It is important to protect high capacity transformers from external and internal electrical faults.

External Faults in Power Transformer:

External Short Circuit of Power Transformer:

A short circuit can occur in two or three phases of an electrical system. The level of fault current is always quite high. It depends on the voltage that is short-circuited and the impedance of the circuit up to the fault point. The copper loss of the fault feeding transformer is suddenly increased. This increased loss of copper causes internal heating in the transformer. Large fault currents also create severe mechanical stresses in the transformer. The maximum mechanical stress occurs during the first cycle of symmetrical fault current.

High Voltage Disturbance in Power Transformer:

High voltage disturbance in power transformer are of two kinds,

1. Transient Surge Voltage.
2. Power Frequency Over Voltage.

Transient Surge Voltage:

High voltage and high frequency surges in a power system can occur due to any of the following reasons,

1. Arcing ground if the neutral point is isolated.
2. Switching operation of various electrical equipment.
3. Atmospheric Lightning Impulse.

Whatever the causes of a voltage surge, it is ultimately a traveling wave with a high and steep waveform and high frequency. This wave travels in the electrical power system network, upon reaching the power transformer, it causes a breakdown of the insulation between the turns adjacent to the line terminal, which can create a short circuit between the turns.

Power Frequency Over Voltage:

There is always a possibility of system over voltage due to sudden disconnection of large loads. Although the amplitude of this voltage is greater than its normal level, the frequency is the same as in the normal state. Over voltage in the system causes stress on the insulation of the transformer. As we know that voltage.

Increasing voltage causes a proportional increase in work flow.
This leads to an increase in iron loss and a proportionally larger increase in magnetizing current. The surge current is diverted from the transformer core to other steel structural parts of the transformer. Core bolts, which normally carry little flux, can also be subjected to a larger flux by turning them through the saturated region of the core. In such a condition, the bolts can quickly heat up and destroy their own insulation as well as the winding insulation.

Under Frequency Effect in Power Transformer:

Internal Faults in Power Transformer: