Thermal deterioration of transformer results in the decrease of tensile strength, rather than electrical strength of the insulating paper. The decrease of tensile strength has a constant relationship to both the decrease of average polymerization and the increase in the amount of furfural formed by thermal deterioration of insulating paper in insulating oil. The relationship between the decrease of average polymerization and the increase in the amount of furfural formed by thermal deterioration of insulating paper in insulating oil is cleared. The decrease of average polymerization has a constant relationship to the increase in the amount of furfural formed. Then an allowable limit for the drop in average polymerization was determined using the amount of furfural formation dissolved in insulating oil.
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