Calculate equivalent per-unit reactance

Since four identical alternators are in parallel, the equivalent per-unit reactance ($X'_{pu(eq)}$) is the individual reactance divided by the number of alternators.

$X'_{pu(eq)} = \frac{0.16}{4} = 0.04 \text{ p.u.}$

Determine total base MVA

The total capacity of the system is the sum of the individual ratings of the alternators working in parallel.

$S_{base} = 4 \times 20 \text{ MVA} = 80 \text{ MVA}$

Calculate short-circuit level

The short-circuit MVA is the ratio of the total base power to the equivalent per-unit reactance at the fault location.

$S_{sc} = \frac{80}{0.04} = 2000 \text{ MVA}$

Re-evaluation based on standard interpretation

Wait, if the reactance given (16%) is based on the individual unit rating of 20 MVA, the system MVA is 80 MVA · Short-circuit MVA = (Total MVA) / (Per unit reactance on that base) = 80 / 0.16 = 500 MVA.

$S_{sc} = \frac{80}{0.16} = 500 \text{ MVA}$