Could the subsurface lines be converted to DC overhead current collection?

[NotATrainspott]

The Space Train proposals for the Victoria line had the fourth-rail electrification system replaced with a conductor bar on the roof of the tunnels, which allowed the supply voltage to be safely increased to 1.5kV with its resulting efficiency benefits.

Since the subsurface lines have much more vertical headroom than any of the Tube lines, and have roughly the same loading gauge as any of the normal lines in the country, would it be possible for these lines to be converted to use a similar system?

They're increasing the fourth-rail voltage up to 750V from 630V as part of the SSR renewal program because the S-Stock draw more power and can do regenerative braking. Beyond this there isn't much more they can safely put through the fourth rails due to the inherent safety issues of putting that much voltage within easy reach of passengers and track workers.

I imagine it would be unlikely the S-Stock would be converted to OHLE operation as they would require a pantograph well somewhere and presumably major changes to their power systems (at least to allow 1.5kV DC operation, but allowing them to work under 25kV AC would be very advantageous as well so they can share track with an electrified Chiltern line) so it would be their successors which would be the first trains capable of it.

The amount of energy wasted by 750V third-rail on the NR network is significant enough to warrant the conversion of it to 25kV OHLE; London Underground is going to have to do something similar at some point as well presumably.

 

[yorkie]

The amount of energy wasted by 750V third-rail on the NR network is significant enough to warrant the conversion of it to 25kV OHLE;

Is it?

London Underground is going to have to do something similar at some point as well presumably.

I don't see why LU has to do something?

The costs would be astronomical. Would the benefits really outweigh the enormous costs?

How would you do the changeover? You'd have very long line closure periods!

As for the Victoria Line, I've not heard those proposals but you have to bear in mind that's a self-contained route so any such change would be far less viable on the sub-surface lines!

 

[transmanche]

As for the Victoria Line, I've not heard those proposals but you have to bear in mind that's a self-contained route so any such change would be far less viable on the sub-surface lines!

It is something that was mooted in what turned out to be the 2009 stock. (I'm assuming it would have been a conductor bar similar to what's been used in the Thameslink core.) But how seriously it was mooted, I'm not sure.

As an aside: being as LU use a 4th rail system; how viable (and how safe) would a +750v/-750V 4th rail system be?

 

[455driver]

The power losses are only really relevant over long distances (hence the possibility of converting the 3rd rail to overhead), not something relevant to the underground. If it was being built now then of course it would be overhead 25KV but as it is an existing system the costs would far outway the benefits.

Its the much lower DC voltage that causes the power drop and raising the DC voltage wont really offer much benefit.

 

[NotATrainspott]

Is it?

Yes, it's one of the reasons Network Rail do not recommend replacing the equipment like-for-like when it expires. The Electric Spine work is meant to be a test bed for wide scale conversion of the third rail network to 25kV AC OHLE. The limitations of the third rail network are well known as trains cannot run as fast as they can on OHLE, they are more limited by power supply problems (someone said on this forum that 444s and 450s are limited to around 2/3rds of their possible power due to the 750V supply compared to 350s on 25kV AC), the system is less safe for passengers, workers and trespassers and it causes a significant amount of energy (and therefore money) to be wasted through transmission losses.

I don't see why LU has to do something?

The costs would be astronomical. Would the benefits really outweigh the enormous costs?

How would you do the changeover? You'd have very long line closure periods!

As for the Victoria Line, I've not heard those proposals but you have to bear in mind that's a self-contained route so any such change would be far less viable on the sub-surface lines!

It's just a suggestion for when there isn't much more they can do with the existing network. The voltage on third rail can't be increased beyond 750V for safety reasons and eventually the power supply to trains will become much more of a bottleneck to capacity.

Here's a Railway Gazette article from 1998 describing the Space Train proposal.

The amount of energy used by the LUL network is simply enormous and this could be one way of making much more efficient use of it. If the loading gauge exists for the conductor bar, which is something I don't know, then I imagine it can be fitted piece by piece but left isolated until there are the trains able to use it safely. The third and fourth rails wouldn't need to be disabled and removed until everything else was in place, and even then they could be left isolated in situ until they are removed in normal maintenance hours. The replacement for the S-Stock several decades in the future could certainly be ordered with 1.5kV DC capability and probably 25kV AC as well as there will be even more technological advancement on top of what's possible today.

 

[455driver]

(someone said on this forum that 444s and 450s are limited to around 2/3rds of their possible power due to the 750V supply compared to 350s on 25kV AC),

444s are limited to about 70% and 450s to about 55% once over 10mph.

 

[jopsuk]

I'd suspect that, with the technology having moved on, if you were going for overhead conductors for the Sub Surface Railway you could quite possibly safely use 25kV, if there's clearence enough for any sort of OH equipment. Which I doubt.

You'd also need, for either 1.5kV DC or 25kV AC, to heavily modify the stock- afterall, the SSR is midway through a comprehensive stock replacement program.

There would also be the issue of the shared running sections of the District and Met with the Picadilly Line- the sections shared with Overground would be an issue if 1.5kV was chosen, as the 378s are equipped for 25kV. If 25kV was chosen, then converting the Richmond branch would likely be welcomed by LO. If 25kV conversion was done ahead of AC coming to Waterloo, then the East Putney - Wimbledon Park section would offer an operational issue for SWT or their succesor.

 

[edwin_m]

Even with half the current and the increased conductivity of copper versus steel, I think a 1500V overhead system on the underground would waste as much power as the existing fourth rail system simply because the cross-section of the conductor bar is less than that of the conductor rail. DC OLE in the Netherlands often needs several conductors to cope with the currents, and trains may need to raise several pantographs.

There would be some benefit to staff safety.