In recent years, problems with sulfur corrosion have been frequently correlated to the corrosion of the silver-plating on-load tap changer (OLTC) contacts. Apart from contamination by oil during regeneration with reactivating adsorbent, other possible causes of oil contamination have been recognized, such as: gasket materials, cements, rubbers, reservoirs, oil production and storage, etc. The problem became amplified when failure investigation records showed that minute concentrations of elemental sulfur (S8) in the oil may be enough to trigger failure. This is due to a high ratio of oil mass to surface area of silver-plated contacts that can be affected, especially in designs with in-tank OLTC selectors. Silver corrosion became a severe global problem, as it was found in other types of insulating liquids, such as esters.

This paper provides inputs for power transformer risk assessment based on the insulating liquids propensity to deposit silver sulfide, based on the concentration of elemental sulfur in the oil, and data on liquid quantity and surface area in the OLTC. Reactivity of S8 to silver is high; reaction starts at ambient temperatures of 30 to 40°C in cases of high concentrations of 10-20 ppm of S8. Metal passivator (tolytriazole derivative) was found to be ineffective at protecting silver surfaces against sulfur corrosion, as shown in previous publications and by service experience [1-4]. This paper provides insights for risk assessment and addresses the introduction of new sulfur corrosion test methods, which aim to adequately detect different types of corrosive sulfur species corrosive to silver that corresponds to service experience. Methods for quantitative detection of minute concentrations of elemental sulfur in mineral oils and synthetic esters are also presented in this paper. Gas chromatography equipped with a micro electron capture detector (μECD) was found to reach low detection limits of elemental sulfur (below 0.1 ppm), both in mineral oils and synthetic esters. Service experiences show that minute concentrations of S8, below 1 ppm can cause severe corrosion of OLTC silver plated contacts. Owing to this, mitigation solutions are needed urgently, both related to the removal of corrosive sulfur from insulating liquids and the cleaning of corroded OLTCs contacts (removal of silver sulfide deposits). Removal of elemental sulfur from mineral oil and synthetic ester in “as delivered” state was successful on a pilot scale. Nikola Tesla Institute (NTI) patented technology was found to be efficient in the removal of high concentrations of DBDS and elemental sulfur from different types of insulating liquids.

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