Infinite Bus X/R

Location
Chicago
Occupation
Electrical Engineer
My issue is that I am getting Single phase line to ground fault values higher than the 3 phase fault values in an SKM short circuit study. I am using the INFINITE BUS option on the utility source and the X/R ratio defaults to 9999. This seems too high and I think thats the issue. The main Switchboard has a 3ph fault of 38,000 amps and a SLG fault of 40,000 amps.

1. Why is my SLG fault higher than my 3ph fault?
2. What value would you assume for the X/R of an infinite bus?
 

steve66

Senior Member
I agree with the others. L-G faults can be slightly higher than the 3 phase faults, even though we normally only look at the 3 phase fault. I just noticed that on a couple of projects the other day.

Types of faults

So you aren't necessarily doing anything wrong.
 

Boerg

Member
Location
Oklahoma City
Occupation
Engineer
My issue is that I am getting Single phase line to ground fault values higher than the 3 phase fault values in an SKM short circuit study. I am using the INFINITE BUS option on the utility source and the X/R ratio defaults to 9999. This seems too high and I think thats the issue. The main Switchboard has a 3ph fault of 38,000 amps and a SLG fault of 40,000 amps.

1. Why is my SLG fault higher than my 3ph fault?
2. What value would you assume for the X/R of an infinite bus?
1. If you're close to a ground fault current source, such as the wye side of a delta-wye transformer, then it's conceivable that SLG fault current is higher.
2. A sufficiently high X/R will do. 9999 is good. I was going to say 100, but those fault-current angles are both close enough to 90 degrees to essentially be the same.
 
Location
Chicago
Occupation
Electrical Engineer
Thanks again guys, helpful as always.

I contacted SKM tech support and they said that the X//R of 9999 is assuming a very small X and an infinitely small R.

I.E. X = 0.001 and R = 0.00000000001
 

NinjaGrin

New User
Location
Mexico
Occupation
hr manager
1. If you're close to a ground fault current source, such as the wye side of a delta-wye transformer, then it's conceivable that SLG fault current is higher.
2. A sufficiently high X/R will do. 9999 is good. I was going to say 100, but those fault-current angles are both close enough to 90 degrees to essentially be the same.




Thank you! This was a big help.
 

winnie

Senior Member
I contacted SKM tech support and they said that the X//R of 9999 is assuming a very small X and an infinitely small R.
I.E. X = 0.001 and R = 0.00000000001
That makes sense. The 'infinite bus' approximation is the assumption that there is _no_ impedance on the primary side of the transformer.

X=0 R=0 would give an undefined X/R (0/0), so they pick some really small numbers that are small enough to be effectively zero but large enough to let the equations converge.

-Jon
 

beanland

Senior Member
Location
Vancouver, WA
Any time you use assumptions about the source fault current, there is the risk of very bizarre results. A typical small transformer has an X/R of ~1 and the typical utility padmount transformer has an X/R of ~10. Utility primary supply, unless adjacent to the substation will have an X/R of ~5. For fault currents, using the infinite bus assumption produces a good indication of the maximum possible available fault current. But, if you are performing arc flash studies, you cannot use the infinite bus because it gives invalid results. Best bet, use reasonable assumptions to get reasonable answers.
 
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