Solve Example 7.3 parts (a) and (c) without using the superposition principle. First calculate the internal machine voltages \(E_{g}^{\prime \prime}\) and \(E_{m}^{\prime \prime}\) using the prefault load current. Then determine the subtransient fault, generator, and motor currents directly from Figure 7.4(a). Compare your answers with those of Example 7.3.

Example 7.3

The synchronous generator in Figure 7.3 is operating at rated MVA, 0.95 p.f. lagging and at 5% above rated voltage when a bolted three-phase short circuit occurs at bus 1. Calculate the per-unit values of

(a) subtransient fault current;

(b) subtransient generator and motor currents neglecting prefault current; and

(c) subtransient generator and motor currents including prefault current.

Figure 7.3

100 MVA 13.8 kV X" = 0.15 T 38 100 MVA Xung 13.8 kV A/138 kVY X = 0.10 = 20 T 38 100 MVA 138 kVY/13.8 kVA X 0.10 M 100 MVA 13.8 kV X" = 0.20