Английский язык. Арланова Т.Л - 8 стр.

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8
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.Vodi-
Kislovodsk, 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.
Exercise 5. a) Make a list of advantages alternating current power supply.
Use the text to help you.
1 higher efficiency (power losses 5-6 % less)
2_____________________
3_____________________

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