4.2. Aging Characteristic Detection and Analysis
4.2.1. Tested Samples
This section mainly selects insulators with different operating years coated with RTV in Qinghai Province and carries out hardness tests, water repellency inspection, and other test items on them, so as to record the deterioration characteristics of RTV coatings in Qinghai with different operating years and ultimately obtain the characteristics of the aging degree of different RTV coatings.
To asses coatings using the RTV anti-pollution flashover coating insulator test project, you can refer to the national standard stipulated in the power industry standards on the performance of silicone rubber materials. The specific provisions of the project are shown in
Table 2.
4.2.2. Hardness Testing
Aging silicone rubber exhibits hardening and a loss of flexibility. Therefore, hardness is often used as an indicator to reflect the aging state of silicone rubber. The hardness of silicone rubber is generally characterized using the Shore hardness method. For silicone rubber materials, a Shore A durometer is typically used for measurement.
The hardness test in this study utilizes the TH200 Shore A hardness tester, with technical specifications of SR44. The equipment is shown below in
Figure 4.
Field insulators operate in complex and unpredictable environments, with each insulator exposed to different natural factors. To refine the assessment of insulator aging and provide more precise aging evaluation recommendations, this experiment considers variations in aging across different positions on the same insulator skirt. This includes differences in aging between various orientations on the same skirt, proximity to or distance from the insulator center, as well as comparisons between the upper and lower surfaces and the internal structure of the skirt. Based on this, each insulator skirt is divided into four equal sections. The section directly below the insulator nameplate is designated as 1, with the remaining sections numbered sequentially in a clockwise direction as 1, 2, 3, and 4, as shown in
Figure 5.
The Shore A hardness test results for the upper surface of insulator #1 are shown in
Table 3.
The results of the hardness test on the positive upper surface of insulator #1 show that the hardness value varies in different locations, but the overall upper surface hardness is high, with an average hardness value between 93.6 and 97.0.
The bottom surface of the insulator was then divided into the inner and outer lower surfaces. The hardness test results for insulator #1 on the inner and outer lower surfaces are shown in
Table 4.
The hardness test results for the outer and inner lower surfaces show that the outer hardness values are slightly lower than the inner ones, with average hardness values ranging from 90.6 to 92.5, while the inner hardness values range from 93.7 to 94.4.
Combined with the graphical analysis, it can be seen that the overall hardness distribution of the insulator is relatively uniform. However, the hardness of the upper surface is relatively large because the upper surface has been subjected to long-term erosion by wind and sun, resulting in serious wear and aging of the surface coating.
The results of the hardness tests show that UV radiation does induce photochemical reactions in silicone rubber, and these reactions lead to the shedding of methyl groups, exposing the polarity of the silicone–oxygen backbone, which triggers further cross-linking and oxidation reactions, ultimately leading to an increase in the polarity of the surface of the material and a decrease in its hydrophobicity.
4.2.3. Boiling Test
The boiling test is mainly used to detect the damage of composite insulators due to water vapor erosion. Using a water-boiling test can accelerate the aging of the insulator’s hidden defects and find defects that are not easily noticed in the daily operation of the insulator.
According to the standard JB/T 5892-1991 on the composite insulator boiling test, part of the RTV insulator specimens was placed in water containing 0.1% NaCl solution in the boiling box to ensure that the composite insulator was submerged in a depth of no less than 10 cm, heated to the boiling point temperature of the water solution, and boiling was maintained for 42 h; the water temperature naturally cooled to 50 °C, the specimen was removed from the water and washed, and an appearance check was immediately carried out on the specimen, respectively, to see whether the RTV coating on the insulator’s appearance showed peeling, cracking, or a peeling off phenomenon.
After the completion of the boiling test, the umbrella skirt was inspected and compared with its appearance before boiling, and the following was observed:
- (1)
The phenomenon of surface and edge shedding was significantly aggravated, and insulators whose surfaces were not shed or slightly shed before boiling also shed more obviously after boiling, accompanied by a peeling phenomenon at the shedding place.
- (2)
The insulators that appeared to be chalked before boiling were boiled, the upper surface coating was basically completely detached, the lower surface was also detached in many large areas, and the insulator material began to become brittle and hard.
- (3)
The surface coating of some insulator specimens showed irregular bumps of different degrees, and there were differences in the aging degree of the insulator specimens.
Statistical analysis of the results revealed that the aged RTV insulators showed significant aging characteristics compared to brand new insulators in high-temperature and humid environments, which mainly included a decrease in the water repellency grade of the RTV coating surface by 2–3 grades, a decrease in the relative dielectric constant, and an increase in the surface hardness by 15%. It was also found that microcracks and surface roughening occurred in the aged RTV coatings, leading to a significant decrease in the water-repellent recovery and water-repellent migration properties, which affects the insulator’s ability to resist fouling flash in complex environments. Further analysis showed that the combined effect of high temperature and high humidity exacerbated the molecular chain breakage and structural deterioration of the RTV material, especially after continuous boiling, and the proportion of silica–oxygen bond breakage within the coating increased significantly, showing a decrease in chemical stability.
The results of the water-boiling experiments show that the aging phenomenon of RTV insulators in a high-temperature and humid environment is more serious, and further research and improvement of the material formula or coating process are needed to improve the durability and reliability of the insulators.
4.2.4. Hydrophobicity Test
Insulator hydrophobicity is one of the important indexes to detect insulator operation status. The so-called hydrophobicity refers to an insulator surface being difficult to moisten; if the adsorption of water in the form of discontinuous isolated small water droplets exists, the insulator does not form a continuous water film, thus limiting the surface leakage current, improving the flashover voltage. Water-repellent migration refers to the silicone rubber surface being stained; silicone rubber itself can migrate to the surface of the dirt layer of water-repellent material so that the surface of the dirt layer also has a water-repellent.
In this paper, the hydrophobicity of the insulator surface was studied using the HC method, which has more applications and relatively good differentiation, and the measurement of the static contact angle of silicone rubber material in two directions was conducted.
- (a)
HC method
The test piece was hung vertically during the experiment, and a sprayer that can produce tiny mist particles was used at a distance of 25 cm from the surface of the test piece. Roughly twice-per-second pressure spraying on the surface of the composite insulator was conducted, with a duration of 20~30 s; 10 s after the completion of the spray, the water droplet contact angle condition was observed using the umbrella group surface water-repellent level of discrimination.
Referring to the standard “DL/T 1474-2015 Measurement Method of Hydrophobicity of Composite Insulators for AC and DC Systems with Nominal Voltage Higher than 1000 V”, the hydrophobicity of the insulator surface was divided into 7 levels, which are expressed as HC1-HC7; the larger the level’s number is, the worse the hydrophobicity is, among which the HC1 level represents the most hydrophobic surface and the HC7 level represents a completely hydrophilic surface. It is generally believed that HC1 and HC2 levels of materials have good hydrophobicity; HC3 level materials show surface aging; HC4 and HC5 material surfaces show more serious aging; HC6 and HC7 levels of material surfaces show complete aging. The specific grading criteria are shown in
Table 5 and
Figure 6 below.
- (b)
Static contact angle
To measure the static contact angle, a small amount of deionized water was dropped onto the surface of the silicone rubber using a micro-sampler, with the angle between the tangent plane of the droplet and the solid plane at the triple line as the contact angle, as shown in the diagrams θ1 and θ2. Generally speaking, a static contact angle of more than 90° is considered to be hydrophobic, while one of less than 90° is considered to be hydrophilic [
16].
An example of the measured contact angle is shown below in
Figure 7.
The hydrophobicity test results for the upper surfaces of Specimen #1 and Specimen #2 are shown in
Table 6 below.
The hydrophobicity test results for the upper surfaces of Specimen #1 and Specimen #2 indicate that Specimen #1 primarily falls within the HC2 and HC3 classifications, while Specimen #2 ranges from HC2 to HC4. This suggests that the upper surface of Specimen #1 generally exhibits better hydrophobic performance than that of Specimen #2.
The hydrophobicity measurement results corresponding to regions 1, 2, 3, and 4 on the lower surface directly beneath the upper surfaces of Specimen #1 and Specimen #2 are presented in
Table 7.
The hydrophobicity test results for the lower surface of Specimen #1 primarily fall within the HC3 classification, while those for Specimen #2 range from HC1 to HC3. This indicates that Specimen #2 exhibits a more favorable hydrophobic distribution in the lower upper surface region, whereas Specimen #1 shows relatively poorer performance.
The hydrophobicity test results for the corresponding lower surface regions 1, 2, 3, and 4 of Specimen #1 and Specimen #2 are presented in
Table 8.
The hydrophobicity test results for the lower surfaces of Specimen #1 and Specimen #2 are generally superior to those of their upper surfaces, with a more uniform distribution. Specimen #1 primarily falls within the HC2 and HC3 classifications, while Specimen #2 ranges between HC1 and HC2, demonstrating better hydrophobic performance. The results of the hydrophobicity measurements are shown in
Figure 8.
The insulator strings are tension-resistant strings, in which the area of #1 has been subjected to sun exposure and rain erosion for a long period of time, resulting in poorer water repellency. This is related to the way the insulators are hung: area #1 is located directly above the insulators when they are hung, which is more likely to be affected by environmental factors; area #3 is located directly below the insulators when they are hung, which is less exposed to the sun and therefore has relatively better water repellency; the lower surface is less exposed to the sun and rain and is able to maintain better water repellency, and the overall distribution of water repellency is more uniform, which is superior to that of the upper surface.
The hydrophobicity test results of Specimen #1 and Specimen #2 show that the hydrophobicity of the lower surface is generally better than that of the upper surface. Specimen #2 has a better distribution of hydrophobicity in the upper surface area directly underneath; while area #1 has a poorer hydrophobicity due to long-term exposure to the sun and rain erosion, and this area requires special attention and maintenance in practical applications.
4.2.5. Hydrophobicity Migration Test
The excellent pollution flashover resistance of insulators is closely related to the superior hydrophobicity migration characteristics of silicone rubber materials. Hydrophobicity migration enables contaminated insulators to maintain a high flashover voltage, effectively suppressing pollution flashover incidents. Therefore, the hydrophobicity migration of insulators serves as a crucial indicator for assessing the condition and aging of insulators, making it a subject of significant research interest.
To investigate the hydrophobic recovery and migration properties of RTV-coated insulators, eight clean specimens were immersed in a container filled with deionized water at room temperature (25 °C) for 96 h. After immersion, the specimens were removed, and any remaining moisture was absorbed using filter paper. The hydrophobic recovery characteristics of the insulators were measured after 24 and 48 h. A contamination solution was prepared by mixing an appropriate amount of diatomaceous earth, sodium chloride, and deionized water, achieving a salt density of 0.1 mg/cm
2 and an ash density of 0.5 mg/cm
2. The solid coating method was used to apply the contamination solution evenly onto the RTV-coated surface. The measurement results are presented in
Table 9.
From the table, it can be observed that most insulators initially exhibited high hydrophobicity. However, after immersion in deionized water and exposure to the contamination solution, both the hydrophobic recovery (at 24 h and 48 h) and hydrophobic migration results generally declined. Different insulators exhibit a partial recovery of hydrophobicity over time, but the extent of hydrophobic recovery (at 24 h and 48 h) and hydrophobic migration varies. Notably, RTV-coated insulators that have been in operation for over 10 years fail to meet the required hydrophobic migration standards.
In summary, the hydrophobic performance of RTV-coated insulators deteriorates under aging and environmental factors. Therefore, further research and improvements in RTV insulator materials and coating technologies are necessary to enhance their hydrophobic performance and long-term stability in harsh environments.