X/R for study source.

[Southern38371]

I am using SKM to analyze a system and the utility gave me the values in the following format..

12.47 kV source

Source Impedance
Z1 = 2.602 + j 45.463% 100MVA base ( 0.040 + j 0.707 Ohms)
Z0 = 0.918 + j 39.123% 100MVA base ( 0.014 + j 0.608 Ohms)

Fault Availability
Three-Phase = 10167 A, 219.6 MVA
Phase-Ground = 10668 A

My understanding is that the X/R would be X(Z1)/R(Z1) for the 3 phase fault and (2*X(z1)+X(z0))/(2*R(z1)+R(z0)) for the line to ground faults but the values are not looking correct to me...

Can someone give me a check on this? I'm getting 3 phase X/R of 17.675.

 

[Kiribanda]

Assuming Z1=Z2,
For 3-ph faults X/R= (X1)/(R1)=17.47
whereas for 1-ph faults,
X/R = [(X1+X2+X3)/(R1+R2+R3)] = 21.2
If there is a fault resistance (Rf), then it should be (R1+R2+R3+Rf).
Hope this helps.

 

[cuky2000]

Be aware that for a high X/R>17, the circuit breaker interrupting current rating need some reduction.

 

[jghrist]

This is a reasonable value for a source just on the low side of the power transformer.