Insulating oil quality diagnosis in exploitation

In the electric equipments with oil insulation, the processes of degradation and ageing of the electroinsulating oil can occur because of the solicitations during the exploitation. These solicitations are produceing a modification of the electroinsulation properties in such a measure that the dielectric breakdown phenomenon can occur having negative consequences on the operational safety of the electric devices. In order to avoid the malfunction, it's mandatory to make a periodically monitoring of the values, so they remain in admissible limits. Evaluating insulating properties of the oil is done by two important electric values: the dielectric rigidity and dielectric loss tangent angle. To avoid the appearance of defects it is necessary to have a monitoring system which can present over time the difference between the values in order to take the necessary actions and keep the values within acceptable limits. By analysing a case study with contradictory results for the profilactic measures of the same sample, one of the parameters having the acceptable values and the other not, the paper work focuses on highlighting the influence factors and understanding the mechanisms that lead to the degradation of electroinsulating oil.