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  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?
    What is esoteric knowledge today will be common knowledge tomorrow.

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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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    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.
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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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  7. #7
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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.
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    Quote Originally Posted by mbrooke View Post
    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.
    i see..

    but this will result to only 1 bus scheme. why not use partial differential? that way, both buses are protected by 87 separately.. and it is dynamic..


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  9. #9
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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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    I think he his saying
    all loads less one on A
    one load on B
    tie closed
    use the ct's on the tie and the one load in a differential scheme
    if they don't match fault on bus

    but why?
    need to see how buses are fed



  10. #10
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    Quote Originally Posted by Ingenieur View Post
    I think he his saying
    all loads less one on A
    one load on B
    tie closed
    use the ct's on the tie and the one load in a differential scheme
    if they don't match fault on bus

    but why?
    need to see how buses are fed

    Normally all loads are on buss A, and the only live connection to Buss B is the coupler.

    The issue I am facing is having the buss bar differential system remaining stable as loads are shifted from buss A to bus B with the coupler closed.
    What is esoteric knowledge today will be common knowledge tomorrow.

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