Question 8.5: Estimate the symmetrical short-circuit current at the second......
Estimate the symmetrical short-circuit current at the secondary side of a
13.8 \ kV-208Y/120-V transformer rated at 1000 \ kVA with Z = 5% and supplied by a utility having 15 \ kVA fault current capabilities.
First, the MVA_{SC} values for both the utility and the transformer are estimated. For the utility (using the utility distribution voltage and the utility fault current),
MVA_{SC,U}=\sqrt{3}*13.8 \ kV*15 \ kVA=358.52 \ MVAFor the transformer using Equations 8.13 and 8.14,
Y=\frac{100}{Z(\%)} (8.13)
MVA_{SC}=\frac{MVA}{Y} (8.14)
MVA_{SC,T}=\frac{1000 \ kVA}{0.05}=20 \ MVASince the utility source and the transformer are in series, the total MVA_{SC-tot} value can be estimated using Equation 8.16:
1/MVA_{SC-tot}=1/MVA_{SC,1}+1/MVA_{SC,2}+ …+1/MVA_{SC,n} (8.16)
1/MVA_{SC-tot}=1/MVA_{SC,U}+1/MVA_{SC,T}=\frac{1}{358.52}+\frac{1}{20}=0.05279Thus,
1/MVA_{SC-tot}=\frac{1}{0.05279}=18.94 \ MVATherefore, the symmetrical short-circuit current at the secondary side of the transformer can be calculated using Equation 8.17 with a line voltage of E_L=208 \ V:
I_{SC}=\frac{MVA_{SC-tot}}{\sqrt{3}E_L } (8.17)
I_{SC}=\frac{18.94 \ MVA}{\sqrt{3}(208 \ V) } =52,583 \ A