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Insulator leakage current is the primary root cause of pole fires; therefore, it is critical to recognize outdoor insulators as potential cause of wildfires. [ABSTRACT FROM AUTHOR]

The viability of fillers for inhibiting the dry-band arcing damage on high temperature vulcanized silicone rubber under the critical AC and the equivalent DC voltages of inclined plane test is investigated. Alumina tri-hydrate and ground silica are employed in the study as common fillers employed in silicone rubber formulation for outdoor high voltage insulation, and thermal analysis is applied in order to understand the protective mechanisms of the fillers. Filler volume effect plays a role in improving the erosion performance, and the dilution effect of water of hydration is shown advantageous for preventing progressive (directed) erosion. Dilution effect promoted with internal oxidation is found only notable at 50 wt% alumina tri-hydrate in this study. On the other hand, alumina tri-hydrate at 10 or 30 wt% may yield a porous residue, which is conducive to thermo-oxidation (combustion) and consequently erosion. The erosion suppression effects reported for the fillers in the composites employed in this study are found less viable under the equivalent DC as compared to the critical AC voltages of the inclined plane test. This study clarifies common perceptions about the viability of fillers in silicone rubber for AC and DC outdoor insulation applications. [ABSTRACT FROM AUTHOR]

This paper investigates the tracking and erosion of silicone rubber and EPDM insulation in the DC inclined plane tracking and erosion test. Silicone rubber filled with alumina tri-hydrate fails due to combustion in the gas phase or erosion paths, but with no evidence of tracking. EPDM fails due to tracking and surface capturing fire, from dry-band arcing leaving a black residue while impinging the surface. Carbonaceous residue on EPDM gives rise to leakage current magnitude, whereas fused silica-based residue on silicone rubber is shown to promote combustion. A high amount of alumina tri-hydrate replacing the polymeric fuel in the composite is shown as a primary factor preventing failure and changing the nature of the residue in the DC inclined plane tracking and erosion test. Depolymerization is the main degradation mechanism reported for silicone rubber and EPDM, with notable degrees of crosslinking reported only for silicone rubber. Crosslinking reduces the amount of silicone rubber undergoing depolymerization, thus, the amount of fuel prone to combustion is reported to promote a coherent residue shield against the eroding dry-band arcing. [ABSTRACT FROM AUTHOR]

This paper discusses the state of the art in the application of self-healing silicone-based materials for outdoor high-voltage insulation. Both the dynamic behavior of the dimethyl side groups of silicone rubber and the diffusion of a bulk siloxane to maintain low surface energy are respectively reported as intrinsic mechanisms responsible for the self-healing of silicone rubber. Localization, temporality, mobility, and the type of synthesis are the aspects defining the efficiency of the self-healing ability of silicone rubber. In addition, the deterioration of the self-healing ability with filler loaded into silicone rubber insulation housing composites is discussed. Taking the self-healing property into consideration among the other properties of silicone rubber insulators, such as tracking and erosion resistance, can be a useful design practice at the material development stage. Hydrophobicity retention, recovery, and transfer measurements are discussed as useful indicators of the self-healing ability of silicone rubber. Nevertheless, there remains a need to standardize them as design tests at the material development stage. The paper is intended to shed the light on the hydrophobicity recovery, a key material design parameter in the development of silicone rubber outdoor insulating composites, similar to the tracking and erosion resistance.

Ethiopia is nowadays in urgent need of developing a modern power grid to meet the growing demand for reliable, cost-effective and environmentally friendly transmission and distribution of electric energy. The population growth, as well as development of various industries and agricultural and mining activities, increases pollution sources and yield new types of pollutants. Ensuring the safety and reliability of the network operation is thus among the top priorities for the local power companies. In addition, proper selection and dimensioning of outdoor insulation play a significant role in preventing or limiting the number of pollution induced flashovers on high voltage (HV) insulators in high voltage overhead lines, power generation plants and substations. [ABSTRACT FROM AUTHOR]

Power girds in different countries face different types of threats and in China the threats severely affecting grid reliability were in the past large power outage accidents caused by pollution flashovers. First records of pollution flashover accidents in China can be traced back to 1950s in industrial areas in Northeast and East China. Thereafter, such accidents occurred occasionally, but after 1970s they started to expand and were recorded in most of the country's provinces. The very first large area outage accident caused by pollution flashover happened in early 1974 in Northeast China around the local largest cities Shenyang and Fushun. Between 1971 and 1994, 3542 pollution-caused line trip-outs (several trip-outs on one overhead line (OHL) happening the same day is counted in China as a single event) were recorded on 35–500 kV OHLs and 1768 substation trip-outs. A total of 44 large area outage accidents caused by pollution flashovers took place during the 30 year period before 2007, as shown in Figure 1, while the outages recorded between 1981 and 2001 accounted for 43% of the total number. Among them, outages of super large character occurred every 5–6 years, e.g., in 1990 in North China and Northeast China, in 1996–1997 in East China and Central China, 2001 in North China and Northeast China [1–3]. Here we refer to the large area outage accidents as flashover events on several main transmission overhead lines and substations, appearing during one or several consequent severe meteorological events and continuing for several days. They affect several big cities within one province and stretch over an area of less than 100 thousand km2. For super large area outage accident, we refer to the accidents stretching across several provinces over the area of 200–300 thousand km2 and resulting in hundreds of pollution flashover events. [ABSTRACT FROM AUTHOR]

This paper investigates the erosion suppression effects of the water of hydration in alumina tri-hydrate and magnesium hydroxide fillers in silicone rubber during the AC and DC dry-bad arcing. Simultaneous thermogravimetric and differential thermal analyses show the hydrated filler type, level and size affecting erosion. Water of hydration in alumina tri-hydrate is shown to be released before the depolymerization of silicone rubber; whereas, the water of hydration in magnesium hydroxide is released during the depolymerization. As such, alumina tri-hydrate could promote a stronger shield of residue against dry-band arcing as compared to magnesium hydroxide, which could further suppress erosion. Increasing the hydrated filler loading raises the amount of the water of hydration released, which could dilute the silicone rubber in the gas phase and suppress combustion. However, insignificant effect on the suppression of depolymerization is obtained at higher levels of the hydrated filler, possibly due to the lack of filler-polymer interactions tethering silicone rubber chains. Internal oxidation promoted with lower sizes of alumina tri-hydrate could promote a radical-based crosslinking, leaving a stronger shield of residue against dry-band arcing. However, hydrolysis may take place and insignificant improvement is shown for the erosion performance with internal oxidation. [ABSTRACT FROM AUTHOR]

This paper investigates the residue formed on silicone rubber filled with natural silica in the inclined plane tracking and erosion test under AC and DC voltages. Simultaneous thermogravimetric and differential thermal analyses reveal residue formation dependent on volatilization and crosslinking in silicone rubber. Thermo-oxidation of volatiles yields silica-based residue protecting the surface during dry-band arcing; whereas, crosslinking leaves carbonaceous residue leading to more severe dry-band arcing. Volume fraction of natural silica replacing silicone rubber is shown to be a main source of silica residue suppressing carbonaceous residue, thus preventing failure in the inclined plane test. More severe erosion under the equivalent +DC voltages as compared to standard AC and equivalent — DC voltages of the inclined plane test is attributed to more severe dry-band arcing heat promoted by carbonaceous residue. Silicone rubber filled with lower particle sizes of natural silica is demonstrated to deteriorate erosion performance, possibly as result of filler-polymer interactions or lower particle size reducing the integrity of the protective silica residue. The following paper highlights the importance of using critical AC and DC voltages during the inclined plane tracking and erosion tests. [ABSTRACT FROM AUTHOR]

In this contribution, a mathematical model is proposed to predict the discharge inception voltage on polluted insulators under a non-uniform field of a high-voltage DC system. The present study is derived from the modified Peek's law used for predicting the threshold voltage of a typical corona discharge. An onset criterion useful for practical electrode geometries was experimentally validated by considering insulators' level of pollution along with the electro-geometrical parameters including inter-electrode distance, HV electrode radius, and width of the ground electrode. It was observed that the proposed mathematical model is much precise for smaller electrode radii and inter-electrode distances with higher conductivity of the pollution layer. Since prediction of the electrical onset on a polluted surface is of general interest for designing HV equipment suitable for heavily polluted regions in the world, it is hoped that the results presented can be viewed as a benchmark and a challenge for further research. [ABSTRACT FROM AUTHOR]

Epoxy and its composites have shown remarkable properties to be used for outdoor high voltage insulation applications. However, outdoor environment that contains UV-A radiations deteriorates the organic structure of epoxy-based insulators. To analyze the surface degradation and chemical changes in structure due to UV radiations, neat epoxy (NEPOX), 15 wt.% epoxy microcomposite (EMC 15) and 5 wt.% epoxy nanocomposite (ENC 5) were prepared and subjected to a specially fabricated UV chamber under accelerated conditions. The samples were analyzed with visual inspection, STRI hydrophobicity classification, and Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy. The analyses confirmed the pattern of gradual degradation of surface structure due to the appearance of ester and carbonyl groups due to oxidation of hydrocarbon chains. FTIR peaks of C=O stretching at ∼1725.52 increased by 157.44%, 75.86% and 33.33% for NEPOX, EMC 15 and ENC, respectively. This shows that ENC-5 expressed better resistance against surface oxidation as compared to EMC 15 and NEPOX. Addition of fillers enhanced the chain intactness and nano filler performed even better due to greater surface area and surface energy.

DC flashover performance of ice-covered composite insulators with a parallel air gap (CI/PAG) is an important technical consideration when such insulators are used to isolate ground wires for the purpose of DC ice-melting. Tests on tension and suspension types of CI/PAG were thus carried out in the artificial climate chamber to investigate their DC icing flashover performance. The influences of parallel air gap, ice thickness, pollution severity and air pressure on DC negative 50% flashover voltage (U50%) of CI/PAG were investigated. Test results show that the parallel air gap affected both the discharge path and U50%. With increasing ice thickness, U50% declined by up to 52%; this effect was more evident when the breakdown occurred in the air gap. The pollution severity affected U50% only when the flashover happened along insulator surface. With a decrease of atmospheric pressure, U50% decreased. U50% and the ratio of air pressure were in a power function relationship with a positive characteristic exponent which was relevant to the discharge path.

Silicone rubber (SR) is the most promising polymer for outdoor insulation but in neat form, it has low thermal stability and poor erosion resistance which restricts its outdoor applications. In this work, we explored the high thermal stability and excellent dielectric properties of SR co-filled with micron-AlN and nano-SiO 2 . Samples of SR composites with different mixture (micro, nano and micro/nano co-filled) are fabricated by mixing techniques of mechanical stirring followed by sonication. Test results show that there is a significant escalation in the corresponding temperature of 5%, 25% and 50% weight loss. Besides, it is noticed a significant increment in residual percentage weight of the polymer component of co-filled composites. Moreover, co-filled composites show no major variations in the real permittivity but they offer lower dielectric loss relative to AlN30m. AlNmn3 and AlNmn4 are found to be the most promising co-filled composites for high thermal stability and dielectric properties, respectively, which makes them suitable for electrical insulation purposes. [ABSTRACT FROM AUTHOR]

Hydrophobicity is the most important property of silicone rubber for outdoor isolation. Controlling wettability, along with interface and adhesion studies, has been widely investigated for technological applications, such as self-cleaning surfaces, by modifying the geometric structure and chemical composition of surfaces. Superhydrophobic surface with self-cleaning and anti-icing properties is ideal for outdoor insulation material. This paper reports on an effective strategy to modify the surface structure of silicone rubber. Microcraters with nanotexture can be fabricated on a mold steel using high-power picosecond laser incorporated with a high-speed scanning mirror. This template is used in direct replication process in which the surface morphology of silicone rubber can be changed during vulcanization. Two accessible methods are used to implement the modification process. Single-point pulse method is used to prepare regular papilla and abundant nanotexture on open architecture-type superhydrophobic surface, with contact angle reaching 151.5°±1.7°. Line scanning method is used to prepare connected papillae on enclosed-type surface, forming the secondary structure. In this case, nanotexture is richer and has a smaller scale (100 nm) compared with open architecture-type surface. Moreover, the contact angle of the enclosed-type superhydrophobic surface is 150.3°±0.9°. The contact model of water on superhydrophobic surfaces follows the Wenzel and Cassie coexisting model as shown by the air pockets, which are found experimentally and by calculation. Sliding angle of superhydrophobic surfaces reaches 5°, which indicates that this insulating surface possesses good self-cleaning properties. Repeated and large-area fabrication of superhydrophobic silicone rubber can be achieved by the proposed methods. These methods can be performed for large-scale application. [ABSTRACT FROM PUBLISHER]

The occurrence of large‐scale haze–fog weather in some developing countries brings a new threat to the safe operation of outdoor insulation. In order to study the effect of the haze–fog on the flashover performance of insulators and the flashover mechanism, kieselguhr, calcium sulphate (CaSO-4) and ammonium sulphate ((NH4)2SO4) in different particle sizes were mixed with ultrasonic fog to simulate the haze–fog environment and quantitative ac flashover experiments on the standard suspension porcelain insulator were carried out. The test results indicate that the flashover voltage is related with the pollution degree, particle component, particle size and fog‐water conductivity, and may decrease by more than 20% under haze–fog environment. The effect of salt aerosols on flashover voltage is determined by the solubility and larger than that of non‐soluble mineral aerosols or organic carbon aerosols. The flashover voltage increases with the increase of the particle size, but decreases with the increase of fog‐water conductivity. The flashover of insulator under haze–fog condition is a special kind of pollution flashover in essence. [ABSTRACT FROM AUTHOR]

Outdoor insulators are often subject to corona discharges and the problem is becoming more prevalent with the increasing use of higher transmission voltage levels. For polymeric insulators, exposure to such discharges can alter the chemical structure of basic polymer and degrade surface properties. This paper investigates the effect of micro and/or nano fillers in silicone rubber composites in suppressing such damage. Four different types of samples are fabricated: pristine silicone rubber (PR), 30wt% micron?sized silica/silicone rubber (MC), 27.5wt% micron + 2.5wt% nano silica/silicone rubber (NMC), and 5wt% nano silica/silicone rubber (NC) composites. Samples are exposed to AC corona using a needle to ground-plane electrode setup. Experimental results are analyzed based on five different measurement methods: phase-resolved partial discharge (PD), hydrophobicity loss-recovery, Scanning Electron Microscopy (SEM), Surface roughness and Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that NC shows a strong resistance to partial discharges and hydrophobicity loss. In the area below the needle tip, higher hydrophobicity loss and higher recovery are observed as compared to the vicinity region. Variations in surface roughness, appearance of crackles, voids, pits, surface splitting into blocky structures and damages to chemical structure of silicone rubber are appreciably retarded in NC as compared to PR, MC and NMC. Based on NMC results, it is found that addition of nano?sized silica can be an attractive approach to improve the corona resistance of micron?sized silica filled silicone rubber. [ABSTRACT FROM PUBLISHER]

In the following paper the development of a semiconductive coating for medium voltage outdoor insulator is presented. The coating based on silicon rubber and semiconductive fibers shows resistivity in range of 10 MΩ/□ which allowed for increasing of the insulator temperature 8 °C above the ambient after energizing with 12 kV. Outdoor testing was performed at Koeberg Insulation Pollution Test Station. The developed coating presented superior performance compared to a standard outdoor epoxy insulator. Moreover in contrary to traditionally made ceramic glazes the semiconductive coating shows positive thermal coefficient of resistance (PTCR) eliminating the risk of thermal runway. [ABSTRACT FROM PUBLISHER]

Wetting of a pollution layer by mist or light rain is inhibited, in the case of silicone rubber (SiR) insulators, by the migration of hydrophobic polymeric chains from the insulator to the layer surface. However, recent laboratory fog-chamber tests have shown that a salt/kaolin layer applied to the surface of an 11kV SiR insulator can reduce the specific creepage distance (SCD) at flashover to as low as 16mm/kV. Even for larger values of SCD, potentially damaging partial arcs can arise along the insulator surface. It has been shown that some mitigation of partial-arc activity and an increase of flashover voltage can be achieved by appropriate texturing of the SiR insulator housing. The present paper describes additional infrared (IR) recording which accompanied these previous tests. Although a reduction of the flashover voltage in polluted environments is generally surmised to be the result of the formation of dry bands in a conducting moistened surface layer, no direct observations of dry bands appear to have been previously demonstrated in the laboratory. Such observations are described here, where details of dry-band location and growth are revealed by IR recording. Dry bands are shown by close-up visual photography to be invariably bridged by small streamer/spark discharges which maintain current continuity in the pollution layer. Local surface heating by these discharges are the probable cause of the delayed rewetting of the bands. Partial-arc channels that may result in flashover develop from and across the dry-band streamers. It has become clear that clean-fog testing with infrared recording and leakage current measurements provide new possibilities for the modeling of dry band discharges and improvement of insulator design. [ABSTRACT FROM PUBLISHER]

A study about the influence of joint use of nanosilica and ATH as fillers on room temperature vulcanized silicone rubber used in the outdoor insulation is carried out. This study is made by comparison between room temperature vulcanized silicone rubber coating with nanosilica and ATH as simultaneous fillers and room temperature vulcanized silicone rubber coating with nanosilica and ATH as independents fillers. The parameters of this comparison are: erosion resistance, circulation of leakage currents under salt fog conditions and hydrophobicity loss. The results of this work show a better development of room temperature vulcanized silicone rubber coating with nanosilica and ATH as simultaneous fillers than room temperature vulcanized silicone rubber coating with nanosilica and ATH as independents fillers. [ABSTRACT FROM AUTHOR]

This paper discusses the state of the art in the application of self-healing silicone-based materials for outdoor high-voltage insulation. Both the dynamic behavior of the dimethyl side groups of silicone rubber and the diffusion of a bulk siloxane to maintain low surface energy are respectively reported as intrinsic mechanisms responsible for the self-healing of silicone rubber. Localization, temporality, mobility, and the type of synthesis are the aspects defining the efficiency of the self-healing ability of silicone rubber. In addition, the deterioration of the self-healing ability with filler loaded into silicone rubber insulation housing composites is discussed. Taking the self-healing property into consideration among the other properties of silicone rubber insulators, such as tracking and erosion resistance, can be a useful design practice at the material development stage. Hydrophobicity retention, recovery, and transfer measurements are discussed as useful indicators of the self-healing ability of silicone rubber. Nevertheless, there remains a need to standardize them as design tests at the material development stage. The paper is intended to shed the light on the hydrophobicity recovery, a key material design parameter in the development of silicone rubber outdoor insulating composites, similar to the tracking and erosion resistance. [ABSTRACT FROM AUTHOR]