Oil oxidation is an ongoing process that occurs in oil in all transformers, the by-products of which attack the transformer’s paper insulation.
Oil oxidation is influenced by heat, oxygen and moisture, and facilitated by the presence of metal – copper in the case of a transformer. Despite best efforts to exclude air from a transformer, all transformer oil will contain oxygen, thus oil oxidation is inevitable. In the presence of heat and electrical stress, oxidation by- products include free radicals, acids, water, alcohols, peroxide, aldehydes, ketones and esters.
An oxidation inhibitor is a substance contained within or added to an insulating fluid to improve its resistance to deleterious attack in an oxidising environment, i.e. inhibitors improve the oxidation stability of oils.
All transformer oils contain inhibitors to some extent. “Uninhibited” oils contain natural inhibitors and/or synthetic inhibitors to a maximum concentration of 0.08% by weight. “Inhibited” oils contain natural inhibitors and/or synthetic inhibitors to a maximum concentration of 0.4% by weight.
IEC 60296 defines inhibited oil as “mineral insulating oil containing a minimum of 0.08% and a maximum of 0.40% antioxidant additive together with other additives”.
The most commonly used synthetic inhibitors are DBPC (2, 6-ditertiary-butyl para-cresol) and DBP (2, 6-ditertiary-butyl phenol). In general, natural inhibitors are various sulfur compounds.
Benzoil recommends and sells transformer oil inhibited with DBPC to a concentration of 0.3% by weight. This complies with standards IEC 60296, ASTM D3487 and AS1767, and is in accordance with the recommendations of Doble Engineering, SD Myers, and Powerlink Oil Testing Services.
It is commonly understood that the oxidation stability of uninhibited oil will decline “gradually” over time, whereas the oxidation stability of inhibited oil will decline sharply, but only after a long stable condition. Although this is true, it should be noted that quality inhibited oil will outperform uninhibited oil by up to 70% or more, i.e. 15 years will become 25+ years. In short, the point at which the oil deteriorates is many years beyond that of uninhibited oil.
We suggest using re-refined or regenerated transformer oil as the ideal replacement oil for transformer top-ups or retrofills. These are fully miscible with other naphthenic mineral oils, have no detrimental effect on oil performance, and will improve the operational longevity performance of the oil mix.
It is interesting to note that while historically in Australia there has been wide use of uninhibited oil, in the world’s largest electricity market (USA), inhibited oils dominate the market by a factor of 10 or 20 to 1. All transformer oil vendors in Australia offer inhibited versions of their products.
Australian power utilities who currently specify inhibited transformer oil include SA Power Networks, Essential Energy, Ausgrid, Jemena, United Energy Distribution, Endeavour Energy and Transgrid. Other power utilities are now in the early stages of amending their standards to specify inhibited oil, and it is widely thought that a “common standard” across the industry is not far off.