# Power Systems Questions and Answers – Sag and Tension, Vibrations and Dampers

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This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses 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

Explanation: 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

Explanation: 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

Explanation: 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.
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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

Explanation: 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

Explanation:
(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

Explanation: 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

Explanation: 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

Explanation:
Resultant stress = √[{(wc + wi )2 + ww2}] = √[{(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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: The conductors are protected by dampers. Dampers prevents the resonant vibrations from reaching the conductors at the clamps or supports.

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