ABSTRACT

The morphology and microstructure of icicles formed around the skirts of insulators energized under both direct and alternating voltages were analyzed in the present study. Wet ice accretion promoting the formation of icicles was grown at −12"C from 80 μm super-cooled droplets at a wind velocity of 4.2 ms−1. In general, the icicles grown under all three energized conditions are shorter than those formed without voltage. Bubble and crystal features show that icicles are very sensitive to heat exchange at the water/ice interface and they grow as hollow ice cores filled with liquid water at the tips. In the absence of applied voltage as well as in dc·, the core is circular, 5 mm in diameter, while in ac and dc+, where ionic wind velocity is stronger, they are deformed and shrink to a size of 4 mm and 3 mm respectively. The decrease in core diameter and increase in the ionic wind velocity at the tip of the icicle in dc+ and ac produce cooling effects.

1. INTRODUCTION

In cold regions, ice accretions on power lines are the origin of a number of problems including a substantial reduction in the electrical insulation strength of insulators, (Kawai, 1970; Phan and Matsuo, 1983). Under atmospheric conditions favouring the formation of icicles, where the surfaces of ice deposits are covered by a water film, the spaces between the insulator units often become bridged by icicles. This considerably reduces the withstand voltages and causes insulator flashover under normal voltages and, consequently, line outages. The growth of icicles around the skirts of energized insulators is a complicated process and not only sensitive to atmospheric conditions but also to corona discharge intensity, affecting the heat exchange rate with the environment.

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