What Is Buchholz Relay And How Does It Work?

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What is Buchholz Relay?

Buchholz Relay is a safety device mounted on oil-filled power transformers and reactors, equipped with an external oil reservoir (called Conservator). It is a mechanically operated device used as protective equipment mostly for transformers. It is the most important part of transformer that protects the transformer from internal faults.

Buchholz Relay is a gas-actuated relay installed on the pipe connecting the Conservator to the main tank of the transformer. Such Relays are used in power transformers having a rating of more than 500kVA, but not used in small transformers because of economic considerations.

Working Principle of Buchholz Relay

Buchholz Relay works on the theory of gas detection. Whenever a fault occurs inside the tank of the transformer such as insulation failure between windings, breakdown, and heating of the core, then heat is produced by fault current. That heat causes the decomposition of transformer oil into gas bubbles of different hydrocarbon gases, CO₂, and CO.

Also Read: What is Relay? Types, Applications, and Working of Relay

The generation of gas depends on the intensity and magnitude of the fault. These gas bubbles move upward through the pipe and accumulate in the upper part of the Relay container causing a fall in oil level. The collected gas displaced the oil level in Relay and the displacement is equivalent to the volume of gas collected. That causes the upper float to close the upper mercury switch and activates the alarm circuit.

Construction of Buchholz Relay

Buchholz Relay consists mainly of two hinges placed in an oil-filled metallic chamber and this chamber is connected through a pipe between the Conservator and the main tank of the transformer. There are two hinges, one is connected to the float at the top and the other is connected to a flap at the bottom of the metallic chamber. Both hinges are accompanied by a mercury switch.

One hinge is placed at the upper portion of the metallic chamber connected to float along with the mercury switch. This mercury switch is connected to an alarm circuit and used for activating the alarm in case of a minor fault.

Another hinge is placed at the lower portion of the chamber and is connected to a flap along with a mercury switch. This mercury switch is used for actuating the trip circuit in case of a severe fault in the transformer. Hence the Construction of Buchholz Relay is shown in the above figure.

How does Buchholz Relay work?

Operation of Buchholz Relay:

The operation of Buchholz Relay is very simple, and it works on mainly two different cases as explained below.

Case 1: A minor fault occurred inside the transformer tank

When a minor fault occurs inside the transformer tank, heat is produced by the fault current. The produced heat causes the decomposition of transformer oil into gas bubbles. These gas bubbles flow upward through the pipe and get collected in the chamber. These gases displaced the oil in the Buchholz Relay chamber and the displacement of oil is equivalent to the volume of gas collected.

The displacement of oil causes the upper float to close the upper mercury switch that is connected to the alarm circuit. Hence during a minor fault in the transformer, the alarm gets activated. The collected amount of gas indicates the severity of the fault. Since there is a minor fault, the production and flow of gas are not enough to move the lower flap of the Buchholz Relay. Hence in case of a minor fault, the lower flap remains unaffected, and only an alarm is generated by the upper float.

Case 2: When a severe fault occurred in the transformer

When a major fault occurs like a phase-to-earth fault (short circuit), the heat generated is high, and a large amount of gas is produced. These gas bubbles similarly flow upward through the pipe, but their motion is high enough to tilt the lower flap in the Buchholz Relay. Hence the lower flap closes the lower mercury switch that is connected to the trip circuit and hence they trip the transformer from supply. Hence in this way, the transformer gets isolated from the supply during major faults or severe fault conditions. Hence Buchholz Relay also provides protection against major faults in transformers.

Advantages of Buchholz Relay

The severity of the fault can easily be identified with the help of Buchholz Relay without even dismantling the transformer.
It indicates the internal faults due to heating and it also helps in preventing the major fault.
In case of a major fault, the transformer can easily be isolated with the help of Buchholz Relay to prevent accidents.

Limitations of Buchholz Relay

The following are some disadvantages or limitations of Buchholz Relay:

As it is a mechanically operated relay, its response time is high as compared to other electrical and electronic relays. Its minimum operating time is 0.1 sec whereas, the average operating time is 0.2 sec.
It doesn’t protect connecting cables. So, a separate protection scheme is required for the protection of cables.
It can be only used in oil-immersed power transformers.
It is not suitable for small rating distribution transformers.
It can only detect the fault below the oil level in the transformer.

When does Buchholz Relay operate?

Buchholz Relay operates mainly under three conditions:

Whenever gas bubbles are formed inside the transformer tank due to heating of Windings.
When the level of transformer oil falls below a certain pre-set value.
Whenever transformer oil flows rapidly from the Conservator to the main tank and vice-versa.

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What is Buchholz Relay and How does it work?