Электрификация железных дорог переменным током (AC) напряжением 15 киловольт (кВ) и частотой 16,7 герц (Гц) применяется на транспортных железных дорогах Германии , Австрии , Швейцарии , Швеции , Норвегии . Высокое напряжение позволяет передавать большую мощность с более низкой частотой, снижая потери тяговых двигателей , которые были доступны в начале 20 века. В электрификации железных дорог в конце 20-го века, как правило, используются системы переменного тока напряжением 25 кВ, частотой 50 Гц , которые стали предпочтительным стандартом для новых электрификаций железных дорог, но расширение существующих сетей 15 кВ не является совершенно маловероятным. В частности, в Готардском базовом туннеле (открытом 1 июня 2016 года) до сих пор используется электрификация 15 кВ, 16,7 Гц.
Из-за высоких затрат на преобразование маловероятно, что существующие системы 15 кВ, 16,7 Гц будут преобразованы в 25 кВ, 50 Гц, несмотря на то, что это уменьшит вес бортовых понижающих трансформаторов на одну треть от массы бортовых понижающих трансформаторов. присутствующие устройства.
На первых электрифицированных железных дорогах использовались двигатели постоянного тока с последовательной обмоткой сначала на 600 В , а затем на 1500 В. В регионах с контактными сетями постоянного тока 3 кВ (в основном в Восточной Европе ) использовались два последовательно соединенных двигателя постоянного тока на 1500 В. Но даже при напряжении 3 кВ ток, необходимый для питания тяжелого поезда (особенно в сельской и горной местности), может быть чрезмерным. Хотя увеличение напряжения передачи уменьшает ток и связанные с ним резистивные потери для заданной мощности, ограничения по изоляции делают тяговые двигатели более высокого напряжения непрактичными. Таким образом, трансформаторы на каждом локомотиве должны понижать высокое напряжение передачи до практического рабочего напряжения двигателя. До разработки подходящих способов эффективного преобразования постоянного тока с помощью силовой электроники эффективным трансформаторам строго требовался переменный ток (AC); таким образом, на электрифицированных железных дорогах высокого напряжения наряду с системой распределения электроэнергии использовался переменный ток (см. Война токов ).
Сеть переменного тока частотой 50 Гц (60 Гц в Северной Америке) была создана еще в начале 20 века. Хотя двигатели с последовательной обмоткой в принципе могут работать как на переменном, так и на постоянном токе (по этой причине они также известны как универсальные двигатели ), у больших тяговых двигателей с последовательной обмоткой были проблемы с такими высокими частотами. Высокое индуктивное сопротивление обмоток двигателя вызывало проблемы с перекрытием коммутатора , а неламинированные магнитные полюсные наконечники, изначально предназначенные для постоянного тока, демонстрировали чрезмерные потери на вихревые токи . Использование более низкой частоты переменного тока облегчило обе проблемы.
В немецкоязычных странах высоковольтная электрификация началась в 16 веке.+2 ⁄ герца , ровно одна треть частоты национальной электросети 50 Гц. Это облегчило работуротационных преобразователейот сетевой частоты и позволило специальным железнодорожнымгенераторамработать с той же скоростью вала, что и стандартный генератор с частотой 50 Гц, за счет уменьшения количества пар полюсов в три раза. Например, генератор, вращающийся со1000 об/мин, будет иметь две пары полюсов, а не шесть.
Отдельные электростанции поставляют железнодорожную электроэнергию в Австрии, Швейцарии и Германии, за исключением Мекленбург-Передней Померании и Саксонии-Анхальт ; Преобразователи, работающие от сети, обеспечивают электроэнергию для железных дорог в этих двух немецких землях, а также в Швеции и Норвегии. В Норвегии также есть две гидроэлектростанции, предназначенные для обеспечения железной дороги с 16+ Выходная частота 2 ⁄ герц .
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 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 electric 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 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.
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 Hz; the voltage is 110 kV in Germany and Austria and 132 kV in Switzerland. This system is called the centralized railway energy supply. A separate single-phase power distribution grid makes the recuperation of energy during braking extremely easy in comparison with 25 kV 50 Hz system tied to 3 phase distribution grid.
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.
The centralized system is supplied by special power plants that generate 110 kV (or 132 kV in the Swiss system) AC at 16.7 Hz and by rotary converters or AC/AC converters that are supplied from the national power grid (e.g. 110 kV, 50 Hz), they convert it to 55-0-55 kV (or 66-0-66 kV) AC at 16.7 Hz. The 0 V point is connected to earth through an inductance so that each conductor of the single phase AC power line has a voltage of 55 kV (or 66 kV) with respect to earth potential. This is similar to split-phase electric power systems and results in a balanced line transmission. The inductance through which the earthing is done is designed to limit earth currents in cases of faults on the line. At the transformer substations, the voltage is transformed from 110 kV (or 132 kV) AC to 15 kV AC and the energy is fed into the overhead line.
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.
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.
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.
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.
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.