Research on Non-destructive Testing Method for Aging of Sheds of Composite Insulators

As an important part of external insulation of transmission and distribution lines and substation equipment, silicone rubber insulators are generally used because of their strong anti-pollution flashover ability, excellent hydrophobicity, and lightweight. It is one of the effective means to obtain the aging degree of sheds quickly and accurately to prevent operation accidents. From the micro and macro perspectives, firstly, this paper uses the traditional scanning electron microscopy and dielectric loss factor test to classify the aging degree of 11 high-voltage side sheds samples of silicone rubber insulators running on-site for 0~11 years. According to the different experimental principles, the aging degree of each experimental method is divided into four grades. Then, the detection method based on terahertz time-domain spectroscopy is proposed to realize a nondestructive evaluation of the aging grades of sheds of silicone rubber insulators. Finally, through comparison, it can be found that the THz absorption coefficient spectra can find the characteristic quantity of classification of aging levels of silicone rubber materials: The aging levels of the silicone rubber samples increase, and the absorption peak at 1.32 THz in the absorption spectra show a downward trend. According to the characteristic quantity, 11 samples can also be divided into four aging grades, and the results are consistent with the previous experiments.


Introduction
China first put domestic composite insulators into use in electrified railways in 1982.In 1985, composite insulators were gradually applied to the power grid.Now, more than 9 million composite insulators are putting into operation in China [1], and the research and production of composite insulators have reached a world-leading level.By analyzing the statistical data of CIGRE on composite insulator failures in recent years, it is found that the most severe component failure in the operation of composite insulators is the destructive failure of the core rod, followed by insulation failure caused by the deterioration of the sheds [2].Therefore, monitoring the service status of the sheds is particularly important.
The low photon energy characteristics of THz waves and their high transmittance to the vast majority of dry dielectric materials determine the prospective applications of THz technology in detecting internal defects in insulation materials and the non-destructive assessment of aging conditions [3].The application research on THz technology for aging detection of silicone rubber composite insulators is less widespread than oil paper insulation and cable insulation.Zhang et al. [4] found that THz waves can effectively distinguish three types of defects: erosion defects, interface delamination, and internal bubbles.Cheng et al. [5] found that the THz dielectric spectrum is greatly affected by the moisture in the air during the experimental process and is unsuitable for on-site detection of the aging levels of silicone rubber.Wang et al. [6] found that the change in dielectric parameters during UV aging is smaller

Classification experiment on the aging degree of composite insulators' sheds in field operation
To truthfully reflect the natural aging rule of sheds of composite insulators in field operation, this section selects 11-rod suspension composite insulator strings of different models and manufacturers from a certain power grid in North China with different operating years as experimental research objects.The selected insulators have a service life ranging from 0 to 11 years, and the specific distribution of service life is shown in Table 1 To make the experimental effect more obvious, the outer edge of the first large shed on the highvoltage side of each insulator is selected as the experimental object.During the experiment, the selected shed will be cut completely along the edge of the core rod, and the area that needs to be tested will be selected and cut into sectors.Then, anhydrous ethanol and deionized water will be used to wipe sample's surface to remove surface dirt and residual charges.Then, the sector-shaped sample will be divided into circular sample with a diameter of 5 cm for dielectric loss factor testing.Finally, according to the cutting method shown in Figure 1, 11 sets of square samples with a length of 2.5 cm, a width of 2 cm, and a thickness of 1.0 mm are made for subsequent experiments.The final cut silicone rubber samples are numbered 1~11#.

Experimental study on the classification of dielectric loss factors for the aging of sheds of composite insulators
The dielectric loss tangent ( tan  ) is the most significant feature in measuring the levels of dielectric aging.Compared to other insulation characteristic parameters, the value can effectively reflect the deterioration of dielectric distribution and moisture and is not affected by the shape and size of the tested sample.
The experiment uses WB6000 fully automatic dielectric loss tester produced by Yangzhou Miaocheng Electric Co., Ltd., as shown in Figure 2. The tester is used in a power frequency environment, with five voltage levels in the 0~10 kV range and strong anti-interference ability.It also has three testing modes: normal, reverse, and external wired methods.The measurement error of the tester is 1.5%  0.09%.

Figure 2. The operation interface of the WB6000 dielectric loss tester
The normal wired method is used during the test while maintaining an indoor temperature of 292 K and relative humidity of 17%.During the measurement, a 4.5 cm diameter tin foil patch is used as the electrode, and the wire is pressed onto the interface between the tin foil electrode and the material.The experiment only tests the values at 2.5 kV.Three sets of results are tested for each sample.After taking the average, the results are shown in Figure 3. Based on the results of the dielectric loss factor test, the aging grades of the silicone rubber insulator samples can be divided into 4 levels, as shown in Table 2

Experimental study on surface micromorphology classification of sheds of composite insulators
The experiment uses a JSM-IT 100 scanning electron microscope (SEM) produced by Japan Electronics Co., Ltd.The experimental equipment is shown in Figure 4.As shown in Figure 5, the secondary electron emission images of silicone rubber samples with a magnification of 1000 can be obtained using a SED probe.The working distance of the probe during the experiment is 18 mm.From the figure, it can be seen that sample 1# has good density and no continuous defects.After aging, the structure of silicone rubber undergoes significant degradation, and the pores of sample 4# further expand.Some pores are formed due to the detachment of fillers in their original positions.New small defects are more likely to occur near large defects than defect-free areas, with defects distributed in an aggregated manner.The surface structure of sample 3# shows a cohesive network, with many protrusions and pores.Many nano fillers with larger particle sizes can be seen from 3# sample.The ferrite particles in these particles are prone to agglomeration due to magnetism, and affect the distribution of surrounding magnetic particles, resulting in a larger crystal block volume and affecting material homogeneity.It is also one of the important ingredients driving the electrochemical aging of silicone rubber [11].At a magnification of 1000, surface cracks can be observed on samples 5#, 10#, and 11#, indicating that a large number of C-C cross-linking bonds in silicone rubber have broken, resulting in an increase in free groups in the samples, and enhancing the conductivity of the material and intensifying the damage to the material of the shed under strong electric fields.

Research on the aging classification of sheds based on THz time-domain spectroscopy(THz-TDS)
A low-temperature THz-TDS facility is independently designed by the Minzu University of China to conduce the THz-TDS experiment.The testing platform and measurement system are shown in Figure 6.Among them, the center wavelength of the ultrafast laser is 800 nm, and the pulse width is 100 fs.The signal-to-noise ratio of the experimental system is better than 5000:1, and the system has a high resolution for the detection results of silicone rubber samples in the frequency range of 0.5~2.0THz.To reduce the absorbance of THz radiations by water vapor in humid air during the test, it is necessary to place the THz optical path in a sealed cover filled with dry air or nitrogen gas, the temperature in cover is controlled below the 296 K, and the humidity is less than 10%.The reference THz-TDS signal in nitrogen and the time-domain spectra of typical experimental samples at four aging grades are shown in Figure 7. Compared to the reference signal, the variations in the microscopic properties of the samples after aging result in changes in the absorption, dispersion, and other processes of THz waves in samples.After passing through the silicone rubber samples, THz waves experience a certain delay in time, and the signal's amplitude also significantly decays.sam ( ) where c is the light velocity.According to Equation (2), the absorption coefficients of samples 1~11# can be calculated.After normalizing all the curves, the absorption coefficient curves of 11 groups of samples can be obtained, as shown in Figure 8.The absorbance peak of all samples is strong at 1.32 THz, while the absorption peak at other frequencies is weak.Therefore, the absorbance peak at 1.32 THz is the main peak of silicone rubber material in the frequency range of 0.5~2.0THz.To verify the correctness of feature frequency selection, the absorption coefficient values at 1.32 THz is extracted in the normalized absorption spectrum, as shown in Figure 9.With the increase in the service life of insulators, the absorption coefficient of silicone rubber samples shows an incomplete downward trend.After aging, silicone rubber's internal molecular network structure is destroyed, and the degree of cross-linking has a certain decline.At the same time, more side groups are separated from the main chain and forming free groups, so the absorption peak of the aged samples at 1.32 THz is lower than that of the new samples.The change in the THz absorption peak of the sample also shows that the THz wave is more sensitive to the change in molecular structure.

Figure 1 .
Figure 1.The preparation process of experimental samples

Figure 3 .
Figure 3.The dielectric loss factor of 11 samples

Figure 5 .
Micromorphology of the sample surface at 1000 magnification Based on the above test results, the classification of the aging degree of 11 groups of samples observed by SEM on the surface microstructure of the material is shown in Table3.Table3.Classification of aging degree of SR samples (SEM) classification results of the above two experiments are consistent, indicating that both SEM and dielectric loss factor testing experiments can effectively reflect the aging levels of silicone rubber samples.

Figure 7 .
Figure 7.The THz reference signal and signals of samples After obtaining the THz time domain signal, the amplitude parameters

Figure 9 .
Figure 9. Absorption coefficient curve of 1~11# samples The diagnostic basis and classification of the aging levels of samples of sheds based on the THz absorbance coefficient spectrum are shown in Table 4.The classification results of the samples are consistent with the classification results of dielectric loss factor experiments and SEM experiments, indicating that THz-TDS technology is feasible for measuring the aging levels of silicone rubber.Table 4. Classification and basis for the aging degree of SR samples (THz-TDS) Aging grades The absorption coefficient at 1.32 THz Sample Number Ⅰ 0.276 and above 1#, 9# Ⅱ 0.252~0.2752#, 4#, 7#, 8# Ⅲ 0.231~0.2513#, 5#, 6# Ⅳ 0.230 and below 10#, 11#

Table 2 .
. Classification and basis of the aging degree of SR samples ( tan  )