Observations of Abnormal Gassing Rise after Metal Passivation in Transformers

Some Sulphur-containing compounds in mineral insulating oil have been found to be corrosive to copper and silver surfaces, especially when exposed to heat. This corrosion and resulting copper sulfide formation have been cited as the cause for many transformer failures worldwide. Hence, the addition of metal passivators to shield copper and other metallic surfaces in transformers is a widely used mitigating action. This paper reports a study performed to determine the influence of metal passivation on DGA (Dissolved Gas Analysis) data. DGA data of 150 in-service and passivated transmission power transformers in the UK were used in this study. A criterion to determine an abnormal rise of gassing level is proposed. This criterion can be used for DGA data of passivated or non-passivated transformers to identify abnormal increments of gases. Using this, abnormal increments of hydrogen and carbon monoxide were observed in some transformers after metal passivation. Generally, hydrogen peaked and stabilized while carbon monoxide peaked and decreased after metal passivation. The trends observed after metal passivation need to be considered to avoid misleading interpretation of DGA data in facilitating asset management decisions on transformers.