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Home / Technical Articles / The Purpose Of Transformer Gas Relay


The transformer gas relay is a protective device installed on the top of oil-filled transformers. It performs two functions. It detects the slow accumulation of gases, providing an alarm after a given amount of gas has been collected.

The Purpose Of Transformer Gas Relay
The Purpose Of Transformer Gas Relay (on photo: Gas actuated relay for oil-filled transformers by Cedaspe S.p.a.)

Also, it responds to a sudden pressure change that accompanies a high rate of gas production (from a major internal fault), promptly initiating disconnection of the transformer. An incipient fault or developing fault, usually causes slow formation of gas.

Examples of incipient faults are:

  • Current flow through defective supporting and insulating structures;
  • Defective joints at winding terminals causing heating;
  • Minor tap changer troubles; and
  • Core faults.

A major fault is one that results in a fast formation of a large volume of gases. Examples of such faults are:

  • Shorts between turns and windings; and
  • Open circuits, which result in severe arcing.

Failure to disconnect the transformer under fault conditions can result in severe equipment damage from high gas and oil pressures and the effect of the electrical fault.

Generation of Gas Due to Faults

Internal transformer electrical faults result in the production of ionized gases. A significant volume of gas is frequently generated in the early stages of a fault by rapid oil breakdown.

The generated gases rise through the oil to the top of the equipment and collect in the gas relay.

Once a sufficient volume of gas has accumulated, an alarm is generated by contacts within the gas relay.

In the event of a gas alarm, it is necessary to sample and analyze the gas being generated. This analysis, together with knowledge of the rate at which gas is accumulating, will determine the proper course of action. If a fault is thought to be developing, the device must be removed from service.

Ignoring this early warning sign can lead to severe equipment damage as the fault progresses.

Operation of a Transformer Gas Relay

A typical transformer gas relay consists of two chambers, each performing a distinctive function. A simplified cross-section of a gas relay is shown in Figure 1.

The relay assembly consists of a gas accumulation chamber mounted directly over a pressure chamber. The accumulation chamber collects slowly produced gases. A float located in this partially oil-filled chamber moves as the gas volume increases. It operates an alarm switch when the amount of gas collected reaches a specified level.

An indicator coupled to the float also provides a means to monitor the rate at which gas is being generated.

Typical Transformer Gas Relay
Figure 1 – Typical Transformer Gas Relay

The second chamber, a pressure chamber, connects directly to the transformer oil circuit. It connects vertically to the accumulation chamber, providing a path for the rising gas.

An air-filled bellows within the pressure chamber acts as the pressure change detector. A sudden pressure surge in the oil compresses the bellows and forces the
air within to move a diaphragm. The moving diaphragm actuates a switch that initiates tripping of the transformer.

Sudden pressures, such as oil circulating pump surges, are normal operating events and the relay must be set to ride through them. In practice, it is necessary to make sure the relay is set to operate at about 7 KPa (1 psi) above the maximum oil circulating pump surge pressure.

Dangerously high pressure increases from major faults are relieved by an explosion vent on the top of the transformer tank.

This is basically a diaphragm sealed pipe with its open end directed away from the transformer.

A significant increase in pressure bursts the diaphragm and discharges gases and hot oil with a possibility of resulting fire.

Buchholz Relay (VIDEO)

Resource: Science and Reactor Fundamentals – Electrical; CNSC Technical Training Group

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Maryam
    May 19, 2019

    Dear Mr.Edvard,

    This is Maryam from Iran Transfo Co. I am responsible for purchasing the Gas Relays.
    We are seeking for Transformer Gas Relays manufacturer, If you are interested in working with us please let us have your catalogs.
    We are looking forward to having your replay ASAP.


    Dec 01, 2012

    Dear Mr Edvard,
    I found your post is like treasure information. I’ve got many information from your website that is useful for my career. I am kindly request you the ” Electrical estimation for buildings and industries “book or related book. I’ve searched that book at the internet many times but I didn’t find. Please share me if you have.

  3. Vilas Jadhav
    Nov 30, 2012

    Dear Edvard,

    Thanks for the detais on Transformer Gas Relay.

    Vilas Jadhav..

  4. kumereshan
    Nov 30, 2012

    Mr Edvard
    thank u for using my Buchholz Relay video

    • Edvard
      Nov 30, 2012

      Hi Kumereshan,

      Nice to meet you! Excellent video I must say. That’s why I chosed your video between several others on Youtube!

      Kind regards

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