What is Buchholz relay and the significance of it in to the transformer?

 Buchholz relay is a device which is used for the protection of the transformer from its internal faults, it is a gas-based relay. whenever any internal fault occurs in a transformer, the Boucholz relay at once gives a horn for some time if the transformer is isolated from the circuit then it stops its sound itself otherwise it trips the circuit by its own tripping mechanism.