Axial Temperature Rise Characteristics of Overhead Line Broken Strand Defects and Identification Method

Defects in operating outdoor overhead transmission lines with broken strands will cause local excess temperature rise. The maximum temperature occurs at the defect and decays rapidly to a defect-free area. Infrared thermography based on temperature distribution can identify the degree of broken strands defects. However, the wind speed will significantly reduce the surface temperature of the observed object, making infrared detection difficult. To study the axial temperature distribution at the broken strand area of overhead lines under low wind speed, the paper took the LGJ-240/30 type steel-cored aluminum strand as an example, and conducted thermal cycling tests in the State Key Laboratory of Power Transmission Equipment & System Security and New Technology of Chongqing University. The broken strand defect was produced by manual destruction, the homemade wind speed was regulated by the air collecting device, and the AC large current generator provided a stable Joule heat source. The influence of the number of broken strands on the maximum temperature rise at the back of the defect and the temperature difference in the axial defect-free area was obtained. And based on this, the infrared identification method of broken strand number when the wind speed is 1~3 m/s was proposed. Finally, the method was verified by natural experiments in the National Field Science Observation and Research Station. The results show that after the occurrence of strand breaks in overhead transmission lines, the axial temperature difference between the extreme value of the defective temperature and the normal temperature in the non-defective area decreases rapidly with the increase of the wind speed; the fitting coefficient b, which describes the heat transfer term in the fitting equation of the axial temperature difference θ and the wind speed u, increases with the increase of current carrying capacity and the number of strand breaks. The proposed method has a recognition rate of more than 90.1% for the number of broken strands and more than 94% for defects under the condition of low wind speed, the load current is 360, 480, and 600 A, and the number of broken strands is more than 3. It solves the problem of infrared thermal inspection project under low wind speed without missing the best maintenance time, greatly improves the maintenance efficiency of line maintenance, guarantees the safe and stable operation of power grid, and has guiding significance for the infrared thermal inspection project of overhead transmission lines.