Category: Power Systems Objective Questions

Power Systems Multiple Choice Questions on “Medium Transmission Line – 1”.

1. Which of the following transmission line can be considered as medium transmission line?
A. Transmission line of length upto 600 Km
B. Transmission line of length upto 500 Km
C. Transmission line of length upto 200 Km
D. Transmission line of length upto 80 Km
Answer: C
Clarification: The transmission lines having length more than 80 Km and less than 200 Km are considered as medium transmission lines.Their operating voltage is more than short transmission lines but less than long transmission lines.

2. Which of the following is correct operating voltage range for medium transmission lines.
A. More than 765 KV
B. More than 400 KV
C. More than 20 KV
D. More than 132 KV
Answer: C
Clarification: Transmission lines having length more than 80 Km and less than 200 Km, operating voltage between 20 KV and 100 KV fall in the category of medium transmission lines.

3. What is the value of the charging current flowing to earth in medium transmission line.
A. Very high
B. Medium
C. Negligible
D. No capacitance
Answer: B
Clarification: Owing to appreciable length and voltage of the line the charging current is appreciable in medium transmission lines. Also the capacitance effect cannot be ignored in medium transmission lines.

4. Performance of short transmission lines depends on which of the following?
A. Resistance and Capacitance
B. Resistance and Inductance
C. Inductance and Capacitance
D. Resistance, Inductance and Capacitance
Answer: D
Clarification: Medium transmission lines have enough value of resistance and inductance. Length and operating voltage of the medium transmission lines are sufficiently large to cause unavoidable charging current and therefore capacitance effect can not be ignored. Hence, performance of Medium transmission lines depends on R, L and C.

5. Performance analysis of medium transmission line is done ________
A. By reactance diagram
B. By symmetrical component analysis method
C. By neglecting line inductance
D. On per phase basis
Answer: D
Clarification: Performance analysis of balance transmission line is done only. And is done on per phase basis. No symmetrical component method or reactance diagram method is required for performance analysis.

6. What is the value of shunt capacitance of medium transmission line?
A. Very high
B. Medium
C. Zero
D. Negligable
Answer: B
Clarification: Length and operating voltage of the medium transmission lines are sufficiently large to cause unavoidable charging current and therefore capacitance effect can not be ignored. Capacitance effect in medium transmission line is less than that in long transmission line.

7. In medium transmission line the reciving end voltage may be greater than sending end voltage due to Ferranti effect.
A. True
B. False
Answer: A
Clarification: Ferranti effect happens when capacitance effect in medium or long transmission lines becomes greater than inductance effect. This happens during low or no load period.

8. Series inductance and series resistance of medium transmission lines are taken as:
A. Distributed and Lumped
B. Lumped and Distributed
C. Distributed
D. Lumped
Answer: D
Clarification: The series inductance and resistance of medium transmission lines are taken as lumped. This makes the performance analysis easier.

9. Which of the following is like equivalent circuit of medium transmission line?
A. Series RLC circuit
B. RLC circuit in pie form
C. Series RL circuit
D. Parallel RL circuit
Answer: B
Clarification: Medium transmission lines have considerable amount of inductance, resistance and shunt capacitance. Medium transmission lines are represented as either pie form or as T form.

10. In medium transmission lines shunt capacitances are considered as lumped.
A. true
B. false
Answer: A
Clarification: In case of medium transmission lines the charging current is unavoidable but less than that in long transmission lines. So it does not cause much error in calculations if shunt capacitances are taken as lumped.

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Power Systems Multiple Choice Questions on “Underground Cables – 1”.

1. Single core cables for AC systems are not provided with armouring.
A. True
B. False

Answer: A
Clarification: Because of the armouring of the cables leads to eddy current in the power cables adding more to the losses.

2. Single core cables for ac systems are provided with armouring.
A. True
B. False

Answer: A
Clarification: This is so because the eddy currents induced in the steel armour cause the additional power losses.

3. For a transmission system of voltage ‘765 kV’ volts having the core radius of ‘5’ cm and outer radius of ‘0.40’ m. The maximum dielectric stress on single core cable is _________
A. 7357.7 kV
B. 919.7 kV
C. 4247 kV
D. 530 kV

4. The ratio of dielectric stress of the on the cable corresponding to the maximum and minimum is ____________
A. R/r
B. r/R
C. rR
D. (R/r)²

5. The condition at which the minimized dielectric stress condition (R/r) will take place is ______________
A. 2.7188
B. 0.7188
C. 1/2.7188
D. 2.7188²

6. Grading of the cables is performed in order to achieve ____________
(i) uniform stress
(ii) reduction in quantity of insulation
(iii) reduction in quality of insulation
A. (i), (ii)
B. (ii)
C. (i),(iii)
D. (ii), (iii)

Answer: A
Clarification: Grading is performed to reduce the stress at one cable and to make it uniform throughout. Also it reduces the insulation material to be used and thus the cost saving.

7. The resistance of the insulation used for the cables is 40 ohms for a length of ‘x’ km. If the proposed plan needs to extend the transmission line cable to extra ‘2x’ m then the resistance of the cable offered will be _____________ (in ohms).
A. 13.33
B. 20
C. 120
D. 40

Answer: A
Clarification: The resistance of the cable is inversely proportional to the length of it.

8. Consider a system having a cable of capacitance 5 nF, with 765 kV star connected at 50 Hz having a power loss in the cable as 82.1 kW. Then the dielectric loss angle is _________(in degrees).
A. 15
B. 7.5
C. 20
D. 4.9

9. Dielectric power loss of a transmission line cable operating for ‘V’ volts, ‘f’ Hz having a capacitance of ‘C’ Farads with the tangent loss angle of δis ____________
A. ωCV² tanδ
B. ωCV²/tanδ
C. (tanδ/ωCV)²
D. ωCV² tanδ²

Answer: A
Clarification: The Dielectric loss = ωCV² tanδ for a cable.

10. In a transmission tower having ‘n’ insulators with equivalent pin-to-earth capacitance of ‘C’ farads. For the static shielding of the conductors, the capacitance from the shield to the kth link from the top is ______

Answer: A
Clarification: The generalized expression for the capacitance of the kth metal ring is,
C(k) = k*pin-to-earth/(n-k).

Power Systems Multiple Choice Questions on “Sag and Tension, Vibrations and Dampers”.

1. What happens to the tension in a conductor hanged between two poles, when temperature varies?
A. Tension increases with increase in temperature
B. Tension decreases with increase in temperature
C. Tension first increases and decreases with decrease in temperature
D. Tension in conductor is independent of temperature variation
Answer: B
Clarification: The relationship between tension and sag is dependent on the loading conditions and temperature variations. For instance, the tension increases when temperature decreases.

2. What is the effect of rise in temperature on sag when Ice and wind effect are eliminated?
A. Sag decreases
B. Sag increases
C. Sag remains constant
D. Sag becomes zero
Answer: B
Clarification: All metallic body expand with rise in temperature and therefore the length of the conductor increases with the rise in temperature and so does Sag.

3. What is the relation between length of span and sag?
A. sag ∝ √span
B. sag ∝ (1/span)
C. sac ∝ span²
D. Sagar ∝ span³
Answer: C
Clarification: Ultimate stress is directly proportional to the square of span length. Other conditions such as type of conductor working tension temperature it is it remains the same affection with longest and we’ll have much greater Sag.

4. What should be the value of sag for proper operation of overhead transmission line?
A. High
B. Low
C. Nither too low nor too high
D. Anything
Answer: C
Clarification: If the sag is too high more conductor material is required resulting more weight on the supports. If the sag is too low, there is more tension in the conductor and thus it is liable to break if any additional stress such as due to vibrations or due to fall in temperature occurs.

5. An overhead transmission line has a span of 220 metres the conductor waiting 0.604 kg/m. What will be the maximum sag if the working tension is 2879 kg.
A. 8.96 m
B. 8.86 m
C. 8.85 m
D. 1.27 m
Answer: D
Clarification:
(Maximum sag ) S = ωL²/8T
Where ,
ω – weight of conductor per meter
L – span
T – Working tension
Therefore, S = (0.604×2202)/(8×2879)
= 1.269 m ~ 1.27 m.

6. What is the value of working stress in overhead conductors?
A. Less than ultimate stress
B. More than ultimate stress
C. Always equal to ultimate stress
D. Should be zero
Answer: A
Clarification: Working tensile strength of the conductor is determined by multiplying the ultimate stress and area of cross section and dividing by a factor of safety. Due to division of ultimate stress by factor of safety the value of working stress is always less than value of ultimate stress.

7. When the sag exceeds 10% of the span length, the shape made by the conductor is similar to which of the following shape?
A. Hyperbola
B. Parabola
C. Catenary
D. Straight line
Answer: C
Clarification: A flexible wire of uniform cross section when string between two suppose at the same level will form a category however if the sag is very small as well as less than 10% of the span length then its shape approximates a parabola.

8. What will be the resultant weight on per meter of length of conductor if weight of conductor is 150 kg/m, weight of ice in per meter of length is 60 kg/m and wind force is 200 Kg/m.?
A. 300 kg/m
B. 468 kg/m
C. 290 kg/m
D. 390 Kg/m
Answer: C
Clarification:
Resultant stress = √[{(w_c + w_i )2 + w_w²}]
= √[{(150 + 60 )² + 200²}]
= 290 kg.

9. What are aeoline vibrations in overhead transmission line conductors?
A. High frequency and low amplitude vibrations
B. High frequency and high amplitude vibrations
C. Low frequency and low amplitude vibrations
D. Low frequency and high amplitude vibrations
Answer: A
Clarification: Aeoline vibrations are high frequency (as high as 500 Hz) and low amplitude (20 mm to 50 mm ) vibrations. They are caused by vortex phenomenon in light winds (5 to 20 Km/hr).

10. What are galloping vibrations in overhead transmission line conductors?
A. High frequency and low amplitude vibrations
B. High frequency and high amplitude vibrations
C. Low frequency and low amplitude vibrations
D. Low frequency and high amplitude vibrations
Answer: d
Clarification: Galloping vibrations in overhead transmission lines are low friquency ( 1 or 2 Hz) and high amplitude ( about 6 m ) vibrations. They occurs during sleet storms with a strong winds. In such situations conductors are said to ‘dance’.

11. Which of the following vibrations causes different conductors to touch due to high swing?
A. Aeoline vibrations
B. Galloping vibrations
C. Aeoline and Galloping
D. Amplitude vibrations
Answer: B
Clarification: Galloping vibrations in overhead transmission lines are high amplitude ( about 6 m ) vibrations. During such vibrations conductors ‘dance’ in horizontal and vertical directions with high amplitude which makes the conductors to touch each other.

12. ‘Dancing’ of overhead conductors occurs during which of the following types of vibrations?
A. Aeoline vibrations
B. Galloping vibrations
C. Aeoline and Galloping
D. Amplitude vibrations
Answer: B
Clarification: During galloping vibrations in overhead transmission lines conductors vibrates with high amplitude ( about 6 m ) and low friquency (about 1 or 2 Hz). Swinging of conductors with high amplitudes in horizontal and vertical directions are called as they are ‘dancing’.

13. What are the method for prevention of low frequency high amplitude vibrations?
A. Horizontal conductor configuration
B. Vertical conductor configuration
C. Horizontal and Vertical conductor configuration
D. There is no method for prevention of such vibrations
Answer: D
Clarification: Horizontal conductor configuration can be used to reduce the danger of low frequency vibrations. But no method can prevent such vibrations.

14. What are the methods used to protect conductors against high frequency resonant vibrations?
A. Horizontal conductor configuration
B. Vertical conductor configuration
C. By using dampers
D. There is no method for prevention of such vibrations
Answer: C
Clarification: The conductors are protected by dampers. Dampers prevents the resonant vibrations from reaching the conductors at the clamps or supports.

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Power Systems online test on “Medium Transmission Line – 2”.

1. In medium transmission lines capacitive current is ________________
A. Less than short transmission lines
B. Equal to short transmission lines
C. More than short transmission lines
D. More than long transmission lines
Answer: C
Clarification: Line to Earth capacitance of medium transmission line is more than that in short transmission line and less than that in long transmission line. Because of this the charging current of medium transmission line is more than that in short transmission line and less than that in long transmission line.

2. In actual the capacitance of line is ____________
A. More at receiving end
B. More at sending end
C. Uniformly distributed over entire length
D. More in middle of sending end and receiving end
Answer: C
Clarification: Actually the capacitance of line is uniformly distributed over its entire length. To make calculations simple the capacitance of the system is assumed to be divided up and lumped in the form of capacitors shunted across the line at one or more points.

3. Charging current in medium transmission line is _________________
A. Maximum at receiving end
B. Maximum at sending end
C. More in between sending and receiving end
D. Equal throughout the line
Answer: A
Clarification: The magnitude of capacitive current flowing at any point along the line is that required to charge the section of the line between the given point and receiving end. Hence, it has maximum value at the sending in and diminished at practically uniform rate down to zero at the receiving end.

4. In end condenser method of solution of medium transmission line, capacitance of the line is assumed to be _____________
A. Distributed uniformly from sending end to receiving end
B. Lumped at the load end
C. Lumped at the generation end
D. Lumped at middle of generation and load end
Answer: B
Clarification: In end condenser method the capacitance of the line is assumed to be lumped at the load end. This method overestimates the effect of capacitance.

5. In nominal-T method of solution of medium transmission line, capacitance is assumed to be __________
A. Distributed uniformly from sending end to receiving end
B. Lumped at the load end
C. Lumped at the generation end
D. Lumped at middle of generation and load end
Answer: D
Clarification: In nominal- T method or middle condenser method of analysis of medium transmission line the whole of the line capacitance is assumed to be concentrated at the middle point of the line and half the line resistance and reactance to be lamp on either side.

6. In nominal-π method of solution of medium transmission line, capacitance is assumed to be:
A. Distributed uniformly from sending end to receiving end
B. Divided into two halves
C. Lumped at the generation end
D. Lumped at middle of generation and load end
Answer: B
Clarification: In nominal-π method of solution of medium transmission line the capacitance of each line conductor is assumed to be divided into two halves. One half being shunted between line conductor and neutral at the receiving in and other half at the sending end.

7. Medium transmission line operates below voltage level of 20 KV.
A. True
B. False
Answer: B
Clarification: The voltage limit of 20 KV is for short transmission lines. Medium transmission lines can operate for voltage more than 20 KV.

8. Which of the following is not a method for solution of medium transmission line?
A. End condenser method
B. Nominal-T method
C. Nominal-π method
D. Nominal-H method
Answer: D
Clarification: Actual method used for solution of medium transmission lines are end condenser method, nominal-T method and nominal π method.

9. In a nominal-π method the resistance of line is assumed to be __________
A. Distributed uniformly from sending end to receiving end
B. Divided into two halves
C. Lumped at the generation end
D. Lumped at middle of the line
Answer: D
Clarification: In nominal-π method the capacitance of each line conductor is assumed to be divided into two halves and the resistance of the line is assumed to be lumped in between them at the middle of the line.

10. In nominal-T method of solution of medium transmission line, resistance and inductive reactance of the line is divided into two half’s.
A. True
B. False
Answer: A
Clarification: In nominal-T method of solution of medium transmission line, the whole of the line capacitance is assumed to be concentrated at the middle point of the line and half the line resistance and reactance to be lumped on either side of it.

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