Concentration dependence of corrosive sulfur on copper-sulfide deposition on insulating paper used for power transformer insulation

Copper-sulfide deposition on coil insulating paper used for power transformer insulation is investigated with the aim of obtaining the concentration dependence of corrosive sulfur. Several mineral insulating oils containing dibenzyl disulfide (DBDS), which is typical corrosive sulfur causing the copper sulfide deposition, are used as sample oils. Thermal aging tests are done at 140degC with an air atmosphere up to 1296 h with Kraft paper-wrapped and bare copper plates. The test results indicate that the deposition rate is proportional to the DBDS concentration and the incubation period before onset of the copper-sulfide deposition is inversely proportional to the DBDS concentration. These concentration dependences can be derived by considering equilibrium between three compounds such as dissolved DBDS-Cu complexes in the oil, adsorbed DBDS-Cu complexes and deposited copper-sulfide on the insulating paper.