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.

A condition assessment of a group of field-aged insulators operated for only 6 years on the island of Rhodes after incidents of intense electrical activity is presented in this paper. The investigated insulators were in service in 150 kV overhead transmission lines operating in proximity to the seacoast, exposed to the action of marine pollution. Although the same type of insulator has been widely used in similar conditions, both on the island of Rhodes and on the island of Crete, incidents of intense electrical activity have only been experienced in a specific area on the southwest side of Rhodes. To understand the deterioration mechanism, in addition to a group of failed insulators, a number of insulators from the same area without indication of deterioration were removed to be tested. In total, 40 insulators were examined: 23 with extensive failure, three without any deterioration and 14 with different levels of tracking and erosion traces along the polymeric housing. A series of tests were performed, including visual inspection, hydrophobicity classification, insulation performance through tanδ measurements, an adhesion test between the polymeric housing and the rod and material identification of the housing material through FTIR-ATR. The results indicate that the main failure mechanism is insulator flashunder due to the poor adhesion between the polymeric housing and the rod, as well as the poor sealing of insulators, favoring the ingress of water on the insulator rod and the initiation of electrical discharges.