- Buchholz Relay Defined: A Buchholz relay is a safety mechanism used in oil-filled transformers, designed to detect internal faults by monitoring gas and oil movements.
- Working Principle: The Buchholz relay working principle involves mechanical detection of oil level changes and gas accumulation to signal possible transformer issues.
- Fault Indication: It detects transformer faults by identifying gas formation due to oil decomposition, with the upper float triggering an alarm.
- Severe Fault Handling: In cases of severe faults, a surge in oil activates the lower baffle plate’s mercury switch, which in turn cuts off the transformer to prevent damage.
- False Alarm Prevention: False alarms can be minimized with mechanical locks used during maintenance or oil refilling to ensure proper relay function.
What is Buchholz Relay?
A Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors that have an external overhead oil reservoir called a conservator. It is used as protection because it responds to dielectric failure inside the equipment. A Buchholz relay is a type of gas-detection relay.
A Buchholz relay has two main elements. The upper element is a float on a hinge so it can move up and down with the oil level in the Buchholz container.
A mercury switch is fixed on the float. Alignment of the mercury switch therefore depends on the position of the float.
The lower element is a baffle plate and a mercury switch. That plate is fitted on a hinge just in front of the inlet (main tank side) of the Buchholz relay in a transformer so that when oil enters the relay from that inlet at high pressure, the alignment of the baffle plate and the mercury switch attached to it will change.
In addition to these main elements, a Buchholz relay has gas release pockets on top. The electrical leads from both mercury switches are taken out through a molded terminal block.
Buchholz Relay Working Principle
The Buchholz relay working principle is a mechanical response to gas and oil movement.
It is mechanically actuated. Whenever there is a minor internal fault in the transformer (an insulation fault between turns, breakdown of the transformer core or core heating) the insulating transformer oil decomposes into hydrocarbon gases, CO2 and CO.
Gases from that decomposition of transformer insulating oil accumulate in the upper part of the Buchholz container and the oil level in it falls.
The video below shows a working animation of a Buchholz relay:
A fall in oil level lowers the float and tilts the mercury switch. The contacts of this mercury switch close and an alarm circuit is energized.
Oil leakage in the main tank can also send air bubbles into the upper part of the Buchholz container, lowering the oil level and triggering the alarm circuit.
By collecting and analyzing gases accumulated in the relay’s release pockets, one can determine the type of fault within the transformer.
More severe faults, such as short circuits between phases or to earth and faults in the tap-changing equipment, are accompanied by a surge of oil that strikes the baffle plate and causes the mercury switch of the lower element to close.
This switch energizes the trip circuit of the circuit breakers linked to the transformer and isolates the faulty unit from the electrical power system by tripping circuit breakers on both the LV and HV sides. That is how Buchholz relay functions.
Buchholz Relay Operation Certain Precaution
The Buchholz relay operation may occur without any fault in the transformer. For instance, when oil is added to a transformer, air may enter with the oil and accumulate under the relay cover. That can cause a false Buchholz relay operation.
That is why a mechanical lock is provided on the relay so the mercury switches can be locked when oil is being topped up in the transformer.
This mechanical locking also helps to prevent unnecessary movement of the breakable glass bulbs of the mercury switches during transportation of the Buchholz relays.
The lower float may also operate falsely if oil velocity in the connection pipe, not caused by an internal fault, is high enough to tip the float.
That can happen on an external short circuit when overcurrents in the winding overheat the copper and the oil and cause the oil to expand.
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