Toshitsugu Ishii

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.

Partial Discharge Characteristics of Silicone Liquids

New safety and environmental considerations in the design and operation of modern public transportation systems exclude the use of materials that can cause fire and ecological hazards. Therefore, the power transformers in trains and subway stations require new safer insulating liquids. The askarels or polychlorinated biphenyl (PCB) liquids have been classified as nonflammable, but recent investigations have indicated that spills of these chlorinated liquids may have adverse ecological effects. The manufacture and use of PCBs in Japan have been prohibited by law. To meet environmental and toxicological requirements, new insulation systems for transformers were developed using silicone insulating liquids. In the evaluation of a high-voltage transformer insulation system, it is important to study partial discharge behavior in the insulating liquid. This paper describes measurements of partial discharge inception voltage and of discharge pulse amplitudes at higher test voltages in silicone liquids. The initial partial discharge inception phenomenon in both silicone liquids and mineral oils is the same and is probably related to the presence of contaminants such as water and particles in the liquids. But the further development and growth of the discharges in the liquids seem to be dependent on the chemical and physical properties of the insulating liquid. During the partial discharge measurements, it was observed that small amounts of solid material were formed on the electrode. Chemical analysis showed the material to be cross-linked polymeric siloxanes. The quantity of gelied material is a function of the partial discharge intensity and the duration of discharges.

Evolution of Hydrogen from Iusulating Oil in Transformers

Gases dissolved in transformer oil are analyzed for the diagnosis of transformers in operation. In some newly installed transformers, we found that the combustible gases dissolved in the oil were mostly hydrogen. From considering the dehydrogenation equilibrium of saturated hydrocarbons it is concluded that oil generates a small amount of hydrogen even at the normal operating temperature of transformers, and there may be catalytic action of metal on the dehydrogenation of oil in these transformers. We found that stainless steel is such a catalyst.