Utility X/R and SC
Utility X/R and SC
(OP)
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?
Since there is no utility impedance?






RE: Utility X/R and SC
RE: Utility X/R and SC
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: Utility X/R and SC
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?
RE: Utility X/R and SC
RE: Utility X/R and SC
"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.
RE: Utility X/R and SC
RE: Utility X/R and SC
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.
RE: Utility X/R and SC
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.