Power Line Construction
A metal support for a transmission line.
It includes wires, insulators, and supports (Fig.). Overhead transmission line wires must have good electric conductivity, mechanical durability, and resistance to atmospheric and chemical effects. Aluminium wires are the main conductor of electric power transmission lines in Russia; aluminium alloys with high mechanical strength (aldrey, almelek, acron) as well as high-temperature zirconium alloys (operating temperature up to 150-210 °C) are widely used abroad. Wires (not insulated) are made by twisting together several layers (turns) of round or shaped wires; mainly the hardened (so-called steel-aluminium) ones with the cores made of wire rope steel wires are used.
Power transmission lines with rated voltages of up to 220 kV use only single wires in each of the three phases. Power lines of 330 kV and above have split phases (instead of one phase wire of larger cross-section, several wires of smaller cross-section are strung together) to eliminate long corona discharges on wires (which cause additional electric power losses).
Minimum number of wires in a split phase increases correspondingly to the increase of nominal voltage of a power line: 330 kV – 2; 500 kV – 3; 750 kV – 4; 1150 kV – 8. Increasing the number of wires in a phase above the minimum allows for a proportional increase in the transmission capacity of the transmission line (i.e., the highest possible active power). Overseas and in Russia, self-supporting insulated wires are widely used on the newly built 35-110 kV power transmission lines, which enables to reduce the phase distance at the poles and the width of clearings in forests.
Electric insulation is provided by either strands of glass-hardened suspended disc insulators that are mechanically connected in chains, or by rod-mounted polymer insulators that are based on fiberglass rods, hermetically protected by a ribbed sheath made of silicone-organic rubber. Advantages of polymer insulation are: low weight; easy storage, transportation and installation; increased resistance to destruction, etc. Wires are fastened to insulation and insulation to poles by means of nodes and products of aerial line fittings (wire clamps, earrings, staples, etc.).
To keep wires at a safe distance from land (water) surface, insulation hangers and supports (wood, concrete, metal) as well as other supporting structures and natural formations (rocks, brackets and poles on other engineering structures) are used. Wooden supports (for overhead power transmission lines up to 220 kV inclusive) in Russia are made of logs (pine, larch), the standard length of which is limited by the greatest size of 16 m. Foreign countries (USA and Canada) have developed support designs consisting of long glued wooden elements, which makes it possible to use wooden poles at nominal voltages up to 500 kV inclusive. The supports in constructions of concrete fittings (up to and including 500 kV) are long (up to 26 meters) conical and cylindrical tubes with inner prestressed armature and centrifugal concrete consolidation. The transversal elements of such supports (traverses) are made of hot-rolled steel angles. Carbon and low-alloy steels, structural aluminum alloys (of Al-Mg-Si system) are used for manufacturing the metal supports (for all voltages). Aluminum supports are most commonly used in the USA and Canada.
The design of metal supports is very diverse: Single and portal supports, both free-standing and retained in a normal spatial position with the help of stretching rods, attached to anchor plates sunk into the ground. The stanchions and traverses of the metal supports can be designed as a 4 or 3-sided obelisk with connected flat lattice trusses on its sides. In Russia, the conical polyhedral steel supports are increasingly used, which are made by bending a billet on a special high-powered press with a computer control. All metal supports are installed on foundations, in contrast to the wooden and concrete supports. Several types of steel and concrete mushroom-shaped foundations are widely used, which have a base plate and a post with exposed anchor bolts to secure the shoe of the support.
The disadvantages of such foundations are their heavy weight and the need to dig a deep foundation pit for the installation, backfill it and then compact the soil. Pile foundations have none of these disadvantages, because they can be made with cast-in-place concrete prismatic piles that are sunk into the ground by vibration pressing, or with steel screw piles. Abroad (USA), steel multifaceted pile foundations are made by concreting in the pit at the site of the pile installation using the formwork and reinforcement. In Russia, the use is found in large-diameter tubular foundations and mushroom-shaped substrates installed in a circle.