Thermal properties of synthetic ester-based transformer oil during ageing in laboratory conditions

Synthetic esters are interesting as transformer insulating liquids for use in densely populated and environmentally sensitive areas because of high flashpoint, good biodegradability, low toxicity, good electrical and thermal properties, resistance to electrical discharges and compatibility with materials in transformers. Monitoring of liquid insulation is a convenient method for assessing general condition of the transformer. Since synthetic esters have only recently been put into use in power transformers, there is still not enough data on change of their properties during exploitation for transformer condition assessment. Monitoring changes of properties in laboratory ageing conditions, as well as the determination of characteristic degradation products will contribute to a better understanding of the insulating liquid and to the assessment of its functionality. In this paper, changes of physico-chemical properties of synthetic ester-based transformer oil in laboratory ageing conditions were investigated. The oil was submitted to 120 °C and 150 °C at different time intervals. After the exposure to high temperature, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed. DSC was performed for the purpose of gathering qualitative and quantitative data about physical and chemical processes that include endothermic and exothermic effects and heat capacity change. Heat capacity is highly affected by viscosity, so temperature dependence of viscosity as well as viscosity change during ageing was monitored. TGA was performed for the purpose of material characterization and gathering of information about thermal stability of oil. The investigation contributes to better understanding of thermal ageing kinetics of environmentally friendly synthetic ester-based insulating liquid.