It is an oil activated relay used for transformer protection. It protects transformers from internal faults.
buchholz relay
buchholz relay
It has two floats and two reed switch inside.
The reed switch is connected to the external trip circuit.
A Buchholz relay is used to detect the faults that occur inside the main tank of the transformer such as inter-turn faults, short circuits, earth faults, overheating of the core, oil leakage, etc.
It is mounted on the pipe connecting the main tank and the conservator of the transformer.