When the incipient faults are occured in the transformer, no relay will detect such faults due to very small magnitude of fault current.
To detect such slow developing faults, Buchholz's relay is used.
When the transformer is totally oil immersed and having kVA rating above 750 kVA; Buchholz relay will provide protection against such faults.
Basically Buchholz relay is a gas-actuated relay installed in oil-immersed transformers for protection against all kind of fault.
When there is incipant fault in transformer Buchholz's relay give an alarm.
When there is severe fault in transformer, it disconnects the transformer from supply.
It is usually installed in the pipe connecting the conservator to the main tank.
Construction :
Fig. shows the constructional details of the Buchholz relay.
It consists of a domed vessel and located between the main tank and the conservator.
Buchholz relay has two elements. The upper element consists of a mercury-type switch attached to a float and used to close an alarm circuit during incipient fault.
The lower element contains a mercury switch mounted on a hinged type flap located in the direct path of the flow of oil from the transformer to the conservator and is arranged to trip the circuit breaker in case of severe internal faults.
Operation :
When an incipient fault occurs inside the transformer, it generates the heat due to fault current.
This heat causes the decomposition of some transformer oil in the main tank.
Decomposition of transformer oil produces more than 70 % of hydrogen gas.
Due to the lightweight of the hydrogen gas, it tries to go into the conservator and in the process gets accumulated in the upper part of relay chamber.
When a predetermined amount of gas get accumulated in the conservator it exert sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it.
The closing of the mercury switch completes the alarms circuit and gives an alarm.
When a major fault occurs inside the transformer, an enormous amount of gas is generated in the main tank.
The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch.
This completes the trip circuit of the circuit breaker and circuit breaker disconnects the transformer by opening its contacts.
Advantages:
1. Incipant faults are difficult to detect but with the help Buchholz relay it is possible to detect it at much earlier stage.
2. Construction of Buchholz relay is much simpler.
Disadvantages :
1. Buchholz relays can only be used with oil-immersed transformers equipped with conservator tanks.
2. Faults at connecting cable have to be provided with a separate protection scheme since the device can detect only faults below oil level in the transformer.