Comparative Analysis of the Dielectric Performance of Insulating Liquids Aged Under Different Conditions

Abstract

This thesis investigates the dielectric performance of transformer oils used as high voltage insulation media in membrane-sealed and fully enclosed transformers and compares the impact of the sealing methods. Oil samples were collected from real transformer units, which were operating under different conditions over several years and the insulating oils in them were exposed to combined electrical-thermal stresses of different duration and intensity. It was anticipated that the intake of air through the sealings of the transformer tanks might initiate chemical reactions under such stresses thus causing accelerated ageing of the insulation. To examine this hypothesis, dielectric measurements and breakdown tests were conducted according to the respective standards. The former included measurements of complex capacitances, loss factor and oil resistivity whereas the latter provided the maximum electric field strength the oil could withstand. The obtained results indicate a rapid initial degradation in some of the measured parameters, while the evidence for storagetime- dependent degradation remains inconclusive. Under the applied test conditions, the oil samples taken from fully enclosed transformer tank showed a slightly better performance than those from membrane-sealed tank. To explicate this finding, an attempt to correlate the measured results with the histories of the respective transformer units is undertaken. Ageing factors affecting performance of the insulating oils, including chemical degradation under electro-thermal stresses and possible contaminations, are discussed based on the performed literature study.