Experimental study on moisture dynamics in transformers insulated with natural-esters

In the last years great attention has been paid to ester fluids as an alternative to mineral oil. At the present the use of these liquids is becoming an habitual practice in distribution transformers and some experiences are also starting to be reported about their application to power transformers. One of the main differences between ester fluids and mineral oils is that those are able to absorb much greater amounts of water when they are in contact with cellulosic insulation. Moisture is one of the key variables that must be kept under control for a safe transformer operation. As is well known the distribution of moisture between transformer solid and liquid insulation depends on transformer temperature, and thus a continuous migration of moisture between both materials takes place during transformer normal operation. In the case of transformers isolated with ester fluids, the equilibrium points, and probably the time constants of the migration processes, would be affected by the different properties of these materials with regard to moisture. In this work the moisture dynamics in ester fluids is compared with that in mineral oils. Drying processes of pressboard has been experimentally simulated using mineral oil and a natural ester as insulating fluid. The results of these experiments will be used in the future to develop dynamic models that will allow to simulate the moisture migration processes in transformer insulation during normal operation as well as transformer drying processes.