Allowable tension levels for overhead-line conductors

Abstract

Optimised design of overhead transmission lines requires the selection of appropriate stringing tensions for conductors. The percentage of rated-tensile-strength and tension-over-mass criteria for determining these stringing tensions are discussed and analysed in terms of the static tensile stresses in the aluminium component of the conductor. It is demonstrated that the CIGRE tension-over-mass method of specifying vibration constraints does not maintain constant initial aluminium stress for conductors with various steel-to-aluminium ratios in all climates. In place of methods based on a percentage of rated strength or the tension-over-mass method, it is proposed that vibration constraints be based on specified values of static tensile aluminium stress. The ‘aluminium-stress method’ may be applied by specifying a stress constraint in a sag-tension program that is equipped to apply one. If such a program is not available to a line engineer, a simple method is provided for computing the initial tension constraint based on the steel and aluminium areas, the average temperature of the coldest month and specified constraints on initial aluminium stress and built-in stress. The effect of built-in stresses on initial stringing tensions is discussed.