Koi Koi Grace
by on March 6, 2019
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Transmission structures, whether wood, steel, lattice, concrete or composite, are one of the most visible elements of an electrical power transmission system. Regardless of material, they serve a single purpose: supporting the insulators, wires and equipment. The choice of a particular type of structure – material and configuration – for a transmission line, however, depends on several factors such as electrical, spatial, structural and economy.

Electrical factors include voltage, number of circuits (single or multiple, including distribution under-build), conductor bundles (single or multiple), communication cables or optical ground wires (OPGW), insulation, lightning protection and grounding.

Spatial constraints involve allowable spans, ROW (Right-of-Way) issues as well as vertical and horizontal clearances mandated by codes for the project. ROW is a function of voltage; so are the clearances required for wires above ground and above other specified wires. The adopted conductor configuration – horizontal, vertical or Delta – influences the nature of clearances. Conductor separation also plays a critical part in determining phase spacing and therefore, pole or structure height.

Structural factors involve strength and stiffness of the structure, its components and foundations in resisting the loads applied on them without excessive stress and deformation. Structure strength is traditionally indicated by horizontal and vertical span (HS and VS) limits, which are in turn dependent on strength of pole material, knee braces, cross braces, cross arms and uplift. Additionally, the ratio of HS/VS is limited by insulator swing (for tangent structures with suspension insulators). Finally, majority of transmission structures in USA must, as a minimum, be designed to the applicable NESC (National Electrical Safety Code) or RUS (Rural Utilities Service) loading guidelines (or utility-specified norms exceeding NESC) and these include climactic loads due to ice, wind and temperature. Some utilities are also specifying special HIW (High Intensity Wind) loading for lattice towers in areas of hurricanes, tornadoes and downbursts.

Economy is associated with the issues of constructability, erection techniques, inspection, assessment and maintenance. As discussed in Chapter 2, accessibility for construction must be considered while deciding on structure types. Mountainous or swampy terrains make access difficult for construction vehicles and use of specialized equipment or a helicopter may be required. Guyed structures also create construction difficulties since a wider area must be accessed to install guys and anchors often impacting the ROW and land use restrictions.

This chapter looks at various types and configurations of structural systems adopted for transmission lines in North America, materials, computer modeling, analysis and design, hardware and development of design drawings. Also discussed are the processes of selecting structural components and compliance with different code regulations for ensuring safety, integrity and strength of structures.

 

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