Overhead Distribution Lines. Lawrence M. Slavin

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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.

This manual is divided into 10 sections or chapters. The background, including a description of the two general categories of construction methods relevant to outdoor utility lines, is provided in Chapter 1, including the overall characteristics and relative advantages and disadvantages represented by each category. Chapter 2 contains a description of pole structures and their physical characteristics, while Chapter 3 provides an overview of their installation, operation, and maintenance procedures. A description of the various types of suspended wires, conductors, and cables is contained in Chapter 4, and Chapter 5 discusses the methods of their installation. Chapters 6 (Strength and Loading) and 7 (Clearances) are primarily based on the rules of the National Electrical Safety Code (NESC^®), which govern the physical design and construction of the overhead lines, to help ensure safe facilities for the public and utility workers. Chapter 8 explains the principles regarding wire sags (and related tensions) and provides methods for determining these values under various weather and operating conditions. Chapter 9 contains a brief description of General Order 95, which governs overhead utility lines in California, and how it compares to the NESC. Example calculations are included in Chapter 10 for estimating the physical status of pole systems subject to storm loads. Additional details and explanatory information supporting the various chapters is contained in Appendices A–E. A glossary of terms and a list of references are also provided.

1.2 Background

      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.

For both these modes of construction, the requirements and guidelines for the installation and operation of power supply and communications lines, including the shared usage of facilities (poles, underground conduit systems, trenches), are provided in utility industry standards and documents, the most significant of which is the NESC. Both construction methods are commonly used in the industry, with an increasing amount of belowground distribution facilities being placed relative to aerial plant in more recent decades, primarily driven by regulations. For example, the large majority of new construction in local (residential) subdivisions deploys belowground facilities, in response to the demands of communities and various levels of government for a greater portion of belowground construction along roads and thoroughfares, primarily due to esthetic considerations and safety concerns. Utility pole collisions account for a significant fraction of automotive fatalities along the nation's roads and highways, for which the U.S. Federal Highway Administration, as well as the individual states, therefore provide appropriate guidelines for their usage (FHA 1993; AASHTO 2011).

A description of belowground cable applications, including conduit and duct applications, is provided in ASCE Manual of Practice No. 118 (ASCE 2009). Figure 1.1 illustrates typical belowground (underground conduit and direct-buried) construction alternatives.

The extensive conduit facilities of underground plant are generally appropriate for limited applications, such as associated with the trunk or feeder portions of the traditional telecommunications network, owing to the high cost of this method of construction. However, the availability of several vacant conduits does provide flexibility, including the capability to postpone installation of expensive trunk or feeder facilities (fiber-optic, etc.) until the need arises. Such expensive underground conduit systems are also the only viable alternative in metropolitan or large urban areas where overhead lines and/or future digging are not practical options. In comparison, direct-buried plant is a lower cost method for placing individual cables belowground between any desired termination points, but lacks any flexibility with respect to future additions or replacements. While the placement of utility lines belowground, using either method of construction, avoids much of the potential damage resulting from extreme weather events, there is nonetheless greater vulnerability during incidents of flooding and accidental damage during excavation work in the area.

Figure 1.1 Typical belowground construction.

      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.

Figure 1.2 illustrates a typical СКАЧАТЬ