Mechanical Design of Transmission Lines

UNIT – III

Chapter -  4

(a) Mechanical Design of Transmission Lines

Syllabus

Mechanical design of OH lines - Line Supports - Types of towers - Stress and Sag Calculation - Effects of Wind and Ice loading.

Contents

4.11ntroduction

4.2 Definition of Sag

4.3 Calculation of Sag and Tension ... May-04, 11, 13, ... Dec.-10,11,12,13,15,16,     Marks 8

4.4 Effect of Atmospheric Conditions on Transmission Line .... Dec.-04, 05, 07, 10, 13, 14, 15, ... April-01, May-16,18, ... Marks 13

4.5 Calculation of Diameter for Stranded Conductor

4.6 Calculation of Sag for Catenary Shape of Conductor       ... Dec.-17, ... Marks 7

4.7 Tension and Sag at Erection       ... May-14, ... Marks 16

4.8 Stringing Chart

4.9 Sag Template and Tower Spotting       .... Dec.-17,  ... Marks 6

4.10 Vibrations of Conductors and Damper

4.11 Types of Towers  ... May-17, ... Marks 5

4.12 Two Marks Questions with Answers 

Introduction

It is well known that when a rope is suspended between the two points then it shows a tendency to sag down. Similarly when a wire or a conductor is suspended between the two points, the wire sags down. The wire takes the shape of a catenary or a parabola as shown in the Fig. 4.1.1. This sag plays an important role in the mechanical design of the overhead transmission lines.

The transmission lines are designed primarily from the economy point of view. To have lowest cost, length of the transmission line must be 7' as shown in the Fig. 4.1.1. But due to sag, it requires more conductor material. Similarly if sag is more, then an extra height of the supporting poles is also necessary. This increases the cost. Hence for keeping cost minimum, sag also must be kept minimum.

But if sag is too low then wire gets subjected to an extra tension. There are some other factors present like temperature, wind pressure etc., due to which conductor gets subjected to the maximum stress. If sufficient sag is not kept then conductor cannot sustain under such tension and there is possibility of the mechanical failure of the transmission lines.

Thus minimum sag and less tension cannot be satisfied simultaneously. Hence it is necessary to design the sag value considering all the severe conditions, such that the tension in the conductor is always well within the safe limits. Such severe conditions may include the ice coating on the lines, high wind pressure etc.

Hence it is necessary to study, how to calculate the sag and the tension in the transmission lines under widely varying environmental conditions. This chapter explains the sag and tension calculations in overhead transmission lines which is nothing but the mechanical design of transmission lines. The chapter also includes the sag template and vibrational damper used in the transmission line design.