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Answer Posted / valluru subba rao
Buchholz Relay (found only on transformers with conservators).
The Buchholz relay has two oil-filled chambers with floats
and relays arranged vertically one over the other. If high
eddy currents, local overheating, or partial discharges
occur within the tank, bubbles of resultant gas rise to the
top of the tank. These rise through the pipe between the
tank and the conservator. As gas bubbles migrate along the
pipe, they enter the Buchholz relay and rise into the top
chamber. As gas builds up inside the chamber, it displaces
the oil, decreasing the level. The top float descends with
oil level until it passes a magnetic switch which activates
an alarm. The bottom float and relay cannot be activated by
additional gas buildup. The float is located slightly below
the top of the pipe so that once the top chamber is filled,
additional gas goes into the pipe and on up to the
conservator. Typically, inspection windows are provided so
that the amount of gas and relay operation may be viewed
during testing. If the oil level falls low enough
(conservator empty), switch contacts in the bottom chamber
are activated by the bottom float. These contacts are
typically connected to cause the transformer to trip. This
relay also serves a third function, similar to the sudden
pressure relay. A magnetically held paddle attached to the
bottom float is positioned in the oil-flow stream between
the conservator and transformer tank. Normal flows resulting
from temperature changes are small and bypass below the
paddle. If a fault occurs in the transformer, a pressure
wave (surge) is created in the oil. This surge travels
through the pipe and displaces the paddle. The paddle
activates the same magnetic switch as the bottom float
mentioned above, tripping the transformer. The flow rate at
which the paddle activates the relay is normally adjustable
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