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Buchholz Relay Construction, Working In Transformer

What is a Buchholz relay?

Buchholz relay is one of the protective device for electrical transformer from internal faults.It works with all gas and oil immersed transformer.Buchholz relay in transformer is an oil contained arrangement. Buchholz relay connected in between pipe from main tank to conservator tank and it is filled with oil throughout transformer operation.It is used for all power transformers,but not economical in the case of low rated transformers.Mostly buchholz relay employed for gas and oil operated transformer(rated >500 kVA).

buchholz relay diagram

Why Buchholz relay is used in transformers?

The buchholz relay works on the principle of gas vaporization from insulating oil in the fault conditions.Buchholz relay protects power transformer from all internal faults such as breakdown of the insulating oil, insulation failure of winding,over heat generated in the coils due to failure of insulation etc.

Working,Construction of Buchholz Relay


Buchholz relay function is based on very simple mechanical phenomenon.It consists of two elements mounted in a metallic chamber located in the pipe connecting the conservator and the transformer tank. When minor fault occur, heat is produced due to current leakage, some of the oil in transformer tank evaporates and some of the vapor gets collected at the top of the chamber while passing it to the conservator.

When a predetermined amount of vapor accumulates at the top of the chamber, the oil level falls, the mercury switch attached to a float is tilted so it closes the alarm circuit and rings the bell. A release cock is provided at the top of the chamber, so that after operation, the pressure in the chamber can be released and the gas emitted allows the chamber to refill with oil.

When severe faults occur a large volume of gas is evaporated, so that the lower elements containing a mercury switch mounted on a hinged type flap is tilted and the trip coil is energized. A drain is provided at the bottom of the chamber to allow air to be pumped into the chamber for test purposes.

The Buchholz relay operation may be actuated without any fault in the transformer. For instance, when oil is added to a transformer, air may get in together with oil, accumulated under the relay cover and thus cause a false Buchholz relay operation.

Buchholz relay is employed for the following types faults, 

1. Core bolt insulation failure
2. Local-overheating
3. Entrance of air into the oil
4. Less of oil due to leakage
5. Short circuited lamination due to burns.

Merits & Demerits of Buchholz Relay

Advantages of  Buchholtz relay

1. The advantage of using a Buchholtz relay is that they indicate incipient faults. For example, It indicates the faults between turns or heating of core which enables us to take the transformer out of service before severe damage occurs.

2. Buchholtz relay gives an alarm when the oil level reduces below a certain level due to leakage of oil from transformer.

3. Buchholtz relay gives an audible warning which informs the operator that there is some fault in the transformer.
4.Type of insulation failure of the transformer can also be detected by testing the gas. 

Drawbacks of  Buchholtz relay

1. This type of relay can be used only for transformers with conservators.
2. The relay does not operate when the faults are above the oil level.
3. Buchholz relay does not protect the cables associated with the transformers below. A separate protection has to be employed for the cables.
4. Due to economic considerations, this relay is not employed for transformers above 500 kVA.
5. Setting of the mercury switch cannot be too sensitive, otherwise there can be a false operation due to vibrations,earthquakes. mechanical shocks to the pipe.
6. The operation of this new is unsatisfactory due to slow operating time. (Nearly 0.2 to 0.1 seconds)

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