Overhead lines—some aspects of design and construction

Abstract

Overhead lines embrace so many engineering disciplines, that only some aspects of design and construction can be considered in the paper. The new trends in assessing design loads for the components are briefly outlined, together with their interrelation with the strength of the components. It is argued that ultimate loading conditions should replace ‘working loads and factors of safety’. Although conductors have been fairly well standardised, new techniques for assessing their performance have recently been developed. However, with the advent of conductors incorporating optical fibres, some basic investigations are required. While there is a better understanding of wind induced phenomena, there is still no universal technique or device to solve galloping problems. After the conductors, the supports represent the second high-cost component of an overhead line, and much of the routine of design can now be performed by computers. Depending on requirements, loadings, ecological factors, economics etc., the supports can adopt many shapes and sizes, from the single member pole for low, medium or even high voltage lines, to the complex lattice types normally reserved for HV or EHV lines. The availability of fewer rolled sections and a limit on larger sizes tax the ingenuity of designers, who have to use, occasionally, compound sections or solid round members. Testing support designs is still a fairly common requirement. Considerable advances have been made in the knowledge of the electrical performance of air gaps, but the problem of insulation co-ordination still remains. Possibly, the greatest innovation in the field of insulators is the improving confidence in the use of composite insulators. The foundations of an overhead line are subjected to loadings seldom encountered with other civil engineering works, i.e. uplift. A review is made of the various shapes and sizes which can be used, together with up-to-date techniques of design. Site investigation is gaining more importance and more acceptance, in view of the need to match designs to actual site conditions with the dual purpose of economy and reliability. The influence of foundation movement is also discussed. Regarding installation, the equipment is adapted to the type of construction. It can be fairly simple for distribution lines and may become very sophisticated for transmission lines, especially when acceleration in the progress of work is desired. In addition, consideration of logistics, transportation and access have become exceedingly important. Work in highly inaccessible terrains, or opposition to access road building for ecological reasons, has focused attention on the application of helicopters for overhead transmission-line construction