Insulating oils

Following a systematic investigation using laboratory procedures previously developed, Piper⁽¹⁾ et al contributed further significant data on the fundamental causes of electrical losses occurring in insulating oils and oil-impregnated papers. Addition of asphalt to a refined oil was shown to cause the oil to have a relatively high power factor and high conductivity. The conductivity appeared to be almost independent of the frequency of the impressed voltage. The asphalt-oil mixtures were similar in properties to those of oil solutions of electrolytic dissociable materials, such as lauryl sulfonic acid. Both asphalt and lauryl sulfonic acid mineral oil solutions are strongly absorbed by paper when used as impregnants. However, :the low frequency (60 cycles) a.c. equivalent conductivity of the paper impregnated with the asphalt-oil mixtures is lower than the conductivity of the solution itself, while the reverse is true for the lauryl sulfonic acid-oil solutions. In the latter system, the increase in the lauryl sulfonic acid-oil-paper conductivity (over that of the base oil-paper conductivity) is shown to be caused by dielectric absorption, while in the paper-asphalt-oil system, the slight increase is largely caused by conduction alone. The authors suggest that this difference in behavior provides a means for determining whether high conductivity in an insulating oil is caused primarily by electrolytically dissociable materials, such as acids, or by colloidally dispersed materials, such as asphalt. The procedure for this determination would be to examine the 60 cycle conductivity of the oil alone and when impregnated in filter paper. An increase in conductivity of the oil-paper compared with the oil alone would indicate predominately electrolytically dissociable types; while a decrease would indicate colloidal types of contaminants in the oil.