Power Systems Test - 2 - PDF Flipbook
Power Systems Test - 2
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GATE
EEE
Power
Systems
Test-02Solutions
POWER SYSTEMS
1. A lossless radial transmission line with surge impedance loading
a) Takes negative VAR at sending end and zero VAR at
receiving end.
b) Takes positive VAR at sending end and zero VAR at
receiving end.
c) Has flat voltage profile and unity power factor at all points
along it.
d) Has sending end voltage higher than receiving end voltage
and unity power factor at sending end.
Answer: (c)
Solution:
When the transmission line is dealing with surge impendence
loading, then such a line is called flat line, in which the
magnitude of the voltage throughout its length is same. For
surge impedance loading, load impedance & line impedance is
purely resistive in nature. Hence the power factor is unity
throughout its length.
2. The permissible voltage drop from supply terminal to any point
on the wiring system should not exceed
a) 1% + 1 Volt
b) 3% + 1 Volt
c) 4% + 1 Volt
d) 2% + 1 Volt
Answer: (b)
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Solution:
The permissible limit is (3% + 1) volt.
3. Steady state operating condition of a power system indicates
a) A situation when the connected load is absolutely constant.
b) A situation when the generated power is absolutely constant.
c) A situation when both connected load and generated power
are equal to each other and remain constant.
d) An equilibrium state around which small fluctuations in
power, both in generation and load, occur all the time.
Answer: (c)
Solution:
Steady state condition implies there is a load generation balance
in the system.
4. Which of the following processes essentially needs direct
current?
a) Welding
b) Heating
c) Refrigeration
d) Electroplating
Answer: (d)
Solution:
In electroplating process DC is used. Heating, welding
refrigeration can be done by AC.
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5. In HVDC transmission systems, filters are installed on
a) AC side only
b) DC side only
c) Both on AC and DC sides
d) Sending end only
Answer: (c)
Solution:
AC to DC converters are source of harmonics on both AC as
well as DC side. Series and shunt filters are installed on both
AC and DC sides.
6. In helium arc welding, the electrode is made of
a) copper
b) carbon
c) mild steel
d) tungsten
Answer: (d)
Solution:
The helium arc welding process uses the tungsten electrode.
This is a type of Gas metal arc welding (GMAW).
7. The inductance of a power transmission line increases with
a) Decrease in line length
b) Increase in diameter of conductor
c) Increase in spacing between the phase conductors.
d) Increase in load current carried by the conductors.
Answer: (c)
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Solution:
Inductance per phase of a transmission line is � ℎ� =
0.2 � 1 �
Where ‘d’ is the space between two conductors, r1 is the
effective radius of conductor
∴ ℎ ∝ ( )
[∵ 1 = 0.7788 = ]
8. A long wire composed of a smooth round conductor runs above
parallel to the ground (Assumed to be a large conducting plane).
A high voltage exists between the conductor and the ground.
The maximum electric stress occurs at
a) The upper surface of the conductor
b) The lower surface of the conductor
c) The ground surfaces
d) Midway between the conductor and the ground
Answer: (b)
9. A three-phase overhead transmission line has its conductors
horizontally spaced with spacing between adjacent conductors
equal to ‘d’. If now the conductors of the line are rearranged to
form an equilateral triangle of sides equal to ‘d’ then
a) Average capacitance and inductance will increase
b) Average capacitance will increase and inductance will
increase
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c) Average capacitance will increase and inductance will
decrease
d) Surge impedance loading of the line increases
Answer: (c)
Solution:
Case (i):
When conductors are horizontally spaced.
Inductance/phase = 0.2 ln � �
Capacitance/phase = 2 0
ln � �
In this case GMD = 3�√2 × √ × √2 ×
= 3√2 = 1.26
= 0.7788
Case (ii):
When conductors are placed in equilateral triangle form.
In this case GMD = d
GMR = 0.7788 r
Since GMR & r are constant in both cases
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L/ph ∝ ( )
C/ph ∝ 1
( )
∴ From case (i) to case (ii) the inductance decreases and
capacitance increases.
10. The function of choke in the fluorescent tube circuit is to
a) eliminate ratio interference
b) reduce noise
c) reduce flicker
d) initiate the arc and stabilize it
Answer: (d)
Solution:
The choke (inductor) provides a high voltage for ionization of
the gas atoms in the tube light (i.e. initiation of arc) and then it
controls or stabilizes it.
11. HFC fuse provides best protection against
a) open circuit
b) overload
c) reverse current
d) All the above
Answer: (b)
Solution:
The HRC fuse i.e. high rupturing capacity fuse is used when the
high amount of fault current flows i.e. short circuit current or
overload current. This is best suited for the protection against
short circuits.
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12. The load carrying capability of a long AC transmission line is
a) Always limited by the conductor size
b) Limited by stability considerations
c) Reduced at low ambient Temperature
d) Decreased by the use of bundled conductors of single
conductors
Answer: (b)
Solution:
For a long line with AC transmission, reactance is more and is
having more effect on stability of system.
13. In arc welding arc is created between the electrode and work
by
a) flow of current
b) voltage
c) material thickness
d) contact resistance
Answer: (d)
Solution:
Arc is produced after the contact is separated by a small
distance, hence the air between the electrode is ionized by the
potential difference and that reduces resistance.
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14. Consider the transformer connections in a part of a power
system shown in the figure. The nature of transformer
connections and phase shifts are indicated for all but one
transformer. Which of the following connections, and the
corresponding phase shift θ, should be used for the transformer
between A and B?
a) Star - Star (θ = 0o)
b) Star - Delta (θ = -30o)
c) Delta - Star (θ = 30o)
d) Star - Zigzag (θ = 30o)
Answer: (a)
Solution:
Transformer is Y – Y
T1 = ΔY
T2 = ΔY
T3 = YY
T4 = Y (Auto transformer)
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15. Arc lamp operates at
a) low lagging power factor
b) high leading power factor
c) unity power factor
d) zero power factor
Answer: (a)
Solution:
Arc lamps have very high lagging circuits i.e. lagging angle ϕ is
very high (near 800), hence cosϕ i.e. lagging pf is very low.
16. Supplier's fuse, which is provided in domestic wiring system is
a) after the energy meter.
b) before the energy meter.
c) before distribution board.
d) after main switch.
Answer: (a)
Solution:
The supplier's fuse, this fuse is provided by the electric supply
company, installed just after the energy meter.
17. An air blast circuit breaker is usually employed for
a) instantaneous voltage
b) intermittent duty
c) repeated duty
d) short duty
Answer: (c)
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Solution:
The air blast circuit breakers have applications in arc furnaces
and in the traction systems because they are best suited for
repeated duty. They are used in range of 132 kV to 400 kV with
breaking capacity upto 7000 MVA.
18. Which of the following is of high importance in case of
induction heating?
a) voltage
b) frequency
c) current
d) all the above
Answer: (b)
Solution:
In induction heating the high frequency AC is passed through an
electromagnet which produces eddy currents inside the
conductor by electromagnetic induction. The high frequency is
important parameter in induction heating.
19. Corona losses are minimized when
a) Conductor size is reduced.
b) Smoothness of conductor is reduced.
c) Sharp points are provided in the line hardware.
d) Current density in conductors is reduced.
Answer: (d)
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Solution:
Current density =
For a given current, if the current density is reduced means the
cross sectional area of conductor is increased.
But corona loss ∝ ( − 0)2
Where V = operating voltage
0 = Critical disruptive voltage
0 ∝ � �
(d = separation between conductors = constant)
0 ∝ (radius of conductor)
As radius of conductor increases, critical disruptive voltage (V0)
also increases and hence corona loss decreases.
(∵ increase in ‘r’ is more effective than decrease in ln� �).
Note: As the radius of conductor increases then corona loss
decreases.
20. Consider a bundled conductor of an overhead line consisting of
three identical sub-conductors placed at the corners of an
equilateral triangle as shown in figure. If we neglect the charges
on the other phase conductors and ground, and assume that
spacing between sub-conductors is much larger than their
radius, the maximum electric field intensity is experienced at
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a) Point X
b) Point Y
c) Point Z
d) Point W
Answer: (b)
Solution:
Electric field intensity at various points are shown as follows:
At point Y:
At point X:
At point Z:
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At point W:
It is clear from the above diagrams that minimum cancellation
of vectors occurs at point Y, hence maximum field intensity.
21. The resistance welding process requires a
a) high value of ac current at low voltage.
b) low value of ac current at high voltage.
c) high value of dc current at low voltage.
d) low value of dc current at high voltage.
Answer: (a)
Solution:
In welding process step down transformer is used which has low
voltage and high current.
22. The ratio of the puncture voltage to the flashover voltage of an
insulator is
a) equal to one
b) lower than one
c) zero
d) greater than one
Answer: (d)
13
Solution:
Puncture voltage: The voltage at which an insulator gets
punctured.
Flashover voltage: Flashover voltage is when the air around the
insulator surface has breakdown and flashover are produced.
Puncture voltage > Flashover voltage
So, > 1
ℎ
23. During arcing ground conditions, the phase voltage of the
system rises to
a) 15 times its normal value
b) 10 times its normal value
c) 5 to 6 times its normal value
d) √3 times its normal values
Answer: (d)
Solution:
During arcing ground conditions, the phase voltage of the
system rises to √3 times its normal values.
24. Buccholz relay cannot be used on
a) 500 kV transformer
b) 1000 kV transformer
c) Three-phase transformer
d) Air-cooled transformer
Answer: (d)
14
Solution:
Buccholz relay is used in an oil container and is a gas activated
relay where oil cooling is used. Hence, it cannot be used in air
cooled transformer.
25. In a motor starter, the electromechanical contactor provides
inherent protection against
a) over-current
b) short-circuit
c) single-phasing
d) under-voltage
Answer: (d)
Solution:
In a motor starter, the electromechanical contactor provides
inherent protection against under voltages.
26. A nuclear power station of 500 MW capacities is located at
300 km away from a load center. Select the most suitable power
evacuation transmission configuration among the following
options.
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Answer: (d)
Solution:
= 2 = 400×400 = 400
400
To carry 500 MW a double circuit 400 kV line is used.
27. The angle δ in the swing equation of a synchronous generator
is the
a) angle between stator voltage and current
b) angular displacement of the rotor with respect to the stator.
c) angular displacement of the stator mmf with respect to a
synchronously rotating axis.
d) angular displacement of an axis fixed to the rotor with respect
to a synchronously rotating axis.
Answer: (d)
Solution:
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28. The knowledge of diversity factor helps in computing
a) plant capacity
b) average load
c) units generated
d) peak demand
Answer: (d)
29. A 200 V lamp takes a current of 1 A, it produces a total flux of
2860 lumens. The efficiency of the lamp is
a) 14.3 lumens/W
b) 8.9 lumens/W
c) 10.9 lumens/W
d) 9.9 lumens/W
Answer: (a)
Solution:
Luminous efficiency = 2860 = 14.3 /
200×1
30. Which of the following is correct?
a) Load factor = capacity factor × utilization factor
b) Utilisation factor = capacity factor × load factor
c) Capacity factor = load factor × utilisation factor
d) Load factor has no relation with capacity factor and
utilisation factor
Answer: (c)
Solution:
Capacity factor = Load factor × Utilization factor
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