Experimental study on vegetable and mineral transformer oils properties

In the last years, the vegetable oil for power transformers has attracted much interest from the manufacturers and users of power transformers especially due to its superior fire point and biodegradability. In this paper an experimental study regarding the properties of vegetable and mineral oils subjected to thermal stress (155 °C, 1500 h) in the presence and absence of paper is presented. During the tests, at different time intervals, conductivity, real part of relative permittivity, total acid number and water content in oils and paper have been measured. It has been concluded that water content of vegetable oil decreases and that of mineral oil increases with ageing time. The acid number and conductivity of vegetable oil with and without paper increases with ageing time. Both properties increase much more for vegetable oil with paper. Also, the influence of acid content on new and aged vegetable oil´s conductivity has been studied; it results that conductivity increases with acid number. Finally, it is shown that for the vegetable oil´s samples with the same values of the total acid number, the conductivity increases if the water content increases.