Utility X/R and SC

[tctctraining]

when performing Arc Flash study inside a factory, 3p and 1p sc values are provided at that location by Hydro. Does this mean that I need to leave out x/r ratio at SKM and assume those values zero?
Since there is no utility impedance?

 

[trosepe]

Since all AC systems have resitance and reactance; all AC systems have a ratio of the two impedance elements X/R. What changes in the type of system and what affects the SC calculation is the ratio. Don't think in terms of 'Utility' impedance but in 'Source' impedance. There is always a source impedance.

 

[alehman]

Hydro may be able to provide X/R if you ask the right person. If not, you will have to make an educated assumption. As a starting point, maybe you can get a look at the nameplate on the transformer serving the facility.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[tctctraining]

What confuses me is that if utility provides me with 3p and 1p at the plant location, it means that they have already considered the source impedance to come up with those values.

Entering x/r ratio will introduce another impedance which has already been accounted for.

My understanding is if the utility provides X/R ratio, then the fault level is at the substation and using X/R will result in Sc at the factory location. Is this right or am I missing something?

 

[davidbeach]

X/R is a ratio of the reactive and real portions of the source impedance, says nothing about the magnitude of the source impedance. Put a source into SKM, enter the magnitudes given by the utility, and put in their X/R if they will give it to you, otherwise come up with reasonable values.

 

[tctctraining]

Davidbeach
"X/R is a ratio of the reactive and real portions of the source impedance, says nothing about the magnitude of the source impedance"
isn't X/R representative of and impedance in series with an ideal supply voltage?
Z=(VL-L^2/MVA
MVA=1.73*Isc(3p)*VL-L
So if Hydro provide me with SC values at the factory , by accounting for X/R again ,I actually increase the impedance and therefore lowering the SC level downstream.

 

[davidbeach]

No. Your Z and MVA are magnitudes, how do you get angle? X/R is what provides the angle. Z²=R²+X²

 

[trosepe]

The X/R is important because it determines the peak asymmetrical fault current. The asymmetrical fault current can be much larger than the symmetrical fault current.

Also, the 'hydro'should have indicated that the SC value they gave you was 'symmetrical'. If the value was symmetrical; they did not factor the X/R.

 

[MikiBg]

Just to check first are we speaking in same terminology:
3p=3 phase fault, all three phases short circuited simultaneously, symmetrical fault
1p=1 phase fault, sc between one phase and earth, asymmetrical fault.
If we are, then:
X/R defines peak current for all types of fault, not just asymmetrical, type of fault defines what is X and what is R.
To determine sc current magnitude, u need X and R magnitudes, not just X/R ratio. For 3p fault u need just direct sequence impedance, so if you have 3p sc current data provided, u can find Z upstream and then go downstream by adding Z. For 1p fault u need and zero sequence impedance Z0. sc current magnitude 3p/1p ratio depends on Z/Z0 ratio.