Sag and Tension

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 shown in the figure below. 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.

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.