Buchholz relay is a gas- actuated relay installed in oil-immersed transformers for protection against all kind of faults. It is used to gives an alarm in case of slow developing faults or incipient faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults. It is installed in the pipe between the conservator and main tank as shown in fig11 below. This relay is used in oil-immersed transformers of rating above 750 kVA.
Fig 12 shows the constructional details of buchholz relay. It consists of a domed vessel placed in the pipe between the conservator and main tank of the transformer. The device has two elements. The upper element consists of a mercury type switch attached to a float. The lower element contains a mercury switch mounted on a hinged type flap located on the direct path of flow of oil from the transformer to the conservator. The upper element closes an alarm circuit during slow developing faults whereas the lower element is arranged to trip the circuit breaker in case of severe internal faults.
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.
- It is the simplest form of transformer protection
- It detects the slow developing faults at a stage much earlier than other forms of
1. It can only be used with oil immersed transformers equipped with conservators
2. The device can detect only faults below oil leveling the transformer. Therefore
separate protection is needed for connecting cables.