Effect of temperature gradient on space charge distribution in oil impregnated papers

Temperature gradient existing in the oil impregnated papers insulation such as converting transformer or DC bushing will strongly impact on the space charge distribution. Consequently, the space charge profiles and field distributions in single layer of oil impregnated papers was measured by the pulsed electro-acoustic (PEA) method under different temperature differences (Δθ=0~40°C), and DC stresses of 10, 20 and 40 kV/mm, respectively. The test results show that the heterocharge always exists near both electrodes under different applied DC stresses and different temperature gradients, suggesting that the ionization of the impurity may play an important role in space charge accumulation. Moreover, the higher the temperature difference, the more the heterocharges at the low temperature side of the sample, which results in the greater field distortion there. However, the maximal field distortion factor decreases with the increase of stress. The results will be useful for the optimum design of the oil impregnated papers insulation in the converting transformer.