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A self-consistent dynamic model allowing the prediction of ac discharge activities leading to flashover on ice-covered insulator surface is presented. This model takes into account the effects of a number of parameters including insulator geometry and applied water conductivity. The instantaneous variations of major parameters are discussed in order to develop a sequential time-dependent simulation of the flashover. The temporal evolution of arc current and axial arc velocity are determined in a consistent manner. The critical flashover voltage characteristics, as a function of surface conductivity, insulator length, and insulator diameter, calculated by the model are quite satisfactory when compared to the experimental results from empirical models reported in the literature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper discusses the mechanisms by which inorganic fillers in silicone rubber dielectrics enhance the properties of thermal conductivity, relative permittivity, and electrical conductivity making them useful in outdoor high voltage insulation applications. The addition of alumina trihydrate or silica fillers to silicone elastomers, forming binary composites with enhanced thermal conductivity, is discussed in relation to filler type, particle size, shape, and concentration, and its use as a housing material for non-ceramic insulators to minimize material erosion at dry band arcing sites by lowering hot spot temperature. Also discussed is the enhanced relative permittivity of silicone dielectrics that is obtained through the addition of barium titanate powder which can be further increased with the addition of aluminium powder forming a tertiary composite, resulting in a significant grading of the surface electric field when applied as a housing material to high voltage bushings. Controlled electrical conductivity of silicone dielectrics is discussed through the use of antimony-doped tin oxide filler binary composites and when applied as a housing material to outdoor bushings, the pollution performance is greatly enhanced. [ABSTRACT FROM AUTHOR]

This paper presents the electrical characterization of antimony-doped semiconducting glazes before and after exposure to ac and dc electric fields under outdoor conditions and In the laboratory using the rotatlng-wheel-dip test (RWDT). Measurements of the I-V characteristics as well as dielectric spectroscopy in frequency domain (EDS) were performed on new and aged samples and results were compared. While I-V data only show a decrease in the current with aging, FDS data show more detailed features. Besides a decrease in dc conductance, a strong increase in low frequency dispersion (LFD) was found and a decrease in the relaxation polarization was indicated. The tatter was more pronounced for samples aged under dc voltage. The observed relaxation polarization could originate from the presence of a low-conductive glassy surface layer on top of the "bulk" semiconducting glaze and it was weakened by the appearance of surface changes, including damage of the glassy surface layer, which thereby opened an alternative path for the current flow from the electrode to the "bulk" semiconducting glaze. The increase in LFD could be attributed to moisture trapped in the surface cracks formed during aging. [ABSTRACT FROM AUTHOR]

The effects of air pressure on the ac and dc arc process on an ice surface, including velocity, parameters and flashover voltage, were studied. The results obtained showed that air pressure has an obvious influence on the arc propagation velocity, arc constant A, electrode voltage drop Ve and consequently on the flashover voltage of the ice surface Vf, This decrease in Vf, is explained by the decrease in the E-I are characteristics and arc propagation velocity, and by the relatively: higher density of Na atoms in the arcs at pressures below standard air pressure. Based on these results, the mechanism underlying the decrease in the flashover voltage of ice-covered insulators at high altitude was analyzed. [ABSTRACT FROM AUTHOR]

This paper presents results on the flashover of ice-covered insulators at low air pressure. The flashover performance of two different types of insulators covered with polluted ice was experimentally determined and the effects of several deter- mining factors, such as the air pressure, ice severity, and pollution rate, were discussed. Based on the laboratory investigations, a mathematical model for predicting the critical flashover voltage of ice-covered insulators at low air pressure was established. There is optimal agreement between the calculated and experimental results. [ABSTRACT FROM AUTHOR]

The correlation between tracking resistances of outdoor polymer insulating materials and ambient environmental stresses has been studied. In this study, the tracking resistance variation of a cycloaliphatic epoxy resin filled with high concentration silica powder is investigated as a function of thermal aging and water absorption. Experimental works show that the tracking resistance of this epoxy system is decreased considerably by thermal aging and water absorption. The decrease due to water absorption is found to be an apparent phenomenon because on complete drying treatment, the tracking resistance recovers to its initial state. The combined effect of thermal aging and water absorption makes the tracking resistance decrease to a greater extent than each stress individually. In addition, the tracking resistance of epoxy that is aged by thermal treatment and further water absorption-drying treatment cannot recover to its pre-water-absorption state. Mechanisms of the tracking resistance variations of this epoxy system resulting from thermal aging and water absorption are discussed. © 2000 Scripta Technica, Electr Eng Jpn, 131(3): 9–18, 2000 [ABSTRACT FROM AUTHOR]

Inclined-plane tests of silicone-rubber samples prepared with textured patterns of hemispherical protuberances are described. These are compared with the results obtained for conventional non-textured plane-surface samples of the same material. A remarkable improvement is achieved by such texturing: a material which normally fails the test is enabled to satisfy the standard test criteria, and the tracking and erosion of the samples caused by the tests are significantly reduced. These texturing techniques are currently being extended to prototype composite insulators. It is shown that this improvement for textured samples is a result of the formation of a stable, controlled dry band on the polluted surface. Although discharge inception occurs across the band, these discharges result in a fall, rather than an increase, of the leakage current. The transition to a higher current arc phase is thus inhibited, compared with non-textured samples for which stable dry bands did not occur. These observations of reproducible dry band breakdown also provide an opportunity to characterize experimentally the electrical properties of these discharges. A recent proposed model represents the dry-band discharge as a streamer-spark structure, in contrast to the standard model of an arc-like interdependence of current and voltage. These two models are tested against the present new data, and the discharge is shown to have streamer characteristics. [ABSTRACT FROM PUBLISHER]

On the way of making efforts to standardize the selection of polymeric materials for high voltage outdoor environments, the resistance to corona and ozone has been recognized as one among twelve parameters to be evaluated within the CIGR?? Working Group D1.14 [1]. Our previous related work reported on the evaluation of the resistance of five commercially available polymeric materials to ac corona-ozone treatment, according to the test procedure recommended by this Working Group. With the same test arrangement in use, this report focuses on comparing the influences of ac and dc corona-ozone exposures on high temperature vulcanized (HTV) silicone rubbers. For this purpose, two types of HTV materials were used, each one characterized by different resistivity levels and tensile strength. For defining the treatment conditions, the ac and dc coronas were characterized by measuring the associated discharge current as well as the power released from the electrode. The intensity of ac corona discharge was found to be higher than that of dc corona, the latter being controlled by electrostatic charging of the treated polymeric surfaces, which also resulted in significant differences of the resulting ozone concentration. Experimental investigations after the treatments concentrated on evaluating dielectric properties (e.g. surface and volume resistivities, dielectric response (DR)), mechanical properties (e.g. tensile strength and elongation at break), as well as chemical and structural analyses (e.g. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR)). Results obtained from the measurements of dielectric properties indicated a strong sensitivity of surface resistivity to the treatments, though the impact of the dc corona remained less pronounced. On the other hand, the dielectric response and volume resistivity did not exhibit a high susceptibility to the exposure. No substantial differences were also found among the ac and dc corona treated samples as regards the mechanical properties. The XPS and FTIR analyses revealed proofs of surface oxidation for the samples treated by ac corona, while the samples treated by dc corona showed much less tendency for oxidation. [ABSTRACT FROM PUBLISHER]

This paper presents the results of a study carried out on the deformation of a water surface and the effect water drops have on both conducting and dielectric solid surfaces subjected to a 50-Hz-volage stress. Also, contained in it are fundamental observations of current discharges characterizing water drops and metal electrode configurations as well as those configurations resulting from a flat water surface and a metal electrode. The measurements taken with both rod and point electrodes also came under investigation. The inception field strength of partial discharges between water surface and metal electrodes is analyzed. The discharge activities between water drops and between a conducting water drop and a small rod electrode on a flat hydrophobic insulator are also investigated. The results obtained characteristics of show typical discharge current during the discharge inception and development with and without water filaments, with regard to drop-drop and drop-rod configurations on the dielectric solid surface. This experimental study provides a better understanding of the processes leading up to flashover of hydrophobic insulators through discharges between water drops resulting of the distortion of these drops. [ABSTRACT FROM PUBLISHER]

This paper describes the hydrophobic transfer of unfilled room temperature vulcanized silicone rubber (RTV) resulting from UV radiation, corona discharges, high temperature, and a deposition of hydrophilic layer. The chemical and morphological structure at the surface layer and the migrating behavior of low molecular weight (LMW) silicone fluid from the bulk on to the surface top layer are investigated in order to elucidate the mechanism responsible for the hydrophobic transfer of RTV silicone rubber. It is shown that UV radiation and high temperature reduce highly mobile LMW silicone fluid, which reduces the recovery ability of the hydrophobicity. On the contrary, corona discharges produce highly mobile LMW silicone fluid that is formed as byproducts of corona aging, which promotes the hydrophobidty recovery and provides stability against other stresses. [ABSTRACT FROM PUBLISHER]