What is Buchholz Relay and Explain its Function?

What is Buchholz Relay?

The transformer is safeguarded against internal faults by the Buchholz relay. It is the relay with gas actuators. Between the conservator and the main tank is where the Buchholz relay is located. Such a relay is utilised in transformers with a rating more than 500KVA. Due to financial factors, it is not utilised in small transformers. As they are sensitive to the impacts of a possible internal dielectric failure, Buchholz relays are utilised as a protective mechanism. One kind of gas detection relay is the Buchholz relay.

What is Buchholz Relay

Features in Buchholz Relay:

There are two essential components to Buchholz relays.

  • A float makes up the upper part. Depending on the oil level in the Buchholz relay container, the float can move up and down thanks to its hinged attachment. The float has a mercury switch fastened to it. Thus, the placement of the float determines how the mercury switch is aligned.
  • A mercury switch and a baffle plate make up the lowest element. This plate is positioned on a hinge right in front of the Buchholz relay’s inlet (on the main tank side) of a transformer so that, when high pressure oil enters the relay through that inlet, the alignment of the baffle plate and the mercury switch attached to it will shift.

A Buchholz relay also contains top gas release pockets in addition to these major components. Both mercury switches have moulded terminal blocks where the electrical leads leave.

Features in Buchholz Relay

Operation Principle of Buchholz Relay:

Excess heat is produced whenever a problem in the transformer occurs, such as insulation failure of the turns, core breakdown, or excessive core heating. The transformer’s insulating oil is broken down by the extra heat, which produces gas. Gas production is influenced by the fault’s severity. Gas bubbles are attracted to the conservator by their natural upward flow, and as a result, they are gathered in the Buchholz relay, which is attached to the line that runs between the transformer tank and the conservator.

Operation Principle of Buchholz Relay

How the Buchholz Relay Act?

The fault currents in the transformer generate heat whenever a minor fault occurs. Transformer oil begins to break down due to the heat created, and gas bubbles start to form. The buchholz relay is where these gas bubbles are gathered after flowing upward. In the Buchholz relay, the collected gas displaces the oil to an amount equal to the volume of gas collected.

  • The upper float closes the upper mercury switch, which is coupled to an alert circuit, as a result of the displacement of oil.
  • Consequently, the connected alarm is activated when a minor malfunction occurs. The quantity of gas that was collected reveals the issue. Gas output.

Hydrocarbon gases,

CO2 and


is insufficient to shift the bottom float during mild faults.
  • The bottom float is therefore unaffected by minor faults. Major defects, such as phase to earth short circuits, emit a lot of gas and generate a lot of heat. Although the speed of this vast amount of gas is high enough to tilt the lowest float in the Buccholz relay, it will flow upward identically.
  • In this instance, the lower float will cause the lower mercury switch to trip, isolating the transformer from the supply.
Parts of Buchholz Relay

Advantages of Buchholz Relay:

  • The Buchholz relay assists in preventing significant defects by identifying inter-turn errors that result from heat in the core.
  • Even after the transformer has been taken apart, the relay is still able to assess the degree of the defect.
  • By using a Buchholz relay, it is simple to isolate the transformers in the event of serious problems and prevent accidents.
  • It is the most basic type of transformer protection.
  • When compared to other forms of protection, it can identify faults in their early stages.

Disadvantages of Buchholz Relay:

  • Only transformers with oil immersion use the relay.
  • Only below the oil level can it find the defect.
  • The cables that connect to this relay are not protected. Therefore, the cables are given distinct protection.
  • The relay’s response time is quick- The minimum operation period for this relay is one second.

Applications of Buchholz Relay:

  • The Buchholz relay, known for its distinctive design, is used most frequently in the following situations:
  • The relay can also be used with faulty and slack electrical contacts.
  • It can be used at the point when air bubbles first enter the oil.
  • The relay is able to locate the locations where the oil level has decreased due to any leaks.
  • The core bolt insulation failure is addressed by the relay.
  • It is capable of spotting transformer winding short circuits.
  • Additionally, it has the ability to recognise short circuits between stages.

Precaution in use of Buchholz Relay:

Without any transformer fault, the Buchholz relay can be activated. As a consequence, when oil is introduced to a transformer, air may enter along with the oil, collect beneath the relay cover, and ultimately result in a false Buchholz relay operation. To lock the movement of mercury switches while topping up the transformer’s oil, a mechanical lock is therefore included in that relay. Additionally, this mechanical locking aids in preventing unneeded movement of mercury switches’ breakable glass bulbs when Buchholz relays are being transported.

The lower float might also incorrectly operate if the connection pipe’s oil velocity is sufficient to knock the float off balance without being caused by an internal problem. When overload current run through the winding, they can overheat the copper and the oil, which will cause the oil to expand in the event of an external short circuit.

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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