Overhead power lines. cable structures include

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

I. By type of current

Drawing. VL DC voltage 800 kV

Currently, the transmission of electrical energy is carried out mainly using 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, for example, solar power plants), and the main consumers are alternating current machines.

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

Drawing. 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, B - rectifier, I - inverter, N - load (consumer).

The advantages of transmitting electricity via overhead lines using direct current are as follows:

1. The construction of an overhead line is cheaper, since the transmission of direct current electricity can be carried out over one (monopolar circuit) or two (bipolar circuit) wires.
2. Electricity can be transferred between power systems that are not synchronized in frequency and phase.
3. When transmitting large volumes of electricity over long distances, losses in direct current power lines become less than when transmitting on alternating current.
4. The limit of transmitted power according to the stability of the power system is higher than that of alternating current 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 widely used at present, so in the future we will consider the installation and operation of AC overhead lines.

II. By purpose

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

III. By voltage

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

For an experienced electrician who has been working with overhead power lines for many years, it will not be difficult to visually determine the voltage of the overhead power line by
the type of insulators, supports, and the number of wires in the line without any devices. Although in most cases, to determine the voltage on an overhead line, you just need to look at the insulators. After reading this article, you will also be able to easily determine the voltage of overhead lines using insulators.

Photo 1. Pin insulators for voltage 0.4, 6-10, 35 kV.

Every person should know this! But why, why should a person far from the electric power industry be able to determine the voltage of an overhead power line using appearance insulators and the number of insulators in an overhead line garland? The answer is obvious, it's all about electrical safety. After all, for each voltage class of overhead lines, there are minimum permissible distances, closer than which approaching the overhead line wires is deadly.

In my practice, there were several accidents associated with the inability to determine the voltage class of overhead lines. Therefore, below is a table from the safety rules, which indicates the minimum permissible distances, the closer of which it is deadly to approach live parts that are energized.

Table 1. Permissible distances to live parts that are energized.

*D.C.

The first incident occurred at the construction site of a country house. For some unknown reason, there was no electricity at the construction site; a 10 kV overhead line ran near the unfinished house. Two workers decided to power an extension cord from this overhead line to connect power tools. After stripping two wires on the extension cord and making hooks, they decided to use a stick to hook them to the wires. On a 0.4 kV overhead line, this scheme would work. But since the voltage of the overhead line was 10 kV, one worker received serious electrical injuries, and miraculously survived.

The second incident occurred on the territory of the production base while unloading pipes. A working slinger was unloading metal pipes from a truck using a truck crane in the coverage area of ​​a 110 kV overhead line. During unloading, the pipes bent so that one end came dangerously close to the wires. And even despite the fact that there was no direct contact of the wires with the load, due to high voltage a breakdown occurred and the worker died. After all, you can be killed by electric shock from a 110 kV overhead line even without touching the wires, you just need to get close to them. I think it’s now clear why it is so important to be able to determine the voltage of overhead lines by the type of insulators.

The main principle here is that the higher the power line voltage, the greater the number of insulators in the garland. By the way, the highest voltage power line in the world is located in Russia, its voltage is 1150 kV.

The first type of line whose voltage you need to know in person is a 0.4 kV overhead line. These overhead line insulators are the smallest, usually pin insulators made of porcelain or glass, mounted on steel hooks. The number of wires in such a line can be either two, if it is 220V, or 4 or more, if it is 380V.

Photo 2. wooden support of 0.4 kV overhead line.

The second type is VL-6 and 10 kV; outwardly they do not differ. 6 kV overhead lines are gradually becoming a thing of the past, giving way to 10 kV overhead lines. The insulators of these lines are usually pin-type, but are noticeably larger than 0.4 kV insulators. Suspension insulators, one or two in a garland, can be used on corner supports. They are also made of glass or porcelain, and are mounted on steel hooks. So: the main thing visual difference VL-0.4 kV from VL-6, 10 kV, these are larger insulators, as well as only three wires in the line.

Photo 3. Wooden support of 10 kV overhead line.

The third type is 35kV overhead line. Suspended insulators, or pin insulators, are already used here, but of a much larger size. The number of pendant insulators in a garland can be from three to five, depending on the support and type of insulators. The supports can be either concrete or made of metal structures, as well as wood, but then it will also be a structure, and not just a pole.

Photo 4. Wooden support of 35 kV overhead line.

110 kV overhead line from 6 insulators in a garland. Each phase, single wire. The supports can be reinforced concrete, wooden (almost never used) or assembled from metal structures.

220 kV overhead line from 10 insulators in a garland. Each phase is carried out with a thick single wire. With voltages above 220 kV, supports are assembled from metal structures or reinforced concrete.

Photo 5. Reinforced concrete support of 110 kV overhead line.

330 kV overhead line from 14 insulators in a garland. There are two wires in each phase. The security zone of these overhead power lines is 30 meters on both sides of the outermost wires.

Photo 7. 330 kV transmission line support.

500 kV overhead line from 20 insulators in a garland, each phase is carried out with a triple wire arranged in a triangle. Security zone 40 meters.

Photo 8. 500 kV transmission line support.

750 kV overhead line from 20 insulators in a garland. Each phase has 4 or 5 wires arranged in a square or ring. Security zone 55 meters.

Photo 9. 750 kV transmission line support.

Table 2. Number of insulators in an overhead line garland.

What do the inscriptions on overhead line supports mean?

Surely many have seen the inscriptions on power transmission towers in the form of letters and numbers, but not everyone knows what they mean.

Photo 10. Designations on power line supports.

They mean the following: a capital letter indicates the voltage class, for example T-35 kV, S-110 kV, D-220 kV. The number after the letter indicates the line number, the second number indicates the serial number of the support.

T means 35 kV.
45 is the line number.
105 is the serial number of the support.
This method of determining power line voltage by the number of insulators in a garland is not accurate and does not provide a 100% guarantee. Russia is a huge country, therefore, for different operating conditions of power lines (cleanliness of the surrounding air, humidity, etc.), designers calculated different numbers of insulators and used different types supports But if you approach the issue comprehensively and determine the voltage according to all the criteria described in the article, then you can quite accurately determine the voltage class. If you are far from the electric power industry, then for a 100% determination of the power line voltage, it is still better for you to contact your local energy company.

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Subtitles

Overhead power lines

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

Composition of VL

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

Documents regulating overhead lines

Classification of overhead lines

By type of current

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

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

Such lines are not widely used.

By purpose

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

By voltage

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

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

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

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

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

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

According to the operating mode of neutrals in electrical installations

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

According to the operating mode depending on the mechanical condition

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

Main elements of overhead lines

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

Installation of overhead power lines

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

Cable power lines

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

Classification

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

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

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

Cable structures

Cable structures include:

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

Fire safety

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

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

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

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

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

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

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

High temperature superconductors

HTSC wire

Losses in power lines

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

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

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

Losses in AC power lines

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

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

Natural power and transmission capacity of power lines

Natural power

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

Bandwidth

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

Characteristics of the capacity of overhead power lines