Over the past years, there has been an increasing interest in natural ester insulating liquids for application in power transformers, thanks to their higher environmental compatibility and lower fire point, as compared to conventional mineral oils. The main function of a transformer oil is its ability to dissipate heat, while ensuring electrical insulation. Among the various influencing parameters, it is well known that heat exchange in a liquid is also affected by its viscosity. Thus, the temperature dependence of the viscosity of an insulating oil should be carefully considered in the design of the cooling system of a power transformer. In this study the variation of the dynamic viscosity with temperature of commercially available natural esters and mineral oils is compared. Steady state rheological measurements are carried out, through a controlled shear rate rotational rheometer, in the 20 degrees C divided by 100 degrees C temperature range, under isothermal conditions. Temperature ramps are also performed, while fixing the shear rate, with the aim of investigating the variation of the steady shear dynamic viscosity over the whole temperature range. Results demonstrate that the viscosity of natural esters, though being higher at low temperatures, compared to mineral oils, strongly decreases for higher temperatures, thus favouring heat transfer and reducing the risk of hot-spots, which might lead to insulation-failures.
