1500V DC OHL Electrification - More Robust?

[edwin_m]

Well there are plenty of systems around with substantial DC operation. Down the length of Italy, across the deserts in South Africa. And then there's the Trans-Siberian in Russia. A bit longer than a trip to Weymouth ...

The DC electrification in all those countries got going well before the advent of the 25kV system. The only viable alternatives at the time was the German-Swiss-Austrian 15kV system, and various three-phase options, all of which had their own issues.

Italy is adopting 25kV for high speed lines. South Africa adopted 50kV for the Sishen-Saldanha heavy haul line and 25kV for the Gauteng line, although neither of these has any need for compatibility with the rest of the network. I don't know about the Trans-Siberian but Russia now has similar mileages electrified at 25kV and 3kV.

It was my understanding that German/Swiss etc used DC motors, this was possible as the current changed direction is did so through both armature and field hence motor still rotated same way. The reason for the 16 2/3Hz supply was to reduce arcing on the commutator, which would have been a serious problem at 50Hz. Can anyone clarify?

I think the problem is that the motor coils are inductors, which conduct DC well but have increasing impedance to AC as the frequency increases. This results in heating of the coils, which would apparently have been too much to cope with at 50Hz.

 

[kermit]

Thanks for the many replies to my original query. I hadn't quite appreciated what a debate it would prompt! I guess the absence of clear-cut answers explains why there are still a number of lower voltage DC systems around the world. One last point from me - I travelled on the Manchester - Hadfield class 506 units many times, and their acceleration was phenomenal - much faster than the AC class 304s with which they shared Manchester Piccadilly!

 

[Taunton]

I don't know about the Trans-Siberian but Russia now has similar mileages electrified at 25kV and 3kV.

Yes, though both continue to be extended. Of course, the mainstream long distance trunk routes were the ones done first, such as Moscow to St Petersburg, or the west end of the Trans-Siberian, so the ton-mileage the DC handles is way more. There haven't been many conversions. The Soviet-era emus and locomotives (still the majority), several types, have the same bodies for AC and DC versions, so are difficult to distinguish in photographs.

One last point from me - I travelled on the Manchester - Hadfield class 506 units many times, and their acceleration was phenomenal - much faster than the AC class 304s with which they shared Manchester Piccadilly!

That's more down to the overall design than the supply type. The most glaring difference is on TfL, where 630v DC Underground trains can achieve amazing acceleration, the rear car often leaving the short platforms at well over 30mph, while the AC Overground Class 378 have notably poor and ponderous starts.

 

[Revaulx]

Thanks for the many replies to my original query. I hadn't quite appreciated what a debate it would prompt! I guess the absence of clear-cut answers explains why there are still a number of lower voltage DC systems around the world. One last point from me - I travelled on the Manchester - Hadfield class 506 units many times, and their acceleration was phenomenal - much faster than the AC class 304s with which they shared Manchester Piccadilly!

We in Altrincham were seriously underwhelmed when the line was converted to 25kV in 1971 and the 304s replaced the 1931 DC units. Both acceleration and ride were worse on the 304s, the first batch of which with their non-corridor compartment coach and small windows felt just as old as the 40 year old DC units.

I’m pretty certain that was down to the 304s being underpowered and generally a bit crap, rather than any inherent superiority of DC. Likewise the 306s may have exploded occasionally after their conversion to AC, but I recall them being pretty nippy when they were entering their fourth decade.

 

[Taunton]

Going back to the original post, another reason for Woodhead to be DC, and why it was never changed over, was it seemed a natural for regenerative power, going up over a gable, trains descending returning current to the line for those ascending. At the time, and indeed until comparatively recently, AC trains have been unable to regenerate at the precise frequency of the overhead line, and I think AC regen is still pretty limited, the various installations around the world mostly all being DC.

 

[edwin_m]

Going back to the original post, another reason for Woodhead to be DC, and why it was never changed over, was it seemed a natural for regenerative power, going up over a gable, trains descending returning current to the line for those ascending. At the time, and indeed until comparatively recently, AC trains have been unable to regenerate at the precise frequency of the overhead line, and I think AC regen is still pretty limited, the various installations around the world mostly all being DC.

As far as I'm aware all post-privatization AC EMUs on Network Rail regenerate, possibly 365s too. Older units with DC motors would need a lot of extra electronics to be able to regenerate into AC and none do so. Network Rail actually allowed regeneration into AC before DC - I believe this was due to concerns about safety if a regenerating train ran into a section of third rail that had been isolated for staff safety. But now the situation is similar, all post-privatization DC EMUs regenerate and again I'm not sure about Networkers.

The factors that make DC less suitable for longer-distance schemes also make regeneration less effective on less busy parts of the railway, as there has to be another train drawing power reasonably close by to take up the regenerated current. Otherwise the voltage drop along the wire or third rail means that the train that is a braking can't regenerate much power before the voltage at its shoes or pantograph reaches the maximum allowed. The current and thus the voltage drop is much lower on high-voltage AC systems, so a train much further away can make use of the regenerated energy.

 

[Springs Branch]

On a tangent - does anyone know why the Tyneside Metro was a new-build 1500V DC system in the 1970s, rather than choosing the more conventional 750V DC (as used on subsequent UK light-rail schemes)?

 

[Taunton]

On a tangent - does anyone know why the Tyneside Metro was a new-build 1500V DC system in the 1970s, rather than choosing the more conventional 750V DC (as used on subsequent UK light-rail schemes)?

The T&W Metro was a notable offshoot of the Hong Kong Metro, for designs done from the UK, some shared staff, vehicle manufacturer, and indeed some initial testing of the Hong Kong cars on the T&W test track. That's a 1.5Kv dc system as well. It's a good halfway point for an urban system between third rail systems and the vehicle complexity of 25Kv.

 

[edwin_m]

A couple of recent Metros have opted for 1500V third rail.

 

[Sir Felix Pole]

The DART system in Dublin opted for 1500V DC overhead in the 1980s, despite 25kV AC being well established by then. It was, and is, a purely suburban system with numerous extensions having opened since. Recent proposals, however, envisage extensions to Maynooth and Drogheda on the Sligo and Belfast main-lines respectively. Main-line electrification to Cork and Belfast is also desired, but at 25kV AC, so it will be interesting to see quite how this develops.

France, of course, has substantial mileage at 1500V DC on the classic main-lines south of Paris, and a lot of work has been done recently on replacing the classic 'Midi' overhead structures and catenary in the SW of the country. I think the only section of 1500V DC that has actually been converted to 25kV AC is Bellegarde to Geneva, in connection with the 'Haut-Bugey' line reopening to shorten the Paris to Geneva TGV timings.

 

[Taunton]

France has an interesting example of voltage flexibility, as they electrified at 25Kv AC along the Riviera, but at the border with 3Kv DC Italy, at Ventimiglia, the somewhat compact station there was a challenge. A lot of French stock, locos and emus, is actually dual voltage 1500v DC/25Kv AC, so about a mile from Ventimiglia there is a changeover from AC to DC, which the French trains run seamlessly through. Meanwhile Italian 3Kv DC locos run adequately around the station area on half voltage, to a further changeover out in Italy. It seems strange to see two seemingly incompatible units sharing the same tracks.

 

[Tunnel Bore]

... 25Kv AC ... 3Kv DC ... 1500v DC/25Kv ... 3Kv DC ...

All those Kv in place of kV are making my eyes hurt.

 

[Fireless]

It was my understanding that German/Swiss etc used DC motors, this was possible as the current changed direction is did so through both armature and field hence motor still rotated same way. The reason for the 16 2/3Hz supply was to reduce arcing on the commutator, which would have been a serious problem at 50Hz. Can anyone clarify?

That was pretty much the reason.
When the german railways made the decision on a standard electrification system in 1912, they already saw the advantages of single-phase AC electrification but had to go for a lower frequency to manage arcing on the commutator of the universal motors (pretty much a DC motor with field coils).

The key disadvantage of the system is that it can't be easily fed from the national grid due to the different frequency either requiring its own grid (DB Energie operates its own 7800 km long 110 kV grid) or expensive converters.

 

[Taunton]

The key disadvantage of the system is that it can't be easily fed from the national grid due to the different frequency either requiring its own grid (DB Energie operates its own 7800 km long 110 kV grid) or expensive converters.

I remember when 25Kv was first being reported on in the 1960s, articles banged on about an "industrial frequency" system. As everything railway up until then in Britain had been DC, this likely went over many heads, but presumably in the technology of the day you would need these approaches to get a different frequency, which had been done on all the European AC systems up to then.

Amtrak on New York-Washington also inherited the old Pennsylvania Railroad 11Kv 25Hz system, and has its own 132Kv 25Hz high voltage distribution system; in their case it is carried trackside, high up (maybe 50 feet above ground level, and right over the top of road overbridges) on long, rather irregular extensions to the catenary masts, accounting for the shambolic mass of wiring seen in photographs of the line. See here: https://fineartamerica.com/featured/blasting-through-perryville-peter-crook.html

 

[kermit]

All those Kv in place of kV are making my eyes hurt.

Sorry! Fixed, at least in thread title and original post.

 

[edwin_m]

That was pretty much the reason.
When the german railways made the decision on a standard electrification system in 1912, they already saw the advantages of single-phase AC electrification but had to go for a lower frequency to manage arcing on the commutator of the universal motors (pretty much a DC motor with field coils).

The key disadvantage of the system is that it can't be easily fed from the national grid due to the different frequency either requiring its own grid (DB Energie operates its own 7800 km long 110 kV grid) or expensive converters.

I remember when 25Kv was first being reported on in the 1960s, articles banged on about an "industrial frequency" system. As everything railway up until then in Britain had been DC, this likely went over many heads, but presumably in the technology of the day you would need these approaches to get a different frequency, which had been done on all the European AC systems up to then.

Amtrak on New York-Washington also inherited the old Pennsylvania Railroad 11Kv 25Hz system, and has its own 132Kv 25Hz high voltage distribution system; in their case it is carried trackside, high up (maybe 50 feet above ground level, and right over the top of road overbridges) on long, rather irregular extensions to the catenary masts, accounting for the shambolic mass of wiring seen in photographs of the line. See here: https://fineartamerica.com/featured/blasting-through-perryville-peter-crook.html

The traditional technology would be a rotary converter - a motor supplied by the incoming frequency, driving an alternator configured to produce the outgoing one. 16.67Hz is a third of 50Hz so the coils would have been in that ratio. I believe some of these are still in use but more recent ones use electronics instead.

Having said that, Swiss Railways has a lot of hydro power from its own plant that generates 16.67Hz from scratch. At the time the Swiss and Germans made their decisions on supply frequency there probably wasn't much in the way of a national grid so they would have had to create most of that whatever frequency they chose.