Assessment of transformer insulation condition by evaluation of paper-oil system

Cellulose paper is the best known solid insulating material used in transformers. It is exposed to different types of stresses under normal/abnormal service conditions. This leads to ageing of the paper, causing depolymerisation of the cellulose material and the formation of decomposition products, like 2-furfuraldehyde and 5-hydroxymethyl, which dissolve in oil. Degradation of paper causes a reduction in the degree of polymerisation. Hence, the chemical analysis of transformer oil and paper gives evidence of changes that are taking place in the winding insulation during the service condition of the transformer. The author discusses the degree of polymerisation of fresh and service aged cellulose paper samples and analyses the oil samples, using HPLC, collected from different site transformers. The oxidation stability of oil samples was determined by RBOT and the insulation condition was assessed taking into consideration the experimental and theoretical furfural values. The condition of cellulose paper was also assessed by SEM.