Tag Archives: Transformer

Fast and Simple Short circuit current calculation

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Quick Methods For Transformer

  • Short circuit current Based on the infinite source (Rated current Method):
    Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
    Rated current(Ir)= S(KVA)*1000 / V / root(3) = 1000*1000 /400 / 1.732 = 1443A
    Short circuit current (Isc) = Rated current (Ir) / Zsc
    Short circuit current (Isc) = 1443 A / 0.05 = 28,900 A = 28.9 KA
  • Short circuit current Based on the infinite source (Impedance Method):
    Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
    Impedance(Z) of the transformer = Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
    Short circuit current (Isc) = V / root(3) / Z
    Short circuit current (Isc) =  400 / 1.732 / 0.008 = 28,900 A = 28.9 KA
  • Short circuit current Based on upstream 500MVAsc (Admittance Method):
    Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
    Transformer MVAsc= MVA / 1000 / Zsc=1000 KVA /1000/ 0.05 = 20 MVA
    1 / MVAsc = 1 / Transformer MVAsc + 1 / Upstream MVAsc
    1 / MVAsc = 1/20+1/500 = 0.052
    MVAsc = 1 / 0.052 = 19.23
    Short circuit current (Isc) = MVAsc / root(3) / V = 19.23 / 1.732 / 0.4 = 27,757 A = 27.8 KA
  • Short circuit current Based on upstream 500MVAsc (Impedance Method):
    Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
    Impedance(Zt) of the transformer = Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
    Impedance(Zu) of the Upstream at MV side = V^2 / upstream MVAsc = 13.8^2 X 500 = 0.381 Ω
    Impedance(Zu’) of the Upstream at LV side= 0.381 x (400V/13.8KV)^2 = 0.00032 Ω
    Impedance(Z) of the Upstream and Transformer=0.008 Ω +  0.00032 Ω = 0.00832 Ω
    Short circuit current (Isc) = V / root(3) / Z
    Short circuit current (Isc) =  400 / 1.732 / 0.00832 = 27,757 A = 27.8 KA
  • Short circuit current Based on upstream 250MVAsc (Impedance Method):
    Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
    Impedance(Zt) of the transformer= Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
    Impedance(Zu) of the Upstream at MV side= V^2 / upstream MVAsc = 13.8^2 X 250 = 0.762 Ω
    Impedance(Zu’) of the Upstream at LV side= 0.381 x (400V/13.8KV)^2 = 0.00064 Ω
    Impedance(Z) of the Upstream and Transformer=0.008 Ω +  0.00064 Ω = 0.00864 Ω
    Short circuit current (Isc) = V / root(3) / Z
    Short circuit current (Isc) =  400 / 1.732 / 0.00864 = 26,729 A = 26.8 KA

Impedance Method For System

Short_Circuit