State and explain the faults occur in the transformers.
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Sr. No.
Type of fault
1 Earth fault
Due to breakdown of insulation between live conducting parts such as winding, terminals etc. and earthed metallic parts such as core, tank etc. the current is diverted to earth, causing earth fault. The causes of earth fault are: i) Electrical breakdown of insulation due to overvoltage surges produced by lightning or switching operations. ii) Mechanical damage of insulation. iii) Electrical or mechanical breakdown of bushings iv) Insulation damage due to over-heating
2 Overloads/ Overheating
Sustained overloads cause additional losses in windings, causing over-heating of windings. Unbalanced loading cause additional magnetic losses leading to over-heating of core and other mechanical parts.
3 Incipient faults: Phase to Phase, Phase to ground, low oil level, decomposition of oil
Incipient faults are slowly developing faults, which if not attended in time, can lead to severe faults. i) Slow insulation degradation due to continuous overheating. ii) Reduction in oil level due to leakage, which leads to unequal cooling of transformer assembly, resulting unequal thermal expansion and stresses. iii) Degradation of oil over a period of service.
4 Saturation of magnetic core
Over-voltage conditions lead to saturation of cores. Since flux level is increased, the iron losses are increased.
5 Faults in tap changer
Mechanical problems in tap changer assembly lead to faults in tap changer.
6 Inter turn fault
When insulation breakdown of a winding takes place and few turns of the same winding are shortcircuited, the inter-turn fault takes place.

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