In the field
of electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled transformers and reactors, equipped with an
external overhead oil reservoir called a conservator.
The Buchholz Relay is used as a protective device sensitive to the effects of dielectric
failure inside the equipment.
PRINCIPLE OF OPERATION
The operation of buchholz relay is as follows:
i. In case of slow developing faults within the transformer, the heat due to the fault
causes decomposition of some transformer
oil in the main tank. The products of decomposition mainly contain 70 %
of hydrogen gas. The hydrogen gas being light tries to go into the
conservator and in the process gets trapped in the upper part of the
relay chamber. When a predetermined amount of gas gets accumulated, it
exerts sufficient pressure on the float to cause it to tilt and close
the contacts of mercury switch attached to it. This completes the alarm
circuit to sound an alarm.
ii. If serious fault occur in 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 it tilts the flap to close the
contacts of mercury switch. This completes the trip circuit to open the
circuit breaker controlling the transformer.
Construction
The cover is a weather-resistant casting of light alloy and is provided with a paint coat. Terminal box (1), test valve (2) and test key, covered by a cap nut (3) as well as a plate for operation of the test key (4) are arranged above the cover. The terminal box has an earthing contact (5) and the electrical connectors (6). The aluminium cap (7) seals the terminal box. If the cap is opened the contact setting
(8) can be seen. The cable may be optionally brought in through one of both cable glands (9).