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Monday, 30 March 2020

what are Buchholz relay, Construction and working

Buchholz relay is used to protect internal faults in large size oil-filled transformers. This relay was invented by Max Buchholz in 1921. The relay is mounted in a pipe leading to the main tank at the conservator. We can also call this type of device a "gas detector relay" because it is operated with gas.
Buchholz relay

Relay is essentially a security device utilized in electrical Power systems like transmission and distribution. Buchholz relay provides excellent protection against archiving, fractional discharge, hot spots, and current leakage in electrical systems. The device has a crucial impact on economic prospects and is additionally improving the lifetime of electrical systems equipment.

Buchholz relay Working Principle

This is a gas-operated relay. When the temperature of the core and windings increases due to short circuits, high eddy currents, local overheating inside the transformer. 

So the oil around the windings gets vaporized which results in the relay operating. In the case of low type faults in the buckles relay, the alarm signal is received and in case of a high type of faults, it separates the transformer from the supply.


The buckles relay consists of a vessel or chamber. The inside of which is filled with oil. The vessel or chamber consists of two parts. The upper part consists of a Mercury type switch or contact C1 and float b1. 

The bottom part is routed from the transformer tank to the conservator. And this part is comprised of Mercury type switches or contact C2 and float b2. The upper part is connected to the alarm circuit while the bottom part is connected to the trip circuit.

Buchholz relay operation

When a low type of fault occurs in the transformer. So the temperature rise causes the oil to evaporate inside the transformer tank. 

Which produces gas. This gas contains 70% hydrogen gas. As the gas is lighter, it rises upward in the tank and collides with the top of the relay as it approaches the conservator.

 From which the oil shakes vigorously. And float b1 collides with the Mercury switch c1, causing the alarm circuit to operate. And the alarm sounds. The downward float b does not process due to low gas volume.

When the transformer produces a high type of fault. So, inside the transformer's main tank, the oil is heated and dispersed, so much gas is produced. 

Hot oil and gas collide with the bottom part of the relay while moving rapidly toward the conservator. Which closes the float b2, Mercury switch c2. So the trip circuit operates and separates the transformer from the supply via a circuit breaker.

Disadvantages of Buchholz Relay

The following are the main disadvantage of the Buchholz relay.

  1. Buchholz relay only is implemented in oil-immersed transformers.
  2. Buchholz relay operating time is too high.
  3. This relay detecting faults when the oil level is below.
  4. This Relay has minimum operating time is 1 second.  

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