What is a network station
Lexicon> letter T> transformer station
Definition: a device for the transmission of electrical energy from a medium-voltage network to a low-voltage network
Alternative terms: transformer station, network station, transformer station, local network station
Category: electrical energy
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 08/16/2020; last change: 20.09.2020
Most small consumers of electrical energy are supplied with low voltage. For this purpose, they are each connected to a low-voltage network, which is fed from a medium-voltage network via one or more transformer stations. Even a small town contains a considerable number of transformer stations, since a single one usually has a nominal power between 250 kVA and 1000 kVA (specified as apparent power). Often one network station is only sufficient for a number of households over a length of a few hundred meters on a residential street. Higher performance is possible, but usually only makes sense where a lot of performance is required in a small area, e.g. B. in high-rise buildings.
In principle, the term transformer station could also be used for substations between high-voltage and medium-voltage networks, since these stations are also based on transformers. However, this term is not common there.
Transformer stations are operated by the distribution network operators. The costs incurred are charged to the consumers via the network usage fees.
Essential components of a transformer station
The core of every transformer station is typically one or two transformers (local network transformers, distribution network transformers) that are supplied on the primary side with medium voltage with an effective value of, for example, 10 kV or 20 kV and emit the low voltage on their secondary side. In addition, there are usually some devices for switching, regulation, distribution and monitoring.
While older transformer stations are based on fairly simple technology, more modern stations are often equipped with complex technology. For example, controllable local network transformers, which are helpful for voltage maintenance, are increasingly being used, especially when variable decentralized feeds, e.g. B. of photovoltaic systems play a major role. Stable operation despite significant unbalanced loads is also necessary, which is why specially optimized transformer designs are used in some cases.Today's transformer stations contain quite a bit of sophisticated technology.
In addition, there is increasingly information and communication technology that enables remote control and remote retrieval of important parameters. That is why one sometimes speaks of “intelligent” network stations. Such technology can make networks more stable and reliable and at the same time reduce personnel costs.
The considerable investments in a large number of transformer stations are financially viable because these systems can usually be operated for many decades and convert large amounts of energy over the course of their service life.
Inlets and outlets
The medium voltage is usually supplied via a three-wire system, i.e. with the three phases but without a neutral conductor. In sparsely populated areas, the power is usually supplied via an overhead line, otherwise via underground cables.
A four-wire system is practically always used for the low voltage output, i.e. three phases plus a neutral conductor. The low voltage is usually supplied via underground cables, but sometimes also via overhead lines. This means that consumers can also be connected in one phase, i.e. to one of the phases and the neutral conductor, with an effective voltage of 230 V (in Germany). At the same time, three-phase current with an external conductor voltage of 400 V can be used.
The low-voltage neutral conductor is earthed in a transformer station, i. H. electrically connected to earth. A separate protective conductor is usually not offered; a so-called TN system is used (usually up to the house transfer point).
Significant energy losses occur primarily in the transformers themselves - partly due to the electrical resistance of the transformer windings, partly also due to eddy currents and other losses caused by magnets (hysteresis losses). A typical local network transformer can be expected to lose hundreds of watts even when idling. However, modern transformers often have significantly reduced energy losses. Incidentally, the degree of efficiency depends considerably on the capacity utilization of the system; it drops massively, especially when the workload is very low and is usually optimal when the workload is rather low, but not too low.
Types of transformer stations
In Germany, until around 1980, many transformer stations were used as Tower stations built, d. H. in a solid brick building with the shape of a tower, where the medium voltage was mostly supplied from above. A station can also be built into a prefabricated garage. Today, however, the common form is the one Compact station, which is industrially manufactured and delivered as a whole. Because of their compact, inconspicuous shape in connection with the supply via underground cables, they are hardly noticeable in the townscape.
Sometimes also come for the connection of small consumers Mast stations in question, d. H. Systems mounted on local power poles. Another possibility is the integration of transformer systems in existing larger buildings, for example commercially used, or in the basements of high-rise buildings.
Effects of transformer stations on the environment
Transformer stations usually only have a limited impact on their immediate surroundings. First of all, in systems that have been supplied via overhead lines, on the dangers of at least z. B. with 10 kV or 20 kV voltage (sometimes more) operated supply lines to pay attention. It must be absolutely avoided that z. B. playing children come into contact with the cables, also indirectly, for example through trees.
The transformers usually generate a clearly audible hum in the immediate vicinity, but this rarely causes significant annoyance to residents.
The magnetic leakage flux of the transformers can be considerable in the direct vicinity of the systems, but is carefully minimized, especially with newer systems, due to concerns about electrosmog. Basically, the strength of such alternating magnetic fields, whose health significance is still largely unexplained scientifically, decreases rapidly with increasing distance.Transformer fires can contaminate the whole area!
The transformers used usually contain a more or less toxic and groundwater-damaging transformer oil as a coolant and insulation agent. (There are also cast resin transformers, but they tend to be less efficient.) Appropriate precautions (e.g. oil retention devices) can almost always prevent this oil from harming the environment.
Exceptions are rare transformer fires. However, these can be very problematic, at least if an old-style transformer still contains transformer oil with highly toxic components such as PCB. There have been cases where such a fire caused massive contamination of the area. Extreme cases are some high-rise buildings that were transformed into contaminated ruins that could never be used again by a transformer fire with distribution of the toxic smoke via a ventilation system.
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See also: transformer, low voltage, medium voltage
as well as other items in the electrical energy category
Question: Why does a transformer station only supply relatively few households?
Correct answer: (b)
Question: How is the potential of the neutral conductor stabilized on the low-voltage side?
Correct answer: (a)
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