A transformer has no moving parts — so how does it fail? The answer: heat, moisture, time, and electrical stresses work slowly on the insulation, oil, and connections. This is the fault map that everyone working with transformers should know.
Main Fault Map
| Fault | Most Common Causes | Detection Tools |
|---|---|---|
| Inter-turn short circuit | Deterioration of conductor insulation due to heat, external short-circuit stresses | TTR, DGA, differential protection |
| Winding-to-ground short circuit | Moisture, insulation deterioration, mechanical displacement | Insulation resistance test, REF protection |
| Poor connections | Looseness, corrosion, worn tap-changer contacts | Winding resistance, thermal imaging |
| Oil deterioration | Moisture, oxidation, contamination | BDV and chemical tests |
| Partial discharge | Gas voids in insulation, contamination, cracks | DGA (hydrogen), specialized PD measurements |
| Tap-changer faults | Contact wear, contaminated compartment oil, worn mechanism | Operation counter, winding resistance per tap |
| External insulator (bushing) faults | Cracks, surface contamination, oil leakage from the bushing | Visual inspection, specialized tan δ measurements |
| Blocked/weak cooling | Failed fans, dirty radiators | Temperature monitoring, periodic inspection |
The Golden Rule of Diagnosis
- Faults are progressive: Most catastrophes started as a small symptom — elevated temperature, gases in DGA, a Buchholz alarm — that was ignored.
- Cross-reference the evidence: Don't diagnose from a single reading; DGA + electrical tests + operating history = a reliable diagnosis.
- Review the full prevention methodology in Preventive Maintenance for Transformers.
Sample answer: Insulation faults in their broad sense are the most common: inter-turn short circuits, ground faults, and oil insulation deterioration. The most frequently recurring root cause is long-term thermal stress (overloading or insufficient cooling) combined with infiltrating moisture, both of which gradually degrade the insulation until it fails under the first additional stress, such as a voltage surge or external short circuit.
Searching for the "moment of failure" while neglecting its history. Today's breakdown is often the result of months of deterioration recorded in temperature logs and oil analyses — good diagnosis reads the record, not just the moment.
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