Aging of pressboard in different insulating liquids

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.

I. Introduction

In the recent years, the variety of insulating liquids for transformers has increased steadily. The reasons for this increase are manifold: •

The use of inhibited mineral oils is becoming more widely accepted especially in Europe. It is used for decades in North America.

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Mineral oils have been adapted in order to overcome the corrosive sulfur problem.

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Natural and synthetic esters have been introduced for transformers which are located in densely populated areas due to their lower flammability.

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The higher biodegradability of synthetic and natural esters is advantageous when transformers are mounted in environmentally sensitive places.

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Natural esters are based on renewable resources and not dependent on fossil raw materials.