Buchholz relays

[Izeldeen]

Buchholz relays are used to protect transformers from:
High currents
Low oil level
High voltage
A and B
B and C

any idea?

[cnowak]

I just had this question on my last practice test. I answered High Voltage and was incorrect. A Buchholz relay is used to pick up on a fault within a liquid filled transformer. With that in mind I think the answer would be High Currents because of how high the currents can get during a fault. If you should figure out the answer for sure please feel free to reply.

Buchholz relays are used to protect transformers from:
High currents
Low oil level
High voltage
A and B
B and C

any idea?

[bec51392]

I would say B, possibly also A depending on how you look at it... but "high currents" and "faults" are two different things. see below. click link for helpful pictures and full article. I think its pg 69.

Stolen from U.S. Department of the Interior
Bureau of Reclamation "Transformers:
Basics, Maintenance,
and Diagnostics"

https://www.usbr.gov/tsc/techreferen...s/Trnsfrmr.pdf

4.7 Buchholz Relay (Found
Only on Transformers with Conservators)
The Buchholz relay (figures 33 and 34) 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 bubbles
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, which decreases the oil level. The top float descends with the
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
continues 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),
the bottom float activates the switch contacts in the bottom chamber.

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, which
trips the transformer. The flow rate at which the paddle activates the
relay is normally adjustable. See your specific transformer’s
instruction manual for details.
(TESTING, GENERAL), while the transformer is de-energized,
functionally test the Buchhholz relay by pumping a small amount of
air into the top chamber with a squeeze-bulb hand pump. Watch the
float operation through the window (center in figure 32). Check to
make sure the correct alarm point has been activated. Open the bleed
valve and vent air from the chamber. The bottom float and switching
cannot be tested with air pressure. On some relays, a rod is provided
so that you can test both bottom and top sections by pushing the floats
down until the trip points are activated. If possible, verify that the
breaker will trip with this operation. A volt-ohmmeter may also be
used to check the switches. If these contacts activate during operation,
it means that the oil level is very low, a pressure wave has activated
(bottom contacts), or the transformer is gassing (top contacts). If this
relay operates, do not re-energize the transformer until you have
determined the exact cause.
If a small amount of gas is found in this relay when the transformer is
new (a few months after startup), it is probably just air that has been
trapped in the transformer structure and is now escaping; there is little
cause for concern.
If the transformer has been online for some time (service aged), and
gas is found in the Buchholz, oil samples must be sent to the lab for
DGA and extensive testing. Consult with the manufacturer and other
transformer experts. A definite cause of the gas bubbles must be
determined and corrected before re-energization of the transformer

[Izeldeen]

I just had this question on my last practice test. I answered High Voltage and was incorrect. A Buchholz relay is used to pick up on a fault within a liquid filled transformer. With that in mind I think the answer would be High Currents because of how high the currents can get during a fault. If you should figure out the answer for sure please feel free to reply.

High current and low oil level