Further studies of a vegetable-oil-filled power transformer

In this paper, data obtained from a transformer fitted with an online monitoring system, and using FR3 vegetable oil, were presented. The concentration of water dissolved in the oil was initially around 15 ppm, increased to 25 ppm, and then decreased to 15 ppm over a period of two years. These levels are higher than the levels expected in mineral oil because of the higher hygroscopy of vegetable oil. Ethane was found in a nominally identical transformer that had not been used for two years. This observation supports the hypothesis proposed in previous work that ethane can be generated in FR3 fluid even under no-fault conditions. In the operating transformer the ethane concentration increased during the first month of energization and then decreased to a constant value. The DDF did not vary significantly between oil samples taken from the operating transformer. In the IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers, a maximum value of 0.005 at 25οC is suggested for unused vegetable oil in new equipment. The corresponding value in the operating transformer was around 0.003 at ambient temperature.