Determine \(\boldsymbol{Y}_{\text {bus }}\) in per unit for the circuit in Problem 7.12. Then invert \(\boldsymbol{Y}_{\text {bus }}\) to obtain \(\boldsymbol{Z}_{\text {bus }}\).

Problem 7.12

Equipment ratings for the four-bus power system shown in Figure 7.14 are as follows: 

A three-phase short circuit occurs at bus 1, where the prefault voltage is \(525 \mathrm{kV}\). Prefault load current is neglected. Draw the positive-sequence reactance diagram in per unit on a \(1000-\mathrm{MVA}, 20-\mathrm{kV}\) base in the zone of generator G3. Determine

(a) the Thévenin reactance in per unit at the fault,

(b) the subtransient fault current in per unit and in kA rms, and

(c) contributions to the fault current from generator Gl and from line 1-2.

Generator Gl: Generator G2: Generator G3: Transformer Tl: Transformer T2: Transformer T3: Each 500 kV line: 500 MVA, 13.8 kV, X"= 0.20 per unit 750 MVA, 18 kV, X"= 0.18 per unit 1000 MVA, 20 kV, X"= 0.17 per unit 500 MVA, 13.8 A/500 Y KV, X = 0.12 per unit 750 MVA, 18 A/500 Y kV, X = 0.10 per unit 1000 MVA, 20 A/500 Y kV, X = 0.10 per unit X 50 . =