SAG AND TENSION CALCULATIONS OF TRANSMISSION LINE

If a conductor is sagged with a given tensile force between two points (A and B) representing the attachments of the conductor at the supports, a curve, the sagging curve or catenary will be formed due to the balance between conductor dead loads and tensile forces.

The vertical distance of the conductor to the line connecting both attachment points A and B is called the conductor sag.

The calculation of sag and tension of transmission lines can be quite useful to the engineer. The calculations are used to select the appropriate size support strand for a given application or to determine if clearance requirements are met.

With today’s clearance requirements the only way to assure clearance requirements are met is by calculation. The calculations serve as an aid to the designers who determine the mechanical stress that the cable must be capable of withstanding or to evaluate such problems as expansion loop cracks and center conductor pullouts.

The following are links to discussion of sag and tension calculations that will be useful as tips and reference to engineers and students alike.

CONDUCTOR SAG AND TENSION CALCULATIONS

The Mechanics of Overhead Distribution Line Conductors

The behavior and movement of a suspended conductor is  the most unpredictable variable in distribution line design.  Since complex equations are used to calculate the conductor sag curve, some simplifications and approximations are used.  The approximations cause small errors.  The accuracy of the final calculated results decreases as the  curve equation is simplified. Read more...

Sag and Tension Calculations for Overhead Transmission Lines at High Temperatures— Modified Ruling Span Method

This paper presents a method to calculate sags and tensions of multi-span line segments at different temperatures based on the rotational stiffness of suspension insulator strings. A simple equation, based on the parabolic approximation, is derived to calculate changes in the span lengths and conductor sags and tensions. Read more...

Sag-Tension Calculations: Refinements and Enhancements Made by Pondera

This paper discusses the modifications and enhancements related to the input data, and explains how these changes improve transmission line planning capabilities. Calculation methods are also provided. Read more...

Sag and Tension Calculations for Mountainous Terrain

While normal sag and tension calculations based on the 'equivalent-span' concept are satisfactory, when applied to transmission lines located in a reasonably undulating terrain, the answers obtained by this method are inaccurate for mountainous terrain. An alternative method of calculation, which is based on the analysis of the change of state equation for each span of a section in turn, is given. Read more...

Sag and Tension Calculations For Conductor/Earth Wire For River Crossing

Sag and tension calculations for conductor earth wire are done for the river crossing by following steps :

Determination of Equivalent Span :

Based on anchor spans L1 & L3 and crossing span L2, the equivalent span for river crossing portion is determined by the following formula :

Eq. Span = ÖL13 + L23 + L33 / L1   + L2   + L3 Read more...

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