Sagging in overhead conductor lines

Introduction:?

In Transmission lines, the sag is considered as the vertical distance between two points of support of the transmission towers and the lowest peak point of conductor. To calculate the sag of a conductor it is necessary to know the span of the overhead line.

In other words, the term “sag” may be described as the gap among the midpoint of the strained transmission line among the two insulators and the assumed direct line among the insulators.

Span of an overhead line is the horizontal distance between two points of the support of two consecutive towers of same height.?

In the above Figure, if we consider A & B as the topmost points of two consecutive transmission towers, AOB is the conductor then S is considered as a sag when vertically measured.??

Why is there a need for sag in overhead conductors?????

The sag in overhead conductors prevents the conductor from excessive tension. To provide protection against this tension on conductors, the conductors are not stretched up to their maximum limits and are allowed to have sagged.?

In case, if the conductor is fully stretched up to its maximum limits during the stringing process, the wind will put pressure on it, hence it may result in the breaking of the conductor or the conductor may get detached from the end support structure of the towers. Thus, allowable sag is necessary during installation.??

Sag values for different voltage ratings:??

• < 66kV - 20 Feet?
• For 66kV to 110kV - 21 Feet?
• For 110kV to 165kV - 22 Feet?
• >165kV – 23 Feet?

??Some points regarding sag are as follows:??

• The Sag span curve is parabolic in shape.?
• Sag is very small as compared to the span of the conductor.?
• The tension in all points of the conductor always acts tangentially.?
• The sag becomes more with time due to weight and wind.???????

Naturals phenomenon like Wind, Sunlight & Ice cause effects on sag of overhead line conductors:?

• The per unit weight of the conductor’s length changes because of the wind which carries a certain force.?
• In some countries ice accumulates over time adding more weight to the conductor and creating more sag.?
• Wind adds weight to the conductor in a horizontal direction.?
• Ice loading adds weight to the conductor in a vertically downwards direction.?
• If both wind and ice together add weight to the conductor. The resultant weight will be downwards.?
• Due to heating also in summer days, the copper and aluminium conductors get expanded due to heavy power consumption and hot sunlight. ?

Over Sagging can be dangerous and hence following points should be kept in mind while constructing transmission lines:?

• The transmission lines must be designed in a manner now no longer damage residing life. The layout and assessment must be in a manner to reduce the conductor height and the damage of electromagnetic effects.?
• The transmission lines must be short, safe, and affordable in terms of length to be economical in maintenance.?
• The energy transmission line's route should be constructed with lease-purchase, risk of life, and economic criteria in mind.?
• The energy transmission line's path should be close to current transportation routes so that it may be reached promptly in the event of a failure.?
• To require less maintenance and more failure, transmission line components should be made of the finest quality and longest-lasting materials.?
• The atmospheric conditions of the region should guide the selection of energy transmission lines and transmission components.?
• Extremely hot, cold, windy, and pulsing places should be identified, and projects should be prepared accordingly.?
• Although these types of parameters are decided with the aid of using worldwide standards, every use differs in line with its weather conditions, power community structure, and the revel in of professional personnel.?

Effects of Sag in Transmission line:?

Reduction in excessive tension:?

It's critical to keep the conductors at safe strain when installing an overhead line. The stress on the conductors may reach a dangerous level if they are overly pressured between the supports (towers, utility poles), and the conductor may break owing to excessive pressure (i.e., tension). The conductors (i.e., the transmission lines) are not fully stretched to allow for safe tension in the conductors but are allowed to dip or sag.?

Over-limit sag can increase the cost:?

The transmission line will sag further if there is more distance between the transmission towers. If a transmission line has too much sag, the amount of conductor used will increase, increasing the cost more than necessary.?

Leads to power failure:?

When a transmission line sags too much, it can result in a power outage. In 1996, the largest power outage in the Western United States was caused by an overheating electrical transmission line sagging into a tree. The latest East Coast blackout is thought to have been caused by a similar incident.?

How can we prevent excessive sagging in the Transmission Line??

Transmission lines that sag due to heavy use and high temperatures are transmission line operators' worst nightmare. Not only are these lines limited in terms of the amount of electricity they can transport, but they also have a larger danger of colliding with trees and shorting out. Last year's blackout was thought to have been caused by a sagging transmission cable in Ohio.?

Utilities have had two fundamental techniques to dealing with line sag up until now. They may choose to:?

• Re-engineer the line and keep an eye on it to ensure that sag restrictions are not exceeded.?
• Re-engineering often entails shortening the distance between transmission towers since the longer the distance between towers, the more the transmission line sags.?
• Raising tower heights and/or re-conducting solves the problem, although it's more expensive than monitoring it to ensure that sag limitations aren't exceeded.?

Conclusion:??

In a nutshell, although sag in the conductor is allowable up to some limits but it has many impacts on the performance & life of overhead conductors. Hence, the conductors should be fixed properly after allowable sag is applied while the stringing of conductors. In this way we will not only reduce the possible accidents with living beings but also will reduce the maintenance cost.?

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