CORROSION OF SILVER CONTACTS OF ON-LOAD TAP CHANGERS AND MITIGATION MEASURES

The presence of elemental sulfur in transformer insulating fluids, even in trace amounts, leads to the formation of electrically conductive silver (I) sulfide at low operating temperatures, posing a significant threat to on-load tap changer (OLTC) contacts. The International CIGRE organization currently has an active task force addressing the issue of silver corrosion in transformers (CIGRE A2 Task Force on Silver Corrosion). The focus is on investigating the origin of corrosive sulfur compounds in mineral insulating oils and esters, detection methods, case studies, risk assessment, and mitigation strategies. Insulating fluids (mineral oils or synthetic esters) are tested for corrosive sulfur compounds using standardized corrosion tests (ASTM D1275, DIN 51353) and quantitative sulfur analysis (IEC 62697-1, IEC 62697-2, IEC TR 62697-3). To enhance silver corrosion assessment, a new testing method was developed at the institute, improving upon existing ASTM and DIN standards. As part of the GreenCleanS project, funded by the Science Fund of the Republic of Serbia, an innovative desulfurization technology was successfully implemented at Thermal Power Plant Kolubara A located in Veliki Crljeni, treating 1.500 liters corrosive oil with elemental sulfur. During the technology demonstration at the Kolubara A Power Plant, process parameters were monitored, and environmental impact was assessed. The results indicated that the measured values are within permissible limits, leading to the conclusion that the developed technology is clean, environmentally enclosed, and aligned with the principles of Best Environmental Practices (BEP). Oil analysis after desulfurization in real operating conditions confirmed the effective removal of elemental sulfur below the quantification limit, with improved physical-chemical and electrical characteristics of oil. These findings highlight the high potential of the developed process for industrial applications.