Thermal aging analysis of mineral oil and natural ester immersed windings

Dielectric oil is part of the power transformer insulation system. There it performs its dual functions of providing cooling and insulation within the transformer. In recent years there has been a resurgence in the use of natural esters (vegetable oils), driven by environmental concerns. In this sense, their biodegradable characteristics and high flash point compared to mineral oils increase their use. However, their limited knowledge of long-term performance in service compared to mineral oil limits their application in power transformers. For these reasons, it is necessary to compare mineral oils and natural esters under different stresses. This work presents a comparison of the performance and behavior of an insulation system impregnated with mineral oil with another impregnated with natural ester under identical thermal stress. Physicochemical parameters, degree of polymerization, dielectric frequency response and gas generation are evaluated and analyzed. As a variant in the models to be analyzed, windings with different geometries were used. It was concluded that the paper impregnated with natural ester has practically not changed its moisture content with the aging time. Paper in cells with natural ester has aged somewhat less, although at low load temperatures the aging may be similar. It has been shown that the temperature reached in the windings is dependent in part on the geometry.