Electrical and Thermal Behaviours of Synthetic Transformer Liquids

New materials, such as synthetic transformer liquids based on designed chemical formulae, could provide better consistency of chemical composition and hence have the potential to have better performance than conventional mineral (hydrocarbon) oils refined from crude oils. Synthetic ester liquid has gained its popularity due to its eco-friendly property and higher fire point. In recent years, an emerging technology that converts gas to liquids (GTL) was developed to produce synthetic hydrocarbon transformer oils. A transformer research consortium in the United Kingdom has been working on both types of synthetic transformer liquids. This paper introduces the electrical, ageing and thermal behaviours of a synthetic ester liquid and a hydrocarbon GTL oil in comparison with a conventional mineral oil.
Partial discharge (PD) characteristics of the three liquids including PD inception voltage (PDIV), PD magnitude and PD pulse repetition rate were compared. The mineral oil and GTL oil have very similar PDIVs, which are slightly higher than that of the synthetic ester. At higher voltages, PD pulse repetition rate is more sensitive than PD magnitude to differentiate the liquids. A laboratory ageing experiment of oil paper insulation at 120 °C was conducted with parameters including moisture, acidity, tensile index and 2-FAL measured. The GTL oil showed similar ageing performance to the mineral oil. The synthetic ester showed higher moisture and acidity values but lower paper ageing rate than the other two oils. The comparison of thermal behaviours was investigated by conducting computational fluid dynamics (CFD) simulations. It was found that under OD (oil directed and forced) cooling mode the synthetic ester, which has higher viscosity compared with the GTL oil and the mineral oil, has a more uniform flow distribution in the oil ducts.