The colloquial term power grid describes a network for the transmission and distribution of electrical energy in electrical power engineering . It consists of electrical lines such as overhead lines and underground cables as well as the associated facilities such as switchgear and transformer stations .
Large, spatially adjacent and electrically connected electricity networks are referred to as interconnected networks , while small, spatially separated electricity networks are referred to as island networks . Electrical networks in vehicles and aircraft are called vehicle electrical systems . A historical name for the electricity network is lighting network , because in the beginning electrical energy was almost exclusively used for lighting with incandescent lamps .
Power grids are used to supply consumers with electrical energy and connect the power plants and other energy converters such. B. Wind energy and photovoltaic systems . This is done at different voltage levels in order to reduce network losses. By increasing the voltage, the required cross-section of the power lines and the effort for the power switchgear decrease, on the other hand, the effort and costs for insulation and separation increases. H. protection against flashover and leakage currents. The power supply network is operated with three-phase alternating current and usually comprises four voltage levels, on the one hand to cover long distances and on the other hand to offer user-friendly voltages. The mains frequency in Europe is 50 Hertz (Hz), in North America 60 Hz. This makes transformer transformers possible; at the same time, these frequencies are easy to generate and use in rotating electrical machines . This is also served by the three- phase three- phase network , which can be divided into single-phase alternating current ("household current") for some of the end consumers and a large part of the electrical appliances .
The traction power network of various countries is operated with single-phase alternating current at a frequency of 16.7 Hz. The reason is that electric drives for locomotives were and often still are series-wound motors . Spark formation on their commutator can only be limited by operating at a low frequency .
Overhead line networks for the distribution of electrical energy are also used for the transmission of messages , in the past using carrier frequency methods on the conductor cables, over the earth cables or over communication cables (mostly fiber optic cables ) laid with them. The message transmission is used by the energy suppliers themselves or offered to other users.
Power grids are classified according to the operating voltage at which they transmit electrical energy. With regard to network usage fees, there is a classification in some countries according to the network level from which electricity is drawn.
- Extra high voltage : In Western Europe usually 220 kV or 380 kV. In Canada and the USA 735 kV and 765 kV are used. There is an extensive 750 kV network in Russia , from which individual lines also lead to Poland , Hungary , Romania and Bulgaria . A 1150 kV line leads from the Ekibastus power plant ( Kazakhstan ) to the city of Elektrostal (Russia). Today, however, it is operated at 400 kV.
- High voltage: 60 kV to 150 kV. In Germany and Austria 110 kV is used almost consistently. In addition, there are lines with 60 kV operating voltage in Schleswig-Holstein, near Winsen (Aller), near Landesbergen, near Philippsthal and in the old network of the Kassel municipal works . In Saarland , a network of 65 kV lines operated. In Switzerland there is no uniform value in the high-voltage network.
- Medium voltage : 1 kV to 35 kV. For networks with a high proportion of overhead lines, extensive rural regions and new installations, 20 kV to 25 kV are common. In urban regions where partially underground cables even older paper-lead embodiment with aluminum or as a ground cable are executed, a lower mean voltage of 10 kV is used.
- Low voltage : 230 V / 400 V. Other low voltages are also common in industry, for example 500 V or 690 V.
The maximum, high and low voltages are largely standardized for Western Europe. In the case of medium voltage, subsequent adjustment to standard voltages can be too time-consuming, as a large number of old underground cables with inconsistent maximum operating voltages would have to be replaced. The majority of the investment costs are incurred in the medium and low voltage levels, in which around 70% of the total electricity network costs are bound. The high voltage level (110 kV) accounts for around 20%, the maximum voltage level (220/380 kV) 10%.
Function of the individual networks
- The transmission network uses three-phase high-voltage transmission (DHÜ, HVAC ). It distributes the energy generated by power plants and fed into the grid across the country to power transformers that are close to the main consumption points. It is also connected to the international network via so-called coupling lines.
- The distribution network , which is usually operated with 110 kV in Europe, provides the rough distribution of electrical energy. Lines lead to different regions, metropolitan areas to their substations or large industrial companies . A power requirement of 10 to 100 MW is covered.
- The medium-voltage network distributes the electrical energy to the regionally distributed transformer stations or larger facilities such as hospitals or factories. Municipal utilities , which also operate smaller power plants, often also with combined heat and power , feed their electricity into the medium-voltage network.
- The low-voltage networks are responsible for the fine distribution. In Europe, the low voltage is transformed to the usual 230 V / 400 V and thus private households , small industrial companies , businesses and administrations are supplied. These lines are also known as the last mile . Small - for example private - photovoltaic systems feed in excess power at this low voltage level.
The distribution transformers in the medium-voltage network generally have a fixed transformation ratio. In order to be able to keep the mains voltage at the consumer more or less constant despite the large load fluctuations that occur over the course of a day, the transformation ratio of the power transformers between high and medium voltage network (e.g. 110 kV / 20 kV) can be varied within limits. For this purpose, several taps are led to the outside from the primary winding. A switch built especially for this, a so-called step switch , allows you to switch between the taps without having to switch off the transformer. This process is called voltage regulation . For many devices to function properly, the mains voltage must be kept within narrow limits. Too high or too low voltages can be caused by interference .
There are also lines with high-voltage direct current for transmission over long distances, in particular submarine cables in the form of high-voltage direct current transmission (HVDC).
Connection of the power grids with each other
Power grids with different voltage levels are connected via transformers that are installed in substations . The current flow through the networks and to networks with the same voltage level takes place via switchgear . Power grids with different frequencies or number of phases or power grids that are not synchronized with each other can be coupled with each other via HVDC systems or motor / generator combinations.
Several power plants and customer centers are combined in an interconnected network, as this enables the local difference between supply and demand for instantaneous power to be better balanced within the network. Interconnected networks thus represent the opposite pole to island networks .
The following advantages result from an interconnected network:
- the energy system becomes more stable, as over- and under-capacities are absorbed or can even out,
- by exchanging power, load fluctuations can be better compensated for in the short term than just by regulating the power plants, and
- the operational reliability of the network is increased.
All producers must work synchronously within a network system. Three-phase alternating current leads to higher transmission losses in the cables, so that it is not used, for example, with submarine cables over 30 km in length. In Central and Western Europe, a European network is operated in the area of the Union for the Co-ordination of Transmission of Electricity (UCTE) ; the organizational issues were taken over in 2009 by the ENTSO-E .
A feed-in network is an electricity network specially designed for the reception and transmission of electricity from renewable energies, which is connected to the supply network, often also to the transmission network, and is not set up and operated by the network operator but by the operator of the energy systems. In the German 50hertz control area , two substations are designated as pilot projects. In the Altentreptow-Nord and Wessin substation, only wind energy is fed into the transmission network. In contrast to the public supply network, feed-in networks are less redundant and designed for lower full load hours and can therefore be set up quickly and inexpensively. Feed-in networks serve in particular to improve the system integration of renewable energies in the electricity sector. One example is the feed-in network from Enertrag in the Uckermark .
The lower costs as well as the easier localization and rectification of errors speak in favor of the use of overhead lines. Overhead lines are exposed to environmental influences (e.g. storms), can affect the landscape and in rare cases can endanger people, animals and property.
Underground cables take up less space, are better protected from environmental influences and are more accepted by the population. However, their construction is significantly more expensive; The maintenance effort in the event of defects is high and there are technical problems when underground high-voltage lines exceed certain cable lengths. For example, the heat dissipation of overhead lines is guaranteed by the surrounding air, but not of underground cables. Further problems arise from the enormous reactive power , which in turn is due to the high capacity of the cable.
The German power grid is around 1.8 million kilometers long (as of 2014). The network kilometers are distributed among the various network voltages as follows:
- 1,156,800 km of low voltage level
- medium voltage 509,900 km
- 96,300 km high voltage level
- 35,000 km of high voltage networks
In 2003, around 71% had been laid underground. A comparison with the value for 1993 - around 64% - shows the tendency, as a result of the line expansion in the area of low-voltage networks and in some cases medium-voltage, to expand the underground power distribution. In the high and particularly high voltage range, the underground cable systems laid underground hardly play a role in terms of length.
The power grid must be continuously adapted to the expansion of renewable energies and the resulting changes in the regional distribution of energy systems. In connection with delays in network expansion, this leads to network loads, which the network operators must intervene to stabilize. This affects the transmission network and, to a lesser extent, the distribution network. The redispatch and feed-in management measures required for this cost around 1.4 billion euros in 2017 (around 880 million euros in 2016, around 1.1 billion euros in 2015). These costs are part of the network charges.
Power grids are structured in different ways. The topology is based on various criteria such as the voltage level, spatial boundary conditions, operating costs or security of supply. The most important network types are:
The network is supplied from a central feed point, the individual lines, known as stub lines, run radially to the individual consumption points. Low-voltage networks are often designed in this topology . The advantage lies in the low planning effort, simple troubleshooting and low requirements for network protection. The disadvantage is a low security of supply, since if a branch line fails, all consumers connected to it suffer a power failure .
Ring networks are fed from one or more points, the individual consumers are supplied in the form of a ring line: A consumer can therefore be supplied from two sides via the ring. In the event of a technical defect, the ring around the fault location can be opened, so that the consumers can continue to be supplied away from the fault location. The advantage is an increased security of supply, the disadvantage is the higher qualification of the maintenance personnel, since the activation of a network section in the ring requires the activation of several switching points. Double ring networks represent a special form with increased reliability, in which two ring networks are spatially parallel: Each consumer can then be supplied by one of the two ring networks. Ring networks are used in larger low-voltage networks, particularly in urban areas, in medium-voltage networks and on the 110 kV distribution network level where double ring lines usually supply several subordinate substations .
Mesh networks represent generalized ring networks, are usually fed at several points and the consumers are distributed in a network that has several nodes and branches. The individual consumption points are usually fed via two or more lines; the specific form depends primarily on the performance requirements and spatial conditions. A mesh network offers the highest security of supply if designed accordingly, but requires significantly more complex network protection . There must also be methods for controlling the individual power flows on individual branches, the connecting lines within the network, since each line has only a limited transport capacity. Mesh networks are used, among other things, in transmission networks with high and maximum voltage, such as the 380 kV level. Network networks are usually a spatial combination of several mesh networks.
In the context of network operation, a distinction is made between different network states, which provide information on whether the supply network can fulfill its task of electrical energy distribution. The rules for network operation of transmission networks distinguish between four different network states, which are run through from top to bottom in the event of faults:
- The safe network status is the desired normal case and is characterized by the fact that the permissible electrical limit values are complied with, the N-1 criterion is met in the entire network, sufficient control power is available to be able to compensate for load fluctuations, and all consumers can be supplied.
- The endangered network status means that although all consumers can be supplied, other criteria such as compliance with the N-1 criterion or the availability of sufficient control power are not guaranteed.
- The disturbed network status is also characterized by the fact that not all consumers can be supplied. There are regional power outages .
- The critical network status is characterized by the fact that there is a high risk of extensive power outages and immediate actions, such as separating the network into individual sub-networks, are necessary.
Transmission system operator (TSO)
Four network operators (TSO, Transmission System Operator) are active in Germany; they have joined forces to form the German network control network : Amprion , TransnetBW , Tennet TSO and 50Hertz Transmission .
The Swiss electricity grid is of great importance for Western European electricity trading; it traditionally serves as a hub for balancing peak demand and peak production in the major continental European countries. In 2009, the network in the narrower sense was spun off from the individual energy supply companies (EVU) into so-called grid companies and was transferred to the nationwide transport network operator (TSO) Swissgrid .
In Austria, the national transmission network is operated by the Austrian Power Grid (APG).
At the beginning of 1958, under the control of Swissgrid Control , the power grids of Germany, France and Switzerland were interconnected for the first time near the Swiss town of Laufenburg am Rhein .
In 2007, the European transmission system operators responsible for the operation of the extra high voltage network formed the ENTSO-E association ; before that there were six old associations ("ETSO"). You were reacting to the European Commission's third internal energy market package ; this was adopted in 2009. ENTSO-E also represents the network operators vis-à-vis the Commission.
Distribution network operator (DSO)
In addition to the transmission system operators, there are a large number of distribution networks. In Germany there are around 900 smaller distribution network operators that supply electricity to end consumers.
The network operators receive network usage fees for the service " Passing through electricity from the electricity producer to the consumer". The Federal Network Agency sets prices for this service in Germany .
Electricity networks of the railways
The railway companies operate another energy supply network in Germany, Switzerland and Austria . The DB Energie operates the largest interconnected 110 kV grid in Germany. It uses single phase alternating current . The overhead line network has a length of approx. 7,600 km of traction power lines . In contrast to the national network, the network frequency in the traction current network is 16.7 Hz. The Rübelandbahn uses a 50 Hz network frequency and is supplied directly from the public electricity network.
In addition, there are small regional power grids such as the Mariazeller Bahn in Austria, which is operated with single-phase alternating current and a frequency of 25 Hz . This railway has its own small 27 kV network.
In the other countries, electric railways are supplied with energy from the public power grid. In the case of direct current railways through rectifiers in the substations, in railways operated with single-phase alternating current at a frequency of 50 Hz, the phases of the three-phase system in the substation are separated and fed individually to different sections of the line.
Offshore network, connection to the interconnected network on land
The rapidly growing power supply for the offshore wind industry with the three feed-in stations Büttel, Dörpen and Dörpen West can be seen on the map of the offshore wind turbines.
The current war was a dispute around 1890, whether the DC voltage favored by Thomas Alva Edison or the AC voltage favored by George Westinghouse was the more suitable technology for the large-scale supply of the United States of America with electrical energy and the construction of power grids.
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