Detection of water leakage defects in cable insulation sheath based on terahertz time-domain spectroscopy

Water leakage of the insulation sheath can lead to a decrease in the insulation performance of the power cable, which poses a hidden danger to the power cable. Accurate detection and positioning of the water leakage area are crucial for improving power safety and stability. In response to the water leakage defect of the cable insulation sheath, this article proposes a method based on reflective terahertz time-of-flight tomography technology to measure the water leakage area of the insulation sheath. A sample of insulation sheath water leakage defect was prepared by simulating real scenarios. By analysing the terahertz reflection time-domain optical spectrum and imaging results of the sample, the location and area of the insulation sheath water leakage defect can be accurately measured, verifying the application potential of reflective terahertz time-of-flight tomography technology in detecting water seepage defects in cable insulation sheaths.


Introduction
With the acceleration of urbanization and smart grid construction in recent years, the output and usage of power cables are also increasing gradually, the outer sheath of power transmission cables is usually made of insulating materials such as silicone rubber and epoxy, and the insulation sheath is easy to be affected by outside factors such as wind, rain, sun, and poor operating conditions during long-term operation [1] .Due to the long-term humid underground environment, the liquid outside the cable insulation sheath will gradually penetrate into the cable through small cracks or diffusion effects, causing cable insulation failure [2] .Serious ones will produce short-circuit, insulation discharge, earth conduction, and other problems, eventually leading to power cable failure, causing short-circuit power supply lines, and even leading to large-scale blackouts and other events [3] .According to the statistics of Beijing Electric Power Company about ten years, there are about 134 faults caused by cable water seepage, accounting for 37.6% of the total cable faults.Therefore, the timely and accurate detection of water seepage in the cable is of great significance to improve the stability of the power system.
The non-destructive testing methods of power cable insulation include ultrasonic, x-ray, infrared, ultraviolet, reflection spectrum, AC withstand voltage, partial discharge, isothermal relaxation current, etc.Because of the problems of material attenuation and signal interference, it is difficult to characterize the inner seepage area of the insulation sheath, so early diagnosis and warning cannot be realized.The position of terahertz time-domain spectroscopy gives it unique advantages in detecting insulating materials.Terahertz wave has the advantages of small attenuation, strong collimation, good penetrability, and high resolution in imaging visualization.Terahertz waves can easily penetrate the outer insulating silicone rubber of power cables and reach the aluminum sheath.The metal is almost totally reflective of the terahertz waves, and the liquid water has a strong absorption of the terahertz waves, terahertz imaging technology can clearly detect water intrusion area and quantify water intrusion area [4][5][6] , which has a good application potential and value in the detection of water intrusion of power cable insulation sheath.
Because the terahertz wave has obvious absorption characteristics for water, the water intrusion defect of the insulation silicone rubber sheath of the cable was detected by the reflective terahertz timeof-flight tomography [7] , the simulated sample of water intrusion was prepared and tested by terahertz time-domain spectroscopy system.The reflection spectra and imaging results of the water intrusion region and normal region were obtained, and the location and area of the leakage defect of the insulation sheath can be precisely located by using the imaging results of the two-dimensional moving platform scanning different areas of the sample.

Research on insulation defect detection of electrical equipment based on terahertz technology
The insulating sheath material of the power cable used in this test was silicone rubber which was taken from the power cable used in the 110 kV power supply line.The sample of the cable insulating sheath material with a thickness of d=4.0 mm was prepared by cutting along the direction of the cable.

The structure and geometry of the specimen
In this paper, a silicon rubber sheet and a metal aluminium sheet of the same size are used to simulate the outer insulation sheath and aluminium sheath.0.

Test system and its principle
The schematic diagram of the terahertz time-domain spectral system is shown in Fig. 2. The central wavelength of the ultrafast femtosecond pulse laser is 800 nm, the pulse width is less than 100 fs, and the repetition rate is 80 MHz.The femtosecond laser is divided into two beams: beam 1 and beam 2, which are pumped by a time-delay device and then incident on a ZnTe photoconductive antenna to excite a THz pulse.The pulse is collimated by an off-axis paraboloid mirror and then incident on the sample to be measured.The sample to be measured has a dispersion and absorption effect on the incident THz pulse, which changes the phase and amplitude of the reflected THz pulse, the reflection pulse of THz carrying the sample signal passes through the ZnTe crystal together with the light beam 2 as the probe light.The electric field of the THz pulse modulates the electro-optic crystal ZnTe to produce birefringence through the linear electro-optic effect, and the THz reflected pulse with sample information can be obtained by detecting the change of the polarization state of the probe pulse.When testing, the samples are placed on the platform of a two-dimensional translation table, and the reflection spectrum and imaging data of the samples are obtained.The principle of reflective time-of-flight imaging is as follows in Fig. 3.A terahertz probe emits a pulsed terahertz wave, and after the terahertz wave reaches the sample, it respectively reflects at each interface, the reflected signal enters the terahertz radiation probe and records the flight time and amplitude.In the process of measurement, the sample is fixed on the two-dimensional electric translation table, and the position of the sample is changed through the two-dimensional translation table so that the terahertz pulse scans the different points of the sample one by one, three-dimensional reconstruction of terahertz signals from different points can obtain three-dimensional data, which can be viewed in Bscan, C-scan mode.

Analysis of reflectance spectrum test results
The prepared insulating sheath with water seepage defect was tested by single point reflectance spectrum, and the reflectance spectrum of the water seepage region and the non-water seepage region was tested respectively.The red curve is the reflection spectrum of the normal area, and the two curves have obvious differences in the position and intensity of the reflection peak.The peak A and peak D represent the upper surface of silicone rubber in the defect sample, and the peaks B represent the interface between the lower surface of silicone rubber and water seepage, the peak C is the reflection peak produced by the terahertz signal incident on the aluminium plate through the water seeping area, peak E is the reflection peak of the interface between the lower surface of silicone rubber and the air, and peak F is the reflection peak of the aluminium plate.The peak G is produced by the reflection of the terahertz signal between the silicon rubber lower surface and the aluminium plate.Because of the strong absorption of terahertz signal by water in the seepage region, the intensity of peaks B and C is obviously weaker than peaks F and G in the non-seepage region.
From the signal of the sample with different medium results shown in Fig. 4, it can be seen that there is a significant difference in the intensity of terahertz reflection peaks between the seepage area and the non-seepage area.The thickness of the seepage area and air gap can be judged by the time interval between the reflection peaks, and the terahertz technique can be used to detect the leakage of the insulating sheath.Fig. 4 Test signal of the sample with different medium

Analysis of tomographic results
A reflective terahertz time-of-flight tomography technique was used to test the water seeping defect sample of the insulating sheath, and the defect sample was placed on a two-dimensional translation table, moving the sample so that the terahertz signal could scan different areas of the sample.The x-axis is 0 to 50 mm, and Y-axis is 0 to 30 mm with a scan step of 0.3 mm.The following Fig. 5(a) shows the peak imaging results after scanning, by which the areas of seepage and normal areas can be clearly distinguished, and by the threshold segmentation of the peak imaging results, the area of the seepage area can be further quantified as shown in Fig. 5(b).

Conclusion
According to the practical application of power cable, the samples of water seepage defect in insulation sheath were prepared, and the samples were tested and analyzed by single-point reflectance spectrum and tomography using terahertz time-domain spectroscopy.The moisture content in the insulating sheath will cause strong absorption of the terahertz signal, which makes the terahertz reflection signal strength in the seepage area decrease, and there is a significant difference between the surrounding normal area and the terahertz reflection signal strength.Terahertz reflective time-of-flight tomography can locate and quantify the area of water seepage in the insulation sheath.The usage of terahertz reflection time-of-flight tomography can locate the location of water seepage defects in the insulation sheath and quantify the area of water seepage, which provides a new method for the detection of water seepage defects in the insulation sheath of cables, it can be applied to the actual detection of water seepage defects in cable insulation sheath.
2 ml of water was dropped into three different areas to simulate water intrusion.The silicon rubber plate size is 50 mm*50 mm*4 mm and the aluminium plate size is 50 mm*50 mm *1 mm.The schematic diagram of the test sample is shown in Fig. 1.

Fig. 1
Fig. 1 Schematic diagram of the test sample

3 Fig. 2
Fig. 2 Schematic diagram of the terahertz time-domain spectral system

Fig. 3
Fig. 3 Principles of reflective time-of-flight imaging

Fig. 5
Fig. 5 Imaging results of terahertz time-domain spectroscopy The location of the defect in the seepage area can be seen more directly from the slice image and the 3D reconstruction image by reconstructing the 2D platform scanning data of the sample, as shown in the following figure.

Fig. 6
Fig. 6 Three-dimensional tomographic results of terahertz time-domain spectroscopy