Evaluation of transformer oil aging under service load from various chemical and dielectric measurements

Oil from transformers under normal service load was analyzed by several techniques in order to determine the degree of aging and the best aging evaluation method. The interfacial tension was found to vary with the antioxidant concentration but it is also affected by exposure to daylight, which implies that samples should be measured without too long a delay. Carbonyl and polar groups were determined by IR and HPLC, respectively. The dissolved capper content in the oil, which modifies the tan o values, was determined by neutron activation analysis. Tan δ and dielectric breakdown are not really conclusive tests because of their sensitivity to contamination. The best techniques to evaluate the aging of transformer oil, and to predict its lifetime, are interfacial tension and antioxidant concentration (above 100–150 ppm) measurements and HPLC analysis for lower concentrations.