Overhead power lines consist of. Overhead power line. Power lines and sanitary zones

Each of us is aware of how important power lines (TL) play in our lives. We can say that the energy they carry feeds our lives. Almost any work is impossible without the use of electricity.

Power lines are one of the foundations of the energy complex

The main advantage of the transmission of electrical energy is the minimum time during which the receiving device will receive power. This is due to the speed of propagation of the electromagnetic field and ensures the wide distribution of power lines. Electricity is transmitted over fairly long distances. This requires additional tricks aimed at reducing losses.

Varieties of power lines

For the convenience of information perception, as well as for proper documentation in the field of electric power industry, a classification of power lines was made according to several indicators. Here is some of them.

Mounting method

The main criterion by which power lines are classified is the constructive way of transmitting energy. Lines are divided into the following types:

• air- the transmission of electric current is carried out through wires suspended on special supports;
• cable- the transmission of electric current is carried out by means of power cables laid in the ground, cable ducts or other engineering structures.

Line voltage

Depending on the characteristics of the network, the length of the line, the number of consumers and their needs, power lines are divided into the following voltage classes:

• lower (voltage less than 1 kV);
• medium (voltage in the range from 1 kV to 35 kV);
• high (voltage in the range from 110 kV to 220 kV);
• ultra-high (voltage in the range from 330 kV to 750 kV);
• ultra-high (voltage above 750 kV).

Type of transmitted current

According to this criterion, power lines are divided into the following types:

1. AC lines;
2. DC lines.

DC lines are not widely used, although they have lower costs when transmitting energy over long distances. This is primarily due to the high cost of equipment.

Composition of transmission lines

The composition of the cable and overhead lines are different. For differentiation, consider each type of transmission line separately.

Components of an overhead power line

Overhead lines in their composition have many devices and designs. We list the main ones:

1. supports;
2. fittings and insulators;
3. grounding devices;
4. wires and cables;
5. bit devices;
6. wire markers;
7. substations.

In addition to their direct purpose, overhead lines are used as engineering structures for hanging a fiber-optic communication cable. In this regard, on some lines, the number of constituent elements is constantly growing.

Components of a cable power line

Cable lines are used to transmit electrical energy in places inaccessible for suspension along overhead line supports. The composition includes a power cable and input nodes at the substation and to end consumers.

Justification for high voltage

It is customary for consumers to deliver electric current with a voltage of 220 and 380 volts. However, in the conditions of extended lines, this is not beneficial, since the losses in sections longer than 2 km may not be comparable with the required power consumption.

In order to reduce losses over long distances, power is increased and high voltage current is transmitted. To do this, step-up substations are used before transmission, and step-down transformers are placed in front of the consumer. So the transmission line looks like this:

What are the power lines

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 type of wire placement - air (located in the open air) and cable (closed in insulation);
• by appointment - ultra-long, 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 ground using various supports. Separation of power lines is important for their selection and maintenance.

Distinguish lines:

• according to the type of current being moved - 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 dead-earthed, isolated, effectively-grounded neutral.

Alternating current

Electric lines using alternating current for transmission are most often implemented by Russian companies. 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, you have to spend an additional resource on energy conversion.

Installation of overhead power lines

The device of overhead power lines includes the following elements:

• Support systems or electric 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), corner (placed in places where wire lines change direction).
• Wires. They have their own varieties, can be made of aluminum, copper.
• Traverses. They are mounted on line supports and serve as the basis for mounting wires.
• 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 norms of the PUE (rules for the installation of electrical installations).
• Lightning protection. Its use provides protection for overhead power lines from voltage that may occur when a discharge occurs.

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 air lines, 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 closing 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, several cables can be used, 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 high-voltage power lines with different voltages is possible inside buildings. The most commonly used designs include:

• Tunnels. They are separate rooms, inside which the cables are located along the walls or on special structures. Such spaces are well protected and provide easy access to the installation and maintenance of the lines.
• Channels. These are ready-made structures made of plastic, reinforced concrete slabs and other materials, inside of which wires are located.
• Floor or mine. Premises specially adapted for the placement of power lines and the possibility of a person being there.
• Overpass. They are open structures that are laid on the ground, foundation, supporting structures with wires attached inside. Closed flyovers are called galleries.
• Placement in the 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 the 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 has good flexibility, so lines made of such material are easy to lay even in hard-to-reach places);
• polyethylene (sufficiently resistant to chemical or other aggressive environments);
• PVC (the main advantage of such insulation is availability, although the material is inferior to others in terms of durability and various protective properties);
• fluoroplastic (highly resistant to various influences);
• paper-based materials (poorly resistant to chemical and natural influences, even if impregnated with a protective compound).

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

Classification by purpose

Another characteristic according to which the classification of power lines takes place, taking into account voltage, is their purpose. Overhead lines are usually divided into: ultra-long, trunk, distribution. They differ depending on the power, type of recipient and sender of energy. These can be large stations or consumers - factories, settlements.

ultra-long

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

Trunk

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

Distribution

The type of distribution lines includes networks under voltage of 35, 110 and 150 kV. With their help, there is a movement of electrical energy from distribution networks to settlements, as well as large enterprises. Lines with a voltage of less than 20 kV are used to ensure the supply of energy to end consumers, including for connecting electricity to the site.

Construction and repair of power lines

Laying 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.
• Preparation of the territory, selection and purchase of materials.
• Assembly of supporting elements or preparation for cable installation.
• Installation or laying of wires, suspension devices, strengthening of power lines.
• Improvement of the territory and preparation of the line for launch.
• Commissioning, official registration of documentation.

To ensure the efficient operation of the line, its competent maintenance, timely repair 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 carried out to replace various elements. Overhaul involves more serious work, which may include the replacement of supports, hauling lines, replacing entire sections. All types of work are determined depending on the state of the power transmission line.

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 that make up one circuit; on DC overhead lines - at least two wires.

By the number of circuits, overhead lines are divided into one, two and multi-circuit. The number of circuits is determined by the power supply scheme and the need for its redundancy. If two circuits are required according to the power supply scheme, 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 the 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 chain line. The distance from the suspension point to the lowest point of the wire is called the sag /. It determines the dimension of the approach of the wire to the ground A, which for a populated area is equal to: up 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 poles

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

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

Enlarged, however, the supports of overhead lines above 1 kV are divided into two types of anchors, which completely perceive the tension of wires and cables in adjacent spans; intermediate, not perceiving the tension of the wires or partially perceiving.

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

Wooden supports VL

Wooden poles of overhead lines are still widespread 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 decay, to reduce which antiseptics are used.

An effective method of combating decay is impregnation of wood with oily antiseptics. In the US, the transition to glued wood poles is underway.

For overhead lines with a voltage 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 transmission lines 6-35 kV with pin insulators, for any arrangement of wires, the distance between them D, m, must be not less than the values ​​​​determined by the formula

where U - lines, kV; - the largest sag corresponding to the overall span, m; b - ice wall thickness, mm (not more than 20 mm).

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

The support stand is made of a composite: the upper part (the stand itself) is made of logs 6.5 ... 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 decay at the point of contact with the ground. The connection of the rack with the stepson is carried out with wire bandages made of steel wire with a diameter of 4 ... 6 mm, tensioned with a twist 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 racks.

Steel transmission poles

Widely used on overhead lines with a voltage of 35 kV and above.

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

1. tower or single-column (see Fig. 5.1, e);
2. portal, which, according to the method of fixing, are divided into free-standing supports and supports on braces.

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

Supports are made of steel corner rolled products (basically, an isosceles corner is used); high transitional supports can be made of steel pipes. In the joints of the elements, a steel sheet of various thicknesses is used. Regardless of the design, steel supports are made in the form of spatial lattice structures.

Reinforced concrete power transmission poles

Compared to metal ones, they are more durable and economical in operation, as they require less maintenance and repair (if we take the life cycle, then reinforced concrete ones are more energy-intensive). 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 concreted at the installation site (for the most part, such supports are used abroad) and prefabricated.

Traverses are fastened to the trunk of a reinforced concrete support post using bolts passed through special holes in the post, or using steel clamps covering the trunk and having trunnions for attaching the ends of the traverse belts to them. Metal traverses are preliminarily hot-dip galvanized, so they do not require special care and supervision during operation for a long time.

The wires of overhead lines are made uninsulated, consisting of one or more twisted wires. Wires from one wire, called single-wire (they are made with a cross section of 1 to 10 mm2), have lower strength and are used only on overhead lines with voltages up to 1 kV. Multi-wire 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, withstand atmospheric influences (in this respect, copper and bronze wires are the most resistant; aluminum wires are susceptible to corrosion, especially on sea coasts, where salts are contained in the air; 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 up to 10 mm in diameter. The limitation of the lower limit is due to the fact that wires of smaller diameter have insufficient mechanical strength. The upper limit is limited due to the fact that bends of a single-wire wire of a larger diameter 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 with any section (they are made with a section from 1.0 to 500 mm2).

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

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 longer 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 wire section by the length and density. All calculations assume the actual weight of the wire as specified in the relevant standards.

Grades of bare wires indicate:

• letters M, A, AC, PS - wire material;
• figures - section in square millimeters.

Aluminum wire A can be:

• Grade AT (hard not annealed)
• AM (annealed soft) alloys AN, AZh;
• AS, ASHS - from a steel core and aluminum wires;
• PS - from 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 section of the aluminum part of 50 mm2, a steel core of 8 mm2 (in electrical calculations, the conductivity of only the aluminum part of the wire is taken into account);
• 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 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 PO kV lines - 50 mm2; on lines of 220 kV and above -70 mm2.

The cross section of stranded wires of various grades is determined for overhead lines with voltages up to 35 kV according to the conditions of mechanical strength, and for overhead lines with a voltage of 1 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 wire cross-sections in crossing spans should be increased (Table 5.2).

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

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

Usually vibration of wires is observed at a wind speed of 0.6 ... 12.0 m / s;

Steel wires are not allowed in spans 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 of the wire in the areas of their exit from the clamps due to an increase in 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.

In spans up to 120 m, vibration protection is not required; sections of any overhead lines protected from transverse winds are 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 with a decrease in vibration amplitude near the clamps.

When there is ice, the so-called dance 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 flight). In a plane perpendicular to the axis of the overhead line, the wire At a voltage of 35 - 220 kV, the wires are insulated from the supports with garlands of suspension insulators. Pin insulators are used for isolation of 6-35 kV overhead lines.

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

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

All conditions can change in the presence of constancy of environmental parameters or change together, affecting the operation of the overhead line wire. During the operation of the overhead line, it is considered that at the rated load current, the temperature of the wire is 60 ... 70 ″С. The temperature of the wire will be determined by the simultaneous effect of heat generation and cooling or heat sink. The heat removal of overhead lines increases with an increase in wind speed and a decrease in ambient air temperature.

With a decrease in air temperature from +40 to 40 °C and an increase in wind speed from 1 to 20 m/s, heat losses vary 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 the increase in losses, first determine

where RQ - wire resistance at a temperature of 02, Ohm; R0] - wire resistance at a temperature corresponding to the design load under operating conditions, Ohm; A /.u.s - coefficient of temperature increase in resistance, Ohm / ° С.

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

An increase in AU loss during overload up to 30% can be expected:

1. when calculating the overhead line for AU = 5% A? / 30 = 5.6%;
2. when calculating the overhead line at A17 \u003d 10% D? / 30 \u003d 11.2%.

With an overload of overhead lines up to 50%, the increase in loss will be equal to 5.8 and 11.6%, respectively. Considering the load schedule, it can be noted that when the overhead line is overloaded up 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 have become widespread, made as a self-supporting system of insulated wires (SIW).

SIP is used in cities as a mandatory laying, as a highway in rural areas with low population density, branches to consumers. Ways of laying SIP are different: pulling on supports; stretching on the facades of buildings; laying along the facades.

The design of SIP (unipolar armored and unarmored, tripolar with 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. The tightness is provided 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 grid of strips and threads. The outer protection is made of PVC, gel-free polyethylene. The spans of the gasket, calculated taking into account its temperature and the cross-section of wires (at least 25 mm2 for mains and 16 mm2 for branches to consumer inputs, 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. Straight Line Sequence Options:

• ASB95 - R = 0.31 Ohm / km; X \u003d 0.078 Ohm / km;
• SIP495 - respectively 0.33 and 0.078 Ohm / km;
• 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 the invariability of the load current can be from 9 to 47%, power losses - 18%.

Complex technical power lines (TL) 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 installation of which require mandatory compliance with the 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, on which the wires are fixed using special fittings (insulators and brackets). Most often - these are racks SK.

The composition of overhead lines includes:

• supports for various voltages;
• bare wires made of aluminum or copper;
• traverses, providing the necessary distance, excluding the possibility of contact of the wires with the elements of the support;
• 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.

As high-voltage poles are used:

• metal structures that are effectively used in any climatic zones and with different loads. They are distinguished by sufficient strength, reliability and durability. Represent 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 influence on them of wind loads and soil characteristics;
• wooden poles, which are the most cost-effective to manufacture and have excellent dielectric characteristics. The light weight of wooden structures allows them to be quickly delivered to the installation site and easy to install. The disadvantage of these power transmission towers 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 be able to determine the voltage of power lines in appearance.

VL are classified:

1. by current - direct or alternating;
2. by 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 the installation of overhead lines is not possible - 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. according to the type of insulation - liquid and solid. The first type is petroleum oil, and the second is the cable sheath, which consists of polymers, rubber and oiled paper.

Their distinctive features are the method of laying:

• 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 transmission lines, the “by construction” option provides for the creation of:

• cable tunnels, in which power cables are laid on special support structures that allow installation and maintenance of lines;
• cable channels, which are buried structures under the floor of buildings in which cable lines are laid in the ground;
• cable shafts - vertical corridors having a rectangular section, which 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 of flyovers - for horizontal or inclined cable laying;
• chambers used for laying couplings of power transmission line sections;
• galleries - the same flyovers, only closed.

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 that defines

At the very beginning of the 20th century, an outstanding inventor of Serbian origin, Nikola Tesla, worked on a wireless option for transmitting electricity, but even a century later, such developments did not receive large-scale industrial application. The main way to deliver energy to the consumer is still cable and overhead power lines.

Power lines: purpose and types

The power line is perhaps the most important component of electrical networks, which is part of the system of power equipment and devices, the main purpose of which is the transfer of electrical energy from installations that produce it (power plants), convert and distribute it (electric substations) to consumers. In general cases, this is the name of all electrical lines that are outside the listed electrical structures.

Historical reference: the first power transmission line (direct current, voltage 2 kV) was built in Germany according to the project of the French scientist F. Despres in 1882. It had a length of about 57 km and connected the cities of Munich and Miesbach.

According to the method of installation and arrangement, cable and overhead power lines are divided. In recent years, especially for the energy supply of megacities, gas-insulated lines have been erected. They are used to transmit high power in very dense buildings to save the area occupied by power lines and ensure environmental standards and requirements.

Cable lines are used where the arrangement of air lines is difficult or impossible due to technical or aesthetic parameters. Due to the relative cheapness, better maintainability (on average, the time to eliminate an accident or malfunction is 12 times less) and high throughput, overhead power lines are most in demand.

Definition. General classification

Electric overhead line (OHTL) - a set of devices located in the open air and intended for the transmission of electricity. The structure of overhead lines includes wires, traverses with insulators, supports. As the latter, in some cases, structural elements of bridges, overpasses, buildings and other structures can act. During the construction and operation of overhead power lines and networks, various auxiliary fittings (lightning protection, grounding devices), additional and related equipment (high-frequency and fiber-optic communication, intermediate power take-off) and component marking elements are also used.

According to the type of energy transmitted, overhead lines are divided into AC and DC networks. The latter, due to certain technical difficulties and inefficiency, have not received wide distribution and are used only for power supply to specialized consumers: DC drives, electrolysis shops, urban contact networks (electrified transport).

According to the rated voltage, overhead power lines are usually divided into two large classes:

1. Low-voltage, voltage up to 1 kV. State standards define four nominal values: 40, 220, 380 and 660 V.
2. High voltage, over 1 kV. Twelve nominal values ​​are defined here: medium voltage - from 3 to 35 kV, high - from 110 to 220 kV, ultra-high - 330, 500 and 700 kV and ultra-high - over 1 MV.

Note: all the given figures correspond to the interphase (linear) voltage of a three-phase network (six- and twelve-phase systems do not have a serious industrial distribution).

From GOELRO to UES

The following classification describes the infrastructure and functionality of overhead power lines.

According to the coverage of the territory, the network is divided into:

• for ultra-long distance (voltage over 500 kV), intended for connection of regional energy systems;
• trunk lines (220, 330 kV) serving for their formation (connection of power plants with distribution facilities);
• distribution (35 - 150 kV), the main purpose of which is the supply of electricity to large consumers (industrial facilities, the agricultural complex and large settlements);
• supply or supply (below 20 kV), providing power supply to other consumers (urban, industrial and agricultural).

Overhead power lines are important in the formation of the Unified Energy System of the country, the foundation of which was laid during the implementation of the GOELRO (State Electrification of Russia) plan of the young Soviet Republic about a century ago to ensure a high level of reliability of energy supply, its fault tolerance.

According to the topological structure and configuration, overhead transmission lines can be open (radial), closed, with backup (containing two or more sources) power.

According to the number of parallel circuits passing along one route, the lines are divided into single-, double- and multi-circuit (a circuit is understood as a complete set of wires of a three-phase network). If the circuits have different nominal voltage values, then such an overhead transmission line is called combined. Chains can be attached both on one support and on different ones. Naturally, in the first case, the mass, dimensions and complexity of the support increase, but the line's security zone is reduced, which in densely populated areas sometimes plays a decisive role in the preparation of the project.

Additionally, separation of overhead lines and networks is used, based on the design of neutrals (isolated, solidly grounded, etc.) and the mode of operation (regular, emergency, installation).

Secured territory

To ensure the safety, normal functioning, ease of maintenance and repair of overhead power lines, as well as to prevent injuries and deaths of people, zones with a special mode of use are introduced along the routes. Thus, the security zone of overhead power lines is a land plot and the air space above it, enclosed between vertical planes, standing at a certain distance from the extreme wires. In security zones, the work of lifting equipment, the construction of buildings and structures is prohibited. The minimum distance from the overhead power line is determined by the rated voltage.

When crossing non-navigable reservoirs, the protective zone of overhead power lines corresponds to similar distances, and for navigable ones its size increases to 100 meters. In addition, the guidelines determine the smallest distances of wires from the surface of the earth, industrial and residential buildings, trees. It is forbidden to lay high-voltage lines over the roofs of buildings (except for industrial ones, in specially specified cases), over the territories of children's institutions, stadiums, cultural and entertainment and shopping areas.

Supports - structures made of wood, reinforced concrete, metal or composite materials to ensure the required distance of wires and lightning protection cables from the earth's surface. The most budget option - wooden racks, which were used very widely in the last century in the construction of high-voltage lines - are gradually being taken out of service, and new ones are hardly installed. The main elements of overhead transmission line towers include:

• foundation foundations,
• racks,
• struts,
• stretch marks.

Structures are divided into anchor and intermediate. The first ones are installed at the beginning and end of the line, when the direction of the route changes. A special class of anchor supports are transitional, used at the intersections of high-voltage power lines with water arteries, overpasses and similar objects. These are the most massive and highly loaded structures. In difficult cases, their height can reach 300 meters!

The strength and dimensions of the construction of intermediate supports, used only for straight sections of the tracks, are not so impressive. Depending on the purpose, they are divided into transposition (serving to change the location of the phase wires), cross, branch, low and high. Since 1976, all supports have been strictly unified, but today there is a process of moving away from the mass use of standard products. They try to adapt each track as much as possible to the conditions of the relief, landscape and climate.

The main requirement for high-voltage transmission lines is high mechanical strength. They are divided into two classes - non-isolated and isolated. They can be made in the form of stranded and single-wire conductors. The latter, consisting of one copper or steel core, are used only for the construction of low voltage lines.

Stranded wires for overhead power lines can be made of steel, alloys based on aluminum or pure metal, copper (the latter, due to their high cost, are practically not used on long routes). The most common conductors are made of aluminum (the letter "A" is present in the designation) or steel-aluminum alloys (grade AS or ASU (reinforced)). Structurally, they are twisted steel wires, on top of which aluminum conductors are wound. Steel, for protection against corrosion, galvanized.

The choice of section is made in accordance with the transmitted power of the allowable voltage drop, mechanical characteristics. The standard sections of wires produced in Russia are 6, 10, 16, 25, 35, 50, 70, 95, 120 and 240. An idea of ​​​​the minimum sections of wires used for the construction of overhead lines can be obtained from the table below.

Branches are often performed with insulated wires (brands APR, AVT). Products have a weather-resistant insulating coating and a steel carrier cable. Wire connections in spans are mounted in areas not subject to mechanical stress. They are spliced ​​by crimping (using appropriate devices and materials) or by welding (with thermite checkers or a special apparatus).

In recent years, self-supporting insulated wires have been increasingly used in the construction of overhead lines. For low voltage overhead power lines, the industry produces SIP-1, -2 and -4 grades, and for 10-35 kV lines - SIP-3.

On routes with voltages over 330 kV, to prevent corona discharges, the use of a split phase is practiced - one wire of large cross section is replaced by several smaller ones fastened together. With an increase in the rated voltage, their number increases from 2 to 8.

Linear reinforcement

VTL fittings include traverses, insulators, clamps and suspensions, strips and spacers, fasteners (brackets, clamps, hardware).

The main function of traverses is to fasten wires in such a way as to provide the necessary distance between opposite phases. Products are special metal structures made of angles, strips, pins, etc. with a painted or galvanized surface. There are about two dozen standard sizes and types of traverses, weighing from 10 to 50 kg (designated as TM-1 ... TM22).

Insulators are used for reliable and safe fastening of wires. They are divided into groups, depending on the material of manufacture (porcelain, tempered glass, polymers), functional purpose (support, through passage, inlet) and methods of attachment to traverses (pin, rod and suspension). Insulators are made for a certain voltage, which must be indicated in the alphanumeric marking. The main requirements for this type of fittings when installing overhead power lines are mechanical and electrical strength, heat resistance.

To reduce line vibration and prevent wire breaks, special damping devices or damping loops are used.

Technical parameters and protection

When designing and installing overhead power lines, the following important characteristics are taken into account:

• The length of the intermediate span (the distance between the axes of adjacent racks).
• The distance of the phase conductors from each other and the lowest one from the surface of the earth (line size).
• The length of the string of insulators according to the rated voltage.
• full height of supports.

You can get an idea about the main parameters of overhead power lines of 10 kV and above from the table.

To prevent damage to overhead lines and prevent emergency shutdowns during a thunderstorm, a steel or steel-aluminum wire lightning rod with a cross section of 50-70 mm 2, grounded on supports, is launched above the phase wires. Often it is made hollow, and this space is used to organize high-frequency communication channels.

Valve arresters provide protection against surges arising from lightning strikes. In the event of an induced lightning impulse on the wires, a breakdown of the spark gap occurs, as a result of which the discharge flows to the support, which has the ground potential, without damaging the insulation. Support resistance is reduced using special grounding devices.

Preparation and installation

The technological process of the construction of the overhead transmission line consists of preparatory, construction and installation and commissioning works. The former include the purchase of equipment and materials, reinforced concrete and metal structures, the study of the project, the preparation of the route and picketing, the development of a PEP (plan for the production of electrical work).

Construction work includes digging pits, installing and assembling supports, distributing rebar and grounding kits along the route. The direct installation of overhead power lines begins with the rolling of wires and cables, making connections. This is followed by lifting them to the supports, stretching, sighting the sag arrows (the greatest distance between the wire and the straight line connecting the points of its attachment to the supports). Finally, wires and cables are tied on insulators.

In addition to general safety measures, work on overhead power lines implies compliance with the following rules:

• Cessation of all work when a thunderstorm front approaches.
• Ensuring the protection of personnel from the effects of electrical potentials induced in the wires (shorting and grounding).
• Prohibition of work at night (except for the installation of intersections with overpasses, railways), ice, fog, with a wind speed of more than 15 m / s.

Before commissioning, check the sag and the dimensions of the line, measure the voltage drop in the connectors, the resistance of the grounding devices.

Maintenance and repair

According to the work regulations, all overhead lines over 1 kV every six months are subject to inspection by maintenance personnel, engineering and technical workers - once a year, for the following faults:

• throwing foreign objects on the wires;
• breaks or burnout of individual phase wires, violation of the adjustment of the sag arrows (should not exceed the design ones by more than 5%);
• damage or overlap of insulators, garlands, arresters;
• destruction of supports;
• violations in the security zone (storage of foreign objects, finding oversized equipment, narrowing the width of the clearing due to the growth of trees and shrubs).

Extraordinary inspections of the route are carried out during the formation of ice, during the period of flooding of rivers, natural and man-made fires, as well as after automatic shutdown. Climbing inspections are carried out as needed (at least once every 6 years).

If a violation of the integrity of a part of the wires of the wire (up to 17% of the total cross section) is detected, the damaged area is restored by applying a repair sleeve or bandage. In case of large damage, the wire is cut and reconnected with a special clamp.

During the current repair of the airway, lopsided supports and struts are straightened, all threaded connections are checked for tightening, the protective paint layer on metal structures, numbering, signs and posters are restored. Measure the resistance of grounding devices.

Overhaul of overhead power lines involves the implementation of all maintenance work. In addition, a complete wiring is carried out with the measurement of the contact resistance of the couplings and post-repair testing.