Low voltage network
Low-voltage networks are part of the electricity network for distributing electrical energy to the majority of electrical end users (low-voltage devices) and are managed by many regional distribution network operators . In order to minimize power losses, low-voltage networks are limited in terms of their spatial extension to a range of a few 100 m to a few kilometers. They are therefore fed regionally via transformer stations from a higher-level medium-voltage network.
In contrast to the other voltage levels, low-voltage networks in large areas of Europe are not three-wire, but four-wire systems to enable the connection of single-phase consumers. They are usually operated with a mains voltage of 230 V / 400 V (single-phase / three-phase) to 1000 V. The rated outputs of individual local network transformers are 250, 400, 630 or 1000 kVA . Outside of Europe, other shapes and operating voltages are also common. In North America and partly in Asia, for example, the single-phase three-wire network and the Red-Leg Delta system based on it are widespread.
species
The types of network design for three-phase alternating current used in Europe are defined by the International Electrotechnical Commission (IEC) and are divided into:
- TN system (French: Terre Neutre): It is the most common version in Central Europe, has a rigidly earthed star point, and in various partial versions the protective conductor or, together, the neutral and protective conductor is used as a so-called PEN conductor from the transformer station to the individual sub-distributions. This means that five or four parallel conductors are required. The TN system is divided into the TN-C system, TN-S system and the most common, the TN-CS system, depending on the specific design of the protective conductor or neutral conductor and their combinations.
- TT system (French: Terre Terre): One point of the power source, usually the neutral point of the transformer, is directly connected to earth via a plant earth. The bodies of the electrical equipment in the consumer system are directly connected to earth via a system earth. In contrast to the TN system, there is no direct connection between the two earth electrodes via a PEN or PE conductor . Further protective measures may be necessary, as the level of the earth resistance of the system earth does not guarantee that the power supply will be switched off automatically in the event of a fault.
- IT system (French: Isolé Terre): So-called isolated network. Use in mostly small-scale industrial networks and in hospitals. As a special feature, the star point is not earthed in this network. IT networks have the advantage that a simple earth fault does not immediately lead to failure. The fault is displayed by an insulation monitoring device and can then be rectified under certain circumstances without interruption. A small reactive current flows at the fault location . The level of the current depends on the capacity of the network cables interconnected in the network area. The use of residual current circuit breakers in this network is only possible in the event of double or multiple faults for immediate shutdown.
topology
Starting from the area of the main distribution, low-voltage networks are usually divided into several cable strands that supply individual houses or groups of houses in the vicinity. The lines are usually implemented in a star shape, with the branches to the sub-distribution in the area of the house connection via a so-called loop box . The overhead lines still common in rural areas are fed out via roof stands. In special cases, a low-voltage network can also be set up as a ring and fed from several points.
In the sub-distribution area , the individual consumers and sockets are star-shaped. In England and in some former English colonies, ring-shaped distributions also occur in dwellings under the BS 1363 standard . The ring topology has the advantage that smaller conductor cross-sections can be used with the same performance, but the installation effort is higher.
Feed
Low-voltage networks are fed from the medium-voltage network by local transformer stations . Sometimes a more economical transformer with the vector group Yy0 (star-star 0) is used, whereby the secondary side of the windings are connected to the star point at one end. The vector group Yz5 (star-zigzag 5) is better than the star connection, in which the asymmetry is largely compensated for by distributing the asymmetrical external conductor current to each of 2 leg halves of a transformer. If this compensation does not take place, then the individual external conductor voltages at the consumer will have unequal voltage values and unbalanced loads are the result.
On the low-voltage side, the neutral point of the local transformer is solidly earthed . A ground resistance of usually R E below 2 Ω is required. If this value is higher due to aging, damage, dry ground or other circumstances, in the event of an error, inadmissibly high contact voltages or step voltages can occur.
Head designations
- Outer conductor (L1, L2, L3): Designation for the potential-carrying end of the transformer winding. These are three conductors, each phase shifted by 120 °, in the three-phase system. Colloquially, the outer conductor is also referred to as phase, or an outer conductor against neutral conductor is also referred to as luminous flux and the three-phase alternating current is also referred to as power current.
- Neutral conductor (N): This describes a conductor that is electrically connected to the neutral point (usually the star point in a three-phase network) and is able to contribute to the distribution of electrical energy. In a symmetrically loaded three-phase system with an ohmic load, the currents in the outer conductors cancel each other out. No electrical current flows in the star point (neutral conductor) . Only when an asymmetrical load arises from a 230 V consumer does the current in the neutral conductor compensate for the asymmetry.
- Protective conductor (PE): This describes the conductor that is used exclusively for protection against dangerous contact voltages. It is always marked green / yellow and is a continuous electrical connection of all easily touchable metal parts that do not belong to the operating circuit.
- PEN conductor (PEN): This is a conductor that simultaneously fulfills the functions of the protective conductor (PE) and the neutral conductor (N). A ladder with such a double function is only possible in a TN-C system. Possible problems: see description in article PEN conductor . In new systems, PEN conductors are only permitted to be permanently laid with a conductor cross-section of at least 10 mm² copper or 16 mm² aluminum, which excludes "classic zeroing" as was previously the case.
- Grounding : When one speaks of grounded or grounding, a conductive connection to the ground is always meant. This is produced by an earth electrode . Foundation earth electrodes in house foundations, rod earth electrodes up to 20 m deep or strip iron earth electrodes that are buried 15 m long and 1 m deep are common. The earth electrodes of the transformer stations should not have a resistance of more than 2 Ω . All other earth electrodes usually have a worse earthing resistance, which is heavily dependent on the nature of the soil and the weather.
- Equipotential bonding : In addition to all protective conductors of the system and the PE conductor of the supply, all electrically conductive bodies (e.g. stair railings, heating pipes, water pipes, sewage pipes, air conditioning systems, gas pipes, etc.) are included in the equipotential bonding. The previous requirement to also connect shower trays or bathtubs to the equipotential bonding is no longer included in the current standards.
The railway must include all metallic parts under its contact lines in the equipotential bonding, including railings, clocks and cladding. In the crack area of the contact line, the lines have to withstand thermal short-circuit currents of even 25 kA / 1 s. Otherwise, the electrical voltage drop caused by contact line short circuits would generate excessively high voltages.
Coloring
In order to differentiate between the outer conductor and the neutral conductor, cable systems for low voltage have standardized colors depending on the region. In the EU, the coloring is specified by the IEC 60446 (EN 60446) standard. Different color schemes are also used in other countries. Some common color schemes in three-phase systems are:
| country | L1 | L2 | L3 | Neutral conductor N |
Earth / protective earth |
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| European Union |
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| Germany |
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| Switzerland |
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| Austria |
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| France |
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| United Kingdom |
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| United States |
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| Canada |
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| Australia and New Zealand |
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| China |
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Notes on the color table:
- ↑ Preferred colors; Covers all countries that apply the CENELEC standard IEC 60446 .
- ↑ a b c d e f g h i j k l m n o p q r s Only for old installations
- ↑ a b c d e Before March 31, 2004, specified in British Standard BS 7671
- ↑ Not standardized. Colors are partly defined in the NEC .
- ↑ Established by the Canadian Standards Association as mandatory
- ↑ Specified in the AS / NZS 3000: 2007 standard
- ↑ Established in Standard GB 50303-2002 Section 15.2.2
Special forms
Low-voltage networks with nominal voltages of 690 V are used, among other things, in industrial plants or power plants to supply larger electric motors for driving pumps, conveyor belts and the like with powers of a few 100 kW to a few megawatts.
Another special application of higher low voltages is extensive low voltage networks in rural areas in Europe with 960 V as intermediate voltage in order to reduce the voltage drop between the medium-voltage transformer station and the 400 V end customer connection. For this purpose, an additional transformer from 960 V to 400 V in the power range of a few 10 kVA is provided in the immediate vicinity of the end customer (for example a remote homestead or individual remote country houses). The transformer station, which is operated with medium voltage, on the other hand, can be a few kilometers away. In addition to reducing the voltage fluctuations on long low-voltage supply lines, the advantage of the intermediate voltage is that there is no need for a medium-voltage line to the remote buildings that is complex and expensive in terms of insulation. The low-voltage lines and electrical installation equipment approved up to 1 kV can be used.
Other uses
Low-voltage networks are used not only to supply electrical energy, but also to transmit messages . In particular, control signals for ripple control technology and, in some countries, higher-frequency data signals are transmitted via low-voltage networks by means of carrier frequency modems ( powerline ). There were also attempts and applications with wire radio .
See also
literature
- Adolf J. Schwab: electrical energy systems. Generation, transport, transmission and distribution of electrical energy . Springer Verlag 2006, ISBN 3-540-29664-6 .
Individual evidence
- ↑ Access to electricity (% of population). In: World Bank Open Data. World Bank , 2019, accessed October 28, 2019 .
- ^ Harmonized Colors and Alphanumeric Marking , in IEE Wiring Matters , Spring 2004
- ↑ according to VDE 0293
- ↑ Information on harmonizing the wire colors ( memento of the original from October 29, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
