Buchholtz relay is also called as gas relay or sudden pressure relay. It is mounted in the
pipe line, and in between, connecting the conservator oil tank and the main tank of the
transformer. This relay is capable of operating the tripping mechanism of the transformer
circuit breaker before the occurring of incipient dielectric failure fault in a transformer and
even will trigger a hooter circuit before the tripping of the electrical circuit.
The relay has two different detection modes. On a slow accumulation of gas, due perhaps
to slight overload, gas produced by decomposition of insulating oil accumulates in the top of
the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm
signal that also serves to detect slow oil leaks.
If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator.
This flow of oil operates a switch attached to a vane located in the path of the moving oil. This
switch normally will operate a circuit breaker to isolate the apparatus before the fault causes
additional damage. Buchholz relays have a test port to allow the accumulated gas to be
withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as
overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak.
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