Dielectric Loss in Insulating Liquids

Abstract

A study has been carried out on the dielectric-loss behavior in mineral oils and silicone liquids under varying frequency, temperature, and voltage-stress conditions. Three distinct types of loss mechanisms, namely those of dipole orientation, space-charge polarization, and ionic conduction were manifest in the oils. It was found that both the dipole orientation and the ionic conduction losses increased markedly with the aromatic content of the oils. The size of the dipoles, which caused the polar loss, was estimated to vary from 1Å in the highest viscosity oil to 5.5Å in the lowest viscosity oil. The effect of viscosity upon the dipole-loss intensity appeared to be considerably less pronounced than that upon the ionic conduction loss. The increase of ionic conduction with the aromatic content was attributed to the presence of ionizable oxidation products and their increased dissociation factor in the more aromatic or polar oils. The space-charge loss was not found to be a simple function of the aromatic content or the viscosity of the oils; its occurreilce was ascribed to the presence of residual moisture within the oils. The silicone liquids exhibited no dipole loss, although their space-charge and ionic-conduction-loss behavior was similar to that in the oils. A peculiar type of absorption loss, whose magnitude increased with falling temperature but remained independent of frequency, was observed in the high-viscosity silicone liquid below -50°C.