Mechanical Design of Overhead Lines Interview Questions and Answers:

1. What are the various components of an overhead line ?

Ans. The main components of an overhead line are supports/towers, cross arms and clamps, insulators, conductors, guys and stays, lightning arresters, earth wire, V-guards, guard wires, phase plates, bird guards, danger plates, anti-climbing devices, vibration dampers etc.

2. What is the function of line support ?

Ans. The function of a line support is to support the line conductors and keep them at a safe distance from the ground.

3. What are the main requirements of line supports ?

Ans. The main requirements of line supports are high mechanical strength, lighter in weight, low maintenance cost, longer life, cheaper in cost, good appearance, easy accessibility for painting and erection of line conductors and immune to damage from lightning and fire.

4. What are four types of line supports ?

Ans. The four types of line supports are wooden poles, RCC or PCC poles, rail poles and steel structures or towers.

5. What are the drawbacks of wooden poles ?

Ans. The main drawbacks of wooden poles are comparatively smaller life, poor mechanical strength, tendency to not below the ground level and requirement of periodical inspection.

6. How do tangent angle tower and deviation tower differ from each other ?

Ans. Steel towers can be broadly classified as tangent towers and deviation towers. Tangent towers can be used for straight runs of the line and up to 2° line deviation from the straight run. The line is straight or along the tangent to the line route. In such towers the stress is because of the weight of the conductors, ice and wind loads. In addition extra forces due to break in the line on one side of the tower is also to be considered in the design of towers. The base of such a steel tower may be square or rectangular. Insulators used with such towers are suspension types.

For deviations exceeding 2°, special angle towers, sometimes called the deviation towers, are used. They are used where the transmission line changes direction. Such towers have broader base and stronger members as they are to withstand the resultant force due to change in direction in addition to the forces to which the tangent towers are subjected. Insulators used with such towers are of strain type. The cost of deviation tower is comparatively larger than that of a tangent tower because it is designed to withstand heavy loading as compared to standard or tangent tower.

7. What are the main requirements of good conductor material used for overhead lines ?

Ans. Main requirements of good conductor material used for overhead lines are high electrical conductivity, high tensile strength, low specific gravity, low cost, easy availability, free from brittleness and the material should not react with atmospheric condition.

8. What for ACSR conductor stands ?

Ans. ACSR conductor stands for aluminium conductor steel reinforced conductor.

9. Why are conductors for transmission line stranded ?

Ans. All conductors used for overhead transmission lines are stranded so as to increase the flexibility. Solid wires, except of smaller sizes, are difficult to handle when used for long spans and tend to crystallize at the points of supports because of swinging in winds.

10. Define term ‘ACSR’.

Ans. ACSR conductor consists of a core of galvanized steel strand surrounded by a number of aluminium strands.

11. Why are ACSR conductors preferred for transmission and distribution lines ?

Ans. ACSR conductors are preferred for transmission and distribution of lines because of the following facts:

  • ACSR conductor being of high tensile strength and lighter in weight produces small sag and therefore longer spans can be used. Because of use of larger span, the number of line supports may be reduced by about 25%. Thus the overall cost of supports, foundations, insulators and erection is considerably reduced. In addition, there is a substantial saving in maintenance cost. Further, since most faults occur at the line supports, so the frequency of occurrence of faults is automatically reduced.
  • The ACSR conductor has a largest diameter than any other type of conductor of same resistance, so corona losses are reduced.
  • Another advantage with ACSR conductor is that, because of reduced corona losses, critical voltage limit of the conductor can be raised by 30 to 50% as compared to copper conductors.

12. Why is galvanised steel wire not suitable for EHT lines for the purpose of transmitting large amounts of power over a long distance ?

Ans. The galvanised steel wire is not suitable for EHT lines for the purpose of transmitting large amounts of power over a long distance because of poor conductivity, high internal reactance and eddy current and hysteresis.

13. What is the function of cross arms ?

Ans. The function of cross arms is to keep the line conductors at a safe distance from each other and from the pole.

14. What are the factors on which conductor spacings and ground clearance depend ?

Ans. Factor affecting conductor spacings is voltage while factors affecting ground clearance are voltage and area (living or non-living, urban or suburban, along the streets or across the streets, river crossings/hill crossings, railway track/bridge etc).

15. Define term ‘sag’ in a transmission line.

Ans. The difference in level between the points of line supports and the lowest point on the conductor is known as sag.

16. What are the factors affecting the sag in a transmission line ?

Ans. Factors affecting the sag in a transmission line are weight of conductor, span length (distance between the poles), working tensile strength, ice coating, wind pressure and temperature.

17. Why is it disadvantageous to provide either too high sag or too low sag ?

Ans. It is disadvantageous to provide either too high sag or too low sag owing to the following facts.

In case the sag is too high, more conductor material is required, more weight on the supports is to be supported, higher supports are necessary and there is a chance of greater swing-amplitude due to wind load. On the other hand in the case of too low sag, there is more tension in the conductor and thus the conductor is liable to break if any additional stress is to be taken, such as due to vibration of line or due to fall in temperature.

18. What is the effect of wind and ice on sag ?

Ans. Combined Effect of Wind and Ice: Due to weight of ice deposits on the line, and the wind pressure, the mechanical stress increases in the conductor and, therefore, the line must be designed to withstand these stresses and tensions. Under this condition, the weight of the conductor, together with weight of ice acts vertically downwards while the wind loading ww acts horizontally.

19. What is stringing chart ?

Ans. Stringing chart gives the data for sag and tension to be allowed at a particular temperature. It is very useful while erecting the transmission line conductors for adjusting the sag and tension properly.