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 electrical machines are needed. 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. Consequently, before that time, a large number of AC power lines had already been built. 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 of 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, wires are the hallmarks. 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. it

• 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. The main structural elements of overhead lines 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 the service life, wooden supports are impregnated with antiseptics, which significantly slow down the process of wood decay. In operation, antiseptic treatment is carried out by applying an antiseptic bandage in places prone to 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 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 view of a 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 are arranged in various 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.

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

Clamps 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 is the meaning of power lines? Is there a precise definition of the wires through which electricity is transmitted? There is an exact definition in the intersectoral rules for the technical operation of consumer electrical installations. So, a power line is, firstly, an electric line. Secondly, these are sections of wires that go beyond substations and power stations. Thirdly, the main purpose of power lines is the transmission of electric current at a distance.

According to the same rules of the MPTEEP, power transmission lines are divided into overhead and cable ones. But it should be noted that high-frequency signals are also transmitted through power lines, which are used to transmit telemetry data, for supervisory control of various industries, for emergency automatics and relay protection signals. According to statistics, 60,000 high-frequency channels today pass through power lines. To put it bluntly, the figure is significant.

Overhead power lines

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

What is included in the composition of overhead power lines:

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

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

The construction of a power transmission line, as well as its design, plus the design features of the supports, are determined by the rules for the installation of electrical installations, that is, the PUE, as well as various building rules and regulations, that is, SNiP. In general, the construction of power lines is a difficult and very responsible business. Therefore, their construction is carried out by specialized organizations and companies, where there are highly qualified specialists in the state.

Classification of overhead power lines

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

By type of current:

• variable,
• Permanent.

Basically, overhead lines are used to transmit alternating current. It is rare to find the second option. It is usually used to power a contact or communication network to provide communication to several power systems, there are other types.

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

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

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

The voltage of power lines corresponds to their purpose.

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

Classification of power lines by neutral

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

Unfortunately, there is also such a separation of power lines, which takes into account the operational state of all elements of the power transmission line. This is a transmission line in good condition, where wires, poles and other components are in good condition. Basically, the emphasis is on the quality of wires and cables, they should not be broken. Emergency condition, where the quality of wires and cables leaves much to be desired. And the installation condition, when repairing or replacing wires, insulators, brackets and other components of power lines.

Elements of overhead power lines

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

• The route is the axis of the power line laying, which runs along the surface of the earth.
• PC - pickets. In fact, these are segments of the power line route. Their length depends on the terrain and on the rated voltage of the route. Zero station is the beginning of the route.
• The construction of a support is indicated by a center sign. This is the center of the support installation.
• Picketing - in fact, this is a simple installation of pickets.
• The span is the distance between the supports, or rather, between their centers.
• The sag is the delta between the lowest point of the wire sag and a strictly stretched line between the supports.
• The wire gauge is again the distance between the lowest point of the sag and the highest point of the engineering structures running under the wires.
• Loop or loop. This is the part of the wire that connects the wires of adjacent spans on the anchor support.

Cable power lines

So, we turn to the consideration of such a thing as cable power lines. Let's start with the fact that these are not bare wires that are used in overhead power lines, these are cables enclosed in insulation. Typically, cable transmission lines are several lines installed next to each other in a parallel direction. The length of the cable is not enough for this, so couplings are installed between the sections. By the way, you can often find oil-filled cable power lines, so such networks are often equipped with special low-fill equipment and an alarm system that responds to oil pressure inside the cable.

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

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

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

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

Conclusion on the topic

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

Overhead power lines are distinguished by a number of criteria. Let's give a general classification.

I. By the nature of the current

Picture. 800 kV direct current overhead line

Currently, the transmission of electrical energy is carried out mainly on alternating current. This is due to the fact that the vast majority of electrical energy sources produce alternating voltage (with the exception of some non-traditional sources of electrical energy, such as solar power plants), and the main consumers are AC machines.

In some cases, direct current transmission of electrical energy is preferable. The scheme for organizing DC transmission is shown in the figure below. To reduce the load losses in the line during the transmission of electricity at direct current, as well as at alternating current, the transmission voltage is increased with the help of transformers. In addition, when organizing a transmission from a source to a consumer at direct current, it is necessary to convert electrical energy from alternating current to direct current (using a rectifier) ​​and vice versa (using an inverter).

Picture. Schemes for organizing the transmission of electrical energy on alternating (a) and direct (b) current: G - generator (energy source), T1 - step-up transformer, T2 - step-down transformer, V - rectifier, I - inverter, N - load (consumer).

The advantages of transmitting electricity through overhead lines at direct current are as follows:

1. It is cheaper to build an overhead line, since DC power transmission can be carried out on one (monopolar circuit) or two (bipolar circuit) wires.
2. The transmission of electricity can be carried out between power systems that are not synchronized in frequency and phase.
3. When transmitting large amounts of electricity over long distances, losses in DC power lines become less than when transmitting on alternating current.
4. The limit of the transmitted power according to the condition of the stability of the power system is higher than that of AC lines.

The main disadvantage of DC power transmission is the need to use AC to DC converters (rectifiers) and vice versa, DC to AC (inverters), and the associated additional capital costs and additional losses for electricity conversion.

DC overhead lines are not currently widespread, so in the future we will consider the installation and operation of alternating current overhead lines.

II. By appointment

• Extra-long overhead lines with a voltage of 500 kV and above (designed to connect individual power systems).
• Main overhead lines with a voltage of 220 and 330 kV (designed to transmit energy from powerful power plants, as well as to connect power systems and combine power plants within power systems - for example, connect power plants with distribution points).
• Distribution overhead lines with a voltage of 35 and 110 kV (intended for power supply of enterprises and settlements of large areas - connect distribution points with consumers)
• VL 20 kV and below, supplying electricity to consumers.

III. By voltage

1. VL up to 1000 V (low voltage VL).
2. Overhead lines above 1000 V (high-voltage overhead lines):

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. By definition 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 by 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