Complex technical power lines (PTL) are used to deliver electricity over long distances. On a national scale, they are strategically important facilities that are designed and built in accordance with SNiP and PUE.

These linear sections are classified into cable and overhead power lines, the installation and laying of which requires mandatory compliance with the design conditions and the installation of special structures.

Overhead power lines

Fig. 1 Overhead high-voltage transmission lines

The most common are overhead lines, the laying of which takes place in the open air using high-voltage poles, to which the wires are fixed using special fittings (insulators and brackets). Most often these are SC racks.

The structure of overhead transmission lines includes:

• supports for various voltages;
• bare wires of aluminum or copper;
• traverse, providing the required distance, excluding the possibility of contact of wires with support elements;
• insulators;
• ground loop;
• arresters and lightning rod.

The minimum sagging point of overhead lines is: 5 ÷ 7 meters in uninhabited areas and 6 ÷ 8 meters in settlements.

The following are used as high-voltage poles:

• metal structures that are effectively used in all climatic zones and with different loads. They are characterized by sufficient strength, reliability and durability. They are a metal frame, the elements of which are connected by means of bolted connections, which facilitate the delivery and installation of supports at the installation sites;
• reinforced concrete supports, which are the simplest type of structures that have good strength characteristics, are easy to install and install overhead lines on them. The disadvantages of installing concrete supports include - a certain effect of wind loads and soil characteristics on them;
• wooden poles, which are the least expensive to manufacture and have excellent dielectric characteristics. The low weight of wooden structures allows them to be quickly delivered to the installation site and easy to install. The disadvantage of these power transmission line supports is their low mechanical strength, which allows them to be installed only with a certain load and susceptibility to biological destruction processes (material decay).

The use of a particular design is determined by the magnitude of the voltage of the electrical network. It will be useful to have the skill to determine the voltage of the power line in appearance.

Overhead lines are classified:

1. for current - direct or alternating;
2. according to voltage ratings - for direct current with a voltage of 400 kilovolts and alternating current - 0.4 ÷ 1150 kilovolts.

Cable transmission lines

Fig. 2 Underground cable lines

Unlike overhead lines, cable lines are insulated and therefore are more expensive and reliable. This type of wire is used in places where the installation of overhead lines is impossible - in cities and towns with dense buildings, in the territories of industrial enterprises.

Cable power lines are classified:

1. by voltage - just like overhead lines;
2. by the type of insulation - liquid and solid. The first type is petroleum oil and the second is a cable sheath made of polymers, rubber and oiled paper.

Their distinctive features are the way of laying:

• underground;
• underwater;
• for structures that protect cables from atmospheric influences and provide a high degree of safety during operation.

Fig. 3 Laying of underwater power lines

Unlike the first two methods of laying cable power lines, the "construction" option provides for the creation of:

• cable tunnels, in which power cables are laid on special supporting structures, allowing installation work and line maintenance;
• cable ducts, which are buried structures under the floor of buildings in which the laying of cable lines takes place in the ground;
• cable shafts - vertical corridors with a rectangular cross-section, which provide access to power lines;
• cable floors, which are dry, technical space with a height of about 1.8 m;
• cable blocks consisting of pipes and wells;
• open type ramps - for horizontal or inclined cable laying;
• chambers used for laying couplings of power transmission lines;
• galleries - the same overpasses, only of a closed type.

Conclusion

Despite the fact that cable and overhead power lines are used everywhere, both options have their own characteristics, which should be taken into account in the design documentation, which determines

What power lines are

A network of power lines is necessary for the movement and distribution of electrical energy: from its sources, between settlements and final consumption objects. These lines are very diverse and are divided:

• by the type of placement of wires - air (located in the open air) and cable (closed in insulation);
• by designation - super-long-distance, trunk, distribution.

Overhead and cable power lines have a certain classification, which depends on the consumer, type of current, power, materials used.

Overhead power lines (VL)

These include lines that are laid outdoors above the ground using various supports. Separation of power lines is important for their selection and maintenance.

Distinguish lines:

• by the nature of the moved current - alternating and direct;
• by voltage level - low-voltage (up to 1000 V) and high-voltage (more than 1000 V) power lines;
• on the neutral - networks with a solidly grounded, isolated, effectively grounded neutral.

Alternating current

Electric lines using alternating current for transmission are introduced by Russian companies most often. With their help, systems are powered and energy is transferred over various distances.

D.C

Overhead power lines providing direct current transmission are rarely used in Russia. The main reason for this is the high cost of installation. In addition to supports, wires and various elements, they require the purchase of additional equipment - rectifiers and inverters.

Since most consumers use alternating current, when arranging such lines, additional resources have to be spent on energy conversion.

Overhead transmission line device

The device of overhead power lines includes the following elements:

• Support systems or electrical poles. They are placed on the ground or other surfaces and can be anchor (take the main load), intermediate (usually used to support wires in spans), angular (placed in places where the wire lines change direction).
• Wires. They have their own varieties, they can be made of aluminum, copper.
• Traverses. They are attached to the line supports and serve as the basis for wiring.
• Insulators. With their help, wires are mounted and isolated from each other.
• Grounding systems. The presence of such protection is necessary in accordance with the rules of the PUE (electrical installation rules).
• Lightning protection. Its use protects the overhead power line from the voltage that may occur when a discharge hits.

Each element of the electrical network plays an important role, taking on a certain load. In some cases, it may use additional equipment.

Cable power lines

Cable power lines under voltage, unlike overhead ones, do not require a large free area for placement. Due to the presence of insulating protection, they can be laid: on the territory of various enterprises, in settlements with dense buildings. The only drawback in comparison with overhead lines is the higher cost of installation.

Underground and underwater

The closure method allows you to place lines even in the most difficult conditions - underground and under the water surface. For their laying, special tunnels or other methods can be used. In this case, you can use several cables, as well as various fasteners.

Special security zones are established near electrical networks. According to the rules of the PUE, they must ensure safety and normal operating conditions.

Laying on structures

Laying of high-voltage power lines with different voltages is possible inside buildings. The most commonly used designs are:

• Tunnels. They are separate rooms, inside which cables are placed along the walls or on special structures. These spaces are well protected and provide easy access to line installation and maintenance.
• Channels. These are ready-made structures made of plastic, reinforced concrete slabs and other materials, inside which wires are located.
• Floor or mine. Premises specially adapted for the placement of power lines and the possibility of finding a person there.
• Overpass. They are open structures that are laid on the ground, foundations, support structures with wires attached inside. Closed overpasses are called galleries.
• Placement in free space of buildings - gaps, space under the floor.
• Cable block. Cables are laid underground in special pipes and brought to the surface using special plastic or concrete wells.

Insulation of cable power lines

The main condition when choosing materials for insulation of power lines is that they should not conduct current. Typically, the following materials are used in the device of cable power lines:

• rubber of synthetic or natural origin (it is distinguished by good flexibility, so lines made of such material are easy to lay even in hard-to-reach places);
• polyethylene (sufficiently resistant to the effects of a chemical or other aggressive environment);
• PVC (the main advantage of such insulation is its availability, although the material is inferior in resistance and various protective properties to others);
• fluoroplastic (highly resistant to various influences);
• paper-based materials (not resistant to chemical and natural influences, even if impregnated with a protective compound).

In addition to traditional solid materials, liquid insulators and special gases can be used for such lines.

Classification by purpose

Another characteristic by which the classification of power lines takes place, taking into account the voltage, is their purpose. It is customary to divide overhead lines into: super-long-distance, trunk, distribution. They differ depending on the power, the type of the receiver and the sender of the energy. These can be large stations or consumers - factories, settlements.

Ultra-long range

The main purpose of these lines is communication between different energy systems. The voltage in these overhead lines starts from 500 kV.

Trunk

This transmission line format assumes a voltage of 220 and 330 kV. Trunk lines provide transmission of energy from power plants to distribution points. They can also be used to connect various power plants.

Distribution

Distribution lines are 35, 110 and 150 kV. With their help, electric energy is transferred from distribution networks to settlements, as well as to large enterprises. Lines with a voltage of less than 20 kV are used to ensure the supply of energy to end users, including for connecting electricity to the site.

Construction and repair of power lines

The laying of networks of high-voltage cable power lines and overhead lines is a necessary way to provide energy to any objects. With their help, electricity is transmitted over any distance.

The construction of networks for any purpose is a complex process that includes several stages:

• Survey of the area.
• Line design, budgeting, technical documentation.
• Site preparation, selection and purchase of materials.
• Assembly of support elements or preparation for cable installation.
• Installation or laying of wires, suspension devices, strengthening of power lines.
• Landscaping and preparation of the line for launch.
• Commissioning, formalization of documentation.

To ensure the efficient operation of the line, its competent maintenance, timely repairs and, if necessary, reconstruction are required. All such activities must be carried out in accordance with the PUE (rules for technical installations).

Repair of electrical lines is divided into current and capital. During the first, the state of the system is monitored, work is performed to replace various elements. Overhaul involves more serious work, which may include replacement of supports, hauling of lines, replacement of entire sections. All types of work are determined depending on the condition of the power line.

How can you indicate the value of power lines? Is there a precise definition of the wires that carry electricity? There is a precise definition in the intersectoral rules for the technical operation of electrical installations of consumers. So, a power transmission line is, firstly, an electric line. Secondly, these are sections of wires that go beyond substations and power plants. Thirdly, the main purpose of power lines is the transmission of electric current over a distance.

According to the same rules of MPTEEP, power transmission lines are divided into overhead and cable ones. But it should be noted that high-frequency signals are also transmitted over power lines, which are used for transmitting telemetric data, for dispatching control of various industries, for emergency automation and relay protection signals. According to statistics, 60,000 high-frequency channels today pass through power lines. Let's face it, the indicator is significant.

Overhead transmission lines

Overhead power lines, they are usually denoted by the letters "VL" - these are devices that are located in the open air. That is, the wires themselves are laid through the air and fixed on special fittings (brackets, insulators). Moreover, their installation can be carried out on poles, and on bridges, and along overpasses. It is not necessary to count "overhead lines" those lines that are laid only along high-voltage poles.

What is included in overhead power lines:

• The main thing is the wires.
• Traverses, with the help of which conditions are created for the impossibility of contact of wires with other elements of the supports.
• Insulators.
• The supports themselves.
• Ground loop.
• Lightning arresters.
• Dischargers.

That is, a power line is not just wires and supports, as you can see, it is a rather impressive list of various elements, each of which carries its own specific load. Here you can add fiber optic cables, and ancillary equipment. Of course, if high-frequency communication channels are carried out along the power transmission line supports.

The construction of a power transmission line, as well as its design, plus the design features of the supports are determined by the rules for the installation of electrical installations, that is, PUE, as well as various building rules and regulations, that is, SNiP. In general, the construction of power lines is not an easy and very responsible business. Therefore, their construction is carried out by specialized organizations and companies, where the staff has highly qualified specialists.

Classification of overhead power lines

Sami overhead high-voltage power lines are divided into several classes.

By the nature of the current:

• Variable,
• Permanent.

Basically, overhead transmission lines are used to transmit alternating current. The second option is rare. Usually it is used to power the network by contact or communication to provide communication for several power systems, there are other types.

By voltage, overhead power lines are divided according to the nominal value of this indicator. For information, we list them:

• for alternating current: 0.4; 6; ten; 35; 110; 150; 220; 330; 400; 500; 750; 1150 kilovolts (kV);
• for constant, only one type of voltage is used - 400 kV.

At the same time, power lines with voltage up to 1.0 kV are considered to be of the lowest class, from 1.0 to 35 kV - medium, from 110 to 220 kV - high, from 330 to 500 kV - ultra-high, above 750 kV ultra-high. It should be noted that all these groups differ from each other only in the requirements for design conditions and design features. In all other respects, these are ordinary high-voltage power lines.

The voltage of the power lines corresponds to their purpose.

• High-voltage lines with voltage over 500 kV are considered ultra-long-distance, they are intended to connect separate power systems.
• High-voltage lines with a voltage of 220, 330 kV are considered trunk lines. Their main purpose is to interconnect powerful power plants, separate power systems, as well as power plants within these systems.
• Overhead transmission lines with a voltage of 35-150 kV are installed between consumers (large enterprises or settlements) and distribution points.
• Overhead lines up to 20 kV are used as power lines that directly supply electric current to the consumer.

Classification of power lines by neutral

• Three-phase networks in which the neutral is not grounded. Typically, such a scheme is used in networks with a voltage of 3-35 kV, where small currents flow.
• Three-phase networks in which the neutral is grounded through inductance. This is the so-called resonant-grounded type. In such overhead lines, a voltage of 3-35 kV is used, in which large currents flow.
• Three-phase networks in which the neutral bus is fully grounded (effectively grounded). This mode of neutral operation is used in overhead lines with medium and extra-high voltage. Please note that in such networks it is necessary to use transformers, and not autotransformers, in which the neutral is permanently grounded.
• And, of course, grounded neutral networks. In this mode, overhead lines operate with voltage below 1.0 kV and above 220 kV.

Unfortunately, there is also such a division of power lines, where the operational state of all elements of the power transmission line is taken into account. This is a transmission line in a normal state, where wires, supports and other components are in good condition. Basically, the emphasis is on the quality of wires and cables, they should not be cut off. An emergency condition where the quality of wires and cables is poor. And the installation state, when the repair or replacement of wires, insulators, brackets and other components of the power transmission line is carried out.

Elements of overhead transmission lines

There are always conversations between specialists in which special terms related to power lines are used. For the uninitiated in the intricacies of slang, it is quite difficult to understand this conversation. Therefore, we offer a decoding of these terms.

• The track is the axis of the power transmission line, which runs along the surface of the earth.
• PC - pickets. In fact, these are sections of the power transmission line. Their length depends on the terrain and on the nominal line voltage. Station zero is the beginning of the alignment.
• The construction of a support is indicated by a center sign. This is the center of the support installation.
• A picket is essentially a simple set up of pickets.
• Span is the distance between the supports, or rather, between their centers.
• The sag arrow is the delta between the lowest point of the wire sag and the strictly stretched line between the supports.
• The size of the wire is, again, the distance between the lowest point of the sag and the highest point of the engineering structures under the wires.
• Loop or loop. This is the part of the wire that connects the wires of adjacent spans on the anchor support.

Cable transmission lines

So, let's move on to considering such a thing as cable power lines. To begin with, these are not bare wires that are used in overhead power lines, they are cables that are enclosed in insulation. Typically, cable transmission lines are several lines installed next to each other in a parallel direction. The length of the cable for this is sometimes not enough, therefore, couplings are installed between the sections. By the way, it is not uncommon to find oil-filled cable power lines, therefore such networks are often equipped with special low-filling equipment and an alarm system that reacts to the oil pressure inside the cable.

If we talk about the classification of cable lines, then they are identical to the classification of overhead lines. There are distinctive features, but there are not so many of them. Basically, these two categories differ from each other in the way of laying, as well as in design features. For example, by the type of laying, cable power lines are divided into underground, underwater and by structures.

The first two positions are clear, but what refers to the position “on structures”?

• Cable tunnels. These are special closed corridors in which the cable is laid along the installed support structures. In such tunnels, you can walk freely, carrying out installation, repair and maintenance of the power line.
• Cable channels. Most often they are buried or partially buried channels. They can be laid in the ground, under the floor base, under the ceilings. These are small channels in which it is impossible to walk. To check or install the cable, you will have to dismantle the ceiling.
• Cable shaft. It is a vertical corridor with a rectangular cross-section. The mine can be a walk-through, that is, with the ability to fit into it for a person, for which it is supplied with a ladder. Or impassable. In this case, you can get to the cable line only by removing one of the walls of the structure.
• Cable floor. This is a technical space, usually 1.8 m high, equipped with floor slabs from below and from above.
• It is also possible to lay cable power lines in the gap between the floor slabs and the floor of the room.
• A cable block is a complex structure consisting of laying pipes and several wells.
• The chamber is an underground structure, closed on top with a reinforced concrete or slab. In such a chamber, the sections of the cable transmission line are connected by couplings.
• An overpass is a horizontal or inclined structure of an open type. It can be above-ground or above-ground, passable or impassable.
• The gallery is practically the same as the overpass, only of a closed type.

And the last classification in cable power lines is the type of insulation. In principle, there are two main types: solid insulation and liquid insulation. The first includes insulating polymeric sheaths (polyvinyl chloride, cross-linked polyethylene, ethylene-propylene rubber), as well as other types, for example, oiled paper, rubber-paper braid. Liquid insulators include petroleum oil. There are other types of insulation, for example, special gases or other types of solid materials. But they are rarely used today.

Conclusion on the topic

The variety of power lines is reduced to the classification of two main types: overhead and cable. Both options are used everywhere today, so you should not separate one from the other and give preference to one over the other. Of course, the construction of overhead lines is associated with large investments, because the laying of the route is the installation of mainly metal supports, which have a rather complex structure. This takes into account what network, under what voltage will be laid.

Transformers carry out a direct transformation of electricity - a change in voltage. Switchgears are used to receive electricity from the supply side of transformers (receiving switchgears) and to distribute electricity on the consumer side.

In the following chapters, the structural implementation of the main elements of power supply systems is considered, the main types and schemes of substations are given, and the basics of mechanical calculation of overhead power lines and bus structures are given.

1. Structures of overhead power lines

1.1. General information

By air line(VL) is a device for transmitting electricity through wires located in the open air and attached with insulators and fittings to the supports.

In fig. 1.1 shows a fragment of the overhead line. The distance l between adjacent supports is called a span. The vertical distance between the straight line connecting the suspension points of the wire and the lowest point of its sagging is called boom sag wire f NS . The distance from the lowest point of the wire sag to the ground is called overhead line size h G . A lightning protection cable is fixed in the upper part of the supports.

The size of the line dimension h g is regulated by the PUE, depending on the voltage of the overhead line and the type of terrain (populated, uninhabited, inaccessible). The length of the string of insulators λ and the distance between the wires of adjacent phases h p-p are determined by the rated voltage of the overhead line. The distance between the suspension points of the upper wire and the cable h p-t is regulated by the PUE based on the requirement for reliable protection of overhead lines from direct lightning strikes.

Conducting materials with high electrical conductivity (low resistance) and high mechanical strength are required to ensure economical and reliable transmission of power. In the structural elements of power supply systems, copper, aluminum, alloys based on them, and steel are used as such materials.

Rice. 1.1. Fragment of an overhead power line

Copper has low resistance and fairly high strength. Its specific active resistance is ρ = 0.018 Ohm. mm2 / m, and the ultimate tensile strength is 360 MPa. However, this is an expensive and scarce metal. Therefore, copper is used, as a rule, for making transformer windings, less often for cable cores and is practically not used for overhead lines.

The resistivity of aluminum is 1.6 times higher, the ultimate tensile strength is 2.5 times less than that of copper. The high prevalence of aluminum in nature and lower than that of copper, the cost led to its widespread use for overhead lines.

Steel has high resistance and high mechanical strength. Its specific active resistance is ρ = 0.13 Ohm. mm2 / m, and the ultimate tensile strength is 540 MPa. Therefore, in power supply systems, steel is used, in particular, to increase the mechanical strength of aluminum wires, to manufacture supports and lightning protection cables of overhead power lines.

1.2. Overhead lines and cables

Overhead lines are used directly for the transmission of electricity and differ in design and used conductive material. Most economically feasible

the material for overhead lines is aluminum and alloys based on it.

Copper wires for overhead lines are used extremely rarely and with an appropriate feasibility study. Copper wires are used in contact networks of mobile transport, in networks of special industries (mines, mines), sometimes when passing overhead lines near the seas and some chemical industries.

Steel wires for overhead lines are not used, since they have high active resistance and are susceptible to corrosion. The use of steel wires is justified when performing particularly large spans of overhead lines, for example, when crossing overhead lines through wide navigable rivers.

Wire cross-sections correspond to GOST 839-74. The scale of nominal cross-sections of overhead lines is the following row, mm2:

1,5; 2,5; 4; 6; 10; 16; 25; 35; 50; 70; 95; 120; 150; 185; 240; 300; 400; 500; 600; 700; 800; 1000.

According to their design, the overhead lines are divided into: single-wire;

multi-wire from one metal (monometallic); multi-wire of two metals; self-supporting isolated.

Solid wires, as the name suggests, is made of one wire (Fig. 1.2, a). Such wires are made of small cross-sections up to 10 mm2 and are sometimes used for overhead lines with voltage up to 1 kV.

Stranded monometallic wires are made with a cross section of more than 10 mm 2 ... These wires are made from individual wires twisted together. Around the central wire, a twist (row) of six wires of the same diameter is performed (Fig. 1.2, b). Each subsequent layer has six more wires than the previous one. The twisting of adjacent layers is performed in different directions to prevent unwinding of the wires and give the wire a more round shape.

The number of twigs is determined by the cross-section of the wire. Wires with a cross-section up to 95 mm2 are made with one bend, with a cross-section of 120 ... 300 mm2 - with two bends, a cross-section of 400 mm2 and more - with three or more bends. Stranded wires are more flexible than single-wire wires, convenient for installation, and reliable in operation.

Rice. 1.2. Constructions of bare wires overhead lines

To give the wire greater mechanical strength, stranded wires are made with a steel core 1 (Fig. 1.2, c, d, e). Such wires are called steel-aluminum. The core is made of galvanized steel wire and can be single-wire (Figure 1.2, c) and multi-wire (Figure 1.2, d). A general view of a steel-aluminum wire of large cross-section with a stranded steel core is shown in Fig. 1.2, d.

Steel-aluminum wires are widely used for overhead lines with voltage above 1 kV. These wires are available in various designs, differing in the ratio of the cross-sections of the aluminum and steel parts. For conventional steel-aluminum wires, this ratio is approximately six, for lightweight wires, eight, and for reinforced wires, four. When choosing one or another steel-aluminum wire, external mechanical loads on the wire, such as ice and wind, are taken into account.

The wires, depending on the material used, are marked as follows:

M - copper, A - aluminum,

АН, АЖ - from aluminum alloys (have greater mechanical strength than A-grade wire);

АС - steel-aluminum; ASO - steel-aluminum lightweight construction;

ACS - steel-aluminum reinforced structure.

The digital designation of the wire indicates its nominal cross-section. For example, A95 is an aluminum wire with a nominal cross section of 95 mm2. In the designation of steel-aluminum wires, the cross-section of the steel core may additionally be indicated. For example,

АСО240 / 32 is a steel-aluminum wire of lightweight construction with a nominal section of the aluminum part of 240 mm2 and a section of the steel core of 32 mm2.

Corrosion resistant aluminum wires of the AKP brand and steel-aluminum wires of the ASKP, ASKS, ASK brands have an inter-wire space filled with a neutral grease of increased heat resistance, which counteracts the appearance of corrosion. For AKP and ASKP wires, the entire inter-wire space is filled with such a grease, for the ASK wire only the steel core, for the ASK wire the steel core is filled with neutral grease and isolated from the aluminum part by two polyethylene tapes. AKP, ASKP, ASKS, ASK wires are used for overhead lines passing near seas, salt lakes and chemical plants.

Self-supporting insulated wires (SIP) are used for overhead lines with voltage up to 20 kV. At voltages up to 1 kV (Fig. 1.3, a), such a wire consists of three phase stranded aluminum conductors 1. The fourth conductor 2 is a carrier and at the same time zero. The phase conductors are twisted around the carrier in such a way that the entire mechanical load is absorbed by the carrier, made of durable aluminum alloy ABE.

Rice. 1.3. Self-supporting insulated wires

Phase isolation 3 is made from thermoplastic light stabilized or crosslinked light stabilized polyethylene... Due to its molecular structure, such insulation has very high thermomechanical properties and high resistance to solar radiation and the atmosphere. In some designs of self-supporting insulated wire, the zero bearing core is made with insulation.

The design of the self-supporting insulated wire for voltages above 1 kV is shown in Fig. 1.3, b. Such a wire is single-phase and consists of

current-carrying steel-aluminum core 1 and insulation 2 made of cross-linked light-stabilized polyethylene.

Overhead lines with self-supporting insulated wires have the following advantages in comparison with traditional overhead lines:

lower voltage losses (improvement of the quality of electricity), due to the smaller, approximately three times, reactive resistance of three-phase SIP;

do not require insulators; ice formation is practically absent;

allow suspension on one support of several lines of various voltages;

lower operating costs due to a reduction of approximately 80% in the volume of emergency and recovery work; the possibility of using shorter supports thanks to

shorter permissible distance from self-supporting insulated wire to the ground; reduction of the security zone, permissible distances to buildings and

structures, the width of the clearing in a wooded area; the practical absence of the possibility of a fire in

wooded areas when the wire falls to the ground; high reliability (5-fold reduction in the number of accidents by

compared with traditional overhead lines); complete protection of the conductor from moisture and

corrosion.

The cost of overhead lines with self-supporting insulated wires is higher than that of traditional overhead lines.

Overhead lines with a voltage of 35 kV and above are protected from a direct lightning strike lightning protection wire, fixed in the upper part of the support (see Fig. 1.1). Lightning protection cables are elements of overhead lines, similar in design to stranded monometallic wires. The cables are made of galvanized steel wires. The nominal wire cross-sections correspond to the scale of the nominal wire cross-sections. The minimum cross-section of the lightning protection wire is 35 mm2.

When using lightning protection cables as high-frequency communication channels, instead of a steel cable, a steel-aluminum wire with a powerful steel core is used, the cross-section of which is commensurate with or greater than the cross-section of the aluminum part.

1.3. Overhead line supports

The main purpose of the supports is to support the wires at the required height above the ground and ground structures. Supports consist of uprights, traverses and foundations. The main materials from which the supports are made are softwood, reinforced concrete and metal.

Supports made of wood easy to manufacture, transport and operate, are used for overhead lines with voltages up to 220 kV inclusive in areas of forestry or close to them. The main disadvantage of such supports is the susceptibility of wood to decay. To increase the service life of the supports, the wood is dried and impregnated with antiseptics that prevent the development of the decay process.

Due to the limited construction length of wood, the supports are made composite (Figure 1.4, a). Wooden rack 1 is articulated with metal bands 2 with a reinforced concrete attachment 3. The lower part of the attachment is buried in the ground. Supports corresponding to Fig. 1.4, a, are applied for voltages up to 10 kV inclusive. For higher stresses, wood supports are made U-shaped (portal). Such a support is shown in Fig. 1.4, b.

It should be noted that in modern conditions of the need to preserve forests, it is advisable to reduce the use of timber supports.

Reinforced concrete supports consist of a reinforced concrete rack 1 and a traverse 2 (Fig. 1.4, c). The rack is a hollow conical pipe with a small slope of the cone generatrix. The lower part of the rack is buried in the ground. Traverses are made of galvanized steel. These poles are more durable than timber poles, are easier to maintain, and require less metal than steel poles.

The main disadvantages of reinforced concrete supports: high weight, which makes it difficult to transport the supports to hard-to-reach places of the overhead line route, and relatively low concrete bending strength.

To increase the bending strength of the supports in the manufacture of a reinforced concrete rack, prestressed (stretched) steel reinforcement is used.

To ensure high concrete density in the manufacture of support racks, they use vibration compaction and centrifugation concrete.

Racks of overhead lines with voltage up to 35 kV are made of vibrated concrete, at higher voltages - of centrifuged concrete.

Rice. 1.4. Intermediate supports of overhead lines

Steel supports have high mechanical strength and long service life. These supports are assembled from separate elements by welding and bolted connections, therefore it is possible to create supports of almost any design (Figure 1.4, d). Unlike supports made of wood and reinforced concrete, metal supports are installed on reinforced concrete foundations 1.

Steel supports are expensive. In addition, steel is susceptible to corrosion. To increase the service life of the supports, they are coated with anti-corrosion compounds and painted. Hot-dip galvanized steel poles are very effective against corrosion.

Supports made of aluminum alloys effective in the construction of overhead lines in the conditions of hard-to-reach routes. Due to the resistance of aluminum to corrosion, these supports do not need a corrosion-resistant coating. However, the high cost of aluminum significantly limits the use of such supports.

When passing through a certain territory, an overhead line can change direction, cross various engineering

structures and natural barriers, connect to substation switchgear buses. In fig. 1.5 shows a top view of a fragment of the overhead line route. From this figure it can be seen that different supports work in different conditions and therefore must have a different design. By design, the supports are divided:

for intermediate(supports 2, 3, 7), installed on the straight section of the overhead line;

angular (support 4), installed at the bends of the overhead line route; end (supports 1 and 8), installed at the beginning and end of the overhead line; transitional (supports 5 and 6), installed in the span

crossing an overhead line of any engineering structure, such as a railroad.

Rice. 1.5. Fragment of the overhead line

Intermediate supports are designed to support wires on the straight section of the overhead line. The wires with these supports do not have a rigid connection, since they are fastened with insulators supporting strings. These supports are acted upon by the gravity forces of wires, cables, insulator strings, ice, as well as wind loads. Examples of intermediate supports are shown in Fig. 1.4.

The end supports are additionally influenced by the tensile force T of the wires and cables, directed along the line (Fig. 1.5). The angular supports are additionally affected by the tensile force T of wires and cables, directed along the bisector of the angle of rotation of the overhead line.

Transitional supports in the normal mode of overhead lines play the role of intermediate supports. These supports take on the tension of wires and cables when they are broken in adjacent spans and exclude unacceptable sagging of the wires in the crossing span.

End, corner and transition supports must be sufficiently rigid and must not deviate from the vertical

positions when exposed to the force of gravity of wires and cables. Such supports are made in the form of rigid spatial trusses or with the use of special cable ties and are called anchor supports... Wires with anchor supports have a rigid connection, as they are fastened with tension strings of insulators.

Rice. 1.6. Anchor corner supports VL

Anchor supports made of wood are made A-shaped at voltages up to 10 kV and AP-shaped at higher voltages. Reinforced concrete anchor supports have special cable ties (Fig. 1.6, a). Metal anchor supports have a wider base (lower part) than intermediate supports (Fig. 1.6, b).

By the number of wires suspended on one support, they are distinguished single-chain and double-chain supports... Three wires are suspended on single-circuit supports (one three-phase circuit), on double-circuit ones - six wires (two three-phase circuits). Single-chain supports are shown in Fig. 1.4, a, b, d and Fig. 1.6, a; double-circuit - in Fig. 1.4, c and fig. 1.6, b.

A double-chain support is cheaper than two single-chain ones. The reliability of electricity transmission over a double-circuit line is somewhat lower than over two single-circuit lines.

The double-chain timber supports are not manufactured. Supports of overhead lines with a voltage of 330 kV and above are manufactured only in a single-circuit design with a horizontal arrangement of wires (Fig. 1.7). Such supports are made U-shaped (portal) or V-shaped with cable braces.

Rice. 1.7. Supports of overhead lines with a voltage of 330 kV and above

Among the supports of overhead lines, supports are separately distinguished, having special design. These are branch, elevated and transposition supports. Branch supports are designed for intermediate power take-off from overhead lines. Raised supports are installed in large spans, for example, when crossing wide navigable rivers. On transpositional supports transposition of wires is carried out.

An asymmetrical arrangement of wires on supports with a long overhead line length leads to an asymmetry of the phase voltages. Balancing the phases by changing the relative position of the wires on the support is called transposition. Transposition is provided for overhead lines with a voltage of 110 kV and above, with a length of more than 100 km and is carried out on special transposition supports. The wire of each phase passes the first third of the length of the overhead line on one, the second third on the other and the third on the third place. This movement of wires is called a complete transposition cycle.

Overhead power line(VL) - a device designed for the transmission or distribution of electrical energy through wires with a protective insulating sheath (VLZ) or bare wires (VL), located in the open air and attached using traverses (brackets), insulators and linear fittings to supports or other engineering structures (bridges, overpasses). The main elements of overhead lines are:

• wires;
• protective cables;
• a support that supports wires and hummocks at a certain height above ground or water level;
• insulators, insulating wires from the body of the support;
• linear fittings.

Linear portals of switchgears are taken as the beginning and end of the overhead line. By design, overhead lines are divided into single-circuit and multi-value, usually 2-chain.

Usually, an overhead line consists of three phases, therefore, the supports of single-circuit overhead lines with voltages above 1 kV are designed to suspend three phase wires (one circuit) (Fig. 1), six wires are suspended on the supports of two-circuit overhead lines (two parallel circuits). If necessary, one or two lightning protection cables are suspended above the phase wires. On the supports of overhead lines of a distribution network with a voltage of up to 1 kV, from 5 to 12 wires are suspended to supply various consumers with one overhead line (external and internal lighting, power facilities, household loads). Overhead lines with voltage up to 1 kV with a dead-grounded neutral, in addition to phase ones, are equipped with a neutral wire.

Rice. 1. Fragments of 220 kV overhead lines:a - single-circuit; b - double-circuit

The wires of overhead power lines are mainly made from aluminum and its alloys, in some cases from copper and its alloys, made from cold-drawn wire with sufficient mechanical strength. However, the most widespread are two-metal stranded wires with good mechanical characteristics and relatively low cost. This type of wire includes steel-aluminum wires with the ratio of the cross-sectional areas of the aluminum and steel parts from 4.0 to 8.0. Examples of the location of phase wires and lightning protection cables are shown in Fig. 2, and the design parameters of the overhead line of the standard series of voltages are given in table. 1.

Rice. 2.: a - triangular; b - horizontal; в - hexagonal "barrel"; d - reverse "tree"

Table 1. Design parameters of overhead lines

Nominal overhead line voltage, kV	Distance between phase wires, m	Length span, m	Height	Overall dimensions
Less than 1	0,5	40 – 50	8 – 9	6 – 7
6 – 10	1,0	50 – 80	10	6 – 7
35	3	150 – 200	12	6 – 7
110	4 – 5	170 – 250	13 – 14	6 – 7
150	5,5	200 – 280	15 – 16	7 – 8
220	7	250 – 350	25 – 30	7 – 8
330	9	300 – 400	25 – 30	7,5 – 8
500	10 – 12	350 – 450	25 – 30	8
750	14 – 16	450 – 750	30 – 41	10 – 12
1150	12 – 19	–	33 – 54	14,5 – 17,5

For all the above options for the location of the phase wires on the supports, an asymmetrical arrangement of wires with respect to each other is characteristic. Accordingly, this leads to unequal reactance and conductivity of different phases due to mutual inductance between the line wires and, as a result, to phase voltage asymmetry and voltage drop.

In order to make the capacitance and inductance of all three phases of the circuit the same, wire transposition is used on the power line, i.e. mutually change their location relative to each other, with each phase wire passing one third of the way (Fig. 3). One such triple movement is called a transposition cycle.

Rice. 3. Scheme of a full cycle of transposition of sections of an overhead power transmission line: 1, 2, 3 - phase wires

The transposition of phase wires of an overhead power transmission line with bare wires is used for a voltage of 110 kV and above and with a line length of 100 km or more. One of the options for mounting wires on a transposition support is shown in Fig. 4. It should be noted that the transposition of conductive conductors is sometimes used in cable lines, in addition, modern technologies for the design and construction of overhead lines make it possible to technically implement the control of line parameters (controlled self-compensating lines and compact ultra-high voltage overhead lines).

Rice. 4.

Wires and protective cables of overhead lines in certain places must be rigidly fixed on the tension insulators of the anchor supports (end supports 1 and 7, installed at the beginning and end of the overhead line, as shown in Fig. 5 and stretched to a given tension. Between the anchor supports, intermediate supports are installed , necessary to maintain wires and cables, using supporting strings of insulators with supporting clamps, at a given height (supports 2, 3, 6), installed on the straight section of the overhead line; corner (supports 4 and 5), installed at the bends of the overhead line; transitional (supports 2 and 3), installed in the span of the intersection of an overhead line of any natural obstacle or engineering structure, for example, a railroad or highway.

Rice. 5.

The distance between the anchor supports is called the anchor span of the overhead power line (Fig. 6). The horizontal distance between the attachment points of the wire on adjacent supports is called the span length L ... A sketch of the overhead line span is shown in Fig. 7. The length of the span is chosen mainly for economic reasons, except for transition spans, taking into account both the height of the supports and the sagging of wires and cables, as well as the number of supports and insulators along the entire length of the overhead line.

Rice. 6.: 1 - a supporting garland of insulators; 2 - tension garland; 3 - intermediate support; 4 - anchor support

The smallest vertical distance from the ground to the wire with its greatest sagging is called the dimension of the line to the ground - h ... The line dimensions must be maintained for all rated voltages, taking into account the danger of overlapping the air gap between the phase conductors and the highest point of the terrain. It is also necessary to take into account the environmental aspects of the impact of high intensities of the electromagnetic field on living organisms and plants.

The greatest deviation of the phase conductor f n or lightning protection cable f t from the horizontal under the action of a uniformly distributed load from its own mass, ice mass and wind pressure is called a sag arrow. To prevent the wires from snagging, the cable sag arrow is made less than the wire sag by 0.5 - 1.5 m.

Structural elements of overhead lines, such as phase wires, cables, strings of insulators, have a significant mass, therefore, the forces acting on one support reaches hundreds of thousands of Newtons (N). The pulling forces on the wire from the weight of the wire, the weight of the tension strings of insulators and ice formations are directed along the normal downward, and the forces due to the wind pressure are directed along the normal to the side of the vector of the wind flow, as shown in Fig. 7.

Rice. 7.

In order to reduce the inductive resistance and increase the transmission capacity of long-distance transmission lines, various versions of compact power lines are used, a characteristic feature of which is a reduced distance between the phase wires. Compact power lines have a narrower spatial corridor, a lower level of electric field strength at ground level and allow technically realizing control of line parameters (controlled self-compensating lines and lines with unconventional split phase configuration).

2. Cable power line

Cable power line (KL) consists of one or more cables and cable fittings for connecting cables and for connecting cables to electrical devices or switchgear buses.

Unlike overhead lines, cables are laid not only outdoors, but also indoors (Fig. 8), in land and water. Therefore, CRs are susceptible to moisture, chemical aggressiveness of water and soil, mechanical damage during earthworks and soil displacement during heavy rains and floods. The design of the cable and structures for laying the cable must provide for protection against these influences.

Rice. eight.

According to the value of the rated voltage, the cables are divided into three groups: cables low voltage(up to 1 kV), cables medium voltage(6 ... 35 kV), cables high voltage(110 kV and above). By the nature of the current they distinguish AC and DC cables.

Power cables are carried out single-core, two-core, three-core, four-core and five-core. High voltage cables are made single-core; two-core - DC cables; three-core - medium voltage cables.

Low voltage cables are available with up to five cores. Such cables can have one, two or three phase conductors, as well as a neutral working conductor. N and zero protective conductor PE or combined zero working and protective conductor PEN .

According to the material of the conductive cores, cables are distinguished with aluminum and copper conductors. Due to the scarcity of copper, cables with aluminum conductors are most widely used. Used as an insulating material cable paper impregnated with oil-rosin composition, plastic and rubber. A distinction is made between cables with normal impregnation, depleted impregnation and impregnation with a non-dripping compound. Cables with depleted or non-dripping impregnation are laid along a route with a large height difference or along vertical sections of the route.

High voltage cables are carried out oil-filled or gas-filled. In these cables, the paper insulation is filled with oil or gas under pressure.

Protection of insulation from drying out and ingress of air and moisture is ensured by imposing a hermetically sealed jacket on the insulation. The cable is protected from possible mechanical damage by armor. An outer protective cover serves to protect against the aggressiveness of the external environment.

When studying cable lines, it is advisable to note superconducting cables for power lines the design of which is based on the phenomenon of superconductivity. In a simplified form, the phenomenon superconductivity in metals can be represented as follows. Coulomb repulsive forces act between electrons as between like charged particles. However, at ultra-low temperatures for superconducting materials (and these are 27 pure metals and a large number of special alloys and compounds), the nature of the interaction of electrons with each other and with the atomic lattice changes significantly. As a result, it becomes possible to attract electrons and form the so-called electron (Cooper) pairs. The emergence of these pairs, their increase, the formation of "condensate" of electron pairs and explains the appearance of superconductivity. As the temperature rises, some of the electrons are thermally excited and pass into a single state. At a certain so-called critical temperature, all electrons become normal and the state of superconductivity disappears. The same happens when the tension rises. magnetic onla... The critical temperatures of superconducting alloys and compounds used in technology are 10 - 18 K, i.e. from –263 to –255 ° С.

The first projects, experimental models and prototypes of such cables in flexible corrugated cryostatting sheaths were realized only in the 70-80s of the XX century. As a superconductor, tapes based on an intermetallic compound of niobium with tin, cooled with liquid helium, were used.

In 1986, the phenomenon was discovered high temperature superconductivity, and already at the beginning of 1987, conductors of this kind were obtained, which are ceramic materials, the critical temperature of which was increased to 90 K. The approximate composition of the first high-temperature superconductor YBa 2 Cu 3 O 7 – d (d< 0,2). Такой сверхпроводник представляет собой неупорядоченную систему мелких кристаллов, имеющих размер от 1 до 10 мкм, находящихся в слабом электрическом контакте друг с другом. К концу XX века были начаты и к этому времени достаточно продвинуты работы по созданию сверхпроводящих кабелей на основе высокотемпературных сверхпроводников. Такие кабели принципиально отличаются от своих предшественников. Жидкий азот, применяемый для охлаждения, на несколько порядков дешевле гелия, а его запасы практически безграничны. Очень важным является то, что жидкий азот при рабочих давлениях 0,8 - 1 МПа является прекрасным диэлектриком, превосходящим по своим свойствам пропиточные составы, используемые в традиционных кабелях.

Feasibility studies show that high-temperature superconducting cables will be more efficient in comparison with other types of power transmission even with a transmitted power of more than 0.4 - 0.6 GVA, depending on the actual object of application. In the future, high-temperature superconducting cables are supposed to be used in the power industry as conductors at power plants with a capacity of over 0.5 GW, as well as deep lead-ins into megacities and large energy-intensive complexes. At the same time, it is necessary to realistically assess the economic aspects and a full range of works to ensure the reliability of such cables in operation.

However, it should be noted that during the construction of new and reconstruction of old cable lines, it is necessary to be guided by the provisions of PJSC "Rosseti", according to which it is prohibited to use :

• power cables that do not meet current fire safety requirements and emit large concentrations of toxic products during combustion;
• oil-insulated and oil-filled cables;
• cables made using the silanol crosslinking technology (silanol crosslinkable compositions contain grafted organofunctional silane groups, and crosslinking of the molecular chain of polyethylene (PE), leading to the formation of a spatial structure, in this case occurs due to the silicon-oxygen-silicon (Si-O-Si) bond , and not carbon-carbon (C-C), as is the case with peroxide crosslinking).

Cable products, depending on designs, are subdivided into cables , wires and cords .

Cable- a completely ready-to-use factory electrical product, consisting of one or more insulated conductive cores (conductors), enclosed, as a rule, in a metallic or non-metallic sheath, over which, depending on the conditions of installation and operation, there may be an appropriate protective cover, which includes may include armor. Power cables, depending on the voltage class, have from one to five aluminum or copper conductors with a cross section of 1.5 to 2000 mm 2, of which up to 16 mm 2 are single-wire, more than multi-wire.

The wire- one uninsulated or one or more insulated conductors, over which, depending on the laying and operating conditions, there may be a non-metallic sheath, winding and (or) braiding with fibrous materials or wire.

Cord- two or more insulated or highly flexible conductors with a cross section of up to 1.5 mm 2, twisted or laid in parallel, over which, depending on the conditions of installation and operation, a non-metallic sheath and protective coatings can be applied.