Question 8.5: Estimate the symmetrical short-circuit current at the second......

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),

For the transformer using Equations 8.13 and 8.14,

$Y=\frac{100}{Z(\%)}$                                 (8.13)

$MVA_{SC}=\frac{MVA}{Y}$                     (8.14)

Since 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)

Thus,

Therefore, 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)