Natural Ester Liquid-Immersed Transformers: Preservation Systems Avoiding Liquid Insulation Aging by Oxidation

The most common degradation process of insulating liquids is oxidation and each type of insulating liquid has its specificities. This paper reviews the mechanisms of oxidation in natural ester insulating liquids doing a comparison with same degradation process in mineral oil. Oxidation by-products in mineral oil produces acids (corrosive to other transformer material) and sludge (which can block cooling ducts). In turn, oxidation by-products in natural ester does not produce sludge, only increases liquid viscosity. An important assessment done in this paper it is regarding the test method for evaluation of oxidation stability in some natural ester international standards, beyond the IEC 62770. National standards in Brazil, Colombia, Ecuador and Mexico have defined test parameters and acceptance criteria to qualify the oxidation stability of new natural esters. The most common types of insulating liquid preservation systems will be reviewed in the paper with focus in analyze the role in preserving the liquid quality by preventing contaminants, mainly the oxygen. A well formulated natural ester using oxidation inhibitors has the robustness to offer years of tolerance to free-breathing conditions and, some cases reviewed in this paper, show it. However, as the natural ester immersed-transformers needs to comply with life expectancy requirement of 180.000 hours, these transformers need to have an insulating liquid preservation system. A real case of a free-breathing mineral oil immersed-transformer retrofilled with a natural ester will be evaluated in detail, including data of physical-chemical and dissolved gases analysis. An external and attachable insulating liquid preservation system was implemented in this retrofilled transformer to prevent the permanent contact of natural ester with oxygen.