On Sun, 7 Jun 2009 14:04:25 +0100 someone who may be tony sayer
wrote this:-

And you wouldn't normally use that high a voltage for rail traction
nowadays DC wise as its rather more awkward to control unlike AC which
of course can be transformed down....

The reasons for going for a high voltage were to do with minimising
the number of feeder stations and other electrical infrastructure.
With a higher voltage there are far fewer and these are fed directly
from the external supply. With a lower voltage there are a few
external supply points, but then there has to be a railway
distribution system (usually 33kV) and frequent feeder stations.

The reason for going with AC was that the locomotive could be fitted
with a transformer to easily convert the supply to a more practical
voltage to feed into the equipment on the locomotive.

In the late 1940s the advantage was still with lower voltage DC
systems. This simplified the equipment on the train, at the expense
of a lot of infrastructure at the lineside. The 1500V DC
electrification from Paris to southern France is a testament to the
relative merits of the systems at the time. It was thought that
developments in (mercury arc) rectifiers had made the position of
relatively low voltage DC systems unassailable.

However, at the same time the French were experimenting with a line
which had come into their posession from the Germans. This used an
industrial frequency [1] supply and a variety of locomotives. From
this came the concept of a locomotive fitted with a transformer and
rectifier, the output of which was fed to standard DC motors. The
locomotives were a bit erratic, due to splashing of mercury within
the rectifiers, but there was promise.

The industrial frequency AC system was developed from that. By the
mid 1950s it was cheaper than DC and so began to be used for new
installations, but it was not so cheap as to make it worthwhile
ripping out DC systems and replacing them with AC [2]. The result in
France is that, roughly speaking, lines south of Paris are 1500V DC
and lines north of Paris are 25000V AC. LGVs, being new lines, have
all been AC lines, even in southern France.

Improvements in rectifiers involving semiconductors improved DC and
AC systems, but did not alter the relative merits. For the
foreseeable future the 25000V AC system will remain the standard.

Eventually semiconductors improved locomotives in another way. These
days it doesn't matter whether the supply is AC or DC, though it
needs to be reduced to a voltage the inverters can cope with. It
comes out of these as variable frequency AC which is fed to
induction motors. No more commutators to maintain, which is great.


[1] 50Hz. There were earlier railway electrification systems at 25
and 16.6Hz AC. The frequency was kept low in order to minimise
problems with the commutator in the motors. Germany railways, and a
number which followed their practice, still have 16.6Hz supplies.

[2] replacing worn out DC systems and standardising on AC are
another matter. Several have been replaced with AC.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54