Parameters affecting the electrical and thermal properties of transformer oils

Transformer oil is expected to function as an insulating medium and heat transfer agent. In this contribution, contaminants that mostly influence the electrical and thermal properties of transformer oil are addressed. To access the influence of the type of impurities on oil properties, various scenarios were considered. Investigations were therefore performed on new oil, new oil submitted to electrical stress, service-aged oil, reclaimed (by Fuller's Earth) service-aged oil, dehumidified service-aged oil, dehumidified and degassed service-aged oil. The influence of each type of “contaminant” is emphasized and discussed. The results obtained from the measurements show that electrical discharge by-products (charges carriers), mostly affect oil insulating properties, and influence the conduction phenomena which in turn increases the dissipation factor. It is also shown that colloidal suspensions in oil affect the heat transfer capability of this later, as assessed by viscosity.