BUCHHOLZ RELAY OR POWER TRANSFORMER BASICS AND TUTORIALS

BUCHHOLZ RELAY FOR POWER TRANSFORMER BASIC INFORMATION

What Are Buchholz Relay? How Buccholz Relay Works?


The Buchholz Relay (Gas Relay) is designed to protect equipments submerged in insulating liquid, by means of supervision of the oil abse nce or abnormal flow, and abnormal gassing caused by the equipment. Buchholz relay is usually fitted on transformers provided with an expansion tank for the insulating liquid.

Buchholz Relay

Buchholz relay is capable to accurately detect, for example, the following problems: Leakage of insulating liquid, short - circuit inside the equipment causing a great displacement of insulating liquid, inside gassing due to intermittent or continuous failures occurring inside the equipment.

Buchholz relay is usually installed between the main tank and the oil expansion tank of the transformer.

Buchholz relay housing is made of cast iron, having two flanged openings and two sight glasses showing a graduated scale of gas volume. There are two inside floats, being that the upper float is fo rced to move downwards (this also happens in case of oil leakage).

On the other hand, in case an excessive gassing causes an oil ci rculation through the relay, the lower float reacts, even before the gas reaches the relay. In both cases, the floats make contacts when they are displaced.

The Buchholz Relay has a device for the inside float testing and locking. To check for proper operation of the relay contacts, when it is installed in the transformer, proceed as follows:

Alarm:
· Connect an Ohmmeter to terminal s + C - D. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Shutdown
· Connect an Ohmmeter to terminals + A - B. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Before supplying po wer to the transformer, the following items should be checked:

· Remove the lid of the relay -testing device.

· Remove the float -locking pin from the inside of the testing device. Both floats should be free to move.

· Replace the cover of the relay -testing device.

· Purge the air from the relay by means of the 1/8” air valve located on the relay lid.

· Check the relay for possible leakage that might have occurred during the installation on the transformer and fix it.

· Check the relay for proper fitting wi th regards to the oil float direction, which arrow should be pointing towards the transformer’s oil expansion tank.

If the alarm sounds without turning off the transformer, it is necessary to turn it off immediately and then test the gas removed from the inside of the relay. In this case, the origin of the failure can be assessed according to the gas testing result, i.e.:

· Combustible gas (contents of acetylene): In this case there must be a failure to be repaired on the electrical part;

· Non-combustible gas (without acetylene) : in this case, it means there is pure air. The transformer can be turned on again without danger after the air is bled out from the relay. When the alarm sounds repeatedly, it indicates that air is penetrating into the transformer. Tur n it off and repair the failure.

· No gassing (the gas level inside the relay is getting lower and an amount of air is being drawn through the open valve), in this case, the oil level is too low, possibly due to a leakage. Top up with oil until the control level and carry out the air tightness essay.

The transformer is turned off without a previous alarm. In this case, the transformer must have been thermally overloaded. Turn it on again after a considerable time interval for cooling. The failure can be found atcthe short-circuit contact in the protection relay system.

The alarm sounds and the transformer is shutdown immediately before or after the alarm sounds. In this case, one of the above mentioned failures must be the cause. Make the gas testing and proceed as described above.

ATTENTION!
Float locking device for transport purpose and testing of contacts : After installing the relay, remove the insert used to lock the floats.

Operation: To test the contacts, press the internal part with the lid pin. The contacts should actuate automatically. If everything is properly working, close the device again in order to prevent any leakage. Now the relay is ready to be put in operation.


NOTE: The insert is used for transport purposes only.