Analysis of the Degradation of Five Esters and the Effect of the Cellulose on Their Properties

The need to replace mineral oils used in the cooling and insulation of transformers has led to the development of various alternatives. The most promising options include synthetic and natural esters of various origins. However, based on the available information and the studies conducted under different conditions, it is not possible to determine if the long-term behaviour and properties of all esters are the same. This work analyses the five main commercial esters (sunflower, rapeseed, soybean, palm and synthetic) and their degradation when combined with kraft and thermally upgraded papers. The insulating materials were aged along with copper at 130 °C in sealed glass vials. The performance of the fluids was studied through the measurement of moisture content, acidity, interfacial tension, and dielectric dissipation factor. Moreover, the ageing of the cellulosic materials was monitored by the reduction of the polymerisation degree. The results showed that there are significant differences between esters of different origins. Furthermore, the properties of the esters were affected by the paper they were combined with.