# Power Systems Questions and Answers – Switchgear and Protection – 4

This set of Tough Power Systems Questions and Answers focuses on “Switchgear and Protection – 4”.

1. An isolated generator connected to a turbine with its continuous maximum power of 20 MW, 50 Hz. Generator connected with two loads of 8 MW, each operate at 50 Hz.. Generator has 5% droop characteristic. If an additional load of 6 MW is added then the change in the frequency will be ____________
a) 0.25
b) 0.75
c) 0.5
d) 1

Explanation: Y11=j2+(1/5j) = 1.8j
Y12=-(1/5j) = 0.2j; Y13=0
Hence option ‘a’ is most applicable.

3. If short TL is delivering a lagging power factor load, sending end p.f. would be ________
a) (VrcosФr+IR)/Vs
b) (VrcosФr+IRsinФr)/Vs
c) (VrsinФr+IR)/Vs
d) (VssinФr+IRcosФr)/Vs

4. If all the sequence voltages at the fault point in a power system are equal, then fault is __________
a) LLG fault
b) LG fault
c) Three phase to ground fault
d) Line to Line fault

Explanation: It is LLG fault.

5. The below mentioned graph is a recorded data of the voltage fluctuation of the network, then it can be concluded that these faults are __________ a) L-G
b) A three phase motor short
c) Feeder fault
d) Any of the mentioned

Explanation: The sudden dip is the LG fault in the graph as the dips are not very large and the system is maintaining its stability.
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6. The inertia constant of two groups of machines which do not swing together are 4 and 20 MJ/MVA, the equivalent inertia constant of the system is _________
a) 2.85
b) 14
c) 6
d) 12

Explanation: For the incoherent system, the equivalent inertia = H1H2/(H1+H2).

7. A fault current of 1500A passing through the primary side of a 400/5 CT. On the secondary side an inverse time over current relay is connected whose current setting is 75%, the plug setting is __________
a) 5
b) 3.75
c) 6
d) 4

8. The three phase line is working at 50 Hz with conductors rearranged as shown. The Cph of TL at 132 kV is ___________ a) 8.75*10-9
b) 8.75*10-12
c) 26.2*10-12
d) 26.2*10-9

9. For a 100 MVA, 11 kV three phase Alternator observes a three phase fault at terminals of it. If the fault current is 2000 A, the pu value of positive sequence reactance ______
a) 2.62
b) 0.46
c) 1.31
d) 0.92

Explanation: PU fault current = 2000/5248.63 = 0.381 pu
0.381 = 1/X
X= 1/0.381 =2.62 pu.

10. A lossless TL having SIL of 3000 MW is provided with uniform distributed series capacitance compensation of 25% and shunt compensation of 30% then the SIL of compensated TL will be _______________
a) 3949.9
b) 3289
c) 2784.9
d) 2877.6

Explanation: SIL=(V2 )/Z
SIL α 1/Z
Zcomp = Zs√((1-kse)/((1+ksh)(1-Klsh)))
Kse = 0.25; Klsh = 0; Kcsh = 0.3,
Zcomp =0.7595
SILcomp = 0.7595*3000 = 3949.96 MVA.

11. A lossless TL having SIL of 3000 MW is provided with uniform distributed series capacitance compensation of 25% and shunt compensation of 30% then the compensated impedance of TL will be(in pu) ____________
a) 0.7595
b) 2.88
c) 0.647
d) 3.27

Explanation: SIL=(V2)/Z
SIL α 1/Z
Zcomp = Zs√((1-kse)/((1+ksh)(1-Klsh)))
Kse = 0.25; Klsh = 0; Kcsh = 0.3,
Zcomp =0.7595.

Sanfoundry Global Education & Learning Series – Power Systems.

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