What is Buchholz Relay? And why it is so important?


As you know, there are many parts in the transformer, like the core, winding, transformer bushing, conservator tank, main oil tank, Buchholz relay, etc. In this post, we will discuss Buchholz relay.

What is a Buchholz relay?

Buchholz relays are protective relays that protect the transformer from internal faults. This relay is used in the transformer above 500 kVA. Rarely we use this relay below a 500 kVA transformer. This relay is situated between the main oil tank and the conservator tank. This relay connects the main oil tank pipe to the conservator tank pipe through itself.


Basically, the Buchholz relay structure is like a chamber, as shown in the figure.

What is Buchholz Relay


This chamber is made up of metallic material. In this chamber, there are two floating hinges with mercury switches. One hinge is on the upper side of the chamber, and the other is on the lower side. The upper side of the mercury switch (contact) is connected to an alarm circuit. The lower side of the mercury switch is linked to a tripping circuit. A testing cock is also located on the chamber’s outer and upper sides. In addition, one drain plug is used at the chamber’s lower surface.

Working principle of the Buchholz relay

if any internal fault occurred in the transformer, like a winding fault. As a result, current will increase in the transformer. and heating will increase in the transformer. Due to the overheating of the transformer, oil will evaporate and vapour will go up the side. Due to evaporation, the oil level in the main oil tank will decrease. As a result, the floating hinge will move, and the mercury switch will sound the alarm.

What is Buchholz Relay

But if the fault level increases, then evaporation will increase. In this condition, the lower side floating hinge will operate the mercury switch, trip the circuit, and separate the transformer from the main supply.

Some demerits of Buchholz Relay:

There are some demerits to the Buchholz relay. which are given below.

  • This relay is only applicable for oil-filled transformers.
  • This relay will only detect the fault when the oil level decreases.
  • This relay has a very fast operating time.
  • This relay does not sense the outer fault of the transformer.

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