How are power transmission towers arranged?
Power transmission line (transmission line) - one of the components of the electric network, a system of power equipment designed to transmit electricity through electric current. Also, an electric line as part of such a system, extending beyond the boundaries of a power plant or substation. Distinguish between overhead and cable power lines. Recently, gas-insulated lines - GIL - are gaining popularity. Information is also transmitted via power lines using high-frequency signals (according to experts, in the CIS about 60 thousand RF channels are used on power lines) and fiber optic links. They are used for supervisory control, telemetry data transmission, relay protection signals and emergency automation. It is also noteworthy that in the 1985s, the development of power lines was carried out, reaching a height of 70 meters.
An overhead power transmission line support (transmission line support) is a structure for holding wires and, if available, lightning protection cables of an overhead power transmission line and fiber-optic communication lines at a given distance from the earth's surface and from each other.
What could be more ordinary than power lines? Power transmission towers are one of the most common engineering structures, and they are always in front of our eyes. However, this area also has its own technological subtleties and even scope for technical progress. Overhead power lines that are not very noticeable to us acquire a new look and new design.
Most often, we imagine the support of power lines in the form of a lattice structure. About 30 years ago, this was the only option, and even today they continue to be built. A set of metal corners is brought to the construction site and, step by step, the support is screwed from these typical elements. Then a crane arrives and sets the structure upright. Such a process takes a lot of time, which affects the timing of laying the lines, and these supports themselves with dull lattice silhouettes are very short-lived. The reason is poor corrosion protection. The technological imperfection of such a support is complemented by a simple concrete foundation. If it is made in bad faith, for example, using a solution of inadequate quality, then after some time the concrete will crack, water will fall into the cracks. Several cycles of freezing and thawing, and the foundation must be redone or seriously repaired.
Handsets instead of corners
We asked the representatives of PJSC Rosseti about what kind of alternative is replacing traditional supports made of ferrous metal. “At our company, which is the largest electric grid operator in Russia,” says the specialist of this organization, “we have long tried to find a solution to the problems associated with lattice supports, and in the late 1990s we began to switch to faceted supports. These are cylindrical racks from a bent profile, in fact pipes, in cross section having the appearance of a polyhedron. In addition, we began to apply new methods of corrosion protection, mainly the method of hot dip galvanizing. This is an electrochemical method of applying a protective coating to a metal. In an aggressive environment, the zinc layer becomes thinner, but the bearing part of the support remains unharmed. ”
Supports of electric overhead lines, we usually imagine it that way. However, the classic lattice design is gradually giving way to more advanced options - multifaceted supports and supports made of composite materials.
In addition to greater durability, the new supports are also characterized by ease of installation. No more corners need to be screwed up: the tubular elements of the future support are simply inserted into each other, then the connection is fixed. You can mount such a design eight to ten times faster than assembling a trellis one. The corresponding transformations have undergone and foundations. Instead of the usual concrete, the so-called pile piles began to be used. The structure sinks into the ground, a counterflange is attached to it, and the support itself is already placed on it. The estimated service life of such supports is up to 70 years, that is, approximately two times longer than that of trellised ones.
Why are the wires humming?
And the wires? They hang high above the ground and from a distance look like thick monolithic cables. In fact, high-voltage wires are retinue of wire. Conventional and universally used wire has a steel core, which provides structural strength and is surrounded by aluminum wire, the so-called external wires, through which the current load is transmitted. Lubricant is laid between steel and aluminum. It is needed in order to reduce friction between steel and aluminum, materials with different coefficients of thermal expansion. But since the aluminum wire has a circular cross-section, the coils are not tight to each other, the surface of the wire has a pronounced relief. This flaw has two consequences. Firstly, moisture penetrates into the gap between the turns and leaches the grease. Friction intensifies and conditions for corrosion are created. As a result, the service life of such a wire is no more than 12 years. To extend the service life, sometimes repair cuffs are put on the wire, which can also cause problems (more on this below). In addition, this design of the wire helps to create near the overhead line a clearly distinguishable hum. It occurs due to the fact that an alternating voltage of 50 Hz gives rise to an alternating magnetic field, which causes individual wires in the wire to vibrate, which entails their collisions with each other, and we hear a characteristic buzz. In EU countries, such noise is considered acoustic pollution, and they are combated. Now such a struggle has begun with us.
“We now want to replace the old wires with wires of a new design, which we are developing,” says the representative of Rosseti PJSC. - This is also steel-aluminum wire, but the wire there is used not of circular cross-section, but rather of a trapezoid. The twisting turns out to be dense, and the surface of the wire is smooth, without cracks. Moisture can hardly get inside, the grease is not washed out, the core does not rust, and the service life of such a wire is close to thirty years. Wires of a similar design are already used in countries such as Finland and Austria. There are lines with new wires in Russia - in the Kaluga region. This is the Orbit-Sputnik line 37 km long. Moreover, there the wires have not just a smooth surface, but also another core. It is not made of steel, but of fiberglass. Such a wire is lighter but tighter than ordinary steel-aluminum. "
However, the latest design achievement in this area can be considered a wire created by the American concern 3M. In these wires, the bearing capacity is provided only by conductive layers. There is no core, but the midwives themselves are reinforced with alumina, thereby achieving high strength. This wire has excellent load-bearing capacity, and with standard supports it can withstand spans of up to 700 m in length due to its strength and low weight (standard 250x300 m). In addition, the wire is very resistant to heat loads, which determines its use in the southern states of the USA and, for example, in Italy. However, the wire from 3M has one significant minus - the price is too high.
Original "designer" supports serve as an undoubted decoration of the landscape, however, they are unlikely to be widely used. Power grid companies prioritize reliability of power transmission rather than costly “sculptures”.
Ice and strings
Overhead power lines have their natural enemies. One of them is icing wires. This disaster is especially characteristic of the southern regions of Russia. At a temperature near zero, drops of drizzle fall on the wire and freeze on it. A crystal cap forms on the top of the wire. But this is only the beginning. The hat under its weight gradually turns the wire, substituting the other side for freezing moisture. Sooner or later, an ice clutch will form around the wire, and if the weight of the clutch exceeds 200 kg per meter, the wire will break and someone will be left without light. Rosseti has its own know-how in the fight against ice. A section of the line with icy wires is disconnected from the line, but connected to a direct current source. When using direct current, the ohmic resistance of the wire can be practically ignored and currents can be transmitted, say, two times stronger than the calculated value for alternating current. The wire heats up and the ice melts. Wires dump unnecessary load. But if there are repair couplings on the wires, then additional resistance arises, and then the wire may burn out.
Another enemy is high-frequency and low-frequency oscillations. An overhead line tensioned wire is a string that starts to vibrate at high frequency under the influence of wind. If this frequency coincides with the natural frequency of the wire and a combination of amplitudes occurs, the wire may break. To cope with this problem, special devices are installed on the lines - vibration dampers, which look like a cable with two weights. This design, which has its own oscillation frequency, upsets the amplitudes and dampens the vibration.
Such a harmful effect as “wire dancing” is associated with low-frequency vibrations. When a break occurs on the line (for example, due to the formation of ice), vibrations of the wires occur, which go the wave further, after several spans. As a result, five to seven supports making up the anchor span (the distance between two supports with a rigid wire attachment) may bend or even fall. A well-known means of combating “dancing” is the installation of interfacial spacers between adjacent wires. If there are spacers, the wires will mutually damp their vibrations. Another option is the use on the line of supports made of composite materials, in particular fiberglass. Unlike metal supports, the composite has the property of elastic deformation and can easily "win back" the vibrations of the wires, bending down and then restoring the vertical position. Such support can prevent the cascade fall of an entire section of the line.
In the photo, the difference between the traditional high-voltage wire and the wire of a new design is clearly visible. Instead of a round wire, a pre-deformed wire was used, and the composite core took the place of the steel core.
Of course, there are all sorts of unique cases associated with laying overhead lines. For example, when installing supports in flooded soil or in permafrost, conventional pile piles for the foundation will not work. Then screw piles are used, which are screwed into the ground like a screw in order to achieve the most solid foundation. A special case is the passage of power lines of wide water barriers. They use special high-rise supports, which weigh ten times more than usual and have a height of 250x270 m. Since the span can be more than two kilometers, a special wire with a reinforced core is used, which is additionally supported by a cargo cable. This is, for example, arranged for the transition of power lines through the Kama with a span of 2250 m.
A separate group of supports are structures designed not only to hold wires, but also to carry a certain aesthetic value, for example, support-sculptures. In 2006, Rosseti initiated a project to develop supports with an original design. There were interesting works, but their authors, designers, often could not appreciate the possibility and manufacturability of the engineering embodiment of these structures. In general, it must be said that the supports in which the artistic design is embedded, such as, for example, the support-figures in Sochi, are usually installed not on the initiative of network companies, but on the order of some third-party commercial or government organizations. For example, in the USA, a support in the form of the letter M, stylized as the logo of the McDonald's fast food chain, is popular.
Via popmech.ru & wiki