# Sathyabama University Previous Years Question Papers BE

# Power system Analysis Fifth Sem

Answer All the Questions

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)

Answer All the Questions

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

admittance (pu)

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.