An Application of Dissolved Gas Analysis (DGA) in the Assessment of Transformer Failures: A Focus on Geomagnetic Disturbances (GMDs)

Abstract

Power transformers are an integral part of transmission and distribution plant and are often characterised by long procurements lead-times as they usually cannot be bought off-the-shelf. The procurement and replacement of a power transformer may require processes of transformer design specification, tendering, bid evaluation, contract award, designing, transformer testing and transformer test certification, before delivery and commissioning of such a central transmission plant. As a result, power transformer failure may be costly to utilities depending on the severity of the failure. Power transformer failures arise due to a number of factors which may include electrical stress, mechanical factors and thermal degradation. Power transformer failure has also been shown to arise due to space weather-related phenomenon in the form of Geomagnetic Disturbances (GMDs) and the associated Geomagnetically Induced Currents (GICs). This study used empirical data from a power utility to investigate and re-assess a number of transformer failure cases from four substations - Substation 1, Substation 2, Substation 3 and Substation 4, in order to ascertain if the failures could have been induced by GMDs and the associated effects of GICs. For each Case-Study a Low Energy Degradation Triangle (LEDT) was developed using Dissolved Gas Analysis (DGA) data. There were 31 events which were reported from the Case Studies and for each event, geomagnetic storm disturbances preceding the fault day were plotted and analysed. The LEDT analys is showed an abnormality early in the lives of the power transformers, especially at Substation 2 and Substation 3. The analysis suggested that most of the power transformer faults were preceded by some geomagnetic activity, hence the Case Studies imply a causal relationship between power transformer faults and geomagnetic activity for these events. There were warning triggers on six (6) transformers in which four (4) of these failed within a year following their first triggers. On average the lifespan of a transformer is 40 years, however, this paper presents that the average lifespan of the power transformer units which failed within the power utility and had to be replaced was found to be 22.6 years. The results seem to suggest that Substation 3 and Substation 2 were the most affected by these geomagnetic storm activities compared to Substation 4 and Substation 1.