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Thread: CT Connection

  1. #1
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    CT Connection

    Gents,

    For a three-phase protection current transformer, is it OK to jumper together polarity marked leads to create a Y and bring out non-polarity conductors to a protective relay? I recently saw a configuration depicted in the attached sketch where the CT upstream of protected equipment had the polarities connected together, which threw me off, since the CT has to be properly installed & wired in the field to function properly in a scheme.

    Please provide your insight. The CT configuration on the left is what I'm used to seeing ..

    Thanks,
    FaradayFF
    Attached Images Attached Images

  2. #2
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    the pdf is blank

  3. #3
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    Quote Originally Posted by Ingenieur View Post
    the pdf is blank
    It displays fine in the Preview application on a Mac and in the Drive PDF viewer and the Acrobat app on my Android phone.
    Did you download the attachment instead of just expecting the diagram to show up in the browser?

  4. #4
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    I can read the pdf and, to answer the OP, it will depend on whether the "protected equipment" induces a phase-shift AND whether the "microprocessor based relay" can be programmed to recognize the phase-shift, if any, along with the CT wiring configuration. Many modern relays would have no problem.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  5. #5
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    Quote Originally Posted by GoldDigger View Post
    It displays fine in the Preview application on a Mac and in the Drive PDF viewer and the Acrobat app on my Android phone.
    Did you download the attachment instead of just expecting the diagram to show up in the browser?
    just tapped it
    all other pdf's just pop open
    iphone 6

  6. #6
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    Quote Originally Posted by Ingenieur View Post
    just tapped it
    all other pdf's just pop open
    iphone 6
    Possibly a non-Adobe PDF that does not play well with iPhone viewer? Try downloading and opening with Adobe app.

    Sent from my XT1585 using Tapatalk

  7. #7
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    very common
    http://www.gegridsolutions.com/multi...tsci/art07.pdf

    still can't see the pdf

    we do it all the time and the common residual/gf

  8. #8
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    Quote Originally Posted by GoldDigger View Post
    Possibly a non-Adobe PDF that does not play well with iPhone viewer? Try downloading and opening with Adobe app.

    Sent from my XT1585 using Tapatalk
    Displayed fine on my iPhone 7.

  9. #9
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    Honesdale, PA USA
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    Common depending on application...

    Quote Originally Posted by FaradayFF View Post
    Gents,

    For a three-phase protection current transformer, is it OK to jumper together polarity marked leads to create a Y and bring out non-polarity conductors to a protective relay? I recently saw a configuration depicted in the attached sketch where the CT upstream of protected equipment had the polarities connected together, which threw me off, since the CT has to be properly installed & wired in the field to function properly in a scheme.

    Please provide your insight. The CT configuration on the left is what I'm used to seeing ..

    Thanks,
    FaradayFF
    Your pdf file shows the supply CT wired 180° different from the load side CTs. This is common for some differential relay applications. You did not indicated the make and model of the relay. Note that the operating coils in the relay are also 180° from each other and this may be the reason the load CTs are wired with opposite polarity. I would need the make and model of the relay used and it would be helpful to know that device is being protected.

  10. #10
    Join Date
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    California
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    The CT is part of a differential scheme protection delta(L)-Y grounded(H) Transformer. The relays are Schweitzer relays. I'm thinking that unless the relay is programmed to account for the reverse polarity connection of the load site CT, it may mis operate.

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