How does a Buchholz relay work?
Buchholz relay belongs to ‘standard’ protection elements of liquid-immersed power transformers including special designs such as converter duty transformers. The simple gas-actuated relay is so popular and reliable that even after development of sophisticated numerical protection relays no one has yet dared to remove Buchholz relay from the list of transformer protection devices. As this year remarks a centenary of the famous relay it is a good opportunity to describe its construction, working principle, performance and comparison with other protection elements.
History of Buchholz relay
Transformer expansion tanks (conservators) have been implemented since 1910. They contributed to lower pace of aging of transformer oil. Buchholz relay was developed by Max Buchholz (1875 – 1956) who was working as a senior councillor at Preussische Elektrizitäts-A.G. (Prussian electricity company) in Kassel, Germany. His task was to examine transformer damages. During his task he figured out that large portion of heat due to electric arc destroys the insulation material and generates gas. He came to an idea to lead the gas bubbles under the cover of the main tank to an appropriate place. After many trials he found a solution: the gas could be collected with a small inclination of the cover. A disposed pipe should lead the gas into the expansion tank. The color and quantity of the gas could be observed and checked whether it is flammable or not. Based on such examination it could be decided about what happened inside the transformer.
Buchholz received his first patent in 1921. Shortly after the company Max Buchholz AG was founded in Kassel and Elektrokustos AG was established in Zurich, Switzerland. Buchholz relays were also manufactured by AEG and Siemens and at Micafil in Zurich for BBC [1].
The Buchholz relay is approximately 100 years old. Nonetheless, it has been steadily improved and further developed. In mid 1920s the lower float was improved to work reliably in case of strong gas flow. In the mid-1930s the lower float was connected to a baffle plate (flap) to achieve a higher sensitivity on flow. The baffle plate then became standard part of the Buchholz relay. In 1960s the relay was improved with regards to vibration and shock to avoid false tripping. At the end of 1990s an electronic Buchholz protection was introduced.
Construction
Buchholz relay is used for conservator type transformers. It is located in the connection link between main tank of the transformer and the conservator tank.The relay consists of following main parts:
– Enclosure (oil chamber)
– Inlet section fitted to the pipe from main tank
– Outlet section fitted to the pipe to the conservator tank
– Gas inspection chamber
– Terminal box (contacts for alarm and trip)
– Test screw
– Upper float with alarm contacts
– Lower float with trip contacts
– Baffle plate with opening
– Bleeding valve
– Sample valve
A good engineering practice is to use “double float” relays as described above. The alarm and trip circuits shall be electrically separated. The relay shall be equipped with shut-off valve. Gas sampling valve is optional but highly recommended.
Protection functions
Buchholz relay provides 3 fundamental protection functions for the transformer:
– Short circuit and internal arc protection
– Protection against smaller internal faults
– Protection against loss of liquid
The relay does this by monitoring the gas build-up caused by degradation or decomposition of insulation material, by monitoring an oil surge caused by an arc fault or a short circuit and by monitoring loss of oil inside the conservator tank.
Principle
Buchholz relay works on a principle of actuation of gas generated by the transformer due to internal or external issue. Therefore, the mechanical relay is either named Buchholz relay after the inventor Mr. Buchholz or gas-actuated relay called after the working principle.
In normal healthy condition there should be virtually no gas in a liquid immersed transformer and the relay is entirely filled with the insulating liquid. However, due to overheating or internal faults certain amount of gas is developed. Basically, any type of fault creates some sort of gas (gas decomposition of overheated insulating liquid à gas bubbles). Such gas flows from the main tank to the conservator tank passing through the Buchholz relay where it causes movement of the floats. Those floats are equipped with contacts wired to alarm and trip circuit.
A) Large faults
Severe faults generate larger quantity of gas that causes the lower float to drop. The gas stream makes the baffle plate to move. Upper float may move or stay in its position. The combination of those events is recognized as a large fault and triggers a trip. Typical range of flow rate to initiate a trip is 1 to 3 m/s.
B) Small faults
In case of small faults, a corresponding smaller amount of gas is produced. The gases slowly accumulate inside the upper section of the relay and displace the oil equivalent inside the chamber. Finally the gas accumulation causes the upper float to drop. Lower float and baffle do not move. Such combination is evaluated as a small fault and an alarm is initiated.
C) Low liquid level
Buchholz relay also detects loss of insulating liquid. In such case both upper and lower floats drop while the baffle plate does not move.
Advantages and limitations
Let’s look at some advantages and limitations of Buchholz relay.
A) Advantages
Advantages of Buchholz relay are simple principle and thus reliable operation, detection of small faults in early stage as well as severe faults and inherent supervision of oil level. The relay was further developed to increase its sensitivity while minimizing false trips.
B) Limitations
Despite being simple and reliable device, the Buchholz relay also comes with certain limitations. A major one is the slow response with typical range of 100 – 200 ms. It may be too slow in case of very severe transformer faults. Another possible limitation is that Buchholz relay only protects against faults associated with components immersed in the insulation liquid.
Older Buchholz relays, especially those with mercury contact switches, were susceptible to false alarms and trips due to vibration and shock. Those relays have been phased out, so it is unlikely to find them (unless it is quite old transformer without any upgrade). Modern Buchholz relays are more robust against vibration.
Buchholz relay today
As we have learned the Buchholz relay celebrates its 100th birthday in 2021. The transformer protection landscape has changed a lot during the last century. Therefore, a logical question may come: Is Buchholz relay still relevant for transformer protection today?
The answer is YES. Regardless of its age Buchholz relay is still used as part of standard protection of most liquid-immersed transformers. To overcome some of its weaknesses the transformer is equipped with other protection elements as well, for instance sudden pressure relief.
References
[1] W. Schossig, Transformer protection history, PAC World, Winter 2009
[2] ETD Power Transformers, http://www.etd-bez.cz/en
[3] ABB Transformer Handbook 1LAC000010, 2004
[4] VFD transformers – Protection in general, https://mb-drive-services.com/protection_of_vfd_transformers/
