home · Networks · A power line is a wire or cable line for transmitting electricity. Cable power lines Power lines interpretation

A power line is a wire or cable line for transmitting electricity. Cable power lines Power lines interpretation

The main elements of overhead lines are wires, insulators, linear fittings, supports and foundations. On overhead lines of three-phase alternating current, at least three wires are suspended, constituting one circuit; on direct current overhead lines - at least two wires.

Based on the number of circuits, overhead lines are divided into single, double and multi-circuit. The number of circuits is determined by the power supply circuit and the need for its redundancy. If the power supply scheme requires two circuits, then these circuits can be suspended on two separate single-circuit overhead lines with single-circuit supports or on one double-circuit overhead line with double-circuit supports. The distance / between adjacent supports is called the span, and the distance between anchor-type supports is called the anchor section.

Wires suspended on insulators (A, - the length of the garland) to the supports (Fig. 5.1, a) sag along the catenary line. The distance from the suspension point to the lowest point of the wire is called the sag /. It determines the clearance of the wire approaching the ground A, which for populated areas is equal to: to the surface of the earth up to 35 and PO kV - 7 m; 220 kV - 8 m; to buildings or structures up to 35 kV - 3 m; 110 kV - 4 m; 220 kV - 5 m. Span length / is determined by economic conditions. The span length up to 1 kV is usually 30...75 m; PO kV - 150…200 m; 220 kV - up to 400 m.

Types of power transmission towers

Depending on the method of hanging the wires, the supports are:

  1. intermediate, on which the wires are secured in supporting clamps;
  2. anchor type, used for tensioning wires; on these supports the wires are secured in tension clamps;
  3. corner ones, which are installed at the angles of rotation of overhead lines with wires suspended in supporting clamps; they can be intermediate, branch and corner, end, anchor corner.

On a larger scale, overhead line supports above 1 kV are divided into two types: anchor ones, which fully support the tension of wires and cables in adjacent spans; intermediate, not perceiving the tension of the wires or perceiving partially.

On overhead lines, wooden supports are used (Fig. 5L, b, c), new generation wooden supports (Fig. 5.1, d), steel (Fig. 5.1, e) and reinforced concrete supports.

Wooden overhead line supports

Wooden overhead line poles are still common in countries with forest reserves. The advantages of wood as a material for supports are: low specific gravity, high mechanical strength, good electrical insulating properties, natural round assortment. The disadvantage of wood is its rotting, to reduce which antiseptics are used.

An effective method of combating rot is impregnation of wood with oily antiseptics. In the USA there is a transition to laminated wood supports.

For overhead lines with voltages of 20 and 35 kV, on which pin insulators are used, it is advisable to use single-column candle-shaped supports with a triangular arrangement of wires. On overhead power lines 6 -35 kV with pin insulators, for any arrangement of wires, the distance between them D, m, must be no less than the values ​​determined by the formula


where U - lines, kV; - the largest sag corresponding to the overall span, m; b - thickness of the ice wall, mm (no more than 20 mm).

For overhead lines 35 kV and above with suspended insulators with horizontal wires, the minimum distance between wires, m, is determined by the formula


The support post is made of a composite: the upper part (the post itself) is made of logs 6.5...8.5 m long, and the lower part (the so-called stepson) is made of reinforced concrete with a section of 20 x 20 cm, lengths 4.25 and 6.25 m or from logs 4.5...6.5 m long. Composite supports with reinforced concrete stepson combine the advantages of reinforced concrete and wooden supports: lightning resistance and resistance to rotting at the point of contact with the ground. The connection of the rack to the stepson is made with wire bands made of steel wire with a diameter of 4...6 mm, tensioned by twisting or a tension bolt.

Anchor and intermediate corner supports for 6 - 10 kV overhead lines are made in the form of an A-shaped structure with composite posts.

Steel transmission towers

Widely used on overhead lines with voltages of 35 kV and higher.

According to their design, steel supports can be of two types:

  1. tower or single-column (see Fig. 5.1, d);
  2. portal, which, according to the method of fastening, are divided into free-standing supports and supports with guy wires.

The advantage of steel supports is their high strength, the disadvantage is their susceptibility to corrosion, which requires periodic painting or the application of an anti-corrosion coating during operation.

The supports are made of rolled steel (usually an isosceles angle is used); high transition supports can be made of steel pipes. Steel sheets of various thicknesses are used in the connection nodes of the elements. Regardless of the design, steel supports are made in the form of spatial lattice structures.

Reinforced concrete power transmission towers

Compared to metal ones, they are more durable and economical to operate, since they require less maintenance and repair (if we take the life cycle, then reinforced concrete ones are more energy-consuming). The main advantage of reinforced concrete supports is a reduction in steel consumption by 40...75%, the disadvantage is a large mass. According to the manufacturing method, reinforced concrete supports are divided into those concreted at the installation site (for the most part, such supports are used abroad) and factory-made.

The traverses are fastened to the trunk of the reinforced concrete support post using bolts passed through special holes in the rack, or using steel clamps that cover the trunk and have pins for attaching the ends of the traverse belts to them. Metal traverses are pre-hot-galvanized, so they do not require special care and supervision during operation for a long time.

Overhead line wires are made uninsulated, consisting of one or more twisted wires. Wires made from one wire, called single-wire (they are made with a cross-section from 1 to 10 mm2), have less strength and are used only on overhead lines with voltages up to 1 kV. Stranded wires, twisted from several wires, are used on overhead lines of all voltages.

The materials of wires and cables must have high electrical conductivity, have sufficient strength, and withstand atmospheric influences (in this regard, copper and bronze wires have the greatest resistance; aluminum wires are susceptible to corrosion, especially on sea coasts, where the air contains salts; steel wires are destroyed even under normal atmospheric conditions).

For overhead lines, single-wire steel wires with a diameter of 3.5 are used; 4 and 5 mm and copper wires with a diameter of up to 10 mm. The lower limit is limited due to the fact that wires of smaller diameter have insufficient mechanical strength. The upper limit is limited due to the fact that bends in larger diameter solid wire can cause permanent deformations in its outer layers that will reduce its mechanical strength.

Stranded wires, twisted from several wires, have great flexibility; such wires can be made of any cross-section (they are made with a cross-section from 1.0 to 500 mm2).

The diameters of individual wires and their number are selected so that the sum of the cross sections of the individual wires gives the required total cross-section of the wire.

As a rule, stranded wires are made from round wires, with one or more wires of the same diameter placed in the center. The length of the twisted wire is slightly greater than the length of the wire measured along its axis. This causes an increase in the actual mass of the wire by 1 ... 2% compared to the theoretical mass, which is obtained by multiplying the cross-section of the wire by its length and density. In all calculations, the actual weight of the wire specified in the relevant standards is taken.

Brands of bare wires indicate:

  • letters M, A, AS, PS - wire material;
  • in numbers - cross section in square millimeters.

Aluminum wire A can be:

  • AT grade (solid unannealed)
  • AM (annealed soft) alloys AN, AZh;
  • AS, ASHS - made of steel core and aluminum wires;
  • PS - made of steel wires;
  • PST - made of galvanized steel wire.

For example, A50 denotes an aluminum wire with a cross-section of 50 mm2;

  • AC50/8 - steel-aluminum wire with a cross-section of the aluminum part of 50 mm2, steel core of 8 mm2 (electrical calculations take into account the conductivity of only the aluminum part of the wire);
  • PSTZ,5, PST4, PST5 - single-wire steel wires, where the numbers correspond to the diameter of the wire in millimeters.

Steel cables used on overhead lines as lightning protection cables are made of galvanized wire; their cross-section must be at least 25 mm2. On overhead lines with a voltage of 35 kV, cables with a cross section of 35 mm2 are used; on kV lines - 50 mm2; on lines 220 kV and above -70 mm2.

The cross-section of stranded wires of various brands is determined for overhead lines with voltages up to 35 kV according to the conditions of mechanical strength, and for overhead lines with voltages up to kV and higher - according to the conditions of corona losses. On overhead lines when crossing various engineering structures (communication lines, railways and highways, etc.), it is necessary to ensure higher reliability, therefore the minimum cross-sections of wires in crossing spans must be increased (Table 5.2).

When an air flow directed across the axis of the overhead line or at a certain angle to this axis flows around the wires, turbulence occurs on the leeward side of the wire. When the frequency of formation and movement of vortices coincides with one of the natural oscillation frequencies, the wire begins to oscillate in the vertical plane.

Such vibrations of a wire with an amplitude of 2...35 mm, a wavelength of 1...20 m and a frequency of 5...60 Hz are called vibration.

Typically, vibration of wires is observed at wind speeds of 0.6 ... 12.0 m/s;

Steel wires are not allowed to fly over pipelines and railways.



Vibration typically occurs in spans longer than 120 m and in open areas. The danger of vibration lies in the breakage of individual wires in the areas where they exit the clamps due to increased mechanical stress. Variables arise from periodic bending of the wires as a result of vibration and the main tensile stresses are stored in the suspended wire.

For spans up to 120 m long, vibration protection is not required; Areas of any overhead lines protected from cross winds are also not subject to protection; at large crossings of rivers and water spaces, protection is required regardless of the wires. On overhead lines with a voltage of 35...220 kV and above, vibration protection is performed by installing vibration dampers suspended on a steel cable, absorbing the energy of vibrating wires and reducing the vibration amplitude near the clamps.

When there is ice, the so-called dancing of wires is observed, which, like vibration, is excited by the wind, but differs from vibration in a larger amplitude, reaching 12... 14 m, and a longer wavelength (with one and two half-waves in the span). In a plane perpendicular to the axis of the overhead line, the wire. At a voltage of 35 - 220 kV, the wires are isolated from the supports with garlands of pendant insulators. To insulate 6-35 kV overhead lines, pin insulators are used.

Passing through the overhead line wires, it releases heat and heats the wire. Under the influence of heating the wire, the following occurs:

  1. lengthening the wire, increasing the sag, changing the distance to the ground;
  2. change in wire tension and its ability to bear mechanical load;
  3. change in wire resistance, i.e. change in electrical power and energy losses.

All conditions can change if environmental parameters are constant or change together, affecting the operation of the overhead line wire. When operating overhead lines, it is considered that at the rated load current the wire temperature is 60...70″C. The temperature of the wire will be determined by the simultaneous effects of heat generation and cooling or heat sink. The heat dissipation of overhead line wires increases with increasing wind speed and decreasing ambient temperature.

When the air temperature decreases from +40 to 40 °C and the wind speed increases from 1 to 20 m/s, heat losses change from 50 to 1000 W/m. At positive ambient temperatures (0...40 °C) and low wind speeds (1...5 m/s), heat losses are 75...200 W/m.

To determine the effect of overload on increasing losses, first determine


where RQ is the resistance of the wire at a temperature of 02, Ohm; R0] - wire resistance at a temperature corresponding to the design load under operating conditions, Ohm; А/.у.с - coefficient of temperature increase in resistance, Ohm/°C.

An increase in wire resistance compared to the resistance corresponding to the design load is possible with an overload of 30% by 12%, and with an overload of 50% by 16%.

An increase in AU loss at an overload of up to 30% can be expected:

  1. when calculating overhead lines at AU = 5% A?/30 = 5.6%;
  2. when calculating overhead lines on A17 = 10% D?/30 = 11.2%.

When the overhead line is overloaded to 50%, the increase in loss will be equal to 5.8 and 11.6%, respectively. Taking into account the load graph, it can be noted that when the overhead line is overloaded to 50%, the losses briefly exceed the permissible standard values ​​by 0.8... 1.6%, which does not significantly affect the quality of electricity.

Application of SIP wire

Since the beginning of the century, low-voltage overhead networks, designed as a self-supporting system of insulated wires (SIP), have become widespread.

SIP is used in cities as a mandatory installation, as a highway in rural areas with low population density, and as branches to consumers. The methods of laying SIP are different: tensioning on supports; stretching along building facades; laying along the facades.

The design of SIP (unipolar armored and unarmored, tripolar with an insulated or bare carrier neutral) generally consists of a copper or aluminum conductor stranded core surrounded by an internal semiconductor extruded screen, then insulation made of cross-linked polyethylene, polyethylene or PVC. Tightness is ensured by powder and compounded tape, on top of which there is a metal screen made of copper or aluminum in the form of spirally laid threads or tape, using extruded lead.

On top of the cable armor pad, made of paper, PVC, polyethylene, aluminum armor is made in the form of a mesh of strips and threads. The external protection is made of PVC, polyethylene without gelogen. The spans of the laying, calculated taking into account its temperature and the cross-section of the wires (at least 25 mm2 for main lines and 16 mm2 on branches to inputs for consumers, 10 mm2 for steel-aluminum wire) range from 40 to 90 m.

With a slight increase in costs (about 20%) compared to bare wires, the reliability and safety of a line equipped with SIP increases to the level of reliability and safety of cable lines. One of the advantages of overhead lines with insulated VLI wires over conventional power lines is the reduction of losses and power by reducing reactance. Line Sequence Options:

  • ASB95 - R = 0.31 Ohm/km; X= 0.078 Ohm/km;
  • SIP495 - 0.33 and 0.078 Ohm/km, respectively;
  • SIP4120 - 0.26 and 0.078 Ohm/km;
  • AC120 - 0.27 and 0.29 Ohm/km.

The effect of reducing losses when using SIP and keeping the load current constant can range from 9 to 47%, power losses - 18%.

Complex technical power lines (PTLs) are used to deliver electricity over long distances. On a national scale, they are strategically important objects 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 require mandatory compliance with design conditions and the installation of special structures.

Overhead power lines

Fig.1 Overhead high-voltage power lines

The most common are overhead lines, which are laid outdoors using high-voltage poles to which the wires are secured using special fittings (insulators and brackets). Most often these are SK racks.

The composition of overhead power lines includes:

  • supports for various voltages;
  • bare wires made of aluminum or copper;
  • traverses that provide the required distance to prevent the wires from coming into contact with the support elements;
  • insulators;
  • ground loop;
  • arresters and lightning rod.

The minimum sag point of the overhead line is: 5÷7 meters in uninhabited areas and 6÷8 meters in populated areas.

The following are used as high-voltage poles:

  • metal structures that are effectively used in any 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 using bolted connections, which facilitate the delivery and installation of supports at 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 influence on them of wind loads and soil characteristics;
  • wooden supports, which are the most cost-effective to produce and have excellent dielectric characteristics. The low weight of wooden structures allows them to be quickly delivered to the installation site and easily installed. The disadvantage of these power line supports is their low mechanical strength, which allows them to be installed only with a certain load, and their susceptibility to processes of biological destruction (rotting of the material).

The use of one design or another is determined by the voltage of the electrical network. It will be useful to have the skill of determining the voltage of power lines in appearance.

Overhead lines are classified:

  1. by 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 power lines

Fig.2 Underground cable lines

Unlike overhead lines, cable lines are insulated and therefore more expensive and reliable. This type of wire is used in places where 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. in terms of voltage - just like overhead lines;
  2. by type of insulation - liquid and solid. The first type is petroleum oil, and the second is a cable braid consisting of polymers, rubber and oiled paper.

Their distinctive features are the laying method:

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

Fig.3 Laying an underwater power line

Unlike the first two methods of laying cable power lines, the “by construction” option involves the creation of:

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

Conclusion

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

Overhead lines (OL) serve to transmit electricity through wires laid in the open air and secured to special supports or brackets of engineering structures using insulators and fittings. The main structural elements of overhead lines are wires, protective cables, supports, insulators and linear fittings. In urban environments, overhead lines are most widespread on the outskirts, as well as in areas with buildings up to five floors. Elements of overhead lines must have sufficient mechanical strength, therefore, when designing them, in addition to electrical ones, mechanical calculations are also made to determine not only the material and cross-section of the wires, but also the type of insulators and supports, the distance between wires and supports, etc.

Depending on the purpose and installation location, the following types of supports are distinguished:

intermediate, designed to support wires on straight sections of lines. The distance between supports (spans) is 35-45 m for voltages up to 1000 V and about 60 m for voltages of 6-10 kV. The wires are fastened here using pin insulators (not tightly);

anchor, having a more rigid and durable design in order to absorb longitudinal forces from the difference in tension along the wires and support (in the event of a break) all the wires remaining in the anchor span. These supports are also installed on straight sections of the route (with a span of about 250 m for a voltage of 6-10 kV) and at intersections with various structures. Wires are fastened to anchor supports tightly to pendant or pin insulators;

terminal, installed at the beginning and end of the line. They are a type of anchor supports and must withstand the constant one-way tension of the wires;

angular, installed in places where the direction of the route changes. These supports are strengthened with struts or metal braces;

special or transitional, installed at the intersections of overhead lines with structures or obstacles (rivers, railways, etc.). They differ from other supports of a given line in height or design.

Wood, metal or reinforced concrete are used to make supports.

Depending on the design, wooden supports can be:

single;

A-shaped, consisting of two posts, converging at the top and diverging at the base;

three-legged, consisting of three pillars converging at the top and diverging at the base;

U-shaped, consisting of two racks connected at the top by a horizontal crossbar;

AP-shaped, consisting of two A-shaped supports connected by a horizontal crossarm;

composite, consisting of a stand and an attachment (stepson), attached to it with a bandage made of steel wire.

To increase their service life, wooden supports are impregnated with antiseptics, which significantly slow down the process of wood decay. In operation, antiseptic treatment is carried out by applying an antiseptic bandage in places prone to rotting, with antiseptic paste applied to all cracks, joints and cuts.

Metal supports are made of pipes or profile steel, reinforced concrete - in the form of hollow round or rectangular posts with a decreasing cross-section towards the top of the support.

Insulators and hooks are used to fasten overhead line wires to supports, and insulators and pins are used to fasten them to a traverse. Insulators can be porcelain or glass, pin or suspended (in places of anchor fastening) (Fig. 1, a-c). They are firmly screwed onto hooks or pins using special polyethylene caps or tow impregnated with red lead or drying oil.

Picture 1. a - pin 6-10 kV; b - pin 35 kV; c - suspended; g, d - polymer rods

Overhead line insulators are made of porcelain or tempered glass - materials with high mechanical and electrical strength and resistance to weathering. A significant advantage of glass insulators is that if damaged, the tempered glass shatters. This makes it easier to locate damaged insulators on the line.

By design, insulators are divided into pin and pendant.

Pin insulators are used on lines with voltages up to 1 kV, 6-10 kV and, rarely, 35 kV (Fig. 1, a, b). They are attached to the supports using hooks or pins.

Suspended insulators (Fig. 1, c) are used on overhead lines with a voltage of 35 kV and higher. They consist of a porcelain or glass insulating part 1, a cap made of malleable cast iron 2, a metal rod 3 and a cement binder 4. Suspended insulators are assembled into garlands, which can be supporting (on intermediate supports) or tensioning (on anchor supports). The number of insulators in the garland is determined by the line voltage; 35 kV - 3-4 insulators, 110 kV - 6-8.

Polymer insulators are also used (Fig. 1, d). They are a rod element made of fiberglass, on which a protective coating with ribs made of fluoroplastic or silicone rubber is placed:

The overhead line wires are required to have sufficient mechanical strength. They can be single or multi-wire. Single-wire steel wires are used exclusively for lines with voltages up to 1000 V; stranded wires made of steel, bimetal, aluminum and its alloys have become prevalent due to their increased mechanical strength and flexibility. Most often, on overhead lines with voltages up to 6-10 kV, aluminum stranded wires of grade A and galvanized steel wires of grade PS are used.

Steel-aluminum wires (Fig. 2, c) are used on overhead lines with voltages above 1 kV. They are produced with different ratios of sections of aluminum and steel parts. The lower this ratio, the higher the mechanical strength of the wire and is therefore used in areas with more severe climatic conditions (with a thicker ice wall). The grade of steel-aluminum wires indicates the cross-sections of the aluminum and steel parts, for example, AC 95/16.

Figure 2. a - general view of a stranded wire; b - cross-section of aluminum wire; c - cross-section of steel-aluminum wire

Wires made of aluminum alloys (AN - not heat-treated, AZh - heat-treated) have greater mechanical strength and almost the same electrical conductivity compared to aluminum alloys. They are used on overhead lines with voltages above 1 kV in areas with ice wall thickness up to 20 mm.

Wires are arranged in different ways. On single-circuit lines they are usually arranged in a triangle.

Currently, so-called self-supporting insulated wires (SIP) with voltages up to 10 kV are widely used. In a 380 V line, the wires consist of a carrier uninsulated wire, which is neutral, three insulated linear wires, and one insulated outdoor lighting wire. Linear insulated wires are wound around the supporting neutral wire. The supporting wire is steel-aluminum, and the linear wires are aluminum. The latter are covered with light-resistant heat-stabilized (cross-linked) polyethylene (APV type wire). The advantages of overhead lines with insulated wires over lines with bare wires include the absence of insulators on the supports, maximum use of the height of the support for hanging wires; there is no need to trim trees in the line area.

For branches from lines with voltages up to 1000 V to inputs into buildings, insulated wires of the APR or AVT brand are used. They have a load-bearing steel cable and weather-resistant insulation.

Wires are fastened to supports in various ways, depending on their location on the insulator. On intermediate supports, the wires are attached to pin insulators with clamps or binding wire made of the same material as the wire, and the latter should not have bends at the point of attachment. The wires located on the head of the insulator are fastened with a head tie, and on the neck of the insulator with a side tie.

On anchor, corner and end supports, wires with voltages up to 1000 V are secured by twisting the wires with a so-called “plug”; wires with voltages of 6-10 kV are secured with a loop. At anchor and corner supports, at crossing points across railways, driveways, tram tracks and at intersections with various power and communication lines, double suspension of wires is used.

The wires are connected using die clamps, a crimped oval connector, an oval connector, or a twisted special device. In some cases, welding is used using thermite cartridges and a special apparatus. For solid steel wires, lap welding can be used using small transformers. In spans between supports it is not allowed to have more than two wire connections, and in spans where overhead lines intersect with various structures, wire connections are not allowed. On supports, the connection must be made in such a way that it does not experience mechanical stress.

Linear fittings are used for fastening wires to insulators and insulators to supports and are divided into the following main types: clamps, coupling fittings, connectors, etc.

Clamps are used to secure wires and cables and attach them to garlands of insulators and are divided into supporting, suspended on intermediate supports, and tension, used on anchor-type supports (Fig. 3, a, b, c).

Figure 3. a - supporting clamp; b - bolt tension clamp; c - pressed tension clamp; d - supporting garland of insulators; d - distance spacer; e - oval connector; g - pressed connector

Coupling fittings are designed for hanging garlands on supports and connecting multi-chain garlands with each other and includes brackets, earrings, ears, and rocker arms. The bracket is used to attach the garland to the support crossbeam. The supporting garland (Fig. 3, d) is fixed on the traverse of the intermediate support using earring 1, the other side of which is inserted into the cap of the upper suspension insulator 2. Eyelet 3 is used to attach the garland of supporting clamp 4 to the lower insulator.

Connectors are used to connect individual sections of wire. They are oval and pressed. In oval connectors, the wires are either crimped or twisted (Fig. 3, e). Pressed connectors (Fig. 3, g) are used to connect large cross-section wires. In steel-aluminum wires, the steel and aluminum parts are crimped separately.

Cables, along with spark gaps, arresters and grounding devices, serve to protect lines from lightning surges. They are suspended above the phase wires on overhead lines with a voltage of 35 kV and higher, depending on the area of ​​lightning activity and the material of the supports, which is regulated by the “Rules for the Construction of Electrical Installations”. Lightning protection cables are usually made of steel, but when used as high-frequency communication channels, they are made of steel and aluminum. On 35-110 kV lines, the cable is fastened to metal and reinforced concrete intermediate supports without cable insulation.

To protect against lightning overvoltages sections of overhead lines with a lower insulation level compared to the rest of the line, tubular arresters are used.

On the overhead line, all metal and reinforced concrete supports on which lightning protection cables are suspended or other lightning protection means (arresters, spark gaps) of 6-35 kV lines are installed are grounded. On lines up to 1 kV with a solidly grounded neutral, the hooks and pins of phase wires installed on reinforced concrete supports, as well as the fittings of these supports, must be connected to the neutral wire.

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    Subtitles

Overhead power lines

Overhead power line(VL) - a device intended for transmitting or distributing electrical energy through wires located in the open air and attached using traverses (brackets), insulators and fittings to supports or other structures (bridges, overpasses).

Composition of VL

  • Traverses
  • Sectioning devices
  • Fiber-optic communication lines (in the form of separate self-supporting cables, or built into a lightning protection cable or power wire)
  • Auxiliary equipment for operational needs (high-frequency communication equipment, capacitive power take-off, etc.)
  • Marking elements for high-voltage wires and power line supports to ensure aircraft flight safety. The supports are marked with a combination of paints of certain colors, the wires are marked with aviation balloons for marking in the daytime. Illuminated fencing lights are used for marking during the day and at night.

Documents regulating overhead lines

Classification of overhead lines

By type of current

Basically, overhead lines are used to transmit alternating current and only in certain cases (for example, for connecting power systems, powering contact networks, etc.) are direct current lines used. Direct current lines have lower losses due to capacitive and inductive components. Several DC power lines were built in the USSR:

  • High-voltage direct current line Moscow-Kashira - Elbe Project,
  • High-voltage direct current line Volgograd-Donbass,
  • High-voltage direct current line Ekibastuz-Center, etc.

Such lines are not widely used.

By purpose

  • Ultra-long-distance overhead lines with a voltage of 500 kV and higher (designed to connect individual power systems).
  • Trunk overhead lines with voltages of 220 and 330 kV (designed to transmit energy from powerful power plants, as well as to connect power systems and combine power plants within power systems - for example, they connect power stations with distribution points).
  • Distribution overhead lines with voltages of 35, 110 and 150 kV (designed for power supply to enterprises and settlements of large areas - connecting distribution points with consumers)
  • Overhead lines 20 kV and below, supplying electricity to consumers.

By voltage

  • Overhead lines up to 1000 V (overhead lines of the lowest voltage class)
  • Overhead lines above 1000 V
    • Overhead lines 1-35 kV (overhead lines of medium voltage class)
    • Overhead lines 35-330 kV (overhead lines of high voltage class)
    • Overhead lines 500-750 kV (overhead lines of ultra-high voltage class)
    • Overhead lines above 750 kV (overhead lines of ultra-high voltage class)

These groups differ significantly, mainly in terms of design conditions and structures.

In the CIS networks of general purpose alternating current 50 Hz, according to GOST 721-77, the following rated phase-to-phase voltages should be used: 380; (6) , 10, 20, 35, 110, 220, 330, 500, 750 and 1150 kV. There may also be networks built according to outdated standards with nominal phase-to-phase voltages: 220, 3 and 150 kV.

The highest voltage power line in the world is the Ekibastuz-Kokchetav line, the rated voltage is 1150 kV. However, currently the line is operated at half the voltage - 500 kV.

The rated voltage for direct current lines is not regulated; the most commonly used voltages are: 150, 400 (Vyborgskaya substation - Finland) and 800 kV.

Other voltage classes can be used in special networks, mainly for traction networks of railways (27.5 kV, 50 Hz AC and 3.3 kV DC), metro (825 V DC), trams and trolleybuses (600 VDC).

According to the operating mode of neutrals in electrical installations

  • Three-phase networks with ungrounded (isolated) neutrals (the neutral is not connected to the grounding device or is connected to it through devices with high resistance). In the CIS, this neutral mode is used in networks with a voltage of 3-35 kV with low currents of single-phase ground faults.
  • Three-phase networks with resonantly grounded (compensated) neutrals (the neutral bus is connected to ground through inductance). In the CIS it is used in networks with a voltage of 3-35 kV with high currents of single-phase ground faults.
  • Three-phase networks with effectively grounded neutrals (high and ultra-high voltage networks, the neutrals of which are connected to the ground directly or through a small active resistance). In Russia, these are networks with voltages of 110, 150 and partially 220 kV, which use transformers (autotransformers require mandatory solid grounding of the neutral).
  • Networks with solidly grounded neutral (the neutral of the transformer or generator is connected to the grounding device directly or through low resistance). These include networks with voltages less than 1 kV, as well as networks with voltages of 220 kV and higher.

According to the operating mode depending on the mechanical condition

  • The overhead line is in normal operation (the wires and cables are not broken).
  • Overhead lines in emergency operation (in case of complete or partial breakage of wires and cables).
  • Overhead lines of installation operating mode (during installation of supports, wires and cables).

Main elements of overhead lines

  • Route- position of the overhead line axis on the earth's surface.
  • Pickets(PC) - segments into which the route is divided, the length of the PC depends on the rated voltage of the overhead line and the type of terrain.
  • Zero picket sign marks the beginning of the route.
  • Center sign on the route of the overhead line under construction, it indicates the center of the support location.
  • Production picketing- installation of picket and center signs on the route in accordance with the list of support placement.
  • Support foundation- a structure embedded in the ground or resting on it and transferring load to it from supports, insulators, wires (cables) and from external influences (ice, wind).
  • Foundation base- the soil of the lower part of the pit, which takes the load.
  • Span(span length) - the distance between the centers of two supports on which the wires are suspended. Distinguish intermediate span (between two adjacent intermediate supports) and anchor span (between anchor supports). Transition span- a span crossing any structure or natural obstacle (river, ravine).
  • Line rotation angle- angle α between the directions of the overhead line route in adjacent spans (before and after the turn).
  • Sag- vertical distance between the lowest point of the wire in the span and the straight line connecting the points of its attachment to the supports.
  • Wire size- vertical distance from the wire in the span to the engineering structures crossed by the route, the surface of the earth or water.
  • Plume (a loop) - a piece of wire connecting the tensioned wires of adjacent anchor spans on an anchor support.

Installation of overhead power lines

Installation of power lines is carried out using the “pull” installation method. This is especially true in the case of difficult terrain. When selecting equipment for installing power lines, it is necessary to take into account the number of wires in a phase, their diameter and the maximum distance between power line supports.

Cable power lines

Cable power line(CL) - a line for transmitting electricity or its individual impulses, consisting of one or more parallel cables with connecting, locking and end couplings (terminals) and fasteners, and for oil-filled lines, in addition, with feeding devices and an oil pressure alarm system .

Classification

Cable lines are classified similarly to overhead lines. In addition, cable lines divide:

  • according to the conditions of passage:
    • underground;
    • by buildings;
    • underwater.
  • by type of insulation:
    • liquid (impregnated with cable petroleum oil);
    • hard:
      • paper-oil;
      • polyvinyl chloride (PVC);
      • rubber-paper (RIP);
      • ethylene propylene rubber (EPR).

Insulation with gaseous substances and some types of liquid and solid insulation are not listed here due to their relatively rare use at the time of writing [ When?] .

Cable structures

Cable structures include:

  • Cable tunnel- a closed structure (corridor) with supporting structures located in it for placing cables and cable couplings on them, with free passage along the entire length, allowing for cable laying, repair and inspection of cable lines.
  • cable channel- a non-passable structure, closed and partially or completely buried in the ground, floor, ceiling, etc. and intended for placing cables in it, the installation, inspection and repair of which can only be done with the ceiling removed.
  • Cable mine- a vertical cable structure (usually rectangular in cross-section), the height of which is several times greater than the side of the section, equipped with brackets or a ladder for people to move along it (through shafts) or a wall that is completely or partially removable (non-through shafts).
  • Cable floor- part of the building limited by the floor and the ceiling or covering, with a distance between the floor and the protruding parts of the ceiling or covering of at least 1.8 m.
  • Double floor- a cavity limited by the walls of the room, the interfloor ceiling and the floor of the room with removable slabs (over the entire or part of the area).
  • Cable block- a cable structure with pipes (channels) for laying cables in them with associated wells.
  • Cable camera- an underground cable structure, covered with a blind removable concrete slab, intended for laying cable couplings or for pulling cables into blocks. A chamber that has a hatch to enter it is called cable well.
  • Cable rack- above-ground or above-ground open horizontal or inclined extended cable structure. The cable rack can be pass-through or non-pass-through.
  • Cable gallery- above-ground or above-ground closed (fully or partially, for example, without side walls) horizontal or inclined extended cable passage structure.

Fire safety

The temperature inside cable channels (tunnels) in summer should be no more than 10 °C higher than the outside air temperature.

In case of fires in cable rooms, the combustion progresses slowly in the initial period and only after some time the rate of combustion propagation increases significantly. Experience shows that during real fires in cable tunnels temperatures of up to 600 °C and higher are observed. This is explained by the fact that in real conditions, cables burn that are under current load for a long time and whose insulation is heated from the inside to a temperature of 80 °C and above. Simultaneous ignition of cables may occur in several places and over a considerable length. This is due to the fact that the cable is under load and its insulation heats up to a temperature close to the auto-ignition temperature.

The cable consists of many structural elements, for the manufacture of which a wide range of flammable materials are used, including materials with a low ignition temperature and materials prone to smoldering. Also, the design of the cable and cable structures includes metal elements. In the event of a fire or current overload, these elements are heated to a temperature of the order of 500-600 ˚C, which exceeds the ignition temperature (250-350 ˚C) of many polymer materials included in the cable structure, and therefore they can be re-ignited by heated metal elements after the supply of fire extinguishing agent has stopped. In this regard, it is necessary to select standard indicators for the supply of fire extinguishing agents in order to ensure the elimination of flaming combustion, as well as to exclude the possibility of re-ignition.

For a long time, foam extinguishing systems were used in cable rooms. However, operating experience has revealed a number of shortcomings:

  • limited shelf life of foam concentrates and inadmissibility of storing their aqueous solutions;
  • job instability;
  • difficulty of setup;
  • the need for special care of the foam agent dosage device;
  • rapid destruction of foam at high (about 800 °C) ambient temperature during a fire.

Studies have shown that sprayed water has greater fire extinguishing ability compared to air-mechanical foam, since it well wets and cools burning cables and building structures.

The linear speed of flame propagation for cable structures (cable burning) is 1.1 m/min.

High temperature superconductors

HTSC wire

Losses in power lines

Electricity losses in wires depend on the current strength, therefore, when transmitting it over long distances, the voltage is increased many times (reducing the current strength by the same number of times) using a transformer, which, when transmitting the same power, can significantly reduce losses. However, as the voltage increases, various discharge phenomena begin to occur.

In ultra-high voltage overhead lines there are active power losses due to corona (corona discharge). Corona discharge occurs when the electric field strength E (\displaystyle E) at the surface of the wire will exceed the threshold value E k (\displaystyle E_(k)), which can be calculated using Peak’s empirical formula:
E k = 30 , 3 β (1 + 0.298 r β) (\displaystyle E_(k)=30(,)3\beta \left((1+(\frac (0(,)298)(\sqrt (r \beta ))))\right)) kV/cm,
Where r (\displaystyle r)- radius of the wire in meters, β (\displaystyle \beta )- the ratio of air density to normal.

The electric field strength is directly proportional to the voltage on the wire and inversely proportional to its radius, so you can combat corona losses by increasing the radius of the wires, and also (to a lesser extent) by using phase splitting, that is, using several wires in each phase held by special spacers at a distance of 40-50 cm. Corona losses are approximately proportional to the product U (U − U cr) (\displaystyle U(U-U_(\text(cr)))).

Losses in AC power lines

An important quantity affecting the efficiency of AC power lines is the quantity characterizing the ratio between active and reactive power in the line - cos φ. Active power is the part of the total power passed through the wires and transferred to the load; Reactive power is the power that is generated by the line, its charging power (the capacitance between the line and ground), as well as the generator itself, and consumed by the reactive load (inductive load). Active power losses in the line also depend on the transmitted reactive power. The greater the flow of reactive power, the greater the loss of active power.

When AC power lines are longer than several thousand kilometers, another type of loss is observed - radio emission. Since this length is already comparable to the length of an electromagnetic wave with a frequency of 50 Hz ( λ = c / ν = (\displaystyle \lambda =c/\nu =) 6000 km, quarter wave vibrator length λ / 4 = (\displaystyle \lambda /4=) 1500 km), the wire works as a radiating antenna.

Natural power and transmission capacity of power lines

Natural power

Power lines have inductance and capacitance. Capacitive power is proportional to the square of the voltage, and does not depend on the power transmitted along the line. The inductive power of the line is proportional to the square of the current, and therefore the power of the line. At a certain load, the inductive and capacitive power of the line become equal, and they compensate each other. The line becomes “ideal”, consuming as much reactive power as it produces. This power is called natural power. It is determined only by linear inductance and capacitance, and does not depend on the length of the line. Based on the amount of natural power, one can roughly judge the capacity of the power transmission line. When transmitting such power on the line, there are minimal power losses, its operating mode is optimal. When the phases are split, by reducing the inductive reactance and increasing the capacitive conductivity of the line, the natural power increases. As the distance between the wires increases, the natural power decreases, and vice versa, to increase the natural power it is necessary to reduce the distance between the wires. Cable lines with high capacitive conductivity and low inductance have the highest natural power.

Bandwidth

Power transmission capacity means the highest active power of three phases of power transmission, which can be transmitted in a long-term steady state, taking into account operational and technical limitations. The maximum transmitted active power of power transmission is limited by the conditions of static stability of generators of power stations, the transmitting and receiving parts of the electric power system, and the permissible power for heating line wires with permissible current. From the practice of operating electric power systems, it follows that the capacity of power transmission lines of 500 kV and above is usually determined by the factor of static stability; for power transmission lines of 220-330 kV, restrictions may arise both in terms of stability and in terms of permissible heating, 110 kV and below - only in terms of heating.

Characteristics of the capacity of overhead power lines

Transportation of electrical energy over medium and long distances is most often carried out via power lines located in the open air. Their design must always meet two basic requirements:

1. reliability of high power transmission;

2. ensuring safety for people, animals and equipment.

When operating under the influence of various natural phenomena associated with hurricane gusts of wind, ice, and frost, power lines are periodically subjected to increased mechanical loads.

To comprehensively solve the problems of safe transportation of electrical power, power engineers have to lift live wires to a great height, distribute them in space, isolate them from building elements and mount them with current conductors of increased cross-sections on high-strength supports.

General structure and layout of overhead power lines


Any power transmission line can be represented schematically:

    supports installed in the ground;

    wires through which current is passed;

    linear fittings mounted on supports;

    insulators attached to the fittings and holding the orientation of the wires in the air space.

In addition to the elements of overhead lines it is necessary to include:

    foundations for supports;

    lightning protection system;

    grounding devices.


The supports are:

1. anchor, designed to withstand the forces of tensioned wires and equipped with tensioning devices on the fittings;

2. intermediate, used to secure wires through supporting clamps.

The distance along the ground between two anchor supports is called the anchor section or span, and for intermediate supports between themselves or with the anchor - intermediate.

When an overhead power line passes over water barriers, engineering structures or other critical objects, supports with wire tensioning devices are installed at the ends of such a section, and the distance between them is called an intermediate anchor span.

The wires between the supports are never pulled like a string - in a straight line. They always sag a little, positioned in the air taking into account climatic conditions. But at the same time, the safety of their distance to ground objects must be taken into account:

    rail surfaces;

    contact wires;

    transport routes;

    wires of communication lines or other overhead lines;

    industrial and other facilities.

The sagging of the wire due to tension is called. It is assessed in different ways between supports because the upper parts of them can be located at the same level or with excesses.

The sag relative to the highest point of support is always greater than that of the lower one.

The dimensions, length and design of each type of overhead power line depend on the type of current (alternating or direct) of the electrical energy transported through it and the magnitude of its voltage, which can be less than 0.4 kV or reach 1150 kV.

Arrangement of overhead line wires

Since the electric current passes only in a closed circuit, consumers are powered by at least two conductors. Using this principle, simple overhead power lines of single-phase alternating current with a voltage of 220 volts are created. More complex electrical circuits transmit energy using a three or four-wire circuit with a solidly insulated or grounded zero.

The diameter and metal of the wire are selected for the design load of each line. The most common materials are aluminum and steel. They can be made from a single monolithic core for low-voltage circuits or woven from multi-wire structures for high-voltage power lines.

The internal inter-wire space can be filled with a neutral lubricant, which increases resistance to heat, or without it.

Stranded structures made of aluminum wires that conduct current well are created with steel cores, which are designed to withstand mechanical tension loads and prevent breaks.


GOST classifies open wires for overhead power lines and defines their markings: M, A, AC, PSO, PS, ACCC, ASKP, ASU, ACO, ASUS. In this case, single-wire wires are designated by their diameter. For example, the abbreviation PSO-5 reads “steel wire. made of one core with a diameter of 5 mm.” Multi-core wires for power lines use a different marking, including designation with two numbers written through a fraction:

    the first is the total cross-sectional area of ​​aluminum conductors in mm sq;

    the second is the cross-sectional area of ​​the steel insert (mm sq).

In addition to open metal conductors, wires are increasingly used in modern overhead lines:

    self-supporting isolated;

    protected by extruded polymer, which protects against the occurrence of short circuits when phases are overwhelmed by wind or when foreign objects are thrown from the ground.

Overhead lines are gradually replacing old non-insulated structures. They are increasingly used in internal networks, made of copper or aluminum conductors covered with rubber with a protective layer of dielectric fiber materials or polyvinyl chloride compounds without additional external protection.


To eliminate the appearance of a long-distance corona discharge, the wires of 330 kV overhead lines and higher voltages are split into additional streams.


On VL-330, two wires are mounted horizontally; for a 500 kV line they are increased to three and placed at the vertices of an equilateral triangle. For 750 and 1150 kV overhead lines, splitting into 4, 5 or 8 streams, respectively, is used, located at the corners of their own equilateral polygons.

The formation of a “corona” leads not only to energy losses, but also distorts the shape of the sinusoidal oscillation. Therefore, they fight it with constructive methods.

Support arrangement

Typically, supports are created to secure the wires of one electrical circuit. But on parallel sections of two lines, one common support can be used, which is intended for their joint installation. Such designs are called double-chain.

Materials for making supports can be:

1. profiled corners made of various types of steel;

2. construction wood logs impregnated with anti-rotting compounds;

3. reinforced concrete structures with reinforced rods.

Support structures made of wood are the cheapest, but even with good impregnation and proper maintenance, they last no more than 50–60 years.


In terms of technical design, overhead line supports above 1 kV differ from low-voltage ones in their complexity and height of wire attachment.


They are made in the form of elongated prisms or cones with a wide base at the bottom.

Any support design is designed for mechanical strength and stability, and has a sufficient design margin for existing loads. But it should be taken into account that during operation, damage to its various elements is possible as a result of corrosion, impacts, and non-compliance with installation technology.

This leads to a weakening of the rigidity of the single structure, deformations, and sometimes falls of the supports. Often such cases occur when people work on supports, dismantling or tensioning wires, creating variable axial forces.

For this reason, the admission of a team of installers to work at height from the support structure is carried out after checking their technical condition with an assessment of the quality of its buried part in the ground.

Construction of insulators

On overhead power lines, to separate the current-carrying parts of the electrical circuit from each other and from the mechanical structural elements of the supports, products made of materials with high dielectric properties with ÷ Ohm∙m are used. They are called insulators and are made from:

    porcelain (ceramics);

    glass;

    polymer materials.

The designs and dimensions of insulators depend on:

    on the magnitude of dynamic and static loads applied to them;

    values ​​of the effective voltage of the electrical installation;

    operating conditions.

The complex shape of the surface, operating under the influence of various atmospheric phenomena, creates an increased path for a possible electrical discharge to flow.

Insulators installed on overhead lines for fastening wires are divided into two groups:

1. pin;

2. suspended.

Ceramic models

Porcelain or ceramic pin single insulators have found greater use on overhead lines up to 1 kV, although they work on lines up to 35 kV inclusive. But they are used under the condition of fastening wires of low cross-sections, creating small traction forces.

Garlands of suspended porcelain insulators are installed on lines from 35 kV.


The single porcelain pendant insulator kit includes a dielectric body and a cap made of malleable cast iron. Both of these parts are held together with a special steel rod. The total number of such elements in the garland is determined by:

    the magnitude of the overhead line voltage;

    support structures;

    features of equipment operation.

As the line voltage increases, the number of insulators in the string is added. For example, for a 35 kV overhead line, it is enough to install 2 or 3 of them, but for 110 kV, 6 ÷ 7 will be required.

Glass insulators

These designs have a number of advantages over porcelain ones:

    the absence of internal defects in the insulating material that affect the formation of leakage currents;

    increased strength to torsional forces;

    transparency of the design, allowing you to visually assess the condition and control the polarization angle of the light flux;

    absence of signs of aging;

    automation of production and smelting.

The disadvantages of glass insulators are:

    weak anti-vandal resistance;

    low resistance to impact loads;

    possibility of damage during transportation and installation from mechanical forces.

Polymer insulators

They have increased mechanical strength and a weight reduction of up to 90% compared to ceramic and glass counterparts. Additional benefits include:

    ease of installation;

    greater resistance to atmospheric pollution, which, however, does not exclude the need for periodic cleaning of their surface;

    hydrophobicity;

    good susceptibility to overvoltage;

    increased vandal resistance.

The durability of polymer materials also depends on operating conditions. In an air environment with increased pollution from industrial enterprises, polymers may exhibit “brittle fracture” phenomena, which consist in a gradual change in the properties of the internal structure under the influence of chemical reactions from pollutants and atmospheric moisture, occurring in combination with electrical processes.

When vandals shoot at polymer insulators with shot or bullets, the material usually does not completely collapse, like glass. Most often, a pellet or bullet flies through or gets stuck in the body of the skirt. But the dielectric properties are still underestimated and damaged elements in the garland require replacement.

Therefore, such equipment must be periodically inspected using visual inspection methods. And it is almost impossible to detect such damage without optical instruments.

Overhead line fittings

To attach insulators to an overhead line support, assemble them into garlands and install current-carrying wires to them, special fastening elements are produced, which are commonly called line fittings.


According to the tasks performed, fittings are classified into the following groups:

    coupling, designed for connecting suspended elements in various ways;

    tension, used for attaching tension clamps to wires and garlands of anchor supports;

    supporting, holding fastenings of wires, cables and screen mounting units;

    protective, designed to preserve the operability of overhead line equipment when exposed to atmospheric discharges and mechanical vibrations;

    connecting, consisting of oval connectors and thermite cartridges;

    contact;

    spiral;

    installation of pin insulators;

    installation of SIP wires.

Each of the listed groups has a wide range of parts and requires closer study. For example, only protective fittings include:

    protective horns;

    rings and screens;

    arresters;

    vibration dampers.

Protective horns create a spark gap, divert the emerging electric arc when an insulation flashover occurs, and in this way protect overhead line equipment.

Rings and screens divert the arc from the surface of the insulator and improve voltage distribution over the entire area of ​​the garland.

Arresters protect equipment from surge voltage waves caused by lightning strikes. They can be used on the basis of tubular structures made of vinyl plastic or fiber bakelite tubes with electrodes, or they can be manufactured as valve elements.

Vibration dampers work on cables and wires to prevent damage from fatigue stresses created by vibrations and oscillations.

Grounding devices for overhead lines

The need for re-grounding of overhead line supports is caused by the requirements for safe operation in the event of emergency conditions and lightning overvoltages. The resistance of the grounding device circuit should not exceed 30 Ohms.

For metal supports, all fasteners and reinforcement must be connected to the PEN conductor, and for reinforced concrete supports, the combined zero connects all the struts and reinforcement of the racks.

On supports made of wood, metal and reinforced concrete, pins and hooks when installing self-supporting insulated wires with a supporting insulated conductor are not grounded, except in cases where it is necessary to perform repeated grounding for surge protection.


Hooks and pins mounted on the support are connected to the ground loop by welding, using steel wire or rod no thinner than 6 mm in diameter with the obligatory presence of an anti-corrosion coating.

On reinforced concrete supports, metal reinforcement is used for grounding. All contact connections of grounding conductors are welded or clamped in a special bolted fastening.

The supports of overhead power lines with voltages of 330 kV and higher are not grounded due to the complexity of implementing technical solutions to ensure safe values ​​of touch and step voltages. The protective functions of grounding in this case are assigned to high-speed line protection.