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Three single phase transformer with different %Z's

  1. Jul 27, 2015 #1

    JWT

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    Hello everyone,

    I have a three phase circuit that consists of a three-phase transformer bank feeding a balanced three-phase delta load. The transformer bank consists of three single phase transformers connected in delta. Each transformer is rated at 333kVA, 2400V. Two of the transformers have percent impedances of 5.35%, while the third has a percent impedance of 4.04%. Also, each transformer has the same X/R ratio of 4.5 (if that matters), and assume the balanced load is 0 ohms, and the line impedance is 0 ohms.

    My question- in this circuit, how would you calculate the three line currents?

    I appreciate your time.
     
  2. jcsd
  3. Jul 27, 2015 #2

    JWT

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    By the way, this is not a homework question. I just want to see the effects of difference in transformer impedances on the line currents, even if there are nothing but three-phase loads in the facility. VFD's and soft starts are tripping on current imbalance.
     
  4. Jul 28, 2015 #3

    jim hardy

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    So you're after short circuit currents?
     
  5. Jul 28, 2015 #4

    JWT

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    In this example, yes. Really what I'm after is getting an idea of what the percentage deviation would be on the line currents if a three-phase delta secondary comprised of single phase transformers that had different percent impedances (which in my case, two cans have 5.35%Z while the third has 4.04%Z).

    Thank you, sir, for your help.
     
  6. Jul 28, 2015 #5

    jim hardy

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    i didn't set down with pencil and paper

    but does this not look soluble by basic circuit analysis?

    3ph.jpg
    Shorting the 3 apexes sets each phase current trivially to V/Z ,
    is each line current not the sum of the two phases to which it's attached ?
     
  7. Jul 29, 2015 #6

    JWT

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    Yes, sir. That does make sense. For some reason I was looking into using sequence impedances and didn't think to just do a simple nodal analysis. Thanks again for your help.
     
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