Forums » Electrical Engineering

short circuit calculation

    • 4 posts
    June 22, 2019 11:03 PM PDT

    I am looking to do the short circuit calculation. The question is the generator  is 10 MVA, and on other hand the generator's turbine can only deliver 7 MVA.

    which power value should be used for short circuit calculation (10 MVA or 7MVA)

    kV = 15 kV
    Based MVA = 10 MVA
    Based kV = 15 kV
    X"d = 20%

    Short circuit current I"d?


    This post was edited by Ed Conner at June 22, 2019 11:04 PM PDT
    • 103 posts
    June 23, 2019 4:17 AM PDT

    I would use 7 MVA.

    • 75 posts
    June 23, 2019 7:25 AM PDT

    You can use either value.  However, X"d is "per unit" and based on the nameplate base (10MVA)

    On the 10 MVA base:

    FLC = 10,000(kVA)/15kV/sqrt(3) = 385A

    I"d = FLC/X"d = 1924A

     

    The size of the driver doesn't change that.

     

    I'm okay if I get corrected - My brain occasionally stalls before the second cup of coffee

     

    carl

    • 29 posts
    June 24, 2019 10:00 AM PDT

    At first thought, I'm with Carl on this one.  If we think about the reasons for including inductive machine ratings in short circuit analysis, we are considering the energy storage capacity of the winding, and that energy gets released back into the system upon a fault, and we must account for that energy when we do our calculation.  The capacity of the winding to store energy is independent of what the mechanical properties of the driver are.  In a short circuit study, I calculate a 1000 horsepower motor on the 1000 horsepower energy capacity is has, not what actual load is that the motor might be driving.  Perhaps that is over conservative, but the consequences of being under conservative can be very drastic.