Overhead Distribution Lines. Lawrence M. Slavin
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Название: Overhead Distribution Lines

Автор: Lawrence M. Slavin

Издательство: John Wiley & Sons Limited

Жанр: Физика

Серия:

isbn: 9781119699200

isbn:

СКАЧАТЬ high-voltage power transmission facilities, along the same transmission line, are relative invariable and unchanging, with relatively uniform spans, and require the efforts of experienced structural engineers, often supported by the use of sophisticated software tools, to cost-effectively design reliable, safe overhead facilities.

      There are two basic modes of construction for outdoor utility lines for electric power supply and communication systems:

      1 Belowground (or underground) plant consisting of an array of parallel conduit paths, spanning the distance between manholes, typically located parallel to a main thoroughfare or highway; or, for more local distribution applications, direct burial of the cables within the soil, possibly placed along a road or street, with buried service drops to the residences. Routine access to such distribution facilities is typically provided by flush-mounted handholes or above ground terminals and pedestals.

      2 Aerial/overhead plant in which the cables are individually suspended between utility structures, including tall structures or towers for high-voltage transmission lines, spaced hundreds, or possibly thousands, of feet apart; or relatively short structures for distribution applications, typically individual poles, spaced up to a few hundred feet apart.

Photographs depicting typical belowground construction alternatives: (left) Cables installed within ducts and (right) cables placed directly in trench.

      Although not esthetically pleasing, the ubiquitous overhead lines throughout the United States – supported by possibly as many as 200 million utility poles – provide many important benefits, and is the reason these structures and suspended lines continue to be widely used. Individual distribution poles, or even lattice transmission towers, require minimum real estate at the ground level, and allow new lines to be readily deployed in available overhead space. This includes otherwise difficult crossing applications, or where expensive belowground construction methods (e.g. directional drilling) would be required, such as at highways, railroads, and waterways. Overhead installations avoid the many possible issues encountered when attempting to perform construction beneath the surface in various or unknown belowground conditions, often in the presence of existing belowground facilities. The latter situation can be particularly hazardous, especially when power or gas lines are in the vicinity. The use of mandatory “call-before-dig” rules, and related utility locating practices, are not infallible, and unfortunate accidents may occur in spite of such precautions.

      Apart from natural or man-made disasters, overhead lines are exposed to environmental stresses that are generally less severe than the persistent wet and corrosive surroundings that can be found belowground. As a result, it is often more of a challenge to design the belowground cables and/or the associated conduit/manhole facilities with sufficient resistance to those degradation forces than where the cables are placed overhead. In addition, if degradation occurs, or water penetrates the belowground plant, their repair and replacement is more difficult, expensive, and time-consuming. In general, overhead lines are inherently significantly less expensive to install and maintain than belowground facilities, as well as being characterized by greater flexibility for the addition, rearrangement, and/or replacement of the supported lines and equipment.