Study On Direction Pilot Protection For Transmission Line Of Large-scale DFIG-based Wind Farms

In recent years,China’s wind power industry has developed rapidly,and many large-scale wind farms have been built and put into use.Transmission lines transmit the electric energy from the wind farm to the grid.In order to improve the efficiency of electric energy transmission,the form of double circuit on the same tower is generally adopted.When a large-scale wind farm is connected to the grid,its fault characteristics are quite different from those of traditional power plants,which has a certain impact on the existing transmission line relay protection devices.This thesis takes the directional polit protection of the double-circuit transmission line of a large-scale doubly-fed-induction-generator(DFIG)-based wind farm as the research object.The adaptability problems of directional polit protection on double-circuit transmission line of the wind farm is studied.A directional polit protection scheme based on the impedance and fault current characteristics of the DFIG is proposed.With the help of the six-sequence component method,the action of the positive-sequence fault component directional element of the traditional synchronizer system under various transmission line faults is analyzed.The results show that when some non-identical-phase cross-line faults adjacent to the busbar occur,the directional element has insufficient sensitivity,and may refuse to act in some special cases.In order to explore the specific influence of DFIG on the fault component direction polit protection,the concept of equivalent calculation impedance of DFIG was introduced to characterize the characteristics of the fault voltage and current at the outlet of DFIG.The transient characteristics of the positive and negative sequence stator currents and impedance of DFIG when the crowbar is put in are analyzed.It is obtained that the positive and negative sequence impedance’s amplitude and phase angle are quite different,and DFIG’s positive sequence impedance angle is not similiar to inductance.Based on the impedance characteristics of DFIG,the six-sequence component method is used to analyze the action of the positive-sequence fault component directional polit protection of the double-circuit transmission line.It is pointed out that the positive-sequence fault component directional polit protection may refuse to act when a fault occurs in the protected area.Theoritically,there will be no maloperation in the event of an external fault.The same-sequence phase angle difference protection criterion suitable for synchronous machine system is constructed by using the double-circuit same-positive-sequence component networks.Based on the differences between the equivalent impedance of DFIG and the system after the fault,regarding the double-circuit line as a whole,a same-sequence impedance difference criterion is proposed.According to the fault current characteristics of DFIG,the current fitting direction criterion is proposed.The standard sine wave is fitted and the similarity between the measured current waveform of each phase at both ends of the transmission line and the fitted wave is calculated.When the fault occurs in the protected area,the waveform similarity of the wind power side will be much smaller than the value of the system side because of the non-power frequency characteristics of the current of DFIG in the early stages of the fault.The source of the fault current can be identified by using this difference,thereby judging the fault direction.An improved directional element is constructed by combining the impedance difference criterion and the current fitting criterion with the synchronous machine direction criterion,and a directional polit protection scheme suitable for the double-circuit transmission line of the DFIG-based wind farm is proposed.On the MATLAB/Simulink platform,a double-circuit transmission line model of the traditional synchronizer system and a double-circuit transmission line model of grid-connected wind farm are built.The analysis of the existing protection’s adaptability problems and the performance of the proposed protection scheme are simulated and verified.The simulation results prove the correctness of the problem analysis in this paper and the rationality of the proposed double-circuit directional polit protection scheme.