Study On Protection Scheme For Collector Lines In Wind Farms Based On Sequence Current Amplitude Comparison

In China,the areas rich in water resources are far away from the load center,as a result of which,the consumption of wind power are required to be transported to areas where the load is concentrated via high-voltage transmission lines after being converged by the collector lines.Collector lines are now generally equipped with Co-ordination current protection,the performance of which would be influenced by the uncertainty of wind power output and the different fault characteristics of DFIG from that of the traditional synchronous generator.More current information available for relay protection is excavated and a new relay protection scheme for collector lines is proposed with the comprehensive consideration of technology and cost.Based on the structure of the collector lines and the characteristics of current protection,the short-circuit current provided by wind farm is analyzed for its negative effect on the selectivity and sensitivity of conventional current protection.Considering the different fault characteristics of DFIG under different low voltage ridethrough(LVRT)strategies,the negative impact on current protection is not the same,the positive and negative sequence current characteristics of DFIG is analyzed in detail during the two types of LVRT strategies: putting crowbar resistance and rotor inverter controlling excitation.On this basis,the sensitivity of the current protection would be reduced by the suppression current supplied by the DFIGs under the LVRT by putting crowbars,on the other hand,the selectivity of the overcurrent protection would be destroyed by the reverse-direction LVRT current on non-faulty lines.Compared with the positive sequence fault current,the negative sequence component the protection can consider the negative sequence component of the fault current.To solve the problem of incorrect tripping and fail-operation of traditional current protection,a new criterion called sequence current amplitude comparison criteria is proposed by comparing the amplitude of positive or negative sequence current,with which,the collector line with short-circuit faults could be selected reliably and sensitively almost uninfluenced by transition resistance,fault location and LVRT strategies.By combining traditional current protection and sequence current amplitude comparison criteria,a new protection scheme for collector lines based on sequence current amplitude criteria is proposed,and the setting of which is explained in detail in this paper.The sensitivity,selectivity and reliability of the protection for collector lines could be improved in this new scheme without using the voltage value,moreover,the cost of alteration is small and the new scheme could be put into pratical use.At last,a simulation model of collector lines with DFIGs is built in MATLAB/SIMULINK and simulation verifies the effectiveness of this scheme.