Research On New Methods Of Fault Line Selection And Location For Large-scale Wind Farms' Collecting And Output Lines

China boasts abundant wind energy resource in the three north,south and southwest regions.Wind power in China is connected to the grid in a large-scale centralized longdistance way.Taking Zhangbei wind farm as an example,there are many collector lines in the wind farm.In additional,these wind turbines at the downstream of the lines are mostly gathered in radiation type or chain type.The off-site delivery lines mostly transfer wind power to the load center in a long distance.When the line of the wind farm fault,the shortcircuit characteristics of the doubly-fed induction generation,the complex topology of the wind farm collector line,and the transmission in a long-distance way of the delivery line all make the system's fault characteristics more complicated.The traditional relay protection of the power system is greatly challenged.Therefore,it is necessary to propose a new relay protection method suitable for large scale wind farm collector lines and long-distance delivery lines according to the structure and fault characteristics of wind farm system as well as the defects of traditional protection.The main components of the doubly-fed induction generation system and the basic principles of constant-frequency variable-speed doubly-fed wind power generation system were illustrated in this paper.The mathematical model of DFIG in the synchronous rotating coordinate system was established.The short-circuit current characteristics of Crowbar circuit connected to the DFIG are considered when asymmetric short-circuit faults and symmetric short-circuit faults occur in the system.Aiming at the problem of unreliable zero-sequence protection in high-impedance faults of small resistance grounding systems in wind power plant,a single-phase ground fault line selection method based on zero sequence current waveform similarity is proposed.Most of collector lines in wind power plants are cable-line.cable-line hybrid lines.Due to the cable capacitance per unit length is larger than the overhead line,the faulty transient component of the line is more abundant.Therefore,temporary zero –sequence current is selected to calculate the Hausdorff distance between all lines,so as to obtain the similarity degree of zero-sequence current waveform between faulty and unfaulty lines,and preliminarily select faulty lines.In view of the complexity of the downstream structure of the collector line and the few detection points,the detection points are reasonably added for the key nodes of the collector line,and the faulty section and faulty branch identification method based on the phase difference of zero sequence current is proposed.Compared with the traditional FFT method,the all-phase FFT(ap FFT)spectrum analysis method is used to calculate the phase difference od zero sequence current between two adjacent measurement points.Simulation results and actual data show that the faulty line selection method,faulty section and faulty branch identification method are accurate and reliable.A two-terminal fault location method suitable for long-distance transmission lines in large wind farms is firstly proposed.The time-domain method is used to solve the problem that the frequency offset characteristic of DFIG makes the traditional distance protection unreliable.The two-terminal ranging method is used to solve the problem that the transition resistance of the single-terminal ranging method that affected by the impedance of the terminal system is weak and easily.The golden section search is used to perform redundant calculation on the sampling values of multiple sampling moments to reduce the ranging error and accelerate the searching speed of the ranging results with high probability.Compared with the lumped parameter line model,the distributed paramemter line model can reduce the error caused by the distributed capacitance of the long-distance line.The voltage along the line is calculated by using the time domain value of the two-terminal voltage and current.Then,the faulty distance is calculated according to the principle of equal voltage at faulty points.Finally,simulation analysis verifies the reliability and applicability of the ranging method under different faulty types,different operation modes of the wind farm.