Code No: 126AG

R13
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
B. Tech III Year II Semester Examinations, December - 2017
COMPUTER METHODS IN POWER SYSTEMS
(Electrical and Electronics Engineering)
Time: 3 hours Max. Marks: 75

Note: This question paper contains two parts A and B.

PART - A
(25 Marks)

1.a) Define the terms tree and co-tree with suitable examples. [2]
b) Explain demerits of Z-bus over Y-bus. [3]
c) What is the effect of acceleration factor in the load flow solution algorithm? [2]
d) What is the necessity of conducting power flow studies? [3]
e) What are the assumptions made in short circuit studies of a large power system
network? [2]
f) What are the applications of series reactors? [3]
g) How can the steady state stability of power system be increased? [2]
h) Distinguish between steady state and dynamic stability of a power system. [3]
i) Write the state variable formulation of swing equation. [2]
j) What is the significance of Critical Clearing Angle? [3]

PART - B
(50 Marks)

2.a) Derive the expression for bus admittance matrix in terms of primitive admittance
matrix and bus incidence matrix.
b) For the power system network shown in Figure 1, use ground as a reference bus. Form
YBUS by direct inspection method. Line and generator reactances are mentioned in the
figure 1. [5+5]

Figure 1
OR
3. Assume the bus impedance matrix for a partial network is known. Now explain the Z
bus building algorithm for the following modifications. (a) Addition of a branch and
(b) Addition of a link. [10]
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4.a) What are the advantages of NR-method over GS-method? Explain.
b) For the system shown in below figure 2. P2 = 0.5 p.u., Q2 = -0.2 p.u., P3 = -1 p.u.,
Q3 = 0.5 p.u., P4 = 0.3 p.u., Q4 = -0.1 p.u., and V1 = 1.04∟00 p.u. Determine the value
of V2 after the first iteration of Gauss Seidel (GS) method. Line admittances are as
shown in the figure 2. [5+5]

Figure 2
OR
5.a) What are the assumptions made in reducing Decoupled method to Fast Decoupled
method of power flow solution?
b) The magnitude of voltage at bus-1 is adjusted to 1.05 p.u, voltage magnitude at bus-3 is
fixed at 1.04 p.u with a real power generation of 2.0 p.u. A load consisting of
Pd2 = 4.0 p.u and Qd2 = 2.5 p.u. is taken from bus-2. Given line admittances
y12= (10- j20) p.u., y13 = (10-j30) p.u.,y23 = (16 - j32) p.u. Obtain the power flow
solution using FDLF method. [5+5]

The voltages across a 3-phase unbalanced load are Va  300 Volts, Vb  300  90
0
6.a)
Volts and Vc  800  143 Volts respectively. Determine the sequence components of
0

voltages. Phase sequence is ABC.

Figure 3
OR
7.a) Derive an expression for the fault current for a line-to-line fault at an unloaded
generator.
b) A 20MVA, 11KV, 3-Ф, 50HZ generator has its neutral earthed through a 5% reactor. It
is in parallel with another identical generator having isolated neutral. Each generator
has a positive sequence reactance of 20%, Negative sequence reactance of 10% and
zero sequence reactance of 15%. If a line to ground short circuit occurs in the common
bus-bar, determine the fault current. [5+5]

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8. Write short notes on:
a) Selection of circuit breakers.
b) Synchronizing coefficient and
c) Transfer reactance. [10]
OR
9. A 50Hz, 4 pole generator rated 100MVA, 11kV has an inertia constant of 8MJ/MVA
a) Find the stored energy in the rotor at synchronous speed
b) If the mechanical input is suddenly raised to 80MW for electrical load of 50MW find
rotor acceleration.
c) i) if the acceleration calculated in ii) is maintained for 10cycles, find the change in
torque angle and rotor speed in rpm at the end of this speed.
d) Another generator 200MVA, 3000rpm having H=6MJ/MVA is put in parallel with
the above generator. Find the inertia constant for equivalent generator on a basis of
100MVA. [10]

10.a) Explain clearly the application of equal area criterion for studying the transient stability
of a system.
b) Why transient state stability limit is less than steady state stability limit? Explain. [5+5]
OR
11. Consider the system shown in Figure 4 below.

Figure 4

Initially the system is operating stable while delivering a load of 1.25 p.u. Determine
the stability of the system when one of the lines is switched off due to a fault. Also
determine the maximum value of the rotor swing. [10]

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