Transmission and Distribution Objective Questions with Easy Explanation Part-6

 51. Guy wire is employed for

(a) providing protection against surges.

(b) providing emergency earth route.

(c) supporting the pole.          

(d) all of the above.

 

Answer: (c) supporting the pole.

 

Explanation:

  • Guy wire or extended cable is needed on some poles to support unbalanced lateral loads due to the utility wires attached to them or to resist ground movement.
  • Guy wire or extended cable is particularly needed on dead-end (anchor) poles, where a long straight section of wireline ends, or angles off in another direction and hence prevent the pole from bending.
  • To protect the public against faults that might allow the extended cable to become electrified, utility guy cables usually either have a ceramic strain insulator ("Johnny ball"), or a fiberglass guy strain insulator inserted near the top.
  • To ensure that any dangerous voltages do not reach the lower end of the wire accessible to the public. The lower end where the cable enters the ground is often encased in a length of the yellow plastic reflector to make it more visible so that people or vehicles do not run into it.

 

52. The sag of a transmission line is least affected owing to

(a) the weight of the conduct

(b) current through the conductor.

(c) atmospheric temperature.

(d) ice deposition on the conductor.

 

Answer: (b) current through the conductor.

 

Explanation:

  • Sag is defined as the difference in level between points of supports and the lowest point on the conductor.
Factors affecting the sag:
  • 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 current passes through the conductor.

 

53. 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.

(d) decrease both.      

 

Answer: (a) increase the sag and decrease the tension.

 

Explanation:

Effect of increase in temperature in overhead transmission line:

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.

S=Wl2/8T

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.


54. 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.

 

Answer: (b) more than that in winter.

 

Explanation:

Factors affecting the sag:

  • 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 comparison to winter.

 

55. In a transmission line, sag depends upon

(a) span length.          

(b) tension in conductors.

(c) weight of the conductor per unit length.

(d) all of the above.

 

Answer: (d) all of the above.

 

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 comparison to winter.
  • The sag of the transmission line is least affected by current passes through the conductor.

 

56. Which of the following statements is correct?

(a) Ice on conductors increases skin effect.

(b) Wind pressure reduces the corona effect.

(c) Wind pressure is taken to act at perpendicular to that of ice,

(d) Ice on conductors reduces sag.

 

Answer: (c) Wind pressure is taken to act at perpendicular to that of ice,

 

Explanation:

  • The physical state of atmosphere: Corona is formed due to the ionization of air surrounding the conductors. The number of ions is more than normal in the stormy weather.
  • The irregular and rough surface cause more corona loss because unevenness of the surface decreases the value of breakdown voltage.
  • A larger distance between conductors reduces the electrostatic stresses at the conductor surfaces. It helps in avoiding the formation of the corona.
  • The operating voltage
  • Air density factor
  • Corona loss is independent of the current in the conductor.

 

57. Wind loading in coastal regions is in the range of

(a) 40-50 kg/ m2        

(b) 150 kg/m2

(c) 96 kg/m2   

 

Answer: (b) 150 kg/m2

 

Explanation:

  • Wind loading in coastal regions is in the range of 150 kg/m2
  • 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.

 

58. The maximum tension in a section of overhead line conductor between two supports of unequal height occurs at

(a) the higher support.

(b) the lower point.

(c) the midpoint of the conductor.

(d) None of the above.           

 

Answer: (a) the higher support.         

 

Explanation:

  • The maximum tension in a section of overhead line conductor between two supports of unequal height occurs at the higher support.
  • For safety purpose, the ground clearance of the conductors at maximum temperature and minimum loading condition should be maintained.
  • Analysis of the sag and tension is important in the transmission line for the continuity and quality of electrical services.
  • If the tension of the conductor is increased beyond the limit, it may get broken, and the power transmission of the system get erupt.
  • The dip of the conductor between the two level supports is called sag.
  • In other words, the vertical distance between the highest point of the electrical pole or tower (where the conductor is connected) and the lowest point of the conductor between the two adjacent level supports is known as sag.
  • The horizontal distance between two electrical supports is called the span.
  • If the weight of a conductor is uniformly distributed along the line, then it is assumed that a freely suspended conductor shape is a parabola.
  • The shape of sag increases with the increase in the length of the span. For a small span ( up to 300 meters) parabolic method and large span ( like river crossings) catenary method is used for the calculation of sag and tension.

 

59. Stringing chart is useful

(a) for finding the sag in the conductor.

(b) in the design of tower.

(c) in the design of insulator string.

(d) finding the distance between towers.

 

Answer: (a) for finding the sag in the conductor.

 

Explanation:

  • Stringing chart is useful in knowing the sag and tension at any temperature
  • Stringing chart gives the data per sag to be allowed and the tension to be allowed for a particular temperature
  • Stringing chart prepared by calculating the sag and tension on the conductor under worst conditions such as maximum wind pressure and minimum temperature by assuming a suitable safety factor

 

60. Hot template curves are plots of

(a) temperature and humidity.

(b) conductor sag and span lengths.

(c) conductor weight and sag.

(d) none of the above.

 

Answer: (b) conductor sag and span lengths.

 

Explanation:

  • Hot Curve – The hot curve is obtained by plotting the sag at maximum temperature against span length. It shows where the supports must be located to maintain the prescribed ground clearance.
  • Ground Clearance Curve – The clearance curve is below the hot curve. It is drawn parallel to the hot curve and at a vertical distance equal to the ground clearance as prescribed by the regulation for the given line.
  • Support Foot Curve – This curve is drawn for locating the position of the supports for tower lines. It shows the height from the base of the standard support to the point of attachment of the lower conductor. For wood or concrete line, pole line this curve is not required to be drawn since they can be put in any convenient position.
  • Cold Curve or Uplift Curve – Uplift curve is obtained by plotting the sag at a minimum temperature without wind price against span length. This curve is drawn to determine whether uplift of conductor occurs on any support. The uplift conductor may occur at low temperature when one support is much lower than either of the adjoining ones.


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