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Contact resistance test producing CT secondary current

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    Contact resistance test producing CT secondary current

    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.

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    How did they test the breaker? I am assuming it was connected in normal position with line bus de-energized and contact-resistance was measured from line bus to load bus.

    As far as finding a peer reviewed article is concerned, I don't know of any but I can ask some relay guys if they have ever seen anything like what your describing. You might be able to contact the manufacturer of the test set for an answer. Your experiment sounds interesting and I would love to see the result.

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    So after talking with some of the guys at my shop, everyone seems to agree that your theory is plausible but more information is needed about the scheme to know what really happened. It's also possible that there is a wiring problem somewhere which caused the breaker to trip.

    Can you provide a sketch of the differential protection scheme along with relay settings and the location of the DLRO test leads? This will help to better understand the various zones of protection.

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    Differential Trip

    Quote Originally Posted by alexlounsbury View Post
    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
    Review your SLD, check the Zone of protection what equipment are associated with the differential scheme.

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    Quote Originally Posted by Skyehaven40 View Post
    Review your SLD, check the Zone of protection what equipment are associated with the differential scheme.
    This is an interesting topic, has anyone discovered the cause of the differential trip?

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    DLOR Demon!

    Quote Originally Posted by Achandler View Post
    This is an interesting topic, has anyone discovered the cause of the differential trip?
    This problem has reared its ugly head many times over the years. While performing this resistance check, the current was increased from 0 to 200 amps. After getting the readings, the test set was turned off. Not ramped down, just shut down. The bus diff saw this 200 amps to zero spike as an input from the out of service breaker and out went the bus.

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    Quote Originally Posted by alexlounsbury View Post
    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
    Hey alexlounsbury, I know its been a while but here is a paper from Megger that discusses your exact problem:

    Effects of Low Resistance Measurement Instruments on Protection and Control Devices, Effect of DLRO on the differential protection scheme

    The DC current used for measuring the low resistance can affect the protection and control circuitry in adverse ways. Typical problems encountered are Current Transformer (CT) magnetization, accidental tripping of differential relays and inconsistent and unreliable low resistance measurements. The paper will address the root cause of these encountered problems. It will recommend precautions to take prior to performing the test and discuss in detail the recommended practices to avoid any misoperation of connected protection and control devices.

    https://content.megger.com/getattach...ol-Devices.pdf
    Last edited by SecondGen; July 12, 2018 at 07:24 PM.

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    Quote Originally Posted by alexlounsbury View Post
    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
    Was this using a 10A ductor or a larger 100-200A unit?

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    M

    Quote Originally Posted by alexlounsbury View Post
    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
    Our crew, when ductoring a breaker associated with any bus config, have isolated the breaker and shorted the CTS
    's indicative of the protection schemes. Just because I've heard of such things happening.

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