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Homework Help: Per unit fault impedance

  1. Apr 21, 2015 #1
    1. The problem statement, all variables and given/known data
    Given the relationships:

    Xpu = MVAbase / MVAfault

    Xpu = ( MVAbase / MVArated ) * ( X% / 100 )

    MVAfault = sqrt(3) * V * Ifault

    Xohms = [ V / sqrt(3) ] * Ifault

    2. Relevant equations

    Show that:

    Xpu = [ MVAbase / V^2 ] * Xohms

    3. The attempt at a solution

    No matter how I approach this problem I keep finding that one of the given relationships must be incorrect. Could someone with better algebra skills then me please take a look and confirm whether (a) there is an error in the set problem or (b) I'm just being dense?

  2. jcsd
  3. Apr 21, 2015 #2


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    Science Advisor

    I have no idea of what you are trying to ask - but "Xohms = [ V / sqrt(3) ] * Ifault" seems very wrong. Resistance (ohms) is always Voltage divided by current.
  4. Apr 21, 2015 #3
    Please see the attachment for the problem in its original format, which should make things clearer.

    I have solved the derivation of equation 5(a).

    I can solve the derivation of equation 5(b) no problem using "V=IX" (ignoring the resistance of the cables for the purposes of the fault calculation, as instructed in the problem).

    What I can't see is how the given equation "Xohms = [ V / sqrt(3) ] * Ifault" fits into the derivation of equation 5(b). This equation seems wrong.

    Attached Files:

  5. Apr 22, 2015 #4


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    Science Advisor

    A typo. It should be "Xohms = V /[sqrt(3) * Ifault]".
  6. Apr 23, 2015 #5


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    Staff: Mentor

    Or, equivalently, Xohms = [ V / sqrt(3) ] / Ifault
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