3rd rail vs OHLE

[JamesT]

I'm surprised to hear that - 16 2/3 seems a more obvious frequency to use, as it's one-third of 50Hz which is the standard mains frequency in Europe.

https://en.wikipedia.org/wiki/15_kV_AC_railway_electrification mentions an issue with some generators being at 1/3rd of the mains frequency. Shifting the frequency slightly alleviated the problem whilst being within tolerance for existing kit.

 

[Wolfie]

I'm surprised to hear that - 16 2/3 seems a more obvious frequency to use, as it's one-third of 50Hz which is the standard mains frequency in Europe.

Given that some variation is always permitted l remain to be convinced that there is any practical difference between 16.7Hz and 16.6666666666667Hz (insert however many decimal places you want with the last being 7) when the latter to one significant figure (my maths degree coming out!) is 16.7Hz anyway.

 

[A0wen]

Possibly another reason why that the LB&SCR only installed overhead electification in the suburban area from 1909 until the 1920s before changing to 3rd rail.

The decision to go 3rd rail was a post grouping one - and the LSWR were the biggest part of the parts which made up the Southern and already had a large network of 3rd rail - larger than the LBSC's overhead network. That's what settled that decision.

 

[gallafent]

Given that some variation is always permitted l remain to be convinced that there is any practical difference between 16.7Hz and 16.6666666666667Hz (insert however many decimal places you want with the last being 7) when the latter to one significant figure (my maths degree coming out!) is 16.7Hz anyway.

Here in the UK the national grid is “obliged by its licence” to maintain grid frequency at 50Hz +/- 1%, i.e. 49.5Hz-50.5Hz … a third of that would give a range of 16.5Hz-16.833333…Hz. I expect similar rules apply on the continent. So either 16.7 or 16 2/3 “nominal” frequency is fine and in range, and picking between the two is indeed completely moot … and if you have two different sources that aren't synced, there's surely always going to be a chance of slightly differing frequencies “beating” against each other, etc.

Relatedly, domestic mains supply is specified at 230V +/- 10% nominal, although in fact there's plenty of grid equipment here still that's working at our previous standard of 240V +/- 10% (and no doubt on the continent that aims at 220V), which is thus also in range … and by a similar token I assume overhead line electrification at 25kV often swings well above and below that nominal value, depending on load.

 

[Wolfie]

Here in the UK the national grid is “obliged by its licence” to maintain grid frequency at 50Hz +/- 1%, i.e. 49.5Hz-50.5Hz … a third of that would give a range of 16.5Hz-16.833333…Hz. I expect similar rules apply on the continent. So either 16.7 or 16 2/3 “nominal” frequency is fine and in range, and picking between the two is indeed completely moot … and if you have two different sources that aren't synced, there's surely always going to be a chance of slightly differing frequencies “beating” against each other, etc.

Relatedly, domestic mains supply is specified at 230V +/- 10% nominal, although in fact there's plenty of grid equipment here still that's working at our previous standard of 240V +/- (and no doubt on the continent that aims at 220V), which is thus also in range … and by a similar token I assume overhead line electrification at 25kV often swings well above and below that nominal value, depending on load.

TY for a valuable and interesting explanation building on my musings.

 

[zuriblue]

I'm surprised to hear that - 16 2/3 seems a more obvious frequency to use, as it's one-third of 50Hz which is the standard mains frequency in Europe.

The problem was that with modern Variable Frequency drives some undesirable harmonics were being fed into the network. Shifting to 16.7 Hz resolved that. This is why the SBB Infra power stations run their turbines at 501 rpm

 

[Bald Rick]

The biggest challenge with the speed of building of any electrification as @Bald Rick keeps pointing out is that it needs a connection to the National Grid - and that's not quick or easy to achieve.

Correct

And 3rd rail installations need more of them, by virtue of the system not transmitting power as far as AC OHLE does

3rd rail doesn’t need any more grid connections. But it does need a railway ‘internal’ distribution system to get the power from the feeder station(s) to the trackside substations. This system has lots of thick cables at 33kV AC running alongside the track, and a substation every few miles.

Are there many third rail failures on the Brighton Main Line. It certainly doesn't seem to make the news - but is that because it rarely happens?

Yes they happen, fairly frequently (certainly every week, can be more often), but most of them are relatively minor faults. Same as with OLE faults.

The LMS had plans also for St Pancras to Harpenden on overhead - with a new station twixt SAC and the latter !

That needs a pub discussion!

The cost of the third rail electricity bill is often inflated because Network Rail let ballast like against the third rail.

That really is the primest of prime boll**ks. The power loss due to such issues is negligible compared to the resistive losses in a short length of 3rd rail.

and by a similar token I assume overhead line electrification at 25kV often swings well above and below that nominal value, depending on load.

Yep, and considerably so.

 

[gallafent]

TY for a valuable and interesting explanation building on my musings.

My pleasure, it's really interesting stuff to my mind! … now as for one side of Japan running 50Hz, and the other at 60Hz, that's another rabbit-hole entirely!

This is why the SBB Infra power stations run their turbines at 501 rpm

What an interesting little nugget of information!

 

[Nicholas Lewis]

I think those that actually work on the railways probably disagree with your first point about it being a 'safe' system. And I'm not sure that failures of the infrastructure are any the less rare on 3rd rail than they are on OHLE - the big difference is that when it happens on OHLE it takes out either the WCML, ECML or Thameslink, which tends to be "front page" news. Whereas a 3rd rail failure which brings the Hastings line or Brighton Mainline to a stand isn't as high profile.

The biggest challenge with the speed of building of any electrification as @Bald Rick keeps pointing out is that it needs a connection to the National Grid - and that's not quick or easy to achieve. And 3rd rail installations need more of them, by virtue of the system not transmitting power as far as AC OHLE does. So whilst chucking down a few miles of 3rd rail on its own *may* be quicker than an OHL installation, if the connection to the National Grid isn't there it's completely academic.

Third Rail systems don't need more grid connections but they do need more trackside substations is maybe what your alluding to which are fed from a lineside HV distribution system. Furthermore, until recently, single phase 25kv systems had to be connected at least 132kV which isn't cheap whereas DC systems are connected at 33kV local distribution systems which are far more numerous.

Displaced conductor rails are rare as are OLE dewirements but repair time of the latter is a lot more involved need access towers or plant whereas conrail off just needs a few p way guys to crow bar it back onto the insulators. Also on OLE especially areas with headspans all roads get impacted.

Thanks. I understand the point you are making but I still don't know if the Brighton Main Line has 'regular' third rail problems. I'm guessing it's far more resilient than many ohl operations.....

It doesn't ive been travelling over it everyday since 1989. It does have numerous signalling issues, lineside fires, trespassers and suicides though.

The decision to go 3rd rail was a post grouping one - and the LSWR were the biggest part of the parts which made up the Southern and already had a large network of 3rd rail - larger than the LBSC's overhead network. That's what settled that decision.

and the fact Herbert Walker ex LSWR became Chairman of the Southern Railway.

 

[WesternBiker]

Just for reference, it was 3,000 DC. The outline project was designed by Merz & McLellan, electrical engineering consultants. It is of course interesting to contemplate what the track layout and arrangements at Taunton would have been (nothing shown in the report).

Ah - thank you for that.

Is the report publicly available anywhere? I have only come across fleeting references to it in the railway press over the years, but it has always interested me.

 

[A0wen]

3rd rail doesn’t need any more grid connections. But it does need a railway ‘internal’ distribution system to get the power from the feeder station(s) to the trackside substations. This system has lots of thick cables at 33kV AC running alongside the track, and a substation every few miles.

Thank you for clarifying - so if we were to compare two similar length lines (let's use Victoria - Brighton and St Pancras - Bedford) as an example where one is OHLE and the other 3rd rail - do they (or conceptually could they) both have the same number of connections onto the National Grid ?

And the 33kv cables - not required for OHL, as these are at ground level do they have an attraction to the metal fairies or do they know better than to even try to take those out ?

It doesn't ive been travelling over it everyday since 1989. It does have numerous signalling issues, lineside fires, trespassers and suicides though.

And trespass and suicides on 3rd rail lines mean the power has to be switched off - which isn't the case with OHLE. So potentially it increases the disruption caused.

 

[36270k]

In Switzerland hydro power on the Gotthard was generated at 16.6hz. Long before any national grid at 50hz existed.
50hz generators did not exist in 1910

On the LBSC 25hz was used and was the frequency generated by Greenwich power station.
The input at LSWR substations was at 25hz
25hz was a common frequency at UK power stations in the early 1900's

 

[Taunton]

The decision to go 3rd rail was a post grouping one - and the LSWR were the biggest part of the parts which made up the Southern and already had a large network of 3rd rail - larger than the LBSC's overhead network. That's what settled that decision.

Arguments we have put before, but I may as well have another shot again.

A key upside of 3rd rail is that it is so easy, and cheap, to install. I saw the installation of it on Stratford-North Woolwich one weekend in the 1980s. I don't know if they did the whole lot in one Sunday, but it looked possible. One works train, ground crew, screw pots in, drop 3rd rail onto pots, on to the next length. Try that with 25kV, GWML lot. Notably it was a (onetime) Eastern Region crew from Chesterton Junction, Cambridge, pw yard doing it, so not a lot of background 3rd rail experience either.

Much is made of the additional lineside substations. But a 12-car overhead emu had three substations, one under each motor coach, and all the hoopla of pantographs, transformers, rectifiers, etc on board each one. Lineside substations are so straightforward compared to putting them on the rolling stock.

And the 33kv cables - not required for OHL, as these are at ground level do they have an attraction to the metal fairies or do they know better than to even try to take those out ?

It seems this is known only too well. The lineside cabling for the DLR extension Canning Town to Stratford International was stolen before it was switched on - twice! But not since.

 

[londonmidland]

Am I right to think that new third rail installations are banned, except for short extensions/new replacements?

Overall OLE is more efficient, as well as not having to worry about power supply constraints*

*Unless you send a 92 down a suburban commuter line

 

[zwk500]

Am I right to think that new third rail installations are banned, except for short extensions/new replacements?

To all intents and purposes, yes. There is a review currently underway into this position by the ORR & RSSB (with DfT, NR and other industry input), but it is not yet published.

Overall OLE is more efficient, as well as not having to worry about power supply constraints*

*Unless you send a 92 down a suburban commuter line

Newcastle-Edinburgh begs leave to disagree. OLE is just as susceptible to power supply constraints as any other system if you don't install enough feeder capacity. If there isn't enough power entering the system, it makes very little difference the method by which the power gets from the supply to the train.

 

[nlogax]

Newcastle-Edinburgh begs leave to disagree. OLE is just as susceptible to power supply constraints as any other system if you don't install enough feeder capacity. If there isn't enough power entering the system, it makes very little difference the method by which the power gets from the supply to the train.

What's the typical result of too many trains entering that particular section? Low power / go slow? Has this happened a lot?

 

[Taunton]

Overall OLE is more efficient

The trouble with such global statements is that the various proponents only give one side of the argument, and gloss over the other. As I described above, 25kV supporters go on about lineside substations but then forget about substations under each motor coach. They describe modern advances in train-borne power systems but ignore that such advances are equally applicable to lineside installations. And so on.

The chaos, disruption and overspend of the recent GWML electrification, ending up with only half the solution anyway, has never had any parallel on the 3rd rail.

 

[zwk500]

What's the typical result of too many trains entering that particular section? Low power / go slow? Has this happened a lot?

At the moment I believe the problem is worked around by TPE Bi-Modes running on Diesel. AIUI a programme is underway to fix the problem (I'm not sure what the problem actually is, @Bald Rick or @59CosG95 may well have more info).

 

[59CosG95]

At the moment I believe the problem is worked around by TPE Bi-Modes running on Diesel. AIUI a programme is underway to fix the problem (I'm not sure what the problem actually is, @Bald Rick or @59CosG95 may well have more info).

The main thing being worked on is a beefed-up power supply at Marshall Meadows FS (at the ENG-SCO border); the existing supply will gain a Static Frequency Converter (SFC) to reduce losses due to phase imbalance. Booster Transformers (which maximise returned traction current at the cost of available power (read: impedance)) will be removed where still extant.
TPE may not feel the benefits at all if the proposed ECML TT changes happen, but the ECML at the Border will still benefit from a far more resilient supply than the existing.

 

[swt_passenger]

At the moment I believe the problem is worked around by TPE Bi-Modes running on Diesel. AIUI a programme is underway to fix the problem (I'm not sure what the problem actually is, @Bald Rick or @59CosG95 may well have more info).

In addition to the previous reply, we have a dedicated thread about the ECML Power upgrade. I posted an update about Marshall Meadows itself a few weeks ago:

ECML Power Supply Upgrade

As the works were undertaken in 2017, I think it was a case of "while you're here..." for AmcoGiffen. It might have been a small part of the signalling power supply upgrade, which may or may not be separate to the traction power supply upgrade. I don't know for certain as I'm not on either...

www.railforums.co.uk

 

[O L Leigh]

And the 33kv cables - not required for OHL, as these are at ground level do they have an attraction to the metal fairies or do they know better than to even try to take those out ?

The “metal fairies” are generally a lot more clued up than most people are prepared to give them credit for. They know precisely which parts of the system are safe to remove and even appear to be aware of isolations to aid their nefarious activities.

The trouble with such global statements is that the various proponents only give one side of the argument, and gloss over the other. As I described above, 25kV supporters go on about lineside substations but then forget about substations under each motor coach. They describe modern advances in train-borne power systems but ignore that such advances are equally applicable to lineside installations. And so on.

With respect, you’re making rather a lot of this point too. I don’t know the relative costs of an AC unit compared to an otherwise identical DC unit, but I can’t imagine that it’s huge. Both types of unit will require systems for the collection, distribution and use of power, whether that’s using shoes or a pantograph and transformer.

However, the difference between a lineside substation and an on-train one is that the lineside version needs connection to the grid which, if earlier discussions regarding this topic are anything to go by, is the time-consuming and expensive part. Even if you allow for the fact that at least a proportion of those lineside installations are fed by an internal network, the cabling and other infrastructure necessary to permit this still has to be provided.

 

[A0wen]

Arguments we have put before, but I may as well have another shot again.

A key upside of 3rd rail is that it is so easy, and cheap, to install. I saw the installation of it on Stratford-North Woolwich one weekend in the 1980s. I don't know if they did the whole lot in one Sunday, but it looked possible. One works train, ground crew, screw pots in, drop 3rd rail onto pots, on to the next length. Try that with 25kV, GWML lot. Notably it was a (onetime) Eastern Region crew from Chesterton Junction, Cambridge, pw yard doing it, so not a lot of background 3rd rail experience either.

Much is made of the additional lineside substations. But a 12-car overhead emu had three substations, one under each motor coach, and all the hoopla of pantographs, transformers, rectifiers, etc on board each one. Lineside substations are so straightforward compared to putting them on the rolling stock.

As I said in post #15

"The biggest challenge with the speed of building of any electrification as Bald Rick keeps pointing out is that it needs a connection to the National Grid - and that's not quick or easy to achieve." " So whilst chucking down a few miles of 3rd rail on its own *may* be quicker than an OHL installation, if the connection to the National Grid isn't there it's completely academic."

 

[mike57]

Railway electrification has been a series of developments over the last 130 years, and some of the current standards are based on decisions taken when there were no alternatives. 50Hz overhead electrification only became possible once rectification to DC became possible within the rolling stock. Hence the lower frequency systems. The original LBSCR overhead system used series wound motors similar to those used on DC systems. These will work on AC as well as DC, but the size of motors used for railway traction will not operate satisfactorally at 50Hz. So put the clock back to the 1920's when the Southern Railway expanded its electrification. Choice was low frequency high voltage OHL or 3rd rail DC. 3rd rail is quicker to install, no transformers are required on board the rail vehicles (and because of the low frequency requirement the transformers are bigger and heavier), and there is a good understanding of the technology as DC at around 500-700v is in use in tramways and on the London Underground. This means there will be a good pool of people with the skills to design, install and maintain the installation. So at that point in time the right choice.

Fast forward to today, 25kV 50Hz is the best for long distance and or high speed, but there is considerable installation of 3rd rail DC. No government is going to approve wholesale replacement of 3rd rail with 25kV OHL. So 3rd rail is going to be around for a while. Nowdays mutli voltage rolling stock is readily available so that solves the 'different standards' problem. New 25kV OHL has proven very expensive recently with cost and time overruns so justiyfing new electrification projects get ever more difficult.

3rd rail DC is probably the right choice for suburban systems anyway, but if you were designing from scratch you would use a higher voltage partially protected 3rd rail. Manchester Bury used 1200v DC side contact for many years.

Installation of 3rd rail is quicker, cheaper and less fraught. With the ready availablility of multi voltage rolling stock maybe new 3rd rail installations have a place as a quick win for electrification of busy routes where the maximum speed is less than 100mph and the line geometry is going to rule out 100mph+ speeds anyway. If the voltage is increased to about 1250v with some physical protection built in then losses are reduced to roughly a third of the 700v DC losses for a given power draw, so it must bear serious consideration for some routes. One that comes to mind are the two Leeds Manchester routes, via Huddersfeild and the Calder valley route. Lots of tunnels which pose problems for OHL, just a thought..

 

[stuu]

The chaos, disruption and overspend of the recent GWML electrification, ending up with only half the solution anyway, has never had any parallel on the 3rd rail.

No, but then the benefits are far higher too, unless you are proposing slowing the GWML down to 100mph

 

[Wolfie]

Railway electrification has been a series of developments over the last 130 years, and some of the current standards are based on decisions taken when there were no alternatives. 50Hz overhead electrification only became possible once rectification to DC became possible within the rolling stock. Hence the lower frequency systems. The original LBSCR overhead system used series wound motors similar to those used on DC systems. These will work on AC as well as DC, but the size of motors used for railway traction will not operate satisfactorally at 50Hz. So put the clock back to the 1920's when the Southern Railway expanded its electrification. Choice was low frequency high voltage OHL or 3rd rail DC. 3rd rail is quicker to install, no transformers are required on board the rail vehicles (and because of the low frequency requirement the transformers are bigger and heavier), and there is a good understanding of the technology as DC at around 500-700v is in use in tramways and on the London Underground. This means there will be a good pool of people with the skills to design, install and maintain the installation. So at that point in time the right choice.

Fast forward to today, 25kV 50Hz is the best for long distance and or high speed, but there is considerable installation of 3rd rail DC. No government is going to approve wholesale replacement of 3rd rail with 25kV OHL. So 3rd rail is going to be around for a while. Nowdays mutli voltage rolling stock is readily available so that solves the 'different standards' problem. New 25kV OHL has proven very expensive recently with cost and time overruns so justiyfing new electrification projects get ever more difficult.

3rd rail DC is probably the right choice for suburban systems anyway, but if you were designing from scratch you would use a higher voltage partially protected 3rd rail. Manchester Bury used 1200v DC side contact for many years.

Installation of 3rd rail is quicker, cheaper and less fraught. With the ready availablility of multi voltage rolling stock maybe new 3rd rail installations have a place as a quick win for electrification of busy routes where the maximum speed is less than 100mph and the line geometry is going to rule out 100mph+ speeds anyway. If the voltage is increased to about 1250v losses are reduced to roughly a third of the 700v DC losses for a given power draw, so it must bear serious consideration for some routes. One that comes to mind are the two Leeds Manchester routes, via Huddersfeild and the Calder valley route. Lots of tunnels which pose problems for OHL, just a thought..

An interesting thought. My reservation is that two standards introduces complexity, hence would bringing a third in as you suggest be sensible?

 

[mike57]

An interesting thought. My reservation is that two standards introduces complexity, hence would bringing a third in as you suggest be sensible?

With multi voltage rolling stock being the norm thats less of an issue, it may be a case of lower cost elctrification or no electrification for many routes.

 

[A0wen]

Installation of 3rd rail is quicker, cheaper and less fraught. With the ready availablility of multi voltage rolling stock maybe new 3rd rail installations have a place as a quick win for electrification of busy routes where the maximum speed is less than 100mph and the line geometry is going to rule out 100mph+ speeds anyway. If the voltage is increased to about 1250v with some physical protection built in then losses are reduced to roughly a third of the 700v DC losses for a given power draw, so it must bear serious consideration for some routes. One that comes to mind are the two Leeds Manchester routes, via Huddersfeild and the Calder valley route. Lots of tunnels which pose problems for OHL, just a thought..

There is *no way* that the Trans Pennine routes are going to be electrified using anything other than 25kv OHLE.

An interesting thought. My reservation is that two standards introduces complexity, hence would bringing a third in as you suggest be sensible?

The bigger problem remains the close cohabitation of 3rd rail and OHLE which causes other problems which others have explained and expanded on on various other threads.

Railway electrification has been a series of developments over the last 130 years, and some of the current standards are based on decisions taken when there were no alternatives. 50Hz overhead electrification only became possible once rectification to DC became possible within the rolling stock. Hence the lower frequency systems. The original LBSCR overhead system used series wound motors similar to those used on DC systems. These will work on AC as well as DC, but the size of motors used for railway traction will not operate satisfactorally at 50Hz. So put the clock back to the 1920's when the Southern Railway expanded its electrification. Choice was low frequency high voltage OHL or 3rd rail DC. 3rd rail is quicker to install, no transformers are required on board the rail vehicles (and because of the low frequency requirement the transformers are bigger and heavier), and there is a good understanding of the technology as DC at around 500-700v is in use in tramways and on the London Underground. This means there will be a good pool of people with the skills to design, install and maintain the installation. So at that point in time the right choice.

There was also a practical consideration - the LSWR 3rd rail network was more extensive than the LBSC's OHLE one - in that sense the decision as to which to standardise on was easier in that converting the LSWR network to OHLE would have been a far bigger activity than what was done.

 

[Taunton]

Nowdays mutli voltage rolling stock is readily available so that solves the 'different standards' problem.

Not totally, because it seems the changeover point is the Achilles' Heel of such mixed systems. The GN suburbans have ever since introduction had issues with trains not changing over satisfactorily at Drayton Park, or trackside issues there causing them to have to divert to Kings Cross. Thameslink, exactly the same, there have been multiple reworks at Farringdon etc to try to minimise the disruption caused by a train failing to change over; in fact it was seriously stated that it was a mistake to build the change point there instead of at Kentish Town, where there are multiple platforms to allow running around failed changeovers.

It's not just overhead/third rail. I believe SNCF have comparable experiences at DC/AC overhead changeovers.

 

[O L Leigh]

Installation of 3rd rail is quicker, cheaper and less fraught. With the ready availablility of multi voltage rolling stock maybe new 3rd rail installations have a place as a quick win for electrification of busy routes where the maximum speed is less than 100mph and the line geometry is going to rule out 100mph+ speeds anyway. If the voltage is increased to about 1250v with some physical protection built in then losses are reduced to roughly a third of the 700v DC losses for a given power draw, so it must bear serious consideration for some routes. One that comes to mind are the two Leeds Manchester routes, via Huddersfeild and the Calder valley route. Lots of tunnels which pose problems for OHL, just a thought..

We won’t be seeing any new 3rd rail schemes, at least not before the outcome of the consultation currently ongoing is known. No matter what it’s perceived advantages you can no longer go around leaving unprotected high voltage conductors where they can be easily accessed by Johnny Q Public.

 

[A0wen]

No government is going to approve wholesale replacement of 3rd rail with 25kV OHL. So 3rd rail is going to be around for a while.

As a single 'let's re-electrify the whole of the 3rd rail network' - no, you're quite correct.

But - there are some lines where extending electrification makes far more sense to be done as OHLE rather than 3rd rail and part of that may also be converting some 3rd rail into OHLE. The obvious ones which keep getting cited are Reading - Basingstoke. Basingstoke - Sailsbury - Romsey. Which in turn may lead to reviewing the 3rd rail between Basingstoke and Southampton and onto Bournemouth. Part of it will be when equipment starts reaching life expiry.

And over time, there will need to be investment in the infrastructure - at which point it *may* make sense to re-electrify on OHLE.