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Electrical Engineering: Calculate the fault level for a 3Ø fault

  1. Sep 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Calculate the fault level for a 3Ø fault at F
    With bus section CB closed
    With bus section CB open


    2. Relevant equations
    The circuit diagram can be found here
    http://s1167.photobucket.com/albums/q629/sayeen1/?action=view&current=fault_level.png

    3. The attempt at a solution
    The solution of this question is provided as follows

    Sbase = 0.5 MVA

    (Xfault)Source = [Sbase/(Sfault)] = 0.5/250 = 0.002 p.u

    Zbase = [(Vbase)2/(Sbase)] = [(11kV)2/(0.5 MVA)] = 242 Ω

    Rcable = [(Rcable)Ω/(Zbase)] = 0.252/242 = 0.001 p.u

    Xcable = [(Xcable)Ω/(Zbase)] = 0.0817/242 = 0.00034 p.u

    With bus section closed:
    *This is the part that i dont understand, how did they get "j0.0273"?*

    (Xfault)Total = 0.001 + j0.0273 → (Xfault)Total = 0.0273 p.u → Sfault = 0.5 MVA / 0.0273 p.u = 18.3 MVA

    With bus section open:
    *This is the part that i also dont understand, how did they get "j0.0523"?*

    (Xfault)Total = 0.001 + j0.0523 → (Xfault)Total = 0.0523 p.u → Sfault = 0.5 MVA / 0.0523 p.u = 9.56 MVA
     
  2. jcsd
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