Kerr electro-optic field mapping and charge dynamics in impurity-doped transformer oil

We measured dc electric field distribution in transformer oil mixed with different impurities and additives so as to simulate practical degradation conditions of transformer oil. The electric field in the oil doped with asphalt was reduced near the cathode and enhanced near the anode. On the other hand, electric field in oil with copper oleate was enhanced near both electrodes and reduced at the center between the electrodes. The space charge density was estimated from the measured electric field profile using a 1-dimensional form of Gauss law; negative ions occurred more than positive ions by 5 to 40 pC/cm³ in the oil with asphalt from 10 to 30 ppm. It was also found that heterocharges with 100 to 300 pC/cm³ existed near both electrodes in the oil with copper oleate. Consequently, the electric field and the charge distribution in the oil proved to change depending on the kind and content of impurities or additives and thus depending on the degradation of the oil