Fault Diagnosis in Power Transformers: A Field Case Analysis Based on Visual Inspection and Electrical Testing

This paper presents an analysis of fault cases in three-phase oil-immersed power transformers diagnosed in an industrial environment between 2024 and 2026. The cases were identified through systematic visual inspection, electrical testing, and dissolved gas analysis (DGA), and were classified according to the fault mechanisms established by IEC 60599. The identified causes include mechanical failure of the tap changer, moisture ingress, lightning surge, thermal aging due to overload, and critically low insulating oil level. Additionally, four DGA analyses of in-service transformers are presented, illustrating different equipment condition stages — from normal to critical. The comparative analysis of the cases demonstrates that the diversity of failure mechanisms reinforces the need for complementary diagnostic approaches to reliably assess equipment condition.