Short Circuit Calculation using Sequence Immedances from the Utility

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DMG_1

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Location
Boston, MA, USA
I requested short circuit data from the utility and was given base voltage and base MVA and per unit values for positive, negative, and zero sequence impedance (impedance is a complex number). Does anyone know how this calculation is done?
 
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Sunny_92

Member
Location
York, PA
Occupation
Electrical Engineer
What exactly are you looking to calculate? If you just need the short-circuit current available at the utility, it's simply:

SCC = (MVA * 1e6) / (sqrt(3) * base voltage)
 
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DMG_1

Member
Location
Boston, MA, USA
I was given base voltage, base MVA, and sequence impedances. The equation you gave me will be the Ibase. After that you need to multiply by a value determined by the impedance to find the short circuit current. I have been given positive, negative, and zero sequence impedance in complex form "R + jX". I think I just need to come up with Z total by take the square root of the sum of the squares, Zt = ((R*R) + (X*X))^1/2.
 
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
In my opinion, if you intend to calculate unsymmetrical fault current-not only three-phase solid short-circuit current as per above mentioned article-a per-unit value can be converted to an actual quantity at any time by multiplying the per-unit value by the base value of that quantity as already Ingenieur proposed.
If you have actual impedances you may calculate unbalanced system fault currents following-for instance:
Analysis of three-phase networks in disturbed operating conditions using symmetrical components- B. de Metz-Noblat
file:///C:/Users/user/Downloads/ECT018.pdf
 
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Ingenieur

Senior Member
Location
Earth
To get a reasonable order of magnitude
convert pu |Z1| to act ohms
convert your xfmr pu to act ohms ph-ph
calc you service feeder ohms for ph-ph loop
convert Z1 to xfmr sec basis
sum Z1, xfmr Z and cond Z
divide into sec Vll
 
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