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The results of erosion resistance, tensile strength, elongation at break, hardness, and thermal stability measurements are presented for silicone nanocomposites prepared using various nano and micro silica and alumina fillers in a two-part silicone rubber (SIR) matrix. The fillers are used to improve the erosion resistance of SiR, which is necessary for outdoor insulation housing applications. Good dispersion of the fillers is achieved using Triton[TM], a common surfactant. An optimal surfactant concentration imparts good erosion resistance to the nanocomposites in laser heating tests without adverse effects, but excess surfactant has a negative impact on the mechanical properties of the silicone. Thermal gravimetric analysis demonstrated that nano fumed silica imparts better heat resistance to silicone than natural nano silica or nano alumina fillers. Fourier transform infrared spectroscopy analysis of the nanofillers indicated a significantly higher concentration of silanol groups in the nano fumed silica filler than in micro silica. The influence of the increased number of silanol groups on the erosion resistance of the nanocomposites and their mechanical properties is discussed. Index Terms--Surfactant, Triton[TM], erosion resistance, mechanical properties, nanofiller, dispersion, dielectrics, silicone, nanocomposites.

In this paper, the degradation processes in stator-bar groundwall insulation that is subjected to steep-front voltage pulses, at different temperatures, are studied. The test conditions include pulse voltage amplitudes up to 6 kVp and pulse switching frequencies up to 3 kHz in ambient temperatures of 23 [degrees]C, 120 [degrees]C, and 155 [degrees]C. During the aging process, the surface temperature is monitored with an infrared camera to determine the regions with highest temperatures and the partial discharge (PD) activity. The visual examination of failed bars confirmed that the groundwall insulation in these regions is degraded. Index Terms--Adjustable-speed drives, converters, dielectric heating, groundwall insulation, induction motor, partial discharge (PD), pulse voltage, solid-state switches.

This paper provides an overview of the electric field (E-field) distribution on transmission line composite insulators applied in alternating current applications. Factors that affect the E-field distribution are discussed as well as the influence of the E-field distribution on the short and long term performance. Modeling and measurement methods are reported and examples of calculated E-field magnitudes determined are presented together with corona ring application information. This paper was developed by the IEEE Task Force on Electric Fields and Composite Insulators. Index Terms--Composite insulator, corona ring, E-field, electric field, grading ring, nonceramic insulator (NCI), polymer insulator.

The paper reports on tests done on four commercial room temperature vulcanized (RTV) silicone rubber high voltage insulator coatings aimed at achieving information on the relative performance of the coatings with respect to aging. The initial characterization of the coatings includes the identification of the primary filler type, concentration of filler, the amount of free fluid, and the molecular species of the free fluid. Soxhlet extraction and gas chromatography tests show considerable depletion of fluid after accelerated aging tests in acid-water and the associated changes to the physical properties is assessed by standard mechanical tests on coupon size specimens. The effect of acid-water aging on the hydrophobicity is assessed by measurements of the static contact angle and the onset of leakage current in an incline plane test. Several field aged coatings are assessed for residual LMW fluid which is compared to accelerated aging tests. Index Terms--RTV silicone rubber coating, HVIC, accelerated aging, depletion of fluid, hydrophobicity, de-polymerization, Soxhlet extraction, lifetime, gas chromatography, LMW fluid.

This paper reports on the heat erosion resistance of various silicone rubber compositions under infrared laser heating that simulates the dry band arcing phenomenon on outdoor insulation. The composites are made using a two-part addition cure silicone rubber, with various amounts of alumina, natural silica, or fumed silica, nano- or micron size, and different amounts of Triton[TM] X-100 surfactant. It is shown that the surfactant greatly improves the dispersion of nanosized particles, yielding composites that are more homogeneous, with improved resistance to heat ablation. Significantly improved dispersion in the presence of surfactant is confirmed by SEM analysis for some nanofillers, while very little effect is observed for others. The surfactant is beneficial up to the point where the filler surface is saturated with the additive. Fumed silica is shown to impart greater heat ablation resistance than natural silica and alumina. Furthermore, a distinct protection mechanism appears to be operative for fumed silica and natural silica as opposed to alumina. Index Terms--Surfactant, Triton[TM] X-100, erosion, nanofiller, dispersion, dielectrics, silicone, composites.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This second part of the paper describes the strength of typical line insulation in these conditions and then deals with selection and mitigation methods based on the in-service stresses described in Part I. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This first part of the paper describes the environmental and electrical stresses encountered in service that influence the risk of flashover on line insulators. Part II of the paper describes the strength of typical line insulation in these conditions and deals with selection and mitigation methods. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

This letter describes a high-resolution technique called thermally stimulated depolarization current (TSDC) that can be used for the measurement of space charge accumulation in the medium voltage magnet wires operating on pulsewidth modulated (PWM) inverters. Index Terms--Fast rise time pulses, induction motors, inverterfed drive motors, magnet wires, space charge injection.

The paper presents the experimental results on accelerated aging of enameled wires with and without nanofiller in the coating under partial discharge (PD) activity. The residual life of the wires, and the surface roughness as measured by a scanning electron microscope (SEM), are used to evaluate the effects of the nanofiller. The surface erosion in the specimens without nanofiller is considerably greater, which is 1120-1270 nm, under pulse aging; whereas, in specimens with ~1%, by wt, fumed silica (Si[O.sub.2]) nanofiller it was measures to be 230-250 nm. An evaluation of the residual life of the wires, aged under high frequency ac waveforms, reveal that the wires with Si[O.sub.2] achieved a life that is twice the life of conventional wires. The two-parameter Weibuil distribution of dielectric strength data show that the value of the shape parameter, [beta], increases from 2.6 to 11.4 for the coatings with nanoffilers indicating that the material becomes more homogeneous and exhibits fewer defects. Finally, the paper discusses possible reasons for the reduction in surface erosion and improvements in breakdown strength of enameled wires filled with various types and concentrations of nanofillers. Index Terms--Adjustable speed drives, dielectric strength, fast rise time pulses, inter-turn insulation, nanofilled enameled wires, residual life, standard deviation.

The paper presents the results of the study on the influence of insulator profile on the aging performance of silicone rubber (SIR) insulators in salt-fog. Experiments have been conducted on various two-shed arrangements at 35 V/mm average stress and 0.25 S/m salt-fog conductivity level. The work is also extended to include commercial 15 kV class insulators with different profiles. Shed spacing, shed diameter, alternate shed design and shed shape are the parameters investigated in this study. The low frequency harmonics of the leakage current, early aging period (EAP), and equivalent salt deposit density (ESDD), are used to evaluate the aging performance of different designs. Insulator profile is shown to greatly influence the aging performance of SIR insulators. Shed shape proves to be the most important parameter to be considered in designing non-ceramic insulators profiles. Also, as the shed spacing decreases, the performance of SIR insulators improves. Simulation results using FEMLAB[C] show that the electric field on insulators is below the corona onset at both dry and wet conditions. Dry band arcing is therefore the main electrical cause for aging in distribution class insulators and it is possible to improve the pollution performance of SIR insulators using appropriate profiles as suggested in this work. Index Terms--Leakage current, silicone rubber insulators, aging.

The paper describes the investigation of the failure mechanisms of medium voltage inter-turn insulation, as a consequence of pulse aging. To study the performance and failure behaviour of enameled wires, models of inter-turn insulation are prepared with enameled wires having polyimide and nano-filled coatings. The aging of such bar samples is attained by sinusoidal (60Hz), fast repetitive unipolar voltage pulses, and high frequency ac waveforms. To establish the residual strength of the bar samples, dc breakdown voltages are determined before and after the aging. In addition, the thermally stimulated depolarization current (TSDC) is used to provide information about the relaxation processes, such as the disorientation of the dipoles and the release of charges from the trapping sites in enameled wires. The various trapping levels, which have discrete energy differences with distinct peaks in the thermograms, are discussed in terms of space charge polarization. Index Terms--Form-wound stator coils, inter-turn insulation, nano-filled enameled wires, partial discharge inception voltage, residual strength, space charge injection, charge trapping levels.

The standard methods available to test tracking and erosion resistance of filled silicone rubber do not allow to delineate the fundamentals of the thermal degradation, although the heat from the dry band arcing is the main degradation factor. In this work a thermal imaging camera is used to investigate the performance of filled silicone rubber in an inclined plane test, and a scheme is established to relate the temperature and the electrical discharge energy. Further, an infrared laser technique, based on constant energy approach, is developed to study the material performance under laser heating. The technique consists of applying a certain amount of infrared laser energy during a set time period, in order to produce erosion in the silicone rubber samples. Through correlation studies the applicability of the proposed laser technique to rank silicone rubber samples with various fillers has been validated. The laser tests show identical ranking of samples when compared to samples ranked using inclined plane tests, yet offering significant advantages of being faster, simpler, and reproducible. Index Terms--Outdoor insulation, composites, erosion resistance, silicone rubber, dry band arcing, diagnostics, high voltage, fillers.

Silicone rubber samples having various concentrations and mean particle sizes of either alumina tri-hydrate or silica fillers, prepared by room temperature and heat cured under pressure (hot pressed), are tested for erosion resistance in the ASTM D2303 inclined plane tracking and erosion test. Their corresponding thermal conductivities are determined using a transient temperature technique in which an infrared laser is employed as the heat source and a thermal imaging camera as a temperature detection device. Scanning electron microscope observations show greater filler bonding to the silicone matrix in the hot pressed samples than in the room temperature vulcanized samples leading higher thermal conductivity and increased resistance to erosion, for both ATH and silica filled samples. The correlation study shows a strong relationship between the erosion resistance and the thermal conductivity of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing. The results can be used to provide guidance on filler selection for silicone rubber compounding for outdoor insulation applications. Index Terms--Thermal conductivity of silicone rubber, erosion resistance of filled silicone rubber, inclined plane tracking and erosion test of filled silicone rubber.

Silicone rubber samples, filled with 30% alumina trihydrate, subjected to the ASTM inclined plane test, show good correlation between the measured harmonic power components of dry band arcing and the surface temperature recorded with a thermovision camera. The simultaneous study of the low frequency Fast Fourier Transform and variations in temperature shows that when the temperature rises above 100[degrees]C there is a well defined increase in the low frequency harmonic components of the measured power; however, this trend is not verified by the fundamental component. The eroded mass of the samples is measured independently by a laser approach and a model to rank the tested samples using the Inclined Plane Test is developed. The model is validated by applying it to both the recorded harmonic power components and temperature, resulting in the correct ranking of damage on the samples tested in the Inclined Plane test. Index Terms--Inclined plane test, dry band arcing, surface temperature, erosion resistance, thermal degradation, filled silicone rubber, erosion resistance.

The paper presents the results of a study on the effect of shed design on the aging performance of silicone rubber (SIR) insulators in salt-fog. The experiments on a single shed design at 35 V/mm average stress and 0.25 S/m salt-fog conductivity level are reported. Shed diameter, location and inclination angle are the parameters investigated in the study. The low frequency harmonics of the leakage current, early aging period, equivalent salt deposit density (ESDD), and physical damage are used to evaluate the aging performance of various designs. Insulators having sheds with 100 mm diameter show a better performance than those having 80, 60 and 40 mm diameter sheds. The inclination angle of the sheds is shown to have a significant effect in reducing the leakage current to a very low value but simulation results using FEMLAB[R] show that a steep inclination angle results in field enhancement beneath the shed that may lead to corona damage of the shed. A cup shaped design for the shed is shown to have a low leakage current level and a !ow field enhancement beneath the shed. Index Terms--Silicone rubber, ageing, electric field calculation, polymeric insulators, non-ceramic insulators, outdoor insulation, leakage current, harmonics, erosion.

The paper presents the results of using the fundamental and the low frequency harmonic components of leakage current to study aging of silicone rubber in salt-fog. Experiments have been conducted on RTV and HTV coated rods at different fields (0.25-0.6 kV/cm) and conductivities (1000 to 2500 [micro]S/cm). The onset of dry-band arcing on samples could be determined by measuring the low frequency harmonic components. A correlation has been found between the fundamental and low harmonic components of leakage current and different forms of aging. Where erosion could be associated with an increase in the level of both the fundamental and low frequency harmonic components of leakage current. For example, surface damage for HTV rods occurred when the fundamental component of leakage current was greater than 2 mA. On the other hand, when the samples approached failure, the fundamental component of leakage current reached relatively high values ( > 6 mA for HTV rods and > 2 mA for RTV rods) and the low frequency harmonic components of the leakage current tended to decrease. The results suggest that both the fundamental and low frequency harmonics of leakage current can be used as a tool to determine both the beginning of aging and end of life of silicone rubber in salt-fog. Index Terms--Silicone rubber, aging, harmonics, leakage current, failure mechanisms, salt-fog, erosion, polymeric insulators, composite insulators, outdoor insulation.

The paper presents an investigation on the use of partial discharge (PD) as a means to study the development of leakage current (LC) on silicone rubber insulators in a salt-fog chamber. Simultaneous measurements of LC and PD on dead-end type silicone rubber insulators exposed to different combinations of electrical stress and salt-fog salinity are performed. The results show that for an electrical stress of 33 V/mm applied to the insulator, the average LC experiences gradual changes and reaches a critical level (around 1 mA) where dry-band arcing is observed after 20 minutes at 2300 [micro]S/cm, and after 25 minutes at 1800 [micro]S/cm. The corresponding PD levels reach hundreds of pC after 15 minutes at 2300 [micro]S/cm, and after 20 minutes at 1800 [micro]S/cm. For a salinity corresponding to 1800 [micro]S/cm and stress of 26 V/mm, the LC changes much more gradually and reaches the critical level only after approximately 60 minutes. The corresponding PD reaches hundreds of pC after 30 minutes and more than 1500 pC after 60 minutes. Pattern recognition techniques have been applied to investigate the transition from corona to dry-band arcing. The features extracted from the maximum and mean apparent charge trends are found to be useful to cluster and classify the two phenomena. Simultaneous visual observations of the discharges on the insulator surface support the results. The investigation, therefore, shows that PD is a sensitive method for detecting the transition from corona from water droplets to dry-band arcing and an alternative method for studying the early aging stage of silicone rubber insulators.

Under wet and contaminated conditions dry band arcing occurs on the surface of HV outdoor insulators coated with room temperature vulcanizing (RTV) silicone rubber. The thermal impact of the arcing on the surface of the RTV causes chemical changes. The gradual loss of hydrophobicity due to dry band arcing is of considerable concern because it results in a reduction of the withstand voltage and subsequent failure of the insulator and power outage. In this study the RTV coatings were subjected simultaneously to salt-fog and electrical stress. attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and electron spectroscopy for chemical analysis (ESCA) were then employed to investigate the chemical changes. Possible chemical reactions and processes occurring under dry band arcing are suggested. The results of ATR-FTIR and ESCA indicate the formation of low molecular weight polydimethylsiloxane (PDMS), the reduction in the surface density of CH3 groups and crosslinking of the PDMS backbone chains at the surface. Chemical derivative analysis was also carried out to further elucidate the chemical changes on the surface of RTV coating.

Room temperature vulcanizing (RTV) silicone rubber coatings are applied to electrical insulators to improve their subsequent insulation strength, particularly under wet conditions. Under prolonged wetting the hydrophobicity of the coating is reduced temporarily and the insulator protection is lost. After a dry period, a recovery of the hydrophobicity takes place. To investigate this phenomenon, the surface was subjected to various wetting conditions with and without electrical stress. Measurements of the contact and the sliding angles were used to determine the state of the surface. Using water, the surface free energy [gamma.sup.s] of the RTV was calculated from the contact angle measurements as a function of time of exposing the surface to salt-fog. The surface free energies due to London dispersion [gamma]SD, and hydrogen bonding [gamma]SH, forces on the surface of RTV coatings were calculated from the contact angle measurements using both water and methylene iodide. Good agreement was obtained with the literature value for a virgin RTV surface. To study the chemical changes on the surface of RTV, attenuated total reflection Fourier transform infrared spectroscopy and electron spectroscopy for chemical analysis were employed. The chemical changes of the surface are shown to be responsible for the loss of hydrophobicity.

This paper presents measurements of partial discharge (PD) from water droplets on a silicone rubber insulating surface in an ac field. Tests are done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface is experimentally and numerically determined. Values in the range of 3.8 to 6.3 are obtained depending on the size and number of droplets. The time resolved analysis of PD from water condensation indicates a threshold field, between 3.0 and 3.5 kV/cm, above which the PD activity increases with time, and brings the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments is between 5.8 and 6.8. The results therefore suggest that the PD activity on the hydrophobic surface can be used effectively to learn about the insulation surface conditions.

This paper deals with the evaluation of the insulation design of polymer housed surge arresters for distribution lines. Five commercial designs were tested in a salt fog chamber for 9 cycles using a cycle consisting of 120 hours of salt fog that was followed by a 48 hour rest period for a total of 1280 hours of salt fog. In these tests, the specification for the 1000-hour salt fog test in IEC 1109 for composite insulators was used as a guide. The results demonstrate that a comprehensive evaluation of the external and internal insulation design of polymer housed surge arresters can be obtained by performing a salt fog test, and as such, the test can be considered as a useful design test. Suggestions for improvements in the insulation design of all polymer housed surge arresters evaluated are given in the paper. Index Terms--Accelerated aging, polymer housed surge arresters, polymer housings, salt-fog testing, surge arresters.

This letter describes a neural network approach to the prediction of the leakage current (LC) on silicone rubber insulators exposed to salt-fog. The validity of the approach was examined by testing several insulators in a salt-fog chamber. Feed-forward back propagation was found as the best method among several training methods evaluated for the prediction of the LC. The predicted LC with this method has less than 12% error for the tested cases. Index Terms--Leakage current, neural network, polymeric insulator, salt-fog test.