The insulating value of transformer oil (dielectric strength) is effected by cellulose and water contaminants. Copper has the highest thermal conductivity amongst heat-transferring media for windings, while oils have a heat conduction capacity approximately 3-4 times higher than cellulose. The coefficient of thermal conductance between the winding insulation and the oil also impedes heat transfer. This means that practically the entire temperature drop from winding to oil takes place in the cellulose.
The aging of oil is of great interest because the oil surrounds the cellulose in the windings. The chemical and physical properties of the oil have a strong influence on the aging of the cellulose, difficult to replace, whereas the oil is easily replaced. In a transformer, the cellulose is significantly more sensitive and susceptible to aging than oil.
(Vastera s PetroleumKemi AB, Sweden)
Some aspects of the measurement, analysis and management of contaminants within the transformer can be read here ….. DTC