Influence of oil electrification on ac breakdown between metallic electrodes

This paper presents experimental investigations concerning the influence of space charge on the ac breakdown of moving mineral oil, between metallic electrodes. Space charges are injected in oil with sharp metallic points, and carried into the breakdown test cell through a perforated ground electrode. Breakdown voltages are measured under ac voltage in rod-sphere or sphere-sphere electrode at 5 and 10 cm gap distances. The results show that, whatever the flow rate of oil and the amount of charge injected, no significant influence on ac breakdown voltage is observed. As a consequence, it appears that the deleterious effect of static electrification of oil in devices like power transformers cannot be simply ascribed to a degradation of the intrinsic oil performance.