15 kV AC railway electrification
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Railway electrification using alternating current (AC) at 15 kilovolts (kV) and 16.7 hertz (Hz) are used on transport railways in Germany, Austria, Switzerland, Sweden, and Norway. The high voltage enables high power transmission with the lower frequency reducing the losses of the traction motors that were available at the beginning of the 20th century. Railway electrification in late 20th century tends to use 25 kV, 50
Due to high conversion costs, it is unlikely that existing 15 kV, 16.7
History
[edit]The first electrified railways used series-wound DC motors, first at 600 V and then 1,500 V. Areas with 3 kV DC catenaries (primarily in Eastern Europe) used two 1,500 V DC motors in series. But even at 3 kV, the current needed to power a heavy train (particularly in rural and mountainous areas) can be excessive. Although increasing the transmission voltage decreases the current and associated resistive losses for a given power, insulation limits make higher voltage traction motors impractical. Transformers on each locomotive are thus required to step high transmission voltages down to practical motor operating voltages. Before the development of suitable ways to efficiently transform DC currents through power electronics, efficient transformers strictly required alternating current (AC); thus high voltage electrified railways adopted AC along with the electric power distribution system (see War of the currents).
The 50 Hz (60 Hz in North America) AC grid was already established at the beginning of the 20th century. Although series-wound motors can in principle run on AC as well as DC (the reason they are also known as universal motors) large series-wound traction motors had problems with such high frequencies. High inductive reactance of the motor windings caused commutator flashover problems and the non-laminated magnetic pole-pieces originally designed for DC exhibited excessive eddy current losses. Using a lower AC frequency alleviated both problems.
In the German-speaking countries, high-voltage electrification began at 16+2⁄3 hertz, exactly one third of the national power grid frequency of 50 Hz. This facilitated the operation of rotary converters from the grid frequency and allowed dedicated railway power generators to operate at the same shaft speed as a standard 50 Hz generator by reducing the number of pole pairs by a factor of three. For example, a generator turning at 1,000 rpm would be wound with two pole pairs rather than six.
Separate plants supply railway power in Austria, Switzerland and Germany, except for Mecklenburg-Western Pomerania and Saxony-Anhalt; converters powered by the grid supply railway power in those two German states plus Sweden and Norway. Norway also has two hydro-electric power plants dedicated for railway power with 16+2⁄3 hertz output.
The first generators were synchronous AC generators or synchronous transformers; however, with the introduction of modern double fed induction generators, the control current induced an undesired DC component, leading to pole overheating problems. This was solved by shifting the frequency slightly away from exactly one third of the grid frequency; 16.7 hertz was arbitrarily chosen to remain within the tolerance of existing traction motors. Austria, Switzerland and Southern Germany switched their power plants to 16.7 Hz on 16 October 1995 at 12:00 CET.[1][2] Note that regional electrified sections run by synchronous generators keep their frequency of 16+2⁄3 Hz just as Sweden and Norway still run their railway networks at 16+2⁄3 Hz throughout.
One of the disadvantages of 16.7
These drawbacks, plus the need for a separate supply infrastructure and the lack of any technical advantages with modern motors and controllers has limited the use of 16+2⁄3 Hz and 16.7 Hz beyond the original five countries. Most other countries electrified their railways at the utility frequency of 50/60 Hz. Denmark, despite bordering only 15 kV territory decided to electrify their mainline railways at 25 kV 50 Hz for that and other reasons.[3][4] Because it is technically very challenging and therefore not cost-effective to provide high-speed passenger services on 1.5 or 3 kV DC lines, newer European electrification primarily in Eastern Europe is mostly 25 kV AC at 50
Simple European unification with an alignment of voltage/frequency across Europe is not necessarily cost-effective since trans-border traction is more limited by the differing national standards in other areas. To equip an electric locomotive with a transformer for two or more input voltages is cheap compared to the cost of installing multiple train protection systems[citation needed] and to run them through the approval procedure to get access to the railway network in other countries. However, some new high-speed lines to neighbouring countries are already intended to be built to 25 kV (e.g. in Austria to Eastern Europe). Although newer locomotives are always built with asynchronous motor control systems that have no problem with a range of input frequencies including DC, the required additional pantographs and wiring are not universally installed in order to offer cost-reduced models like the Siemens Smartron. Likewise, newer regional passenger trainsets such as the Bombardier Talent 2 series are not certified for additional electrification systems. Despite the Deutsche Bahn train operator does not use older models from the standard electric locomotive series anymore, many smaller private rail companies do, though some are now as much as 60 years old. Even as these obsolescent models are decommissioned, it still may not be easier to unify. Meanwhile, the Deutsche Bahn tends to order rolling stock that are capable of running multiple electrification systems, especially freight locomotives and high-speed passenger trainsets as these operate across Europe.
Distribution networks
[edit]In Germany (except Mecklenburg-Western Pomerania and Saxony-Anhalt), Austria and Switzerland, there is a separate single-phase power distribution grid for railway power at 16.7
In Sweden, Norway, Mecklenburg-Western Pomerania and Saxony-Anhalt, the power is taken directly from the three-phase grid (110 kV at 50
Generation and conversion
[edit]The centralized system is supplied by special power plants that generate 110 kV (or 132 kV in the Swiss system) AC at 16.7
Asynchronous converters
[edit]The frequency of 16.7
Power plants providing 110 kV, 16.7
Synchronous converters
[edit]The power for the decentralized system is taken directly from the national power grid and directly transformed and converted into 15 kV, 16+2⁄3 Hz by synchronous-synchronous-converters or static converters. Both systems need additional transformers. The converters consist of a three-phase synchronous motor and a single-phase synchronous generator. The decentralized system in the north-east of Germany was established by the Deutsche Reichsbahn in the 1980s, because there was no centralized system available in these areas.
Facilities for 15 kV AC railway electrification in Germany, Austria and Switzerland
[edit]Germany, Austria and Switzerland operate the largest interconnected 15 kV AC system with central generation, and central and local converter plants. However, there are islands with alternative electrification systems. For example, the Rübeland Railway is the largest 25 kV AC line in Germany.
Norway
[edit]In Norway all electric railways use 15 kV 16+2⁄3 Hz AC[5] (except the Thamshavnbanen museum railway which uses 6.6 kV 25 Hz AC). The Oslo T-bane and tramways use 750 V DC power.
Sweden
[edit]In Sweden most electric railways use 15 kV 16+2⁄3 Hz AC. Exceptions include: Saltsjöbanan and Roslagsbanan (1.5 kV DC), the Stockholm Metro (650 V and 750 V DC) and tramways (750 V DC). The Oresund Bridge linking Sweden and Denmark is electrified at 25 kV, Danish standard; the split is located on the Swedish side near the bridge. Only two-system trains (or diesel trains; rare) can pass the point.
See also
[edit]References
[edit]- ^ Bahnstromsystem (German) railway electrification systems
- ^ C. Linder (2002). "Umstellung der Sollfrequenz im zentralen Bahnstromnetz von 16 2/3 Hz auf 16,70 Hz" [Switching the frequency in train electric power supply network from 16 2/3 Hz to 16.70 Hz]. Elektrische Bahnen (in German). 12. ISSN 0013-5437.
- ^ "Siemens to electrify Denmark's rail network | Press | Company | Siemens".
- ^ "ELECTRIFICATION OF THE DANISH RAILWAY".
- ^ "Bane Energi". jernbaneverket. Archived from the original on 5 October 2015. Retrieved 29 July 2015.