Online Transformer DGA Monitoring Case Studies in Condition Assessment

Dissolved Gas Analysis (DGA) is a crucial tool for evaluating the condition of a transformer. Electrical and thermal fault activities in the active part of the transformer lead to the production of fault gases. When trended and analyzed, these gases provide insights into the transformer’s health. Online devices offer the added advantage of high data resolution, with samples taken up to hourly, enabling not only the analysis of gas trends but also correlations with parameters such as load, temperature, and tap position. Furthermore, most online DGA monitors include an inline moisture sensor, aiding in the assessment of moisture in the solid insulation of the transformer. This paper presents a number of case studies focusing on data from online DGA monitors. First, a case study highlighting the value of online DGA in detecting internal faults is discussed. Secondly examples of thermo-oxidative stray gassing is given. Mineral oil and natural esters can exhibit differences in stray gassing. Stray gassing involving hydrogen production can be mistakenly interpreted as Partial Discharge (PD), and ethane production may be misinterpreted as a low-temperature fault. Additionally, online moisture monitoring, combined with top oil and load data, can be employed to assess moisture in paper insulation. The approach and a case study supporting this methodology are briefly explained. The final case studies focus on the trending of the carbon oxide ratio (carbon dioxide and monoxide) as a supplementary diagnostic tool.