How much capacity will transmission lines have

High voltage line

Lexicon> Letter H> High voltage power line

Definition: Lines for the long-distance transmission of electrical energy with high electrical voltage

More specific terms: overhead line, underground cable, submarine cable

English: high voltage transmission line

Category: electrical energy

Author: Dr. Rüdiger Paschotta

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Original creation: 03/19/2010; last change: 08.05.2021


Power lines are used to transmit electrical energy over long distances in the interconnected network. They are operated with particularly high electrical voltages of at least 60 kV (kilovolts) up to maximum voltages of around 1 MV = 1000 kV. From approx. 1 kV alternating voltage or 1.5 kV direct voltage, one speaks of high voltage. High voltage means at least 220 kV conductor voltage with three-phase current.

With high voltage, a high electrical power can be transmitted in spite of a moderate current strength, and this in turn allows the use of thinner cables with nevertheless not too high energy losses. The voltage tends to be higher, the higher the transmitted power and the longer the cable.

Most of the power lines are Overhead lines with ladders suspended from high masts. But there are also high-voltageUnderground cablesthat are laid in the ground as well Submarine cables for use on the seabed.

Power lines that operate at lower voltage levels are known as medium-voltage or low-voltage lines.

Overhead lines

High-voltage overhead lines usually contain several (at least three) thick conductor cables that can withstand high currents. Typical conductor cables, consisting of aluminum with a steel core, can with a cross-sectional area of ​​40 mm2 are loaded with up to approx. 2 kA, but are less loaded in continuous operation (see below).

Usually not simple conductors, but Bundle ladder Typically consisting of 3 to 6 conductors, because the tendency to glow discharges can be reduced, which makes a higher operating voltage possible. The conductors are suspended from high masts so that a safety distance of several meters not only from the ground, but also from buildings, trees, etc. is maintained under all circumstances. Because of the high voltages, electrical insulation with plastic coatings is possible (as with low-voltage cables), but would require a very high thickness of the insulation layer. That is why air gaps are simply used for insulation, i. H. a sufficient distance. The conductor cables must be attached in such a way that they can never collapse or come too close, even in strong winds, as this could otherwise lead to a short circuit or an electric arc.

The tips of the masts are usually connected by an earth cable that is at earth potential (by connection via Earth electrode). It usually hardly carries any current, but it will absorb most of the lightning strikes that could otherwise hit the conductors and cause damage.

How much power can a three-phase high-voltage line transmit?

As an example, consider a 400 kV three-phase line with three conductor cables, each carrying 2 kA. The total power transmitted is then 3 · 230 kV · 2 kA = 1.4 GW. (Note that the voltage to earth and not the voltage between the conductors is relevant here, i.e. 230 kV and not 400 kV.) This corresponds to the output of a large power plant and is the output common in Germany today in the extra-high voltage network. The active power can be slightly lower if a reactive current component occurs. Higher capacities are possible by using additional conductors.

Due to the high currents and the (albeit low) electrical resistance of the cables, the cables get quite hot at full load (e.g. 80 ° C, with high-temperature cables even 150 ° C). The maximum power loss is typically a few hundred watts per meter of transmission length, but when designing the system, the aim is to achieve a significantly lower load on average in order to limit the energy losses. The line mentioned in the example above will usually only have a capacity of a few hundred megawatts. When using bundled ladders, more is of course possible - even a permanent load of the order of 1 GW.

The conductor cables are cooled by the ambient air. The heating leads to an expansion of the material and thus to greater sagging of the cable. The required minimum distance to the ground can limit the transferable power. As part of overhead line monitoring (FLM), the maximum load on lines is adapted to the ambient temperature and wind conditions in order to be able to make maximum use of the available transport capacities. The heating power generated naturally means lost electrical power; the energy efficiency would be higher with less utilization of the lines (i.e. also with greater design of the maximum power of the line).

Energy losses in transmission lines occur in different ways:

How do energy losses occur in high-voltage lines?
  • The electrical resistance of the cables leads to heating of the cable and a corresponding drop in the transmitted voltage (ohmic losses). The power lost increases with the square of the power transmitted, and it limits the maximum power that can be transmitted due to the heating of the cables. In cold, windy weather, the cables of overhead lines are better cooled, so that the transmitted power can then possibly also be selected to be significantly higher, although increased relative power losses occur.
  • Eddy currents are only induced in the lines with alternating current, and the skin effect tends to displace the current somewhat from the inside to the outside, so that it does not flow evenly. Both lead to additional ohmic losses.
  • In addition, the high voltages lead to corona discharges (glow discharges), which are also responsible for the often audible noises (humming and crackling). This part of the losses occurs regardless of the current load, but depends on the weather conditions. In connection with these discharges, there is also a partial ionization of the air, which leads to the formation of ozone.
  • Indirectly, the reactive power required by a cable can lead to additional losses at other points in the power grid, e.g. B. in devices for reactive current compensation. However, this problem only occurs with alternating current or three-phase lines, not with direct current transmission.
  • Certain losses also occur in the transformers, which are required for reclamping (increasing and decreasing the electrical voltage). In the case of high-voltage direct current transmission, there are losses in converters that are higher than in transformers.

The total power losses are often a few percent per 100 km. However, in modern projects with high-voltage direct current transmission, it is also possible to reduce the losses to a few percent per 1000 km.

To increase the security of supply through redundancy, routes with two independent line systems are often used; if one of them fails, the other can still take over full power. In this respect, such a solution makes more sense than, for example, using only one line system with twice the cross-sectional area.

Underground cables

Underground cables are based on cables that are provided with thick electrical insulation. The cables are embedded in a trench, often with a protective layer of sand. The insulation layer usually consists of plastic.

Underground cables have several advantages over overhead lines:

Underground cables offer various advantages over overhead lines - even lower energy losses.
  • The landscape is hardly disturbed.
  • Dangers z. B. for low-flying aircraft or helicopters are avoided.
  • The susceptibility to damage from lightning strikes, hail and storms is largely avoided. This greatly reduces the failure frequency. However, for example, dredging work occasionally leads to breakdowns, which then take much longer to repair than is the case with overhead lines.
  • The energy losses are lower because less heat can be dissipated and therefore a larger cable cross-section is used.
  • The exposure to so-called electrosmog (see below) is greatly reduced (except in the immediate vicinity of the cable), and the ionization of the air, which leads to the formation of ozone, is avoided.
Why not use underground cables everywhere?

On the other hand, underground cables are usually several times more expensive than overhead lines, which usually limits their use to particularly sensitive areas. Incidentally, the laying of underground cables is not entirely possible without impairing the landscape; in particular, there is increased interference in the soil and accessible socket structures are required.

Gas-insulated pipe ladder

The technology of gas-insulated pipe conductors is an alternative to overhead lines and also to conventional underground cables. Such a pipe conductor consists of a thinner pipe which is guided inside a thicker pipe. The two pipes are electrically insulated from each other: The inner pipe is suspended with insulators, and the cavity is filled with a gas that is particularly dielectric-proof, i.e. enables a very high electrical voltage (hundreds of kilovolts) between the pipes. The outer pipe is at ground potential during operation, so that it is not dangerous to touch it from the outside. The inner tube carries the high voltage.

Sulfur hexafluoride (SF6) or a mixture of the same with nitrogen is used. Its pressure can e.g. B. 5 bar, so correspond to five times the atmospheric pressure, because this increases the dielectric strength and thus the possible voltage. Sulfur hexafluoride is a powerful greenhouse gas, so its release would be very harmful to the climate. In practice, however, such a gas leak is hardly to be expected.

Gas-insulated pipelines can be laid directly in the ground or in a tunnel. Since the energy losses in them are very low, no active cooling is usually necessary. Such a purely passive system is very robust, requires little maintenance and has a long service life.

Compared to conventional underground cables, there are not only lower energy losses, but also lower capacities and correspondingly lower capacitive reactive power (with AC operation). The transferable power can be several gigawatts, corresponding to the output of several large power plants. However, the costs are several times higher than for an overhead line.

AC or DC operation

Most high-voltage lines are operated with alternating current: current and voltage oscillate periodically with a frequency of mostly 50 Hz (in Europe). The use of alternating current has the great advantage that transformers can be used to raise the voltage level for the line and then lower it again for fine distribution. For operation with direct current (→High voltage direct current transmission, HVDC), technically more complex power electronics are required, which have only been available for very high outputs for a few years. In particular, high-voltage rectifiers and converters are required to connect DC and AC systems.

AC operation also has a number of disadvantages:

Most high-voltage lines are operated with alternating current (or three-phase current). This has advantages, but also a number of disadvantages.
  • Kick it Reactive currents that have to be compensated with additional measures and cause additional losses. Such losses are particularly high for submarine cables and underground cables.
  • Because the strength of corona discharges limits the peak value of AC voltage, this is feasible Rms value the AC voltage is lower than for DC voltage, which means higher losses or a significantly lower power limit of the line.
  • The concerns about electrosmog (see below) are much more pronounced with AC voltage lines.

Other disadvantages of alternating current occur when operating interconnected networks. In particular, the electrical oscillations in the entire network must be synchronized, which requires some effort.

Natural performance

When the current load is low, a high-voltage line causes capacitive reactive currents, whereas when the current load is high, it generates inductive currents. In between, at the natural performance, there is an operating point at which the line does not cause or compensate for reactive power.

In the case of overhead lines, the natural output is often a small fraction of the maximum output. That's why they are often used in supernatural realm operated where they cause inductive reactive power. Their compensation is possible with systems for reactive power compensation.

The situation is different for underground cables. Because it is more difficult to dissipate heat, they are usually used in sub-natural realm operated where they generate capacitive reactive power. In the case of gas-insulated pipelines, the natural output is also usually higher than the operating output, but in any case significantly less reactive power is generated here than with an underground cable insulated with solids.

Embedding in the power grid

There is a coarse-meshed network of extra-high voltage lines, including several lower voltage levels. The article on power grids explains this in more detail.

A relatively large-meshed network of extra-high voltage lines with mostly 400 kV forms the backbone z. B. the European interconnected network. Large power plants usually feed directly into such extra-high voltage lines. The fine distribution in the regions takes place at lower voltage levels of z. B. 110 kV. Transformers are used in the usual alternating voltage networks for “reclamping” to other voltage levels.

Up to now, high-voltage direct current transmission has mostly only been used sporadically for point-to-point connections with high transmitted powers. When embedded in an alternating current or three-phase network, a rectifier system is required at one end and an inverter at the other end.

economic aspects

Is the construction of high voltage lines an important factor in the price of electricity?

The construction of high voltage lines is costly. In absolute terms, HVDC versions are particularly expensive for overhead lines. Because of the very high power transmitted, the costs per megawatt can still be lower. In general, the amortization of high-voltage lines is not a problem due to their very long service life (around 100 years) and the high energy throughput, except for the very expensive underground cables. Compared to the costs of electricity generation and also to those for the distribution networks (medium and low voltage level), the costs for the high voltage transmission network are low.

In order to avoid expensive investments in new high-voltage lines, attempts are often made to increase the transmission capacity of existing lines at low cost. One option for this is the equipment with high-temperature conductor cables. These may get hotter during operation (e.g. by adding zirconium) and therefore allow e.g. B. 50% higher performance. (The energy losses then increase sharply, but this is bearable if the full load only occurs temporarily.) Another option is sometimes to equip masts with additional conductor cables, provided there is still space available.

A significantly greater increase in capacities is possible if a switch is made to high-voltage direct current transmission. At the same time, even visually less conspicuous masts can be used. If the same route is used as before for the AC transmission, the implementation is much easier compared to a completely new route.

Electrosmog and other pollution

Because of the presence of electric and magnetic fields (often as Electrosmog There are widespread health concerns about staying near high-voltage lines for long periods of time, especially against living under such lines. Extensive epidemiological studies have been carried out in various countries, but without any clear indications of hazards. In addition, it is not clear which mechanisms of action could cause possible damage. Conceivable (but without scientifically proven harmful effects) are effects based on the following effects:

In what ways could electric fields or magnetic fields from power lines be harmful?
  • The strong currents in the lines generate magnetic fields. In the case of alternating current, these are oscillating magnetic fields that can cause so-called eddy currents in electrically conductive objects and also in organisms. The magnetic field strengths decrease rapidly with increasing distance from the lines, especially in line systems with several phases. With direct current transmission only temporally constant magnetic fields arise, which can hardly be assumed that they have any other effect than z. B. the natural magnetic field of the earth, so that harmfulness is not to be feared.
  • The high electric field strengths in the immediate vicinity of the lines lead to a partial ionization of the air and thus also to the formation of toxic ozone. Such effects can occur with overhead lines, especially at very high voltages, but not with underground and submarine cables.
  • Direct effects of the electric fields on organisms are also conceivable. However, the electrical field strengths also decrease rapidly with increasing distance from the lines to a level that various other sources can also cause.
Scientifically, health hazards from electrosmog can neither be proven nor disproved. Other threats that are definitely real deserve more attention.

The scientific clarification of the question of whether and, if so, how electrical or magnetic fields from high-voltage lines could cause damage to health is very difficult and will likely take a long time. This is due, among other things, to the fact that those affected are also exposed to a large number of other stresses (so that the cause of health problems is not clear), that negative effects may only appear after a very long time and that a good control group (without any electromagnetic pollution , but with a similar way of life) does not exist. Because of the uncertainties that will probably remain for a long time, it seems advisable, in accordance with the precautionary principle, to limit or reduce the field strengths to which people are exposed in the long term as much as possible with a reasonable amount of effort. In addition, it should be noted that the electricitygeneration It has been shown to cause massive environmental and health problems, especially in coal-fired power plants, and therefore deserves far more attention than the conceivable but unproven health hazards caused by electrosmog. For example, it would certainly be detrimental to the health of the population if coal-fired power plants were operated longer than actually necessary, because high-voltage lines used for the energy transition are not being built due to fears of electrosmog.

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See also: electrical energy, high-voltage direct current transmission, underground cables, security of supply, power grid, power failure, reactive power compensation, Ferranti effect
as well as other items in the electrical energy category

Understand everything?

Question: What effects would an increase in the operating voltage have for a given transmitted power of an overhead line?

Correct answers: (a) and (c)

Question: Why do underground cables tend to have lower energy losses than overhead lines?

Correct answer: (b)

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