Impedance

Te quantity Z is called the impedance of the circuit, X the reactance, XL inductive reactance and XC capacitive reactance.
Impedance and reactance are expressed in ohms. The general term for a device possessing reactance is a reactor.
The maximum current is seen to be related to the maximum potential difference by an equation having the same form as Ohm's law for steady current, impedance Z corresponden to resistance R.



Picture above refer to http://idpowrstation.blogspot.com/2009/03/500kv-line-switching.html as impedance mapping.
Impedance calculation based on 100 MVA, 10.5 kV and fault current 11,324.04A
Impedance at 525kV system [100 MVA / 500kV x 11,324.04 A X (3^0.5)] x 100% = 1.02% R << X so %IZ = j 1.02 Impedance at Generator Reactance sub-transient saturated Xd'' = 25.8% (at 802 MVA) Reactance transient saturated Xd' = 32.7% (at 802 MVA) (Xd'' + Xd')/2 = 29.3% 29.3% x (100MVA / 802MVA) = 3.65% R << X so %IZ = j 3.65 Impedance at Generator Transformer %IZ = 15% X/R = 88 %IR = (100MVA / 786MVA) x 15% [1/(1+88^2)^0.5] = 0.02% %IX = 0.02% x 88 = 1.91% %IZ = 0.02 + j 1.91 Impedance at Unit Transformer %IZ = 12.5% x 0.95 = 11.56% X/R = 28.2 %IR = (100 MVA / 54 MVA) x 11.56% x [1/(1+28.2^2)^0.5] = 0.759% %IX = 0.759% x 28.2 = 21.4% %IZ = 0.759 + j 21.4 Impedance at Total Motor %IZ = 9.7 + j 6.45 It will describe on next post. For calculation of short circuit current: i1 = current supplied from generator at the three phase short circuit on the isolated phase bus duct. i2 = current supplied from generator transformer at the three phase short circuit on the isolated phase bus duct. i3 = current supplied from the unit transformer at the three phase short circuit on the isolated phase bus duct. ix = (100 x 10^6)/[(3^0.5) x 22.8 x 10^3] = 2.53 x 10^3 A i1 = ix x (100/Xg) = 2.53 x 10^3 x (100/3.65) = 69.3 x 10^3 A i2 = ix x [100/(Xe+Xmt)] = 2.53 x 10^3 x [100/(j 1.02 + 0.02 + j 1.91)] = 86.3 x 10^3 A i3 = ix x [100/(Hht/2 + XM/2)] = 2.53 x 10^3 x [100/(0.759 + j 21.4 + 97 + 64.5)] = 5.8 x 10^3 A