Answer: Transformer is one of the most expensive equipments in the Power System. Failure of the transformer can cause outage of power supply for even days. However, multiple things can go wrong with the transformer: There can be two types of failure - Internal Faults and External Faults.
•Internal Faults (the faults within the transformer):
1.Core Failure
2.Winding Failure
3.Lamination Failure
4.Bushing Failure
5.Terminal Board Failure
6.Tap Changer Failure (OLTC failures)
7.Transformer Oil
8.Oil level low
9.Moisture absorption
10.Transformer cooling system
11.Failure of insulation between Lamination and Core Bolt
12.Badly made joints and connections.
13.Frequent exposure to Lightning strokes
•External Faults (the faults outside the transformer):
1.OverLoad ... increases copper losses and consequence temperature rise.
2.Through Faults or System Faults ... not detected by differential protection of transformer, however, if through
faults persist for longer duration, transformer gets damaged by thermal and electromagnetic mechanical stress
... detected by overcurrent relays.
3.Short Circuits
4.Open Circuits
5.Earth Faults
6.Over Voltage
7.Reduced system frequency