Research On Wind Farm Outgoing Transmission Line Protection Scheme

With the increasingly serious energy crisis and environment problems,wind energy,as an important form of renewable energy,is gradually getting more attention.In China,wind power resources mainly concentrate in the northwest region,we often transport wind power by large scale and in a centralized form.Outgoing transmission line is a channel which connect wind farm to power system,the correct operation of its relay protection has great significance on safety and stability of power system.At present,doubly fed induction generator is the major form of wind generator,so when short circuit faults happen on a wind farm outgoing transmission line,it will show quite different fault characteristic from traditional synchronous generator and bring a great challenge on relay protection.In this paper,we first introduce the network structure of wind farm connected to the grid.Based on theoretical derivation on transient process and sequence network of wind farm,we get three conclusions:the fault current in the wind farm side has rotor speed frequency component,especially in three-phase or phase-to-phase short circuit;the positive and negative sequence impedance of wind farm are no longer approximately equal;wind farm has weak power characteristics due to that the zero sequence impedance is quite smaller than positive and negative sequence impedance.Simulation results have proved those conclusions.Secondly,according to the configuration schema of wind farm outgoing transmission line relay protection,a detailed study has put forward on the influence of fault data,which take fiber current differential protection element,direction pilot protection element,distance protection element and fault phase selector element as the objects of research.Simulation results show that direction protection and distance protection element may not operate correctly and fault phase selector may miss the fault phase.Next,aimed at solving phase selection failure of fault phase selector,this paper presents an improved fault phase selection method using current symmetrical components based on negative sequence impedance angle correction.Simulation results have shown that this improved method is able to select the correct phase in different fault types.This paper also verifies the applicability of fault phase selector using voltage symmetrical components in the outgoing transmission line protection,which is a traditional fault phase selector of week feed side.In view of the problem that the distance protection may not operate correctly,this paper proposes an improved distance protection scheme based on TLS-ESPRIT parameter identification method,by the means of analysis of rotor speed frequency component circuit.The method can identify the number of AC components of fault data,acquire the amplitude,frequency information of these AC components,and then calculate the measuring impendance accurately.Simulation has proved that the improved distance protection algorithm can work well on various fault types,positions and transition resistances,thus ensuring the selectivity and reliability of distance protection element.Finally,in order to solve the problem of protection setting caused by large-scale wind power integration,this paper presents an equivalent method for wind farm based on the monkey algorithm and capacity-weighted aggregation,which can be used to calculate optimal parameters of the equivalent wind power generator.By comparing dynamic response of the detailed model and the equivalent model,it is verified that this method can be applied to the protection setting of outgoing transmission line.