Resistivity of transformer oil at low and medium field strengths

In certain terminal equipment used in high-voltage d.c. transmission schemes, transformer oil is subjected to a d.c. stress. The behaviour of the oil under this stress is important in the design of equipment. The volume resistivity ¿ of five samples of transformer oil, ranging from new oil to oil after 10 years´ service in a transformer, was measured over a range of stress levels E and electrode spacings d. The resistivity increased with increasing field strength up to a critical field strength, above which it fell sharply. Below this critical value, the resistivity was found to be a function of the ratio E/d. The curve of ¿ against E/d consisted of two parts: (a) for low values of E/d, ¿ was constant, (b) for higher values, the resistivity followed the equation ¿ = ¿ ¿ = ¿ (E/d)n where ¿ and n are constants. Some potential-distribution measurements were also made, and the stress in the oil was found to be uniform and approximately equal to the applied stress over the bulk of the oil in the interelectrode space. The results of the resistivity measurements are interpreted in terms of electrolytic conduction due to dissociation of weak electrolytes in the oil.