Lightning Performance of Selected Dielectric Liquids After Thermal Aging

The aim of this paper is to investigate impact of thermally forced accelerated aging process on lightning properties of synthetic ester fluid (SE) compared to conventional inhibited mineral oil (IMO). Research plan implemented in this paper involves examining lightning properties of liquids after accelerated aging process. Process will consist in affecting liquid with a high temperature of 150° C for a period of 168 hours, and then cooling liquid and conducting tests for lightning impulse breakdown voltage (in accordance with IEC 60897) taking into account the initiation voltage (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{i}$</tex>) and acceleration voltage (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{a}$</tex>). Tests were carried out in a pointsphere electrode system for a 25 mm inter-electrode gap and negative polarity of lightning impulse voltage. Tests are performed on fresh liquid and then after two aging cycles. Liquids will also be verified in terms of other parameters such as kinematic viscosity, dielectric loss coefficient, acidity, and interfacial tension, which are indicators of liquid aging. Based on measurements performed, it will be possible to correlate the level of aging with changes in lightning properties of liquids under consideration. The tests were carried out using a 6 -stage lightning impulse generator (LIG) with a system for recording pre-breakdown phenomena based on a photomultiplier system for recording light emitted by discharge. In context of tracking discharge processes also used an high speed camera, which enable a visual assessment of developing discharges in interelectrode gap and check whether aging process affects not only initiation and breakdown parameters, but also the spatial shape of discharges. Results will allow us to verify how alternative dielectric liquids to mineral oil behave after longterm operation.