1. The most common type of fault being the winding to core faults because of weakening of insulation. Phase faults inside the transformers are rare. However, such faults may occur on terminals, which fall within the transformer protection zone.
2. Power transformers are generally provided with on-line tap changing (OLTC) gear. This is another major area of occurrence of fault.
3. All large transformers are oil immersed type. There is a possibility of oil leakage.
4. Transformers experience large inrush currents that are rich in harmonic content at the time of switching if they happen to be unloaded.
5. A transformer may develop inter turn faults giving rise to local hot spots within the winding.
6. Transformers may suffer from over fluxing due to under frequency operation at rated voltage. Over fluxing may also be caused when the transformer is subjected to over voltage at the rated frequency.
7. In case of sustained overload conditions, the transformer should not be allowed to operate for long duration.
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