AC/DC overhead lines mixing

[ShadowKnight]

Hey, with a recent network rail report talking about potentially electrifying alot of the network, I came to thinking what solutions could be had in places where there are existing DC overhead electrification in Sunderland and Rotherham for the local tram/metro systems.

Let's say the national network WILL be electrified in these areas how can this solution be reasonably made?

Turning on and of the AC/DC wires?
Adding transformers to the tram/train?
Something else?

 

[Bill_Lestrange]

You would just need dual voltage rolling stock that can work at 1500V DC and 25kV AC OHLE which, I think, would effectively mean having an AC circuit and a DC circuit to a common source - AC circuit would then be rectified and transformed to the working voltage for the motor which the DC current would just need to be transformed (if necessary) then both output to the same motor.

Eurostar, TGVs, JR East Joban line etc. all use dual/trivoltage AC/DC OHLE rolling stock. I mean, class 319s, 700s, 717s, 395s etc. all are dual DC/AC sets.

Personally, I'm in favour of more 1500V DC electrification, especially around urban areas to increase the amount of electric traction and reduce the costs of electrification but that is my personal preference.

 

[eastwestdivide]

The Rotherham system at least is designed to be convertible to 25kV if the wires come to the area. The tram-trains themselsves were specified as dual voltage and the overheads are designed to 25kV standards, as I understand it.
They currently come to a halt on the new Tinsley chord in any case for signalling handover, so presumably if the Network Rail side was re-electrified, the section on the chord is designed to be able to accommodate a voltage change.

 

[AM9]

You would just need dual voltage rolling stock that can work at 1500V DC and 25kV AC OHLE which, I think, would effectively mean having an AC circuit and a DC circuit to a common source - AC circuit would then be rectified and transformed to the working voltage for the motor which the DC current would just need to be transformed (if necessary) then both output to the same motor.

Eurostar, TGVs, JR East Joban line etc. all use dual/trivoltage AC/DC OHLE rolling stock. I mean, class 319s, 700s, 717s, 395s etc. all are dual DC/AC sets.

Personally, I'm in favour of more 1500V DC electrification, especially around urban areas to increase the amount of electric traction and reduce the costs of electrification but that is my personal preference.

Current practice for ac/DC stock is to have a 750V DC bus which is then used to create a 3 phase drive to the motors. On a 1500VDC set the motors are usually wired as serial parallel pairs. It might be simpler to use a conventional 750V DC bus by electronically stepping the 1500V down before sending it to the bus.

 

[ainsworth74]

I wondering about how you can separate a section of 25kv AC and 1500v DC? Can you get away with just having a neutral section between them or would you require rather more?

 

[edwin_m]

I wondering about how you can separate a section of 25kv AC and 1500v DC? Can you get away with just having a neutral section between them or would you require rather more?

Yes, neutral sections are used for the tram-trains in Germany (at 15kV but the principle is the same) and as mentioned the Rotherham route makes provision for one to be added if converted. I think it's a longer neutral section than usual, and triggers the on-board equipment to check what the voltage is before re-closing the breakers.

Current practice for ac/DC stock is to have a 750V DC bus which is then used to create a 3 phase drive to the motors. On a 1500VDC set the motors are usually wired as serial parallel pairs. It might be simpler to use a conventional 750V DC bus by electronically stepping the 1500V down before sending it to the bus.

It does however introduce a non-standard feature into any train that needs to use the Newcastle-Sunderland route. I'm quite surprised they appear to have abandoned the idea of giving the new Metro stock a 25kV capability, so we are stuck with that problem for many years unless the Metro can run that far on battery.

 

[ainsworth74]

Yes, neutral sections are used for the tram-trains in Germany (at 15kV but the principle is the same) and as mentioned the Rotherham route makes provision for one to be added if converted. I think it's a longer neutral section than usual, and triggers the on-board equipment to check what the voltage is before re-closing the breakers.

Thank you

I'm quite surprised they appear to have abandoned the idea of giving the new Metro stock a 25kV capability, so we are stuck with that problem for many years unless the Metro can run that far on battery.

If I remember rightly it boiled down to money. It was get the new stock in the numbers required but with battery rather than dual voltage capability or don't get the new stock in the numbers required and have dual voltage. I suppose, depending on how complex/expensive it is, the section will be fairly short. Just Pelaw to Sunderland (around eight and a bit miles) with the ability to recharge just beyond Sunderland to South Hylton and back. Which should be doable on battery I'd have thought? Obviously if you cannot have the two systems co-existing on tracks next to each other then the length in question will grow to somewhere around ten miles each way but I think that might still be doable with the chance to recharge at the Sunderland end (around twenty five minutes including turn around time at South Hylton).

 

[Bill_Lestrange]

Current practice for ac/DC stock is to have a 750V DC bus which is then used to create a 3 phase drive to the motors. On a 1500VDC set the motors are usually wired as serial parallel pairs. It might be simpler to use a conventional 750V DC bus by electronically stepping the 1500V down before sending it to the bus.

I think this would be a thoroughly workable solution for these units. I must confess, I'm talking from my background in EE - I don't know the exact specifics of these units so to get this insight is extremely valuable. Perhaps a cl. 319 could be modified to work in this way?

I wonder whether it would be possible to expand tramway electrification systems onto the existing network even more so. For example the Robin Hood line would benefit from electric traction and also shared sections between NET and EMR services would increase capacity around Nottingham. So new rolling stock would be able to work on the 750V DC tramway system as well as either working with battery or hybrid. But this is a definite aside.

Manchester, Sheffield and Birmingham would also benefit from this too I think.

 

[edwin_m]

I think this would be a thoroughly workable solution for these units. I must confess, I'm talking from my background in EE - I don't know the exact specifics of these units so to get this insight is extremely valuable. Perhaps a cl. 319 could be modified to work in this way?

I wonder whether it would be possible to expand tramway electrification systems onto the existing network even more so. For example the Robin Hood line would benefit from electric traction and also shared sections between NET and EMR services would increase capacity around Nottingham. So new rolling stock would be able to work on the 750V DC tramway system as well as either working with battery or hybrid. But this is a definite aside.

Manchester, Sheffield and Birmingham would also benefit from this too I think.

Like the third rail system, tramway overhead is optimized for fairly small vehicles running fairly frequently at low speeds with closely spaced stops. The vehicle can be lighter and when it brakes there is probably another one nearby to take up the regenerated power. However tramways have a substation every couple of km typically.

Trains tend to be heavier, faster, further apart and stopping less frequently. Threrefore they need more power and with the same voltage they would need more current, but energy loss varies as the square of the current so is hugely more in a 750V system than in 25kV. This saving (also meaning that regenerated power can travel further) outweighs the extra power needed because the train is heavier, in effect carrying its own substation around. Since the 25kV system became viable in the 1950s, I don't think there's a single railway starting afresh with an electrification system that has gone for any lower voltage*, and quite a few railways have started using 25kV even though they already had a different system in use. So I think it's pretty clear 25kV is the optimum for most railways.

*A few heavy haul railways have gone for 50kV.

 

[Bill_Lestrange]

Like the third rail system, tramway overhead is optimized for fairly small vehicles running fairly frequently at low speeds with closely spaced stops. The vehicle can be lighter and when it brakes there is probably another one nearby to take up the regenerated power. However tramways have a substation every couple of km typically.

Trains tend to be heavier, faster, further apart and stopping less frequently. Threrefore they need more power and with the same voltage they would need more current, but energy loss varies as the square of the current so is hugely more in a 750V system than in 25kV. This saving (also meaning that regenerated power can travel further) outweighs the extra power needed because the train is heavier, in effect carrying its own substation around. Since the 25kV system became viable in the 1950s, I don't think there's a single railway starting afresh with an electrification system that has gone for any lower voltage*, and quite a few railways have started using 25kV even though they already had a different system in use. So I think it's pretty clear 25kV is the optimum for most railways.

*A few heavy haul railways have gone for 50kV.

I see your point, but I was thinking purely of commuter routes. I agree with you that longer distance routes would require more substantial electrification (ie. I think that electrifying say the midland at 1500V DC would be a silly idea - power loss would be 10s of thousands times higher!) but for those routes around cities, it might be useful to use a lower voltage DC system to allow cross running between tram-trains and trains and reduce the amount of weight track-wear (ie. a reduction in fixed track access costs) and allow for an increase of the frequency of commuter services. I am only thinking about around cities and certainly not as an solution for intercity traffic.

There would be a requirement for more substations and I am all too aware of how raising the costs of an infrastructure project makes it significantly less palatable but if the reduction in operational costs and the opportunity for interoperation balanced it out this might make it more appealing.

 

[edwin_m]

I see your point, but I was thinking purely of commuter routes. I agree with you that longer distance routes would require more substantial electrification (ie. I think that electrifying say the midland at 1500V DC would be a silly idea - power loss would be 10s of thousands times higher!) but for those routes around cities, it might be useful to use a lower voltage DC system to allow cross running between tram-trains and trains and reduce the amount of weight track-wear (ie. a reduction in fixed track access costs) and allow for an increase of the frequency of commuter services. I am only thinking about around cities and certainly not as an solution for intercity traffic.

There would be a requirement for more substations and I am all too aware of how raising the costs of an infrastructure project makes it significantly less palatable but if the reduction in operational costs and the opportunity for interoperation balanced it out this might make it more appealing.

Plenty of people have looked at this. Basically 750Vdc overhead works for trams but not for anything bigger. It's probably even more inefficient than 750V third rail (smaller cross-section of conductor even if it's copper not steel) which is still pretty much standard for metros. But for inter-running the main line trains would need a 750Vdc mode as well - far easier to keep the hundreds of trains that might use a terminus as standard 25kV and build in dual-voltage capability in a small number of tram-trains that have to be non-standard anyway.

 

[Energy]

DC overhead is only for the T&W network, I believe their new stadler trains will be able to work at both 1500vDC and 25kvAC. The super tram in rotherham is electrified to 750vDC but the class 399 tram-trains are dual voltage and can also work at 25kvAC, keep the supertram at 750vDC (I don't think the older Siemens trams can work at 25kvAC) and have them switchover to 25kvAC when they go onto the national rail bit (which is only served by the dual voltage class 399). The rest of DC overheads are tram networks not national rail.

 

[Philip Phlopp]

Thank you

If I remember rightly it boiled down to money. It was get the new stock in the numbers required but with battery rather than dual voltage capability or don't get the new stock in the numbers required and have dual voltage. I suppose, depending on how complex/expensive it is, the section will be fairly short. Just Pelaw to Sunderland (around eight and a bit miles) with the ability to recharge just beyond Sunderland to South Hylton and back. Which should be doable on battery I'd have thought? Obviously if you cannot have the two systems co-existing on tracks next to each other then the length in question will grow to somewhere around ten miles each way but I think that might still be doable with the chance to recharge at the Sunderland end (around twenty five minutes including turn around time at South Hylton).

The Metro stock will still be 25kV capable, it's just they won't have the necessary capability as delivered.

 

[edwin_m]

keep the supertram at 750vDC (I don't think the older Siemens trams can work at 25kvAC) and have them switchover to 25kvAC when they go onto the national rail bit (which is only served by the dual voltage class 399). The rest of DC overheads are tram networks not national rail.

All tramways are 750V or occasionally less, and no tram (unless it's a tram-train) has a 25kV capability. As well as the practical advantages of a lower voltage for tramways, a higher voltage wouldn't be considered safe for use in a tramway environment, where it's at risk of being hit by over-high vehicles or falling onto people if damaged.

 

[ainsworth74]

The Metro stock will still be 25kV capable, it's just they won't have the necessary capability as delivered.

Oh interesting. I was under the impression they'd chucked that out entirely in favour of batteries.

 

[MarkyT]

...a higher voltage wouldn't be considered safe for use in a tramway environment, where it's at risk of being hit by over-high vehicles or falling onto people if damaged.

That's got to be the main reason trams anywhere in the world don't use higher voltages. While 25kV might be tolerably safe above a road over a short level crossing, stringing it down the high street with some of the supporting catenary attached to buildings etc seems fraught with risk. The extent of earth bonding of anything metallic nearby in the streetscape would be prohibitive. I think it has only fairly recently become practical to accommodate dual voltage step-down transformers on tram sized vehicles like the Sheffield tram-trains. Traditional heavy rail equipment for this would have been far too bulky.

The Metro stock will still be 25kV capable, it's just they won't have the necessary capability as delivered.

I guess they could convert a limited subfleet to dual voltage for the Sunderland line if and when neccessary. That might be a different subfleet to those vehicles equipped with additional traction batteries for new off wire extensions elsewhere.

 

[eastwestdivide]

In Switzerland, the RhB Arosa line has street running of trains through Chur at 11kV (AC), while the RhB Bernina line also has street running, but at 1000V (DC).
Any other high-voltage examples?

 

[MarkyT]

In Switzerland, the RhB Arosa line has street running of trains through Chur at 11kV (AC), while the RhB Bernina line also has street running, but at 1000V (DC).
Any other high-voltage examples?

1kV isn't significantly above the norm for street railways but 11kV certainly is. I'd always assumed the Arosa line was an isolated branch at the lower voltage, but then I've also seen standard RhB locos in the street in Chur but didn't join up the dots. Thanks for the correction. The new multiple unit trains must be dual voltage as I've seen them on all parts of the network.
The standard gauge Seethalbahn, also in Switzerland between Luzern and Lenzburg is standard 15kV and much of it is like a rural roadside tramway in character, passing through fairly small villages rather than the urban streets of downtown Chur.

 

[Taunton]

I wondering about how you can separate a section of 25kv AC and 1500v DC? Can you get away with just having a neutral section between them or would you require rather more?

Plenty of these in France, which has a (roughly) equal amount of the system on each. The RER has a changeover right in the middle of Paris at Chatelet.

A curious arrangement at Ventimiglia, on the Mediterranean coastal line, between France (25Kv AC) and Italy (3Kv DC). Both run equally around the station area. The French have a lot of dual voltage stock, that almost seems their norm now, which is allocated to this line, and about a mile west of Ventimiglia there is a changeover to an isolated 1.5Kv DC section, which the French do. Meanwhile a mile on the Italian side of the station there is another neutral section and without any switching they just run at half voltage in and out of the station.

It's notable that despite railways with existing DC often changing to AC for newer area electrifications in recent decades, there's been hardly any movement (outside Britain) to change long established DC systems to AC. And just about every urban Metro system built in recent times has gone for 3rd rail dc, I can't think of any new one that has gone for AC overhead. Even BART in San Francisco, where they challenged just about every established rail standard (gauge, wheel profile, etc) went for 3rd rail DC.

 

[edwin_m]

It's notable that despite railways with existing DC often changing to AC for newer area electrifications in recent decades, there's been hardly any movement (outside Britain) to change long established DC systems to AC. And just about every urban Metro system built in recent times has gone for 3rd rail dc, I can't think of any new one that has gone for AC overhead. Even BART in San Francisco, where they challenged just about every established rail standard (gauge, wheel profile, etc) went for 3rd rail DC.

SNCF converted its third rail routes around Paris, the last being Issy-Plaine to Puteaux which became a tramway in the 1990s.

The Netherlands has looked at converting their 1.5kV network to a higher voltage but doesn't appear to be taking it forward.

When I did some work for Latvia a few years ago they were considering changing their 3kV to 25kV.

Delhi Metro is 25kV (highly unusual).

 

[MarkyT]

A curious arrangement at Ventimiglia, on the Mediterranean coastal line, between France (25Kv AC) and Italy (3Kv DC). Both run equally around the station area. The French have a lot of dual voltage stock, that almost seems their norm now, which is allocated to this line, and about a mile west of Ventimiglia there is a changeover to an isolated 1.5Kv DC section, which the French do. Meanwhile a mile on the Italian side of the station there is another neutral section and without any switching they just run at half voltage in and out of the station.

That's a clever set up. Have they ever done something similar at the Belgian border? Certainly a lot simpler than some of the border station arrangements elsewhere I remember from youthful travels with OHLE voltage switching capability through platforms to allow traction changes.

It's notable that despite railways with existing DC often changing to AC for newer area electrifications in recent decades, there's been hardly any movement (outside Britain) to change long established DC systems to AC. And just about every urban Metro system built in recent times has gone for 3rd rail dc, I can't think of any new one that has gone for AC overhead. Even BART in San Francisco, where they challenged just about every established rail standard (gauge, wheel profile, etc) went for 3rd rail DC.

3rd rail can work well on highly segregated railways where fencing and security make public incursions to the trackside all but impossible, and sectioning and remote switching is sophisticated enough to allow quick, easy isolations to protect maintenance staff while stopping a minimum of otherwise unaffected traffic nearby. On the other hand, as the tentacles of BART spread ever further from the bay, the bizzare choice of the ibero-indian track gauge together with the 3rd rail electrical supply is an increasing handicap to infrastructure sharing with other local transit at the extremities of the system where the BART frequency is comparatively low. For info, BART is 1000V DC, and from photos it looks like top contact with a continuous protective covering clear of the very thin shoe contacts on the trains. That arrangement is not dissimilar to trad. U.S. subways in New York for example.

 

[eastwestdivide]

I'd always assumed the Arosa line was an isolated branch at the lower voltage

Apparently it was originally at a lower voltage, 2400V DC, and converted to 11kV as late as 1997, according to the RhB history page at: https://www.rhb.ch/en/company/portrait/history

 

[Taunton]

That's a clever set up.

I've always thought so too. It's one of those things that make you remember that the words "engineer" and "ingenious" come from the same root.

 

[eastwestdivide]

That's a clever set up. Have they ever done something similar at the Belgian border? Certainly a lot simpler than some of the border station arrangements elsewhere I remember from youthful travels with OHLE voltage switching capability through platforms to allow traction changes.

Yes, pretty sure the Belgian EMUs that run to Rosendaal and Maastricht go at half voltage on the Dutch side.
Paul Steane's EGTRE (Enthusiast's Guide to Travelling the Railways of Europe) has a list of border crossings including electrification arrangements at:

Category:Border Crossings - EGTRE

 

[stuu]

In Switzerland, the RhB Arosa line has street running of trains through Chur at 11kV (AC), while the RhB Bernina line also has street running, but at 1000V (DC).
Any other high-voltage examples?

The South Shore Line in Chicago has some street running sections with 1500V DC

 

[robvulpes]

Nobody in this thread has yet mentioned the Sihltal Zürich Uetliberg Bahn in Switzerland which has one track approaching it's Zurich terminus dual-electrified at 15kV AC and 1200v DC, the latter with offset catenary! The latest Be510 Uetliberg EMUs are dual voltage with a pantograph that can be slid across (though, I believe, only by a fitter in the depot, not automatically while in service). This is because the Uetlibergbahn is due to be converted to Swiss standard AC operation (timing uncertain).

 

[Bald Rick]

Nobody in this thread has yet mentioned the Sihltal Zürich Uetliberg Bahn in Switzerland which has one track approaching it's Zurich terminus dual-electrified at 15kV AC and 1200v DC, the latter with offset catenary! The latest Be510 Uetliberg EMUs are dual voltage with a pantograph that can be slid across (though, I believe, only by a fitter in the depot, not automatically while in service). This is because the Uetlibergbahn is due to be converted to Swiss standard AC operation (timing uncertain). View attachment 83848

Gosh, I hope the ORR’s Swiss division doesn’t take its standard Golfing umbrella there.

 

[Philip Phlopp]

Gosh, I hope the ORR’s Swiss division doesn’t take its standard Golfing umbrella there.

Funny, I was going to post the exact same thing. I'd love to see their safety case for that.

 

[dm1]

The offset pantograph is the DC one, so the 15kV line is at normal clearances, whereas the 1200V DC is relatively common on Swiss railways, and is fine. Both of the stations with dual electrification are underground, with a station design such that I don't think it would be that simple to poke an umbrella at the wires (or rather, conjductor bars) unless you were doing it deliberately. I don't know for certain, but I imagine there would be some kind of safety systems to detect the leakage current.

 

[edwin_m]

The offset pantograph is the DC one, so the 15kV line is at normal clearances, whereas the 1200V DC is relatively common on Swiss railways, and is fine. Both of the stations with dual electrification are underground, with a station design such that I don't think it would be that simple to poke an umbrella at the wires (or rather, conjductor bars) unless you were doing it deliberately. I don't know for certain, but I imagine there would be some kind of safety systems to detect the leakage current.

The station pictured in #26 would appear to be very much not underground, and although it may not have dual electrification the wires that are there are on the platform side and the pantograph looks to be easily accessible with the proverbial selfie stick (the lower part of the frame won't be live but everything above the insulators will be).

Interested to see if anyone has pictures of the undergound stations and the arrangement described. I've wondered if having a lower/deeper canopy valance would be one way of addressing the hazard in the UK, as well as doing a better job of keeping people dry!