(DFIG)is different from traditional power generation,which brings problems to the current protection.In this paper,by studying the influence of fault current characteristics of DFIGs on the collector lines,the protection scheme for collector lines considering the influence of wind speed variation is proposed.On the basis of studying the fault characteristics of DFIG and the characteristics of current protection,the positive and negative sequence equivalent circuit models of DFIG gridconnected with crowbar protection are analyzed.Due to the influence of wind speed variation,different fault current characteristics on non-fault lines bring problems to the current protection.The actual wind speed of each DFIG on the line is obtained by Jensen Wake Algorithm considering the factors of wind speed,wind direction angle and shielded area.In the fault equivalent circuit,the characteristics of positive and negative sequence impedances of DFIGs on the line under wind speed variation are analyzed.It is found that the amplitude and phase angle of negative sequence impedances are basically unchanged,while the amplitude and phase angle of positive sequence impedances have a large variation range.The sequence impedance characteristics of DFIGs on the line under wind speed variation may cause the fault line current to be less than the setting value of the I segment current protection,and the non-fault line current to be greater than the II segment current protection.The selectivity and rapidity of current protection will be affected.In order to improve the influence of wind speed variation on the current protection,the current information of the fault line and non-fault lines under wind speed variation and different fault types is analyzed.According to the basic stability characteristics of the negative sequence current,the difference criterion of the amplitude comparison of negative sequence current between lines is constructed.According to the steady-state and transient characteristics of the full current,the difference criterion of the full current amplitude comparison with spectrum index between lines is constructed.The difference criterion is used to overcome the influence of wind speed variation,identify the fault line and improve the selectivity of the protection.By considering the characteristics of the transformer fuse,a matching inverse time action setting method is proposed to improve the rapidity of the protection.The improved protection scheme consists of two parts.Considering the characteristics of the fuse,the main protection is the inverse time current protection scheme based on the action of difference criterion.The overcurrent protection scheme based on the difference criterion combined with the existing current protection is taken as backup protection.The simulation model of the collector lines of doubly fed wind farm is built on MATLAB/Simulink platform.The improved protection scheme is verified from the following aspects: the action coordination between the scheme and the fuse,the selectivity of the difference criterion and the rapidity of the protection scheme.The simulation results show that the reliability of the criterion and the effectiveness of the proposed scheme are improved.Compared with the traditional protection scheme,the selectivity and rapidity of the proposed scheme are improved.