Aging of kraft paper by acid catalyzed hydrolysis

Abstract

Hydrolysis of transformer insulating Kraft paper is experimentally investigated. Paper moisture content was varied and five different acids -representative of what may be found in an aging transformer -were added to the oil. The low molecular weight acids were to a large degree absorbed by the paper, contrary to the high molecular acids that remained in the oil. The three low molecular acids accelerated the aging of the paper, and a clear synergy between these acids and water content was found. The high molecular acids did not influence the paper aging significantly. The results support a theory explaining the hydrolysis as governed by acid catalysis. Examples from measurements on real transformers are shown. It is concluded that low molecular acids constitute an important aging factor for transformer windings and should be included in diagnostic schemes, and when estimating the pay-back from maintenance actions.

A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

“…The low molecular acids accelerate the ageing of the paper, but the high molecular acids do not influence the paper ageing significantly [15,22,35]. The high molecular acids produced by hydrolysis of natural ester can react with the cellulose via transesterification [22,23,34,36], as shown in equation (3).…”

“…The pressboard aged in natural ester has a much higher acid content than the pressboard aged in mineral oil before 93 days. L. E. Lundgaard et al studied the acids in mineral oil-paper insulation system and proposed that the hydrophilic acids are mostly concentrated in the insulation paper [15,35]. The lower the molecular weight of acids, the more easily it is absorbed by insulation paper [15,35].…”

“…From the components of natural ester and mineral oil, it can be known that the main type of acids in natural ester is high molecular acids, such as oleic acids, while the main type of acids in mineral oil is low molecular acids, such as formic, acetic and levulinic acids [15,22,[32][33][34][35]. The low molecular acids accelerate the ageing of the paper, but the high molecular acids do not influence the paper ageing significantly [15,22,35].…”

“…Therefore, the different chemical reaction in oils results in the natural ester having much higher acidity than mineral oil. On the other hand, due to the differences in the chemical structures of acids formed by natural ester and mineral oil, the acids formed in mineral oil are detrimental [15,22,35], while the acids produced by esters are beneficial [22,23,[32][33][34].…”

The Investigation on Thermal Aging Characteristics of Oil-Paper Insulation in Bushing

“…The outermost and innermost It is obviously that the interaction or transference of water between paper and oil seems like more frequent than that of middle layer of paper. On the other hand, acids including high and low molecular weight acids generated by aging of oil-paper affect the degradation of paper greatly [19][20] due to the ionization of H + . Part of acids can dissolve into the bulk oils which are close to the first and fifth layers of paper.…”