15 kV AC railway electrification
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Due to high conversion costs, it is unlikely that existing 15 kV, 16.7 Hz systems will be converted to 25 kV, 50 Hz despite the fact that this would reduce the weight of the on-board step-down transformers to one third that of the present devices.
History
The first electrified railways used
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
Separate plants supply railway power in Austria, Switzerland and Germany, except for
The first generators were synchronous AC generators or synchronous transformers; however, with the introduction of modern
One of the disadvantages of 16.7 Hz locomotives as compared to 50 Hz or 60 Hz locomotives is the heavier transformer required to reduce the overhead line voltage to that used by the motors and their speed control gear. Low frequency transformers need to have heavier magnetic cores and larger windings for the same level of power conversion. (See effect of frequency on the design of transformers.) The heavier transformers also lead to higher axle loads than for those of a higher frequency. Theoretically, in turn, this leads to increased track wear and increases the need for more frequent track maintenance while in practice elelctric locomotives must not become too lightweight in order to preserve traction effort at low speeds. The Czech Railways encountered the problem of the reduced power handling of lower frequency transformers when they rebuilt some 25 kV AC, 50 Hz locomotives (series 340) to operate on 15 kV AC, 16.7 Hz lines. As a result of using the same transformer cores (originally designed for 50 Hz) at the lower frequency, the transformers had to be de-rated to one third of their original power handling capability, thereby reducing the available tractive effort by the same amount (to around 1,000 kW).
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 Hz. Conversion to this voltage/frequency requires higher voltage insulators and greater clearance between lines and bridges and other structures. This is now standard for new overhead lines as well as for modernizing old installations.
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
Distribution networks
In Germany (except
In Sweden, Norway, Mecklenburg-Western Pomerania and Saxony-Anhalt, the power is taken directly from the three-phase grid (110 kV at 50 Hz), converted to low frequency single phase and fed into the overhead line. This system is called the decentralized (i.e. local) railway energy supply.
Generation and conversion
The centralized system is supplied by special
Asynchronous converters
The frequency of 16.7 Hz depends on the necessity to avoid synchronism in parts of the rotary machine, which consists principally of a three phase asynchronous motor and a single phase synchronous generator. Since synchronism sets in at a frequency of 16+2⁄3 Hz (according to the technical details) in the single phase system, the frequency of the centralized system was set to 16.7 Hz.
Power plants providing 110 kV, 16.7 Hz, are either dedicated to generating this specific single phase AC or have special generators for the purpose, such as the Neckarwestheim nuclear power plant or the Walchensee hydroelectric power station.
Synchronous converters
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
Germany, Austria and Switzerland operate the largest interconnected 15 kV AC system with central generation, and central and local converter plants.
Norway
In Norway all electric railways use 15 kV 16+2⁄3 Hz AC
Sweden
In Sweden most electric railways use 15 kV 16+2⁄3 Hz AC. Exceptions include:
See also
References
- ^ 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.