Impact Of Doubly-fed Wind Farm Integration On Transmission Line Relay Protection And Improvement Measurements

In recent years,the problem of fossil energy depletion and environmental pollution has become more serious in the global scope,as an important representative of renewable energy,wind power has been developed rapidly and the doubly-fed induction generator has been widely used because of its high efficiency,easy to realize decoupling control of power and other advantages.The wind farm in china is mainly grid-connected on large-scale with the characteristics of cluster development,weak grid integration and long distance transmission.The unique fault characteristics of wind farm have brought some new problems to the distance protection and auto-reclosure of transmission line,so it is necessary to study the doubly-fed wind farm effects on relay protection of transmission line and is important to improve the reliability of relay protection,enhance the fault-withstand ability and guarantee safe and stable operation of power grid.Firstly,this paper studied the operation principle and control strategy of doubly fed induction generator,a detailed model of doubly-fed wind farm and equivalent model in DIg SILENT/PowerFactory was built combined with the typical doubly-fed wind farm diagram,and the feasibility of the equivalent model was verified in steady and transient conditions.The impedance,weak-infeed and non-industrial frequency characteristics of doubly-fed wind farm were analyzed in theory,and simulation in different types of fault proved the correctness of the theory analysis.Then,the influence of doubly-fed wind farm on transmission line distance protection was studied.Affected by weak-infeed characteristic,the additional impedance when fault with resistance may result in relay misstrip or maloperation and relay of the wind farm side is effected more seriously.Because the zero sequence impedance is far less than the positive and negative sequence impedance,the ability to withstand interphase fault is weaker than the grounding fault,and will be reduced after crowbar inserted.The non-industrial frequency of current leads to the measuring impedance fluctuation,so the relay action time will be extended,and when the fault is located near the boundary of the protection scope,relay may operate wrongly.A distance protection scheme with fault type and phase selector based on two terminals phase current of transmission line was proposed,this method is not effected by non-all-phase operation and system oscillation.On the other side of transmission line,current has been measured by distance relay and can be transmitted by the installed longitudinal differential channel.This scheme is of high practicability.Finally,in view of the influence of double-fed wind farm on reclosure,an improved coordination scheme with synchronization-check and voltage-check was given.Also,automatic self-synchronization scheme can be adopted.In some case,the line voltage transformer is not installed or out of service due to fault,a reclosure time calculation method was put forward.