Question

Problem 2 (10 pts) 132 kV TL 40 km 1 pu mid-point L-E fault 60 ΛΤΑ TL: Transmission line. L-E fault: Line-to-earth fault. Z. 0.07 pu 0.1 pu Generator 0.1 pu 0.1 pu 0.1 pu Transformers 2.5x Z] Transmission Line 0.7 Ω/km 0.7 Ω/km Suppose a single L-E fault happens at the mid-point of the transmission line. Determine the per unit phase voltage and current at point P (Choose Sbase-60 MVA and Vbase-132 kV). (4 pts) * The phase impedance seen at point P is defined as the ratio of phase voltage to current at point P. Show that the phase-a impedance seen at point P is independent of the generator source impedance. (6 pts)

Answer 1

Generator Z 0.1 p.u Z2 0.07 p.u Transformer: Z Z, Z 0.1 p.u Transmission line: Z, Z, 0.7 O/km=0.7x40=28 Q Zo 2.5x Z 2.5 x 28 = 70 МVA base = Z 2pu. X 1(actual) kV, 2 60 = 0.0964 p.u. 1322 -28x 60 = 0.241p.u 1322 Zop.u 70x 0.0964 =0.0482 p.u. 2 0.241 Zo 0.1205 p.u 2 Z-0.10.1+0.0482 0.2482 p.u. Draw the positive sequence circuit diagram

0.1 0.0482 0.0482 0.1 2 T/L Draw the negative sequence circuit diagram 0.1 0.0482 0.0482 0.1 T, 0.07 G N Zeg 0.07 0.1+0.0482 0.2182 p.u. Draw the zero sequence circuit diagram

0.1 0.1205 0.1205 0.1 Tr 3X 0 G Z 0.1 0.1205 0.2205 p.u (a) Calculate the fault current 3E I f leg 2eg 0eg 3x1.0 0.2482 0.2182+0.2205 4.3674 p.u Fault impedance is, Z, 0 f Hence V I,Z -0 V Calculate the actual fault current - I I f(actual) Ibase f(pu) _ h: 60 = 4.3674 x 3 x132 1.1461 kA

b) Vf Impedance at point P If 0 1.1461 =0 0 As impedance at point P is zero, it is independent of the generator source impedance