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✪ Dance of the wires of the overhead power line 110 kV

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Overhead power lines

Overhead power line(VL) - a device designed for the transmission or distribution of electrical energy through wires located on outdoors and attached with the help of traverses (brackets), insulators and fittings to supports or other structures (bridges, overpasses).

Composition VL

• Traverses
• Partitioning devices
• Fiber-optic communication lines (in the form of separate self-supporting cables, or built into a lightning protection cable, power wire)
• Auxiliary equipment for the needs of operation (equipment for high-frequency communication, capacitive power take-off, etc.)
• Elements for marking high-voltage wires and power transmission line poles to ensure the safety of aircraft flights. Supports are marked with a combination of paints of certain colors, wires - with aviation balloons for marking in the daytime. To indicate in the daytime and at night, the lights of the light fence are used.

Documents regulating overhead lines

VL classification

By type of current

Basically, overhead lines are used to transmit alternating current, and only in some cases (for example, for connecting power systems, powering a contact network, and others), direct current lines are used. DC lines have lower capacitive and inductive losses. In the USSR, several DC power lines were built:

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

Such lines were not widely used.

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, 110 and 150 kV (intended for power supply of enterprises and settlements in large areas - connect distribution points with consumers)
• VL 20 kV and below, supplying electricity to consumers.

By voltage

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

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

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

The highest voltage transmission line in the world is the Ekibastuz-Kokchetav line, with a nominal voltage of 1150 kV. However, at present the line is operated under half the voltage - 500 kV.

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

Other voltage classes can be used in special networks, mainly for railway traction networks (27.5 kV, 50 Hz AC and 3.3 kV DC), underground (825 V DC), trams and trolleybuses (600 in direct current).

According to the mode of operation of neutrals in electrical installations

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

According to the mode of operation depending on the mechanical condition

• Overhead line of normal operation (wires and cables are not broken).
• Overhead lines of emergency operation (with a complete or partial breakage of wires and cables).
• VL of the installation mode of operation (during the installation of supports, wires and cables).

The main elements of overhead lines

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

Installation of overhead power lines

The installation of power transmission lines is carried out by the "Mounting" "pull-up" method. This is especially true in the case of complex terrain. When selecting equipment for the installation of power transmission lines, it is necessary to take into account the number of wires in the phase, their diameter and the maximum distance between the power transmission line supports.

Cable power lines

Cable power line(KL) - a line for the transmission of electricity or its individual impulses, consisting of one or more parallel cables with connecting, locking and end sleeves (terminals) and fasteners, and for oil-filled lines, in addition, with feeders and an oil pressure alarm system .

Classification

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

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

Gaseous insulation and some types of liquid and solid insulation are not indicated here due to their relatively rare use at the time of writing [ When?] .

cable structures

Cable structures include:

• cable tunnel- a closed structure (corridor) with supporting structures located in it for placing cables and cable boxes on them, with free passage along the entire length, which allows cable laying, repair and inspection of cable lines.
• cable channel- an impassable structure, closed and partially or completely buried in the ground, floor, ceiling, etc., and intended for placing cables in it, the laying, inspection and repair of which can only be done with the ceiling removed.
• cable shaft- a vertical cable structure (usually of a rectangular section), whose height is several times greater than the side of the section, equipped with brackets or a ladder for people to move along it (walk-through shafts) or a completely or partially removable wall (non-passage mines).
• cable floor- a part of the building bounded by the floor and the floor or cover, with a distance between the floor and the protruding parts of the floor or cover of at least 1.8 m.
• double floor- a cavity bounded by the walls of the room, the interfloor overlap and the floor of the room with removable plates (on the whole or part of the area).
• cable block- cable structure with pipes (channels) for laying cables in them with wells related to it.
• cable camera- an underground cable structure closed with a deaf removable concrete slab, designed for laying cable boxes or for pulling cables into blocks. A chamber having a hatch to enter it is called cable well.
• cable rack- above-ground or ground open horizontal or inclined extended cable structure. Cable overpass can be passable or non-passage.
• cable gallery- above ground or ground closed (in whole or in part, for example, without side walls) horizontal or inclined extended cable structure.

Fire safety

The temperature inside the cable channels (tunnels) in summer time should not be more than 10 °C above the outside temperature.

In case of fires in cable rooms, in the initial period, combustion develops slowly and only after some time does the combustion spread rate increase significantly. Practice shows that during real fires in cable tunnels, temperatures up to 600 ° C and above are observed. This is explained by the fact that in real conditions, cables burn, which are under current load for a long time and the insulation of which warms up from the inside to a temperature of 80 ° C and above. Simultaneous ignition of cables in several places and over a considerable length can occur. This is due to the fact that the cable is under load and its insulation is heated to a temperature close to the self-ignition temperature.

The cable consists of many structural elements, for the manufacture of which a wide range of combustible materials are used, including materials that have low temperature flammable, materials prone to smoldering. Also, the design of the cable and cable structures includes metal elements. In the event of a fire or current overload, these elements heat up to a temperature of about 500-600 ˚C, which exceeds the ignition temperature (250-350 ˚C) of many polymeric materials included in the cable structure, and therefore they can be re-ignited from heated metal elements after stopping the supply of fire extinguishing agent. In this regard, it is necessary to choose the normative indicators for the supply of fire extinguishing agents in order to ensure the elimination of fiery combustion, as well as to exclude the possibility of re-ignition.

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

• limited shelf life of the foaming agent and the inadmissibility of storing their aqueous solutions;
• instability in work;
• complexity of setup;
• the need for special care for the foam concentrate dosing device;
• rapid destruction of the foam at high (about 800 ° C) ambient temperature during a fire.

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

Line speed flame propagation for cable structures (cable burning) is 1.1 m/min.

High temperature superconductors

HTS wire

Losses in power lines

The loss of electricity in the wires depends on the strength of the current, therefore, when transmitting it over long distances, the voltage is increased many times (by the same amount reducing the current strength) with the help of a transformer, which, when transmitting the same power, can significantly reduce losses. However, as the voltage increases, various discharge phenomena begin to occur.

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

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

Losses in AC power lines

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

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

Natural power and transmission capacity of power lines

natural power

Power lines have inductance and capacitance. Capacitive power is proportional to the square of the voltage, and does not depend on the power transmitted over the line. The inductive power of the line is proportional to the square of the current, and hence the power of the line. At a certain load, the inductive and capacitive powers of the line become equal, and they cancel each other out. The line becomes "ideal", consuming as much reactive power as it produces. This power is called natural power. It is determined only by the linear inductance and capacitance, and does not depend on the length of the line. By the value of natural power, one can roughly judge the transmission capacity of the power line. When transmitting such power on the line, there is minimal power loss, the mode of its operation is optimal. When splitting the phases, by reducing the inductive resistance and increasing the capacitance of the line, the natural power increases. With an increase in the distance between the wires, the natural power decreases, and vice versa, to increase the natural power, it is necessary to reduce the distance between the wires. Cable lines with high capacitive conductivity and low inductance have the highest natural power.

Bandwidth

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

Characteristics of the throughput capacity of overhead power lines

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, power lines are, firstly, electrical line. Secondly, these are sections of wires that go beyond substations and power stations. Thirdly, the main purpose of power lines is the transmission electric current on 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 control signals and relay protection. 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; 10; 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 there are, but not many. Basically, these two categories differ from each other in the way they are laid, as well as 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 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 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 in terms of length, it is precisely air lines. 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 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

Content:

One of the pillars modern civilization is the power supply. 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 most high voltages that only exist at the current stage of development of technology. 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. Because the energy flow already close to the settlements, the tension 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, information about overhead power lines, we mention those additional elements, which are found on supports and are clearly visible. This

• 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 power lines.

An overhead electric line is a device that serves to transmit electrical energy through wires located in the open air and attached to supports with the help of insulators and fittings. Overhead power lines are divided into overhead lines with voltage up to 1000 V and above 1000 V.

During the construction of overhead power lines, the volume earthworks insignificant. In addition, they are easy to operate and repair. The cost of building an overhead line is approximately 25-30% less than the cost of a cable line of the same length. Air lines are divided into three classes:

class I - lines with a rated operating voltage of 35 kV for consumers of the 1st and 2nd categories and above 35 kV, regardless of the categories of consumers;

class II - lines with rated operating voltage from 1 to 20 kV for consumers of the 1st and 2nd categories, as well as 35 kV for consumers of the 3rd category;

class III - lines with a rated operating voltage of 1 kV and below. characteristic feature overhead line with a voltage of up to 1000 V is the use of supports for the simultaneous fastening of radio network wires, outdoor lighting, telecontrol, and signaling on them.

The main elements of an overhead line are supports, insulators and wires.

For lines with a voltage of 1 kV, two types of supports are used: wooden with reinforced concrete attachments and reinforced concrete.
For wooden poles use logs impregnated with an antiseptic, from the forest of II grade - pine, spruce, larch, fir. It is possible not to impregnate logs in the manufacture of supports from hardwood winter felling. The diameter of the logs in the top cut must be at least 15 cm for single poles and at least 14 cm for double and A-shaped poles. It is allowed to take the diameter of the logs in the upper cut at least 12 cm on the branches leading to the inputs to buildings and structures. Depending on the purpose and design, intermediate, angular, branch, cross and end supports are distinguished.

Intermediate supports on the line are the most numerous, as they serve to maintain the wires at a height and are not designed for the forces that are created along the line in the event of a wire break. To perceive this load, anchor intermediate supports are installed, placing their "legs" along the axis of the line. To absorb forces perpendicular to the line, anchor intermediate supports are installed, placing the "legs" of the support across the line.

Anchor supports have a more complex design and increased strength. They are also divided into intermediate, corner, branch and end, which increase the overall strength and stability of the line.

The distance between two anchor supports is called the anchor span, and the distance between the intermediate supports is called the support pitch.
In places where the direction of the overhead line route changes, corner supports are installed.

For power supply to consumers located at some distance from the main overhead line, branch supports are used, on which wires are fixed connected to the overhead line and to the input of the consumer of electricity.
End supports are installed at the beginning and end of the overhead line specifically for the perception of one-sided axial forces.
The designs of various supports are shown in fig. 10.
When designing an overhead line, the number and type of supports are determined depending on the configuration of the route, the cross-section of wires, the climatic conditions of the area, the degree of population of the area, the relief of the route and other conditions.

For overhead lines with voltages above 1 kV, reinforced concrete and wooden antiseptic supports on reinforced concrete attachments are mainly used. The structures of these supports are unified.
Metal supports are mainly used as anchor supports on overhead lines with voltages above 1 kV.
On the VL supports, the arrangement of wires can be any, only the neutral wire in lines up to 1 kV is placed below the phase ones. When suspended on outdoor lighting wire supports, they are placed below the neutral wire.
Wires of overhead lines with voltage up to 1 kV should be hung at a height of at least 6 m from the ground, taking into account the sag.

The vertical distance from the ground to the point of greatest sagging of the wire is called the gauge of the overhead line wire above the ground.
Overhead line wires can come close to other lines along the route, intersect with them and pass at a distance from objects.
The approach dimension of the overhead line wires is the permissible smallest distance from the line wires to objects (buildings, structures) located parallel to the overhead line route, and the intersection gauge is the shortest vertical distance from the object located under the line (intersected) to the overhead line wire.

Rice. 10. Structures of wooden poles for overhead power lines:
a - for voltages below 1000 V, b - for voltages of 6 and 10 kV; 1 - intermediate, 2 - angled with a brace, 3 - angled with a brace, 4 - anchor

Insulators.

The overhead line wires are fastened to the supports using insulators (Fig. 11) mounted on hooks and pins (Fig. 12).
For overhead lines with a voltage of 1000 V and below, insulators TF-4, TF-16, TF-20, NS-16, NS-18, AIK-4 are used, and for branches - SHO-12 with a wire cross section of up to 4 mm 2; TF-3, AIK-3 and SHO-16 with a wire cross section of up to 16 mm 2; TF-2, AIK-2, SHO-70 and ShN-1 with a wire cross section of up to 50 mm 2; TF-1 and AIK-1 with a wire cross section of up to 95 mm 2.

Insulators ShS, ShD, USHL, ShF6-A and ShF10-A and suspension insulators are used to fasten wires of overhead lines with voltages above 1000 V.

All insulators, except for suspension ones, are tightly screwed onto hooks and pins, on which tow is preliminarily wound, soaked in minium or drying oil, or special plastic caps are put on.
For overhead lines with voltages up to 1000 V, KN-16 hooks are used, and above 1000 V - KV-22 hooks made of round steel with a diameter of 16 and 22 mm 2, respectively. On the traverses of the supports of the same overhead lines with a voltage of up to 1000 V, when attaching wires, pins ШТ-Д are used - for wooden traverses and ШТ-С - for steel ones.

When the voltage of overhead lines is more than 1000 V, the pins SHU-22 and SHU-24 are mounted on the traverses of the supports.

According to the conditions of mechanical strength for overhead lines with a voltage of up to 1000 V, single-wire and multi-wire wires are used with a cross section of at least: aluminum - 16 steel-aluminum and bimetallic -10, steel stranded - 25, steel single-wire - 13 mm (diameter 4 mm).

On an overhead line with a voltage of 10 kV and below, passing in an uninhabited area, with an estimated thickness of an ice layer formed on the surface of the wire (ice wall) up to 10 mm, in spans without intersections with structures, the use of single-wire steel wires is allowed if there is a special instruction.
In spans that cross pipelines not intended for flammable liquids and gases, it is allowed to use steel wires with a cross section of 25 mm 2 or more. For overhead lines with voltages above 1000 V, only stranded copper wires with a cross section of at least 10 mm 2 and aluminum wires with a cross section of at least 16 mm 2 are used.

The connection of wires to each other (Fig. 62) is carried out by twisting, in a connecting clamp or in die clamps.

The fastening of wires of overhead lines and insulators is carried out with a knitting wire in one of the ways shown in Fig. 13.
Steel wires are tied with soft galvanized steel wire with a diameter of 1.5 - 2 mm, and aluminum and steel-aluminum wires with aluminum wire with a diameter of 2.5 - 3.5 mm (multi-wire wires can be used).

Aluminum and steel-aluminum wires at the attachment points are pre-wrapped with aluminum tape to protect them from damage.

On intermediate supports, the wire is fixed mainly on the head of the insulator, and on the corner supports - on the neck, placing it on the outside of the angle formed by the line wires. The wires on the head of the insulator are fixed (Fig. 13, a) with two pieces of knitting wire. The wire is twisted around the head of the insulator so that its ends different lengths were on both sides of the neck of the insulator, and then two short ends are wrapped 4-5 times around the wire, and two long ones are transferred through the head of the insulator and also wrapped around the wire several times. When attaching the wire to the neck of the insulator (Fig. 13, b), the knitting wire loops around the wire and the neck of the insulator, then one end of the knitting wire is wrapped around the wire in one direction (from top to bottom), and the other end - in the opposite direction (from bottom to top).

On anchor and end supports, the wire is fixed with a plug on the neck of the insulator. In places where overhead lines cross railways and tram tracks, as well as at intersections with other power lines and communication lines, double fastening of wires is used.

All wooden details when assembling the supports, they are tightly adjusted to each other. The gap in the places of cuts and joints should not exceed 4 mm.
Racks and attachments to overhead line supports are made in such a way that the wood at the junction does not have knots and cracks, and the joint is completely tight, without gaps. The working surfaces of the cuts must be continuous cut (without grooving wood).
Holes are drilled in logs. It is forbidden to burn holes with heated rods.

Bandages for pairing attachments with a support are made of soft steel wire with a diameter of 4 - 5 mm. All turns of the bandage must be evenly stretched and fit snugly to each other. In the event of a break in one turn, the entire bandage should be replaced with a new one.

When connecting wires and cables of overhead lines with a voltage above 1000 V, no more than one connection for each wire or cable is allowed in each span.

When using welding to connect wires, there should be no burnout of the wires of the outer layer or violation of welding when the connected wires are bent.

Metal poles, protruding metal parts of reinforced concrete poles and all metal parts of wooden and reinforced concrete poles of overhead lines are protected with anti-corrosion coatings, i.e. paint. Places of assembly welding of metal supports are primed and painted to a width of 50 - 100 mm along the weld immediately after welding. Parts of structures that are subject to concreting are covered with cement laitance.

Rice. 14. Ways of fastening wires with viscous to insulators:
a - head knit, b - side knit

During operation, overhead power lines are periodically inspected, as well as preventive measurements and checks are made. The value of wood decay is measured at a depth of 0.3 - 0.5 m. The support or attachment is considered unsuitable for further use if the depth of decay along the radius of the log is more than 3 cm with a log diameter of more than 25 cm.

Extraordinary inspections of overhead lines are carried out after accidents, hurricanes, in case of fire near the line, during ice drifts, ice, frost below -40 ° C, etc.

If a break is found on the wire of several wires with a total cross section of up to 17% of the wire cross section, the break is blocked by a repair sleeve or bandage. A repair sleeve on a steel-aluminum wire is installed when up to 34% of aluminum wires break. If broken large quantity lived, the wire must be cut and connected using a connecting clamp.

Insulators can suffer punctures, glaze burns, melting of metal parts, and even destruction of porcelain. This occurs in the event of breakdown of insulators by an electric arc, as well as in the deterioration of their electrical characteristics as a result of aging during operation. Often breakdowns of insulators occur due to severe contamination of their surface and at voltages exceeding the operating voltage. Data on defects found during inspections of insulators are entered in the defect log, and plans are made based on these data. repair work air lines.

Cable power lines.

A cable line is a line for the transmission of electrical energy or individual impulses, consisting of one or more parallel cables with connecting and end sleeves (terminals) and fasteners.

Protective zones are installed above underground cable lines, the size of which depends on the voltage of this line. So, for cable lines with voltage up to 1000 V, the security zone has a platform size of 1 m on each side of the extreme cables. In cities, under sidewalks, the line should run at a distance of 0.6 m from buildings and structures and 1 m from the carriageway.
For cable lines with voltages above 1000 V, the security zone has a size of 1 m on each side of the outermost cables.

Submarine cable lines with voltage up to 1000 V and above have a security zone defined by parallel straight lines at a distance of 100 m from the outermost cables.

The cable route is chosen taking into account its lowest consumption and ensuring safety from mechanical damage, corrosion, vibration, overheating and the possibility of damage to adjacent cables in the event of a short circuit on one of them.

When laying cables, it is necessary to observe the maximum permissible bending radii, the excess of which leads to a violation of the integrity of the core insulation.

Cable laying in the ground under buildings, as well as through basements and storage facilities is prohibited.

The distance between the cable and the foundations of buildings should be at least 0.6 m.

When laying the cable in the plantation zone, the distance between the cable and tree trunks must be at least 2 m, and in the green zone with shrub plantings, 0.75 m is allowed. less than 2 m, to the axis of the railway track - at least 3.25 m, and for an electrified road - at least 10.75 m.

When laying the cable parallel to the tram tracks, the distance between the cable and the axis of the tram track must be at least 2.75 m.
At the intersection of railways and highways, as well as tram tracks, cables are laid in tunnels, blocks or pipes across the entire width of the exclusion zone at a depth of at least 1 m from the roadbed and at least 0.5 m from the bottom of drainage ditches, and in the absence of a zone alienation cables are laid directly at the intersection or at a distance of 2 m on both sides of the roadbed.

Cables are laid in a "snake" with a margin equal to 1 - 3% of its length in order to exclude the possibility of dangerous mechanical stresses arising from soil displacements and temperature deformations. It is forbidden to lay the end of the cable in the form of rings.

The number of couplings on the cable should be the smallest, so the cable is laid in full construction lengths. For 1 km of cable lines, there can be no more than four couplings for three-core cables with voltage up to 10 kV with a cross section of up to 3x95 mm 2 and five couplings for sections from 3x120 to 3x240 mm 2. For single-core cables, no more than two sleeves per 1 km of cable lines are allowed.

For connections or cable terminations, the ends are cut, i.e. stepwise removal of protective and insulating materials. The dimensions of the cut are determined by the design of the coupling that will be used to connect the cable, the voltage of the cable and the cross section of its conductive cores.
The finished cutting of the end of a three-core cable with paper insulation is shown in fig. 15.

The connection of the ends of the cable with voltage up to 1000 V is carried out in cast iron (Fig. 16) or epoxy couplings, and with a voltage of 6 and 10 kV - in epoxy (Fig. 17) or lead couplings.


Rice. 16. Cast iron coupling:
1 - upper sleeve, 2 - resin tape winding, 3 - porcelain spacer, 4 - cover, 5 - tightening bolt, 6 - ground wire, 7 - lower half sleeve, 8 - connecting sleeve

The connection of the conductors of the cable with voltage up to 1000 V is carried out by crimping in the sleeve (Fig. 18). To do this, a sleeve, a punch and a matrix, as well as a crimping mechanism (press tongs, hydraulic press, etc.), are selected according to the cross section of the connected conductive wires, the inner surface of the sleeve is cleaned to a metallic sheen with a steel brush (Fig. 18, a), and the connected wires - with a brush - on carded tapes (Fig. 18, b). Round multi-wire sector cable cores with universal pliers. The cores are inserted into the sleeve (Fig. 18, c) so that their ends touch and are located in the middle of the sleeve.

Rice. 17. Epoxy coupling:
1 - wire bandage, 2 - coupling body, 3 - bandage of harsh threads, 4 - spacer, 5 - core winding, 6 - ground wire, 7 - core connection, 8 - sealing winding


Rice. 18. Connection of copper conductors of the cable by crimping:

a - cleaning the inner surface of the sleeve with a steel wire brush, b - stripping the core with a brush made of cardolent tape, c - installing the sleeve on the connected cores, d - crimping the sleeve in a press, e - finished connection; 1 - copper sleeve, 2 - ruff, 3 - brush, 4 - core, 5 - press

The sleeve is installed flush in the matrix bed (Fig. 18, d), then the sleeve is pressed with two indentations, one for each core (Fig. 18, e). The indentation is made in such a way that the punch washer at the end of the process abuts against the end (shoulders) of the matrix. The residual cable thickness (mm) is checked using a special caliper or caliper (H value in Fig. 19):

4.5 ± 0.2 - with a cross section of the connected cores 16 - 50 mm 2

8.2 ± 0.2 - with a cross section of the connected cores 70 and 95 mm 2

12.5 ± 0.2 - with a cross section of the connected cores 120 and 150 mm 2

14.4 ± 0.2 - with a cross section of the connected cores 185 and 240 mm 2

The quality of the pressed cable contacts is checked by external inspection. At the same time, attention is paid to the indentation holes, which should be located coaxially and symmetrically with respect to the middle of the sleeve or the tubular part of the tip. There should be no tears or cracks at the points of indentation of the punch.

To ensure the appropriate quality of cable crimping, the following work conditions must be met:
use lugs and sleeves, the cross section of which corresponds to the design of the cable cores to be terminated or connected;
use dies and punches corresponding to the standard sizes of tips or sleeves used in crimping;
do not change the cross section of the cable core to facilitate the insertion of the core into the tip or sleeve by removing one of the wires;

do not pressurize without preliminary cleaning and lubrication with quartz-vaseline paste of the contact surfaces of the tips and sleeves on aluminum conductors; finish crimping not earlier than the punch washer comes close to the end of the die.

After connecting the cable cores, a metal belt is removed between the first and second annular notches of the sheath and a bandage of 5-6 turns of harsh threads is applied to the edge of the belt insulation under it, after which spacer plates are installed between the cores so that the cable cores are held at a certain distance from each other. friend and from the clutch housing.
The ends of the cable are laid in the sleeve, having previously wound I onto the cable at the points of its entry and exit from the sleeve 5-7 layers of resin tape, and then fasten both halves of the sleeve with bolts. The grounding conductor, soldered to the armor and cable sheath, is led under the fixing bolts and thus firmly fixed to the sleeve.

The operations of cutting the ends of cables with a voltage of 6 and 10 kV in a lead sleeve are not much different from similar operations of connecting them in a cast-iron sleeve.

Cable lines can provide reliable and durable operation, but only if the technology is followed installation work and all requirements of the rules of technical operation.

The quality and reliability of the mounted cable glands and terminations can be improved if the installation kit is used. necessary tool and devices for cutting the cable and connecting the cores, heating the cable mass, etc. The qualification of the personnel is of great importance for improving the quality of the work performed.

For cable connections, sets of paper rollers, rolls and bobbins of cotton yarn are used, but they are not allowed to have folds, torn and crumpled places, or be dirty.

Such kits are supplied in cans depending on the size of the couplings by numbers. The jar at the installation site must be opened and heated to a temperature of 70 - 80 °C before use. Heated rollers and rolls are checked for the absence of moisture by immersing paper tapes in paraffin heated to a temperature of 150 ° C. In this case, crackling and foaming should not be observed. If moisture is detected, the set of rollers and rolls is rejected.
The reliability of cable lines during operation is supported by the implementation of a set of measures, including cable heating control, inspections, repairs, preventive tests.

To ensure long-term operation of the cable line, it is necessary to monitor the temperature of the cable cores, since overheating of the insulation causes accelerated aging and a sharp reduction in the service life of the cable. The maximum allowable temperature of the conductors of the cable is determined by the design of the cable. So, for cables with a voltage of 10 kV with paper insulation and viscous non-flowing impregnation, a temperature of not more than 60 ° C is allowed; for cables with a voltage of 0.66 - 6 kV with rubber insulation and viscous non-flowing impregnation - 65 ° C; for cables with voltage up to 6 kV with plastic (made of polyethylene, self-extinguishing polyethylene and polyvinyl chloride plastic compound) insulation - 70 ° C; for cables with a voltage of 6 kV with paper insulation and depleted impregnation - 75 ° C; for cables with a voltage of 6 kV with plastic (from vulcanized or self-extinguishing polyethylene or paper insulation and viscous or depleted impregnation - 80 ° C.

Long-term permissible current loads on cables with insulation made of impregnated paper, rubber and plastic are selected according to the current GOSTs. Cable lines with a voltage of 6 - 10 kV, carrying loads less than the nominal ones, can be temporarily overloaded by an amount that depends on the type of laying. So, for example, a cable laid in the ground and having a preload factor of 0.6 can be overloaded by 35% for half an hour, 30% for 1 hour and 15% for 3 hours, and with a preload factor of 0.8 - by 20% for half an hour, by 15% - 1 hour and by 10% - 3 hours.

For cable lines that have been in operation for more than 15 years, the overload is reduced by 10%.

The reliability of the cable line depends to a large extent on the correct organization of operational supervision of the condition of the lines and their routes through periodic inspections. Scheduled inspections make it possible to identify various violations on cable routes (excavation work, warehousing, planting trees, etc.), as well as cracks and chips on the insulators of the end sleeves, weakening of their fastenings, the presence of bird nests, etc.

A great danger to the integrity of the cables is the excavation of the earth, carried out on the routes or near them. An organization operating underground cables must provide an observer during the excavation in order to prevent damage to the cable.

According to the degree of danger of damage to cables, earthworks are divided into two zones:

I zone - a piece of land located on the cable route or at a distance of up to 1 m from the extreme cable with a voltage above 1000 V;

Zone II - a piece of land located at a distance of more than 1 m from the outermost cable.

When working in zone I, it is prohibited:

use of excavators and other earth-moving machines;
the use of impact mechanisms (wedge-women, ball-women, etc.) at a distance closer than 5 m;

the use of mechanisms for excavating soil (jackhammers, electric hammers, etc.) to a depth of more than 0.4 m at a normal cable laying depth (0.7 - 1 m); earthworks in winter time without preliminary heating of the soil;

performance of work without supervision by a representative of the organization operating the cable line.

In order to timely identify defects in cable insulation, connecting and terminations and prevent sudden cable failure or destruction by short circuit currents, preventive tests of cable lines with increased DC voltage are carried out.