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What is Buchholz Relay:

Buchholz relay is a safety device which is generally used in large oil immersed transformers (rated more than 500 kVA). It is a type of oil and gas actuated protection relay. 

Buchholz relay in a transformer is an oil container housed the connecting pipe from the main tank to conservator tank.

Why is Buchholz relay used in transformers?

Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer. Short circuit faults such as
  • inter-turn faults,
  • incipient winding faults,
  • core faults may occur due to the impulse breakdown of the insulating oil
  • simply the transformer oil.

Buchholz relay will sense such faults and closes the alarm circuit.

Working Principle Of Buchholz Relay:

Whenever a fault occurs inside the transformer, such as We mentioned above, the fault is accompanied by the production of excess heat. This excess heat decomposes the transformer insulating oil which results in the production of gas. The generation of gases depends on intensity the of fault. Gas bubbles tend to flow in the upward direction towards conservator and hence they are collected in the Buchholz relay which is placed on the pipe connecting the transformer tank and conservator.

How Does a Buchholz Relay Work?

Whenever a minor fault occurs inside the transformer, heat is produced by the fault currents. The produced heat causes decomposition of transformer oil and gas bubbles are produced. These gas flow in the upward direction and get collected in the Buchholz relay. The collected gas displaces the oil in Buchholz relay and the displacement is equivalent to the volume of gas collected. The displacement of oil causes the upper float to close the upper mercury switch which is connected to an alarm circuit. Hence, when a minor fault occurs, the connected alarm gets activated.

The collected amount of gas indicates the severity of the fault occurred. During minor faults, the production of gas is not enough to move the lower float. Hence, during minor faults, the lower float is unaffected.

During major faults, like phase to earth short circuit, the heat generated is high and a large amount of gas is produced. This large amount of gas will similarly flow upwards, but its motion is high enough to tilt the lower float in the Buchholz relay. In this case, the lower float will cause the lower mercury switch which will trip the transformer from the supply and transformer is isolated from the supply.

The Buchholz relay operation may be actuated without any fault in the transformer. For instance, when oil is added to a transformer, air may get in together with oil, accumulated under the relay cover and thus cause a false Buchholz relay operation.
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