Overhead lines - Line layout and calculation

This first article on overhead lines is divided into two distinct parts: a general presentation, on the one hand, and theoretical calculation concepts, on the other.

• An overhead line consists mainly of a set of conductors ensuring electrical continuity and providing a connection between two nodes of an electrical network.

  Overhead lines can be classified according to the functions they perform in the network.

  • Main transmission lines: these are used to evacuate the energy produced by a generation center to a consumption center or to major interconnection substations.

  • Interconnection lines: these pool the generation resources of several regions or countries, thus facilitating mutual back-up in the event of special situations affecting consumption, water availability, equipment availability, etc. Interconnection lines form a meshed network.

  • Distribution lines: direct energy from major interconnection substations to substations supplying distribution networks or those of major national customers.

  • Distribution lines: these carry energy to the various consumers; they can be broken down into framework lines with large conductor cross-sections and gap lines, which are mainly radial and generally lighter.

The entire article applies to the three-phase, alternating-current overhead lines commonly used in power grids in France and other countries.

It is possible, however, to build large transmission or interconnection lines with DC links, but these are currently few and far between.

• Line calculations involve a number of highly specific theoretical concepts, used almost exclusively by the manufacturers and operators of electrical networks.

These theoretical concepts (static equilibrium of the span, deflection, change-of-state equation, control canton, etc.) are reviewed in the second part of the article.

The reader's attention is particularly drawn to the approximations used to establish the usual formulas. While these approximations are perfectly acceptable for calculating a line without any particular difficulties, they can lead to significant errors when calculating long spans or very uneven spans.