4. (15 points) A 4-bus system is shown in Figure 1. z (0) = [/0.1553 j0.1407 j0.0493 j0.03471 j0.1407 j0.1999 j0.0701 j0.0493 bus j0.0493 j0.0701 j0.1999 j0.1407 Lj0.0347 j0.0493 j0.1407 j0.1553] [j0.1437 j0.1211 j0.0789 j0.05631 j0.1211 j0.1696 j0.1104 j0.0789 j0.0789 j0.1104 j0.1696 j0.1211 Lj0.0563 j0.0789 j0.1211 j0.1437] bus Z = Z(2) bus = Let's assume that Z, = j0. 12 (per unit) V, = 120° (per unit). A single line-to-ground fault occurs on phase A at bus 3. 2 3E Generator Figure 1 3 (a) Draw the Thevenin equivalents of the sequence (i.e., symmetrical components) networks for the single line-to-ground fault; (b) Determine the subtransient current to the ground at the fault (at bus 3); (c) Determine the line-to-ground voltages at bus 2. Machine