Effect of temperature on the breakdown voltage of paper-covered conductor impregnated with various liquids

Lengths of flat copper conductor insulated with aromatic polyamid (aramid) tape or cellulose tape were bent into rings and secured around the edge of discs. Two discs were placed edge-to-edge in one plane and breakdown voltages were measured between the conductors. The conductor assemblies were dried under vacuum, impregnated and immersed in several low-flammability liquids at temperatures up to 180°C under air. Comparisons were made with mineral oil which was tested up to 130°C under nitrogen. With the discs set to provide a 1-mm liquid gap between the insulated conductors, the variation of breakdown voltage with temperature is limited to a few percent and values tend to their highest at 80°C for both AC and lightning impulse. With the conductors in contact, alternating breakdown voltages vary little, whereas impulse voltages fall appreciably with temperature rise, especially for cellulose