Question

A transmission line made of copper has a resistance of 100 at OC . calculate the change in resistance between summer and winter, knowing that temperature varies from +35C° to -30Cº. Assume the temperature co-efficient of copper to be 0.00427/Cºat 0Cº.
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Answer 1

INTRODUCTION

This document describes the calculation and methods which are used for determining thermal

ratings for transmission lines. It should be noted that thermal ratings are only one of several

aspects of transmission network operation which determine the network capability. Other

aspects are not discussed in this document but include transient stability, dynamic stability and

voltage support capability. All of these elements must be maintained in operating a

transmission system. As such the actual capability of the system is determined by the lowest

of these elements and therefore, in many cases transmission capability is not actually

determined by the thermal transmission line ratings.

An overhead transmission line thermal rating (“line rating”) is a limit on the combination of the

line-current magnitude and duration, for the purpose of restricting conductor temperature.

Conductor temperature, in turn is restricted in order to limit one or more of the following:

 The clearance between the conductor and ground

 The clearance to other conductors

 Protection from loss of tensile strength or permanent conductor damage by heat.

There are some significant differences in networks that operate in the NEM. These differences

are derived predominantly from the geography, climate, topography, distribution of load and

the distribution and types of generation source available in each NEM region.

While recognising the constraints imposed by network differences, a common approach to the

calculation of line ratings across the NEM has been agreed so that all TNSPs in the NEM use

the same equations and the same secondary environmental variables (i.e. excluding wind

speed and ambient temperature) when calculating line ratings. The only source of differing

rating outcomes in each region or state arises from differing ambient conditions.