Investigation on the Pyrolysis of Mineral Oil and Decane Stressed by Thermal Transformer Faults

The aging mechanism that occur during various stresses in a complex insulating system of oil-filled power transformer, mainly consisting of insulation liquids and paper, are so intricately structured and dependent on several influences that no clear reaction mechanism taking place can be defined. Only thermodynamic and thermochemical considerations can be used to calculate an estimation of the probabilities of the various aging reactions occurred. The precise knowledge of the aging reactions and secondary reactions taking place during the operation of oil-filled power transformers can increase the reliability of diagnostic methods for condition monitoring or fault detection in order to extend the lifespan. The overall aim is an improved assessment of the severity and activity of the transformer fault. Thus, this investigation offers a first estimate of pyrolysis reactions occurring in mineral oil. The reaction enthalpies of the pyrolysis of pure decane and cyclohexane under standard conditions are calculated and an estimation of the probability of the occurring reactions and thus generated fault gases is determined. Using laboratory investigations with a thermal fault model, the generated fault gases in mineral oils and n-decane, cyclohexane are detected and compared. There are large differences in the gassing behavior of naphthenic and paraffin-based mineral oils and their pure components. In particular, the paraffins have a higher gas formation rate than naphthenes. Furthermore, a high temperature dependency was observed on the formation of gas type and rate.