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 an overhead line?

2. The factors affecting sag.

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

    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. Material Properties

The material of the structure or cable plays a significant role in determining the sag. The elasticity, strength, and weight of the material dictate how much it will deform under load. For example:

• Elastic modulus: A material with a higher modulus will resist stretching, leading to less sag.

• Density: Heavier materials tend to sag more due to their own weight.

2. Tension in conductor.

    The amount of tension applied to a cable is one of the most critical factors affecting sag. There is an inverse relation between the sag and conductor tension. Increased tension reduces sag but can introduce higher stresses, potentially leading to structural failure. Conversely, low tension results in excessive sag, which may impair functionality or safety Therefore, in practice, a compromise is made between the two.

3. Weight of the conductor.

    The weight per unit length of a conductor is an important factor in calculating the sag; more weight will lead to more sag.

  

4. Wind & Ice Loading.

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

5. Ambient temperature.

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

6. Span length.

    The span length also contributes to the variation of sag. Longer spans typically exhibit more sag due to the increased length of the unsupported section. Engineers must carefully balance span length with material selection and tension to achieve optimal performance. Two times increase in the length of the span causes sag to be four times increased. 

7. Support Conditions

The type and rigidity of the supports influence the amount of sag. Rigid supports provide better stability, reducing sag, while flexible or movable supports may allow for greater displacement.

Why is sag important in the design of overhead transmission lines?

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.

​​Sag plays a critical role in the design and functionality of overhead transmission lines for several reasons:

• Safety: Proper sag ensures that conductors remain at a safe distance from the ground, buildings, and other objects, minimizing the risk of electrical hazards.

• Reliability: Excessive sag can cause conductors to touch trees or other obstacles, leading to power outages. Conversely, insufficient sag increases tension, potentially damaging the conductors.

• Thermal Expansion: Conductors expand with heat. Designing appropriate sag accommodates these changes, preventing structural stress and maintaining functionality.

• Wind and Weather Resilience: Adequate sag allows the line to withstand wind loads and ice accumulation without excessive stress or failure.

Conclusion

Sag is a complex phenomenon influenced by material properties, tension, span length, environmental factors, load distribution, and support conditions. Engineers must consider all these factors to design systems that are safe, efficient, and durable. By understanding and addressing the causes of sag, we can build structures and systems that stand the test of time and perform reliably under varying conditions.