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Buchholz relays

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    Buchholz relays

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

    any idea?

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    Just had this question.

    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.

    Quote Originally Posted by Izeldeen View Post
    Buchholz relays are used to protect transformers from:
    High currents
    Low oil level
    High voltage
    A and B
    B and C

    any idea?

  4. #3
  5. bec51392 is offline NETA Level III Pro Subscriber
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    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

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    Quote Originally Posted by cnowak View Post
    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

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