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Thread: CT reversed in bus differential zone

  1. #1
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    CT reversed in bus differential zone

    How does bus protection behave when a CT is reversed polarity wise? Say normally all feeders are kept on the lower bus B. Then all feeders are placed on the upper buss A except one remaining on B with current flowing from A to B via bus coupler and then out the feeder.




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  2. #2
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    MBrooke,

    When 3 CTs, Ia, Ib, and Ic, are configured to derive a residual-current, 51N, then 51N-Amps will be exactly twice that of the line-current measured with a reversed CT!

    Following are 3 cases given Ia, Ib, and Ic:

    a) Covers the case when the CT in Line A is reversed; then, 51N is twice Ia!

    b) Covers the case when the CT in Line B is reversed; then, 51N is twice Ib!

    c) Covers the case when the CT in Line C is reversed; then, 51N is twice Ic!

    Hopefully, this will aid you!

    Regards, Phil Corso

  3. #3
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    Quote Originally Posted by Phil Corso View Post
    MBrooke,

    When 3 CTs, Ia, Ib, and Ic, are configured to derive a residual-current, 51N, then 51N-Amps will be exactly twice that of the line-current measured with a reversed CT!

    Following are 3 cases given Ia, Ib, and Ic:

    a) Covers the case when the CT in Line A is reversed; then, 51N is twice Ia!

    b) Covers the case when the CT in Line B is reversed; then, 51N is twice Ib!

    c) Covers the case when the CT in Line C is reversed; then, 51N is twice Ic!

    Hopefully, this will aid you!

    Regards, Phil Corso

    Thanks!

    So if the feeder current is 1000amps, it will appear as a 2000amp bus fault?
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    sorry but im having a hard time understanding the scheme. basically, when designing the 87b if should cover all possible normal connections.

    when you transfer feeders, so basically, it should be stable.

    when you combine bus, the bus coupler is close and there is only one bus. the CT in the bus coupler is not included.


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    if you are using partial 87b, the same thing will happen, only that the relay takes the polarity of CTs used.


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    Quote Originally Posted by rian0201 View Post
    sorry but im having a hard time understanding the scheme. basically, when designing the 87b if should cover all possible normal connections.

    when you transfer feeders, so basically, it should be stable.

    when you combine bus, the bus coupler is close and there is only one bus. the CT in the bus coupler is not included.


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    Normally this system would have two dynamic differential zones with half the circuits on bus one and half on bus two. A fault on either bus clears only half the circuits.

    In this case all the circuits are on one buss and buss two is simply a spare, used only when bus one needs to be taken out of service. So under normal operation there is only one zone.


    Technically I do not need to include the CT coupler, but one of the busses (the reserve) Id like to be protected with over current by the bus coupler, so technically under normal operation I need to exclude the reserve bar out of the differential zone. Either that my bus differential zone would need an 8 cycle delay, something I'd like to avoid.
    What is esoteric knowledge today will be common knowledge tomorrow.

  7. #7
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    CT reversed in bus differential zone

    To answer your question: they add making a through current appear 2x actual through current. Differential relaying works by applying Kirchoff's current law. Things should add to zero, but with reversed CTs you've multiplied by -1.


    Your diagram is lousy. Check out the SEL document. Relevant stuff starts on page 7 but all is good. SEL has so many good white papers.

    https://cdn.selinc.com/assets/Litera...0150812-080535


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    Quote Originally Posted by gray.one View Post
    To answer your question: they add making a through current appear 2x actual through current. Differential relaying works by applying Kirchoff's current law. Things should add to zero, but with reversed CTs you've multiplied by -1.


    Your diagram is lousy. Check out the SEL document. Relevant stuff starts on page 7 but all is good. SEL has so many good white papers.

    https://cdn.selinc.com/assets/Litera...0150812-080535


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    Thanks


    What if I simply opened the bus coupler test switch CTs?


    I think the challenge here is that while this substation is a single breaker double bus (which are usually dynamic and run with an SEL487B) we have the case where under normal operation only a single zone is needed.
    What is esoteric knowledge today will be common knowledge tomorrow.

  9. #9
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    Quote Originally Posted by mbrooke View Post
    Normally this system would have two dynamic differential zones with half the circuits on bus one and half on bus two. A fault on either bus clears only half the circuits.

    In this case all the circuits are on one buss and buss two is simply a spare, used only when bus one needs to be taken out of service. So under normal operation there is only one zone. [/QOUTE]

    this is what i am saying..

    [QOUTE]
    Technically I do not need to include the CT coupler, but one of the busses (the reserve) Id like to be protected with over current by the bus coupler, so technically under normal operation I need to exclude the reserve bar out of the differential zone. Either that my bus differential zone would need an 8 cycle delay, something I'd like to avoid.
    im curious, you mean that the reserve bar is protected by overcurrent? how? is the bus coupler always closed? what coordination with the down stream OCs?



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    Quote Originally Posted by rian0201 View Post
    im curious, you mean that the reserve bar is protected by overcurrent? how? is the bus coupler always closed? what coordination with the down stream OCs?



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    Yup, the bus coupler is always closed. The reserve bus is protected by over current via the bus coupler. The bus coupler is set to have an SEL-351 relay which gives the definite and inverse time over current.


    Normally no bays are connected to the reserve bus. The reserve bus is only used when the main bus must be taken out of service for repair or maintenance.


    The reasoning behind keeping the reserve bus energized and protected is so faults on it can be detected as they happen. Otherwise the reserve bus may become faulty and nor be discovered until its needed.
    What is esoteric knowledge today will be common knowledge tomorrow.

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