Effects of Aging on Condition of In-Service Transformer Oil: The Study Case of 150 kV/20 kV Substations in Crete

The effect of aging on the condition of the oil and the insulating paper of transformers, based on the oil analysis of a fleet of 150 kV/20 kV power transformers, is presented in this paper. Transformer loading is recognized as the main stress mechanism and oil analysis is the primary assessment technique. The transformers investigated have been in operation for 4 to 51 years. Oil color, oil water content, breakdown voltage, oil quality index, the degree of polymerization and the sulfur corrosivity, are parameters investigated to assess the transformer insulation condition. The results indicate that due to aging, the color of the oil darkens and there is an increase in water content and a decrease in breakdown voltage, oil quality index and degree of polymerization. A threshold of 136,000 h of equivalent full-load operation can be considered for the deterioration of oil to an unacceptable level of quality. Considering the transformer’s overall health, which is associated with the condition of the insulating paper, it is shown that the extensive deterioration of the insulating paper starts when the transformer load index exceeds a threshold of 158,000 h of equivalent full-load operation.