Assessment of Transformer Fault Severity from Online Dissolved Gas Analysis Using Positive CUSUM

Dissolved gas analysis (DGA) is one of the transformer testing methods that has been widely used for a long time because it does not require the transformer to be offline for testing. The inspection can be conducted while the transformer is operating normally, and it is cost-effective. In research on fault severity, thermodynamic theory has been applied to find the energy index of the hydrocarbon gases generated by faults, CH4, C2H6, C2H4 and C2H2, referred to as the Normalised Energy Intensity (NEI). This study examines the application of the Positive Cumulative Sum of Difference (CUSUM) method to hydrocarbon NEI and carbon dioxide NEI for fault severity assessment. The study demonstrates the effectiveness of this approach by means of case studies on the basis of DGA results from two steel plants. In the case of NEI from hydrocarbon gases (NEIHC), the positive CUSUM is compared with the NEI, the NEI score, and the cumulative NEI. In the case of NEI from carbon oxide gases (NEICO), Positive CUSUM is compared with NEI and the ratio of CO2 to CO. It was found that Positive CUSUM for NEIHC is a much better indicator of the severity of defects than the NEI, the NEI score, and the cumulative NEI. In contrast, Positive CUSUM for NEICO gave excessively high values. However, combining NEI with the CO2/CO ratio gave better monitoring results. Therefore, from an online DGA perspective, Positive CUSUM can be used to effectively monitor fault severity.