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Homework Help: Transformer connected in parallel question

  1. Jan 15, 2014 #1
    1. The problem statement, all variables and given/known data
    The question is as per my attachment

    2. Relevant equations

    3. The attempt at a solution

    I found the following help of the internet but do not know where to start any assistance would be appreciated:
    you don't need to be concerned about matched turns ratio. You can assume that the turns ratio is 1:1 and that the transformers are single-phase or that you have per-phase, wye data. Since the answer is to be given as a percentage, you can assume a convenient voltage like 1000 V. That reduces the problem to a relatively simple network of (0.09 + j0.18) in parallel with (0.09 + j0.23) and that parallel combination in series with (R + jX). With the assumed voltage, the power factor of the current in one branch, and the kva of the load supplied by that branch.

    Attached Files:

  2. jcsd
  3. Jan 15, 2014 #2

    rude man

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    Gold Member

    Can't open your file. (I have office 2003).
    Convert to pdf?
  4. Jan 16, 2014 #3
    For some reason i can't upload the attachment anyways the question is as follows:

    Transformer A is rated at 300 kVA with an impedance of (0.09+j0.18)Ω

    Transformer B is rated at 200 kVA with an impedance of (0.09+j0.23)Ω

    if these transformers are connected in parallel to supply a common load, determine the percentage overload on B when A supplies its full load at p.f. 0.45 lag

    Attached Files:

  5. Jan 16, 2014 #4

    rude man

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    We need input and output impedances of both transformers.
  6. Jan 16, 2014 #5
    This is the complete question as i have it
  7. Jan 24, 2014 #6
    Looking at this again and i have found

    Full load current presuming 1000v and at single phase:
    Transformer A = 300000/1000 = 300A
    Transformer B = 200000/1000 = 200A

    I know that power factor is 0.45 lag so this is cos^-1 0.45 = 63.25 degrees

    kVA a = kVA total (Zb/Za+Zb)
    kVA b = kVA total (Za/Za +Zb)

    I think I have everything to solve but cannot figure out the methodology on this one can anyone assist?
  8. Jan 26, 2014 #7


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    Staff: Mentor

    With any luck, this may not be as complicated as it might seem. You know the voltage drops across the impedances must be identical, since the transformers are working into a common load voltage.

    Focus on just magnitudes of the drop across the impedances. These must be equal.

    Are you provided with the correct answer, so we can check?
  9. Jan 29, 2014 #8
    Thanks for your help.

    I worked out the loads on the two Transformers connected in parallel and then worked out the difference in percentage overload and submitted my answer and this came back correct.

    I think it was just the question that was puzzling me I was not sure what it was asking. :)
  10. Jan 24, 2015 #9
    Hi eddievic I'm stuck on the same question have been for days, how did you work out the loads with out the total KVA or total wattage or is it just as simple as adding the 2 KVA's of the transformers together
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