Sag in overhead transmission line, Sag

calculation & string chart

Contents

• Introduction about sag• Sag calculation• String chart

Introduction

• The straight distance between the two poles gives the shortest distance. Therefore to minimise the length of conductor, one may stretch conductor to make it straight.

• But we also look after that the conductors are in safe tension. in order to permit safe tension in the conductors, they are not fully stretched but are allowed to have some dip.

• Thus the difference in level points of supports and lowest point on the conductor is called as Sag.

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Sag calculation

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• The tension on the conductor depends on the following factors:

Weight of the conductor Wind effects Ice loadingTemperature variation

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When the supports are

at equal ground level

While calculating the sag there are two consideration

When the supports are at unequal

ground level

Supports are at equal ground level

Continued…• Refer fig. in which ‘O’ is the lowest point of the

conductor spacing.• L=length of span in meters • W=weight of the conductor per unit length • T=tension on the conductor• Consider any point ‘P’ on the conductor whose

coordinate are ‘x’ and ‘y’.• There are two forces acting on the portion ‘OP’1. Weight of the portion ‘OP’ acting downward at a

distance x/2 from origin ‘O’.2. Tangential tension T acting at points ‘O’.

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Supports are at unequal level

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• Fig. shows the position of lowest points ‘O’ of conductor which is not exactly at centre of distance l.

• Therefore ‘x1’ is the distance of support at lower level from

lowest point ‘O’. ‘x2’ is the distance of support at high level from lowest

point ‘O’‘l’ is span length = x1 + x2

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String chart

• Stringing chart is useful in knowing the sag and tension at any temperature. Stringing chart gives the data per sag to be allowed and the tension to be allowed for a particular temperature.

• Stringing chart prepared by calculating the sag and tension on the conductor under worst conditions such as maximum wind pressure and minimum temperature by asuming a suitable factor safety.

Equation for determining string chart

• Let L1, T1, t1, W1 are the total length of conductor, tension, temperature and total load per meter under first set of physical conditions respectively.

• Similarly L2, T2, t2, W2 are the same quantities under the second set of physical conditions.

• Increase to tension from T1 to T2 elongates the conductor by L1(T2-T1)/aE

where, a = conductor cross sectional area E = Young’s modulus of elasticity

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Graph

• Now the graph of tension verses temperature and sag verses temperature can be plotted as shown in fig.

• This graph is plotted for a fixed span and is called as stringing chart.