1.2 Failures in transformers

Failures in transformers can be classified into

a)    Winding failures resulting from short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding)

b)    Core faults (core insulation failure, shorted laminations)

c)    Terminal failures (open leads, loose connections, short circuits)

d)    On-load tap changer failures (mechanical, electrical, short circuit, overheating)

e)    Abnormal operating conditions (over fluxing, overloading, overvoltage)

f)     External faults

Besides of above there are some other causes of transformer failure which may include.

Overloading: Transformers that experience a sustained loading that exceeds the nameplate capacity often face failure due to overloading.

Line Surge: Failure caused by switching surges, voltage spikes, line faults/flashovers, and other T&D abnormalities suggests that more attention should be given to surge protection, or the adequacy of coil clamping and short circuit strength.

Loose Connections: Loose connections, improper mating of dissimilar metals, improper torquing of bolted connections etc can also lead to failures in transformers.

Oil Contamination: Oil contamination resulting in sludging, carbon tracking and humidity in the  oil can often result in transformer failure.

Design/Manufacturing Errors: This includes conditions such as: loose or unsupported leads, loose blocking, poor brazing, inadequate core insulation, inferior short circuit strength, and foreign objects left in the tank.

Improper Maintenance/Operation: Inadequate or improper maintenance and operation are a major cause of transformer failures. It includes disconnected or improperly set controls, loss of coolant, accumulation of dirt & oil, and corrosion.

External Factors: Several external factors like floods, fire explosions, lightening and moisture can be established as the causes of the failure as well.