Abstract

The solid insulation materials widely used in the transformer are paper, pressboard, and transformer board, which are formed from the cellulose found in plants. The main concern with using dry paper as an insulating material is that it is very hygroscopic. Insulating papers and pressboards must have high dielectric strength and high tensile strength. However, these materials are gradually degraded due to thermal stress, oxygen, and moisture. Because the tensile strength decreases owing to degradation, the degradation of insulating material may affect the life of transformers. In our laboratory, through accelerated tests using models of oil-impregnated insulating systems, changes in characteristics of insulating cellulose materials have been investigated. These data were compared with data obtained from insulating papers of transformers with long service life. From these investigations, good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and of degree of polymerization. Using this correlation, the degree of degradation of insulating papers in transformers may be known from the amount of gas. The average characteristic curves of insulating papers from transformers coincides with the degradation curve of 90 ° C obtained in experiments. And if the life of insulating papers is considered to be equal to the life of transformers, the life of transformers is between 20 and 40 years.

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