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phasing out three phase

Discussion in 'Electrical Engineering' started by Bill Schafer, May 31, 2004.

  1. Bill Schafer

    Bill Schafer Guest

    I have a question as to wether or not I can parallel two three phase
    transformers with a common tie on a double ended switchboard arrangement.

    We have two identical dry indoor transformers 13.8kV - 480/277V with primary
    sources from same utility line. In order to do service on each tansformer
    the tie breaker will be required to close, parallel the switchgear (xfmrs),
    and open one transformer and asscociated main breaker without disruption.

    I have heard this can be accomplished if each of the three phases voltage
    angles are with 15 degrees of each other. I am not planning on performing
    this task until I am educated enough on this. Can anyone help or provide
    some additional information pertaining to the subject.

    Thanks in advance.
     
  2. Don Kelly

    Don Kelly Guest

    ------
    You should have no problem. This has been done and is still done routinely
    in many places and with identical or nearly identical transformers you
    should have no circulating current problems (and the length of time that
    they will be in parallel will be small) . - provided that you make sure that
    you connect corresponding phases together (don't trust the labels) - This
    will require checking of the phases as phase A on one may not correspond to
    phase A on the other. You can check the voltages between corresponding
    phases on the secondary with a voltmeter- these should be small (unlikely 0
    unless the loads are fully balanced). A competent inductrial electrician
    should be able to check this out.
    Then the next consideration is the load- so the thing to remember is that
    you will be carrying the full load on one transformer when the other is out
    of service- best to do such maintenance at a low load time of day.
     
  3. SQLit

    SQLit Guest

    Everyone has given good ideas.
    Do you know if there is a control scheme in the main tie mains that prevents
    just what your doing? Most will have this hardwired and/or software
    controlled as well.
    What I have seen is that the MOC and TOC contacts are series and/or
    paralleled to prevent all of the breakers from closing. You will have to
    change all of the this wiring and or software to allow a close of the tie.
    Next you had better be talking to the utility folks. They can be of great
    assistance for doing this.
    Lastly it will be time for intensive and compressive training for the worker
    bees.

    I wrote a main tie main scheme for a 15 kv system once. After 3-8 hour days
    of training the workers did as I had feared. They tried to do the process
    out of step. My safety circuit caught the mistake and took over for them.
    This was a live test with no load.
    Go very carefully into this situation.
     
  4. Bill Schafer

    Bill Schafer Guest

    Thanks all for the quick reponse!

    I have not seen the job yet, information was handed to me from the facility
    electrician. The question was raised if I was able to tie the boards
    together and take transformer offline with no interuption to the building.

    They are hiring us to perform de-energized maintenance tasks to each
    transformer as well as infrared thermography previous while energized. I
    will be able to review their tie scheme and take appropriate voltage
    measurements as suggested by s. falke.

    If I find out that the transformer primary sources are not from the same
    utility line but phased correctly between both switchboards, would there
    still be a possibility of pulling this off?

    Bill
     
  5. Don Kelly

    Don Kelly Guest

     
  6. If the source is from the same bus, perform the phasing check once. Then
    recheck whenever a transformer is removed and returned to service, or other
    maintenance work is performed that could possibly affect phasing.

    If planning to tie together two utility feeds, better check with them first.
    You may be subjecting your transformers to large circulating currents,
    possibly overloading them. With this case, since part of the circuit is
    outside of your control, perform the phasing check every time prior to
    closing a tie switch.
     
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