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Article

Aging of Polymeric Insulators under Various Conditions and Environments: Another Look

by
Xinhan Qiao
1,2,
Yue Ming
3,
Ke Xu
4,
Ning Yi
5 and
Raji Sundararajan
6,*
1
School of Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, China
2
Sunten Electric Equipment Co., Ltd., Foshan 528300, China
3
School of Mechanical & Electrical Engineering, Xuzhou University of Technology, Xuzhou 221018, China
4
State Key Laboratory of Environmental Adaptability for Industrial Products, China National Electric Apparatus Research Institute Co., Ltd., Guangzhou 510663, China
5
State Grid Zhenjiang Power Supply Company, Zhenjiang 212000, China
6
School of Engineering Technology, Purdue University, West Lafayette, IN 47907, USA
*
Author to whom correspondence should be addressed.
Energies 2022, 15(23), 8809; https://doi.org/10.3390/en15238809
Submission received: 20 September 2022 / Revised: 10 November 2022 / Accepted: 11 November 2022 / Published: 22 November 2022
(This article belongs to the Special Issue Smart Materials and Devices for Energy Saving and Harvesting)
Browse Figures
Versions Notes

Abstract

:
Polymeric insulators have lightweight, excellent hydrophobicity and convenient transportation and installation. They are widely used in the external insulation for distribution and transmission lines. However, due to the long-term effects of pollution, ultraviolet radiation, discharge, temperature, humidity, altitude and other natural and complex environmental and service factors, the silicone rubber and other materials of polymeric insulators gradually age and lose their hydrophobicity and electrical insulation characteristics. The operability is significantly reduced, which seriously affects the safety and reliability of the power system. Hence, there is a need for assessing and evaluating the long-term aging and degradation of polymeric insulators under various operating conditions and environments. In this review, the various aging and characterization techniques of the polymeric insulators and their aging performance under the action of multiple factors are discussed. To enhance the performance of polymeric insulators, nano-coating, surface treatment and other techniques are also indicated. In addition, future potential fields that should be explored from a high-voltage electrical insulation perspective are also presented.

1. Introduction

Polymeric insulators have the advantages of being lightweight, excellent hydrophobicity and convenient for transportation and installation [1,2,3,4]. They are widely used in external insulation for distribution and transmission lines [5,6].
However, in the long term, they can fail. The numerous failures of polymeric insulators around the world drew attention to the shortcomings of the material and design and the need for diagnostic techniques to determine their state [7]. The damage and failure mechanisms of polymeric insulators differ significantly from ceramic and glass insulators.
The failures of polymeric insulators can be mainly divided into two categories. One is the mechanical fault of the insulator core rod breaking caused by typhoons and other factors, which is less likely to occur. The second is the electrical failure caused by the aging phenomenon [8,9,10], such as the decline of the hydrophobic performance of polymeric insulators. Such incidents will cause the surface flashover of polymeric insulators and a large-scale power outage.
Figure 1 shows various examples of the aging and degradation of polymeric insulators [11]. Table 1 shows a list of the various failures, such as brittle fractures that are experienced by polymeric insulators, as identified by the Electric Power Research Institute (EPRI) [7].
The aging of polymeric insulators is studied extensively [10,12,13,14]. The research on aging mainly focuses on the aging mechanisms and characterization methods. With the development of advanced material engineering technology, the performance of polymeric insulator materials can be improved in various ways to enhance their long-term use [10,15,16,17,18].
This article reviews the various aging methods of polymeric insulators and the analysis of the aging mechanisms in combination with environmental factors. The application of nano and micro fillers, nano-coating and other state-of-the-art techniques are discussed, alongside which technologies can enhance the material’s performance. For this purpose, a literature survey was conducted using the keywords polymeric insulator aging, silicone rubber insulator aging, hydrophobicity loss and aging in standard scholarly databases and Google Scholar.

2. Characterization Techniques of Electrical Performance and Aging Characteristics of Polymeric Insulators

2.1. Electrical Performance

After being in service for several years, the power operation department conducts various withstand study tests, such as a salt-fog flashover [19], a natural pollution flashover, an artificial pollution flashover [4,20,21,22,23,24,25] and a 50% lightning impulse flashover [26,27] to evaluate the degree of deterioration for polymeric insulator performance according to the flashover voltage level. Different flashover voltage tests simulate the electrical performance of insulators under different climatic and operating conditions of the system. This is the most direct method to judge the operating characteristics of polymeric insulators, but it is a destructive test. The insulator needs to be removed from the transmission line for the test, so this method has limitations.
The trap characteristic test [28] can also be used to identify the aging of polymeric insulators. According to reports, when the corona time increases, the sample’s trap energy level gradually increases and the trapped charge rises in general [29].

2.2. Physical Appearance

(1) Visual inspection.
Various obvious and serious aging problems can be identified through a manual visual inspection of the appearance of the insulators (Figure 2.), to be replaced over time [29]. However, the main disadvantage of this method is that it is difficult to identify any internal defects or early aging signs in time. Visual inspections can be conducted by a manual patrol inspection and unmanned aerial vehicle patrol inspections, which are a non-contact inspection method [30].
(2) Hydrophobicity classification (HC) test.
Hydrophobicity [31,32,33] is a crucial factor in determining the level of deterioration of silicone rubber and other polymeric materials. With an increase in aging, the hydrophobicity of polymeric insulator surface decreases or is lost completely, leading to the decrease in flashover voltages.
To comprehensively evaluate the hydrophobicity changes of the silicone rubber and other surfaces of polymeric insulators in time, the HC water spray classification method and static contact angle method can be used [32]. Table 2 and Figure 3 illustrate the HC obtained in the 345 kV EPDM insulators, in-service aged at a coastal environment for 5 years [34]. Here, the middle was the mid part of the 2.862 m insulator. The top surface of the shed was white, which was exposed to UV radiation for 5 years and, therefore, aged and discolored, while dark was the original color of the bottom surface of the shed, which was unexposed to UV radiation. Figure 3 shows how, for the same shed, the side protected from the sun is dark in color with an HC class of 2–3, compared to the other exposed (white) side, with HC 5–7.
The static and dynamic contact angle methods are the two most common techniques used to study the HC conditions. If the HC level is less than four and the static contact angle is larger than ninety degrees, the hydrophobicity of the sample surface is good. The dynamic contact angle error increases with the water drop volume [35]. Thus, the water drop velocity [36] could be a measure of the hydrophobicity change. The above are direct methods used to evaluate hydrophobicity. Other non-contact indirect methods can also be used to evaluate hydrophobicity. The non-contact methods include the DC discharge-induced acoustic wave [32] and the PD-induced electromagnetic wave [37]. These can be used to assess the hydrophobicity of polymeric insulators by recognizing the surface discharge characteristics.
(3) Mechanical properties.
The mechanical properties, such as hardness, tensile strength and elongation at break, are important parameters to characterize the aging of silicone rubber and other polymeric insulators. A Shore hardness tester can be used to characterize the hardness of silicone rubber, which increases with aging [38]. A new approach is reported in ref [39]. The findings suggest that laser-induced breakdown spectroscopy (LIBS) can be a reliable source of hardness information for polymeric materials, which is useful for ensuring the reliability of power lines. However, the tensile strength and elongation at break (tested by a tension machine with a load sensor) decrease with aging [40].
(4) Surface morphology study using scanning electron microscope.
A scanning electron microscope (SEM) [41,42] is useful to visually explore the micromorphology changes on the surfaces of polymeric insulators and identify the defects or deformation, such as roughness change, that are not easily seen by the naked eye. This helps to provide a general overview of the micromorphology properties of the aged polymeric insulator and establish whether the laboratory-accelerated aging test and the in-service aging of actual samples are comparable.
Figure 4 illustrates the SEM surface morphology changes of new, 3, 6 and 9-year-old in-service insulators [43]. Figure 5 indicates the SEM images of the surface morphologies of 345 kV EPDM insulators that were installed in 1995 and removed from service in 2000, in a coastal environment in the USA [34]. Here, bulk indicates the SEM morphology of the bulk of the material (equivalent to new/unaged). The white surface indicates the discoloration caused by UV exposure and the black surface indicates the original surface—all from the high-voltage end of the insulator.
It was also verified that the aging evaluation of silicone rubber insulators could be realized through a portable digital microscope-B011 [30], as shown in Figure 6. The unmanned-aerial-vehicle-mounted electron microscope is expected to be used to conduct the non-contact evaluation of the aging of composite insulators in transmission lines.

2.3. Chemical Performance

FTIR spectroscopy and EDX analyses [44,45,46] are the two most common methods used to analyze the material contents of polymeric insulators. The analyses of these polymeric insulators using EDX and FTIR demonstrated their superiority over other methods [47].
FTIR is useful to identify the number of typical functional groups, and the aging degree can be analyzed based on this. For silicone rubber materials used for polymeric insulator skirts, the corresponding bands of the functional groups in the absorption band diagram are shown in Table 3 [44,45,46]. The height of the absorption peak associated with each functional group is proportional to the functional group’s composition. The greater the absorbance at the absorption peak in a particular band, the greater the absorption spectrum capacity and the greater the number of functional groups that correspond to that band. The lower the absorption peak or the smaller the absorption area of Si–(CH3)2 and Si–O–Si functional groups, the more serious the molecular chain fracture and the more serious the aging degree. Similar results were also reported for 345 kV in-service aged EPDM insulators [34].
EDX analyses are used to obtain the surface element percentage of the polymeric insulators. The working principle of EDX is that the incident beam scans the surface line by line. The X-rays generated by the electron beam and material interaction are collected and classified according to the characteristic X-ray energy spectrum. Since each element has characteristic X-rays, the chemical composition of a given sample can be found according to the energy scale of the abscissa. With an increase in aging, the contents of Si and C usually decrease, while the content of oxygen increases [44,45,46].
The dielectric constant [48] is used to indicate the relative ability of a dielectric to store electrostatic energy in an electric field. It can also indicate the degree of polarization of the dielectric. The smaller the dielectric constant, the weaker the ability of the dielectric to store static electricity, which, to some extent, means the better the insulation performance. Therefore, the dielectric constant measured by the broadband dielectric spectrometer can also be used as a parameter to characterize the aging of polymeric insulators.
In addition, a variety of thermal analysis techniques, including differential scanning calorimetry (DSC) and thermal gravity analysis (TGA), were utilized in order to investigate the transformation of organic components in polymeric insulators [49]. According to the findings of the tests described in ref [49], thermal analysis techniques have the potential to significantly contribute to the performance evaluation of polymeric insulators.
To summarize, there are many methods to characterize the aging of polymeric insulators, and they can be divided into two main categories from the technical means. One is the non-contact characterization method. The insulator in operation is suitable for non-contact characterization. The other is to take samples and return them to the laboratory for testing. This method usually requires a power cut and more time and human resources. In addition, the polymeric insulator has hydrophobic recovery properties [50,51]. When the sample is returned to the laboratory, the characteristics of the polymeric could change. Therefore, the non-contact aging characterization method is preferred to evaluate the aging of polymeric insulators.

3. Aging Mechanisms and the Performance of Polymeric Insulators under Various Operating Conditions and Environments

3.1. Aging under UV, Acidic and High Field Environments

Ultraviolet is one of the dominant factors that influence the aging of polymeric insulators. Although the atmospheric ultraviolet intensity is not enough to interrupt the main chain of silicone rubber (the shortest wavelength of ultraviolet transmitted to the ground is 290 nm, and the energy is 396.3 kJ/mol), it can oxidize the methyl group of the side chain by interacting with other factors, resulting in the aging of the material surface. The Si—O bond energy in the main chain of silicone rubber is 444 kJ/mol. As shown in Figure 7, under ultraviolet light, the C–Si bond or the C–H bond on the silicone rubber surfaces can break and form free radicals. These free radicals have high energy and are prone to cross-linking reactions. Generally, oxygen in the air will react with free radicals to form hydrophilic OH and other polar groups on the silicone rubber surface, generating methane and other gases [52] and leading to long-term aging and degradation.
The other factors that cause the chemical aging of silicone rubber include acid and alkali, ozone and nitrogen oxides where the nitrogen oxides will be converted into nitric acid by moisture absorption. Previous studies have shown that the surface of the composite insulators will be seriously damaged when they are in a strong acidic environment for a long time [11,53,54,55,56,57]. The acidic substance will cause the polar, Si–O bond to break on the main chain of silicone rubber and generate the polar Si–OH bond, as shown in Figure 8. This is different from the breaking of the Si–C bond and the C–H bond on the surface of silicone rubber. The breaking of the main chain will greatly damage the silicone rubber material and the generated Si–OH bond will produce the silicone rubber hydrophilic. Table 4 shows the various compounds studied in ref [57] for enhancing the acidic stability of silicone rubber insulators.
Silicone rubber composite insulators are not only affected by natural conditions but also by high electric fields [28,58,59]. The aging effect of the electric field on silicone rubber is more severe and faster. During the operation of silicone rubber, the impact of charged particles generated by corona or arc discharge, such as the impact of ions and electrons, causes the main chain of silicone rubber to break and the molecules to depolymerize. At the same time, the energy generated by the discharge decomposes and reacts with oxygen and other gases in the surrounding air to produce highly oxidizing substances, such as active oxygen atoms, ozone and nitrogen-oxygen compounds, which can also cause the molecular chain of silicone rubber to break and damage its performance. Therefore, the electrical aging process is the most complex, often accompanied by physical and chemical aging, significantly impacting the silicone rubber’s performance [60].

3.2. Aging under Various Environments

According to the aging mechanism of polymeric insulators analyzed above, the focus was on the effects of the polymeric insulators in tropical, coastal high humidity, plateau strong ultraviolet, acid-based, electric stress, low temperature, icing and high altitude environments [53,58].
(1) Tropical environment.
To determine if insulators can withstand electrical stress beyond the prescribed limits in polluted and clean tropical regions, a field and lab investigation was conducted [61]. During the field experiment, two sets of insulators were erected and activated in coastal and inland Sri Lanka. None of the tested field SiR insulators flashed over. However, a tree-like surface discoloration was most likely caused by some discharge activity. Thus, biological growth was identified on the installed insulators. Another study [62] summarizes fieldwork. Microbiological development is unlikely to cause substantial degradation on non-ceramic insulator rubber housings, according to the collected data. Growth appears to have no effect on insulator performance.
Algae have been observed on polymeric and RTV-coated insulators in southwest and southern China. The research results showed that the algae enhanced salt dispersion and decreased insulator hydrophobicity. When algal covering was less than 20%, the effects on insulators were limited [63].
(2) Salt-fog environment.
In ref [64], salt fog was used to simulate the high humidity environment with salt in a coastal area, and it was found that the sample’s performance deteriorated after the salt-fog treatment. The salt-fog-treated samples became rough and porous under electric and thermal stress. The hydrophobic absorption peak decreased, indicating that the SR molecular chain was broken, and the filler was consumed, lowering the sample’s arc resistance. Moisture absorption affected the insulating performance and electrical strength. The physicochemical deterioration will reduce the electrical strength. Thus, in salt-fog environments, samples with higher conductivity are more deteriorated, showing that salt fog accelerates silicone rubber aging [65].
(3) Radiation environment.
Polymeric materials exposed to gamma rays in a radiation environment [59] have two impacts. First, the bond scission reduces the molecular weight by cross-linking. Second, the surface oxygenation forms oxygenated molecules. As irradiation doses increase, the transmitted energy and oxidation index lead to polymeric cross-linking. Similar conclusions have been obtained from the experiments conducted in central Saudi Arabia where the UV radiation level is high [50].
The hydrophobicity transfer was also affected by the contaminated species, pollution layer thickness, pollution level, temperature, UV radiation and corona activity. The study’s key conclusions [66] note that UV radiation can accelerate the migration of LMW molecules from the SiR bulk to the contamination layer surface and the pollution and layer thickness can increase the hydrophobicity transfer time.
(4) Electric stress.
Further, the hydrophobicity of the SiR surface worsens with increasing the corona discharge voltage and treatment time. The vertical wind can accelerate the hydrophobicity loss of SiR, while the parallel wind inhibits the hydrophobicity loss. The wind with higher speed has greater influence [67]. Thus, positive polarity has a greater deleterious effect on aging than negative polarity [68].
(5) Low temperature icing environment.
The electrical characteristic test of the aging composite insulator in a low-temperature, icing environment was conducted in ref [6]. The test environment is shown in Figure 9. Three kinds of ice were observed (Figure 10A–C), including rime ice (at below 0 °C), glazed ice (at below 0 °C) and continuous water film due to the loss of hydrophobicity (at above 0 °C).
The rime ice and the glazed ice greatly reduced the flashover voltage of the insulators. In low-temperature and dense-fog environments, the hydrophobicity of aging composite insulators is more likely to be lost, as shown in Figure 10C.
(6) High altitude.
There are a few studies on the influence of altitude (air pressure) on the aging of composite insulators, and in ref [69,70], the influence of altitude on the flashover voltage of insulators were investigated. Polymer insulators have a 48.72% larger flashover voltage gradient than porcelain and a 72.35% higher voltage than glass at 59 kPa (corresponding to 4484 m height) [70].
To summarize, the aging of silicone rubber can be divided into three main types: physical, chemical and electrical. The factors causing physical aging include ultraviolet irradiation and local high temperature. The factors causing chemical aging include acid and alkali, ozone and nitrogen oxides. The electrical aging process often accompanies physical and chemical aging. However, the aging effect on silicone rubber insulators is more serious and faster. Polymeric insulators are most prone to aging with corona discharge under high humidity, especially in salt-fog, acid-based and strong radiation environments. Therefore, in the above environments, more attention should be paid to online monitoring of polymeric insulators and improving the anti-aging performance of silicone rubber materials.

4. Enhanced Performance of Polymeric Insulators

4.1. Enhancement Methods

The nano-modified coating is considered to be an effective method to increase the long-term durability of polymeric insulators. A multifunctional nano-coating based on SiO2/PDMS/EP are proposed in ref [71]. SEM images of nano-coated SiR samples at various magnifications are shown in Figure 11. Nanoparticles and aggregates form hierarchical patterns on the covered surface.
The multiscale structure and low surface energy modification of this coating can improve water repellency. Coated silicone rubber has a large contact angle (>160) and a tiny rolling angle (around 0), enabling self-cleaning. Using this nano-coating, the electric field distortion caused by residual water droplets on the surface can be reduced, and the aging of the silicone rubber insulator caused by the corona can also be slowed. Different test samples coated with ATH and SiO2 were subjected to multi-stress aging under both DC voltage polarities [68]. Considering the combined effect of numerous aging characteristics, a hybrid composite is the most age-resistant. Furthermore, the results of ref [72] showed that the micro and nano SiR compounds had better dielectric properties than plain SiR in terms of the dielectric constant and the dielectric loss. A typical polymer nanocomposite composed of three primary constituents—the polymer matrix, the nanofiller and the interaction zone— is claimed to play a significant role in the improvement of polymer nanocomposite properties [73], as shown in Figure 12.
The surface properties of polymeric insulators can also be improved by surface treatment techniques, such as plasma. For example, an atmospheric plasma jet can rapidly increase the hydrophobicity of dirty silicone rubber. According to study findings in [74], plasma can accelerate the transfer of the hydrophobicity of silicone rubber coated in a wet pollution layer. However, the hydrophobicity recovery of contaminated silicone rubber following the plasma treatment is reduced by the high humidity environment. In contrast, it is accelerated by prolonged plasma therapy. According to the findings in [75], the crosslinking density of polymeric materials increased by 8.8 times after a modest dose of electron-beam irradiation (40 kGy) treatment. The crosslinking density of low-irradiated (40 kGy) polymeric material is still six times greater than that of nontreated polymeric material after 1000 h of aging treatment.
Additionally, in particular settings such as acidic situations, inert fillers may be used. The silicone rubber’s acid stability can be improved by the inert fillers. Barium-sulfate-loaded silicone elastomers have better acid-aging resistance [57]. The reduced mass loss in concentrated nitric acid decreased the fracture development and improved the mechanical stability with comparable erosion resistance. A comparison of the various surface treatments are shown in Table 5 [71].
The above research shows that the surface properties of the polymeric insulators can be enhanced by nano-coating, surface treatment technology and inert fillers. However, in the case of a corona caused by the electric field distortion, the effect of these methods will be greatly weakened or even ineffective. Therefore, these treatment methods to improve the surface characteristics of polymeric insulators should be combined with electric field control methods to effectively enhance the long-term use of polymeric insulators.
For example, by combining nano-modified coatings with functionally gradient materials, their performance could be enhanced. The design of functionally gradient materials [76,77,78] can be realized using a finite-element electric field simulation before fabrication. In this way, the polymeric insulator will have better surface hydrophobicity and a lower surface electric field, improving the material’s performance and reducing the effect of electrical stress.

4.2. Future Research Directions

(1) The non-contact aging characterization method is preferred for studying the aging of polymeric insulators. However, the research on non-contact characterization methods is usually based on a single method, such as the infrared thermometer, ultraviolet imager or unmanned aerial vehicle patrol inspection. These non-contact monitoring methods are greatly affected by environmental factors and are more error-prone. Therefore, in the future, online monitoring can be carried out through multi non-contact data fusion, and the change characteristics of monitoring data with environmental parameters can be studied.
(2) The surface properties of polymeric insulators can be enhanced by nano-coating, surface treatment technology and inert fillers. It will be beneficial to combine these treatment methods with electric field control techniques to effectively increase the long-term use of polymeric insulators by enhancing their surface characteristics. However, the aging mechanisms of the various environments are different, and different application environments should adopt different material optimization methods. Therefore, the anti-aging improvement methods for different environments should be studied in the future.

5. Conclusions

The aging performance of polymeric insulators under various conditions and environments is reviewed, and the following conclusions are derived.
(1) The aging characteristics of polymer insulators can be expressed as electrical, physical and chemical characteristics. There are different detection methods for different characteristics, but to facilitate the field application, a non-contact monitoring method is preferable.
(2) Polymeric insulators are most prone to aging with a corona discharge under high humidity, especially in salt-fog, acid-based and strong radiation environments. Therefore, in the above environments, more attention should be paid to online monitoring and improving their anti-aging performance.
(3) Combining surface treatment methods, such as nano-coating, surface treatment technology and inert fillers combined with electric field control methods, will help enhance the long-term performance of polymeric insulators effectively.
(4) More online monitoring should be carried out through multi non-contact data fusion, and the change characteristics of monitoring data with environmental parameters. However, different application environments should adopt different material optimization methods. Therefore, the anti-aging improvement methods for different environments should be studied in the future.

Author Contributions

Conceptualization, X.Q., Y.M., R.S., N.Y. and K.X.; methodology, X.Q. and Y.M.; investigation, N.Y. and K.X.; resources, K.X.; writing—original draft preparation, X.Q. and Y.M.; writing—review and editing, R.S.; software, X.Q.; validation, K.X.; supervision, R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to express their gratitude to external insulation research group of Chongqing University for their assistance with this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Illustrations of the damage, degradation and aging of polymeric insulators [11].
Figure 1. Illustrations of the damage, degradation and aging of polymeric insulators [11].
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Figure 2. Deteriorated insulators that can be found by visual inspection [11].
Figure 2. Deteriorated insulators that can be found by visual inspection [11].
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Figure 3. The hydrophobicity classification of the white and dark surfaces.
Figure 3. The hydrophobicity classification of the white and dark surfaces.
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Figure 4. SEM images of the surface morphology changes of a (a) new (unaged) sample, (b) 3 years, (c) 6 years and (d) 9 years of in-service polymeric insulators [43].
Figure 4. SEM images of the surface morphology changes of a (a) new (unaged) sample, (b) 3 years, (c) 6 years and (d) 9 years of in-service polymeric insulators [43].
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Figure 5. SEM images of the surface morphology changes of 345 kV EPDM insulators removed from service after 5 years in a coastal environment [34].
Figure 5. SEM images of the surface morphology changes of 345 kV EPDM insulators removed from service after 5 years in a coastal environment [34].
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Figure 6. Characterization of the aging surface seen through the portable digital microscope-B011 [30].
Figure 6. Characterization of the aging surface seen through the portable digital microscope-B011 [30].
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Figure 7. Chemical reactions in silicone rubber under UV irradiation [52].
Figure 7. Chemical reactions in silicone rubber under UV irradiation [52].
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Figure 8. The formation of Si–OH bonds due to the reaction between the acid and silicone rubber [52].
Figure 8. The formation of Si–OH bonds due to the reaction between the acid and silicone rubber [52].
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Figure 9. Iced test for transmission lines and insulators [6].
Figure 9. Iced test for transmission lines and insulators [6].
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Figure 10. Iced insulators with various types of ice coverage, such as (A) rime rice (at below 0 °C), (B) glazed-ice (at below 0 °C) and (C) continuous water film due to loss of hydrophobicity (at above 0 °C) [6].
Figure 10. Iced insulators with various types of ice coverage, such as (A) rime rice (at below 0 °C), (B) glazed-ice (at below 0 °C) and (C) continuous water film due to loss of hydrophobicity (at above 0 °C) [6].
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Figure 11. SEM images of nano-coated SiR samples at various magnifications (1–3) and the cross-sectional morphology [71].
Figure 11. SEM images of nano-coated SiR samples at various magnifications (1–3) and the cross-sectional morphology [71].
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Figure 12. Simple diagram demonstrating the components of polymer nanocomposites [73].
Figure 12. Simple diagram demonstrating the components of polymer nanocomposites [73].
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Table
Table 1. List of the various types of failures of polymeric insulators identified by EPRI [7].
Table 1. List of the various types of failures of polymeric insulators identified by EPRI [7].
Failure Type% of Failure Recorded (221)
Brittle fracture51.1%
Flashunder24.9%
Mechanical failure: rod failure10.4%
Rod destruction by discharge activity8.1%
End fitting pullout0.5%
Table
Table 2. HC of the 345 kV EPDM insulator [34].
Table 2. HC of the 345 kV EPDM insulator [34].
WeathershedSurfaceHC
High voltage endWhite (aged/discolored)
Dark		HC5
HC2–3
MiddleWhite (aged/discolored)
Dark		HC4
HC2
Low voltage endWhite (aged/discolored)
Dark		HC5
HC3
Table
Table 3. Characteristic peaks of silicon rubber in IR analysis.
Table 3. Characteristic peaks of silicon rubber in IR analysis.
Characteristic GroupWave Number/cm−1
O–H3700–3200
CH3(C–H)2960
C–H1440–1410
Si–CH3(C–H)1270–1255
Si–O–Si(Si–O)1100–1000
O–Si(CH3)2–O(Si–O)840–790
Si(CH3)3800–700
Table
Table 4. Silicone rubber compounds tested for enhancing acid stability [57].
Table 4. Silicone rubber compounds tested for enhancing acid stability [57].
Silicone Rubber CompoundsATH
Parts Per Hundred Rubber/Weight%		Inert
Parts Per Hundred Rubber/Weight%
52 wt% untreated ATH110/52-
52 wt% precoated ATH110/52-
52 wt% insitu coated ATH vinyl silane110/52-
ATH + 25 wt% SiO255/2555/25
ATH + 9 wt% BaSO4100/920/9
ATH + 13.5 wt% BaSO490/13.530/13.5
ATH + 18 wt% BaSO480/1840/18
ATH + 36 wt% BaSO440/3680/36
54 wt% BaSO40/0120/54
Table
Table 5. Comparison of the various surface treatments [71].
Table 5. Comparison of the various surface treatments [71].
TreatmentContact Angle, oSelf-CleaningSurface Charge
Optimization		Flashover
Improvement
PDMS + ZnO coating162YesYes16.7% increase in dry flashover
Liquid silicone rubber + SiO2 coating161.8Not mentionedNot mentioned10.5% increase in wet flashover
DBD plasma treatments~100YesYesNot mentioned
Raisin-based primer + SiO2 topcoat161Not mentionedNot mentioned29% increase in
pollution flashover
PDMS + ZnO + MWCNT
coating		152YesYes28.8% increase in dry flashover
Pico second laser-ablated
template + PDMS/SiO2 coating		150.3YesYesNot mentioned
Al2O3 + CNT + Polyamide mesh160YesYes30% increase in dry flashover
EP + PDMS + grafted SiO2162YesYes60% increase in wet flashover
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Qiao, X.; Ming, Y.; Xu, K.; Yi, N.; Sundararajan, R. Aging of Polymeric Insulators under Various Conditions and Environments: Another Look. Energies 2022, 15, 8809. https://doi.org/10.3390/en15238809

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Qiao X, Ming Y, Xu K, Yi N, Sundararajan R. Aging of Polymeric Insulators under Various Conditions and Environments: Another Look. Energies. 2022; 15(23):8809. https://doi.org/10.3390/en15238809

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Qiao, Xinhan, Yue Ming, Ke Xu, Ning Yi, and Raji Sundararajan. 2022. "Aging of Polymeric Insulators under Various Conditions and Environments: Another Look" Energies 15, no. 23: 8809. https://doi.org/10.3390/en15238809

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