Effects of Thermal Aging on Blended Oil Characteristics in Comparison to Mineral Oil and Synthetic Esters

The article aims to investigate the long-term behavior of insulating oil in high-voltage applications by analyzing and correlating the changes in its physicochemical properties over the high-temperature range. To comprehend the rate of oil degradation, uninhibited mineral oil (MO), blended oil (MSO), and synthetic ester (SE) have been exposed to enhanced thermal stresses in combination with solid insulation. In compliance with IEEE C57.100, a five-point aging mechanism for thermal aging was taken into account. Thermal stresses ranging from 100 °C to 195 °C were applied continuously for 168 h with a temperature variation of 25 °C. The effects of thermal stress have been deliberated by analyzing changes in physicochemical properties, such as density, kinematic viscosity (KV), acidity, flash point, and visual observation (VO) of the MO, SE, and mixed oil. The results demonstrated that the inclusion of esters into MO enhances its characteristics and functionality and accentuates its suitability for assessing the state of high-voltage transformers.