Monitored Migration of Additives from Thermally Upgraded Paper into Various Insulation Liquids

To determine the long-term persistence of stabilizing additives in thermally upgraded Kraft paper insulation material under operation conditions, the nitrogen migration from the solid material into the encircling insulation material was monitored as part of a large-scale ageing study. Therefore, thermally upgraded as well as normal Kraft paper, combined with pressboard, copper and sheet metal in representative amounts and ratios, was immersed in different types of insulation liquids (mineral oil, G-t-L oil, synthetic as well as natural ester). A continuous 56-day ageing experiment under oxygen-free conditions at elevated temperatures (130 and 150°C) was conducted, with a continuous sampling interval of 2 weeks. The nitrogen concentration of the embedding insulation liquids stayed constant for every insulation liquid aged with natural Kraft paper material, whereas a clear increase of nitrogen levels could be observed over time as soon as thermally upgraded Kraft paper was immersed into the insulation liquids. For prolonged ageing periods at 150°C the measured nitrogen levels in the insulation liquid could rise by as much as 120 ppm, corresponding to nearly 0.3% of dry insulation paper mass, indicating a significant loss of nitrogen-rich stabilizing agents into the corresponding insulation liquid just within weeks of ageing.