Overhead distribution lines¿some reflections on design

While existing regulations regarding the design of overhead lines have served well the electricity supply industry, recent happenings, when many networks suffered damage in Europe, have highlighted the need for a critical review of design practices. It is argued that, provided some statistical data on meteorological phenomena are available, attempts could be made to design overhead lines to suit subsequent performance requirements. The ultimate load design concept is favoured, in which an anticipated load with a quantified probability of occurrence is compared with the strength of a component having a selected probability of being achieved. Besides, it is suggested that, by the use of suitable coefficients, a preconceived coordination of strengths of the components can be obtained. Due to electrical constraints, there is a limit on conductor sizes, beyond which there is no measurable improvement in electrical performance. There is obviously an economic impact which must be appreciated. For a rapid assessment of a suitable voltage and conductor size the technique of kVA-km is a handy design tool. Analysis of many km-years of performance of overhead lines in general has proved useful in defining guidelines for insulation selection. A similar exercise on the lightning performance of distribution lines seems to indicate the beneficial influence of earthwires, although some simplified theoretical considerations would have led to a somewhat different conclusion. Notwithstanding the many efforts by various suppliers to provide alternative types of materials for the supports, these are normally made of either wood or steel. To satisfy specific requirements they can take many shapes to obtain a variety of conductor configurations. Foundations designed for overturning are most commonly used for distribution lines, although the design techniques are seldom taught to students. Designing foundations for overhead lines should remain a challenge to civil engineers.