Investigation on Degradation of Power Transformer Solid Insulation Material

Power transformers are vital parts of electric energy transmission and generation systems. Numerous power transformers in the European power grid have reached an age of 30 years or even more and are possibly close to their end of life. Because of economic reasons it is demanded to keep current assets as long as possible in service without taking the risk of a failure. The DP (degree of polymerization) value is an indicator for the mechanical stability of Kraft paper insulation. The ageing process depends on the transformer's operation temperature and is accelerated by the presence of moisture and acids in the insulation system. During the decomposition reaction of cellulose chains furanic compounds and water is generated. In this paper investigation results from samplings of solid insulation material and their correlation to the concentration of furfurals in oil are presented. These investigations have been carried out during scraping of power transformers that have been put out of service due to failure or replacement. The distribution of the DP value over the insulation system shows a non-uniform ageing of the paper material.