Buchholz Relay
Buchholz relay is a gas-actuated relay mounted on the pipe connecting the main tank and conservator tank. It operates due to the emission of gases due to decomposition of insulating oil due to heat produced during internal faults. It is a most important part of transformer.
It is a safety device used in oil-immersed transformer and reactors of power rated over 500kva. It is not used in small transformer due to its high price. It can be used only in the transformer that have conservator tank.
Function of the Buchholz Relay
The main function of the Buchholz relay is to protect the transformer from all kinds internal faults such as short circuit fault, inter-turn fault etc. It protects the transformer from all kinds of internal faults that produces heat including minor faults. It send signal to trip and alarm circuits to sound alarm and activate circuit breaker.
Construction of Buchholz Relay
Buchholz Relay is consists of two hinged float placed in a metallic oil-filled chamber. This metallic oil-filled chamber is connected through the pipe between conservator tank and main tank. It has a gas release pocket on the top of the metallic chamber.
One hinged float is placed at the upper portion of the metallic oil-chamber along with a mercury switch which is connected to the alarm circuit. Other hinged float is placed at the lower portion of the metallic oil chamber along with a baffle plate and mercury switch which is connected to the Trip Circuit.
This baffle plate is fitted on lower hinged float just in-front of the inlet (main tank side) of the buchholz relay. This is fitted in such a way that when oil enters in the relay in high pressure the alignment of baffle plate along with mercury switch will change.
Working Principle of Buchholz Relay
When the fault like short circuit occurred in the winding then it generates enough heat that decompose the insulating oil into gases. The amount of generation of gas depends upon the magnitude of the fault. These gases form bubbles and these bubbles move upward toward the conservator tank through the connecting pipe and get collected in the Buchholz relay.
Then, Buchholz relay senses the fault and sends signal to trip and alarm circuits to activate alarm and circuit breaker. Then, alarm sounds and the circuit breaker disconnects the transformer from the supply. This is the basic working principle of buchholz relay.
Operation of Buchholz Relay
The operation of buchholz relay is very simple. Whenever any minor internal faults (like insulation failure, core break-down, core heating, oil leakage e.t.c) occurs, fault current generates heat. The generated heat is enough to decompose the insulating oil into gas (like hydrogen, carbon monoxide, carbon dioxide, methane e.t.c.). These gases form bubbles and these bubbles moves upward toward the conservator tank through the connecting pipe and get collected in the buchholz relay.
This collected gas displaces the insulating oil in the metallic chamber and displacement of insulating oil is equivalent to volume of gas collected. This displacement of insulating oil tilts the upper hinged float which closes the attached upper mercury switch which is connected to alarm circuit. Then, upper mercury switch closes the alarm circuit and alarm starts to sound.
The amount of gas collected is viewed through the window provided on the wall of the metallic chamber. The sample of the gas is taken and analyzed. The amount of gas collected indicates severity of the fault and its colour indicates nature of the fault. In the case of minor fault, the hinged float which is at bottom of the metallic chamber remains unaffected because the generated gas is not sufficient to move it. Hence, alarm sounds during the minor fault without affecting trip circuit.
But when major fault like phase to earth fault and fault in tap changer occurs, it generates huge amount of heat in the winding and core of the transformer. Then, temperature of the insulating oil increases gradually and heat starts to decompose the insulating oil so rapidly. Then, huge amount of bubbles generated which is enough to move baffle plate and tilts lower hinged float. Then, the lower hinged float activates the attached lower mercury switch which is connected to the trip circuit.
Then, lower mercury switch closes the trip circuit and trip circuits activates the circuit breaker. Then, circuit breaker operates and immediately isolate the faulty transformer from the supply and load. Hence, transformer isolate during the major fault and also alarm sounds.
Advantages of Buchholz Relay
- It indicates all kinds of internal fault that generates heat (i.e. both minor or major).
- It isolate the transformer from the supply during major fault that prevent accident.
- It detect severe fault without dis-melting of transformer.
Limitation of Buchholz Relay
- It can be used only in oil-immersed transformer.
- It can only detect the fault below oil level.
- It cannot protect the connecting terminals so external protective equipment is needed.
- It’s response time is high about 0.1 second.
Conclusion
Hope, you have clearly understand about the buchholz relay, it’s function, construction, working principle, operation, advantages and limitations. Make sure to leave your opinion and suggestion about this post in the comment section below.