Learning from success and failure in transformer fault gas analysis and interpretation

The importance of transformer fault gas analysis and interpretation is well recognised and documented. However, unlike the diagnosis of human blood where rules have been well established, the diagnosis of faults in transformer oil is not so clearly defined. Whilst some transformer faults are straightforward to diagnose, with dissolved gas analysis (DGA) results pointing to the type and severity of fault, in many other cases it is often difficult to diagnose what type of transformer fault is involved and even harder to determine the cause. In some cases the transformer may be damaged but this may not be obvious from DGA results. This paper introduces the causes of transformer faults and failure; presents an overview of development of DGA diagnostics and discusses the merits and limitation of DGA diagnostic method; and describes five case examples, showing that knowledge from forensic inspection of faults and failures in transformers are of vital importance in understanding results from DGA of oil samples, and an in-depth understanding of the transformer design and life-limiting faults enable better interpretation of DGA results.