Post by M***@dastardlyhq.comOn Wed, 6 Dec 2023 12:50:08 +0100
Post by BobAs a high voltage AC electrification system, the 15 kV system, on
direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
system, and does not have the kinds of power limitations that make DC
systems unsuited to higher speed running or heavy freight haulage (300
Actually DC is actually better for high voltage for transfering power as
there are lower losses. National Grid has built some DC interconnects to
other countries for example. These days if building a railway from scratch
you could probably do worse than 25KV *DC* given converting it down to lower
DC voltages is now a solved problem with modern electronics. Of course you
wouldn't get the nice hum for the catenary in the rain :)
High voltage DC has some very specific use cases where it makes sense,
but also comes with some very specific drawbacks that makes it unsuited
for railway traction purposes.
DC has lower losses for long distances where inductance and capacitance
pose a challenge to transmission. Making use of high voltage DC comes
with the challenge of producing a high voltage DC supply, and of doing
something useful with a high voltage DC supply.
Getting mechanical work out of electricity relies on the interaction
between electric current and magnetic fields. The forces involved depend
on the current flowing. Creating coils of wire increases the magnitude
of the forces by the number of turns, but the lower the current, the
more turns in the coils are required. As electrical power is the current
multiplied by the voltage drop, if you start with a very high voltage,
for a given power there is a very low current.
Having a high voltage also creates a significant challenge on
insulators. If you attempt to transition from a bare conductor to an
insulated cable with an earth potential screen (needed to be able to
safely have it near conductive objects at near earth potential like the
frames of a railway vehicle) starts to become challenging at around 10
kV, and by the time you get to tens of kV, you have to design equipment
quite carefully in terms of switchgear, insulation terminations and joints.
These two factors combined means that to create a controlable electric
motor within a railway vehicle, the voltage that the motor itself
utilises can not readily exceed single digit kV. This is why railway DC
electrification at above about 3 kV is practically unheard of.
To make a high voltage DC railway system viable would involve on board
power electronics to convert high voltage DC to high voltage AC, then
step that down to low voltage AC in a power transformer, rectify that
back to DC, invert it to VVVF 3 phase AC and feed that to a traction motor.
Likewise a substation feeding a high voltage DC installation would
involve taking an AC supply from the grid, transforming it to the right
volage, then rectifying it to high voltage DC.
Each conversion step incurs losses. While the OHLE itself, if fed with
AC, will experience inductive and capacitative losses not present with
DC, for the distances and voltages that are relevant to a railway
traction environment, these are less than the losses of the additional
conversion steps.
In short, to use high voltage DC, there is extra complexity in the
supply and on the vehicles, and associated losses, compared with mains
frequency AC, and their efficiency penalties are greater than the very
small gains from DC transmission. Power transmission lines operate at
hundreds of kV and thousands of A. The electrical effects that are
important in that range are not important at the tens of kV and tens of
A that railway electrification involves.
Robin
