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Today, the transformer industry uses alternative insulating liquids more and more. Possible alternatives are vegetable esters such as rape seed, soy bean or sunflower oils or synthetic esters. The aim is to replace the flammable and environmentally harmful mineral oils. Another advantage of these liquids is the higher water solubility, which is claimed to be responsible for a reduced aging rate of cellulosic insulation compared to mineral oil. In a comprehensive aging experiment 3 mm thick, high density pressboard was aged side by side in nine different insulating liquids; four vegetable oils, one synthetic ester, one isoparaffinic and three naphthenic mineral oils. The materials were aged at 120, 135 and 150°C for up to twelve months in vessels with open, as well as with closed, expansion systems. To simulate the load cycles in a real transformer, the oil was cooled to about 80°C for two hours each day. The effect of the thermal aging on the mechanical, chemical and electrical properties of the pressboard and the different liquids are discussed.

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