Question: A three-phase synchronous generator is connected to a step-up transformer T1, which is connected to a 60km long transmission line. As the far end of the line, a step-down transformer bank T2 is connected. The secondary of T2 supplies two loads L1 & L2.
The ratings for the various parts of the system are as follows:
Generator
10 MVA, 12kV, X = 20%, star connected
T1
5 MVA, 12/69 kV, X = 10%
T2
5 MVA, 69/4.16 kV, X=10%
L12000 kVA at 0.8 pf lagging
L2
1000 kVA at 0.75 pf lagging
Transmission Line
X = 0.5 W/km (R is negligible)
[a.] Draw the per-phase equivalent circuit of the power system showing all reactance in per unit & show the base voltages used at various parts of the network. Use 10 MVA & 12 kV as base quantities in the generator circuit.
[b.] If the voltage on the load bus bar must be maintained at 4.16 kV, find the required excitation on the generator.
[c.] For this excitation, determine the short circuit current (in amperes) when a three-phase fault occurs on the secondary winding of transformer T2.