High voltage line
High-voltage lines are power lines used to transmit electrical energy over long distances. Since the transmission losses are lower, the higher the electrical voltage used for the transmission and the smaller the electrical current transmitted with it, the lower the transmission losses, with the same electrical power , they are operated with voltages above 10 kV to about 1 MV .
In many countries, high-voltage lines are mainly built as overhead lines because, under certain conditions, they are cheaper, more maintenance-friendly and have lower losses than underground or submarine cables . High-voltage lines are usually operated with three-phase alternating current, which, compared to direct voltage transmission, offers the advantage of a simple voltage change with transformers . Because in order to be able to transmit high electrical power with low losses, high voltages are required. On the other hand, alternating voltage transmission - especially over long distances - also entails higher transmission losses due to capacitive and inductive effects.
The current-carrying conductor cables are attached to masts with insulators . The conductors usually consist of aluminum wires, which have a high electrical conductivity , and a steel core, which guarantees a high mechanical tensile strength . The ropes do not have their own insulation, they are bare and are only insulated by the surrounding air. The line resistance is determined by the cross-section of the lines and the electrical conductivity of the material used.
A maximum current of around 2 kiloamps can be transported per conductor cable . The improvement in the transmission efficiency with higher voltages due to a relatively lower ohmic loss is opposed by other losses such as that due to corona discharge . Extra high voltage lines for the transmission of electrical power over long distances have predominantly a voltage of 380 kV.
In contrast to adequately insulated underground cables, however, high-voltage overhead lines represent an increased risk, since electric arcs are already possible with the contactless approach and these (with sufficient current strength) can seriously injure people or other living beings or cause fires in objects. Therefore, the recommended safety distances must be observed, which increase with increasing voltage.
When operating a three-phase high-voltage line , its reactive power must also be compensated for ( reactive power compensation ). The reactive power requirement of the line results from the capacitance and inductance load, which among other things depends on the shape of the overhead line pylons , the conductor arrangement on the pylon and the conductor cross-section. Typical values of the operating capacities for 380 kV high-voltage lines are in the order of magnitude of 5 nF / km to 10 nF / km; higher values are usual for lower voltage levels. However, unlike cables, the inductance layers predominate in overhead lines. In order to keep the reactive power requirement in all conductors the same, overhead lines are balanced at regular intervals by twisting masts .
Are for power factor correction of the line for the power transformers at tertiary windings special static VAR compensators , the reactive current at the beginning of the line causes additional resistive losses in the conductor, thus reducing the total current share of the lead. A reactive power compensation of the line exists when the natural power is transmitted, which means that the load impedance corresponds to the characteristic impedance of the line. The higher the voltage level, with approximately the same operating capacity, the higher the reactive power requirement of a high-voltage line, which is why the upper operating voltage is not only limited by the losses such as the corona discharges in AC voltage operation. The 380 kV network is therefore implemented almost exclusively in the form of overhead lines, underground cables are only used in exceptional cases and over short distances.
With the technically more complex high-voltage direct current transmission (HVDC) there is no reactive power due to the direct voltage. HVDC is used where construction-related high operating capacities occur along the line, such as underground high-voltage cables and especially submarine cables . Another area of application is the transmission of electrical energy with maximum voltage over long distances.
Given: line resistance
d. This means that the power loss decreases with the square of the voltage for the same effective power . However, the greater the voltage, the greater the cost of the insulation . The transmission losses are around 6% per 100 km with a 110 kV line and can be reduced to around 0.5% per 100 km with 800 kV extra-high voltage lines.
Extra high voltage lines are operated within the framework of the European interconnected system (formerly “UCTE interconnected network”) with voltages of up to 1.15 times the nominal voltage , which is specified as the effective value . In a 380 kV system, this leads to an operating voltage of up to 437 kV; With the sinusoidal shape used, there is a peak value of around 620 kV between the conductors. Whenever this peak value is reached, the electric field strength around the line is so great that the dielectric strength of the air is almost reached. Then the air in the immediate vicinity of the conductor is ionized , i.e. weakly conductive, and power is lost. Since this effect is particularly pronounced at the tips , the field strength at these points is reduced by corona rings . The larger the radius of curvature, the lower the electric field strength on the surface and the resulting corona discharge . With the help of the corona camera , the ultraviolet light component of the corona discharges can be optically recorded.
Another means of reducing peak discharge is to increase the radius of curvature of the cable by connecting two to four individual cables in parallel to form a bundle conductor . The individual conductors of the bundle conductor are kept at an exact distance from one another by spacers. The increased radius of the conductor assembly reduces the electric field strength on the surface of the conductor bundle.
Despite all these measures, the losses due to corona discharges increase considerably above an operating voltage of 500 kV. For the corona discharge, the voltage between the conductor and the earth potential is particularly decisive. In three-phase networks, this voltage is lower by the concatenation factor than the specified voltage between two external conductors. For 380 kV extra-high voltage lines, it is 220 kV, for example.
Corona discharges lead to the emission of mostly ultraviolet flashes of light that are invisible to humans. However, many animals can perceive UV light. This is seen as a cause of some animals avoiding high voltage power lines.
Underground and submarine cables
In addition to overhead lines, there is also the option of laying high-voltage lines underground over relatively short distances of up to a few 10 km as underground cables or as gas-insulated pipelines (GIL). This mainly applies to upper voltage levels with operating voltages of around 380 kV and above. At a mains frequency of 50 Hertz (i.e. with alternating current ), underground cables may be a maximum of 70 km long, because the capacitive reactive currents become too large with longer lengths .
Submarine cables are preferred for the transmission of high voltages over long stretches of water, in turn the method for high-voltage direct current transmission (HVDC) with voltages between 100 kV and 1 MV is often used to avoid losses due to high capacitive coating when using alternating current to counteract. In contrast to three-phase systems, there are no standard voltages in HVDC systems. ± 500 kV (i.e. 1 MV between forward and return conductors) were implemented several times.
High voltage lines in Germany
The following information relates to the rms value of the line-to-line voltage between the conductors. The so-called highest operating voltage, which may be applied permanently, must be distinguished from the nominal voltage. Usual nominal voltages in Germany are:
- Medium voltage
- High voltage
- 60 kV (only rarely in Germany, but still in some cable networks)
- 65 kV (STEAG, Saarland)
- 110 kV (overhead lines, traction current )
- In the transmission network, the nominal voltages at the maximum voltage level are:
- 220 kV
- 380 kV
Significant / special lines
- Railway power line Neckarwestheim – Zazenhausen , 1977, only railway power line with four bundles
- 110 kV line Lauchhammer – Riesa , 1912, first high-voltage line in Europe to go into operation with 110 kV
- North-south line Bürs / Tiengen – Brauweiler, 1929, world's oldest network line and first line with 220 kV
- 380 kV line Etzenricht – Hradec , 1992, until 1995 the only line in Germany that was not synchronized with the UCPTE network
- 380-kV-Transversale Berlin , 1970s, high-voltage line designed as an overhead line and underground cable across the center of Berlin
- 380 kV line Wolmirstedt –Greifswald, 1970s, longest power line in Germany
- HVDC Baltic Cable , 1994, longest submarine cable in Europe and highest transmission voltage in Germany with 450 kV
- 380 kV line Lübeck-Siems - Lübeck-Herrenwyk (Baltic Cable), the only 380 kV line not connected to the rest of the 380 kV network
- HVDC Kontek , 1996, 400 kV underground and submarine cable connection between Bentwisch and Bjæverskov Sogn (Denmark)
- 380 kV line Mecklar-Vieselbach, 1995, bundles of four on the Hessian side, bundles of three and other mast construction on Thuringian, synchronous connection of the power grids of East and West Germany.
High-voltage lines in Switzerland
The voltage levels customary in Switzerland are 380, 220, 110 and 50 kV. Today the power supply is guaranteed nationwide with high voltage lines of 380 kV. When the city of Zurich began to obtain electricity from Graubünden , a new high-voltage line of around 120 kilometers was required. They had no experience in transporting electricity over such great distances. Today this route is designed for 380 kV along its entire length.
In the 1950s and 1960s, the north-east Swiss power plants built their identical 380-kV lines Bonaduz -Breite (near Nürensdorf ), Tavanasa-Breite (known as preliminary discharge ), Breite- Beznau and Beznau- Laufenburg . The masts were built too low for today's conditions. As early as the late 1960s and 1970s, efforts were made to build masts as high as possible and to make better use of the area under management. The lines of the transport network (380 and 220 kV) with a length of 6,700 km have been completely owned by the national transport network company Swissgrid , which also operates the control area CH within the European electricity network, since 2014 .
Significant / special lines
- the longest high voltage line in Switzerland between Laufenburg and Creux-de-Chippis ,
- the Gotthard line ,
- the Lukmanier line ,
- the preliminary derivation ,
- the Sils – Fällanden line of the EWZ.
High voltage lines in Austria
In Austria , too , the high-voltage network is made up of the voltage levels 380 kV, 220 kV and 110 kV. The 380 kV network in Austria is not fully developed, but divided into several segments that are connected to each other via 220 kV lines. For historical reasons, the 220 kV level is used in the power plants of the Austrian Danube Power Plants (DOKW). The western part of the 380 kV network in Vorarlberg and West Tyrol is primarily used for the exchange of electricity between the neighboring countries of Germany, Italy and Switzerland and, as in Vorarlberg, is directly assigned to the control area of EnBW Energie Baden-Württemberg .
The eastern and largest control area in Austria, which is operated by Austrian Power Grid AG (APG), includes all federal states except Vorarlberg. The planned 380 kV high voltage ring forms the central supply. In addition to supplying power to the metropolitan area around Vienna, the 380 kV network also serves to exchange electricity between the neighboring countries of the Czech Republic , Hungary and Slovenia in the south. The distribution network operator Wien Energie Stromnetz has a 380 kV network in combination with a fine-meshed 110 kV distribution network in the urban area as the top voltage level in the federal capital Vienna. The 220 kV level is not used in the federal capital. In spring 2006, the 400 kV north feed in Vienna was put into operation, which, in addition to the south feed in the Vienna Southeast substation, represents a second connection to the national 380 kV high voltage ring.
Customary country features
In Germany, Austria and Switzerland, railway operations are mainly tailored to operation with single-phase alternating current of 15 kV and 16 2 ⁄ 3 Hz (the frequency has now been set to the decimal fraction of 16.7 Hz). In addition to the usual three-phase long-distance lines, there are therefore separate traction power lines in these countries to ensure nationwide supply with this power system . Electric railways that use the widespread single-phase alternating current system with 25 kV / 50 Hz could theoretically be supplied from the 50 Hz three-phase system, but this is usually not done because heavily asymmetrical load distributions can occur in the long-distance lines of the three-phase system.
In Central Europe, high-voltage lines in densely built-up urban areas are almost exclusively implemented as underground cable systems, even if their operation is more cost-intensive than overhead lines. In Turkey, such as the Bosphorus crossing in Istanbul, high-voltage lines are also routed through urban areas as overhead lines.
In Germany, both in the GDR and in the Federal Republic of Germany , almost no delta masts were erected. The reason is that delta masts only offer space for a three-phase system, while the barrel masts and Danube masts can accommodate two independent three-phase systems. These can be operated independently of one another, which is important in the event of malfunctions or maintenance work. In the case of delta masts, two separate routes with a correspondingly larger space requirement must be provided for two independent three-phase systems, which is a problem especially in more densely populated rural regions. In the GDR almost all lines on the 110 and 220 kV level were laid on masts with a single level arrangement with two earth cables. In Great Britain, on the other hand, almost all high-voltage lines are laid on pylons with a three-level arrangement .
In the USA, lines with voltages above 100 kV (up to 345 kV) are sometimes laid on wooden or plastic masts with an insulating crossbar . The power grid is operated there in several sub-networks that are not synchronized with one another. In many sparsely populated countries with little agriculture, lines are partly laid on rope-anchored portal masts .
Voltage specifications for high-voltage lines
The voltage specifications in the high and extra high voltage network always relate to the nominal values of the network voltage. These are specified in the network and system rules of the German transmission system operators at 110, 220 and 380 kV, depending on the voltage level. In the European guideline for transmission network operation, a voltage range of 0.90 pu - 1.118 pu for normal operation of the network in the 110 kV network and in the 220 kV network and a voltage range of 0.90 pu in the 380 kV network - 1.05 PU. The following voltages are therefore permissible in normal operation:
- 110 kV network: 99 kV - 123 kV
- 220 kV network: 198 kV - 246 kV
- 380 kV network: 342 kV - 399 kV
Highest transmission voltage
An HVDC test line for 1.33 MV was built near Celilo, Oregon , USA. It was to become part of a 1.33 MV direct current line between Celilo and Hoover Dam , but it was never built. The highest DC voltage for a system in use is currently ± 800 kV (1.6 MV between the two conductors). This voltage is used in a number of systems in China, with the highest transmission capacity being achieved on the southern HVDC Hami-Zhengzhou with 8000 MW.
Technical features / overhead line construction
The 91.7 km long overhead line over the Eagle River in the Tongass National Forest in Southeast Alaska connects the Lake Tyee power station with the Swan Lake Dam power station via 243 pylons . The line was built with helicopters without road construction. The twelve-sided masts were press-fitted onto conical supports that were anchored via rock drillings. The longest span of the line (2.1 km) requires a tensile force of 66.7 kN for the conductor cable .
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