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 PCB's 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.