How are power transmission towers constructed? The movement of electrons why high-voltage wires hum

Most often we imagine a power line support 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, a support is screwed together from these standard elements. Then a crane arrives and places the structure vertically. This process takes quite a lot of time, which affects the timing of laying 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 done in bad faith, for example using a solution of poor quality, then after some time the concrete will crack and water will get into the cracks. Several freeze-thaw cycles, and the foundation needs to be redone or seriously repaired.

Tubes instead of corners

We asked representatives of Rosseti PJSC about what kind of alternative is replacing traditional ferrous metal supports. “In our company, which is the largest power grid operator in Russia,” says a specialist from 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 made of a bent profile, actually pipes, in cross section having the shape of a polyhedron. In addition, we began to use new methods of anti-corrosion protection, mainly the hot-dip galvanizing method. This is an electrochemical method of applying a protective coating to metal. In an aggressive environment, the zinc layer becomes thinner, but the load-bearing part of the support remains unharmed.”

In addition to greater durability, the new supports are also easier to install. There is no need to screw together any more corners: the tubular elements of the future support are simply inserted into each other, then the connection is secured. You can assemble such a structure eight to ten times faster than assembling a lattice structure. The foundations also underwent corresponding transformations. Instead of conventional concrete, they began to use so-called shell piles. The structure is lowered into the ground, a counter flange is attached to it, and the support itself is placed on it. The estimated service life of such supports is up to 70 years, that is, approximately twice as long as that of lattice supports.

This is how we usually imagine electrical overhead line supports. However, the classic lattice design is gradually giving way to more progressive options - multifaceted supports and supports made of composite materials.

Why are the wires humming?

What about the wires? They hang high above the ground and from a distance look like thick monolithic cables. In fact, high-voltage wires are twisted from wire. A common and widely used wire has a steel core, which provides structural strength and is surrounded by aluminum wire, the so-called outer layers, through which the current load is transmitted. There is a lubricant between the steel and aluminum. It is needed in order to reduce friction between steel and aluminum - materials that have different coefficients of thermal expansion. But since aluminum wire has a round cross-section, the turns do not fit tightly against each other, and the surface of the wire has a pronounced relief. This deficiency has two consequences. Firstly, moisture penetrates into the cracks between the turns and washes away the lubricant. Friction increases 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, repair cuffs are sometimes put on the wire, which can also cause problems (more on this below). In addition, this wire design helps create a clearly visible hum near the overhead line. It occurs due to the fact that an alternating voltage of 50 Hz creates an alternating magnetic field, which causes the individual cores in the wire to vibrate, which causes them to collide with each other, and we hear a characteristic hum. In EU countries, such noise is considered acoustic pollution and is dealt with. Now such a struggle has begun among us.

“We now want to replace the old wires with wires of a new design that we are developing,” says a representative of Rosseti PJSC. — These are also steel-aluminum wires, but the wire used there is not round, but rather trapezoidal. The layering is dense, and the surface of the wire is smooth, without cracks. Moisture almost cannot get inside, the lubricant is not washed out, the core does not rust, and the service life of such a wire is approaching 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, the wires there have not just a smooth surface, but also a different core. It is made not of steel, but of fiberglass. This wire is lighter, but more tensile strength than conventional steel-aluminum wire.”

However, the most recent design achievement in this area can be considered the wire created by the American concern 3M. In these wires, the load-bearing capacity is provided only by conductive layers. There is no core, but the layers themselves are reinforced with aluminum oxide, which achieves high strength. This wire has excellent load-bearing capacity, and with standard supports, due to its strength and low weight, it can withstand spans up to 700 m long (standard 250-300 m). In addition, the wire is very resistant to thermal stress, which determines its use in the southern states of the USA and, for example, in Italy. However, the 3M wire has one significant drawback - the price is too high.

Original “designer” supports serve as an undoubted decoration of the landscape, but they are unlikely to become widespread. Electric grid companies prioritize reliable energy transmission rather than expensive “sculptures.”

Ice and strings

Overhead power lines have their natural enemies. One of them is icing of wires. This disaster is especially typical for the southern regions of Russia. At temperatures around 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 just the beginning. The cap, under its weight, gradually rotates the wire, exposing the other side to the freezing moisture. Sooner or later, an ice sleeve will form around the wire, and if the weight of the sleeve exceeds 200 kg per meter, the wire will break and someone will be left without light. The Rosseti company has its own know-how to deal with ice. The 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 carry currents, say, twice as strong as the calculated value for alternating current. The wire heats up and the ice melts. Wires shed unnecessary weight. But if there are repair couplings on the wires, then additional resistance arises, and then the wire may burn out.

Another enemy is high- and low-frequency vibrations. A stretched wire in an overhead line is a string that, when exposed to wind, begins to vibrate at a high frequency. If this frequency coincides with the natural frequency of the wire and the amplitudes combine, 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 vibration frequency, detunes the amplitudes and dampens vibration.

Low-frequency vibrations are associated with such a harmful effect as “wire dancing”. When a break occurs on the line (for example, due to ice formed), vibrations of the wires occur, which travel further in a wave, through several spans. As a result, five to seven supports that make up the anchor span (the distance between two supports with rigid wire fastening) may bend or even fall. A well-known means of combating “dancing” is to install interphase spacers between adjacent wires. If there is a spacer, the wires will mutually cancel out their vibrations. Another option is to use supports on the line made of composite materials, in particular fiberglass. Unlike metal supports, composite supports have the property of elastic deformation and can easily “play out” the vibrations of the wires by bending and then restoring the vertical position. Such a support can prevent a cascading fall of an entire section of line.

The photo clearly shows the difference between the traditional high-voltage wire and the new wire design. Instead of round wire, pre-deformed wire was used, and a composite core took the place of the steel core.

Unique supports

Of course, there are various unique cases associated with the laying of overhead lines. For example, when installing supports in waterlogged soil or in permafrost conditions, conventional shell piles are not suitable for the foundation. Then screw piles are used, which are screwed into the ground like a screw to achieve the strongest possible foundation. A special case is the passage of power lines across wide water obstacles. They use special high-rise supports, which weigh ten times more than usual and have a height of 250-270 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 load cable. This is how, for example, the crossing of a power line across the Kama with a span of 2250 m is arranged.

A separate group of supports are structures designed not only to hold wires, but also to carry a certain aesthetic value, for example, sculpture supports. In 2006, the Rosseti company initiated a project with the goal of developing supports with an original design. There were interesting works, but their authors, designers, often could not assess the feasibility and manufacturability of the engineering implementation of these designs. In general, it must be said that supports in which artistic design is embedded, such as the figure supports in Sochi, are usually installed not on the initiative of network companies, but by 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 fast food chain McDonald's, is popular.

Why are power line wires humming? Have you ever thought about this? But the answer to this question may be by no means trivial, although quite simple-minded. Let's look at several explanation options, each of which has a right to exist.

Corona discharge

This is the idea most often given. An alternating electric field near a power line wire electrifies the air around the wire, accelerates free electrons, which ionize air molecules, and they in turn generate. And so, 100 times per second, the corona discharge around the wire lights up and goes out, while the air near the wire heats up - cools down, expands - contracts, and in this way a sound wave is produced in the air, which is perceived by our ear as the buzzing of the wire.

The veins vibrate

There is also this idea. The noise comes from the fact that alternating current with a frequency of 50 Hz creates an alternating magnetic field, which forces individual cores in the wire (especially steel ones - in wires of the AC-75, 120, 240 types) to vibrate, they seem to collide with each other, and we hear a characteristic noise.

In addition, wires of different phases are located next to each other, their currents are in each other’s magnetic fields, and according to Ampere’s law, forces act on them. Since the frequency of field changes is 100 Hz, the wires vibrate in each other’s magnetic fields from Ampere forces at this frequency, and we hear it.

Resonance of a mechanical system

And such a hypothesis is found in some places. Vibrations with a frequency of 50 or 100 Hz are transmitted to the support, and under certain conditions the support, entering resonance, begins to emit sound. The volume and resonant frequency are affected by the density of the support material, the diameter of the support, the height of the support, the length of the wire in the span, as well as its cross-section and tension force. If there is a hit in resonance, noise is heard. If there is no resonance, there is no noise or it is quieter.

Vibration in the Earth's magnetic field

Let's consider another hypothesis. The wires vibrate at a frequency of 100 Hz, which means that they are constantly subject to a variable transverse force associated with the current in the wires, its magnitude and direction. Where is the external magnetic field? Hypothetically, it could be that magnetic field that is always underfoot, which orients the compass needle - .

Indeed, the currents in the wires of high-voltage power lines reach an amplitude of several hundred amperes, while the length of the wires of the lines is considerable, and the magnetic field of our planet, although relatively small (its induction in central Russia is only about 50 μT), nevertheless it acts everywhere across the planet, and everywhere it has not only a horizontal, but also a vertical component, which crosses perpendicularly both power transmission line wires laid along the Earth's magnetic field lines, and those wires that are oriented across them or at any other angle.

To understand the process, everyone can conduct this simple experiment: take a car battery and a flexible speaker wire with a cross-section of 25 sq. mm and at least 2 meters long. Connect it momentarily to the battery terminals. The wire will jump! What is this if not an impulse of the Ampere force acting on a current-carrying wire in the Earth's magnetic field? Unless the wire jumped in its own magnetic field...

Breathes coolness

There is an evening wind, and it rustles in the leaves

And the branches sway

And he kisses the harp... But the harp is silent...

And suddenly. .. out of silence

A long, thoughtful ringing sound arose.

V. Zhukovsky

Aeolian harp

Even the ancient Greeks noticed that a string stretched in the wind sometimes begins to sound melodious - to sing. Perhaps even then the Aeolian harp was known, named after the wind god Aeolus. The Aeolian harp consists of a frame on which several strings are stretched; it is placed in a place where the strings are blown by the wind. Even if you limit yourself to one string, you can get a whole range of different tones. Something similar, but with much less variety of tones, occurs when the wind sets telegraph wires in motion.

For quite a long time, this and many other phenomena associated with the flow of air and water around bodies were not explained. Only Newton, the founder of modern mechanics, provided the first scientific approach to solving such problems.

According to the law of resistance to the movement of bodies in a liquid or gas, discovered by Newton, the resistance force is proportional to the square of the speed:

Here is the speed of the body, is the area of its cross-section perpendicular to the direction of speed, and is the density of the liquid.

Later it turned out that Newton's formula is not always correct. In the case when the speed of movement of a body is small compared to the speed of thermal movement of molecules, Newton's law of resistance is no longer valid. As we have already discussed in previous sections, when a body moves sufficiently slowly, the drag force is proportional to its speed (Stokes' law), and not to its square, as is the case with fast movement. This situation arises, for example, when small drops of rain move in a cloud, when sediment settles in a glass, or when drops of substance A move in the “Magic Lamp”. However, in modern technology with its rapid speeds, Newton's law of resistance is usually valid.

It would seem that since the laws of resistance are known, the humming of wires or the singing of an aeolian harp can be explained. But that's not true. After all, if the resistance force were constant (or grew with increasing speed), then the wind would simply pull the string, and not excite its sounds.

What's the matter? To explain the sound of a string, it turns out that the simple ideas about the force of resistance that we have just discussed are not enough. Let's discuss in more detail some types of fluid flow around a stationary body (this is more convenient than considering the movement of a body in a stationary fluid, and the answer, of course, will be the same). Look at fig. 17.1. This is the case of low fluid velocity. The fluid flow lines go around the cylinder (the figure shows a cross-section) and smoothly continue behind it. This flow is called laminar. The resistance force in this case owes its origin to internal friction in the fluid (viscosity) and is proportional to the velocity of the fluid at any location, as well as the resistance force, does not depend on time (stationary flow). This case is of no interest to us.

Rice. 17.1: Lines of slow laminar flow around a cylindrical wire.

But look at fig. 17.2. The flow speed increased, and liquid whirlpools - vortices - appeared in the area behind the cylinder. Friction in this case no longer completely determines the nature of the process. More and more

Changes in momentum begin to play a role, occurring not on a microscopic scale, but on a scale comparable to the size of the body. The resistance force becomes proportional

Rice. 17.2: At high speeds, vortices appear behind the wire.

And finally, in Fig. 17.3 the flow speed increased even more, and the vortices lined up in regular chains. Here it is, the key to explaining the riddle! These chains of vortices, periodically breaking off from the surface of the string, excite its sound, just as the periodic touch of a musician’s fingers causes the sound of guitar strings.

Rice. 17.3: In fast flows behind a streamlined body, a periodic chain of vortices is formed.

The phenomenon of the correct arrangement of vortices behind a streamlined body was first studied experimentally by the German physicist Benard at the beginning of our century. But only thanks to the work of Karman that soon followed, this trend, which at first seemed very peculiar, received an explanation. After the name of this scientist, the system of periodic vortices is now called the Karman track.

As the velocity increases further, the vortices have less and less time to spread out over a larger area of fluid. The vortex zone becomes narrow, the vortices mix, and the flow

becomes chaotic and irregular (turbulent). True, at very high speeds, recent experiments have revealed the appearance of some new periodicity, but its details are still not clear.

It may seem that the Karman vortex street is just a beautiful natural phenomenon with no practical significance. But that's not true. Power line wires also oscillate under the influence of wind blowing at a constant speed due to eddy shedding. In places where wires are attached to supports, significant forces arise, which can lead to destruction. Tall chimneys sway in the wind.

Rice. 17.4: The rocking of vibrations by turbulent vortices led in 1940 to the destruction of the Tacoma Bridge in the USA.

However, the vibrations of the Tacoma Bridge in America are undoubtedly the most widely known. This bridge stood for only a few months and collapsed on November 7, 1940. In Fig. Figure 17.4 shows the view of the bridge during vibrations. The vortices came off the supporting structure of the bridge roadway. After lengthy research, the bridge was erected again, only the surfaces blown by the wind had a different shape. Thus, the reason that caused the bridge to oscillate was eliminated.

The evening wind breathes coolness there, and rustles in the leaves, and sways the branches, and kisses the harp... But the harp is silent... ......................... ............ And suddenly... out of the silence A long, thoughtful ringing rose.

V. Zhukovsky. "Aeolian Harp"

For quite a long time, this phenomenon and many others associated with the flow of air and water around bodies were not explained. Only Newton, the founder of modern mechanics, provided the first scientific approach to solving such problems.

According to the law of resistance to the movement of bodies in a liquid or gas, discovered by Newton, the resistance force is proportional to the square of the speed:

F = Kρv 2 S.

Here v is the speed of the body, S is the area of its cross-section perpendicular to the direction of speed, ρ is the density of the liquid.

As we have already discussed in previous sections, when a body moves sufficiently slowly, the drag force is proportional to its speed (Stokes' law), and not to its square, as is the case with fast movement. This situation arises, for example, when small drops of rain move in a cloud, when sediment settles in a glass, or when drops of substance A move in the “Magic Lamp”. However, in modern technology with its rapid speeds, Newton's law of resistance is usually valid.

Look at fig. 1. This is the case of low fluid velocity. The fluid flow lines go around the cylinder (the figure shows a cross-section) and smoothly continue behind it. Such a flow is called laminar. The resistance force in this case owes its origin to internal friction in the fluid (viscosity) and is proportional to v. The speed of the fluid at any place, as well as the resistance force, does not depend on time (flow stationary). This case is of no interest to us.

But look at fig. 2. The flow speed increased, and liquid whirlpools - vortices - appeared in the area behind the cylinder. Friction in this case no longer completely determines the nature of the process. Changes in momentum, occurring not on a microscopic scale, but on a scale comparable to the size of the body, begin to play an increasingly important role. The resistance force becomes proportional to v 2 .

And finally, in Fig. 3, the flow speed increased even more, and the vortices lined up in regular chains. Here it is, the key to explaining the riddle! These chains of vortices, periodically breaking off from the surface of the string, excite its sound, just as the periodic touch of a musician’s fingers causes the sound of guitar strings.

As the velocity increases further, the vortices have less and less time to spread out over a larger area of fluid. The vortex zone becomes narrow, the vortices mix, and the flow becomes chaotic and irregular ( turbulent). True, at very high speeds, recent experiments have revealed the appearance of some new periodicity, but its details are still not clear.

However, the vibrations of the Tacoma Bridge in America are undoubtedly the most widely known. This bridge stood for only a few months and collapsed on November 7, 1940. In Fig. Figure 4 shows a view of the bridge during oscillations. The vortices came off the supporting structure of the bridge roadway. After lengthy research, the bridge was erected again, only the surfaces blown by the wind had a different shape. Thus, the reason causing the vibrations of the bridge was eliminated.

chicco - conducted a standard two-rank examination of the ear using S. Shumakov’s method of radiating surfaces? Which surfaces conditionally emit more - sometimes you can find the direction of the search this way.. Especially if you go through such a standard INVESTIGATION on the walls of the corridor, staircase and the floor above and below.
NOT always - but sometimes you can determine the approximate direction.. But - not always.. Closed volumes and resonant distortions often mask the picture of the intensity distribution.
And - you didn’t specify a little - the whistle has a sound character (from a pulsed IP, for example, which often occurs), or - a low-frequency hum (harmonics in both the midrange and high-frequency - but excitation from 50-60 Hz)

Oleg Perfilov wrote:

Still, apparently the problem is not in the cable itself, the cable cannot hum, but the point is that apparently the electricians installed powerful starters or chokes for street lighting lamps.

I have heard more than once the hum of a decrepit starter that supplies power to several 150-500 watt halogen lamps. This is not a weak sound from a magnetic starter - a powerful nasty buzz. And if such starters stand RIGIDLY on surfaces close to the top starter’s apartment, then all sorts of resonant coincidences are possible..
It is likely that if the starters are on one of the surfaces, they are attached. especially if the old stuff or their cores are loose (as in some trances.)
However, this is only a version.. Based on the fact that only THESE circuits are the source (not air conditioners, water pumping motors, store or home ventilation, etc.. Based on the irrefutability and evidence of the observation -

chicco wrote:

I have discovered a pattern: when the entrance lights are turned on, for the entire the period of their glow until the moment of switching off There is a high-frequency hum in the apartment. .

But on the ZI forums there are sounds from the block starters elevator motors hanging on the walls of the engine compartment room - quite excited sound vibrations in the apartments below the floor (according to reviews)
I have also heard more than once how semi-functional(!) chokes of low-power LDS lamps hum and vibrate (those 16-20 watts that are still widespread in the form of long and shorter lamps under the ceiling. (An interesting case - having removed the protective grille, I hit the metal tray lamp for two LDS under the ceiling - the resonant opposite disappeared. It turns out that something in the metal sheets also influenced it..."tension - in the sense of freedom of vibration")
So your version, Oleg, is completely objective.
After all, the topicstarter did not write what floor it is on, where the starters are located (and chokes - if LDS - lamps .., what types of lamps and ballasts, etc.)
...If the lamps are not powered by 220-V - I don’t know - standard power supplies for 12-volt halogen lamps have not heard their noisy operation - the simplest pulse power supply units immediately fail, just as I don’t know how other types hum lamps and PRU with 12(!)-volt power supply. I will not lie)
Above is the version..
Not being familiar with the power supply system, one can also assume that the top starter is on the FIRST floor - and it has resonant coincidences from the transformer in the nearest room - three-phase imbalances below, which arise when the lamps are turned on, etc. Although - it always seemed to me that on interior entrance lamps, unlike street lamps, do not have a lot of power. And it’s hard to imagine the impact of the connected [b]small power in such a consequence. However, having some knowledge in electronics, I am not an expert in electrical engineering, three-phase power supply, etc., and even more so in MKD input-supply circuits)
(Contacting the RPN with a complaint about excessive noise at NIGHT (!!) time (standards for the night are stricter!) can be beneficial?)