Buchholz relay working principle

The Buchholz relay working principle is very simple. Buchholz relay function is based on a very simple mechanical phenomenon.

It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation fault between turns, break down of core of the transformer, core heating, the insulating transformer oil will be decomposed in different hydrocarbon gases, CO2 and CO.

The gases produced due to the decomposition of transformer insulating oil will accumulate in the upper part of the Buchholz container which causes a fall of the oil level in it.

Fall of oil level means lowering the position of the float and thereby tilting the mercury switch. The contacts of this mercury switch are closed and an alarm circuit energized.

Sometimes due to oil leakage on the main tank air bubbles may be accumulated in the upper part of the Buchholz container which may also cause a fall of oil level in it and the alarm circuit will be energized.

By collecting the accumulated gases from the gas release pockets on the top of the relay and by analyzing them one can predict the type of fault in the transformer.

More severe types of faults, such as short circuits between phases or to earth and faults in the tap changing equipment, are accompanied by a surge of oil that strikes the baffle plate and causes the mercury switch of the lower element to close.

This switch energized the trip circuit of the circuit breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter-tripping the circuit breakers associated with both LV and HV sides of the transformer. This is how Buchholz relay functions.