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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]

The design principles and prototype manufacture of 11kV silicone-rubber composite textured insulators are described. The moulded surface texture increases the maximum creepage length from 375 mm to 503 mm for the four-shed profile used. The trunk sections of the insulator are patterned with a square-section array of hemispherical protuberances, and the shed undersides with interlaced logarithmic spirals of hemispherical ridges. Both textures are sufficiently shallow to facilitate fabrication, to allow effective washing and to avoid an increased salt deposit density (SDD) in artificial pollution tests. The test procedures which were developed in Part 1 of the paper are applied here to the textured prototypes. These procedures had been shown to quantify reliably the performance of conventional non-textured insulators of the same profile: the variation of flashover voltage and leakage current due to the silicone-rubber formulation, SDD and wetting rate could be measured. For textured insulators, it is found that the increased creepage distance improves the flashover voltage by up to 27%, with a corresponding reduction of pre-flashover leakage current. Comparative tests for two silicone-rubber formulations of different hydrophobicity and pollution performance were performed. Although very severe pollution causes a large reduction in flashover voltage, surface texturing mitigates this reduction effectively. This is particularly relevant to recent concerns expressed in IEC-TS 60815-3 (2008) relating to the selection and dimensioning of high-voltage a.c. polymeric insulators for use in heavily polluted conditions. [ABSTRACT FROM AUTHOR]

The development of polymeric materials for outdoor insulation has been much aided by existing standard laboratory tests such as inclined-plane procedures. The position is less satisfactory in respect of the optimization of insulator profiles and the choice of the minimum unified specific creepage distance (USCD) for a given site pollution severity (SPS). Standard tests currently under revision are stated to be 'not directly applicable to polymeric insulators'. Such tests are, therefore, urgently needed to discriminate the pollution performance of silicone-rubber insulators of different housing materials and profiles, to quantify the inverse relationship of pollution severity and flashover voltage and to assess the influence of wetting rate. A suitable test strategy would also avoid housing damage from excessive flashovers. A new test procedure is presented here to achieve these objectives. This is not confined to flashover voltage measurements. A high-resolution data acquisition system is used to monitor and store the waveforms of the applied test voltage and leakage current throughout tests on 11kV silicone-rubber insulators. This enhancement provides a further basis for the quantitative comparison of insulators in artificial pollution testing, because the results corroborate the inverse relationship between flashover voltage and pre-flashover leakage current. Part 2 demonstrates the pollution performance of 11kV silicone-rubber insulators when surface texturing is used to increase USCD values. [ABSTRACT FROM AUTHOR]

Outdoor insulators are exposed to a wide range of conditions, leading to build-up of pollution from the air by dry deposition. Following rain, most insulator surfaces are cleared of preexisting deposit, but the electrical conductivity of the rain itself may lead to wet flashover problems. Insulator standards and application guides suggest appropriate metal-to-metal dry-arc and leakage distances along the insulator profile to ensure that the normal ac line voltage does not follow a conductive path, across the surfaces or through a heavy cascade of water drops, from line to ground. Ice and snow accretion are special cases of exposure to pollution. There is usually no significant difference between the electrical conductivity of various forms of precipitation so that the same issues of wet flashover occur when icicles bridge the sheds of an insulator. Freezing will reduce the conductivity of a water jet by a factor of 100 or more, but the process will also reject all the impurities to the ice surface. A thin ice layer can also stabilize preexisting surface pollution, acting as an effective and thick nonsoluble deposit, and thus depressing flashover strength across insulator leakage distances. [ABSTRACT FROM PUBLISHER]

A review of the literature on nanotechnology for the electrical power sector shows considerable hype in the technology, but the actual usage of nanomaterials in the sector is far below other industries. As an example, the automotive industry has made great strides in using nanomaterials because of their lower weight and higher physical strength. There are several reasons for this, the obvious one being the quantity of nanodielectrics is relatively small compared to nanomaterials in other industries. The second reason is that although there are clear improvements in some dielectric properties, for example resistance to partial discharges, the improvements in other properties are not so significant, or sometimes not even evident. Uniform dispersion of nano-sized inorganic fillers in nanodielectrics during manufacture is critical for repeatable electrical properties improvements, and this has been a problem in some of the reported properties such as breakdown strength. This comes about due to the fact that nanoparticles agglomerate easily because of the high surface energy, and conventional mixing techniques do not break apart the nanoparticle agglomerates. Another problem is the incompatibility of hydrophilic nanoparticles with hydrophobic polymers, such as silicone rubber, which results in poor interfacial interactions. [ABSTRACT FROM PUBLISHER]

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This paper is aimed at the determination of the characteristics of discharge propagating over discontinuously polluted insulators under bi-exponential waves simulating switching impulse voltages, using a predictive dynamic model based on an equivalent electrical network. This model enables to compute the current, electrical charge, instantaneous propagation velocity and channel radius of the discharge, and the flashover voltage for different distributions and conductivities of deposits. The simulated results are first validated with experimental data obtained with an electrolytic model representing a surface with a deposit uniformly distributed and then extended to discontinuously polluted surface. [ABSTRACT FROM AUTHOR]

In this study a two kinds of silicone/SiO2 nanocomposites were tested with respect to their resistance to high-voltage arcing, the resistance to tracking and erosion and their TGA performance. The silicones with filler type F1 show a high improvement of the resistance to high-voltage arcing and the resistance to tracking and erosion at very low filler contents. Furthermore, a maximum resistance to high-voltage arcing is observed for these nanocomposites at a filler content of 5 % wt. In contrast to that the silicones with filler type F2 show no significant improvement for both resistances. The effects are explained by the ratio of interphase to matrix material, which is estimated by the Interphase Volume Model. The thermogravimetric analysis (TGA) shows that the resistance to thermal degradation is significantly higher for the silicones with filler type F1 already at a filler content of 0.5 wt. % compared to the material without any fillers. The resistance to thermal degradation for the materials with filler type F2 is significantly lower. This result is also explained by the high interphase content. [ABSTRACT FROM AUTHOR]

The paper discusses the mechanism by which nanofillers improve the erosion resistance of silicone rubber nanocomposites under simulated conditions of dry band arcing. Various micro-nano composites are studied, including microfiller, nanofiller, combinations of both, and with a commercial surfactant to improve the dispersion of the nanofiller. The thermal conductivity of the composites, measured using a standard method, shows higher thermal conductivity when the nanofiller is well dispersed. A thermal model is developed and solved with finite element method (FEM) to examine the temperature profile in the modelled nanocomposites under laser heating, which simulates the hot spot from dry band arcing. The hot spot is compared to the measured temperature profile by an infrared camera. SEM, EDAX, and XRD techniques are used to analyze the thermally decomposed silicone residue after laser heating to elucidate the protective mechanism in the silicone rubber nanocomposites. [ABSTRACT FROM AUTHOR]

This paper presents a review discussing methods of determination the resistance to corona discharges and ozone of polymeric materials for applications in outdoor insulation. It also describes a part of international activities within CIGRE WG D1.01 aiming to develop suitable methodology for the testing, which includes designing the necessary test arrangement, defining test conditions and finally exemplifying results of the treatment by monitoring changes of electrical, mechanical and structural properties of some materials. [ABSTRACT FROM AUTHOR]

The arc characteristics on contaminated insulator surfaces covered with ice were investigated. The arc constants (A, n), as well as the ac re-ignition parameters (K, b) were determined using a simplified cylindrical physical model. Based on identification methods, a new approach for calculating the residual voltage is introduced. These results will serve to extend the previous mathematical model developed at CIGELE for its application to predicting the flashover voltage of contaminated insulators covered with ice. The model was successfully applied to a five-unit IEEE standard insulator string, and the calculated results were in good agreement with the experiments. [ABSTRACT FROM AUTHOR]

The established model of the flashover of an insulator, whose surface is covered with a moist, conducting layer of pollution, envisages that it develops from the formation and bridging of a dry band by a partial-arc pre-discharge. This pre-discharge is assumed to possess arc characteristics, so that the electric field across the dry band will fall as the partial arc current increases. Observations of insulators which are only lightly polluted, however, show that pre-discharges are of low luminosity and rich in ultraviolet, and resemble the spark leader inception and extension sequence in air gaps. This is because the current limitation by low values of surface-layer conductance can inhibit the transition to an arc at the pre-discharge stage. An alternative model using simplified voltage-current equations is developed to represent these spark properties rather than an arc characteristic. The predictions of the partial-arc model and this new approach are both tested here, using published experimental data from light-pollution fog tests in the authors' laboratory. For this purpose, the partial-arc theory is further developed in order to reveal its implied prediction for the increase of pre-discharge length with applied voltage before flashover. A corresponding relationship is obtained for the increase of spark leader length using the new model. It is found that for light pollution, the dry-band spark model better represents the test data than the partial-arc model. Consequent predictions are made for the variation of flashover voltage with pollution severity in this regime, which may account for anomalous insulation failures. [ABSTRACT FROM AUTHOR]

This paper explores how the addition of inorganic nanofiller and microfiller to silicone rubber can impart resistance to erosion for overhead insulation applications. Various micro-nano composites were prepared for this study, namely microfiller, nanofiller, and the combination of microfiller and nanofiller with a commercial surfactant to improve the dispersion of the nanofiller. The effect of the surfactant on the surface wettability of the composites was analyzed with contact angle measurements. Salt-fog, inclined plane and laser ablation tests were conducted to evaluate the erosion resistance of the micro-nano filled composites. The results in all three tests showed that the combination of microfiller and nanofiller with surfactant resulted in composites with improved erosion resistance to dry band arcing, with the exception of the case where calcinated filler was used in the formulation. [ABSTRACT FROM AUTHOR]

The studies have been presented on spatial and temporal distributions of surface charge on samples of ethylene propylene diene monomer (EPDM) and silicon rubber (SIR). The charge was deposited from a positive impulse corona source located at 1 mm above the polymeric surfaces. During the experiments, the number of pulses and their voltage level were varied. It was observed that the spread of the charge over the surface of the samples as well as its magnitude were both increasing with increasing number of impulses applied. EPDM accumulated charge easier and had a longer relaxation time than SIR. A common feature frequently observed in both cases was that the resulting charge distributions had a saddle-like pattern. Performed computer simulations showed that this feature can be attributed to peculiarities of the corona charging in the considered system, which took place in a form of a burst corona, and the dynamics of the deposited charges could be related to burst corona pulses. [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]

A study of the loss of hydrophobicity of high temperature vulcanized (HTV) silicone rubber (SIR) after immersion in saline solutions as a function of time (0 to 3000 h) at different temperatures is reported. The hydrophobicity is determined by measuring the static contact 0 between the tangent to the droplet of distilled water and the horizontal surface of the HTV SIR. The conductivity of the saline solution was varied from 0.005 to 100 mS/cm and the temperature from 0 to 98 °C. The contact angle which gives a measure of the hydrophobicity of the surface decreased sharply during immersion. The contact angle decreased from the original value of 100±4.6° to low value of 15° after 3000 h of immersion at 98 °C in a solution of 0.005 mS/cm. The lower the conductivity, the larger was the decrease in the contact angle. The changes in the weight of the HTV SIR due to the uptake of water and the average surface roughness (ASR) are determined during the immersion. The increase/decrease in the weight and surface roughness of the HTV SIR are correlated with the contact angle and hence with the loss of hydrophobicity. The surface free energies γsd, γsh and γs, the interfacial energy of HTV SIR and water γsl, and the energy of adhesion Wsl of water on HTV SIR are determined as a function of time during the immersion and related to the loss of hydrophobicity. [ABSTRACT FROM AUTHOR]

Electric discharge between the tip of an icicle and a plane electrode has been studied because of its relevance to the flashover of high voltage ice-covered insulators. A photomultiplier was used to scan the high voltage rod simultaneously with current measurement technique. The basic properties of streamers, namely the onset voltage, leakage current, mean propagation velocity and breakdown voltage, have been measured and compared with propagation from a metal rod as a reference. The effects of several experimental parameters such as freezing water conductivity, surrounding temperature and air gap length on the discharge characteristics have been examined. Possible mechanisms, which control discharges from ice points, have also been discussed and empirical relationships to predict breakdown voltage have been derived. [ABSTRACT FROM AUTHOR]

This paper presents the results of fundamental investigations on the inception and propagation of corona discharge on an ice surface stressed with a standard lightning impulse voltage. High-speed photography and photomultiplier techniques were used to observe and record the propagation of the streamers. The effects of several experimental parameters such as freezing water conductivity and HV rod electrode radius on the streamer inception parameters were investigated. Moreover, time to first streamer, inception voltage and corresponding field, as well as streamer propagation velocity and charge deposited by a streamer on ice surface were measured. The results are discussed and emphases are laid on the main factors influencing the development of positive streamers on ice surface. [ABSTRACT FROM AUTHOR]

A mathematical model for predicting the ac flashover voltage of snow-covered insulators is presented. The arc constant parameters in air gaps and inside snow, as well as the arc reignition condition are determined using a cylindrical model. The effects of the arc length on the arc constants parameters are also investigated. The model is then applied to an EPDM insulator artificially covered with natural snow. There is good concordance between the flashover results determined from the mathematical model and those obtained experimentally. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the effect of insulator dimensions as well as applied water conductivity on flashover voltage under icing conditions, using a dynamic arc model. The electrical performance of several insulators with various diameters and dry arcing distances were evaluated using this model. Moreover, in order to assess the influence of meteorological variables, different applied water conductivities were considered for the laboratory experiments. The model was found to be in good agreement with the experimental results. The results obtained indicate that flashover prediction is possible under ac voltage, using the proposed model. This contribution is also intended to provide a fresh perspective by contributing an innovative approach to this domain of research. [ABSTRACT FROM AUTHOR]

A method is introduced for testing insulators under icing conditions. The method is based on and developed from those described in the standard of IEC60-1, IEC 60507 and IEEE Task Force on Insulator Icing Test Methods, which give instructions to test insulators under pollution. Then, the paper explores the effects of various factors, including ice thickness, pollution severity on the surface of insulators before ice accretion, atmospheric pressure, and shed profiles, on the flashover performance of short samples of two different types of silicone rubber (SIR) composite long rod insulators intended for ultra high voltage (UHV) ac transmission lines. The experiments were carried out in the multi-function artificial climate chamber in the High Voltage and Insulation Technological Laboratory of Chongqing University, China. The experimental results reveal the regularity of the effects of ice thickness, pollution severity and atmospheric pressure on the average flashover voltage of two types of composite insulators. Those can provide reference for the outdoor insulation design for UHV ac transmission lines. [ABSTRACT FROM AUTHOR]

Polymer insulators made of silicone rubber (SIRs) can suffer from algal fouling in tropical and even temperate regions. The purpose of the present study was to understand the hydrophobic behavior and leakage current characteristics of algal- fouled SIR materials when exposed to wet and contaminated conditions. Salt-fog tests were conducted on SIR samples colonized with the treatable alga Chlorella vulgaris. SIR samples covered with non-soluble kaolin powder at a deposition density comparable to that for the alga-treated SIRs were also evaluated. Results show that algal fouling considerably reduced the SIR hydrophobicity compared to the kaolin powder, which resulted in the development of a large leakage current. A major component of the leakage current produced on algal-fouled SIR is dry-band discharge, which leads to material degradation involving tracking and erosion. [ABSTRACT FROM AUTHOR]

Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided. [ABSTRACT FROM AUTHOR]

The present study evaluated the leakage current suppression and the resistance to tracking and erosion of high-temperature vulcanized silicone rubbers (HTV-SIRs) containing different levels of silicone plasticizer. Slab HTV-SIRs unfilled and filled with alumina trihydrate (ATH, 50 wt %) and with added linear silicone plasticizer (0-6.0 wt %), which was terminated with hydroxyl groups and of which repetition number was 13-18, were tested. Evaluation using the IEC 60587 inclined-plane method indicated that the plasticizer had no influence on the tracking and erosion behavior. Salt-fog test results indicated that a higher level of plasticizer in the ATH-filled HTV-SIR showed smaller leakage current. Gas chromatography and mass spectrometry suggested that the enhanced suppression of the leakage current for ATH-filled HTV-SIR can be attributed to migration of the linear silicone plasticizer onto the surface contaminants. It was concluded that the addition of silicone plasticizer is useful for improving the leakage current suppression ability of HTV-SIR without reducing its tracking and erosion resistance in the above laboratory tests. [ABSTRACT FROM AUTHOR]

A salt fog test and an inclined plane test have been combined in order to trace changes in electrical and mechanical properties as well as in chemical stability of high temperature cured silicone rubber (HTV) samples containing interfaces. The interfaces were created in two different ways. The first technique, named adding/curing, was based on pressing together an uncured HTV silicone plate towards a piece of already cured one, followed by curing at atmospheric pressure and high temperature. The second type of interface was made by joining two pieces of cured HTV rubber by silicone glue (RTV-I). Thereafter both types of samples were divided in two parts, one of which was additionally exposed to UV-radiation before testing. The combined salt fog — inclined plane test did not reveal significant differences in the development of leakage currents, in the change of surface roughness, or in the degree of surface oxidation in comparison to reference samples of cured HTV rubber without interfaces. At the same time the leakage currents became lower on the UV-exposed samples. It is suggested that the reduction in leakage currents resulted from a combination of increased surface roughness and increased hydrophobicity. Reduction of mechanical properties of UV-exposed samples was also observed. The tensile testing showed that samples containing the adding/curing interfaces maintained their mechanical strength after ageing while the strength of the glued samples was significantly reduced, indicating that the adding/curing system has better long-term mechanical stability compared to the glued system. [ABSTRACT FROM AUTHOR]

Silicone rubbers and porcelain intended for outdoor insulation were tested under electric fields at ac 60 V/mm on average for about two years at the field test site at Chubu University. Leakage currents on all materials were characterized and separated into three categories: sinusoidal, local arc, and their transition components. The porcelain allowed a larger leakage current than the silicone rubber samples, for which the cumulative charge of Ca. 160 C was recorded in about two years. The cumulative charge of the silicone rubbers was less than 12 C; the cumulative charge of the local arc component, which was likely to be related to surface damage, was less than 3 C. Gas chromatography - mass spectroscopy indicated that the chemical structure change at the silicone rubber surface layer closely resembled that of de-energized surfaces. The dominant aging factor of the silicone rubber in this field test was not electrical stress, but weathering stress such as that resulting from UV radiation. We also performed salt-fog tests of which conditions were adjusted by incorporating non-soluble deposits (kaolin powder) and by changing the salt-fog input duration. The ranking in the salt- fog test was opposite that of the field test. The salt-fog test emphasized the superiority of SIRs to porcelain when compared with the field test. The difference between artificial salt-fog and natural rainfall probably caused such differences. [ABSTRACT FROM AUTHOR]