Mechanism behind Increasing Electrostatic Charging Tendency in Aged Mineral-Oil-Immersed Pressboards

The surface chemical properties of aged pressboards (PBs) were evaluated to investigate the mechanism behind the increasing tendency of electrostatic charges for diagnosing power transformers. The PBs were taken from several aged transformers, and their surface elemental composition and chemical structures were measured by X-ray photoelectron spectroscopy. The PBs properties were measured after being washed by an organic solvent to remove impregnated mineral oil. Carbon (C) and oxygen (O) atoms were mostly observed on virgin PBs surfaces; these are the atoms that compose cellulose polymer, the main component of kraft pulp. In contrast, sulfur (S) and calcium (Ca) atoms were observed on the surfaces of aged PBs in addition to the C and O atoms. Moreover, the surface densities of the Ca atoms had a good correlation with those of the S atoms. While we detected no S atoms inside the aged PBs, we observed almost the same amount of Ca atoms as in the virgin PBs. Mineral oils mainly consist of hydrocarbons, with very small amounts of components such as sulfur compounds derived from crude oil. Heat stress degrades these sulfur compounds. On the other hand, PBs made of kraft pulp contain a very small amount of metal ions, such as Ca ions. We conclude that S atoms observed on aged PB surfaces are a part of the aged sulfur compounds adsorbed from the mineral oil. The adsorption of aged sulfur compounds induced the migration of Ca ions from the inside of the PBs to the surface, which caused the surface density of the Ca ions to increase. This suggests that the accumulation of Ca ions along with the sulfur compounds led to the increasing tendency of electrostatic charges in aged PBs.