The investigation of the temperature and electric field dependency of mineral oil electrical conductivity

Abstract:
The main insulation of power transformers consist of oil and oil-impregnated paper. This contribution deals with conduction mechanisms in insulating oil at DC stresses. Due to the increasing use of high voltage DC transmission and associated mixing stresses the knowledge of the basic behavior of insulating oil conductivity during operation is required. The temperature dependency of electrical conductivity is investigated in this approach. The electrical conductivity is measured using a plate-shaped test cell that allows a variation of oil sample temperature from 20 °C to 90 °C. The reason of variation of conductivity with temperature could be given using the physical phenomena which occur in oil. Higher temperatures lead to increasing of mobility and ion concentration and these changes have an influence on dissociation process. The long duration conductivity and the conductivity values at 60 s after beginning of measurements are determined based on IEC standard for higher electric field strengths. The results discuss the possible mechanisms for observed electric field dependence of mineral oil.