Heat is the number-one enemy of a transformer's life: a well-known rule in insulation engineering states that every 8-10 degree continuous increase above the allowed limit roughly halves the insulation's life. So a rise in temperature is a symptom that must be diagnosed, not tolerated.
Main Causes, Arranged Logically
| Category | Cause | How to Verify? |
|---|---|---|
| Overload | Loading above the rated value or above the current cooling rating | Compare the operating current with the rated value (see current calculation) |
| High-harmonic loads increasing losses | Power quality analyzer | |
| Insufficient cooling | ONAF fans malfunctioning or radiators dirty | Visual inspection and trial run of the fans |
| Low oil level (leak) | Level gauge and signs of leakage | |
| A closed radiator valve or blocked oil path | Abnormal temperature difference between radiators | |
| Internal fault | Poor connections (localized hot spot) | Winding resistance test and thermal imaging |
| Insulation deterioration or incipient winding short | DGA analysis | |
| Environment | Poor room ventilation or ambient temperature above design | Measure ambient temperature and review ventilation |
Brief Field Methodology
- Start with the easiest: load, then cooling, then environment, and finally suspect the inside.
- Thermal imaging immediately distinguishes: distributed heating (load/cooling) versus a hot spot (poor connection).
- Document the readings: top-oil temperature, winding temperature, load, and ambient temperature together — a number without context doesn't diagnose anything.
Sample answer: Order the inspection from outside to inside: first, the cooling system — stopped fans, dirty radiators, a closed radiator valve, or a low oil level due to a leak. Second, the environment — room ventilation and ambient temperature. Third, load quality — high harmonics raise the losses despite a normal apparent current. Finally, the inside — poor connections or an incipient fault, settled by thermal imaging, the winding resistance test, and DGA analysis.
Treating the symptom instead of the cause: increasing ventilation or reducing the load while leaving a poor connection to deteriorate further. A localized hot spot is not solved by an extra fan, and it will develop into a major failure.
Want to master electrical transformers step by step?
Follow trainer Fahad Refai's Electrical Transformers course — a practical walkthrough from the fundamentals to testing and reading catalogs.
Browse Fahad Refai's Courses