Two types of faults occur in power transformer i.e. external and internal electrical faults.
External Faults in Power Transformer:
(a) External Short – Circuit of Power Transformer:
The short – circuit may occur in two or three phases of electrical power system. The level of fault current is always high enough. It depends upon the voltage which has been short – circuited and upon the impedance of the circuit up to the fault point.(b) High Voltage Disturbance in Power Transformer:
High Voltage Disturbance in Power Transformer has two types;I) Transient Surge Voltage II) Power Frequency over Voltage
I) Transient Surge Voltage:
High voltage and high frequency surge may arise in the power system due to any of the following causes;(A) Arcing ground if neutral point is isolated.
(B) Switching operation of different electrical equipment.
(C) Atmospheric Lightening Impulse.
II) Power Frequency over Voltage:
There may be always a chance of system over voltage due to sudden disconnection of large load. Though its amplitude is increased, but its power frequency remains same. Over voltage in the system causes an increase in stress on the insulation of transformer. This stress happens due to increase in flux magnitude, which can strike transformer’s steel body & causes damage.Internal Faults in Power Transformer:
The principle faults which occur inside a power transformer are categorized as;(1) Insulation breakdown between winding and earth.
2) Insulation breakdown in between different phases.
3) Insulation breakdown in between adjacent turns i.e. inter – turn fault.
4) Transformer core fault.