Overhead and cable power lines (TL)

General information and definitions

In the general case, we can assume that a power transmission line (TL) is an electric line that goes beyond the power plant or substation and is designed to transmit electrical energy over a distance; it consists of wires and cables, insulating elements and load-bearing structures.

The modern classification of power lines according to a number of features is presented in Table. 13.1.

Classification of power lines

Table 13.1

sign	line type	Variety
Type of current	Direct current
	Three-phase AC
	Polyphase AC	six-phase
		Twelve-phase
Rated voltage	Low voltage (up to 1 kV)
	High voltage (over 1 kV)	MV (3-35 kV)
		HV (110-220 kV)
		SVN (330-750 kV)
		UVN (over 1000 kV)
constructive performance	aerial
	Cable
Number of circuits	single chain
	double chain
	multi-chain
topological characteristics	Radial
	Trunk
	Branch
functional appointment	Distribution
	Nourishing
	Intersystem communication

In the classification, the type of current is in the first place. In accordance with this feature, direct current lines, as well as three-phase and multi-phase alternating current, are distinguished.

lines direct current compete with the rest only with a sufficiently large length and transmitted power, since a significant share in the total cost of power transmission is the cost of building terminal converter substations.

The most widely used lines in the world three-phase AC, and it is air lines that are leading among them in terms of length. lines polyphase AC(six- and twelve-phase) are currently classified as non-traditional.

The most important feature that determines the difference in the design and electrical characteristics of power lines is the rated voltage U. Category low voltage include lines with a rated voltage of less than 1 kV. Lines with U hou > 1 kV belong to the category high voltage, and lines stand out among them medium voltage(CH) with Uiom = 3-35 kV, high voltage(VN) with U know= 110-220 kV, extra high voltage(SVN) U h(m = 330-750 kV and ultrahigh voltage (UVN) with U hou > 1000 kV.

According to the design, air and cable lines are distinguished. A-priory overhead line is a transmission line whose wires are supported above the ground by poles, insulators and fittings. In its turn, cable line is defined as a transmission line made with one or more cables laid directly into the ground or laid in cable structures (collectors, tunnels, channels, blocks, etc.).

By the number of parallel circuits (l c) laid along a common route, they distinguish single-stranded (n =1), double-chain(and c = 2) and multi-chain(and q > 2) lines. According to GOST 24291-9 b a single-circuit AC overhead line is defined as a line having one set of phase wires, and a double-circuit overhead line is defined as two sets. Accordingly, a multi-circuit overhead line is a line that has more than two sets of phase wires. These kits may have the same or different voltage ratings. In the latter case, the line is called combined.

Single-circuit overhead lines are built on single-circuit supports, while double-circuit ones can be built either with the suspension of each chain on separate supports, or with their suspension on a common (double-circuit) support.

In the latter case, obviously, the right-of-way of the territory under the line route is reduced, but the vertical dimensions and mass of the support increase. The first circumstance, as a rule, is decisive if the line passes in densely populated areas, where the cost of land is usually quite high. For the same reason, in a number of countries of the world, valuable supports are also used with suspension chains of the same rated voltage (usually c and c = 4) or different voltages (s i c

According to the topological (circuit) characteristics, radial and trunk lines are distinguished. Radial a line is considered in which power is supplied only from one side, i.e. from a single power source. Trunk a line is defined by GOST as a line from which there are several branches. Under offshoot refers to a line connected at one end to another power line at its intermediate point.

The last sign of classification - functional purpose. Here stand out distribution and nourishing lines, as well as lines of intersystem communication. The division of lines into distribution and supply lines is rather arbitrary, because both of them serve to provide electrical energy to consumption points. Usually, distribution lines include lines of local electrical networks, and supply lines - lines of networks of regional significance, which supply power to power centers of distribution networks. Intersystem communication lines directly connect different power systems and are designed for mutual power exchange both in normal modes and in case of accidents.

The process of electrification, creation and integration of energy systems into the Unified Energy System was accompanied by a gradual increase in the nominal voltage of transmission lines in order to increase their throughput. In this process, two systems of nominal voltages have historically developed on the territory of the former USSR. The first, most common, includes the following series of values U Hwt: 35-110-200-500-1150 kV, and the second - 35-150-330-750 kV. By the time of the collapse of the USSR, more than 600 thousand km of 35-1150 kV overhead lines were in operation on the territory of Russia. In the subsequent period, the increase in length continued, although less intensively. The corresponding data are presented in table. 13.2.

Dynamics of changes in the length of overhead lines for 1990-1999

Table 13.2

and, kV	Length of overhead lines, thousand km
	1990	1995	1996	1997	1998	1999
Total

Air lines are called lines intended for the transmission and distribution of EE through wires located on outdoors and supported by supports and insulators. Overhead power lines are constructed and operated in a wide variety of climatic conditions and geographical areas, subject to atmospheric influences (wind, ice, rain, temperature changes).

In this regard, overhead lines should be built taking into account atmospheric phenomena, air pollution, laying conditions (sparsely populated areas, urban areas, enterprises), etc. From the analysis of overhead lines, it follows that the materials and designs of lines must meet a number of requirements: economically acceptable cost , good electrical conductivity and sufficient mechanical strength of the materials of wires and cables, their resistance to corrosion, chemical attack; lines must be electrically and environmentally safe, occupy a minimum area.

Structural design of overhead lines. The main structural elements of overhead lines are supports, wires, lightning protection cables, insulators and linear fittings.

According to the design of the supports, single- and double-circuit overhead lines are most common. Up to four circuits can be built on the line route. Line route - a strip of land on which a line is being built. One circuit of a high-voltage overhead line combines three wires (sets of wires) of a three-phase line, in a low-voltage line - from three to five wires. In general, the structural part of the overhead line (Fig. 3.1) is characterized by the type of supports, span lengths, overall dimensions, phase design, and the number of insulators.

The span lengths of overhead lines l are chosen for economic reasons, since with an increase in the span length, the sag of the wires increases, it is necessary to increase the height of the supports H so as not to violate the permissible size of the line h (Fig. 3.1, b), while the number of supports will decrease and line insulators. Line gauge - the smallest distance from the lowest point of the wire to the ground (water, roadbed) should be such as to ensure the safety of people and vehicles under the line.

This distance depends on the rated voltage of the line and the conditions of the area (populated, uninhabited). The distance between adjacent phases of a line depends mainly on its rated voltage. The design of the overhead line phase is mainly determined by the number of wires in the phase. If the phase is made by several wires, it is called split. The phases of the overhead lines of high and ultra-high voltage are split. In this case, two wires are used in one phase at 330 (220) kV, three - at 500 kV, four or five - at 750 kV, eight, eleven - at 1150 kV.

Overhead lines. VL supports are structures designed to support wires at the required height above the ground, water, or some kind of engineering structure. In addition, grounded steel cables are suspended on supports, if necessary, to protect the wires from direct lightning strikes and related overvoltages.

The types and designs of supports are varied. Depending on the purpose and placement on the overhead line, they are divided into intermediate and anchor. The supports differ in material, design and method of fastening, tying wires. Depending on the material, they are wooden, reinforced concrete and metal.

intermediate supports the most simple, serve to support wires in straight sections of the line. They are the most common; their share on average is 80-90% of the total number of overhead line supports. The wires to them are fastened with the help of supporting (suspended) garlands of insulators or pin insulators. Intermediate supports in normal mode are loaded mainly from the own weight of wires, cables and insulators, hanging garlands of insulators hang vertically.

Anchor supports installed in places of rigid fastening of wires; they are divided into terminal, angular, intermediate and special. Anchor supports, designed for the longitudinal and transverse components of the tension of the wires (the tension garlands of the insulators are located horizontally), experience the greatest loads, therefore they are much more complicated and more expensive than intermediate ones; their number on each line should be minimal.

In particular, end and corner supports, installed at the end or at the turn of the line, experience constant tension of wires and cables: one-sided or by the resultant of the angle of rotation; intermediate anchors installed on long straight sections are also calculated for one-sided tension, which can occur when part of the wires break in the span adjacent to the support.

Special supports are the following types: transitional - for large spans crossing rivers, gorges; branch lines - for making branches from the main line; transpositional - to change the order of the location of the wires on the support.

Along with the purpose (type), the design of the support is determined by the number of overhead lines and the relative position of the wires (phases). The supports (and lines) are made in a single- or double-circuit version, while the wires on the supports can be placed in a triangle, horizontally, a reverse Christmas tree and a hexagon or a barrel (Fig. 3.2).

The asymmetric arrangement of the phase wires with respect to each other (Fig. 3.2) causes the unequal inductances and capacitances of different phases. To ensure the symmetry of a three-phase system and phase alignment of reactive parameters on long lines (more than 100 km) with a voltage of 110 kV and above, the wires in the circuit are rearranged (transposed) using appropriate supports.

With a full cycle of transposition, each wire (phase) evenly along the length of the line occupies in series the position of all three phases on the support (Fig. 3.3).

wooden supports( fig. 3.4) are made of pine or larch and are used on lines with voltage up to 110 kV in forest areas, now less and less. The main elements of the supports are stepchildren (attachments) 1, racks 2, traverses 3, braces 4, under-traverse bars 6 and crossbars 5. Supports are easy to manufacture, cheap, and easy to transport. Their main drawback is their fragility due to the decay of wood, despite its treatment with an antiseptic. The use of reinforced concrete stepchildren (attachments) increases the service life of the supports up to 20-25 years.

Reinforced concrete supports (Fig. 3.5) are most widely used on lines with voltage up to 750 kV. They can be free-standing (intermediate) and with braces (anchor). Reinforced concrete supports are more durable than wooden ones, easy to operate, cheaper than metal ones.

Metal (steel) supports ( fig. 3.6) are used on lines with a voltage of 35 kV and above. The main elements include racks 1, traverses 2, cable racks 3, braces 4 and foundation 5. They are strong and reliable, but quite metal-intensive, occupy a large area, require special reinforced concrete foundations for installation and must be painted during operation for corrosion protection.

Metal poles are used in cases where it is technically difficult and uneconomical to build overhead lines on wooden and reinforced concrete poles (crossing rivers, gorges, making taps from overhead lines, etc.).

In Russia, unified metal and reinforced concrete supports of various types have been developed for overhead lines of all voltages, which makes it possible to mass-produce them, speed up and reduce the cost of line construction.

Overhead line wires.

Wires are designed to transmit electricity. Along with good electrical conductivity (possibly less electrical resistance), sufficient mechanical strength and resistance to corrosion must satisfy the conditions of economy. For this purpose, wires are used from the cheapest metals - aluminum, steel, special aluminum alloys. Although copper has the highest conductivity, copper wires due to the significant cost and the need for other purposes, new lines are not used.

Their use is allowed in contact networks, in networks of mining enterprises.

On overhead lines, predominantly uninsulated (bare) wires are used. According to the design, the wires can be single- and multi-wire, hollow (Fig. 3.7). Single-wire, mainly steel wires, are used to a limited extent in low-voltage networks. To give flexibility and greater mechanical strength, the wires are made of multi-wire from one metal (aluminum or steel) and from two metals (combined) - aluminum and steel. The steel in the wire increases the mechanical strength.

Based on the conditions of mechanical strength, aluminum wires of grades A and AKP (Fig. 3.7) are used on overhead lines with voltages up to 35 kV. Overhead lines 6-35 kV can also be made with steel-aluminum wires, and above 35 kV lines are mounted exclusively with steel-aluminum wires.

Steel-aluminum wires have layers of aluminum wires around the steel core. The cross-sectional area of ​​the steel part is usually 4-8 times less than aluminum, but the steel takes about 30-40% of the total mechanical load; such wires are used on lines with long spans and in areas with more severe climatic conditions(with a greater thickness of the ice wall).

The brand of steel-aluminum wires indicates the cross section of the aluminum and steel parts, for example, AC 70/11, as well as data on anti-corrosion protection, for example, AKS, ASKP - the same wires as AC, but with a core filler (C) or all wires (P) with anti-corrosion grease; ASC - the same wire as AC, but with a core covered with a polyethylene film. Wires with anti-corrosion protection are used in areas where the air is polluted with impurities that are destructive to aluminum and steel. The cross-sectional areas of the wires are normalized by the State Standard.

An increase in the diameters of the wires with the same consumption of the conductor material can be carried out using wires with a dielectric filler and hollow wires (Fig. 3.7, d, e). This use reduces corona losses (see Section 2.2). Hollow wires are mainly used for busbars of switchgears 220 kV and above.

Wires made of aluminum alloys (AN - non-heat-treated, AJ - heat-treated) have greater mechanical strength compared to aluminum and almost the same electrical conductivity. They are used on overhead lines with a voltage above 1 kV in areas with an ice wall thickness of up to 20 mm.

Overhead lines with self-supporting insulated wires with a voltage of 0.38-10 kV are finding increasing use. In lines with a voltage of 380/220 V, the wires consist of a carrier bare wire, which is zero, three insulated phase wires, one insulated wire (any phase) for outdoor lighting. Phase insulated wires are wound around the carrier neutral wire (Fig. 3.8).

The carrier wire is steel-aluminum, and the phase 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 supports, the maximum use of the height of the support for hanging wires; there is no need to cut trees in the area where the line passes.

Lightning cables, along with spark gaps, arresters, voltage limiters and grounding devices, serve to protect the line from atmospheric overvoltages (lightning discharges). The cables are suspended above the phase wires ( fig. 3.5) on overhead lines with a voltage of 35 kV and higher, depending on the area for lightning activity and the material of the supports, which is regulated by the Electrical Installation Rules (PUE).

Galvanized steel ropes of grades C 35, C 50 and C 70 are usually used as lightning protection wires, and steel-aluminum wires are used when using cables for high-frequency communication. The fastening of cables on all supports of overhead lines with a voltage of 220-750 kV should be carried out using an insulator shunted with a spark gap. On 35-110 kV lines, cables are fastened to metal and reinforced concrete intermediate supports without cable insulation.

Air line insulators. Insulators are designed for insulation and fastening of wires. They are made of porcelain and tempered glass - materials with high mechanical and electrical strength and resistance to weathering. An essential advantage of glass insulators is that when damaged, the tempered glass shatters. This makes it easier to find damaged insulators on the line.

According to the design, the method of fixing on the support, the insulators are divided into pin and suspension insulators. Pin insulators (Fig. 3.9, a, b) are used for lines with voltages up to 10 kV and rarely (for small sections) 35 kV. They are attached to the supports with hooks or pins. Suspension insulators (Fig. 3.9, in) used on overhead lines with a voltage of 35 kV and above. They consist of a porcelain or glass insulating part 1, a ductile iron cap 2, a metal rod 3 and a cement binder 4.

Insulators are assembled into garlands (Fig. 3.9, G): supporting on intermediate supports and tension - on anchor. The number of insulators in a garland depends on the voltage, the type and material of the supports, and the pollution of the atmosphere. For example, in a 35 kV line - 3-4 insulators, 220 kV - 12-14; on lines with wooden supports, which have increased lightning resistance, the number of insulators in a garland is one less than on lines with metal supports; in tension garlands operating in the most difficult conditions, 1-2 more insulators are installed than in supporting ones.

Insulators have been developed and are undergoing experimental industrial testing using polymer materials. They are a rod element made of fiberglass, protected by a coating with ribs made of fluoroplast or silicone rubber. Rod insulators, in comparison with suspension insulators, have less weight and cost, higher mechanical strength than those made of tempered glass. The main problem is to ensure the possibility of their long-term (more than 30 years) work.

Linear reinforcement is designed to fasten wires to insulators and cables to supports and contains the following main elements: clamps, connectors, spacers, etc. (Fig. 3.10).

Supporting clamps are used for suspension and fastening of overhead lines on intermediate supports with limited termination rigidity (Fig. 3.10, a). On anchor supports for rigid fastening of wires, tension garlands and tension clamps are used - tension and wedge (Fig. 3.10, b, c). Coupling fittings (earrings, ears, brackets, rocker arms) are designed for hanging garlands on supports. The supporting garland (Fig. 3.10, d) is fixed on the traverse of the intermediate support with the help of an earring 1, inserted with the other side into the cap of the upper suspension insulator 2. Eyelet 3 is used to attach the supporting clip 4 to the lower insulator of the garland.

Distance spacers (Fig. 3.10, e), installed in spans of 330 kV and higher lines with split phases, prevent whipping, collisions and twisting of individual phase wires. Connectors are used to connect individual sections of wire using oval or pressing connectors (Fig. 3.10, e, g). In oval connectors, the wires are either twisted or crimped; in pressed connectors used to connect steel-aluminum wires of large cross-sections, the steel and aluminum parts are pressed separately.

The result of the development of EE transmission technology over long distances is various options compact transmission lines, characterized by a smaller distance between the phases and, as a result, smaller inductive resistances and the width of the line path (Fig. 3.11). When using supports of the "covering type" (Fig. 3.11, a) distance reduction is achieved due to the location of all phase split structures inside the “enveloping portal”, or on one side of the support rack (Fig. 3.11, b). The convergence of the phases is ensured with the help of interphase insulating spacers. Various options for compact lines with non-traditional wire layouts of split phases have been proposed (Fig. 3.11, in and).

In addition to reducing the width of the route per unit of transmitted power, compact lines can be created to transmit increased power (up to 8-10 GW); such lines cause less electric field strength at ground level and have a number of other technical advantages.

Compact lines also include controlled self-compensating lines and controlled lines with an unconventional configuration of split phases. They are double-circuit lines in which the phases of different circuits of the same name are shifted in pairs. In this case, voltages shifted by a certain angle are applied to the circuits. Due to the mode change using special devices angle of the phase shift, the line parameters are controlled.

Content:

One of the pillars of modern civilization is electricity. A key role in it is played by power lines - power lines. Regardless of the remoteness of the generating facilities from the end consumers, long conductors are needed to connect them. Next, we will tell in more detail about what these conductors, referred to as power lines, are.

What are overhead power lines

The wires attached to the poles are the overhead power lines. Today, two methods of transmitting electricity over long distances have been mastered. They are based on AC and DC voltages. The transmission of electricity at direct voltage is still less common in comparison with alternating voltage. This is because direct current is not generated by itself, but is obtained from alternating current.

For this reason, additional electric cars. And they began to appear relatively recently, since they are based on powerful semiconductor devices. Such semiconductors appeared only 20–30 years ago, that is, approximately in the 1990s. Therefore, before that time, they had already been built in in large numbers AC power line. The differences in power lines are shown in the schematic below.

The greatest losses are caused by the active resistance of the wire material. It does not matter if the current is direct or alternating. To overcome them, the voltage at the beginning of the transmission is increased as much as possible. The level of one million volts has already been overcome. Generator G feeds AC power lines through transformer T1. And at the end of the transmission, the voltage drops. The power line feeds the load H through the transformer T2. The transformer is the simplest and most reliable voltage conversion tool.

A reader who is not familiar with the power supply is likely to have a question about the meaning of direct current electricity transmission. And the reasons are purely economic - the transmission of electricity at direct current in the transmission line itself gives great savings:

1. The generator generates three-phase voltage. Therefore, three wires for AC power supply are always needed. And at direct current, the entire power of the three phases can be transmitted over two wires. And when using the earth as a conductor - one wire at a time. Consequently, the savings only on materials are threefold in favor of direct current transmission lines.
2. AC electrical networks, when combined into one common system, must have the same phasing (synchronization). This means that the instantaneous value of the voltage in the connected electrical networks must be the same. Otherwise, there will be a potential difference between the connected phases of the electrical networks. As a consequence of the connection without phasing - an accident comparable to a short circuit. For DC power networks is not typical at all. For them, only the current voltage at the time of connection matters.
3. For electrical circuits operating on alternating current, impedance is characteristic, which is associated with inductance and capacitance. The impedance is also available for AC power lines. The longer the line, the greater the impedance and the losses associated with it. For DC electrical circuits, the concept of impedance does not exist, as well as losses associated with a change in the direction of electric current.
4. As already mentioned in paragraph 2, synchronization of generators is necessary for stability in the power system. But the larger the system running on alternating current, and, accordingly, the number of generators, the more difficult it is to synchronize them. And for DC power systems, any number of generators will work fine.

Due to the fact that today there are no sufficiently powerful semiconductor or other systems for voltage conversion that are sufficiently efficient and reliable, most transmission lines still operate on alternating current. For this reason, we will only focus on them below.

Another point in the classification of power lines is their purpose. For this reason, the lines are divided into

• ultra-long,
• trunk,
• distribution.

Their design is fundamentally different due to different voltage values. So, in ultra-long power transmission lines, which are backbone, the highest voltages are used that only exist at the current stage of technology development. The value of 500 kV is the minimum for them. This is due to the significant distance from each other of powerful power plants, each of which is the basis of a separate energy system.

Within it there is its own distribution network, the task of which is to provide large groups end consumers. They are connected to 220 or 330 kV distribution substations on the high side. These substations are the final consumers for the main transmission lines. Since the energy flow has already come close to the settlements, the voltage must be reduced.

The distribution of electricity is carried out by power lines, the voltage of which is 20 and 35 kV for the residential sector, as well as 110 and 150 kV for powerful industrial facilities. The next point in the classification of power lines is by voltage class. On this basis, power lines can be identified visually. Corresponding insulators are characteristic for each voltage class. Their design is a kind of power line certificate. Insulators are made by increasing the number of ceramic cups according to the increase in voltage. And its classes in kilovolts (including voltages between phases, adopted for the CIS countries) are as follows:

• 1 (380 V);
• 35 (6, 10, 20);
• 110…220;
• 330…750 (500);
• 750 (1150).

In addition to insulators, hallmarks are wires. As the voltage increases, the effect of the electric corona discharge becomes more pronounced. This phenomenon wastes energy and reduces the efficiency of the power supply. Therefore, to attenuate the corona discharge with increasing voltage, starting from 220 kV, parallel wires are used - one for every approximately 100 kV. Some of the overhead lines (VL) of different voltage classes are shown below in the images:

Power transmission towers and other notable elements

In order for the wire to be securely held, supports are used. In the simplest case, these are wooden poles. But this design is applicable only to lines up to 35 kV. And with the increase in the value of wood in this stress class, reinforced concrete supports are increasingly being used. As the voltage increases, the wires must be raised higher, and the distance between the phases must be increased. In comparison, the supports look like this:

In general, supports are a separate topic, which is quite extensive. For this reason, we will not delve into the details of the topic of power transmission line supports here. But in order to briefly and concisely show the reader its basis, we will demonstrate the image:

In conclusion of information about overhead power lines, we will mention those additional elements that are found on the supports and are clearly visible. This is

• lightning protection systems,
• as well as reactors.

In addition to the listed elements, several more are used in power lines. But let's leave them outside the scope of the article and move on to cables.

cable lines

Air is an insulator. Air lines are based on this property. But there are other more effective insulating materials. Their use allows you to significantly reduce the distance between the phase conductors. But the price of such a cable is so high that it is out of the question to use it instead of overhead power lines. For this reason, cables are laid where there are difficulties with overhead lines.

Overhead lines (VL) are used to transmit electricity through wires laid in the open air and fixed on special supports or brackets of engineering structures using insulators and fittings. Main structural elements VL are wires, protective cables, supports, insulators and linear fittings. In urban conditions, overhead lines are most widely used on the outskirts, as well as in building areas 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 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 structure 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. Fastening of wires on anchor supports is carried out tightly to suspension or pin insulators;

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

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

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

For the manufacture of supports used wood, metal or reinforced concrete.

Wooden supports, depending on the design, can be:

single;

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

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

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

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

composite, consisting of a rack and a prefix (stepson), attached to it with a steel wire bandage.

To increase service life wooden poles impregnated with antiseptics, significantly slowing down the process of wood decay. In operation, antiseptic treatment is carried out by applying an antiseptic bandage in places prone to decay, with antiseptic paste smearing all cracks, junctions and cuts.

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

Insulators and hooks are used to fasten overhead lines to supports, and insulators and pins are used to fasten them to a traverse. Insulators can be porcelain or glass pin or suspension (in places of anchoring) execution (Fig. 1, a-c). They are firmly screwed onto hooks or pins using special polyethylene caps or tow soaked in red lead or drying oil.

Picture 1. a - pin 6-10 kV; b - pin 35 kV; in - suspended; g, e - rod polymer

Overhead line insulators are made of porcelain or tempered glass - materials with high mechanical and electrical strength and resistance to weathering. An essential advantage of glass insulators is that when damaged, the tempered glass is sent out. This makes it easier to find damaged insulators on the line.

By design, insulators are divided into pin and suspension.

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 with hooks or pins.

Suspension insulators (Fig. 1, c) are used on overhead lines with a voltage of 35 kV and above. They consist of a porcelain or glass insulating part 1, a ductile iron cap 2, a metal rod 3 and a cement binder 4. Suspension insulators are assembled into garlands, which are support (on intermediate supports) and tension (on anchor supports). The number of insulators in a string is determined by the line voltage; 35 kV - 3-4 insulators, 110 kV - 6-8.

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

The requirements for sufficient mechanical strength are imposed on the wires of overhead lines. They can be single or multi-wire. Single-wire steel wires are used exclusively for lines with voltage up to 1000 V; stranded wires made of steel, bimetal, aluminum and its alloys have become predominant 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 smaller this ratio, the higher the mechanical strength of the wire and therefore it is used in areas with more severe climatic conditions (with a greater thickness of the ice wall). The grade of steel-aluminum wires indicates the sections of aluminum and steel parts, for example, AC 95/16.

Figure 2. a - general form stranded wire; b - section of aluminum wire; in - section of steel-aluminum wire

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

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

Currently, the so-called self-supporting insulated wires (SIP) with voltage up to 10 kV are widely used. In a 380 V line, the wires consist of a carrier bare wire, which is zero, three insulated linear wires, one insulated outdoor lighting wire. Linear insulated wires are wound around a carrier neutral wire. The carrier wire is steel-aluminum, and the line 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 supports, the maximum use of the height of the support for hanging wires; there is no need to cut trees in the area where the line passes.

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

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

On anchor, corner and end supports, wires with voltage up to 1000 V are fixed by twisting the wires with the so-called "plug", wires with a voltage of 6-10 kV - with a loop. On anchor and corner supports, at the points of transition through railways, driveways, tram tracks and at intersections with various power lines and communication lines, a double suspension of wires is used.

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

Line 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 serve to fix wires and cables and attach them to the 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; g - supporting garland of insulators; d - remote strut; e - oval connector; g - pressed connector

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

Connectors are used to connect individual sections of wire. They are oval and pressed. In oval connectors, wires are either crimped or twisted (Fig. 3, f). Compressible connectors (Fig. 3, g) are used to connect wires of large cross sections. In steel-aluminum wires, the steel and aluminum parts are pressed 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 above, depending on the area for lightning activity and the material of the supports, which is regulated by the "Rules for Electrical Installations". Lightning 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 overvoltage sections of overhead lines with a reduced insulation level compared to the rest of the line, tubular arresters are used.

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

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

D.C

Overhead power lines providing direct current transmission are rarely used in Russia. main reason 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.

Near electrical networks are installed special security zones. According to the rules of the PUE, they must ensure the safety and normal conditions operation.

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 in the presence of impregnation with a protective composition).

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 provide effective work the line requires its competent maintenance, timely repair and, if necessary, reconstruction. All such activities must be carried out in accordance with the PUE (rules for technical installations).

Repair electrical lines 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, the hauling of lines, the replacement of entire sections. All types of work are determined depending on the state of the power transmission line.