# Power system Analysis Fifth Sem

1. What is meant by a fault in a power system?
2. What are the assumptions made in drawing reactance diagram?
3. Define bus incidence matrix.
4. Draw p model of a transmission line.
5. Define symmetrical fault.
6. Name any two methods of reducing short circuit current.
7. Write the steps involved and assumptions in load flow solution.
8. How do you improve the steady state stability?
9. Write a short technical note on critical clearing angle.
10. State the assumptions made in solution of the swing equation.
PART – B (5 x 12 = 60)
11. Fig. 1 shows a single-line diagram of a power system. The
ratings of generators and transformers are:
Generator G1 : 20 MVA, 13.8 kV, jX = j0.2pu
Generator G2 : 30 MVA, 18 kV, jX = j0.2pu
Generator G3 : 30 MVA, 20 kV, jX = j0.2pu
Transformer T1: 25 MVA, 13.8 D – 220 Y kV, jX = j0.1 pu
Transformer T2: Single phase units each rated at 10 MVA, 127/18
kV, jX = j0.1pu
Transformer T3: 35 MVA, 22 D – 220 Y kV, jX = j0.1 pu
Draw impedance diagram with all values in pu on a base of 50
MVA, 13.8 kV on the circuit of generator G1.

(or)
12. The parameters of a 4 – bus system are as follows.
Bus code Line impedance (pu) Half Line Charging
1- 2 0.2 + j 0.8 j 0.02
2- 3 0.3 + j 0.9 j 0.03
2- 4 0.25 + j 1.0 j 0.04
3- 4 0.2 + j 0.8 j 0.02
1- 3 0.1 + j 0.4 j 0.01
Draw the network and find bus admittance matrix.
13. Two transformers (Ta & Tb) are connected in parallel to an
impedance to neutral per phase of 0.8 + j0.6 per unit at a voltage
of V = (1 + j0) per unit. Transformer Ta has a voltage ratio equal
to the ratio of the base voltages on the two sides of the
transformer. This transformer has an impedance of j0.1 per unit
on the appropriate base. The second transformer Tb also has an
impedance of j.01 per unit on the same base but has a step – up
toward the load of 1.05 times that of Ta (Secondary windings on
1.05 tap). Find the complex power transmitted to the load
through each transformer.
(or)
14. Fig.2 shows the one-line diagram of a simple three- bus power
system with generators at buses 1 and 3. The line impedances are
marked in per unit o a 100 MVA base. Find out the bus voltages
after two iteration using Gauss seidel method.

15. The per unit bus impedance matrix of a four bus power system
shown in Fig.3 is given by,
Zbus =
   
2470.135 0.0975 0.21 0. 5
0.14 0.09 0.2533 0.21
0.075 0.1875 0.09 0.0975
0.15 0.075 0.14 0.135
j j j j
j j j j
j j j j
j j j j
Calculate the fault current for a solid three phase symmetrical
fault at bus 4. Also calculate the post fault bus voltages and line
currents.
(or)
16. (a) Derive the relationship for fault currents in phase a and b in
terms of symmetrical components when there is a double line to
ground fault on phase a and b
(b) Show that positive and negative sequence currents are equal
in magnitude but out of phase by 180 deg. In a line-to-line fault.

17. A fault occurs at point P on the short line between breakers A and
B of the system shown in the Fig.4 and cleared after time “tc” by
opening of the breaker of A. Analyse the transient stability of the
system using equal area criterion clearly specifying the values of
rotor angle, accelerating power and speed at significant operating
points of the generator G during initial steady – state, during fault
and post – fault periods.
(or)
18. (a) State and explain one limitations of equal criterion.
(b) State and explain whether stability limit is increased or
decreased by (i) adding one or more transmission circuit in
parallel and (ii) having fast acting circuit breakers.
19. The synchronous machine shown in Fig. 5 is generating 100 MW
and 75 MVAR. The voltage of the infinite bus q is 1 + jo pu.
The generator is connected to the infinite bus through a line of
reactance 0.08 p.u. on a 100 MVA base. The machine transient
reactance is 0.2 pu and the inertia constant is 4 pu on a 100 MVA
base. A 3-f fault occurs at bus ‘p’ for a duration of o.1 sec.
Compute the rotor angle at t=0.02 sec (Dt = 0.02 sec) by applying
Modified Eulers method. The frequency of the supply is 60Hz.
(or)
20. Derive the swing equation for a single machine connected to
infinite bus system. Validate the nonlinearity of this equation.