Research On Send Out Line And Distribution Line Protection Based On Fault Characteristics Of Double Fed Wind Electric Power Source

Clean and renewable energy,high penetration and distributed access network has become one of the main directions of power grid development,it has changed the original single end power grid structure of distribution network.In particular,unlike the synchronous generator,the distributed wind power presents different fault transient characteristics.Therefore,the protection principle,method and configuration of distributed wind power sent out line and distribution line need special study.First of all,the paper analyzes the adaptability of differential protection of distributed wind power sent out lines and current protection of distribution lines.Due to the crowbar circuit input of doubly-fed wind turbines,distributed wind power source fault current frequency offset.It may cause the sent out line differential protection to be unable to act reliably.The distributed wind power has shunting effect on the fault current in the upstream line.The sensitivity of current protection for distribution lines is affected.Furtherly,In the case of crowbar circuit input or not,the expression of short circuit current of doubly fed wind turbine is derived,and analyzed the significant differences in the short-circuit transient current waveform changes with the power distribution system power supply.The principle of pilot protection based on the transient current waveform alienation on both sides of the sent out line is proposed.Then,the equivalent order impedance characteristics of doubly fed induction generator(DFIG)under grid asymmetric fault and crowbar circuit input are analyzed.The amplitude of the equivalent positive sequence impedance is much larger than its equivalent negative sequence impedance amplitude.The obvious difference is equivalent to the equivalent positive and negative sequence impedance amplitudes of the distribution system.Therefore,the principle of pilot protection based on the difference of magnitude of equivalent sequence impedance between two sides of distribution lines is presented.Finally,A simulation model of doubly fed wind farm to distribution system is built on the electromagnetic transient analysis software PSCAD/EMTDC of power system.A number of simulation tests were performed on the longitudinal protection based on the alienation of the transient current waveform on both sides of the send out line and the longitudinal protection based on the difference in the equivalent impedance of the power supply on both sides of the distribution line.The results show that the proposed two protection schemes can operate correctly and reliably in different fault conditions.It has very important theoretical significance and application value for the protection of the distributed power supply after sending power to the distribution system.