Design of overhead power transmission line supports
Support design
The designs of overhead power transmission line supports are very diverse and depend on the material from which the support is made (metal, reinforced concrete, wood, fiberglass), the purpose of the support (intermediate, corner, transposition, transition, etc.), and on local conditions on the line route ( populated or unpopulated area, mountainous conditions, areas with marshy or soft soils, etc.), line voltage, number of circuits (single-circuit, double-circuit, multi-circuit), etc.
The following elements can be found in the design of many types of supports:
- The stand is the main integral element of the support structure, in contrast to other elements that may be missing. The rack is designed to provide the required dimensions of the wires (wire size is the vertical distance from the wire in the span to the engineering structures crossed by the route, the surface of the earth or water). The support structure can have one, two, three or more posts.
- Struts – used for corner, end, anchor and branch supports of overhead lines with voltage up to 10 kV. They take on part of the load of the support from the one-sided pull of the wire.
- Attachment (stepson) - partially buried in the ground, the lower part of the structure of a combined overhead line support with voltage up to 35 kV, consisting of wooden racks and reinforced concrete attachments.
- Braces are inclined elements of a support that serve to strengthen its structure and connect several support elements to each other, for example, a post with a traverse, or two support posts.
- Crossbar – provides fastening of power line wires at a certain (permissible) distance from the support and from each other.
- Foundation is a structure embedded in the ground and transferring loads from supports, insulators, wires and external influences (ice, wind) to it.
- Crossbar - increases the lateral surface of the underground structure of reinforced concrete racks and footboards of metal supports. Crossbars increase the ability of the foundation to withstand horizontal loads acting on the support, preventing it from tipping over from the gravitational forces of the wires when constructing supports in soft soil.
- Guys - designed to increase the stability of supports and absorb the forces from the tension of the wire.
- Cable stand - the upper part of the support, designed to support the lightning protection cable. Usually it is a trapezoidal spire at the top of the support. The support may have one or two cable supports (on U-shaped supports); there are also supports without cable supports.
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Drawing. Overhead line supports: a – two-post support; b – three-post support.
A stand of lattice-type metal supports is called a trunk. The barrel is usually a tetrahedral truncated lattice pyramid made of rolled steel profiles (angle, strip, sheet), and consists of a belt, a lattice and a diaphragm. The lattice, in turn, has bracing rods and struts, as well as additional connections.
Drawing. Structural elements of a metal support: 1 – belt of the support post; 2 – brace rods forming the rack lattice; 3 – diaphragm; 4 – traverse; 5 – cable support.
Drawing. Corner support with two struts: 1 – stand; 2 – strut.
Drawing. Structural elements of a combined support: 1 – wooden support post; 2 – reinforced concrete attachment (stepson); 3 – brace; 4 – traverse.
Drawing. Support traverses: a - for reinforced concrete supports 10 kV; b - for reinforced concrete supports 110 kV.
Most often you can find traverses in the form of a rigid metal structure, but there are also wooden traverses and traverses made of composite materials.
Drawing. Cross beam of 110 kV overhead line support made of composite materials
In addition, on V-shaped supports of the “nabla” type and U-shaped supports, you can find so-called flexible traverses.
Drawing. Overhead line support with a “flexible” crossarm
In some designs of supports, traverses may be absent, for example, wooden or reinforced concrete overhead line supports with voltages up to 1 kV, overhead line supports with self-supporting insulated wires with voltages up to 1 kV, and anchor supports of overhead lines of any voltage, where each phase is mounted on a separate stand.
Drawing. Support without traverse
Drawing. Mushroom-shaped reinforced concrete foundation
For single-post supports, in which the lower end of the post is embedded in the ground, the bottom of the post serves as the foundation; for metal supports, pile or prefabricated mushroom-shaped reinforced concrete ones are used, and when installing transitional supports and supports in swamps, monolithic concrete foundations are used.
Drawing. Reinforced concrete piles used in single-pile and multi-pile foundations for overhead line supports
Drawing. Power transmission line support on a pile foundation
Drawing. Mushroom-shaped reinforced concrete foundation (1) with three crossbars (2)
Drawing. Support secured with guy wires
The upper part of the guy is attached to the post or crossbeam of the support, and the lower part to the anchor or reinforced concrete slab. In addition, the guy structure may include a tension coupling - a lanyard.
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Drawing. Lower part of the guy
Power transmission line supports are structures that serve to support live wires and lightning protection cables above the earth's surface. They come in various shapes and sizes. The supports can be reinforced concrete, wood, metal or even composite materials. The main elements of power transmission line support are racks, foundations, traverses (crossbars on which the wires are held), cable supports and guy wires are also often used.
ANCHOR SUPPORTS FOR POWER LINES
There are anchor and intermediate supports for power lines. The robust design of the anchor supports can withstand significant forces from wire tension; anchor supports of power lines are installed at the beginning and end of power lines, at turns, when power lines cross small rivers, railways, roads and bridges.
A type of anchor supports - transition supports are used when crossing power lines of rivers and other large obstacles. It is the transition supports that bear the heaviest loads and can themselves reach a height of 300 meters! These poles are the heaviest and tallest of all power line poles; they are often painted in bright colors, for example, red and white poles are often found, and orange, gray and other colors are also used. For more details about transition supports, see the corresponding essay http://io.ua/s73072.
INTERMEDIATE POWER LINE SUPPORTS
Intermediate supports have a less durable structure than anchor ones; they usually serve to support wires and cables on straight sections of power line routes. Most of the supports on the routes are intermediate. As a rule, an intermediate support can be distinguished from an anchor one by the following feature: if the garlands of insulators hang perpendicular to the earth’s surface, then the support is intermediate. And on the anchor supports, the wires are fixed in the clamps of tension garlands; these garlands are like a continuation of the line and are located at an acute angle to the surface of the earth, and sometimes almost parallel.
Also, power line supports are divided into:
- transpositional (to change the order of phases),
- branch,
- cross,
- increased, decreased, etc.
Based on the number of suspended wires (circuits), supports are divided into single- and multi-circuit; by design - single-post, A- and AP-shaped, U-shaped, V-shaped (for example, “Nabla” type), “shot glass” type, etc.
WOODEN POWER SUPPORTS
Today, mainly reinforced concrete and metal power line supports are used. Wooden transmission line supports were installed on power lines with voltages up to 220 kV. Pine and larch poles impregnated with an anti-rot composition (antiseptic) were usually used to make power line supports. Often wooden supports were strengthened on reinforced concrete attachments (stepchildren) or piles. Wooden power line supports were cheap, relatively easy to manufacture and reliable in operation. The first large Soviet power transmission line - Kashirskaya State District Power Plant - Moscow - with a voltage of 110 kV and a length of 120 km was built on wooden supports. Today, power lines with wooden poles are no longer built.
REINFORCED CONCRETE POWER LINE SUPPORTS
Reinforced concrete power transmission line supports, the designs of which were developed in the USSR in 1933, have higher mechanical strength. However, due to the lack of an industrial base, their mass use in the construction of power lines of all voltages began only in 1955. The advantages of reinforced concrete transmission line supports are simplicity of design and manufacturability of factory production. These power transmission towers are usually of a circular or rectangular cross-section and are made primarily of prestressed reinforced concrete.
The most common are intermediate single-post reinforced concrete power line supports with metal cross-arms, which are installed directly in the ground. In addition, on power lines with a voltage of 110-500 kV, intermediate and anchor-corner reinforced concrete transmission line supports with guy wires were widely used.
METAL POWER LINE SUPPORTS
Metal transmission line supports have less weight than reinforced concrete ones and high mechanical strength. This allows you to create supports of considerable height, designed for heavy loads. They are used on power lines of all voltages, often in combination with reinforced concrete intermediate supports. Metal transmission line supports are indispensable on lines with heavy loads (for example, on crossings).
Metal power transmission line supports are made mainly of steel, and in some cases of aluminum alloys. According to the manufacturing method, metal transmission line supports are divided into welded ones, which come from factories in the form of finished sections, and bolted ones, which are assembled on the route from individual elements (braces, rods, chords) on bolts.
Metal supports are divided into two broad groups - lattice and MGS (multifaceted bent posts). If the first ones are well known to everyone, then MGS are just beginning to become widespread in the CIS countries. A lot of useful information about these supports can be found on the website www.energobud.com.ua
By voltage, power lines within the CIS are divided into 35 kV, 110 kV, 154 kV (150 kV), 220 kV, 330 kV, 400 kV, 500 kV, 750 kV, 800 kV, 1150 kV and 1500 kV. Most of the power lines in the world operate on alternating current, but there are also lines that operate on direct current, for example, the Volgograd-Donbass DC power line (you can read about these power lines here http://io.ua/s91331).
POWER LINE VOLTAGE CLASSES
It can be difficult for a non-specialist to accurately determine the voltage in a power line, but, as a rule, this can be done in a simple way - counting how many insulators in a garland are suspended on a crossarm. So 35 kV power lines have three to five insulators in each garland. But in the garlands of 110 kV power lines there are already six to ten insulators. If there are ten to fifteen insulators, then it is a 220 kV power line.
If the power line wires are split into two (this is called splitting), then the line can have a voltage of 330 kV. If there are three wires in each phase, then 500 kV, if there are four wires, then 750 kV.
There are exceptions to every rule. Thus, 220 kV and 150 kV lines have splitting, although this is typical for 330 kV lines. 330 kV power lines, in special cases, can operate without splitting.
35 kV -110 kV power lines are used everywhere as distribution networks (for example, a 110 kV power line can supply a substation that powers a small village or microdistrict). The 150 kV class is a more advanced analogue of the hundred-tens; this voltage is used in the Dneproenergo power system and some adjacent areas, as well as in the Kola power system (Kola Peninsula). This voltage class came to the USSR in the early 30s, along with the American equipment of the General Electric company for the Dnieper Hydroelectric Power Plant.
220 kV power lines are mainly used to connect power plants with substations and large consumers. 330 kV lines are often built over long distances, for communication between powerful power plants and substations (interconnections), and sometimes for the needs of very energy-intensive enterprises. Lines with voltages of 400 kV, 500 kV, and 750 kV and higher are also used for intersystem connections and for transmitting electricity over long distances, including to neighboring countries.
UNIFICATION OF POWER LINE SUPPORTS IN THE USSR
In 1976, in connection with the unification of power transmission line supports in the USSR, the following system for designating metal and reinforced concrete supports of 35-330 kV was adopted:
the letters P and PS indicate intermediate supports,
PVS—intermediate with internal connections,
PU or PUS - intermediate corner,
PP - intermediate transitional,
AN US - anchor-corner,
K or KS - end ones.
The letter B indicates reinforced concrete supports, and its absence indicates that the supports are steel. The numbers 35, 110, 150, 220, etc., following the letters indicate the line voltage, and the numbers behind them indicate the standard size of the supports. The letters U and T are added respectively to the designation of intermediate supports used as corner supports and with cable support. And in modern power grid construction, “de-unification” is observed, new original supports are being developed, designed for the conditions of a specific power line route. Thus, in developed countries they have already abandoned the mass use of standard projects. Each line must be built taking into account all the nuances of the relief, climate, etc.
CLASSIFICATION OF POWER LINE SUPPORTS BY GENERAL APPEARANCE
Tower supports
Classic, the most common of all high voltage power line supports. They can have from one to 9 parallel traverses, and are used for single-, double- or multi-circuit power lines. All lattice tower supports have a common feature - their trunk narrows from the base to the top. Divided into two families:
- wide-barreled lattice (if the base of the mast is wider than a freight car, see photo 1). These are the most common supports. They can be single-chain (“Crimean type”), double-chain (“barrel” type) and multi-chain.
The most interesting representatives of single-circuit tower supports are T-shaped supports for DC lines.
- narrow-base lattice (accordingly, their base is somewhat narrower in size than the base of a freight car).
Portal supports
Supports made of metal, wood or reinforced concrete, resembling the letter “P” or the letter “N”. They are widely used on 330-750 kV power lines. As a rule, single-chain.
AP-shaped supports
Single-circuit supports created using welded metal pipes, MGS or wood, resembling the letter “A” in profile and the letter “P” in front. The cross-section of pipes in these supports can reach 1300 mm, and the height can be over 80 m.
Photo 4 shows an example of such a tubular support when crossing a 330 kV line across the Dnieper, in Ukraine. Inside its racks there are stairs for climbing to the top, and in total the support has four legs, each 21 meters high (they are painted in different colors), the total height of the mast is about 85 meters. You can read more here - http://io.ua/s93360.
Three-post freestanding lattice supports
Three-post lattice supports, as a rule, stand at turns and transitions of 500 kV and 750 kV power lines and are used as anchors (photo 5).
L-shaped supports
They are flat L-shaped lattice structures, articulated with two foundations. At the top of the support there is a traverse for attaching 4 load-bearing cables that hold the support in a vertical position. Below there are three more (rarely two) traverses for hanging wires. L-shaped towers were used, in particular, as transition towers for two 110 kV or 220 kV overhead line circuits. Their use allowed us to save metal and simplify the foundation. It was advisable to use such supports in areas flooded with water during floods. Design features prevented these supports from becoming widespread.
Y-shaped supports, "shot glasses"
Single-chain masts resembling the letter “Y” or a glass (photo 6). There are different types and have been used for quite a long time both here and abroad, including as transitional ones (for example, PS-101). These supports are always made of metal, usually lattice, less often they consist of multifaceted bent posts.
V-shaped,"Nabla"
Intermediate pores with guys are used on power transmission line routes of 330-1150 kV, for example, Nabla type supports for 750 kV. They resemble an inverted triangle - nabla. Exclusively single-chain.
Class: "Cat" type supports
Very interesting original supports are very popular in Western Europe, especially in France (photo 10).
Pillar supports(i.e. not lattice)
These are supports based on wooden, metal or reinforced concrete pillars. There are single-post and portal ones. Single-post reinforced concrete supports are the most widely used intermediate power transmission line supports at voltages of 35-220 kV. Relatively recently, a progressive type of metal single-column pole supports has become widespread - using MGS. To be more precise, in the USA such supports have been used for quite a long time, but in the CIS they are just beginning to gain popularity. The use of MGS made it possible to create multi-chain pillar supports (see photo 8).
Portal pillar supports consist of two pillars (wooden, reinforced concrete or MGS) connected by a common cross beam. Particularly widespread in our country are pole-mounted single-circuit portal reinforced concrete supports (with internal connections) for 220 and 330 kV lines (photo 9).
Non-standard supports
These include various non-standard supports and exotic ones that do not belong to this classification, for example, numerous decorative supports.
2011 "POWERLINER"
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The main elements of overhead power lines include: wires, traverses, insulators, fittings, supports, lightning protection cables, arresters, grounding, sectioning 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 operational needs (high-frequency communication equipment, capacitive power take-off, etc.), as well as marking elements for high-voltage wires and power line supports 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, Illuminated fence lights are used to indicate daytime and nighttime).
Types and classification of power transmission line supports
Power transmission line supports are designed for the construction of power lines and are one of its main structural elements, responsible for fastening and hanging electrical wires at a certain level.
Depending on the method of hanging wires, supports are divided into two main groups:
- § intermediate supports on which the wires are secured in supporting clamps;
- § anchor-type supports used to tension wires; on these supports the wires are secured in tension clamps.
- § These types of supports are divided into types that have a special purpose.
- § Intermediate straight supports are installed on straight sections of the line. On intermediate supports with suspended insulators, the wires are fixed in supporting garlands hanging vertically; on supports with pin insulators, the wires are secured by wire knitting.
Figure 4. Scheme of the anchored section of the overhead line
- § In both cases, intermediate supports perceive horizontal loads from the wind pressure on the wires and on the support, and vertical loads from the weight of the wires, insulators and the own weight of the support.
- § Intermediate corner supports are installed at the angles of rotation of the line with wires suspended in supporting garlands. In addition to the loads acting on intermediate straight supports, intermediate and anchor-corner supports also perceive loads from the transverse components of the tension of wires and cables. At power line rotation angles of more than 20°, the weight of the intermediate corner supports increases significantly. At large rotation angles, anchored corner supports are installed.
When installing anchor supports on straight sections of the route and suspending wires on both sides of the support with equal tensions, the horizontal longitudinal loads from the wires are balanced and the anchor support works in the same way as an intermediate one, that is, it perceives only horizontal transverse and vertical loads. If necessary, the wires on one and the other side of the support can be pulled with different wire tensions. In this case, in addition to horizontal transverse and vertical loads, the support will be affected by horizontal longitudinal load.
When installing anchor supports at corners, the anchor-corner supports also absorb the load from the transverse components of the tension of wires and cables.
Figure 5. Anchor-corner support of a multi-chain line
End supports are installed at the ends of the line. Wires extend from these supports and are suspended on substation portals.
In addition to the listed types of supports, special supports are also used on lines: transpositional, used to change the order of the wires on the supports; branch lines - for making branches from the main line; supports for large crossings across rivers and bodies of water, etc.
Wooden, steel and reinforced concrete supports are used on power lines. Experimental structures made of aluminum alloys have also been developed.
Steel is the main material from which metal supports and various parts (traverses, cable supports, guy wires) of the supports are made. The advantage of steel supports compared to reinforced concrete ones is their high strength with low weight.
According to the design of the shaft, steel supports can be classified into two main schemes - tower (single-column) and portal; according to the method of fastening to the foundations - to free-standing supports and guyed supports; according to the method of connecting elements they are divided into welded and bolted.
The supports are made of rolled steel, and in the vast majority of cases an isosceles angle is used; high transition supports can be made of steel pipes.
In the CIS there are several main centers for the production of steel structures for power transmission line supports - central, Ural and Siberian
Classification of power transmission line supports
Power line supports are divided according to the following criteria.
By purpose:
- § Intermediate supports are installed on straight sections of the overhead line route, are intended only to support wires and cables and are not designed for loads from the tension of wires along the line. Typically they make up 80-90% of all overhead line supports.
- § Corner supports are installed at the angles of rotation of the overhead line route; under normal conditions, they perceive the resultant tension forces of wires and cables of adjacent spans, directed along the bisector of the angle that complements the angle of rotation of the line by 180°. At small rotation angles (up to 15-30°), where the loads are small, angular intermediate supports are used. If the rotation angles are greater, then corner anchor supports are used, which have a more rigid structure and anchor the wires.
- § Anchor supports are installed on straight sections of the route for crossing engineering structures or natural obstacles; they take the longitudinal load from the tension of wires and cables. Their design is rigid and durable.
- § End supports are a type of anchor and are installed at the end or beginning of the line. Under normal operating conditions of overhead lines, they perceive the load from one-sided tension of wires and cables.
- § Special supports: transpositional - to change the order of wires on the supports; branch lines - for installing branches from the main line; cross - when overhead lines intersect in two directions; anti-wind - to enhance the mechanical strength of overhead lines; transitional - when crossing overhead lines through engineering structures or natural barriers.
According to the method of fixation in the ground
- § Supports installed directly into the ground
- § Supports installed on foundations
By design
- § Free-standing supports
- § Supports with guys
- a) intermediate single-circuit on guy wires 500 kV
- b) intermediate free-standing support
By number of circuits
- § Single-chain
- § Double circuit
- § Multichain
Figure 6. Metal supports
By voltage
The supports are divided into supports for lines 0.4, 6, 10, 35, 110, 220, 330, 500, 750 kV. These groups of supports differ in size and weight. The greater the tension, the higher the support, the longer its traverse and the greater its weight. The increase in the size of the support is caused by the need to obtain the required distances from the wire to the body of the support and to the ground, corresponding to the PUE for different line voltages.
According to the material of manufacture
- § Reinforced concrete - made of concrete reinforced with metal. For lines 35-110 kV and above, supports made of centrifuged concrete are usually used. The advantage of reinforced concrete supports is their resistance to corrosion and the effects of chemical reagents in the air.
- § Metal - made from special grades of steel. Individual elements are connected by welding or bolts. To prevent oxidation and corrosion, the surface of metal supports is galvanized or periodically painted with special paints.
Figure 7. Reinforced concrete power line support
Figure 8. Metal power line support
- § Metal supports are in turn divided into:
- § Metal lattice supports
- § Metal polyhedral supports
- § Wooden - made from round logs. The most common are pine supports and somewhat less common are larch supports. Wooden poles are used for lines with voltages up to 220 kV inclusive in the CIS and up to 345 kV in the USA. The main advantages of these supports are their low cost (if local wood is available) and ease of manufacture. The main disadvantage is wood rotting, especially intense at the point of contact of the support with the soil. Impregnation of wood with special antiseptics increases its service life from 4-6 to 15-25 years. To increase service life, a wooden support is usually made not from a whole log, but from a composite one: from a longer main post and a short chair, stepson, or reinforced concrete post. The chair is secured to the main post using a wire band. Composite wooden supports with reinforced concrete chairs are widely used. Wooden supports are made A-shaped or U-shaped. The U-shaped structure is more stable, but requires greater investment due to increased material consumption compared to the A-shaped one.
Figure 9. U-shaped wooden power line support
The service life of reinforced concrete and metal galvanized or periodically painted supports reaches 50 years or more. The cost of metal and reinforced concrete supports significantly exceeds the cost of wooden supports. The choice of a particular material for supports is determined by economic considerations, as well as the availability of appropriate material in the area where the line is being constructed.
Based on many years of practice in the construction, design and operation of overhead lines, the most appropriate and economical types and designs of supports for the corresponding climatic and geographical regions are determined and their unification is carried out.
What associations arise when mentioning overhead power lines? Of course, the wires are stretched through the air from support to support or from pillar to post. Moreover, visually, the greater the span between the supports, the higher the wires are stretched, therefore, the higher the support itself should be. In fact, there is no direct relationship between the height of the support and the length of the span.
The basis for designing power lines is the voltage of the overhead line and its power. Using them, the cross-section and type of wire (cable) are calculated, the weight of the cable is determined from the cross-section, the lengths of anchor and intermediate spans, as well as the types and sizes of supports are calculated from the weight. Also, the type of support depends on the number of “threads” of wires that are planned for the power line section, what branches will have to be made, etc.
Types of transmission line supports
In the process of development of power transmission lines, four types of supports have been established based on the material from which they are made:
- Wooden supports;
- Reinforced concrete supports;
- Metal supports;
- Prefabricated supports.
First things first.
Wooden power transmission poles
The wooden support is historically the oldest of all types of supports. By design, a wooden support is a pole made of coniferous timber, using the rounding method, 8.5 - 13 meters long. Parts for wooden supports are also made from wood: traverses (a wooden horizontal beam on a support), struts (attaching a traverse to a support), crossbars (a crossbar on the edge of a support and a strut dug into the ground).
Advantages of wooden supports
Wooden supports, like any building material, have their advantages and disadvantages. The advantages of wooden supports include their low cost, light weight and flexibility during an earthquake. We must not forget about the general availability of wooden supports. The low weight of the supports makes it easier to install them, and also simplifies the delivery and unloading/loading of the supports at the preparatory stage of work. But wooden supports have more than enough disadvantages.
Disadvantages of wooden supports
- Firstly, wooden supports burn well;
- Being biological material, they rot, mold, and are corroded by bugs;
- In the rain they get wet, swell, and crack.
But in defense of wooden poles, it is worth noting that modern technologies for impregnating poles, and this is impregnation of 100% sapwood of a pole, manufacturers guarantee a 50-year service life of wooden poles, even buried in the ground.
Note: Sapwood is the weak layer of wood found between the bark and the core of a log.
For more details on the designs of wooden poles, read the article: Wooden power line poles.
- Standards: GOST 9463-88, GOST 20022.0-93.
To reduce the contact of wood with the ground, prefabricated supports were used.
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Prefabricated supports
The prefabricated support consists of two parts. The lower part is called the stepson and is made of reinforced concrete, the upper part is a wooden pole. The two parts are connected with steel wire in two places. It is worth noting that instead of a reinforced concrete stepson, a wooden stepson can be used. Prefabricated supports also include supports assembled from a reinforced concrete frame and a metal upper part.
For more details on the designs of prefabricated poles, read the separate article: Prefabricated power line poles.
Reinforced concrete supports, reinforced concrete pillars
Reinforced concrete supports have long replaced wooden supports. They have firmly won the love and recognition of both electricians and customers. And there are several reasons for this.
- Reinforced concrete supports are not subject to damage typical of wooden supports;
- The service life of reinforced concrete supports is practically unlimited;
- Inside the concrete support there is reinforcement, which is used to re-ground the overhead lines. Moreover, the ends of the grounding reinforcement are brought out, above and below the pole. The outlet of the reinforcement simplifies installation, and the protection of the grounding descent with concrete increases electrical safety.
Reinforced concrete supports are marked as SV 95/105/110/164 and are intended for overhead lines of various capacities. Let's look at the photo.
- Regulatory documents: TU 5863-007-00113557-94
Metal power line supports
For overhead power lines of high power and extremely high currents, metal supports are used. Despite the fact that this type of support is made of special steel, they are “afraid” of corrosion and to protect against it, metal supports are coated with an anti-corrosion compound. Depending on the size of the support, the metal support can be prefabricated or welded. The prefabricated support is delivered to the site separately.
They are assembled locally and installed on a pre-prepared foundation. Installation of a metal support is a complex technological process, using traction mechanisms, usually tractors. The support is bolted to the foundation, pre-aligned strictly vertically. Metal supports are practically not used in private housing construction and in countryside partnerships of various types, with the exception of round metal poles.
There are so many designs of metal supports that I had to write a separate article: Metal supports and their designs.
Electricity is the main form of energy used everywhere today. Its widespread use became possible thanks to electrical networks that connect sources and consumers of electricity. Power lines, or power lines for short, perform the function of transporting electricity. They are laid either above the surface of the earth and are called “aerial”, or buried in the ground and or under water and are called “cable”.
Overhead power lines, despite their complex infrastructure, are cheaper than cable lines. The high-voltage cable itself is an expensive and complex product. For this reason, these cables are laid only in some sections of the overhead power line route in places where it is impossible to install supports with wires, for example through sea straits, wide rivers, etc. Cables are used to lay electrical networks in populated areas, where the construction of supports is also impossible due to the urban infrastructure.
Power lines, despite their great length, are still the same electrical circuits for which Ohm’s law applies in the same way as for others. Therefore, the efficiency of power transmission lines is directly related to the increase in voltage in it. The current strength decreases, and with it the losses become less. For this reason, the further consumers are located from the power plant, the higher the high-voltage power line should be. Modern ultra-long-distance power lines transmit electrical energy with voltages of millions of volts.
But increasing voltage to reduce losses has limitations. They are caused by corona discharge. This phenomenon manifests itself, causing noticeable energy losses, starting with voltages above 100 kilovolts. The buzzing and crackling of high-voltage wires is a consequence of corona discharge on them. For this reason, in order to reduce corona losses, starting from 220 kilovolts, two or more wires are used for each phase of an overhead power line.
The length of power lines and their operating voltage are interconnected.
- Ultra-long-distance power lines operate with voltages from 500 kilovolts.
- 220 and 330 kilovolts are voltages for main power lines.
- 150, 110, and 35 kilovolts are the voltages of distribution power lines.
- Voltages of 20 kilovolts and less are typical for local power grids through which end consumers are supplied with electricity.
Wire supports
In addition to wires, power transmission lines also include supports as the main structural elements. Their purpose is to hold wires. Each power line has several types of supports, as shown in the image below:
Anchor supports bear heavy loads and therefore have a strong, rigid structure, which can be very diverse. All supports come into contact with soft or damp soil through a concrete foundation. Wells are made in solid soil into which power line supports are directly immersed. Examples of metal anchor support designs are shown in the image below:
The supports can also be made using concrete or wood. Wooden supports, although less durable, are one and a half times cheaper in comparison with metal and concrete structures. Their use is especially justified in regions with severe frosts and large reserves of wood. Wooden poles are most widely used in electrical networks with voltages up to 1000 Volts. The design of such supports is shown in the image below:
Power line wires
The wires of modern power lines are mainly made of aluminum wire. Pure aluminum wires are used for local power lines. The limitation is the span length between the supports is 100 - 120 meters. For longer spans, aluminum and steel wires are used. Such a wire has a steel cable inside, covered with aluminum conductors. The cable takes mechanical load, aluminum – electrical load.
All-steel wires are used only in short-term areas where maximum strength is required with minimal wire weight. All power lines with voltages above 35 kilovolts are equipped with a steel cable to protect against lightning strikes. Wires made of copper and bronze are currently used only in power lines for special purposes. Copper and aluminum wire are used to make hollow tubular wires. This is done to reduce corona losses and reduce radio interference. Images of wires of various designs are shown below:
The wire for power lines is selected taking into account the operating conditions and the resulting mechanical loads. In the warm season, it is the wind that swings the wires and increases the breaking load. In winter, ice is added to the wind. The weight of a layer of ice on the wires significantly increases the load on them. Moreover, a decrease in temperature leads to a decrease in the length of the wires and increases the internal stress in their material.
Insulators and fittings
Insulators are used to safely connect wires to supports. The material for them is either electrical porcelain, or tempered glass, or polymer, as shown in the image below:
Glass insulators under the same conditions are smaller and lighter than porcelain insulators. Structurally, insulators are divided into pin and pendant. The pin design is not used for power lines with voltages above 35 kilovolts. The mechanical loads absorbed by suspension insulators are greater than those of pin insulators. For this reason, the suspended structure can also be used at lower voltages instead of pin insulators.
The hanging insulator consists of individual cups connected into a garland. The number of cups depends on the power line voltage. To connect the cups into a garland and all other fastenings of wires and insulators, special fittings are used. Reliability, strength and durability in an open environment are determined by such materials for the manufacture of fittings as steel and cast iron. If it is necessary to obtain increased corrosion resistance, parts are coated with zinc.
The fittings include various clamps, spacers, vibration dampers, coupling connectors, intermediate insulator links, and rocker arms. A general idea of the fittings is given by the image below:
Protective devices
Another component of power transmission lines is structures that protect equipment connected to power lines from atmospheric and switching overvoltages. Protection against lightning strikes is provided by a cable stretched above all the wires of the power line and lightning rods, which are usually installed near substations. Protective gaps are located on power transmission line supports. An example of such a gap is shown in the image on the left. Tubular arresters are installed near substations, in which there is a spark gap inside. If it breaks through and an arc occurs, powered by a short circuit current, a gas is released that extinguishes this arc.
All technical and organizational nuances for the installation of power lines are regulated by the Rules for the Construction of Electrical Installations (PUE). Any deviations from these rules are strictly prohibited and may be considered a crime of varying severity depending on the consequences.