Redefining Transformer Operating Requirements: A Temperature Based Approach toMitigate Silver Sulphide

The study examines the interaction between insulating oil chemistry, operating conditions, and selector contact behaviour. Particular attention is given to the role of corrosive sulphur compounds: dibenzyl disulphide (DBDS) and elemental sulphur (S₈) together with passivator concentration, oxygen availability, and selector contact temperature. Laboratory investigations were carried out using standard and modified ASTM D1275B silver strip tests on oils with varying DBDS, S₈, and passivator levels under sealed and free-breathing conditions. Heat-stress regimes at 150 °C, 100 °C, and 60 °C were applied, and silver strip degradation was assessed using visual inspection and energy dispersive X-ray spectroscopy (EDS).

The results demonstrate that oxygen availability is one of the key factors accelerating sulphur reactivity. Oils aged under free-breathing conditions exhibited rapid passivator depletion, enhanced sulphur activity, and significantly higher silver sulphide deposition compared with sealed samples. Elemental sulphur was found to be substantially more aggressive than DBDS, producing severe silver corrosion and flaking even at relatively low temperatures. While passivators were shown to reduce sulphur reactivity, their effectiveness diminished markedly in the presence of oxygen ingress. The study also shows that reliance on visual assessment alone, as prescribed in the ASTM D1275B standard, may underestimate silver corrosion severity.

A structured risk assessment methodology is proposed, integrating oil chemistry, oxygen exposure, passivator amount, selector contact material, diagnostic indicators, and selector contact temperature. To support this approach, a thermal model for selector contacts is introduced, based on electro-thermal analogy and aligned with IEC 60076-7 methodology, enabling estimation of contact temperature under dynamic loading.

The practical applicability of the framework is demonstrated through a detailed case study of a 220/66/22 kV transformer exhibiting repeated OLTC maloperations and abnormal DGA results.

Although bulk thermal conditions suggested low risk, inspection revealed silver sulphide formation caused by contact misalignment and localised overheating. Corrective actions including contact realignment, cleaning, passivation, and oil preservation remediation successfully eliminated thermal gas generation post intervention.

Overall, the paper shows that silver sulphide formation is governed by a complex interaction of chemical, thermal, and mechanical factors. A combined risk based approach, supported by thermal modelling and targeted diagnostics, is essential for preventing OLTC related failures and improving transformer reliability.