"Contribution on the investigation of the physicochemical and electrical properties of vegetable oils as dielectric liquids in high voltage equipment under the ecodesign framework"

The aim of the doctoral thesis is to study specific characteristics of vegetable oils and to identify the factors that allow successful replacement of conventional insulating oils with plant origin oils, in high voltage electrical equipment and especially in high voltage transformers. Two of the main challenges for power transformers, as already outlined in European eco-design legislation, are the reduction of power losses and the prolongation of their operating time. Both of these challenges relate, among other things, to the insulating oils used in this electrical equipment.
Conventional insulating oils for high voltage electrical equipment are products of fractional distillation of crude oil. Mineral oils are almost fully recyclable and can be regenerated with satisfactory results, but are not biodegradable, have dangerous flammability characteristics, as well as toxic properties for both humans and the environment.
Lubricants based on vegetable oils are biodegradable and recyclable while not toxic and flammable. In addition, they are considered a viable solution as they are products of farming and their stocks can practically be considered unlimited, unlike petroleum origin, which are inevitably subject to the limitations of crude oil reserves.
Although replacing conventional insulating oils of mineral origin with vegetable oils is a one-way in the electrical equipment market, there are in practice issues of incompatibility between these two categories of insulating oils, due to their different chemical composition. Their mixing, especially in real conditions of aging, heating and oxidation, creates unwanted by-products that reduce the performance, functionality and service life of the equipment.
This doctoral research will address the physicochemical and functional (electrical) parameters not only of the oil itself but also of the transformer elements, with emphasis on the combination of insulating oil and insulating paper. This emphasis is important, because the insulating paper in the transformers is a critical material for the transformer operational condition, especially when replacing the oil. This is due to the impregnation of the paper with the oil, resulting in problems of compatibility between residues of the old and the new oil, as well as between the new oil and the insulating paper.