Dr. Jelena Janković

For more than two decades, corrosion of metals caused by sulfur contamination has been recognized as a significant issue in liquid filled power transformers (PT). Initially, the problem was mainly associated with copper corrosion, which the power industry has since successfully managed and mitigated. However, in the past decade, new concerns have emerged due to reported transformer failures and inspection findings revealing corrosion of silver components inside PT. This paper focuses on better understanding the phenomenon of silver corrosion, particularly in relation to testing methods and mitigation strategies. Resent research has revealed that significant number of synthetic ester insulating liquids have also shown corrosiveness on silver according the ASTM D1275-15 silver strip corrosion test, due to presence of elemental sulphur (S8) in sub ppm concentrations (0.3 to 0.8 ppm), while dibenzyl disulfide (DBDS) was not detected in the oils. For accurate silver corrosion risk assessment, it is essential to combine sulphur quantification techniques with performance silver strip corrosion tests. Gas chromatography equipped with a micro electron capture detector (µECD) proved to be an effective analytical method for quantifying elemental sulphur in insulating liquids. The technique offers high sensitivity, with detection and quantification limits of 0.05 ppm for synthetic esters and 0.07 ppm for mineral oils, making it suitable for quantifying even trace amounts of elemental sulphur in insulating liquids. Another important finding is that commonly used metal passivators, such as tolyltriazole and benzotriazole, are ineffective in protecting silver surfaces from corrosion. Consequently, remedial actions are often limited to fluid replacement or treatment to remove corrosive compounds from the liquid. Traditional re-refining and recycling methods were found to be inefficient in removal elemental sulphur from synthetic esters. Moreover, these methods struggle to meet modern European Union “Green Deal” directives aimed at reducing carbon footprints. In cases where silver-plated components have already been affected, inspection and maintenance are recommended. Corroded silver contacts should either be replaced or cleaned. This paper introduces Tesla’Ssorb and TeslaCleanS as a new integrated technological solution. Tesla’Ssorb presents a new tailor-made adsorbent, designed to remove elemental sulphur from synthetic ester fluids, without requiring fluid replacement. Using a specialized mobile rig that connects directly to the transformer, the oil treatment with Tesla’Ssorb can be performed while the transformer remains energized and in operation. This approach minimizes downtime and reduces waste. In addition to eliminating corrosive sulphur, the treatment improves key fluid properties such as dielectric dissipation factor, acidity, breakdown voltage. To complement oil treatment, TeslaCleanS provides a non-destructive chemical solution, specifically formulated to remove silver sulfide deposits from on-load tap changer (OLTC) components. Unlike abrasive mechanical cleaning, this method preserves the integrity of silver-plated surfaces through simply wipe cleaning process, restoring the components to their original condition and ensuring long-term reliability. Together, Tesla’Ssorb and TeslaCleanS technologies provide a comprehensive, sustainable and highly efficient approach for permanently eliminating sulphur induced silver corrosion, while ensuring the long-term reliability of power transformers.

Keywords: Silver corrosion, elemental sulphur, synthetic ester, silver-sulphide, OLTC, treatment, reclamation