Analysis of Cellulose and Copper Contamination Effects on the Dielectric and Physicochemical Properties of Natural Ester Oil Under Thermal Aging

Transformers depend strongly on insulating fluids for efficient and reliable operation. Current trends are changing towards natural esters as environmentally friendly alternatives. Understanding how these oils degrade is essential, as changes in oil quality can directly affect the performance and reliability of transformers. This work analyses the effect of degradation due to oxidative aging and the influence of contaminants (copper and cellulose) on natural ester density, viscosity, absorbance and dissolved decay products, as well as dielectric properties (real part of relative permittivity, dissipation factor and conductivity). Four aging periods (0, 164, 332 and 500 hours) and four contaminant conditions (without contaminants, with copper, with cellulose kraft paper and with copper and cellulose kraft paper combination) are considered. The results show that the combination of copper and cellulose exhibits a synergistic effect, causing the most pronounced degradation of all measured properties, causing the gelation of the 500-hour sample.