Changes in the activation energy of thermal decomposition of mineral oil during accelerated thermal ageing

Changes in the activation energy of thermal decomposition of mineral oil during accelerated thermal ageing were studied utilizing thermogravimetry. The main aim was to investigate possibilities of such approach for precise diagnostics of structural changes in mineral oil along the transformer lifetime. The examined oil was a commercially available Nytro Lyra X manufactured by Nynas AB. The accelerated thermal ageing was performed in a laboratory oven under a limited access of air at a temperature of 110, 120 and 130°C. The Fourier transform infrared spectroscopy (FT-IR) was also used to support and interpret observations obtained from thermogravimetry. Values of activation energy were calculated based on standard ASTM E1641. Results of FT-IR revealed that the intensity of the infrared spectral band of the low-temperature inhibitor decreases along thermal ageing within all of the applied temperatures. Evolution of activation energy values over ageing times and temperatures contains three linear regions separated by local minimum and maximum points. Observed changes in activation energy accompanying the ageing process of mineral oil, especially achieving its local maxima are very promising. Together with results of our previous experiments, the activation energy values can present potential observables and its maxima can potentially be considered as a new endpoint criterion for determining the residual lifetime of mineral transformer oil.