Buchholz relay is a protective and gas actuated relay . It is used in that transformer whose winding is immersed in oil and rating having more than 500KVA. Buchholz relay is not used in small transformer because it increase the cost. It is used to protect transformer against internal or incipient faults only. Basically, it is used for detection of winding faults.
Buchholz relay is invented by Buchholz in 1921.
Buchholz Relay Construction
Buchholz relay is installed in the pipe connecting the conservator to the main tank. There are two mercury switches used in the relay. One of the mercury switches is attached to the upper float. This switch send signal to the alarm circuit when relay operates. Whereas the other switch is mounted on lower hinged type flap. This switch send signal to the trip circuit. A release cock is mounted top of the chamber.
Operation or Working of Buchholz Relay
When a minor fault occurs in a transformer, heat is produced, oil gets heat up. Hydrogen gas is produce in transformer. The gases being light, try to go into conservator tank but gas is trapped in Buchholz-Relay which is connected between main tank and conservator tank. It is collected in top of the chamber while passing to the conservator tank. When the predetermined amount of gas is accumulate in the upper part of the relay, oil level falls due to gas pressure. This tilt the upper float downward and closes the alarm circuit contacts through mercury switch attached to the float arm. The alarm rings bell and gives the warning signal.
In case of series fault within the transformer, large amount of gas in evolved in the main tank. In this time, lower mercury switch of the buchholz relay operates and sends the signal to the trip circuit. This completes the trip coil circuit of the circuit breaker. Thereafter, the transformer is disconnected automatically.
A release cock is provided at the top of the relay chamber so that after operation, gas pressure is released through this cock.
‘Buchholz Relay’ indicates internal faults or incipient faults in the transformer, such as inter turns fault, insulation failure. It does not respond to external faults.