Crossrail and Crossrail 2 tunnel lengths

[Mikey C]

Converting the south of the river lines to OHLE at the same time as the rest of Crossrail would surely add massive amounts of cost and complexity to the project, especially when the main benefits of switching to AC are on the higher speed sections rather than on inner London lines where the trains stop regularly?

 

[Bald Rick]

Will there be an extra pair of tracks where the changeover takes place so that the running tracks don't get blocked (unlike Thameslink) whenever it goes wrong? Money well-spent, I'd have thought.

As it happens there will, but not for that reason. It would be a phenomenal amount of cash to spend to mitigate something that rarely happens.

When the changeover fails on TL (about once every 2-3 months in my experience*), the contingency plan results in, typically, no more than 10 minutes delay. The layout is specifically designed to deal with it.

* this is from reading the logs. In over 12 years of using the line, and probably around 5,000 trips, I have only ever personally experienced one train that didn't change voltage as planned; the driver 'rebooted' the train and it worked after that. Delayed 5 mins.

 

[HSTEd]

We could also keep regeneration and lift the nominal substation voltage to 800+V if we simply have enormous busbars on trackside, both on the contact rail and running rail sides of the circuits.

20cm square aluminium busbars (for example) would still be relatively cheap but would have a resistance an order of magnitude lower than a standard conductor rail settup. That would mean for the same settup your necessary regeneration headrump drops from 150V (900-750) to about 15V.
Which means you could potentially run the substation feeds at close to 880V, reducing currents by ~15%, combined with the lower resistance it would drop losses by nearly 90%.
With current controlled rectifiers you could also run the entire system paralleled and thus spread current more efficiently.

Aluminium only costs 1500 dollars per tonne and only has a specific gravity of about 2.7

 

[Philip Phlopp]

Converting the south of the river lines to OHLE at the same time as the rest of Crossrail would surely add massive amounts of cost and complexity to the project, especially when the main benefits of switching to AC are on the higher speed sections rather than on inner London lines where the trains stop regularly?

It doesn't need to be done at the same time, it just needs to be ready for use at the start of Crossrail 2 services.

There's still huge benefits of converting DC to AC, particularly those routes that suffer the highest levels of transmission losses, 5% v 35% is a huge energy saving, and there's guaranteed regeneration, which would further cut energy consumption. There's lots of regenerative braking on stop-start services, it's not just about a Pendolino slowing down from 125mph.

The energy saving of DC to AC conversion can be enough to power 2 to 4 extra carriages in a train.

 

[HSTEd]

There is no particular reason that a DC system can not regenerate to the grid if you want it to. Also regeneration to the grid is less likely to be important in suburban areas since there are likely to be motoring trains nearby.
And with the drastically shrinking cost of supercapacitors it is almost becoming viable to simply accept a drastic loss of capacitance from series connection and simply connect banks directly across the conductor rail.

 

[jopsuk]

with careful design it should be possible to put OHLE on just the SWML section of Clapham Junction to Waterloo, with some short sections of dual voltage to change over if trains need to transfer from one side to the other. The Southern section is operationally separate, as are the Overground platforms- though a further interesting step would be to make platforms 1 & 2 and 16 & 17 dual voltage when the Windsor side goes OHLE, with a moving-changeover section between Latchmere #3 and Longhedge junctions

 

[HSTEd]

If you really need more power in the system you would probably be better off adding 3kVdc for the Crossrail 2 trains, since the equipment would only be needed for the limited crossrail fleet and would not adversely affect the signalling equipment and third rail gear for all the other trains.

If you designed your OLE properly it might even be usable for 25kV later on.

EDIT:

Loop resistance of 3rd rail is roughly ~25mohm/km (~7 for the third rail and ~18 for running rails) and 3kV overhead (2x120mm2 copper conductors) ocmes out at about ~95mohm/km (~75 for the overhead and ~18 for the running rails). This means the voltage drop per unit current is four times the size the current drops by four times so the voltage drop will be the same in absolute terms.
Which means total losses drop by a factor of four because 75V is only 2.5% of the total voltage instead of 10%.
Adding parallel conductors will further improve this result as the resistance of the 3kV system is far higher and thus benefits more from parallel conductors.

 

[Emblematic]

There's going to be no interest at all in introducing a third electrification system, particularly for Crossrail 2 which has to interface with both existing systems. It would have to have substantial advantages over both systems, and I don't see any advantage at all over 25kV OHLE. A key advantage of our present system is that stock designed for either system can safely be used in dual-electrified sections. Add another system and that benefit disappears.
Several countries are in the process of switching their present DC systems to 25kV AC, I think it's just a matter of time before we join them.

 

[Mikey C]

There is no particular reason that a DC system can not regenerate to the grid if you want it to. Also regeneration to the grid is less likely to be important in suburban areas since there are likely to be motoring trains nearby.
And with the drastically shrinking cost of supercapacitors it is almost becoming viable to simply accept a drastic loss of capacitance from series connection and simply connect banks directly across the conductor rail.

Yes, wasn't it the Networkers which introduced this?

 

[HSTEd]

There's going to be no interest at all in introducing a third electrification system, particularly for Crossrail 2 which has to interface with both existing systems. It would have to have substantial advantages over both systems, and I don't see any advantage at all over 25kV OHLE.

It doesn't require contradictory bonding arrangements?
A 3kV system could be partially be superimposed over third rail without any difficulties with massive corrosion or problems with trackside bonding.

A key advantage of our present system is that stock designed for either system can safely be used in dual-electrified sections. Add another system and that benefit disappears.

Dual track areas don't work properly - just look at the messy history of electrification on the NLL.
Also the idea would be that the Crossrail 2 trains only would use 3kVdc - everything else would be third rail still.
They would not need third rail capability and nothing else would require 3kV capability.

Several countries are in the process of switching their present DC systems to 25kV AC, I think it's just a matter of time before we join them.

And who will pay for this scheme with uncertain rewards and enormous costs?
IT is indeed unlikely 3kV would be implemented, since 750Vdc upgrades are almost certainly cheaper for what is required.
However a 3kV overlay is going to be far cheaper than total conversion to 25kV. Stray currents would be a killer in the complex layouts scattered across South London.

Yes, wasn't it the Networkers which introduced this?

Well in theory regeneration to the grid only requires modifications on the substation side, but it was about that time I think.

 

[hwl]

Yes, wasn't it the Networkers which introduced this?

They had the capability but I suspect it might actually have Southern electrostars which actually used it first.

Any BR DC unit with Rheostatic braking could easily be modified to do it.

 

[D365]

As it happens there will, but not for that reason. It would be a phenomenal amount of cash to spend to mitigate something that rarely happens.

https://consultations.tfl.gov.uk/crossrail2/october2015/user_uploads/s13.pdf

Will those additional tracks be for access to the "Wier Road" depot?

 

[hwl]

https://consultations.tfl.gov.uk/crossrail2/october2015/user_uploads/s13.pdf

Will those additional tracks be for access to the "Wier Road" depot?

Turning back the 10tph that don't go beyond Wimbledon so 4 (or possibly 3) CR2 track station

 

[Philip Phlopp]

Stray currents would be a killer in the complex layouts scattered across South London.

That's only the case if you persist in running 750V DC and 25kV side by side, as every unit will be dual voltage capable by the time this is a serious problem, the simple solution is to either convert the complex layouts to 25kV first, and change back to 750V DC either side of somewhere like Clapham Junction, or you convert the track upto Clapham Junction to 25kV first and switch back to 750V DC on approach.

The complex layout then transitions at Christmas or Easter - DC switched off, AC switched on, testing and commissioning, and job done. I'd prefer Christmas, and if there are any serious show stopping issues, AC off, DC on and we'll try again at Easter.

The stock just needs electric, it doesn't matter if it's 25kV AC or 750V DC, one can be swapped for the other at any time until the 750V DC rail, switchgear and cabling is actually recovered. It's quite different to electrification of non electrified route, where it's all different stock to deal with.

These are all details which will need to be considered in detail in future years, when this becomes a real problem to contend with.

 

[Busaholic]

As it happens there will, but not for that reason. It would be a phenomenal amount of cash to spend to mitigate something that rarely happens.

When the changeover fails on TL (about once every 2-3 months in my experience*), the contingency plan results in, typically, no more than 10 minutes delay. The layout is specifically designed to deal with it.

* this is from reading the logs. In over 12 years of using the line, and probably around 5,000 trips, I have only ever personally experienced one train that didn't change voltage as planned; the driver 'rebooted' the train and it worked after that. Delayed 5 mins.

Pleasantly surprised to read that - I've had the impression (from this Forum!) that it happened more regularly. Sod's Law, I suppose, that it happens on the same day that something major occurs at London Bridge or East Croydon as well

 

[D365]

Turning back the 10tph that don't go beyond Wimbledon so 4 (or possibly 3) CR2 track station

Ah ok, so in the same manner as the (future proposed) Old Oak Common station on the 'Elizabeth'?