Preliminary Investigations on the Gassing Tendency and Breakdown strength of Low Pourpoint Transformer Liquids under Selective Conditions

The overall performance of ester liquids is comparable to mineral insulating oils with a reduced environmental impact. However, the application of ester fluids in cold regions is still a challenge to the utilities and transformer owners. In this paper, the water behavior of three low pour point (less than -50 degrees C) insulating liquids (two synthetic esters and a mineral oil) is reported. Non-aged and aged liquid samples underwent electrical discharges of 7 kV (as per a modified ASTM D6180) for 5 hours to understand the gassing tendency and change in water content in the liquid under corona discharges. Also, the water uptake rates of non-aged low pourpoint liquids have been verified at 60 degrees C. Further, non-aged liquids with three different initial water contents have been subjected to 20 repeated AC breakdowns to understand the influence of water on their breakdown failure probability. The obtained results indicate that the gassing tendency under electrical stress of the investigated liquids increases with aging. Similar to the typical ester liquids, the two low pour point ester liquids have higher water saturation limit than that of the mineral oil. In addition, the breakdown failure rates of the low pour point liquids increase with increase in water content.