Electric filed distribution in transformer oil under DC electric field

Due to the accumulation of surface and space charges, which were created by the dissociation of neutral molecules in oil and by the ion injection from the metal electrodes in oil-paper insulation system under DC electric field, the field induced by these charges will distort the original applied fields. So the DC electric field distribution in oil shows some different features compared to its AC counterparts. As one of the key parameters, the DC field distribution in the oil bulk must be considered during the design of the insulation structure for oil-paper insulation system under DC electric field. In this paper, the space electric field distribution in a 6 mm oil gap with two plane metallic electrodes were measured using a contactless electric field measurement method based on the Kerr electro-optic effect. The results show that the DC electric field in the oil gap is a non-uniform field and its ratio to average DC field ranges from 0.87 to 1.05. Furthermore, the field values near the anode are large than that near the cathode and it increase monotonically from cathode to anode under the circumstances of equally applied voltages. Moreover, these experiments show that the normalized electric field by E₀ could be distinguished by three stages: a. at low electric field stage (E<0.5 kV/mm), the normalized field increase monotonically with the applied voltage. b. at high electric field stage (E>4.5 kV/mm), the normalized filed decrease monotonically with the applied voltage. c. at medium electric field stage (0.5 kV/mm<E<4.5 kV/mm), the field change in the relative moderate manner.