Buchholz Relay Working Principle

A relay is an electrical switch that controls a high voltage circuit using a low voltage source. Effectively. A relay electrically separates a low voltage circuit from a high voltage circuit. Most relays are actuated by sensors and hence commonly used in the control system. Buchholz relay is a special device that detects faults inside an oil-filled power transformer. It is a safety component that prevents the severity of an internal faulty by raising an alarm to operators or automatically tripping the switch to disconnect the transformer. The common type of internal fault is a dielectric failure of insulating oil.

Construction and operation

The relay is used with transformers rated 500kVA and above, farther the cooling system must use oil and a conservator be in place. Buchholz relay is mounted along the pipe that connects the transformer oil tank and the conservator tank. It consists of two main elements, namely;

  • Gas detector chamber
  • Oil surge chamber

The two metallic chambers hinged on one end and a mercury floater switch is free to move. The upper chamber activates an alarm when the switch is closed while the lower chamber energizes the transformer trip circuit. A flexible flap is used to detect oil surges. The relay is enclosed with a release cock at the top and a test valve at the bottom. An inlet and outlet opening allows the flow of oil from the main transformer tank to the conservator.

Working Principle

Normally dielectric failure occurs by decomposition of cooling oil into gases. Gas has been lighter it rises above the liquid oil and escapes upwards through the relay to the conservator. Dielectric failure of the insulating oil is caused by the decomposition of the liquid into a gas such as carbon IV oxide. The gas being light rises above the tank and off it escapes to the relay. Inside the relay, the as collects at the upper chamber, displacing the oil. Once it is displaced the float switch moves downwards to the new oil level thereby closing an alarm switch.  An alarm is raised notifying the operators of an internal fault. This fault does not cause the breaker to trip since it is a minor one. 

For oil surge operation, the trip mercury switch is attached to a movable flap. Once the velocity of oil flow at the relay is above a set value, the floater switches in pushed and it closes the contacts of a trip circuit. The circuit breaker is tripped cutting out the faulty transformer from damaging other systems.

The presence of gas in the transformer oil may be caused by one of the following;

  • Overload – this heats up the transformer oil which makes it decompose to gases.
  • Air entry into the tanks due to the presence of an opening
  • Air is trapped inside the tank during the change of oil.

Sparking generated within the transformer cause oil decomposition into gas.

Another reason that may cause an alarm to ring is the vibration of the transformer that can disturb the mercury switch forcing it to close.

Causes of oil surge switch being activated include;

  • Low oil levels could be caused by leakage.
  • Dielectric breakdown of insulation oil in a rapid manner causes fast decomposition and gas production.
  • Oil pump operation- at the instance the pump is turned on, the flow is being at a high velocity to appoint of closing the trip floater switch.
  • Shock and vibration of the transformer may cause fast than normal oil flow closing trip switch.

The oil surge detector is a major fault and it disconnects power from the transformer. This calls for a thorough inspection, testing, and fault finding before re-energizing the transformer.

As we have seen Buchholz relay is a very critical protection device for a transformer. A fault occurring upstream of an electrical network should be cleared as fast as possible to prevent a cascade of the fault downstream.


  • It detects internal faults of equipment without technicians having to open up the closed chambers.
  • It automatically initiates trip operation when a fault is major.
  • The gases collected at the top of the relay can be taken for a lab test to establish the fault.


  • It is a slow-to-operate relay – it has an average of 0.2 seconds to respond to a fault.
  • It can only detect faults below the oil level of the transformer tank.
  • Can only work with oil-cooled transformers which have a conservator.


100 years since the invention of the Buchholz relay, the device remains one of the most critical protection gear for power transformers. It is a maintenance-free relay that stays on guard for years protecting our expensive transformers.  It is a smart device that can be automated to know remote users know the status of their machine. Certainly, the device is a masterpiece.

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