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Buchholz Relay: Working Principle, Advantage, Disadvantage And Where is it used?

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In the last article, we discussed about “Transformer Protection System“. In this article, we will discuss in detail about, what is buchholz relay? how is it work? that is, working principle of buchholz relay. Also, it’s Advantage, Disadvantage And Where Is It Used? So, Let’s get started.

What is Buchholz Relay?

Buchholz relay is a gas actuated relay which is used to protect oil transformer from any fault. It is usually installed in the connecting pipe between the conservator and the tank. Buchholz relays consist of two floating valves in a pot mounted by two nickel leaf rings. Two float valves are usually placed one above and one below. Each of the two float valves is fitted with a mercury switch. One of them is connected to the alarm circuit and the other to the relay tripping circuit.

Under normal conditions the float valve floats above the two oils and cannot cause mercury switch connection. Also contains release cork and test cork. Figure shows the structure of a Buchholz relay.

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Fig: Structure of a Buchholz relay

Working principle of buchholz relays

A higher-than-normal current flow for a light fault leads to heat build-up and decomposition in the oil in the transformer tank. This decomposition produces about 70% hydrogen gas. As this gas is lighter than the oil, it wants to go to the conservator through the gas relay in the form of bubbles. Because of this, gas accumulates on the surface of the relay chamber and lowers the oil level. The upper float valve then moves down and makes contact with the mercury switch, terminating the alarm circuit. As a result, the signal sounds. Errors can be detected easily.

The trip circuit breaker does not disconnect the supply even though the alarm circuit operates for light faults. Because, due to this fault, the amount of current flowing is relatively less, the relay cannot work fully.

Fig: A Buchholz relay of Transformer

If a major fault such as a phase-to-phase short occurs inside the transformer, the amount of gas produced is very high. As a result, due to the high pressure of the gas, the oil from the transformer tank passes through the relay to the conservator. As it moves, the oil pushes against the flap, causing the lower valve to move upwards and make mercury switch contact. When the mercury switch contacts, the trip circuit is complete and the trip coil is energized, opening the circuit breaker of the transformer. Hope you understand, how does Buchholz relay work? Let us now know about some advantages and disadvantages of Buchholz relay.

Advantages of buchholz relay

Two advantages of buchholz relay are mentioned below:

(1) It works very commonly in the protection system of transformers.

(2) Capable of giving an early fault warning alarm within a very short time of any other security system.

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Disadvantages of buchholz relay

Six disadvantages of buchholz relay are mentioned below:

(1) Conservators cannot be used in relays without oil-immersed transformers.

(2) In this method separate protection measures are to be taken for the cables connected to the transformer.

(3) It may not indicate all faults when the oil level drops below a certain level.

(4) The setting of the mercury switch cannot be accomplished with too fine a feeling.

(5) Such relays are slow-type and have a minimum operating time of 0.1 seconds.

(6) buchholz relays below 500 KVA are not used for economic reasons.

Where buchholz relay is used?

Buchholz relay is used in the pipe connecting the conservator to the transformer tank for the protection and warning system of various transformer faults.

How many float valves are there in buchholz relay and what switch is attached to the valve?

Buchholz relays have 2 float valves and a mercury switch attached to each valve.

What circuit is connected to the two valves in the buchholz relay ?

One of them is connected to the alarm circuit and the other to the relay tripping circuit.

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