Answer Happy

Q4. (total marks 20) A three phase 25 KV 100 MVA Generator with a series impedance of jo.20 p.u. is connected to a 25 kV / 110 kV 125 MVA transformer with an impedance of j0.15 p.u. The high voltage side of the transformer is connected to a transmission line of serial impedance Z = 0.04 +0.05 p.u. on a 100 MVA base. The transmission line is then connected to a 110 kV / 25 kV 150 MVA transformer with an impedance of jo.10 p.u. The generator, and both transformers primary and secondary windings, are star connected to earth. Note: Use a power base Sease of 100MVA a) Convert the one-line diagram in part (a) to an equivalent impedance diagram and draw the impedance diagram. (5 Marks) b) If an unbalanced single line-to-ground fault of resistance 8 ohms occurs on the red phase at the end of the line, i.e. after the low voltage secondary terminals of the 110 kV / 25 kV transformer, draw the positive, negative, and zero phase sequence networks, and the overall circuit equivalent impedance diagram. (Hint: remember to convert the 8 ohms fault resistance to per unit values). (10 Marks) c) Calculate the fault current It. (5 Marks)