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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. Index Terms--Wet snow, snow density and conductivity, snow conductivity, flashover, outdoor insulation.

Several major parameters affecting the electrical behavior of natural snow, namely volume conductivity and density, liquid water content and conductivity of water melted from snow, are crucial for the characterization of the electrical performance of snow-covered HV insulators. However, little study has been devoted to this subject, despite of its importance. These parameters are found to vary significantly with snow composition and purity as well as with other parameters, such as temperature, airborne pollutants, electric field strength and polarity. From laboratory experiments carried out on a large number of snow samples, it was found that dc conductivity of snow shows a peak at about -2[degrees]C. This apparently curious behavior near the melting temperature is attributed to important changes to the microstructure of snow. A correlation between dc conductivity and snow temperature was established. Index Terms--snow, conductivity measurement, density, heating, cooling, insulation.

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. Index Terms--Corona streamer, inception voltage, breakdown voltage, water drop, field enhancement, outdoor insulation; atmospheric icing.

Abstract: In the last decades, dielectric testing techniques are being used and investigated as potential tools for condition assessment of oil-paper insulation. From fields and laboratory investigations these techniques were found to be highly operating conditions (moisture, ageing, temperature, etc.) dependant. Because field measurements (generally performed after de-energizing the transformer), last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care is required to interpret the results when performing tests at surrounding low temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behaviour of the insulation system in response to the ambient conditions. In the current research project, a series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. This paper reports the effects of low temperature on the time domain dielectric response of oil impregnated paper insulation. [Copyright &y& Elsevier]