Protection for a Power Transformer

Power Transformer is an essential electrical component. Engineer like me, who only depend on transformer then it’s really important to keep the transformer active for 24/7. If there is any problem with transformer then all machinery inside of the factory floor will be silent and zero production will come.

When you are having problem with your transformer then it may take long time to repair it. So it’s better to protect power transformer at first place. In this post I will discuss all about the power transformer protection procedure that you can apply in your daily work. So let’s go.

Protection for a Power Transformer:

Abnormal Condition Protection Remarks
Incipient faults for below oil level resulting in decomposition of transformer oil. The Buchholz relay sound alarm.

(The GAS activated relay)

Sudden Pressure Relay.

The pressure relief valve.

Normally Buchholz relay used for a power transformer when its rating is 500 KVA or above.
Large amount of internal faults like phase to phase, phase to ground, below oil level.


Fault while tap changing.

The Buchholz relay trips the circuit breaker. The Buchholz relay is too slow and less sensitive.

Buchholz relay for the tap changing also.

Percentage of differential protection.

High Speed high set over current relay

Percentage of differential protection used for a transformer when its above 5 MVA.
Saturation of transformer magnetic circuit. The over fluxing protection.


The over voltage protection.

It’s important for generator transformer and the feeder transformer.

Earth Fault

Differential Protection For the transformer and above 5 MVA.

Earth Fault Relay

Instantaneous resistive earth fault relay.

The time lag earth fault relay.


Trough Fault

Graded time lag over current relay.


HRC fuses.

It’s the protection of distribution transformer.

For small distribution transformer up to 500 KVA.



Transformer Overloads

The thermal overload relay.



Temperature relays sound alarm.

Normally temperature indicators are provided with the transformer.

Temperature increases is also indicated on the control board then fans start at certain temperature.


High Voltage surges due to switching, lightning.

Horn gap.



The lightning arrestor.

It’s not favored for important transformers.

In addition to lightning arrestors for main incoming line.

Small Distribution Transformer.


Transformer in important locations and ratings is above 500 KVA.

Only for high voltage fuses for earth faults protection and the phase fault protection. Overload protection normally not provided.

For an important transformer of about 500 KVA.

Overload relay and instantaneous earth fault relays.

The restricted earth fault relays.

The over current protection, Buchholz relay.

The transformer is about 5 MVA and above Differential Protection.

The restricted earth fault protection.

Over fluxing protection.

Sudden pressure relays.

Over current protection.

Buchholz relay

The under voltage and over voltage relays: when necessary.

Related: Transformer Installation, Inspection and Commissioning

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