Oil-filled transformers at risk

Understanding the risk

Transformer failure caused by copper corrosion is a multi‑stage process. By breaking the process into smaller parts, it not only becomes easier to understand, but also helps in defining the necessary actions. 

According to Sakari Vuorinen, Risk Engineer, at If Insurance, “The corrosive sulphur compounds in oil react with the transformer copper components, forming copper sulphide, which penetrates the oil transformer’s paper insulation. This way the paper insulation becomes electrically conductive, which will eventually lead to transformer failure. It’s important to understand that the corrosion itself is not the main danger. The danger comes from where the copper sulphide contamination ends up.”

Preventing transformer failure

First, it is critical to use proper oil in oil-filled transformers, which is fit for purpose and free from corrosive additives. Even if there are no corrosive compounds present, it is important to perform adequate inspection, measures, and maintenance to ensure the operational reliability of the transformer. However, without knowledge or understanding of the corrosion risk, some of these preventive actions might not be performed. Hence, education of the relevant personnel is one of the key aspects in risk prevention.  

Sakari Vuorinen highlights, “Regular personnel training combined with systematic monitoring helps protect your transformer. Whenever there is uncertainty regarding the presence of corrosive sulphur compounds in the insulating oil, the oil analysis should be performed according to the latest international standards (e.g. IEC 62535 – Detection of Potentially Corrosive Sulphur in Insulating Oil or ASTM D1275 Method B – Standard Test Method for Corrosive Sulphur in Electrical Insulating Oils).” 

Dissolved Gas Analysis (DGA) is a last line of defence regarding corrosive sulphur. A variety of gas components can be formed from the oil, which indicate the severity of the problem. Signature indicator for partial discharge in transformer is a high hydrogen (H2) gas content, whereas for electrical arcing in transformer it is high acetylene (C2H2) and hydrogen (H2) gas contents. The aim is to detect the symptoms of the copper corrosion before transformer becomes irreparably damaged. 

Many industrial plants perform DGA on oil‑filled transformers annually, which is considered recommended “standard testing interval”. However, the optimal testing frequency should be determined by factors such as the transformer’s criticality, age, and emerging gas trends. Industry best practices further emphasize increasing the frequency of DGA once gas concentrations begin to rise or when a transformer is identified as high risk. Additionally, in some cases, it might be justified to perform furan analysis to reveal localized insulation breakdown that doesn’t produce enough gas to show up in DGA analysis.  

Preventing transformer losses

Preventing losses means being ahead of any issues that may lead to the failure of your transformer. In cases where the risk of having corrosive sulphur present in the transformer emerges, be aware of the following actions:  

• Oil replacement: Replacing the contaminated oil with fresh, non-corrosive oil. 
• Oil reclamation: Heating, degassing, adsorption, filtration, re-inhibition 
• Metal Passivators: Additives can be added to the oil to passivate the copper surface and prevent corrosion, but they do not remove the corrosive sulphur compounds. 
• Monitoring and process control: DGA annually, Furan analysis and corrosive sulphur analysis case by case. Temperature control and load optimization based on the results of the analyses.   

Please note that this article provides general insights into the risks, issues, and actions relating to preventing losses caused by copper sulphur in oil transformers. For a deep dive into the proper maintenance of transformers, including when to conduct technical analyses, annual surveys and checks, including an optimal maintenance schedule to prevent losses and manage risks relating to your transformer, please contact If Insurance.