The factors affecting the sag in an overhead line.

    The overhead line is supported by mechanical structures made up of parts like insulators, cross arms, poles, or towers. Even in the worst weather, the structures need to be mechanically strong. Forces acting on the conductor include wind pressure, tension, and the conductor's own weight.

    Even with a 12.7mm radial coating of ice and wind pressure on the order of 380 N/m2, the tension in the conductor is often expected to be less than 50% of its ultimate tensile strength. The diameter of the conductor, the distance between supports, the material of the conductor, the amount of sag in the conductor, wind pressure, and temperature all affect the tension in the conductor. 

Table of Contents:

1. What is sag in overhead line?

2. The factors affecting sag.

3. Why sag is important in the design of overhead transmission line?

    Sag and tension are influenced by the loading circumstances and temperature changes. Tension increases and sag diminishes as the temperature drops.

What is sag in Overhead lines?

Sag is the term used to describe the difference in level between the lowest point on the conductor and the points of the conductor supports.

Factors affecting sag

1. Tension in conductor.

    There is an inverse relation in between the sag and conductor tension. Low sag means a tight wire and high tension, on the other hand, low tension means a loose wire and increased sag. Both more loose and more tight are not desirable for proper design of transmission line. Therefore, in practice, a compromise is made between the two.

2. Weight of the conductor.

    The weight per unit length of conductor an important factor in calculating the sag, more weight will leads to the more sag.

  

3. Wind & Ice loading.

    A conductor may actually have an ice coating and be under wind pressure at the same time. Just like weight of a conductor, the weight of ice also works vertically downward. It is expected that the wind's force will act horizontally.

4. Ambient temperature.

    There is also a temperature effect on the transmission line sag. This is because, the temperature variation may change the length of conductor.

5. Span length.

    The span length also contribute to the variation of sag. Two times increase in length of span causes sag to be four times increased.  

Why Sag is Important in the design of overhead transmission line?

    Sag is a crucial element in the design of an overhead line of the power transmission system. More conductor material is required in the event of more sag, more weight must be supported by the supports, higher supports are required, and there is a possibility of a stronger swing-amplitude owing to wind load. 

    If there is less sag, the conductor is under higher tension and is more likely to break with more stress. Therefore, when calculating sag-tension, two conditions should be primarily taken into consideration:

1. At minimum temperature: When the temperature is the lowest and the wind is the strongest, the conductor sags the least and has the most stress.

2. At maximum temperature: When the temperature is at its highest and there is no wind pressure, the sag is greatest.

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