3rd rail north London

[adamskiodp]

Hi folks

A thought just struck me

National rail lines south of the river Thames are predominantly 3rd rail and north of the river predominantly OHLE/Diesel. Any ideas how the lines from Euston (and formerly Broad Street) to Watford (Overground) use 3rd rail?

Regards,

Adam

 

[MarlowDonkey]

Any ideas how the lines from Euston (and formerly Broad Street) to Watford (Overground) use 3rd rail?

As built, it was four rail electrification, being a combined scheme of the London & North Western Railway and the company/companies then running London Underground. There were shared tracks with the Bakerloo between Queens Park and Watford Junction. Trains also ran between Willesden Junction and Earls Court over the West London line.

If the Great Western and the Tilbury line of the LMS had electrified with the four rail system, a 1920s version of Crossrail could have been set up using both north and south sides of the Circle. From the west, you can connect to the south side at Ealing Broadway and the north side at Paddington.As it was, there was a through Windsor to Southend service for a while.

 

[QueensCurve]

Hi folks

A thought just struck me

National rail lines south of the river Thames are predominantly 3rd rail and north of the river predominantly OHLE/Diesel. Any ideas how the lines from Euston (and formerly Broad Street) to Watford (Overground) use 3rd rail?

Regards,

Adam

Just historical Early electrifications used the 3rd or 3rd/4th rail systems. The London area saw most early electrification and the Southern paid the price for being an early adopter.

 

[30907]

Putting it another way, early electrification schemes in the UK were urban or suburban in character. Almost all of them made the "obvious" choice of 3rd/4th rail dc, and the same was true in other countries. The exceptions being the LBSC and the Midland's trial scheme at Lancaster.

Only the NER was interested in main line electrification at an early stage, and nothing came of that until the LNER Woodhead and Shenfield schemes, by which time mainland Europe had advanced the technology considerably.

 

[MarkyT]

Putting it another way, early electrification schemes in the UK were urban or suburban in character. Almost all of them made the "obvious" choice of 3rd/4th rail dc, and the same was true in other countries. The exceptions being the LBSC and the Midland's trial scheme at Lancaster.

Only the NER was interested in main line electrification at an early stage, and nothing came of that until the LNER Woodhead and Shenfield schemes, by which time mainland Europe had advanced the technology considerably.

Lower voltage DC was the only show in town pre-WW2 unless you went for the low-frequency AC already used widely in Germanic and Scandinavian countries. Unlike 50Hz standard industrial frequency, 16 2/3 Hz can be used 'neat' without rectification in series wound universal DC motors without significant commutator flashover damage. Onboard voltage transformers could be used to drop the line supply but were not as efficient as 50Hz examples so were larger and heavier. However, the big advantage of the lower frequency was the avoidance of onboard rectifiers, which at the time were expensive, large, fragile and frankly rather dangerous mercury arc devices. Many European railways configured their DC as 1500V with OHLE. Post WW1 a Government committee recommended this as a standard for mainlines and the NER and GWR had plans for this system on parts of their networks. Although they abandoned the NER scheme after grouping the LNER resurrected the idea for the Woodhead route, a project stalled by WW2 but eventually completed by BR in peacetime. The early LB&SC suburban electrification took a very different approach in the early 1900s , employing a German supplier to deliver a higher voltage AC OHLE system running at 6,600V AC at a low frequency of 25Hz, again to avoid rectifiers on board. Post-grouping, the Southern Railway decided to standardise on the LSWR 3-rail system however and the OHLE had all been removed before the 1930s. The L&Y Lancaster-Morecambe-Heysham branch was also electrified around the same time using 6.6kV AC OHLE at 25Hz but it remained an isolated trial system, surviving until the 1950s nevertheless. Various largely segregated suburban railways were electrified in cities outside London using the 3 or 4 rail DC system at around 600V, including The Mersey and Wirral Railways, Tyneside suburban and the ex L&Y Manchester-Bury line using an unusual higher voltage (1200V) side contact system.

 

[HSTEd]

If 6.6kV 25Hz had been selected as the standard by the southern might have been able to see off line frequency electrification in the 50s thanks to the lack of rectifiers which proved very unreliable.
Could have had 25kV 25Hz.

 

[Searchlight]

AC trains were heavier and more complicated than DC trains and Universal Motors were heavier, less efficient and needed more maintenance than DC motors. Mercury Arc Rectifiers had problems on trains......Better to put them in substations.
DC motors also gave faster acceleration than when running on AC. Some countries electrified at 3Kv DC overhead, indeed English Electric exported trains for this system. Pity we didn t use it......It would have avoided many problems .

 

[HSTEd]

AC trains were heavier and more complicated than DC trains and Universal Motors were heavier, less efficient and needed more maintenance than DC motors. Mercury Arc Rectifiers had problems on trains......Better to put them in substations.
DC motors also gave faster acceleration than when running on AC.

Well Universal motors are essentially series wound DC motors so are capable of enormous starting torques - just like conventional DC motors.
And whilst they require some more maintenance I believe the quantity is not entirely ridiculous.
The question t hen becomes - how heavy is a 370kW, 6.7kV/0-750V 25Hz transformer? if its only a couple of tonnes its still feasible to build multiple units in the southern style.
And whilst the motors themselves might be less efficient - with a transformer available it becomes possible to use tap-change control to give stepped motor control, reducing the need to use resistors and thus reducing energy consumption significantly.

I believe Universal motors are also capable of regeneration with relatively little work, I believe just an auxiliary excitation winding on the transformer but I might be wrong.

Some countries electrified at 3Kv DC overhead, indeed English Electric exported trains for this system. Pity we didn t use it......It would have avoided many problems .

3kV only really appeared quite late in the DC era - long after 750V had become the de facto Southern standard, there was only a relatively short window where it would be the ideal before the rectifiers become feasible for utility frequency high voltage electrification.

6.7/27kV 25Hz probably would have been the best solution.

 

[AM9]

Well Universal motors are essentially series wound DC motors so are capable of enormous starting torques - just like conventional DC motors.
And whilst they require some more maintenance I believe the quantity is not entirely ridiculous.
The question t hen becomes - how heavy is a 370kW, 6.7kV/0-750V 25Hz transformer? if its only a couple of tonnes its still feasible to build multiple units in the southern style.
And whilst the motors themselves might be less efficient - with a transformer available it becomes possible to use tap-change control to give stepped motor control, reducing the need to use resistors and thus reducing energy consumption significantly.

I believe Universal motors are also capable of regeneration with relatively little work, I believe just an auxiliary excitation winding on the transformer but I might be wrong.


3kV only really appeared quite late in the DC era - long after 750V had become the de facto Southern standard, there was only a relatively short window where it would be the ideal before the rectifiers become feasible for utility frequency high voltage electrification.

6.7/27kV 25Hz probably would have been the best solution.

The 6.7kV would by now be 25kV with the clearance and creepage requirements that were introduced in the late '60s.

 

[HSTEd]

Would probably be 27kV because it would have facilitated 6.7kV capable units.

And there would have been far more 6.7kV than there was 6.25kV, so conversion might not have happened due to inertia. I also don't know how generous the original 6.7kV clearences were relative to later BR ones.

 

[MarkyT]

AC trains were heavier and more complicated than DC trains and Universal Motors were heavier, less efficient and needed more maintenance than DC motors. Mercury Arc Rectifiers had problems on trains......Better to put them in substations.
DC motors also gave faster acceleration than when running on AC. Some countries electrified at 3Kv DC overhead, indeed English Electric exported trains for this system. Pity we didn t use it......It would have avoided many problems .

The only thing really wrong with the Southern system is that exposed power rail at ground level. Even that is not too bad so long as the railway is very well segregated. Its exposure and position limit the voltage and practically the power available however. 3000V OHLE could improve this no end with copper conductors whilst being much safer for maintenance workers, lost wandering children and trespassers and escaped livestock. There would be fewer problems obtaining satisfactory insulation clearance for the overhead than with 25kV and surrounding metalwork would be much less exposed to nasty induced effects from high power AC (although not forgetting ripple and switching effects). It's a nice compromise as far as return current goes as well. Significantly less current for a given power than with 750V and, just as with 3rd rail, the trend away from track circuits means a good parallel bonded return setup can be achieved without all the paraphernalia of impedence bonds. That also means a big heavy parallel strengthening conductor can be bonded in frequently to all the rails simply where required to help keep the loop resistance low and any touch potential to earth under control.

It's an interesting thought. Would a 3kV DC OHLE conversion be more practical than 25kV AC in the south? Most newer stock has a pantograph well for OHLE operation, and a little spare space and weight allocation for additional power equipment. Perhaps a state of the art converter module capable of 3kV DC or 25kV AC input could occupy the space envisaged originally for the 25kV transformer, so a multi-system unit could draw power from either overhead mode as well as classic 3rd rail DC which would have to remain in places during ongoing route conversion projects and as a 'bridge mode' between separated areas of AC and DC OHLE.

 

[Searchlight]

Well Universal motors are essentially series wound DC motors so are capable of enormous starting torques - just like conventional DC motors.
And whilst they require some more maintenance I believe the quantity is not entirely ridiculous.
The question t hen becomes - how heavy is a 370kW, 6.7kV/0-750V 25Hz transformer? if its only a couple of tonnes its still feasible to build multiple units in the southern style.
And whilst the motors themselves might be less efficient - with a transformer available it becomes possible to use tap-change control to give stepped motor control, reducing the need to use resistors and thus reducing energy consumption significantly.

I believe Universal motors are also capable of regeneration with relatively little work, I believe just an auxiliary excitation winding on the transformer but I might be wrong.


3kV only really appeared quite late in the DC era - long after 750V had become the de facto Southern standard, there was only a relatively short window where it would be the ideal before the rectifiers become feasible for utility frequency high voltage electrification.

6.7/27kV 25Hz probably would have been the best solution.

The single phase 25Hz system did indeed have a long innings in the USA on the Pennsylvannia Railroad and the New Haven Railroad, only converted to 25Kv 60Hz system in 1980s. It had originally been promoted by Westinghouse for trolleycars (Interurbans) in 1904. No interurban installed it after 1910, however. It was estimated in 1910, that a DC car weighing 43tons would weigh 54tons if equipped for AC. (source; The Electric Interurban Railways in America). Most AC interurban lines converted to DC (often at 1200v) and immediately noted cost savings. While the Universal Motor is similar to a DC motor, for traction work, there are substancial differences in practice.
The AC version is nowhere on a par with the DC version.....Which, is the main reason why DC has survived so long! 2400vDC systems were developed from 1915......But, the 3Kv DC system really started with the advent of steel-tank rectifiers in 1930, which could be put in unmanned substations that were widely spaced, and, run on any frequency.

 

[HSTEd]

The single phase 25Hz system did indeed have a long innings in the USA on the Pennsylvannia Railroad and the New Haven Railroad, only converted to 25Kv 60Hz system in 1980s. It had originally been promoted by Westinghouse for trolleycars (Interurbans) in 1904. No interurban installed it after 1910, however. It was estimated in 1910, that a DC car weighing 43tons would weigh 54tons if equipped for AC.

There is still some 25Hz in place after all. It was only partially converted, apparently the cost of the conversion gave the people in charge of the purse strings a heart attack.

 

[Searchlight]

The only thing really wrong with the Southern system is that exposed power rail at ground level. Even that is not too bad so long as the railway is very well segregated. Its exposure and position limit the voltage and practically the power available however. 3000V OHLE could improve this no end with copper conductors whilst being much safer for maintenance workers, lost wandering children and trespassers and escaped livestock. There would be fewer problems obtaining satisfactory insulation clearance for the overhead than with 25kV and surrounding metalwork would be much less exposed to nasty induced effects from high power AC (although not forgetting ripple and switching effects). It's a nice compromise as far as return current goes as well. Significantly less current for a given power than with 750V and, just as with 3rd rail, the trend away from track circuits means a good parallel bonded return setup can be achieved without all the paraphernalia of impedence bonds. That also means a big heavy parallel strengthening conductor can be bonded in frequently to all the rails simply where required to help keep the loop resistance low and any touch potential to earth under control.

It's an interesting thought. Would a 3kV DC OHLE conversion be more practical than 25kV AC in the south? Most newer stock has a pantograph well for OHLE operation, and a little spare space and weight allocation for additional power equipment. Perhaps a state of the art converter module capable of 3kV DC or 25kV AC input could occupy the space envisaged originally for the 25kV transformer, so a multi-system unit could draw power from either overhead mode as well as classic 3rd rail DC which would have to remain in places during ongoing route conversion projects and as a 'bridge mode' between separated areas of AC and DC OHLE.

You raise many interesting points, which I would agree with......IF they seriously contemplate converting Southern Region to 25Kv AC, I would have thought that they should also look at 3Kv DC for comparison purposes first......Only 1 in four substations would need replacement.....No immunisation issues with signalling......easier clearances for the wiring through tunnels and overbridges. And, remember, a lot more railway to electrify yet!

 

[AM9]

You raise many interesting points, which I would agree with......IF they seriously contemplate converting Southern Region to 25Kv AC, I would have thought that they should also look at 3Kv DC for comparison purposes first......Only 1 in four substations would need replacement.....No immunisation issues with signalling......easier clearances for the wiring through tunnels and overbridges. And, remember, a lot more railway to electrify yet!

The GEML was originally electrified at 1500VDC OLE. Most of the then existing infrastructure I believe gave adequate clearance for tha voltage. When the Liverpool St lines were converted to ac, the OLE as far as Mountnessing (just beyond Shenfield) and all the way to Southend Victoria were fed with 6.25kV ac, using the same OLE including insulators which meant that they must have been good for that voltage. There were, of course problems with the tapped transformer primaries and switchgear needed for 6.25/25kV auto-changeover but that would have been solved in a next generation of EMUs anyway. Signal immunity to ac has been truly conquered, but the currents of DC traction will always be an issue, particularly with adjacent structures such as the miles of viaduct in South London.
At 3KVDC the additional power capability would be a temptation rather than throttling the new EMUs, so the return current would need very careful management and maintenance. 25kV ac has been truly conquered and represents a quite low actual safety threat to the public, apart from to graffiti taggers on a death-wish.

 

[edwin_m]

I think it's significant that most countries that initally adopted DC systems have gone for 25kV in later schemes. Even the Netherlands, which has almost its entire network on 1500V, is I believe looking at conversion. This suggests that once rectifier technology became good enough to install them reliably on trains, the economies of the higher voltage make it worth converting dispite the extra disruption of having two voltages. I don't think this is driven by international standardisation, because multi-voltage traction these days costs very little more than single voltage.

By contrast there doesn't appear to be any intention to convert the 15kV low frequency supplies in Germany, Switzerland or Austria.

 

[30907]

By contrast there doesn't appear to be any intention to convert the 15kV low frequency supplies in Germany, Switzerland or Austria.

Correct. The 20kV 50Hz Hoellental line in the Black Forest (now converted) and the Ruebeland line in the Harz which is isolated from the rest of the DR/DB electric network are one offs.

 

[Searchlight]

I think it's significant that most countries that initally adopted DC systems have gone for 25kV in later schemes. Even the Netherlands, which has almost its entire network on 1500V, is I believe looking at conversion. This suggests that once rectifier technology became good enough to install them reliably on trains, the economies of the higher voltage make it worth converting dispite the extra disruption of having two voltages. I don't think this is driven by international standardisation, because multi-voltage traction these days costs very little more than single voltage.

By contrast there doesn't appear to be any intention to convert the 15kV low frequency supplies in Germany, Switzerland or Austria.

Yes, but, most of these countries don t have to cope with our restricted loading gauge, tight tunnels and many low bridges! I still say the 25Kv system is not suitable for British conditions......This is one of the reasons costs are enormous and progress at a snails pace here! Yes 25Kv CAN be done, but at what overall cost?
Just compare the hardware needed for 25Kv compared to 3Kv DC.
Heavier trains mean more energy consumption. Over engineering is now endemic in the British rail industry.......Meaning the taxpayer is getting ripped-off and passengers are waiting decades for electric trains to materialise.

 

[jopsuk]

One reason for converting legacy DC systems over is that the standard for HS is 25kV 50Hz. If your "local lines" are also 25kV AC it means that more international HS services can run to "local" destinations without need of a multitude of voltage systems on the stock- and ideally if re-power can be combined with ETCS implementation there's then a move to true interoperability rather than various networks with interfaces and limited cross-interface services.

 

[edwin_m]

Just compare the hardware needed for 25Kv compared to 3Kv DC.
Heavier trains mean more energy consumption. Over engineering is now endemic in the British rail industry.......Meaning the taxpayer is getting ripped-off and passengers are waiting decades for electric trains to materialise.

The trains may be a bit lighter on DC, but it needs more infrastructure and wastes more power. Energy losses in the OLE vary inversely as the square of the voltage, so other things being equal a 3kV system would waste about 70 times as much power as a 25kV system. Of course other things aren't equal, the DC system will have to have thicker wires and more frequent feeders, but this all adds to cost. From the evidence of overseas railways I think we can conclude that, if structural clearances are ignored for the moment, 25kV has a lower whole-life cost than any of the DC voltages.

On structural clearances the original British 25kV schemes reduced the voltage to 6.25kV in tight clearance areas and this was considered not to require any more clearance than the 1500V equipment previously proposed and in some cases actually installed. Some years later it was found that the 6.25kV could be updated to 25kV without significant extra structure work. So the amount of structure work doesn't depend very much on the voltage chosen. There are probably few structures with enough clearance for any sort of OLE so most of them need some sort of work on them anyway.

 

[AM9]

The trains may be a bit lighter on DC, but it needs more infrastructure and wastes more power. Energy losses in the OLE vary inversely as the square of the voltage, so other things being equal a 3kV system would waste about 70 times as much power as a 25kV system. Of course other things aren't equal, the DC system will have to have thicker wires and more frequent feeders, but this all adds to cost. From the evidence of overseas railways I think we can conclude that, if structural clearances are ignored for the moment, 25kV has a lower whole-life cost than any of the DC voltages.

On structural clearances the original British 25kV schemes reduced the voltage to 6.25kV in tight clearance areas and this was considered not to require any more clearance than the 1500V equipment previously proposed and in some cases actually installed. Some years later it was found that the 6.25kV could be updated to 25kV without significant extra structure work. So the amount of structure work doesn't depend very much on the voltage chosen. There are probably few structures with enough clearance for any sort of OLE so most of them need some sort of work on them anyway.

To add to that, the structure clearance for 25kV is less than 6 inches. A large part of that is the dynamic allowance for contact wire movement caused by passing pantographs. The total clearance for 3000VDC would be marginally less as the dynamic allowance would be the same with the flashover allowance forming a small part of the total.
Contrary to comments here, there are often clearance issues on continental lines where old tunnels and bridges were built well before the advent of OLE. As rolling stock has grown larger within the loading gauge, (e.g. double deck coaches), the clearance for OLE become marginal. On the RER lines around Paris where they run on pre-existing surface routes, there are sections of overhead contact rail which provides a tighter mechanical intallation, less subject to movement and realxed on dynamic allowance. The voltage is a red herring.

 

[contrex]

I have been wondering - there must be a lot of copper in a DC overhead electrification scheme - contact wire, transformer windings, motors, and a certain amount in the AC distribution system that might also be recovered - I guess the value of the recovered copper must offset some of the costs of an AC scheme?

 

[Trog]

A lot of work also needs to be done to many overbridges to allow the passage of freight at the larger W11/12 gauges, the electrification clearances can be provided at the same time at an incremental cost. Also with arched structures the height of the trains particularly for freight tends to be set by the outside corners. The overheads being further in over the four foot can then have their supports etc up in the top of the arch, which depending on the shape of the arch is sometimes enough to get them through.

See picture to left.

 

[Class 170101]

There must be a clearance issue for the OLE to ensure live parts don't come into contact with structures though?

Isn't that why some of the structures on the Paisley Canal line have (extended) neutral sections under bridges and only certain traction is permitted on the line whilst the voltage is live? Isn't that why the FOCs got upset about access to Hawkshead terminal (albeit disused)?

 

[HSTEd]

Even a low frequency AC system has a far simpler way of distributing power at main line power requirements than DC.
You just need big transformers and a single phase pole line at >100kV like the ones they have in Germany.

DC only really works at low traffic densities where you can use existing grid facilities.

 

[AM9]

DC only really works at low traffic densities where you can use existing grid facilities.

Or with high densities over slower shorter routes where very short power sections can be fed, e.g. metro systems. that's would have been the way to do the Southern area. It was the daft ideas of running long heavy trains at high speeds over longer (up to 100 miles) routes that showed the problems up in a modern railway.

 

[Searchlight]

Or with high densities over slower shorter routes where very short power sections can be fed, e.g. metro systems. that's would have been the way to do the Southern area. It was the daft ideas of running long heavy trains at high speeds over longer (up to 100 miles) routes that showed the problems up in a modern railway.

SNCB Belgium railways electrified most of its lines at 3Kv DC......Are they really regretting that decision? Are they going to spend billions of Euros converting to 25Kv ? While Belgium got on with it, we have dithered, reversed decisions (eg 1.5Kv DC) prevaricated (stop-go) eventually settling on 25Kv AC......Yet, this is taking forever to install.
Look at the little GOBLIN line fiasco.....I believe clearance issues were involved there too? I believe 25Kv requires 200mm mininium clearance,
50mm should be adequate for 3Kv.....especially if bar conductor is used?
All the talk of DC resistance loss ignores "Skin effect", "Eddy Current"
losses on AC......especially involving steel rails. Long Distance transmittion of electricity worldwide is increasingly being done at very high DC voltages, because, even with the cost of inverters, it is more practical than AC transmittion. DC cables are just more efficient!

 

[edwin_m]

All the talk of DC resistance loss ignores "Skin effect", "Eddy Current"
losses on AC......especially involving steel rails. Long Distance transmittion of electricity worldwide is increasingly being done at very high DC voltages, because, even with the cost of inverters, it is more practical than AC transmittion. DC cables are just more efficient!

Yes, DC at hundreds of kV not at 3kV and below! A 25kV DC system might be a bit more efficient than a 25kV AC system but the trains would all need some kind of solid state voltage converter instead of a simple transformer.

According to Wikipedia the skin effect at 50Hz gives an effective depth of about 10mm for copper and a bit more than 1mm for steel. However high voltage AC systems only use the rail to carry return currents for a maximum of a mile or so, before the current is diverted (using transformers again) into a return conductor.
--- old post above --- --- new post below ---

SNCB Belgium railways electrified most of its lines at 3Kv DC......Are they really regretting that decision? Are they going to spend billions of Euros converting to 25Kv ?

I don't know about Belgium but the Netherlands are certainly looking at converting their 1500V network and have gone for 25kV not 3kV.

http://www.railway-technology.com/projects/netherlands/

Nederlandse Spoorwegen (Netherlands Railways - NS) runs trains over 2000km of electrified railway, and conversion of the overhead from 1500kV dc to 25kV ac is planned to massively improve the quality of service. The changes are needed due to acute, and worsening, capacity problems. Since the 1980s, the drive to increase the number of passenger trains has overloaded the power supply.

Throughout Europe, ac systems have been preferred over dc in recent years because of perceived improvements which are possible in terms of reliability and maintenance costs, as well as the benefits arising from standardisation with the systems of neighbouring countries.

 

[adamskiodp]

Thanks guys for the interesting answers, (even if a lot is over my head )

Adam

 

[Taunton]

The Soviet Union, and their Russian successors, are another to have made the change from DC to AC. A substantial amount of the network was electrified at 3,000v DC up to the 1960s, when the policy change was made to 25Kv AC. All around Moscow and St Petersburg, and the busiest trunk routes, were already done at DC, but newer schemes moved to AC. From Moscow to Vladivostok the Trans Siberian western half is DC but its eastern, more recent electrification is AC. It can be difficult to identify in casual photographs which is which, as the loco and emu building industry have tended to use the same bodies and mechanical parts for both systems. The Russian Railways typically change locos at the substantial number of interface points between the two systems rather than use dual voltage locos. The recent high speed Sapsan emus built by Siemens are 3,000v DC on their initial Moscow to St Petersburg route, but now dual voltage units are being delivered for other lines.