Line with high voltage

The lines with high voltage are the principal lines of the grid systems of electricity. They can be as well air as underground or underwater, though the professionals reserve rather the term with the air links. They are used for transport on the long distances of the electricity produced by various the powerplants, like with the interconnection of the electrical communications.

Why use the high voltage?

The choice to use lines with High voltage is essential, as soon as it is a question of transporting electrical energy at distances higher than some Kilomètre S. the goal is to reduce the voltage drops on line, the on-line losses, and also to improve the stability of the networks.

On-line losses are due to the Joule effect, which depends only on two parameters: the resistance and the running ($P=R.I^2$). The use of the high voltage allows, equivalent transported power, to decrease the current and thus the losses. In addition, to decrease the resistance, at the industrial frequencies, there are only two factors, the Résistivité of the Matériaux used to manufacture the cables of transport and, of the Section of these cables. With material of manufacture and section equivalents, the losses are thus equal, in theory, for the air lines and the underground lines.

The lines with high voltage belong to the field “High voltage B” which includes/understands the values higher than 50  Kv in alternative course. The expression “very high voltage” is sometimes used, but does not have an official definition. The tensions used vary from one country to another. Schematically, in a country, one will find tensions about 63  Kv with 90  Kv for urban or regional distribution, about 110 with 220  Kv for the exchanges between areas, and about 345 with 500  Kv for the principal national and international interconnections. In certain countries, one uses also 800  Kv (as with the Canada), and even of the higher tensions as in China (: 1100  Kv), India (project: 1200  Kv), Japan (project 1100 Kv) and in the ex- the USSR where tests of transport in “ultra high voltage” were carried out in: 1500  Kv - but this type of tension is justified only for one transport at a distance about the thousand of kilometers, for which a transport in D.C. current can be an interesting alternative.

The following table gives the evolution of the tension of the networks to alternative course since 1912, year of the startup of the first line of tension higher than 100  Kv.

Lines with D.C. current

In the vast majority of the cases, these lines with high voltage are fed in alternative course Triphasé; but within the particular framework of certain underwater crossings or buried lines, transport is done in D.C. current (HVDC) for reasons of economy, obstruction and reliability. Here two more detailed examples:

• for the connection France - England IFA 2000, transport is done using two conducting pairs of whose tension continues compared to the ground is worth respectively +270 Kv and -270 Kv, that is to say a potential difference between the two drivers of each pair equal to 540 Kv;
• with Grondine, 100 km in the south-west of Quebec, the crossing of the river the St. Lawrence is carried out by means of two pairs of cables whose tension continues compared to the ground is more or less 450 Kv, that is to say a potential difference between the two drivers of each pair equal to 900 Kv.

Components

Pylons

For the air lines, the operators of electricity transmission, use pylons, in general made out of lattice of Acier. Their function is to support and maintain the conducting S at a sufficient distance from the ground and obstacles: this makes it possible to guarantee safety and the Isolement compared to the ground, the cables being naked (not isolated) to limit the weight and the cost of it.

Drivers

See also: Electric cable with high voltage

The electric current is transported in drivers. Electrical energy being transported in form Three-phase E, one will find at least 3 drivers per line. For a phase, one can also find a beam of drivers (from 2 to 4) in the place of a simple driver in order to limit the losses (see low). The drivers in Cuivre are less and less used. One in general uses drivers out of aluminum alloy, or combination aluminum-steel for the older cables; they are conductive made up of a central heart out of steel on which are braided aluminum bits. The drivers naked, i.e. are not covered with an insulator.

The drivers high voltage are air or underground (and sometimes submarines). The aerial lines are subjected to the action of the atmospheric factors: temperature, wind, rain, glazes etc These factors intervene in an important way in the choice of the parameters of a line high-tension: type of driver electric (materials and geometry), height and distance from the pylons, maximum mechanical tension on the driver in order to maintain a ground clearance sufficient etc the choice of these parameters has a great influence on the costs of construction and maintenance of a line of transmission, like on its reliability and its longevity.

Insulators

The insulation between the drivers and the pylons is ensured by Isolateur S. Those are made in Verre, ceramic, or synthetic material. The insulators out of glass or ceramics in general have the form of a plate. One associates them between them to form strings insulators . The higher the tension of the line is, the more the number of insulators in the chain is important. On a line 400 Kv (400 000V), the insulator strings comprise 19 plates. One can then guess the tension of the lines by multiplying the number of insulators by 20kV approximately.

Earth wires

The earth wires do not transport the current. They are located above the drivers. They play a part of Paratonnerre above the line, by attracting the blows of Foudre, and by avoiding the blasting of the drivers. They are in general carried out in Almelec - steel. In the center of the earth wire one places sometimes a cable Fiberoptic which is used for the communication of the owner. If one decides to install fiberoptic on an already existing earth wire, one then uses a robot which will come to roll up in spiral fiberoptic around the earth wire.

Electric characteristics

The electricity transmission poses several problems, in particular those of the losses of energy and the voltage drops between the entry and the exit. A study using a simplified ideal model makes it possible to include/understand the effect of various parameters on the behavior of the line.

The diagram above represents a summary but easy to use model for a phase of a line not too long: it constitutes an approximation sufficient for lengths from 200 to 300 km. A longer line could be comparable with a succession of basic cells of this type.

One will be able to notice that the electric line of this type is connected with a low-pass Filtre.

Resistance of the line

The resistance of a thread-like driver is written:

$R=\; \backslash \; rho\; \backslash \; frac\; L\; S\; \backslash ,$

In order to limit the losses by Joule effect, it is wished that resistance be weakest possible. The length $l\; \backslash ,$ of the line being imposed, one can play only on the Résistivité $\backslash \; rho\; \backslash ,$ and on the section $s\; \backslash ,$.

Resistivity of materials used for the lines

The copper, whose Résistivité is worth 1,72 X 10 -8 Ω∙m, is not used because too expensive, but also too heavy for the air lines. One prefers to him aluminum-steel units or alloys aluminum, magnesium and silicon whose resistivity is about 3 X 10 -8 Ω∙m

Section of the lines

The section of an aerial line of a line with high voltage is about 500 mm 2 : it is not advantageous to increase the Section more conducting .

Indeed, at the frequency of 50 Hz (and a fortiori at a frequency of 60 Hz), it is advantageous to use 2 drivers of 500 mm 2 to replace one of section 1000 mm 2 because of the skin effect or Effect of skin.

In addition, on lines of tension higher or equal to 345 Kv, it is necessary to envisage at least 2 drivers per phase to limit the losses by effect crowns.

Order of magnitude of the linear electrical resistances

For a line of section 500 mm 2 realized with a material of resistivity 3 X 10 -8 Ω∙m, the resistance of an aerial line is about 6 X 10 -2   Ω/km. This value is given as an indication because we saw that resistance strongly depended on the section.

For the lines with high voltage, the values of the linear electrical resistances lie between 0,01  Ω/km (line 735 Kv of hydroquébec) and 0,1  Ω/km.

Transported powers

Losses of power

In spite of the effort made to limit resistance, the electricity transmission generates important losses of energy, mainly by Joule effect. As example, for the Grid system of electricity in France, these losses are estimated on average at 2,5% of total consumption, that is to say 13 TWh per annum.

Not to undergo important losses, two techniques are thus used:

• to increase the number of drivers: certain lines comprise for each phase to four distant cables of a few centimetres;
• to decrease the intensity of the current by raising the tension: for a identical power transported, if one increases the tension, the intensity of the electric current decreases and the losses due to the passage of the current in the wire will be reduced according to the square of the intensity.

However, the tension been used for the private individuals must remain unchanged (230 V in France or 120 V with the Quebec for the domestic facilities) and in the field of the low tension in order to limit the risks for the users. It thus should be lowered closest to those. As one cannot do it in a simple way with the D.C. current (cf HVDC), one has recourse to the Alternative course (of Fréquence 50 Hz in France or 60 Hz in Quebec and North America) and to Transformateur S.

It is also necessary to take into account the risk of Electric arc between two drivers. This risk is all the more important as the tension is high. That imposes stronger constraints of Isolement and requires in particular:

• for the air lines, to draw aside the drivers sufficiently, (typically 1  cm/kV), which results in proportionally to increase the dimension of the associated materials (Isolateur S, pylons…) ;
• for the cables (buried or not), to increase the thicknesses of insulators, to add screens of Mass, to even resort to different technologies (for example cables with gas insulation).

Intensity of the current

The resistance of the line is the primary reason of the limiting value of the intensity of the current which one can transport. However, it is not economically interesting to reach the bearable heat limit by the driver.

Let us recall first of all that the heating is proportional to the square of the intensity of the current. For a line of 500 mm 2 , a density of current of 1,6 has per mm 2 is a limit which makes it possible not to exceed the temperature of 60°C. But it is more economic to carry out two lines transporting half of the current, because the losses of each line are divided by 4 - thus the total of the losses is divided by 2. The made saving makes it possible to deaden the realization of the second line. Moreover, one preserves the possibility of doubling the intensity of the current where necessary (maintenance actions, breakdowns on the other line,…).

The density of the current in the air lines high voltage is approximately 0,7 - 0,8 A/mm 2

Impedance of the line

the reactive parameters of the line depend little on the tension and of the section but, on the other hand, they are very different for the air lines and the cables posed or buried.

Inductance of the line

• From 1 to 2 mH/km for the air lines is Réactance S ranging between 0,3 and 0,7  Ω/km, therefore definitely higher than the linear electrical resistances.

• From 0,2 to 0,7 mH/km for the cables is reactances ranging between 0,06 and 0,25  Ω/km

Capacity of the line

• Near to 10 nF/km for the air lines.

• From 30 to 800 nF/km for the cables.

Voltage drops

With vacuum

If one considers the model in π when the output current null east, one notices that the condenser of exit east then in series (i.e. crossed by exactly the same intensity) with the resistance and the inductance of line.

One can write: $\backslash \; frac\; \{\backslash \; underline\; U\_e\}\; \{\backslash \; underline\; Z\_L\; +\; \backslash \; underline\; Z\_R\; +\; \backslash \; underline\; Z\_C\}\; =\; \backslash \; frac\; \{\backslash \; underline\; U\_s\}\; \{\backslash \; underline\; Z\_C\}\; \backslash ,$, is: $\backslash \; underline\; U\_e\; =\; \backslash \; underline\; U\_s\; +\; \backslash \; frac\; \{\backslash \; underline\; Z\_L\; +\; \backslash \; underline\; Z\_R\}\; \{\backslash \; underline\; Z\_C\}\; \backslash \; cdot\; \backslash \; underline\; U\_s\; \backslash ,$

from where one draws: $\backslash \; frac\; \{\backslash \; underline\; U\_e\; -\; \backslash \; underline\; U\_s\}\; \{\backslash \; underline\; U\_s\}\; =\; \backslash \; underline\; Y\_C\; \backslash \; underline\; Z\_C\; =\; RC\; \backslash \; Omega\; -\; jLC\; \backslash \; omega^2\; \backslash ,$

For an air line, we saw that , therefore the second term is prevalent, which leads to a output voltage higher of a few percent than the tension of entry. This phenomenon is called Effet Ferranti .

In load

The f.é.m of an alternator is constant and equal to the vectorial sum of resistance time the current interns which crosses it more the impedance interns time the same one running more the sum (resistance and impedance) of the line time the current plus the terminal voltage of the load which is in parallel with the capacity of the line.

$E\; =\; (R\; +\; jl\; \backslash \; Omega).\; I\; -\; (R\; +\; jL\; \backslash \; Omega).\; I\; +\; U\_S\; \backslash ,$, is: $E\; =\; (R\; +\; R).\; I\; -\; J\; (L\; \backslash \; Omega\; +\; L\; \backslash \; Omega).\; I\; +\; U\_S\; \backslash ,$

If the intensity called I increases the two $terms\; (r+R).\; I\; \backslash ,$et $j\; (L\; \backslash \; Omega\; +L\; \backslash \; Omega).\; I\; \backslash ,\; thus$augmentent $U\_S\; \backslash \; Omega\; \backslash ,$ decreases at the end of the line. To cure it, there are two possibilities: either to ask the groups to provide more reagent or to insert the capacitor batteries in the network or the two solutions at the same time.

Controversies

Health

The lines with high voltage are suspectées harmful effects on the human organism, in particular because of the magnetic fields which they emit.

The results of the epidemiological studies like are often contrasted. However it arises from the whole of the studies carried out that it is highlighted no on-risk of cancer in the adult in the event of residential exposure to the fields of the lines to hautre tension (in particular for cerebral leukemias and tumors). For the children, the question remains posed. The British Medical Journal of June 4th, 2005 publishes a study showing a limited but real relative risk of infantile Leucémie for the children residing in the vicinity (from 0 to 600 meters) of a line with high voltage. No increase in the relative risk was highlighted for the other tumors (cerebral tumors for example with a risk relative lower than 1, which does not indicate obviously a protective effect). This study, carried out by a researcher of the Université of Oxford, precise that any social skew was isolated (the risk of leukemia would be higher in the easiest families). However, as for all the studies case-witnesses retrospectives the risks of skew are many and difficult to control: for example only half of the cases of leukemias had not moved between the birth and the diagnosis. No rational explanation was found to explain this on-risk. In particular one did not know yet to define with exactitude if that is due to the magnetic fields or other causes.

Certain studies in laboratory on animals showed that the exposure to the electric fields and magnetic can be associated with the increase with incidence with certain cancers (but not leukemias); The studies not showing any association are more numerous. But the levels of fields necessary to the appearance of the harmful phenomena are without common measurement with those measured near the lines with high voltage. In France, the international Center of research on cancer of Lyon class however the magnetic fields of very low frequency produced by the lines with high voltage in the goupe 2B of the potentially carcinogenic agents, but only for the particular case of leukemias of the child.

The subject thus remains highly polemical. Let us note finally that the hiding of the lines with high voltage is not inevitably the miracle solution with this problem. The Magnetic field plumb with a cable high voltage buried can sometimes be higher than that of an air line of the same tension.

Environment and harmful effects of the lines high voltage

The lines called to very high voltage , 225 or 400 Kv, are highly criticized by associations of environmental protection and in the media, because of:

• impact on the landscapes and the creation of sliced deforestation;
• impact on tourism, the habitat, noise pollutions, as well as the effects on the avifauna;
• medical aspects evoked above.

Associations ecologists require:

• to suspend the whole of the projects of extension of lines VHV;
• to hide existing lines VHV;
• to undertake epidemiological studies near lines VHV;
• to reduce the electric needs.

The obstacles with the hiding of the lines can be either technical, or economic: a line 400 Kv buried costs approximately ten times the price of an air line. But, on the one hand, this approximate evaluation does not take account of the economies of scale obtained which would be possible thanks to the generalization of the techniques of hiding, on the other hand, on-line losses for the air lines are higher than those of the buried cables (¹). Lastly, the air lines are extremely vulnerable in the event of storm: in France, the storm of 1999 involved a overcost of 30% only for the setting to the standards of lines VHV so that they resist strong winds of 170 km/h.

The theoretical overcost put forward by the operator of French network RTE occults the awaited benefit of a hiding while disregarding negative externalities implicitly, namely the impact about the landscape, tourism, the habitat, noise pollutions, as well as the consequences about the avifauna.

Notes & References

See too

Related articles

• Electricity

• Tension
• Electrical communication
• Electrocution

External bonds

• Article on the risks of the lines with high voltage

• Cancer and radio frequencies (CyberSciences)
• Drop voltage on the site of Socomec

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