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Composite insulators have gradually become the preferred approach for electrical insulation in power systems, especially in polluted areas. Composite insulators consist of three main components: the shed, rod, and end fitting. Insulators withstand mechanical stresses via rods that are composed of glass-fiber-reinforced epoxy (GFRE). However, regardless of the high tensile strength of GFRE rods, in real-life operation, abnormal fractures have frequently been reported all over the world, which substantially increase the risk of major accidents in power systems. Fractural accidents mainly consist of brittle and decay-like fractures, which exhibit rather different morphologies at the cross sections. Brittle fracture has been effectively eliminated, while the mechanism of decay-like fracture has still not been clearly revealed. In this study, surface discharge tests were applied to investigate the discharge influence on the degradation of GFRE. The test successfully simulated the composition variation of the rods in real-life composite insulators with decay-like fractures. Moreover, it confirmed that the distinction between the characteristics of brittle fracture and decay-like fracture stems from epoxy degradation due to hydrolysis and carbonization. In addition, the respective influences of the resin type, glass fiber type, and acid liquid immersion on the degradation process were probed, and the degradation mechanism proposed in this research was verified. Based on the results, measures for preventing the development of decay-like fractures in real-life operations were determined. [ABSTRACT FROM AUTHOR]

The brittle fracture (BF) surface of a 500 kV composite insulator was analyzed. It was operated for 6 years. Chemical compositions on the field failed BF surface were examined by X-ray fluorescence spectrum and FTIR spectroscopy. Test results showed that the chemical functional groups N[O.sub.3.sup.-], N[O.sub.2.sup.-], together with S[O.sub.4.sup.2-], were formed both on the BF surface of FRP rod and in the rust on end fitting near BF crack. Laboratory simulated BF by stress corrosion under nitric acid or sulfuric acid were all analyzed and compared with the field failed BF surface. Analysis of the field failed insulator reveals that the BF is influenced not only by nitric acid, which may be formed by electrical discharge, but also by sulfuric acid, which may come from acid rain. Index Terms--Composite insulator, brittle fracture, stress corrosion, FTIR.

Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space. Index Terms--Composite insulation, corona, electric fields, numerical analysis.

Analyzing the nonuniformity of electrical parameters of ZnO varistors is useful to select and coordinate the ZnO varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we designa SPD with ZnO varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO varistors. Index Terms--Discharge withstand current, electrical parameter, Kolmogorov-Smirnov test, minimum gobble distribution, nonuniformity, statistic analysis, surge-protection device, surge residual voltage, ZnO varistor, 1-mA dc voltage.

The interface between silicone rubber (SR) and fiber reinforced plastic (FRP) is a weak point in composite insulator, which is easy to be affected by moisture. However, the corresponding evaluation methods and detection means are not complete yet. Based on the newly proposed method for measuring the interface resistivity with a four-electrode system, the influence of liquid type, filler and coupling agent on the interface resistivity are studied in this paper. It is found that the interface resistivity reduced by 2–4 orders of magnitude after permeation by deionized water, saline and low concentration nitric acid, but the variation trend of interface resistivity is specific when permeated in high concentration of nitric acid solution (1 mol/L). The experimental results of samples with and without silicone coupling agent (SCA) are compared and it is found that the interface without silane coupling agent is easier to be penetrated by liquid, and the change of interface resistivity is more obvious. Besides, the influence of moisture-sensitive Alumina Tri-hydrate and SiO2 fillers, which could provide paths for water and hydrated ions to permeate, on interface resistivity are analyzed. The experimental results are of great significance for the selection of the appropriate formula system and the prevention of liquid penetration for composite insulators. [ABSTRACT FROM AUTHOR]

Capacitors in electrical and electronic systems composed of thin film dielectrics operate under extremely high electric fields. Dynamics of charge transport and relaxation are closely correlated to the energy storage properties and electrical degradation of thin film dielectrics for capacitive applications. In this paper, the dielectric relaxation properties of poly (vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films are investigated over a broad frequency range from 10 mHz to 10 MHz and temperature range from −130 °C to 130 °C. The dielectric responses correlated to bulk and interfacial processes are clarified by varying the thickness and interfacial contact status of P(VDF-HFP) films. A low frequency process originating from the quasi-DC (QDC) process and a dielectric dispersion process correlated to the interfacial polarization are identified based on the Dissado-Hill dielectric response model and the equivalent circuit analysis. The high permittivity and high operation electric field of P(VDF-HFP) for capacitive energy storage give rise to improved blocking capacitance associated with the interface, suggesting the enhanced interfacial polarization. This work provides insights into the bulk and interface charge dynamics of polymer thin film dielectrics. [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]

In this paper, a four-electrode system is developed to determine the interface resistivity of the specially made specimens containing an interface between silicone rubber (SR) film and a fiber reinforced plastics (FRP) substrate. Both simulation and experiment show that the shield electrodes can successfully shield a considerably part of the bulk current. With the shield electrodes, the current flowing through the interface can be obtained and the interface resistivity is calculated. The influence of liquid permeation of SR as well as the recovery process on the interface resistivity is studied. The results show that interface resistivity is a sensitive electrical parameter reflecting the interface status and the newly-proposed test method can provide a means of evaluating the interface between the SR housing and FRP rod in a composite insulator. [ABSTRACT FROM AUTHOR]

The decay-like fracture is an emerging mode of abnormal mechanical failure of composite insulators the study of which is still in its infancy. The degradation of epoxy resin of glass fiber reinforced polymer (GFRP) rod caused by discharge and current distinguish the decay-like fracture from both normal mechanical fracture and brittle fracture. In the present work, the detailed degradation of epoxy resin of GFRP rod in the decay-like fracture of composite insulator were examined using scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The results show that the generation, expansion and release of pyrolysis gas is responsible for the formation of the bubbles and holes in the epoxy resin and the initial formation of holes in the silicone rubber housing during the degradation process. The erosion and pyrolyzation of epoxy resin results in mass loss of epoxy resin leading to degradation of the GFRP rod. High resolution XPS spectra of C1s and N 1s in GFRP rod reflects that oxidization process, exposure of glass fiber, and the introduction of nitrogen-containing compounds occurs during the degradation process. Microscopic photographs to explain the degradation of the epoxy resin and the initial formation of puncture holes in the silicone rubber housing are proposed based on the experimental results. [ABSTRACT FROM AUTHOR]

Laser-ablated template and fluoroalkyl silane modified composite coatings are used to prepare a micro- and nano-structure on silicone rubber surfaces. By testing the surface potential accumulation and dissipation process, we assess the suppression of surface charge on superhydrophobic silicone rubber. The surface physicochemical properties and resistivity are examined. The electron and hole trap distribution are analysed using isothermal current decay theory. Peak density of traps associated with the superhydrophobic surfaces are found to be promoted to lower energy levels relative to unmodified silicone. Surface micro-structure increases trap density and reduce trap depth; nano coatings are shown to further reduce surface trap depth. The improvements shown to suppress surface charge accumulation is beneficial to a range of electrical industries and helps mitigate surface flashover on insulation. [ABSTRACT FROM AUTHOR]

A field brittle fracture composite insulator was examined and analyzed in detail in terms of chemical property and fractography with the purpose of obtaining the cause and mechanism of the failure as well as providing more detailed information on acid source and classification of fracture surface feature responsible for brittle fracture. The Fourier transform infrared spectroscopy (FTIR) analysis was employed to investigate the chemical properties of the rust on the surface of high voltage end fitting and fractured FRP rod. Furthermore, the scanning electron microscopy (SEM) was employed to obtain microscopic pictures of fractured FRP rod with a high magnification. The results showed that the acid source responsible for the initial failure of seal were sulfuric acid and nitric acid from acid rain. The acid source responsible for the development of crack within the FRP rod was the nitric acid generated by the discharge in the presence of moisture. In addition to the classical appearance of mirror, mist and hackle zones on the fracture surface of individual glass fiber, the appearance of mirror and wedge (or wedges) zones was also a common feature of fracture surface of individual glass fiber and can be used as an indication of brittle fracture. The crack propagation directions at micro scale showed some extent of irregular, this was caused by the local variation in acid environment as well as the resin wrapping of fiber and may resulted in the appearacne of matric scarp on the fractrue surface. An axially perforative defect was detected on the fracture surface of FRP rod, this indicated an insufficient resin infiltration process in the production of FRP rod. The results of this research provide the experimental basis for the further understanding of brittle fracture. [ABSTRACT FROM AUTHOR]

In recent years, a new kind of abnormal fracture phenomena of composite insulator occurred in high voltage transmission lines in several countries, which was totally different from brittle fracture and normal fracture. According to the general characteristic features of the fracture, this type of failure was named decay-like fracture of composite insulator by the author. In this paper, a decay-like fractured composite insulator was investigated and microscopic physic-chemical properties of decay-like fractured Fiber Reinforced Plastic (FRP) core rod were analyzed. Scanning Electron Microscope (SEM), Thermal Gravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) Spectroscopy methods were employed to explain the microscopic morphology characteristics and chemical component changes of decay-like fractured composite insulator, and to further explain the reasons for the name of decay-like fracture in terms of "crisp" and "decay-like". The main microscopic feature of decay-like fracture is the degradation and deterioration of the epoxy resin matrix in FRP rod. The mechanism of decay-like fracture of composite insulator could be summarized as follows. Due to the liquids penetration into the sheath-core interface, hydrolysis of silane coupling agent with bad quality leads to poor adhesion strength and interface voids, which results in distortion of electric field at the sheath-core interface. Under the combined action of damp condition and high electric field, discharge and current appear along the FRP rod surface. Epoxy resin matrix is eroded in the presence of discharge and current, and then is soften, melt, evaporated. Meanwhile, ion-exchange and hydrolysis process happen between glass fibers under the combined action of mechanical stress and nitric acid produced by discharge in the presence of moisture. Fiber-resin interface in FRP rod is separated owing to the erosion of epoxy resin. The degradation direction of the decay-like fracture is from outside to inside of FRP rod, and the interface between silicone rubber sheath and FRP rod is confirmed to be the initial deterioration area of the decay-like fracture. The criterion to distinguish the differences between the decay-like fracture, brittle fracture and normal fracture depends on whether the epoxy resin matrix is degraded and deteriorated or not. [ABSTRACT FROM AUTHOR]

The effects of liquids immersion and subsequent drying on the surface properties of filled high temperature vulcanized (HTV) silicone rubber have been reported in this series of paper. In this first paper, the surface hydrophobicity characterized by static contact angle and surface chemical properties characterized by a comprehensive analysis of Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) and thermal desorption gas chromatography mass spectrometry (TD-GC/MS) have been studied. The liquids employed in this study were deionized water, NaCl solution and nitric acid solution, which represented the actual liquids that composite insulators could encounter in practice use. As for the NaCl solution, the lower the concentration, the greater its impact on the hydrophobicity of the silicone rubber (the deionized water can be considered as the NaCl solution of concentration of 0 mol/L). As for the nitric acid solution, the higher the concentration, the greater its impact on the hydrophobicity of silicone rubber. During the permeation process of deionized water and NaCl solution into HTV silicone rubber, some Alumina Tri-hydrate (ATH) filler and low molecular weight (LMW) could be dissolved into the liquids. Nitric acid solution not only dissolved the ATH filler, but also interacted with the ATH filler and degraded the polymer matrix. [ABSTRACT FROM AUTHOR]

The tracking wheel test specified in IEC/TR 62730:2012 provides a method that can be used to evaluate the resistance of composite insulators to tracking and erosion for rejecting inadequate materials or designs. Currently, most of the tests have been conducted on composite line insulators and some other insulator types with relatively small diameters. As the usage of large-diameter composite insulators, such as composite station insulators and composite cross-arms, increases rapidly, it is urgent to investigate if the tracking wheel test method is also suitable for their evaluation. In this study, a tracking wheel device for composite insulators with different diameters, designed and constructed in accordance with the IEC standard, is presented. The relationships between the leakage current and NaCl content as well as the average diameter of the specimen are investigated. It is shown that the test parameters specified in the IEC standard are not suitable for specimens of relatively large diameters. Accordingly, a relevant modification of test parameters has been introduced and verified, showing similar phenomena during the verified test as those observed during previous studies on composite line insulators. It is thus claimed that the modified test procedure is feasible for being applied on large-diameter insulators. [ABSTRACT FROM AUTHOR]

This study utilized a multi-needle corona electrode system to investigate the influence of AC and DC coronas on the surface properties of high temperature vulcanized (HTV) silicone rubber. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) analysis were performed to characterize the chemical structure and hydrophobicity changes in state of the silicone rubber surface after corona exposure. The results indicated that the effects of AC corona were greater than that of DC corona, and the effects of negative DC corona were slightly greater than that of positive DC corona. XPS results showed that the C elemental content on the silicone rubber surface decreased, whereas the O elemental content increased. High-resolution scan on Si 2p XPS spectra proved that surface oxidation occurred during corona exposure and caused the high binding energies of Si atoms and silica-like layer. High-resolution C 1s XPS spectra showed that corona exposure destroyed the Si-CH3 groups on the surface. Some C-O and C=O bonds appeared following the corona exposure. High-resolution O 1s XPS spectra confirmed the formation of Si-OH groups and chemically absorbed water, which was ignored in the past research. Chemically absorbed water was detected as one of the essential factors that caused the loss of hydrophobicity of silicone rubber after corona exposure, together with Si-OH groups and silica-like layer. [ABSTRACT FROM PUBLISHER]

A large number of silicone rubber (SR) insulators have been used in inland areas. Long-term performance evaluation, pre-ranking and selection of these insulators are highly significant. IEC and other institutes have proposed several 5000 h tests with multiple stresses to evaluate long-term performances of composite insulators. However, these procedures have their own characteristics and limitations. None of the existing tests could provide a satisfactory simulation of the condition with severe contamination and high non-soluble deposit density (NSDD) in inland areas as well as the hydrophobicity change process of SR insulators. Based on modifications of the IEC test procedure, a novel 5000 h test procedure is proposed. There are two main modifications. The first one aims to replace the salt fog with a mixed contamination fog including both soluble and non-soluble pollutants. The second one aims to provide sufficient time for SR to recover and to transfer its hydrophobicity. A set of test apparatus based on this novel 5000h test procedure was designed and constructed. Two tests were conducted on 16 SR insulators with different material compositions and shed designs, including an AC and a negative DC test. Contamination accumulation on the surfaces of the specimens during both tests were measured. The NSDD and equivalent salt deposit density (ESDD) were close to those measured in the field in contaminated inland areas. Hydrophobicity reduction and recovery were well reproduced. Discharge activities were observed and leakage current was measured, which were consistent with the hydrophobicity statuses of the specimen. Tracking and erosion were found for some specimens, and the damage pattern was the same with that found in the field. The poor material properties and inappropriate shed designs could be identified successfully through the test. It was concluded that the modified 5000 h procedure could provide a satisfactory simulation of the desired contamination condition and hydrophobicity change process, and the insulators with different design defects are successfully screened out. [ABSTRACT FROM PUBLISHER]

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]

This paper investigates the liquid permeation performance of silicone rubber containing alumina tri-hydrate (ATH) that has been modified with N-[3-(trimethoxysilyl) propyl] aniline (Y-9669). From the change in weight during immersion in water, saline solution or nitric acid, and during subsequent drying in air as well as a second immersion test, it is revealed that the majority of the absorbed liquid exists in filler and interface for silicone rubber with original ATH. Surface modification of ATH filler can protect silicone rubber from liquid permeation. It is easier for liquid to permeate into silicone rubber than to escape from it due to the hydrophobicity of the silicone rubber matrix. A cycle of immersion and drying is found to have little impact on subsequent immersion in the case of water and saline solutions; however, damage inflicted on the matrix and filler by nitric acid means that the rate of permeation increases during subsequent immersion. The volume resistivity and surface resistivity of samples with unmodified ATH also decrease more appreciably than those with modified ATH after a cycle of immersion and air drying due to the amount of the residual absorbed liquid. [ABSTRACT FROM AUTHOR]

Silicone rubber is widely used for outdoor insulation because of its hydrophobicity. Meanwhile, superhydrophobic surfaces with self-cleaning property are ideal as outdoor insulating materials. An efficient technique to fabricate superhydrophobic silicone rubber with a contact angle of 151° and a sliding angle of 5° is reported. Long-term stability of polymeric superhydrophobic insulating surfaces was examined under electrical and non-electrical stresses. Results show that silicone rubber transforms from superhydrophobic to superhydrophilic after corona aging. Superhydrophobic characteristic is fully recovered through a self-healing process. After water immersion, it results in large contract angle hysteresis, although the static contact angle on the surface is large. The increase in surface free energy caused by aging for self-healing superhydrophobic material affects the stability of superhydrophobic properties. Once the hydrophobicity decreases and Wenzel contact mode is formed, the sliding angle and contact angle hysteresis are difficult to recover because of a large energy barrier between the Wenzel-Cassie transitions. [ABSTRACT FROM AUTHOR]

The dielectric response of three kinds of filled high temperature vulcanized (HTV) silicone rubber - SiO2 based nanocomposite with different alumina tri-hydrate (ATH) micro-filler contents are investigated over a wide range of frequency and temperature. The results are analyzed in terms of the many-body Dissado-Hill dielectric response theory. In all three kinds of silicone rubber, a quasi-DC (QDC) process, a Dissado-Hill loss peak process and a diffusion process are observed. Specifically, the ATH filler contributes a QDC electrical transport/hopping process, which at higher temperatures causes the formation of a diffusion layer at the electrodes. The nano-SiO2 particles combined with rubber matrix give a Dissado-Hill loss peak dielectric relaxation. These two dispersion processes are shown to be associated with the interior interfaces of filled HTV silicone rubber. The effects of ATH filler content and temperature on the dielectric response are also analyzed in depth. [ABSTRACT FROM AUTHOR]

Capacitance and gravimetric measurements are used to study the diffusion of water into high temperature vulcanized (HTV) silicone rubber. Silicone rubber with different Alumina Tri-hydrate (ATH) filler contents was used in this research, and deviations from the classical Fickian diffusion were observed. A model is presented for the calculation of the diffusion coefficient of water in silicone rubber by capacitance method in which the anomalous diffusion mechanism and the water concentration profile are considered. The present model is compared to other models and the difference between the diffusion coefficients obtained is analyzed. [ABSTRACT FROM PUBLISHER]