Principle :
The incipient faults in transformer tank below oil level actuate Buchholz relay so as to give an alarm. The arc due to fault causes decomposition of transformer oil.
The product of decomposition contain more than 70% of hydrogen gas, which being light, risks upwards and tries to go into the conservator.
The Buchholz relay is fitted in the pipe leading to the conservator.
The gas gets collected in the upper portion of the Buchholz relay, thereby the oil level in the Buchholz relay drops down. The float, floating in the oil in the Buchholz relay drops down.
The float, floating in the oil in the Buchholz relay tilts down with the lowering oil level.
While doing so, the mercury switch attached to the float is closed and the mercury switch closes the alarm circuit.
Thereby the operators know that, there is some incipient fault in the transformer.
The transformer is disconnected as early as possible and the gas sample is tested.
The testing of gas gives clue regarding the type of insulation failure. Buchholz relay gives an alarm so that the transformer can be disconnected before the incipient fault grows into a serious one.
