Effect of DBDS concentration and heating duration on copper sulfide formation in oil-immersed transformer insulation

Copper sulfide is found to be more likely formed on Kraft paper in oil-immersed transformers with an increase in either dibenzyl disulfide (DBDS) concentration or heating duration, which is verified by heating tests stipulated by IEC 62535. Two effects of DBDS concentration on the formation rates of copper sulfide were obtained. The formation rates of investigated oils are proportional to the DBDS concentration when the concentration is low. However, the amounts of copper on the Kraft papers are constant when the DBDS concentrations are more than certain values, which is considered to reflect the saturation of dissolving DBDS-Cu complex in the oil. These concentrations depend on oil brand. Copper sulfide begins to form after some duration of heating and the duration decreases with an increase in the DBDS concentration. Copper sulfide formed on the Kraft paper is a portion of the total copper reacted with DBDS by comparing the amount of copper formed on the Kraft paper with the total amount of reacted copper calculated from the amount of consumed DBDS.