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Discussion Starter · #1 ·
We're getting there slowly....

From http://www.therailwaycentre.com/News May 2008/190508_Southern.html

Train operators Southern and Southeastern have become the first train operators in the country to introduce regenerative braking on the third rail DC network. Southern and Southeastern are both operated by Govia, the partnership between the Go-Ahead Group and Keolis.
After almost two years of planning and testing, the first Class 375, 377 and 376 Electrostar trains are now returning electricity back into the rail system when braking, allowing other trains to draw on that energy for power.
Until now, energy released by trains during braking has been wasted in heating the braking resistors on the train. Now any other train in close proximity will benefit from the electricity transferred back to the third rail.
Southern’s engineering director, Gerry McFadden said: “This represents the culmination of an intensive 18 month project delivered in a model cross-industry partnership between Southern, Southeastern, Network Rail, and train manufacturer Bombardier, with project management from Booz Allen.” He added: “We also owe a debt of gratitude to the DfT and ATOC who have been instrumental in preparing the industry to take on the green agenda.”
Keith Ludeman, chairman and chief executive of Govia said: “I am delighted that Govia is leading the way in the introduction of DC regenerative braking in UK rail. Southern and Southeastern’s success in this area is an excellent example of the industry working together effectively to reduce rail’s carbon footprint and deliver genuine innovation.”
One three-car Electrostar train can put enough power back into the system per year to save around 14 tonnes of carbon.
the rest is on the website.
 

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doesnt this make another argument for electrification and additional cost benefits that the govt didnt take into account when rejecting electrifying the system? why is it such a low amount though when the french seem to be managing between a quarter and a third return on the power?
 

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doesnt this make another argument for electrification and additional cost benefits that the govt didnt take into account when rejecting electrifying the system? why is it such a low amount though when the french seem to be managing between a quarter and a third return on the power?
In basic terms it is a lot easier to put power back into overhead lines rather than via a 3rd rail. I don't know what the percentage it is that these units put back, but the Pendolinos return around 17%. I didn't know the French figures were that high, having said that, the higher the speed travelling the more braking means the more power put back, so for example a TGV may not put back more as it had to use more to achieve the speed to start with.

That might make sense after you read it a few times :D

In short, a TGV may have a higher percentage, but it had to use more power to start with.
 

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yes i know. the faster it goes the more it can generate in stopping... which suggests to me that perhaps there are cost benefits in running trains faster and again the govt saying it isnt environmentally efficient are fudging the figures. i think though that was the point i was making for proper electrification rather than third rail - we'd get more back.
 

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yes i know. the faster it goes the more it can generate in stopping... which suggests to me that perhaps there are cost benefits in running trains faster and again the govt saying it isnt environmentally efficient are fudging the figures. i think though that was the point i was making for proper electrification rather than third rail - we'd get more back.
I mentioned 3rd rail because the original post mentioned EMUs which use 3rd rail. Any future electrification must be overhead anyhow, apart from short extensions which are still allowed.

Obviously there is an optimal speed range for electric high speed trains, I'm curious to know what it is.
 

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Discussion Starter · #6 ·
Also - returning power ac overhead the returned power can go a long way to find a train that needs it, whereas DC 3rd rail doesn't travel far, so you need a much more local train to use the power generated.
 

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Also - returning power ac overhead the returned power can go a long way to find a train that needs it, whereas DC 3rd rail doesn't travel far, so you need a much more local train to use the power generated.
Two reasons why OHLE works better than 3rd rail for doing this.
(1) Higher voltage is used so distance-dependent "copper losses" are less. It works even better on +25-0-25- where copper losses are still less. **

(2) In AC systems the power can feed back through supply transformers, so the entire distribution network can be used as an active sink rather than merely a nearby train.

Having said that, even though the benefits are greater with OHLE there are still worthwhile benefits with DC. As someone else pointed out a very important benefit is reduction of dumping of unwanted heat into tube tunnels.

** Copper losses is a term of art in EE to refer to resistive losses, there isn't necessarily any copper metal involved (although there usually is but steel/aluminium rails may be an exception).
 

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Discussion Starter · #9 ·
What on Earth are you on about?
The overhead 25kV ac is used precisely because it goes a long way down the cables. DC third rail is an older system that was used because they knew how do it cost effectively then, however low voltage high current DC is very lossy over long distances which is why our third rail network has more distribution sub-stations for a given length of track than the high voltage low current AC systems.

In Kent and Sussex there could be over 15km to the nearest train making regen braking useless, but in busier areas it works fine.
 
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