Effects of electric fields on copper sulfide deposition and the properties of insulating oils in oil-immersed transformers

The failures of high-voltage transformers and reactors can result from the deposition of copper sulfide on the oil-impregnated paper insulation. This study explored the influence of electric fields on copper sulfide deposition and the properties of insulating oils as a result of the reaction of dibenzyl disulfide (DBDS) with copper in oil-immersed transformers. Accelerated aging experiments were conducted for pig-tail windings under two sets of conditions, namely, thermal aging only at 140 °C and AC/DC electrical and thermal aging at 5 kV/140 °C under low oxygen condition. This was combined with oil containing dibenzyl disulfide (DBDS) added to the non-corrosive oil to a concentration of 200 mg/kg. The corrosive sulfur deposits were examined by observing the surface morphology of the windings and measuring the copper and sulfur contents on the windings via inductively coupled plasma-atomic emission spectrometry and a precipitation method. The properties of the insulating oils were measured and analyzed at intervals. The results indicate that electric fields increased the amount of deposited copper sulfide on the windings. The higher the electric field strength, the higher the amount of deposited copper sulfide on the insulating paper surface. Subjecting the oil-paper insulation to electric field and elevated temperature generates dissolved copper in the insulating oil. The electric field can promote the dissolution of copper in oil. The ageing oil under thermal and electrical stresses promote copper dissolution and increase corrosion rate.