SKM Per Unit Contribution in SKM*PTW

[isu20cpree]

I am looking to solve for the source sequence impedance of an x utility. This is what I've done so far.

Parameters:

3-phase utility contribution = 7708 A
1-phase utility contribution = 7708 A
Base/Rated MVA = 100 MVA
Base Rated Voltage(L-L) = 117875 V
X/R = 8

Working towards solution:

Short Circuit Capacity (MVA)

3ph (MVA) = sqrt(3)*7708*117.875kV = 1573 MVA

1ph (MVA) = 7708*117.875kV/sqrt(3) = 524.57 MVA

Utility Positive Sequence Impedance

Z_utility = 100 MVA / 1573 MVA = 0.063 pu

Converting to Rectangular Form:

R_pu = Z_utility * cos(arctan(8) = 0.007885 pu Ohms

X_pu = Z_utility * sin(arctan(8) = 0.06308 pu Ohms

Z_utility = (0.007885 + j 0.06308) pu Ohms <--Positive Seq Impedance

The above checks out with SKM but when I calculate the zero sequence impedance I fail to obtain the same answer as SKM.
Please see the work below:

Utility Zero Sequence Impedance

Zo = 3/If_SLG - (Z1 + Z2)


I_faultSLG = I_base / (sqrt(3 * 117875 V)

SKM Z0 = is same as positive sequence



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