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Thread: CT reversed in bus differential zone

  1. #11
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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



  2. #12
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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.

  3. #13
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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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  4. #14
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    MBrooke…

    I apologize for misinterpreting your query, i.e., reverse CT (1 of 3 in a set), when you really meant reverse-current flow thru the coupler’s CT!

    Presuming it is a conventional system (not CT’s with air-core toroidal cores), then reverse fault-current thru the coupler’s CT will produce a current 180⁰ out of phase with that of any feeder CT monitoring the same fault current. Stability is then dependent on resultant current thru the 87 relay!

    Remember, the circuit is simply a resistive one, including secondary resistance of both CT’s and their connected burdens! If the components are matched, then, the most likely outcome is that the ‘87’ won’t work! Thus, to accomplish the desired result make sure the coupler-CT polarity matches the feeder CT’s in the bus-section you want protected!

    Personally, I would not do it!

    Regards, Phil

  5. #15
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    Quote Originally Posted by Phil Corso View Post
    MBrooke…

    I apologize for misinterpreting your query, i.e., reverse CT (1 of 3 in a set), when you really meant reverse-current flow thru the coupler’s CT!

    Presuming it is a conventional system (not CT’s with air-core toroidal cores), then reverse fault-current thru the coupler’s CT will produce a current 180⁰ out of phase with that of any feeder CT monitoring the same fault current. Stability is then dependent on resultant current thru the 87 relay!

    Remember, the circuit is simply a resistive one, including secondary resistance of both CT’s and their connected burdens! If the components are matched, then, the most likely outcome is that the ‘87’ won’t work! Thus, to accomplish the desired result make sure the coupler-CT polarity matches the feeder CT’s in the bus-section you want protected!

    Personally, I would not do it!

    Regards, Phil

    Make sense.

    What if I opened the coupler differential CTs when all the loads are being transferred from bus one to bus two?
    What is esoteric knowledge today will be common knowledge tomorrow.

  6. #16
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    Mike...

    "What if I opened the coupler-breaker differential CTs .... ?"

    Much too dangerous to open CT's! Some systems use limit-switches fitted on the feeder-breaker transfer switch to essentially prevent response of the '87' during transfer!

    Phil

  7. #17
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    Quote Originally Posted by Phil Corso View Post
    Mike...

    "What if I opened the coupler-breaker differential CTs .... ?"

    Much too dangerous to open CT's! Some systems use limit-switches fitted on the feeder-breaker transfer switch to essentially prevent response of the '87' during transfer!

    Phil


    Done everyday, see this:

    http://www.statesproducts.com/common..._DS_en_V07.pdf
    What is esoteric knowledge today will be common knowledge tomorrow.

  8. #18
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    Mike...

    I didn't say you couldn't do it! I said I wouldn't do it!

    Phil

  9. #19
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    Quote Originally Posted by Phil Corso View Post
    Mike...

    I didn't say you couldn't do it! I said I wouldn't do it!

    Phil
    But you made it sound like it was dangerous. It is only dangerous if you do not have shunts. Test switches in substations have shunts that short the CT together when opened. No harm would be done opening the buss coupler CT under load.
    What is esoteric knowledge today will be common knowledge tomorrow.

  10. #20
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    hihi.. is the OC backup in your scheme? it is always to do the backup.. for me, i would use partial 87 with check zone and finish..


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