Measurements of DC resistivity of insulating oils

Insulating oils are widely used in power transformers. The oil insulation degrades under multiple stresses during the transformer operation, which affects the insulation integrity and consequently could cause transformer failures. DC resistivity is one of the dielectric parameters of insulating oil for assessing transformer insulation conditions. This paper presents results of DC resistivity measurements of oil samples including a mineral oil and a synthetic ester in fresh and thermally-aged conditions as well as two in-service aged oil samples in a cylindrical test cell under temperature range of 20
^o
C to 100
^o
C. Two standards of DC resistivity measurement namely IEC 60247 and ASTM D 1169 were applied. For mineral oil, the results of ASTM D 1169 are higher than those of IEC 60247 which is caused by polarity dependence of DC resistivity. For the other samples, the results are the same for both standards. Further investigation on polarity dependence shows that it might be caused by different ion injection strength of the inner and outer electrodes.