Influence of Thermally Accelerated Aging on Inception and Breakdown Voltage in Mineral Oil Under Insulated Electrode Configuration

This article presents the results of the original studies concerning initiation and breakdown in mineral oil under the lightning impulse voltage of negative polarity in the system of insulated electrodes subjected to the thermally accelerated aging. Within the studies the comparison is done between the measurement results obtained under the initial condition (no aging) and during the three steps of thermally accelerated aging. After each aging cycle (120 hours of aging of mixture of oil, copper and paper/pressboard at 150°C) the 8 electrode models are tested in terms of inception voltage, breakdown voltage and time to breakdown to assess the potential influence of aging process on the parameters mentioned. The results concerning three aging cycles set with the results connected with initial conditions show that the aging decreases inception and breakdown voltage. The explanation of the differences noted can be put into changes in the chemical structure of the mineral oil itself that reduce dielectric strength of the oil as well as the loss of dielectric properties of aged cellulose based solid components. Both processes are equally important for explanation of relationships observed.