Re: Regenerative Braking

Cathode Ray wrote:
On Jan 29, 5:49?pm, "Paul Scott" <notvalidpmsc...@xxxxxxxxxxxxxx>
wrote:

That's correct, it all operates in parallel as a big system, which is
fundamentally different to the 25kV, which has neutral sections to
avoid paralleling parts of the transmission system. There are
various safety devices to cope with short circuits or overloads, but
as I understand it there are not normally breaks in supply as seen
by a train. The 'track paralleling huts' allow isolation of large
sections, but there are not enough of them, so more isolators are
being put into the DC network for more localised isolations for
engineering work or after fatalities etc.

Well, yes and no. On an AC electrification, the regenerated powwer can
flow back into the grid via the traction transformers and thus the
system has guaranteed receptivity as well as allowing the use of
export metering (in simple terms it winds back the incoming feeder's
metering and thus the bill is made up of units consumed minus units
exported) so that you have a lower electricity bill.

On a DC system you can only regenerate power if there is another load
nearby. Whilst the system is connected in a large parallelled mesh,
the system votage will be about 790V DC under light load conditions.
The limits of traction supply voltage for a 750V DC system stipulate a
maximum of 900V DC for short periods.

Thus you only have a 110V headroom from the light load voltage to the
maximum. When trying to export say 2000A under regen braking with only
110V to drive it round the system it doesn't go very far and hence
only a modest amount of current can be exported before the 900V limit
is reached. The train's braking system then dumps the balance of power
into the rheo brake resistor.

DC Regen braking works best in high traffic density areas (say 6TPH in
each direction) because the system voltage is pulled down by the
heavy load to say 620VDC (giving more headroom to export current
before 900V is reached) and there are motoring trains close by which
can use the exported current. Once the voltage at the substation
caused by a regenerating train goes above the light load voltage of
the rectifier set, the rectifier ceases to provide current and hence
that of the braking trains is used.

As the good Captain says, on a DC system you don't export any power
back to the grid (*), but use it to slow down the incoming energy
meter by using less power from the grid, the balance coming from
braking trains when they operate in densely trafficked areas.

Tests undertaken by BR at the time of the introduction of the
Networkers and 323's, provided the following rules of thumb:

DC System:

6- 10 TPH regen braking saves between 12 and 16% energy
4-6 TPH regen braking saves between 8 and 12%
less than 4 TPH regen saves only the system losses and auxiliaries,
say 1 to 2% at best

AC System:

Under light load (1-2 TPH suburban) savings were 10 to 14%
Under heavy load (2 - 10 TPH suburban) savings were typically 12 %
(less export to the grid is possible)

Intercity trains: typically 12 to 18%

The Pendolino fleet is achieving a 14% energy saving as of course they
brake relatively infrequently when compared to suburban duties, but
they can export 4 to 5MW of power under favourable regen braking
conditions.

The 357 fleet is saving 18 to 20% due to the intensive nature of the
suburban railway and using trains which can export 1MW or so per 4 car
unit.

By the way track parallelling huts on a DC system are not there to
make isolations easier, although that is a fringe benefit. Their
purpose is to parallel the conductor rails, thereby reducing the
effective loop impedance and raising the average voltage in the
section. This allows the substations to be spaced at larger intervals,
keeping the costs down as fewer of the substaions are needed.

(*) You can export power from a DC system by using inverting
substations as pioneered by South African Railways on the Natal scheme
in 1927 using mercury arc inverters. They are in use today in
Singapore, Pusan Metro and South Africa. BR/ Railtrack / NR have
investigated their use but they give maximum benefit in two
contrasting situations: Where you have occassional very heavy freight
trains going down long gradients and a very high voltage distribution
system so that you can export the resulting AC power over long
distances, this is what happens in SA, where they have an 88kV HV
railway distribution system.

The other is a very dense metro system such as Singapore and Pusan
where you don't want to equip your trains with heavy and bulky fully
rated rheo brake resistors which generate a lot of heat in an already
hot climate and underground. Thus you need to guarantee that the DC
system is always going to be highly receptive. Even then only one in
three substations is so equipped and then only provides an advantage
when trafic levels are light - in the heavy traffic periods the
regenerated energy gets used by other trains and the amount of
inverted power is negligible.

Ray.

P.S Guess what I've been doing over the last 4 weeks? It involves a
lot of test instrumentation, a DC train with special software, night
shifts and some head scratching!


Thanks for adding the detail and the figures to my recollections of various
posts and magazine articles over the last few years!

Paul


.

Relevant Pages

  • Re: Regenerative Braking
    ... On a DC system you can only regenerate power if there is another load ... Thus you only have a 110V headroom from the light load voltage to the ... When trying to export say 2000A under regen braking with only ... that of the braking trains is used. ...
    (uk.railway)
  • Re: Regenerative Braking
    ... On a DC system you can only regenerate power if there is another load ... Thus you only have a 110V headroom from the light load voltage to the ... When trying to export say 2000A under regen braking with only ... that of the braking trains is used. ...
    (uk.railway)
  • Re: Regenerative Braking
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