Whenever a minor fault occurs inside the transformer, the fault current produces heat. The produced heat decomposes the transformer oil and gas bubbles are produced. These gas bubbles moves upward direction and get collected in the buchholz relay. The collected gas displaces the oil in buchholz relay and the displacement is equivalent to the volume of gas collected. The displacement of oil causes the upper float to close the upper mercury switch which is connected to an alarm circuit. Hence, when minor fault occurs in the transformer, the connected alarm gets activated.
The collected amount of gas indicates the severity of the fault occurred. During minor faults the production of gas is not enough to move the lower float. Hence during minor faults, the lower float remains unaffected.
During the case of major faults, the heat generated is high and a large amount of gas bubbles are produced. This large amount of gas bubbles similarly moves upward but, in this case its motion is high enough to tilt the lower float in the buchholz relay which is connected trip circuit through lower mercury switch. Hence, in case of major, the lower float will activates the lower mercury switch which closes the trip circuit of the transformer resulting isolation of the transformer from the supply.
In short it can be said that buchholz relay protect the transformer in case of both major and minor internal faults that generates heat in the transformer.
Conclusion
Hope, you have clearly understand about the buchholz relay working in the transformer. Make sure to leave your opinion and suggestion about this post in the comment section below.