Power System Analysis
Assignment-1
 It’s not a suggestion, so if you follow it blindly for exam, you may fail there!
 Dead line is 2.30 pm in March 2, 2017 (Thursday). After that time, please don’t even come
to submit!!
1. Prepare an Impedance diagram and a Reactance diagram from the single line diagram
shown in figure 1.
2. A transformer is rated 20kV/200kV, 350MVA and has an internal impedance of j6.4Ω as
seen from the low voltage side. Consider 22kV, 400 MVA as base and show that Vpu, Spu
and Zpu is same for the both sides of this transformer.
3. A 300 MVA, 20 kV three-phase generator has a sub transient reactance of
4. 20%. The generator supplies several synchronous motors over a 64 km (4-mi) transmiss io n
line having transformers at both ends, as shown on the one-line diagram of Fig. 6.29. The
motors, all rated 13.2 kV, are represented by just two equivalent motors. The neutral of one
motor M1 is grounded through reactance. The neural of the second motor M2 is not
connected to the ground (an unusual condition). Rated inputs to the motors are 200 MVA
and 100 MVA for M1 and M2, respectively. For both motors X” =20%. The three-phase
transformer T1 is rated 350 MVA, 230/20kV with the leakage reactance of 10%.
Transformer T2 is composed of three single-phase transformers each rated 127/13.2 kV,
100 MVA with leakage reactance of 10%. Series reactance of the transmission line is 0.5
Ω/km. Draw the reactance diagram with all reactance marked in per-unit. Select the
generator rating as base in the generator circuit.
5. Show If base quantities are changed then Per Unit value of impedance can be obtained as
follows:
S B ,new V 2 B ,old
Z PU ,new  Z PU ,old 

S B ,old V 2 B ,new
6. Find per unit impedances for the following figure.
7. Make the admittance matrix, Ybus, for the following network. Given parameters are
impedance in per-unit.
8. For the previous system, if P2=80 MW and Q2=60MVar, then find P1, Q1, V2, V3 and
P3, Q3. Do the calculations for at least 3 iterations.
9. Define Slack bus, Generator bus and Load bus
10. Write down the expression for total fault current through transmission line on the
occurrence of a symmetric short circuit. From the expression derive the values of
momentary fault current through the transmission line.
11. A transmission line of inductance L = 0.8 H and resistance R = 12 Ω is suddenly short
circuited at t=0, at the far end of the line. If the voltage source is v = 400sin(140πt + 60)
obtain the following,
(i)
Expression for the total short circuit current, i(t)
(ii)
Instant of short circuit so that DC offset current is zero.
(iii)
Instant of short circuit so that DC offset current is max.
12. For a synchronous generator, the fictitious reactance (X_a), reactance of field winding
(Xf) and damper winding Xdw are given as j3Ω, j2.4Ω and j1.5Ω. If the sub transient
reactance of this generator is j6Ω, compute leakage reactance, transient reactance and
steady state reactance
13. Two generators are connected in parallel to the low-voltage side of three-phase ∆-Y
transformer as shown in Fig. Generator 1 is rated 50,000 kVA, 13.8 kV. Generator 2 is
rated 25,000 kVA, 13.8 kV. Each generator has a subtransient reactance of 25%. The
transformer is rated 75,000 kVA, 13.8∆/69Y kV, with a reactance of 10%. Before the
fault occurs, the voltage on the high-tension side of the transformer is 66 kV. The
transformer is unloaded, and there is no circulating current between generators. Find the
subtransient current in each generator when the three-phase short-circuit occurs on the
high-tension side of the transformer.
14. In the figure 11 the motor is drawing 200MW at 0.8 power factor lagging at a terminal
voltage of 17.8kV, when a bolted three phase short circuit occurs at bus 2. Consider the
rating of generator G, as base at generator circuit for per unit calculation of the system
and find total sub-transient fault current and sub-transient generator and motor current in
per unit neglecting pre-fault current.
1
G
18kV
300MVA
X= j0.15 pu
2
Transmission Line
X= j30Ω
20kV/220kV
350MVA
X= j0.1 pu
M
220kV/20kV
400MVA
X= j0.1 pu
18kV
250MVA
X=j0.17 pu
15. What will be the sub-transient generator and motor current in per unit if pre-fault current
is considered?