1. Effect of increase in temperature in an overhead transmission line is
A. to increase stress and length both
B. to decrease stress and length both
C. to decrease stress and increase the length
D. None of the above
Answer
C. to decrease stress and increase the length
Explanation:
With the increase in temperature Sag increases.
Sag is the distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.
Where,
S is the sag of the conductor
W is the weight of the conductor
L is the span length of the conductor
T is the working tension on the conductor
An increase in temperature in overhead lines leads to expansion of length and causes sag thereby decreasing the stress.
2. If span length is doubled with no change in other factors, the sag of the line will become?
A. 2 times
B. 8 times
C. 4 times
D. 0.5 times
Answer
C. 4 times
Explanation:
The sag is given by
Where w is the weight per unit length of the conductor
T is the tension of the conductor
L is the length of the span
If the If span length is doubled with no change in other factors, the sag of the line will become 4 times as sag is directly proportional to the square of the span length.
3. Find the approximate value of sag for a 66 kV line that has a span of 200 m between towers.
A. 0.02 m
B. 2 m
C. 0.2 m
D. 20 m
Answer
B. 2 m
Explanation:
While erecting an overhead line, it is very important that conductors are under safe tension.
If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension.
In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag.
The difference in level between points of supports and the lowest point on the conductor is called sag.
Due to sag between two supports, the conductor takes the form of the catenary.
The value of sag for a 66 kV line which has a span of 200 m between towers is 2 meters.
4. The effect of wind pressure is more predominant on
A. Supporting tower
B. Neutral wires
C. Transmission lines
D. insulators
Answer
A. Supporting tower
Explanation:
The effect of wind pressure is more predominant on supporting towers than transmission lines.
The towers do have a reasonable surface area that would receive the force generated by any constant wind. It would also have the forces at the suspension point generated by the wind on the transmission lines.
The transmission towers have flat surfaces whereas the transmission lines are circular. Then there is the possibility of the wind being just at the right speed to cause the towers to oscillate.
5. The sag of a transmission line depends upon
A. Weight of conductor and directly proportional to span length
B. Weight of conductor and inversely proportional to span length
C. Weight of conductor and directly proportional to the square of span length
D. Weight of conductor and inversely proportional to the square of span length
Answer
C. Weight of conductor and directly proportional to the square of span length
Explanation:
Conductor weight – Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span – Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension -The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind – It increases sag in the inclined direction.
Temperature – The sag is reduced at low temperatures and is increases at higher temperatures.
6. Sag of the conductor’s between two poles can be determined by
A. 
B. 
C.
D. 
Answer
C.
Explanation:
The sag is given by
Where w is the weight per unit length of the conductor
T is the tension of the conductor
L is the length of the span
7. The sag of the conductors of a transmission line is 2.5 m when the span is 250 m. Now if the height of the supporting towers is increased by 2.5%, the sag will
A. reduce by 25%
B. increase by 25%
C. reduce by 12.5%
D. remain unchanged
Answer
D. remain unchanged
Explanation:
The distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.
Span length: It is the shortest distance between two towers or poles.
Where,
S is the sag of the conductor
W is the weight of the conductor
l is the span length of the conductor
T is the working tension on the conductor
Here in the question height of the supporting towers is increased by 2.5 % which is independent of sag. Hence there will be no change in sag.
8. Effect of temperature rise in overhead lines is to
A. increase the sag and decrease the tension
B. decrease the sag and increase the tension
C. increase both the sag and tension
D. decrease both the sag and tension
Answer
A. increase the sag and decrease the tension
Explanation:
With the increase in temperature Sag increases.
Sag is the distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.
Where,
S is the sag of the conductor
W is the weight of the conductor
L is the span length of the conductor
T is the working tension on the conductor
An increase in temperature in overhead lines leads to expansion of length and causes sag thereby decreasing the Tention.
9. The sag of a transmission line conductor in summer is
A. Less than that in winter
B. More than that in winter
C. Same as in winter
D. None of these
Answer
B. More than that in winter
Explanation:
Conductor weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span: Sag is directly proportional to the square of the span length. A longer span gives more sag.
Tension: The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind: It increases sag in the inclined direction.
Temperature:The sag is reduced at low temperatures and is increases at higher temperatures.
In summer due to the increase in average temperature tension decreases and hence sag increases comparison to winter.
10. The sag of a transmission line is least affected owing to
A. weight of the conductor
B. current through the conductor
C. atmospheric temperature
D. ice deposition on the conductor
Answer
B. current through the conductor
Explanation:
Conductor weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span: Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension: The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind: It increases sag in the inclined direction.
Temperature: The sag is reduced at low temperatures and is increases at higher temperatures. In summer due to the increase in average temperature tension decreases and hence sag increases in comparison to winter.
The sag of the transmission line is least affected by the current that passes through the conductor.