DC third rail upgrades

[Joseph T]

Is there any reason, minus cost of upgrading lines and rolling stock, why the existing 750v DC third rail system can't be upgraded to a higher voltage to increase speed?

 

[Deepgreen]

Some sections had, or have, the voltage up to 850v for that very reason (and to allow greater spacing of sub-stations, I think), but there is a limit to how much you can do this without the losses making it not worthwhile (again, I think). In any case, speeds above 100mph or so tend to be unsuitable for third rail use anyway.

 

[swt_passenger]

Some sections had, or have, the voltage up to 850v for that very reason (and to allow greater spacing of sub-stations, I think), but there is a limit to how much you can do this without the losses making it not worthwhile (again, I think). In any case, speeds above 100mph or so tend to be unsuitable for third rail use anyway.

The ‘850V upgrade’has been debunked in these forums many times, a search on the term finds a number of threads where it’s been denied. Here’s an example:

What are the differences between Class 507s and 508s?

The issue with gaps causing lights to flicker off on 508s is intermittent and doesn't seem to affect all examples, and again this only happened following the reduction to three carriages. It will happen to all 508’s, it’s a design feature. The lighting is directly fed from the MA set. On...

www.railforums.co.uk

 

[Deepgreen]

The ‘850V upgrade’has been debunked in these forums many times, a search on the term finds a number of threads where it’s been denied.

Fair enough.

 

[hwl]

Is there any reason, minus cost of upgrading lines and rolling stock, why the existing 750v DC third rail system can't be upgraded to a higher voltage to increase speed?

Elf in High Vis : running rail i.e. current return (similar to neutral) to ground (earth) voltage become potentially lethal max distances from supply points (same reason why 110V with isolating transformers so +/55V max vs ground is used on building sites)

Alternatively you could:

1. upgrade substations to supply higher current
2. increase the number of substations
3. Increase substation quality (e.g. 18 or greater pulse or SFC)
4. increase conductor rail cross section (if not already at max)
5. increase running rail cross section (if not already at max)
6. improve cabling and bonding (if not already a good as can be)

The latter three reduce loop impedance and significantly improve efficiency.

It is worth point out that it is nominally 750V DC:

• the DC is as rough as the north sea during a winter storm
• voltage range is +/-150V with a further derating range (complicated) of -350V. Regenerative braking effectively stops when the measured voltage is 875V.

Three phase drives on the EMUs would much prefer to get extra current @ 750V rather than extra volts above 750V.

 

[Bigfoot]

I'm sure there were tests at above 100mph and the shoes started to float and not maintain contact with the 3rd rail leading to current collection issues. Or at least that's what Im sure I remember reading, feel free to tell me I'm wrong.

 

[edwin_m]

Regenerative braking effectively stops when the measured voltage is 875V.

This also means that as the voltage increases even below 875V, regenerative braking becomes possible for less of the time. This would increase energy use by up to 15%. It might be partly offset by the traction drawing a lower current at the higher voltage for the same power ... but that would result in no performance benefit and negate the point of changing the voltage in the first place.

 

[Nicholas Lewis]

Some sections had, or have, the voltage up to 850v for that very reason (and to allow greater spacing of sub-stations, I think), but there is a limit to how much you can do this without the losses making it not worthwhile (again, I think). In any case, speeds above 100mph or so tend to be unsuitable for third rail use anyway.

The 850V is often associated with the 67 Bournemouth Electrification extension which was actually the no load voltage at the rectifier terminals. This was due to the low rating of the rectifiers used on the extension which were first generation silicon diode. Most were rated at 1MW (there were 2x1MW rectifiers at each substation) so to deliver 750V DC at full load the no load voltage was c850V DC due to the regulation across the rectifier. Previously the nominal rating of rectifiers had been 2.5MW and no load voltage was around 790V to deliver 750V DC.

Separately ive seem over 900V DC when we doing testing at night time as the no load voltage increased further as the high voltage tends to creep overnight to 34kV.

I'm sure there were tests at above 100mph and the shoes started to float and not maintain contact with the 3rd rail leading to current collection issues. Or at least that's what Im sure I remember reading, feel free to tell me I'm wrong.

Shoes dont float they weigh far too much. The main issue is the forces involved when transitioning onto ramp ends would tend to accelerate them up in the air and cause arcing but ultimately the shoes are designed to shear off at a certain point to avoid damage so there is a limit proportional to speed as to how fast you could go.

 

[hwl]

This also means that as the voltage increases even below 875V, regenerative braking becomes possible for less of the time. This would increase energy use by up to 15%. It might be partly offset by the traction drawing a lower current at the higher voltage for the same power ... but that would result in no performance benefit and negate the point of changing the voltage in the first place.

To an extent but increasing the voltage reduces resistive losses the and increases the overall system efficiency.

The 850V is often associated with the 67 Bournemouth Electrification extension which was actually the no load voltage at the rectifier terminals. This was due to the low rating of the rectifiers used on the extension which were first generation silicon diode. Most were rated at 1MW (there were 2x1MW rectifiers at each substation) so to deliver 750V DC at full load the no load voltage was c850V DC due to the regulation across the rectifier. Previously the nominal rating of rectifiers had been 2.5MW and no load voltage was around 790V to deliver 750V DC.

Separately ive seem over 900V DC when we doing testing at night time as the no load voltage increased further as the high voltage tends to creep overnight to 34kV.

Shoes dont float they weigh far too much. The main issue is the forces involved when transitioning onto ramp ends would tend to accelerate them up in the air and cause arcing but ultimately the shoes are designed to shear off at a certain point to avoid damage so there is a limit proportional to speed as to how fast you could go.

The higher the max line speed the shallower the ramp angle needs to be but the shallower the ramp angle the greater the degree of arcing especially if you are well below line speed.

 

[edwin_m]

To an extent but increasing the voltage reduces resistive losses the and increases the overall system efficiency.

That's what I've tried to cover by mentioning that the current would be less at a higher voltage, but only if the overall power isn't increased to compensate. The OP suggested the reason to do this would be to increase speed, which would require an increase in power.

 

[Recessio]

I'm sure there were tests at above 100mph and the shoes started to float and not maintain contact with the 3rd rail leading to current collection issues. Or at least that's what Im sure I remember reading, feel free to tell me I'm wrong.

The world record for third-rail systems was set by the Class 442. It reached 109mph on a test run. Though it didn't do this in regular service!

 

[Snow1964]

I'm sure there were tests at above 100mph and the shoes started to float and not maintain contact with the 3rd rail leading to current collection issues. Or at least that's what Im sure I remember reading, feel free to tell me I'm wrong.

It was never really tested, certainly not at 125mph

The shoes are fairly basic, just sit there under gravity. There has been no attempt at fitting small aerofoils to the brackets like the Bracknell-Willis pantograph has.

I suspect the higher speed sections would need longer lead in ramps on third rail to avoid jolting and bouncing the shoes.

 

[JJmoogle]

I once had a very enlightening chat with a track engineer out from Weymouth in about 2011 that was largely about this, the biggest barrier was the network capacity constraining increase of linespeeds,
There was no point in most areas, even if they could, of raising the linespeed to its current 100mph limit if it's just going to get signal checked or have to stop at a station as soon as it gets there.
The investment required in building(with all that would entail in a regulatory sense adding several additional miles of third rail) new fast lines or reopening avoiding routes to add that capacity just for some minor speed increases for direct trains was so great as to make it pointless against rebuilding stations and junctions(which is what he worked on) to make existing journeys flow better.

This also lead into why we're never likely to see third rail over 100, it'd be possible(apparantly) but the development effort required for very few services to be able to use it for long enough to make it worthwhile. He thought if BR(or Southern) had ever extended the rail down to Salisbury or Exeter then it probably would have been looked at seriously.

 

[Trackman]

I'm sure there were tests at above 100mph and the shoes started to float and not maintain contact with the 3rd rail leading to current collection issues. Or at least that's what Im sure I remember reading, feel free to tell me I'm wrong.

Could be something to do with short 3rd rail ramps. If the shoe hits them at speed, power could be lost. The faster you go, the longer the ramp.

 

[Pigeon]

It was never really tested, certainly not at 125mph

The shoes are fairly basic, just sit there under gravity. There has been no attempt at fitting small aerofoils to the brackets like the Bracknell-Willis pantograph has.

Exactly. People seem to forget that pantographs were themselves not suitable for high speed operation with acceptably low maintenance until a considerable amount of research had been done on the dynamics of the pantograph/catenary system. The bald dogma that "third rail can't do >100mph" fails to acknowledge that pantographs used not to be able to either. It only appears to be true because nobody has done the equivalent research on what could be done for third rail current collection to improve upon dinosaur feet.

(I doubt you'd end up using aerofoils because the situations are rather different, but your point stands, that equivalent measures have not been attempted.)

Thing is the reason it hasn't been tried is that there's so little of the third-rail region where there's any point trying to exceed 100mph in the first place that nobody's interested in finding out how to do it.

 

[edwin_m]

Exactly. People seem to forget that pantographs were themselves not suitable for high speed operation with acceptably low maintenance until a considerable amount of research had been done on the dynamics of the pantograph/catenary system. The bald dogma that "third rail can't do >100mph" fails to acknowledge that pantographs used not to be able to either. It only appears to be true because nobody has done the equivalent research on what could be done for third rail current collection to improve upon dinosaur feet.

(I doubt you'd end up using aerofoils because the situations are rather different, but your point stands, that equivalent measures have not been attempted.)

Thing is the reason it hasn't been tried is that there's so little of the third-rail region where there's any point trying to exceed 100mph in the first place that nobody's interested in finding out how to do it.

The situation itself is also more limiting compared with a pantograph. The inner side of the shoe is close to the bogie frame, the outer side is close to the gauge limit, and the top is constrained by the body above it. The whole thing is mounted to the axleboxes so is unsprung mass, and needs to carry much more current than an AC pantograph so must provide a larger cross-sectional area of conductor.

These factors between them would place severe limits on any re-design of the shoegear for higher speeds. A pantograph has a large amount of movement space to play with, and the ability to mount various aerofoils, springs and dampers on the pan itself or on the roof around it, all of which can be used to improve dynamics.

 

[LBMPSB]

The Lancashire & Yorkshire Railway had a third rail system out of Mancheter to Bury. It was a 1200v DC side contact rail system. The cross-section of the third rail was half that of the Southern's system. And it was encased, so it was extremely difficult to get electrocuted from it. Does anyone know what ampage was drawn by the units that used this system?
Unfortunately the line no longer exists, and there is just no way that the Southern can be converted to side contact, likewise to OHL, conversion of signalling, traction units etc are too costly.

 

[snowball]

The Lancashire & Yorkshire Railway had a third rail system out of Mancheter to Bury. It was a 1200v DC side contact rail system. The cross-section of the third rail was half that of the Southern's system. And it was encased, so it was extremely difficult to get electrocuted from it. Does anyone know what ampage was drawn by the units that used this system?
Unfortunately the line no longer exists

The line, of course, still exists, but is now part of Metrolink, 750V DC overhead.

 

[MarkyT]

The situation itself is also more limiting compared with a pantograph. The inner side of the shoe is close to the bogie frame, the outer side is close to the gauge limit, and the top is constrained by the body above it. The whole thing is mounted to the axleboxes so is unsprung mass, and needs to carry much more current than an AC pantograph so must provide a larger cross-sectional area of conductor.

UK loading gauge is also unusually constrained low down below platform level, in areas also intruded by bridge beams and signals. The UK 3rd rail design had to be compact to avoid large-scale work to change that limited clearance. Many European and American railways were able to place their third rails further from the running rail because their loading gauge already allowed it. That then gave more space onboard in that area to design a more sophisticated yet robust arm to carry the contact shoe, more easily incorporating side or bottom contact if desired, pressure increase for better contact at speed etc.

These factors between them would place severe limits on any re-design of the shoegear for higher speeds. A pantograph has a large amount of movement space to play with, and the ability to mount various aerofoils, springs and dampers on the pan itself or on the roof around it, all of which can be used to improve dynamics.

Research could be done clearly but the consensus seems to always have been that it was not worth the effort.

The other fundamental third rail problem is inefficiency. There is a lot of resistance loss in feeder cables, conductor rail and running rail return, which gets significantly worse at higher currents unless you spend a fortune on parallel strengthening conductors, extra substations etc. The side effect of high resistive losses and voltage drop at the shoe in peak demand is reduced performance of the train.