Thermodynamic Study on the Thermal Decomposition of Insulating Oil

The thermal decomposition of hydrocarbons is considered on the basis of thermodynamic equilibrium. This consideration gives a theoretical interpretation of the analysis of dissolved gases in transformer oil. This kind of analysis is now widely used for the diagnosis of transformers in operation. The thermal decomposition of hydrocarbons is divided into primary and secondary stages. From the thermodynamic equilibrium follows that more saturated hydrocarbon gases are evolved than unsaturated ones in the primary decomposition which is dominant at a lower temperature. At a higher temperature, methane, ethylene, and propylene are evolved more than other hydrocarbon gases in the secondary decomnposition. Overheating of transformers may be estimated by the ratio of the three gases mentioned above. The gas composition is calculated in the state of equilibrium. Experimental data on the thermal decomposition of hydrocarbon and gases in transformer oil, although not obtained in the state of equilibrium, show good agreement with the theoretically calculated result.