WAZIPOINT Engineering Science & Technology: Buchholz Relay for Power Transformer Protection Device

Sunday, March 10, 2024

Buchholz Relay for Power Transformer Protection Device

What Is A Buchholz Relay?

How does a Buchholz Relay Protect Power Transformer?

A Buchholz relay is a protective device used to protect power transformers from internal faults and incipient failures. It is an essential component of a power transformer's protection and monitoring system. The Buchholz relay operates on the principle of detecting abnormal conditions within the transformer's oil-filled tank. Here's how it works and how it protects power transformers:

Detection of Gas Accumulation: Buchholz relay is installed in the oil-filled conservator tank of the transformer. This tank is connected to the transformer's main tank through a pipe. Inside the Buchholz relay, there are two float switches and a gas collection chamber. These switches are designed to float on the oil surface.

Normal Operation: During normal transformer operation, the oil remains stable, and the float switches remain at the bottom of the relay, indicating that everything is operating within the specified parameters.

Abnormal Conditions: When an internal fault occurs within the transformer, such as a short circuit, arcing, or partial discharge, it generates heat and gas. The heat causes the oil to expand, and the gas generated rises to the top of the transformer tank. This gas can accumulate in the Buchholz relay's gas collection chamber.

Gas Accumulation Detection: As gas accumulates in the Buchholz relay, it causes the float switches to rise. When the float switches reach a certain level, they trip a set of contacts in the Buchholz relay.

Alarm and Transformer Trip: The tripped contacts in the Buchholz relay can trigger an alarm and/or initiate a trip signal to the transformer's protective relay system. The alarm is usually a warning that there may be a fault within the transformer. If the fault is severe or hazardous, the trip signal is activated to disconnect the transformer from the power system to prevent further damage and reduce the risk of fire or explosion.

Buchholz relays provide several benefits for transformer protection:

Early Warning:
Buchholz relays can detect incipient faults and minor issues before they escalate into major problems, allowing for preventive maintenance and minimizing downtime.

Safety: They help prevent catastrophic failures, which can be dangerous and expensive.

Fault Identification: The relay can provide valuable information about the type and severity of the fault based on gas accumulation and float switch behavior.

Remote Monitoring: Buchholz relays can be integrated into remote monitoring and control systems, allowing for real-time monitoring of transformer health.

In summary, Buchholz relays protect power transformers by detecting abnormal gas accumulation within the transformer's oil-filled tank, providing early warning of faults, and initiating protective actions to prevent further damage or hazards. This makes them a crucial component in ensuring the reliability and safety of power transformer operation.

What Is A Buchholz Relay?

Could you please provide me with a clear explanation of what a Buchholz Relay is? 

Buchholz relay is a safety device to protect a transformer from internal faults, like- impulse breakdown of the transformer insulating oil or transformer coil turns insulation failure, etc. Buchholz relay in the field of power transmission and distribution systems is a kind of relay that is a little different from other conventional relays. Buchholz relay is also a safety device that is mounted on large size (normally more than 500kVA) oil-immersed transformer reactor and equipped with a conservator or overhead oil reservoir. The Buchholz relay was first developed by Max Buchholz in 1921.

What is Buchholz in chess?  “The Buchholz system (also spelled Buchholtz) is a ranking or scoring system inches developed by Bruno Buchholz (died ca. 1958) in 1932, for Swiss system tournaments (Hooper & Whyld 1992). ... It was probably first used in the 1932 Bitterfeld tournament.”

Working Principle Of Buchholz Relay

It is interesting that the transformer is electrical equipment but the protecting device Buchholz relay does use any electrical signal itself. An internal fault may occur inside transformer due to
  1. insulation oil overheating;
  2. insulation failure of coil turns;
  3. excess core heating or core breakdown;
  4. any fault may result in excess heat; 
Any fault inside the transformer causes excess heat and heat decomposes the transformer oil and generates gas. Gas bubbles ahead upward direction to conservator, Buchholz relay collected these gas bubbles through connected pipe and operate the relay to safe the transformer from destructive damages.Buchholz Relay Constructional Diagram 

Buchholz relay actually consists of two floated hinges accomplished with a mercury switch, one at the top and other at the bottom, both are in an oil-filled chamber. The upper float mercury switch is connected to an alarm circuit and the lower float mercury switch is connected to an external trip breaker, the details working principle are shown in the constructional diagram of a Buchholz relay.

For major faults, like earth faults or phase-to-earth short circuits, the generated heat is high to produce a large amount of gas inside the transformer that will similarly flow upwards, but its motion is high enough to tilt the lower float in the Buchholz relay. Finally, the lower float will cause the lower mercury switch will break or trip the transformer means isolated from the supply.
These are typically connected to cause the transformer to trip and this relay also serves a third function, similar to the sudden pressure relay.

Common Features of a Buchholz Relay

Buchholz relays are protective devices used in oil-filled power transformers and other oil-filled electrical equipment to detect and respond to faults and abnormal conditions within the equipment. These relays are named after their inventor, Max Buchholz, and serve as an essential component for ensuring the safe and reliable operation of such equipment. Here are some common features and functions of Buchholz relays:

Gas Accumulation Detection: Buchholz relays primarily detect the presence of abnormal gas accumulations, typically hydrogen or other combustible gases, within the oil-filled enclosure. These gases can be generated as a result of various faults, such as overheating, partial discharges, or arcing.

Two Floats: Most Buchholz relays are equipped with two floats or vane-type mechanisms suspended in the oil. These floats are designed to move in response to changes in gas levels within the oil.

Alarm Function: When the lower float detects a smaller amount of gas accumulation, it triggers an alarm signal, indicating a minor fault or early warning of a potential problem. This alarm can alert maintenance personnel to investigate the issue.

Trip Function: If the upper float detects a significant increase in gas accumulation, it triggers a trip signal. This trip signal initiates a shutdown of the transformer or other equipment, preventing further damage and potentially catastrophic failures.

Remote Monitoring: Buchholz relays often include provisions for remote monitoring and signaling, enabling operators to receive alerts and take appropriate action even when they are not physically present at the equipment location.

Oil Flow Monitoring: In addition to gas detection, some Buchholz relays are equipped with a second set of contacts that monitor the flow of oil within the equipment. A sudden or severe reduction in oil flow can also trigger an alarm or trip signal.

Installation Position: Buchholz relays are typically installed in the oil-filled pipe connecting the main transformer tank and the conservator tank, which allows them to monitor the gas content in the oil. The position of installation is critical for their proper functioning.

Manual Reset: After a trip is initiated, most Buchholz relays require a manual reset to restore normal operation. This manual reset ensures that a qualified technician can inspect the equipment and address any issues before resuming operation.

Maintenance and Inspection: Buchholz relays require periodic maintenance and inspection to ensure their continued reliability. This includes checking float operation, verifying proper oil levels, and testing the alarm and trip functions.

Fault Analysis: The information provided by Buchholz relays can help identify the type and location of faults within the equipment, allowing for more efficient and targeted maintenance.

Buchholz relays are an essential safety feature in oil-filled electrical equipment, as they provide early warning and protection against potential catastrophic failures. Regular maintenance and testing of these relays are crucial to ensure their effectiveness and the overall reliability of the equipment they protect.

Why Buchholz Relay is Used In Transformers?

It is common for a transformer to have an internal fault due to a short circuit to transformer coil turns, any incidental fault in transformer winding, or any impulse breakdown of the transformer oil or insulating oil. Buchholz relay is used for the protection of transformers from such a kind of fault.

Know detail about safety relay in another post: 

Advantages and Disadvantages Of Buchholz Relay

The major advantages and disadvantages of Buchholz are as below:


      1. Buchholz relay indicates the internal faults due to heating and safe transformer from any major faults;
      2. To determine the fault severity no need to dismantling the transformer;
      3. Buchholz relay prevents accidents from any major fault isolating the transformer.


      1. Buchholz relay can only be used with oil immersed transformers equipped with conservators;
      2. Buchholz relay may operation may be actuated without any fault in the transformer;
      3. During oil fill in the transformer, some air also get in, later this air may accumulate under the relay and false operation may happen;
      4. Buchholz relay mercury switch may lock its movement due to mechanical lock;
      5.  Buchholz relay can detect faults only below the oil leveling the transformer.
Hope this article helped you to increase for learning more about the Buchholz relay and related similar items. If you think it will be helpful for your friends then don’t forget to share with them. Also, you can share your better knowledge about this in the comments box below.

Top 5 Famous Brands of Buchholz Relays

Buchholz relays are protective devices used in oil-filled transformers and other electrical equipment to detect and respond to faults such as internal arcing or gas accumulation. While there aren't many brands that exclusively manufacture Buchholz relays, several well-known electrical equipment and relay manufacturers produce high-quality Buchholz relays. Here are five famous brands that offer Buchholz relays:

  1. Siemens: Siemens is a global leader in electrical engineering and automation technology. They offer a wide range of protection relays and equipment for power systems, including Buchholz relays.
  2. ABB (ASEA Brown Boveri): ABB is another major player in the field of electrical equipment and protection relays. They provide Buchholz relays as part of their comprehensive product lineup.
  3. Schneider Electric: Schneider Electric is a prominent manufacturer of electrical distribution and automation equipment. They produce Buchholz relays for various applications.
  4. Toshiba: Toshiba is known for its high-quality electrical equipment, including transformers. They also offer Buchholz relays to complement their transformer protection systems.
  5. Eaton: Eaton is a global supplier of power management solutions, and they manufacture protective relays, including Buchholz relays, for use in transformers and other oil-filled equipment.

Please note that the availability and specific models of Buchholz relays may vary over time, and it's essential to check with the respective manufacturers for the latest product offerings and specifications.


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