The main risk for short-time failures is the reduction in dielectric stren9th due to the possible presence of gas bubbles in a region of high electrical stress, that is the winJings and leads. These bubbles are likely to occur when the hot-spot temperature exceeds 140 °C for a transformer with a winding insulation moisture content of about 2 %. This critical temperature will decrease as the moisture concentration increases. Gas bubbles can also develop (either in oil or in solid insulation) at the surfaces of heavy metallic parts heated by the leakage flux or be produced by super-saturation of the oil. However, such bubbles usually develop in regions of low electric stress and have to circulate in regions where the stress is higher before any significant reduction in the dielectric strength occurs. Bare metallic parts, except windings. which are not in direct thermal contact with cellulosic insulation but are in contact with non-cellulosic insulation (for example, aramid paper, glass fibre) and the oil in the transformer, may rapidly rise to high temperatures. A temperature of 180 ”C should not be exceeded. Temporary deterioration of the mechanical properties at higher temperatures could reduce the short-circuit strength.

The main risk for short-time failures is the reduction in dielectric stren9th due to the possible presence of gas bubbles in a region of high electrical stress, that is the winJings and leads. These bubbles are likely to occur when the hot-spot temperature exceeds 140 °C for a transformer with a winding insulation moisture content of about 2 %. This critical temperature will decrease as the moisture concentration increases. Gas bubbles can also develop (either in oil or in solid insulation) at the surfaces of heavy metallic parts heated by the leakage flux or be produced by super-saturation of the oil. However, such bubbles usually develop in regions of low electric stress and have to circulate in regions where the stress is higher before any significant reduction in the dielectric strength occurs. Bare metallic parts, except windings. which are not in direct thermal contact with cellulosic insulation but are in contact with non-cellulosic insulation (for example, aramid paper, glass fibre) and the oil in the transformer, may rapidly rise to high temperatures. A temperature of 180 ”C should not be exceeded. Temporary deterioration of the mechanical properties at higher temperatures could reduce the short-circuit strength.