Towers, Power Transmission Line

structures that are used to suspend power lines and lightning-protection cables in overhead power transmission lines. The basic structural components of such towers are posts, footings, crossarms, cable struts, and guy wires.

A distinction is made between the anchor towers and the intermediate towers of a power transmission line. The rigid and strong construction of the anchoring towers is intended to withstand considerable stresses caused by tension in the lines. Anchor towers are installed at the beginning and end of a power transmission line, at turns, and at points where the line passes over water or through mountains. In the last two cases, towers of the pass-over type are used. Intermediate towers are not as strongly constructed, serving mainly to support the power lines and cables along straight sections.

Power transmission line towers can also be classified by type as transpositional (used for changing the sequence of phase conductors), branching, crossing, elevated, and lowered towers. The towers can be classified, according to the number of lines or circuits that they carry, as single-circuit or multicircuit types. From a design standpoint, the towers can be categorized as being of the single-post type; as A-frames, H frames, or a combination of the two; and as freestanding or guyed. Towers can be mounted on reinforced-concrete footings, or they can be bedded directly into the ground.

Towers for power transmission lines can be made of wood, reinforced concrete, or metal. Wood towers are installed in power transmission lines that operate at up to 220 kilovolts (kV). They are used mainly in lines carrying up to 20 kV and in forested areas. Wooden towers are usually built with pine or larch poles impregnated with wood preservatives. Wooden transmission line towers are frequently mounted on reinforced-concrete piers or piles. Toward the end of the 1960’s, the manufacture of towers from bonded lumber was started outside the USSR. This type of design makes possible the use of short lengths of lumber and increases the strength of the towers.

Wooden towers for transmission lines are inexpensive. Their construction is comparatively simple and they are quite reliable in service. The first large power transmission line in the USSR, from the Kashira State Regional Power Plant to Moscow, operating at 110 kV over a total length of 120 km, was suspended on wooden towers.

Towers made of reinforced concrete have higher structural strength. Plans for such towers were first developed in the USSR in 1933. However, because at that time industrial facilities for the production of reinforced-concrete towers were still lacking, they were not widely used in the construction of power transmission lines until 1955. The advantages of reinforced-concrete towers are the simplicity of design and the technological efficiency with which they can be manufactured.

Reinforced-concrete towers usually have a circular or rectangular cross section and are primarily made from prestressed reinforced concrete. The most widely used type is the single post, reinforced-concrete, intermediate tower with metal crossarms, set directly in the ground. Some other designs, used extensively for power transmission lines operating at 110–550 kV, are the reinforced-concrete intermediate towers and the anchor towers, set at turns in the line and equipped with guy wires.

Metal towers for power transmission lines are lighter and have greater structural strength than reinforced-concrete towers. These properties make it possible to build towers of considerable height, designed to withstand large loads. Used on power transmission lines of all voltages, often in combination with reinforced-concrete intermediate towers, metal towers are essential in lines subjected to the great structural loads that occur, for instance, at crossover points. Metal towers for power transmission lines are usually made of steel; outside the USSR, they are sometimes made from aluminum alloys. To provide protection from corrosion such towers are painted or galvanized.

Metal towers are classified according to the method used to fabricate them. Thus there are welded towers, delivered by the factory in preassembled sections, and bolted towers, which are bolted together on location from individual members (braces, bars, chords). Metal towers for transmission lines are mounted on footings.