Identification of affecting factors of copper sulfide deposition on insulating paper in oil

Effects of oil brand, temperature and atmosphere conditions on copper sulfide deposition on insulating paper were investigated by thermal aging test. Copper sulfide deposition occurred in a certain oil brand and dibenzyl disulfide (DBDS) was found in the oil. There was an incubation period observed before copper sulfide deposition occurred. Copper sulfide deposition was accelerated by temperature. The deposition rate after the incubation period became twice when the heating temperature was increased by approximately 10degC. The incubation period and the deposition rate were the same at 140degC whether the thermal aging test was done in air or nitrogen atmosphere. On the other hand, the incubation period in air was shorter than that in nitrogen at 120degC. The deposition rate was not affected by atmosphere condition at 120degC. Intermediate substances due to copper dissolution into oil are introduced to explain the incubation period and the atmosphere dependence of incubation period observed at 120degC.