The impact of corrosive sulfur concentration in transformer oil on the electrical properties of insulated windings

Dibenzyl disulfide (DBDS) is considered as one of the sulfur compounds that can cause copper corrosion in transformers. The deposition on insulated windings is found to be more likely formed with an increase in DBDS concentration which changes the electrical properties of insulated windings dramatically. In this paper, three transformer insulating oils were prepared by adding with different DBDS concentrations in laboratory condition, subsequently a paper-wrapped copper winding model was immersed in oil followed by an accelerated thermal aging at 130°C. Spectrum analysis was adapted to determine the elements content that were deposited on the surface of insulating paper. A high resistance meter (keithley 6517B) was used simultaneously to test volume/surface resistivity of paper-wrapped insulated windings which describes the frequency dependence of permittivity. The results showed that higher DBDS concentration in transformer insulating oil accelerated both the deposition on the surface of the insulating paper and on copper. Furthermore, the volume/surface resistivity decreased significantly. Our study examines the change of electrical properties of insulated winding. The results should be and beneficial to understand the sulfur corrosion mechanism in transformer oils.