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.

Surface hydrophobicity is a critical property of silicone rubber composite insulators. It is the primary advantage in comparison to ceramic and therefore is the first to be investigated when field aged insulators are examined. Initially hydrophobicity was evaluated by static contact angle measurements. However field and laboratory investigations have revealed that in the case of outdoor high voltage insulation, it is the dynamic performance that is critical and especially the capability of hydrophobicity recovery, available due to a low molecular weight molecules migration mechanism. This mechanism efficiency is related to the material formulation but also to the experienced service conditions and the ageing degree of the insulator. Therefore laboratory evaluation of field aged insulators can be considerably beneficial for both researchers and utilities. In this direction the hydrophobicity recovery efficiency of three field aged insulators, with two different base polymers is investigated in this paper. Both static and dynamic measurements are performed, according to the CIGRE guidelines. The results verify the superiority of silicone rubber polymer and reveal a recovery time exceeding 10 hours. Not considerable differences have been observed due to aging in this case. Further investigation is required, in order to be able to correlate this behavior to the insulator efficiency.

In this study, we have investigated the relationship between the plasma temperature in remote laser-induced breakdown spectroscopy (LIBS) experiments and the hydrophobic properties of silicone rubber insulators (SIRs). Contact angle and LIBS measurements were conducted on both artificially-aged (accelerated aging) and field-aged SIRs. This study reveals a clear connection between plasma temperature and the properties of aged SIRs on artificially-aged SIR specimens. Specifically, the plasma temperature exhibits a consistent increase with the duration of the accelerated aging test. The hydrophobicity of the artificially-aged SIRs was assessed by performing contact angle measurements, revealing a decrease in the hydrophobicity with increased aging test duration. Furthermore, we extended our investigation to the study of nine field-aged SIRs that had been in use on 150 kV overhead transmission lines for 0 to 21 years. We find that the laser absorption and hardness of the material do not relate to the plasma temperature. In summary, we observe a direct connection of plasma temperature to both contact-angle measurements and operation time of the in-service insulators. These results strongly suggest the potential use of LIBS for remotely evaluating the hydrophobicity and aging degree of silicone rubber insulators, thus assessing their real-time on-site operational quality.

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.