Observation of Post-Reclamation Ethylene Production in Transmission Power Transformers

Dissolved Gas Analysis (DGA) is a widely used transformer diagnostic technique with proven capabilities to indicate transformer thermal and electrical faults. However, DGA analyses may encounter anomalies when applying numerical methods to interpret the results. This case study considers one anomaly identified concerning unusual ethylene production following oil reclamation for a utility in the UK. A data analysis method that can be used to determine such unusual gas production is described. 84 oil-treated and 292 non-oil-treated in-service power transformers were used in this study. Firstly, the historical gas records of non-oil-treated transformers were analysed to establish a benchmark. The benchmark was set using the 90
^th
percentile gas level and 90
^th
percentile gas generation rate observed from the non-oil-treated transformers. The oil-treated transformers were then analysed against the determined benchmark for the post-oil-treatment period. The results showed that 66% of oil reclaimed units have unusual ethylene production without other gas productions. This observation of unusual ethylene production needs to be considered to avoid misinterpretation of DGA-based fault assessments.