# Power Systems Questions and Answers – Comparison of Angle and Voltage Stability – 2

This set of Power Systems Questions and Answers for Campus interviews focuses on “Comparison of Angle and Voltage Stability – 2”.

1. For a given system consisting of two generators
Units       Rating(MW)        Speed Drop(%R)
1.              400                     0.04
2.              800                     0.05
The units share load of P1=200MW and P2=600MW. The units are operating in parallel to share a load of 700 MW at 50 HZ. The load is increased by 130MW with B=0. Then the value new steady state frequency variation is ______________
a) 49.75 Hz
b) 51.25 Hz
c) 49.25 Hz
d) 48.75 Hz

2. A 50 bus power system Ybus has 80% sparsity. The total number of transmission lines will be ___________
a) 225
b) 500
c) 475
d) 100

Explanation: Number of non zero elements = 50*50*20/100 = 500 non zero
Number of TL = (500-50)/2 = 225 transmission lines.

3. The given graph is the depiction of ________ on a large power system network.
a) L-G fault
b) Three phase motor getting short
c) Fault in feeder
d) Any of the mentioned

Explanation: A LG fault will create a sudden dip in the voltage profile and it will be recovered.

4. A single core cable is graded by using three dielectrics with relative permittivity 5,4,3 respectively. The diameter of the conductor is 2cm and the overall diameter is 8 cm. If the three dielectric work at the same maximum stress of 40kV/cm, then the safe working rms voltage of cable is ______ kV
a) 57.72
b) 81.63
c) 84.67
d) 71.63

Explanation: d=2cm, d1 = ?, d2= ?, D = 8cm
Ԑ1 = 5; Ԑ2 = 4; Ԑ3 = 3
gmax =40 kV/cm
Ԑ1d1 = Ԑ2d2 = Ԑ3d3
d1 = 2.5cm, d2 = 3.33 cm

Rms value of the peak voltage = 81.63/√2 = 57.72 kV.

5. A generator delivers power of 1 pu to an infinite bus through a purely reactive network. The maximum power that could be delivered by generator is 2 pu. A three phase fault occurs at the generator which reduces the generator output to zero. The fault is restored after ‘tc’ seconds. The maximum swing of rotor angle is found to be δmax = 110o electrical. The rotor angle at ‘tc’ is ________ electrical deg.
a) 69.14o
b) 159.14o
c) 63.08o
d) 65.7o

6. A system consists of an alternator having reactance of 0.5pu connectedto an infinite bus through a series of reactance of 1 pu. The generator terminal voltage of IBB is 1 pu and that of 1.2 pu. The steady state power system limit (in pu) is ___________
a) 1.152
b) 1.167
c) 1.765
d) 1.729

7. A system consists of an alternator having reactance of 0.5pu connected to an infinite bus through a series of reactance of 1 pu. The generator terminal voltage of IBB is 1 pu and that of 1.2 pu. The Steady state occurs at power angle of ________degree.
a) 0
b) 90
c) 180
d) 45

Explanation: It occurs at δ=90o.

8. A system consists of an alternator having reactance of 0.5pu connectedto an infinite bus through a series of reactance of 1 pu. The generator terminal voltage of IBB is 1 pu and that of 1.2 pu.The emf induced in the alternator for the maximum power transfer will be __________
a) 1.729
b) 1.152
c) 1.2
d) 1.6

9. A system consists of an alternator having reactance of 0.5pu connected to an infinite bus through a series of reactance of 1 pu. The generator terminal voltage of IBB is 1 pu and that of 1.2 pu. The emf induced in the alternator will have the phase difference with respect to reference for the maximum power transfer is _______________
a) 90
b) 0
c) 73.87
d) 86.25

Explanation: It occurs at δ=90o for the maximum power transfer.

10. A three phase transmission line is having a three unit suspension insulation string. The voltage at the insulator unit nearest to the line is 20kV and that across the adjacent unit is 15 kV. The ratio of mutual to ground capacitance is ________________
a) 0.18
b) 0.2
c) 0.333
d) 0.16