Railway electrification
The key to the increase of train capacity in recent years has undoubtedly been the impressive spread of electrification. Numerous studies and tests as well as practical railway experience show that electric traction has many advantages, especially on lines carrying high-density traffic. The replacement of diesel traction by electric one leads to an all-round improvement in the main operating parameters and the reduction in cost. There is an increase in the average speed of trains, a useful increase in traffic capacity., line capacity., increased automation and hi gher productivity and a reduction of rolling stock requirements. Thus, as compared to diesels, under average conditions, the use of eight-axle electric locomotives results in the increase in the speed of trains by 15-25 per cent.
Undoubtedly, railway future lies in electrification. The conversion of railways to electric traction provides the improvement of labour conditions, the reduction ofair pollution the electrification of settlements situated along the railway lines. Electric traction promotes the automation and mechanization in track laying and repairs.
The reliability of electric traction is a function of combined reliability of the locomotives and power supply, especially the overhead contact wire.
Along its whole length the track on which electric locomotives run must have either an overhead contact wire {catenary) or an extra rail (third or third and fourth rail) from which the train can continuously pick up power. Overhead contact wire is connected with electrical substations and cables to bring the power to the track. As to the extra rail, it is placed in the track between the two running rails. This power supply system makes railway electrification a costly business, therefore it is justified only on railways with a high density of traffic. It should be noted, that the third rail system is particularly essential for underground railways because of a limited space available in tunnels.
Electric railways may use either direct or alternating current (D.C. or A.C). The current supplied from power stations is always alternating one which can be transmitted at high voltage. This high voltage is lowered at a substation to one suitable for transmission over the railway network. If the railway uses the third rail distribution the substation also has to rectify the supply turning it into direct current at a comparatively low voltage.
When alternating current is supplied, it is distributed by an overhead catenary at a high voltage, 25 kV, and at the normal industrial frequency of a country. High voltage is first transformed to a lower voltage in the locomotive and used to feed a.c. or d.c. motors. (It is important that apparatus for rectification should be small because of the limited space of the locomotive.)
At the present state of development almost all electrified railways are supplied by alternating current at 25 kV having frequency of 50 Hz (Herz). Direct current supply causes restrictions on speed, weight of trains. Hence - limited traffic and line capacity, increased power supply and high cost. Electrified lines of alternating current have higher efficiency (power losses for traction are 5-6 % less), weights of trains are practically unlimited. Fulfilling the same work a.c. lines require 15-20 less locomotives and locomotive crews. As has been calculated, traffic expenses on a.c. lines are almost 20 less than on d.c. lines. Instead of formerly used direct current of 3 kV and 1,5 kV many countries nowadays use alternating current as tractive power for trains. In our country according to the Programme of Electrification of Railways adopted at the sitting of the Presidium of Scientific-Tecnical Unity of the Ministry of Ways of Communication which took place in December 2000, 7640 kms of railways will be electrified and 1000 kms of railways will be transformed from direct current to alternating current during the period of 2001 -2010. Such lines as Murmansk-Louchi, Danilov-Alexandrov, Min.VodiKislovodsk, the section of the Transsiberian railway "Zima-Sludyanka), the Volgograd junction and others have recently been transformed to alternating current.
At present there are the following tractive lines in Russia: 3000 V d.c. line and 3000 V a.c. line, frequency 50 Hz (high-voltage lines); 100 V d.c. and 650 V d.c. (having voltage after the primary transformation of high voltage). The former two lines are customary for our railways, the 650 V d.c. line is introduced additionally for reserve supply in case of damage of the main power supply systems. The line 100 V d.c. using the power of accumulated batteries is designed for use in emergency situations.
4. a ) Составьте список преимуществ блока питания переменного тока. Используйте текст , чтобы помочь вам .
1 higher efficiency (power losses 5-6 % less) 2_____________________ 3_____________________
b ) Составьте список недостатков источника питания постоянного тока. Используйте текст, чтобы помочь вам.
1 causes restrictions in speed 2 __________________ 3 _________________
5. Опишите процесс передачи электрического тока в систему железнодорожной сети посредством третьего рельсового распределения. Ваше объяснение будет состоять из нескольких шагов. Вам помогут глаголы.
1 _____________ place 2_____________ connect 3 ______________transform 4 _____________ rectify 5______________turn into 6_______________feed
IV . Закрепление новых знаний.
1. Попробуйте угадать значение слов, выделенных жирным шрифтом. Проверьте свои ответы с помощью словаря.
It is a well-known fact that electric trains perform valuable service on trunk lines with dense traffic. But electric haulage is costly because it needs a whole system of electrical substations and either a catenary or a conductor rail. The third rail system is particularly essential for underground railways because of a limited space available in tunnels. The conductor rail system proves quite successful under normal conditions, but a great difficulty arises on open sections. The matter is that under unfavorable winter conditions the third rail may be covered with ice and it becomes difficult to transfer electric current from the conductor rail to the train. To eliminate this difficulty a special equipment was devised which could remove ice. At first this equipment was manually operated. But the process was relatively slow and inefficient. In recent years, the manually-operated equipment has been converted to semi-automatic working.
2. Определите функцию глагола «иметь» в следующих предложениях, переведите на русский язык.
1. Every engineer working in the field of signaling has to know the principles of track circuit operation. 2. Many new electronic devices have been developed to provide the safety of high-speed traffic. 3. One of the new systems proposed for carrying passengers in towns is the “Never-Stop” Railway which has no rails but runs with rubber-tired wheels. 4. Our country has many locomotive building works producing all types of locomotives. 5. Technical investigations have shown that today it is possible to build very powerful diesel locomotives. 6. The old bridge had to be moved from piers to carry the diverted traffic during the reconstruction. 7. With the introduction of Computer Traffic Control the safety of railway operation has greatly increased. 8. It was reported by foreign press that multiple-wheel cars would have a bright future. 9. All types of locomotives have to satisfy a number of technical and economic requirements.
IV . Закрепление новых знаний.
V . Этап подведения итогов.
VI . Информация о домашнем задании: выучить новые слова, прочитать и перевести
Текст “Railway Electrification”.
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