Longitudinal Protection Scheme For Transmission Lines Of Double-fed Wind Farm

Under the background of "double carbon",the installed capacity of wind turbines has been growing steadily and continuously due to the widespread distribution of wind energy,green and mature wind power generation technology,and low prices.With the advantages of high wind energy utilization rate,wide speed regulation range and small required frequency conversion capacity,doubly-fed induction generators(DFIG)have become one of the mainstream models commonly used in wind farms.Both the stator and rotor of DFIG are connected to the power grid.The rotor exchanges power with the grid in both directions through power electronic components such as rotor-side converters.The fault characteristics of DFIG are significantly different from those of synchronous motors,showing the characteristics of nonlinearity and complexity,which brings new challenges to the setting of relay protection of wind farm outgoing line.A decline in the performance of protective actions may even lead to the risk of refusal and misoperation.Therefore,it is particularly important to analyze the fault characteristics of DFIG and their impact on the relay protection of wind farm outgoing lines,and to study suitable relay protection scheme suitable for wind farm outgoing lines.Firstly,this paper introduces the working principle of DFIG,including the structure,control strategy and mathematical model of DFIG.The two control strategies of rotor-side converter(RSC)control and crowbar protection action of DFIG are mainly briefly introduced.Based on the mathematical model of the DFIG,the expressions of the positive and negative components of the short-circuit current of the DFIG under the two control strategies are derived by Laplace,inverse Laplace transform,or solving differential equations.The content of the short-circuit current of DFIG is analyzed,which explains the reason why the traditional relay protection performance decreases due to the connection of DFIG from the principle.It provides strong theoretical support for the study of relay protection scheme suitable for wind farm outgoing lines with DFIG and for improving the action performance of the protection scheme.Through the analysis of the short-circuit current of the DFIG,it can be seen that when the short-circuit fault occurs on the outgoing line of the doubly-fed wind farms,the short-circuit current on the wind farm side is quite different from the short-circuit current of the traditional power sources,which may cause the performance of traditional distance longitudinal protection based on Fourier algorithm to decline,and even result in protection rejection or misoperation.a class-quadrilateral distance Ⅱ stage directional element that considers the frequency deviation and transition resistance effects was tailored for the outgoing line of the wind farm.On the basis of a comprehensive analysis of its action performance,it is applied to the pilot protection,and a distance pilot protection scheme based on quadrilateral is constructed.The simulation analysis of the action of the protection scheme under various influencing factors,such as the frequency deviation characteristics of the short-circuit current,transition resistance,wind speed change,noise interference,aliasing frequency,reactive power compensation,output change,and data delay.The simulation results show that the protection scheme can accurately determine the fault direction and reliably identify faults inside and outside the area,providing a relatively reliable protection scheme for the outgoing line of the wind farm.According to the grid connection method,doubly-fed wind farms can be divided into AC grid connection and DC grid connection.In DC grid connection systems,flexible DC grid connection has the potential for wind power grid connection due to its advantages such as providing reactive support,good stability,good economy,and strong anti-interference ability.When the AC outgoing line or flexible DC outgoing line of the wind farm is faulty,the protection of the wind farm side is affected by the fault characteristics of the DFIG.When the AC transmission line fails under the two grid-connected modes of AC and flexible DC,the wind field side protection of the transmission line is affected by the fault characteristics of the doubly-fed wind turbine.In the flexible DC grid connection system,the line protection is also affected by the power electronic components of the flexible DC converter on the wind farm side,which leads to a decrease in the performance of traditional relay protection.Therefore,firstly,the short-circuit current and negative sequence impedance characteristics of the doubly-fed wind turbine under two control strategies,namely RSC control and crowbar protection input time,and the negative sequence current characteristics of the flexible DC converter were analyzed when the AC outgoing line or flexible DC outgoing line of the doubly-fed wind farm was faulty.The effects of the two short-circuit current characteristics on traditional relay protection were studied.Then,the characteristic differences of the current on both sides of the AC outgoing line in wind farms with AC and flexible DC grid connections were further analyzed,and a new method for longitudinal connection protection of negative sequence current components applicable to the outgoing line of doubly-fed wind farms was proposed.This method uses the amplitude ratio and phase difference of the negative sequence current on both sides of the AC outgoing line of the doubly-fed wind farm as the protection action criteria.At the same time,the negative sequence component is constructed under symmetrical fault by using data delay,so that it can be applied to various types of faults on the outgoing line of the wind farm,and has advantages such as high sensitivity and strong ability to lerance transition resistance.Finally,a simulation model of DFIG is built in Matlab/Simulink and PSCAD/EMTDC,and the adaptability of the protection scheme on the AC and DC outgoing lines is analyzed through simulation.