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all rights reserved transformers are critical elements for the transmission and distribution of energy in electricity power systems. they are expected to operate for up to 40 years and a transformer's life is actually the life of the internal insulation system, which consists of Kraft paper immersed in insulating mineral oil. Kraft paper is composed of cellulose (about 90%), hemicelluloses and lignin. cellulose degrades slowly with the ageing of the transformer and the polymer chains break down, releasing degradation products into the oil. 1 Insulating oils are composed of naphthenic, paraffinic and minor amounts of aromatic hydrocarbons, and can be degraded producing carboxylic acids. the main transformer ageing accelerators are moisture, oxygen and heat. thermal ageing of the Kraft paper is the critical life-limiting process, and its main