Ultrasonic Critical Longitudinal Wave Detection Method and Application for Microdefects in Tri-Post Insulators
Li Xuan1, Yin Yichun1, Shi Wei2, Wang Jian1, Ren Hanwen1, Li Qingmin1
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China; 2. Electric Power Research Institute of State Grid Shandong Electric Power Company Jinan 250003 China
Abstract：Tri-post insulators as key components of ultra/extra-high voltage gas-insulated transmission lines (GIL), are susceptible to defects degradation, discharge brittle cracking breakdown and other potential risks due to latent defects generated by installation processes such as casting and assembly, and by the composite stresses such as electric-thermal-mechanical in the pipeline, so it is imperative to realize the early detection and identification of microdefects to ensure the safe and stable operation of GIL. This paper focuses on the method of microdefect detection under the complex surface profile of tri-post insulator, calculating the geometrical formation conditions based on the critical longitudinal wave refraction principle between the epoxy-wedge coupling layers, then obtaining the fluctuating displacement quantity by bringing in the insulator characteristic surface profile equation, establishing the regularized correlation matrix between the vibration wave strain and the defective structural parameters under the stress field excitation, and finally generalizing to the amount of longitudinal stress under the defective boundary condition constraints. Corresponding relationship with the electrical signal, a more accurate three-dimensional acoustic wave conduction calculation model is formed. The critical longitudinal wave and body wave are used to detect cracks and air-gap microdefects by dividing the superposition state of tri-post insulator fluctuations into ultrasonic artefacts, propagation and absorption zones in a multi-area measurement method. An optimized design model for critical longitudinal wave detection of microdefects in tri-post insulator is further established to obtain the correlation characteristics of longitudinal wave conduction law and echo signals under different locations, types and scales of defects. The results show the strain energy density of the tri-post insulator with crack defects is bimodal, and the strain energy increases with the increase of crack length, which leads to the decrease of echo energy, and the signal of critical longitudinal wave detection grows by 22.9%, and the echo energy decays by 27.5%. The closer the crack defects are to the legs, the greater the signal decays, and the better the detection effect; there is a polar effect when detecting cracks of different lengths, short crack echo attenuation is weaker, but longer cracks will lead to compensation of energy loss after superposition interference, and the echo amplitude attenuation is the largest when the crack length is 17.6% of the cross-sectional radius of the leg. In addition, the offset angle of the tri-post insulator surface cracks, thickness increase will hinder the incident propagation, resulting in further wave attenuation and time delay, linear fitting of the characteristic peak equation can be entered into the signal energy attenuation and echo time delay to determine the specific parameters of the microdefects; body wave reflection detection of defects in the air gap when the echo was a double-spike distribution, the amplitude of the amplitude of all the more than 50%, with the increase in the frequency of defects in the waveform is significant. Thus, the optimized design of this detection device has a transmitting frequency of 500 kHz in the abdomen and 2 MHz in the legs, a critical angle of incidence of 9°, a radius curvature of 15 mm, can detect microdefects in the size range of 0.41 mm or more, which defect size sensitivity up to 5.8 dB/mm, with a combined sensitivity assessment of 46.8 dB. Finally, accordingly, a detection system was designed based on the combined detection method of critical longitudinal and body wave, and preliminary experimental verification of microdefect detection in tri-post insulator was carried out, in which potential microdefects in insulator standard specimens were measured and found to have average error of less than 5%, which realized the quantitative detection and effective analysis of microdefects in tri-post insulator.
李玄, 尹奕淳, 师伟, 王健, 任瀚文, 李庆民. 三支柱绝缘子微缺陷超声临界纵波探测方法与应用[J]. 电工技术学报, 2025, 40(21): 6905-6921.
Li Xuan, Yin Yichun, Shi Wei, Wang Jian, Ren Hanwen, Li Qingmin. Ultrasonic Critical Longitudinal Wave Detection Method and Application for Microdefects in Tri-Post Insulators. Transactions of China Electrotechnical Society, 2025, 40(21): 6905-6921.
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