25kV AC vs 750V DC

[TheEdge]

Just a little thought I had walking to work at 5 this morning.

As we know 750V DC will happily send a decent sized train up to 100mph (what about beyond?) and 25kV AC will also happily power a train up to 100mph (and beyond).

What is/was the reasoning between using 25kV on the overheads rather than the much smaller 750V? Surely using 750V rather than 25kV would require much simpler and lower spec equipment which would be cheaper to operate and maintain.

Or is it far more complex than that?

 

[Sir_Clagalot]

25Kv AC is cheaper overall as there is no need for a chunky conductor rail, and the number of substations on 750V DC needed to power lots of train is quite high compared to 25Kv AC.
Plus 25Kv AC is safer for those who work on the ground, as they don't have to worry about tripping over it!

 

[O L Leigh]

Ah, this again.

No, 3rd rail isn't necessarily any cheaper or more efficient. The lower voltage means a higher current is required. The voltage drop over distance is also much more pronounced. Both of these things means that there are many more feeder stations needed. It is also much more prone to bad weather, such as flooding and ice formation. It's also lying around on the floor where anyone can step on it or trip over it.

Of course there are downsides to 25kV AC OHLE. It is also prone to bad weather during high wind and there can be failures due to be snagged by pantographs. However, on balance it is a more efficient and, arguably, cheaper system. It's also much safer because you can't trip over it.

O L Leigh

 

[snowball]

To deliver a given power, increasing the voltage reduces the current, so it reduces the cross-section of the wire or conductor rail you need to feed it. So you save money on the copper for the wire, and the structures to support it don't have to be so heavy, so you save money on them too. Energy lost to heat is proportional to the square of the current, so that goes down a lot, so your electricity bills go down.

Edit: time overlap with the post above.

 

[TheEdge]

Fine, I suspected there was probably much more to it.

 

[Lockwood]

Aren't trams 750DC overhead?

How does that compare with 3rd rail from the physics point of view?

 

[O L Leigh]

Not all. Some are 1500V DC.

Being lighter and slower, their power demands are much lower than heavy rail. Therefore the current can be lower.

O L Leigh

 

[thenorthern]

The world record for 3rd rail is 108 MPH which was achieved with a Class 442 which is much slower than overhead can achieve.

 

[starrymarkb]

There have been faster runs with 432 units running solo on the Tonbridge straight as part of the Eurostar development (but no officials/timekeeping on board to validate a record) - 117mph I believe was the speed one was radared at.

 

[snowball]

Aren't trams 750DC overhead?

Not all. Some are 1500V DC.

The Tyne & Wear Metro is 1500V DC but has no on-street running. I don't think a higher voltage than 750V would be allowed for on-street running in the UK.

When Manchester's Metrolink was at an early stage of planning there was talk of using 1000V but it ended up as 750V.

I don't know whether the rules are different in other countries.

 

[DownSouth]

The first of the commuter lines in Adelaide (South Australia) to get electrified recently began service with the first five EMUs now running. The electrical system chosen was 25kV AC overhead lines - and this is a significant choice to be noted for three reasons:

1. It's a brand new system with no connections to any other electrified lines to influence the decision - the nearest other electrified lines are separated by 800 kilometres, two breaks of track gauge and an incompatible loading gauge.
2. The electrical system was chosen before the rolling stock manufacturer - and even if the resulting manufacturer* was chosen first, they work with all the different systems anyway.
3. 25kV AC was chosen despite there being no electric freight, speeds no higher than 130km/h and the longest passenger trains being a double unit at six cars long - all of which would have put it within the performance envelope of 1.5kV DC or 3kV DC but would not have had the advantage of less infrastructure being set up.


* unfortunately it's Bombardier, assembled in Australia with the high value components brought from Europe and India. In eight months they've only been able to deliver six units, and one of those is restricted to driver training duties only until a major defect (the floor is flexible) is fixed.

Aren't trams 750DC overhead?

How does that compare with 3rd rail from the physics point of view?

Trams can be anything from 400V up to 1500V DC, some tram systems go even lower than that but they are anomalous. 600V DC is just as common for tram systems as 750V, most notably the largest tram system in the world (Melbourne) runs on 600V.

The physics is pretty similar to a conductor rail, but with issues of insulation being mercifully simpler and less chance of a pantograph welding itself on than with a conductor shoe. Wiring for DC electrification (whether for trams, light rail or heavy rail) needs to be thicker than for AC, because DC is carried in the core of the conductor while AC is carried on the skin.

3kV DC overhead is the national standard for rail electrification in numerous countries, including Belgium which necessitates London-Brussels trains carrying the kit for 3kV DC.

 

[starrymarkb]

AC vs DC across Europe (map from Wikipedia - if you want to fix anomalies feel free to edit and upload a fixed version)

(Original Link - http://en.wikipedia.org/wiki/File:Europe_rail_electrification_en.svg )

 

[Rapidash]

I'm fairly sure that anything past Weymouth should be grey on that.

 

[starrymarkb]

I'm fairly sure that anything past Weymouth should be grey on that.

I think originally whole countries were filled in, fire up photoshop if you feel it needs fixing

 

[43106]

O.L. Leigh has condensed it nicely. Power is the product of voltage and current, so if one is low, the other is high. A high current needs thicker conductors, AND can cause power losses in those conductors (a.k.a. I-squared losses). That's why the National Grid sends its power round the country at high voltages (132kV, 275kV and 475kV).
At 600V minimum, safety isn't an issue, as both AC and DC can fry the human body, though there are subtle differences between the two - DC can cause muscles to cramp and lock. Sorry to be so morbid, but you did ask!

 

[edwin_m]

Trams can be anything from 400V up to 1500V DC, some tram systems go even lower than that but they are anomalous. 600V DC is just as common for tram systems as 750V, most notably the largest tram system in the world (Melbourne) runs on 600V.

The physics is pretty similar to a conductor rail, but with issues of insulation being mercifully simpler and less chance of a pantograph welding itself on than with a conductor shoe. Wiring for DC electrification (whether for trams, light rail or heavy rail) needs to be thicker than for AC, because DC is carried in the core of the conductor while AC is carried on the skin.

3kV DC overhead is the national standard for rail electrification in numerous countries, including Belgium which necessitates London-Brussels trains carrying the kit for 3kV DC.

Trams being smaller and lighter than trains don't need so much power, and copper wire is more conductive than steel or aluminium third rail so they can get away with a wire of smaller cross-section (the wires on the two tracks are usually also electrically connected). For both trams and trains the actual supply voltage can differ quite a lot from the nominal figure, going down when many trains are accelerating and going up when they are using regenerative braking.

According to Wikipedia the skin depth at 60Hz is 8.5mm, and since railway overhead cables are less than twice that the conductivity is the same as DC. The skin effect only becomes significant at higher frequencies or larger conductors.

 

[WatcherZero]

One of the modern reasons for favouring 25kv (though I believe doubts over the future of DC go right back to the 50's) was several incidents in recent years of under supply when a 10 (or 12, I forget which) carriage train went past greedily sucking up all the juice causing other shorter trains on the line to stall from under voltage.

 

[edwin_m]

Another problem with DC is that at the low voltage the short circuit current is also relatively low, so if several trains are motoring simultaneously in the same area the substations may trip out for over-current. By contrast the current flowing through a short circuit at 25kV would be huge and therefore quickly and reliably detected.

 

[DaveNewcastle]

Another problem with DC is that at the low voltage the short circuit current is also relatively low, so if . . . . . .

Indeed.
Another 'problem' is that of electrolysis. And all the corrosion of vital parts that can so readily follow. This is also one of the several benefits of distributing power by AC.

Another benefit of AC is the relative simplicity of voltage conversion of an AC network (by transformers).

Another benefit of AC power supplies is that they follow naturally from the supply grid power (which is AC), without the need for capacitive conversion to DC.

 

[JB_B]

I think originally whole countries were filled in, fire up photoshop if you feel it needs fixing

I think the sort of anomalies you see there are beyond what you could fix with photoshop - I like the way Shetland and Orkney are 25kV - but you really need something vector based to give a sensible picture. (France looks particularly weird sliced in half with but no sign of the LGVs - likewise the AV/AVE routes in Italy and Spain).

 

[stanley T]

How much rebuilding of overbridges is required with 25kv, given that the UK loading gauge isn't the most generous on the planet.

 

[JB_B]

I appreciate the arguments in favour of 25kV from a transmission efficiency, conversion efficiency, infrastructure cost and track safety point of view) but as a passenger OHLE failure seems to cause far more delays than 3rd rail.

I can recall just one occasion of a 3rd rail specific failure (a shoe ripped off by obstruction on line back in the mid-90's) but even though I live and mostly travel in 3rd rail land I've experienced many more delays due to OHLE failure (high winds (ECML/WCML) , saggy wires (GEML), + pans ripping the wires the down for unspecified reasons).

Is this just down to how delays are reported to passengers or poor implementation of OHLE in the UK (or is it something else)?

 

[RichardN]

One of the big problems with conversion of third rail to DC is the complexity and expense of the changeover points due to the different earth strategies. Third rail you want everything going back down the rails to reduce electrolysis from the DC. 25kv would be dangerous unless earth strongly bonded. I'm not really sure why we couldn't put rectifiers In all stock, switch third rail to AC bit by bit, then strongly bonded to earth, then wires put in as a way of phasing out DC though.

Another solution could be if the battery electrostar could be applied as a mod to the existing fleet, we could keep the DC and AC systems apart by a few miles and use batteries in the gap.

 

[O L Leigh]

Is this just down to how delays are reported to passengers or poor implementation of OHLE in the UK (or is it something else)?

No. It's because you have forgotten all those winter mornings when the entire network south of the Thames was paralysed due to snow and ice while everything north of the Thames that ran off the OHLE ran normally. The same when parts of the south of the UK suffered from flooding.

One of the big problems with conversion of third rail to DC is the complexity and expense of the changeover points due to the different earth strategies. Third rail you want everything going back down the rails to reduce electrolysis from the DC. 25kv would be dangerous unless earth strongly bonded. I'm not really sure why we couldn't put rectifiers In all stock, switch third rail to AC bit by bit, then strongly bonded to earth, then wires put in as a way of phasing out DC though.

Not sure what you're driving at here. There are places where there is both 3rd rail and OHLE on the same lines and yet there aren't any issues.

The return current from OHLE is not so great because the voltages are stepped down by the on-train equipment. No train uses the entire 25kV AC. The voltage is only that high because it facilitates efficient and economical power transmission. The sockets in your house may give you 240V AC, but the power lines that you see straddling the countryside are energised at a very much higher voltage of perhaps several hundred thousand volts. The return current from 25kV AC traction is probably not that much different from the return current from 750V DC traction. Even at flat chat a Cl317 EMU probably uses only about 600V. Therefore the return current going back through the rails to the feeder station is not so great as to cause any issues.

O L Leigh

 

[Bald Rick]

I appreciate the arguments in favour of 25kV from a transmission efficiency, conversion efficiency, infrastructure cost and track safety point of view) but as a passenger OHLE failure seems to cause far more delays than 3rd rail.

I can recall just one occasion of a 3rd rail specific failure (a shoe ripped off by obstruction on line back in the mid-90's) but even though I live and mostly travel in 3rd rail land I've experienced many more delays due to OHLE failure (high winds (ECML/WCML) , saggy wires (GEML), + pans ripping the wires the down for unspecified reasons).

Is this just down to how delays are reported to passengers or poor implementation of OHLE in the UK (or is it something else)?

But apart from the capital cost, running cost, safety and performance issues, what has 25kV ever done for us?

Part of your perception is down to how stuff is reported, part is perhaps you not being aware.

Just on the Southeastern and Southern routes, there have been shoes off trains half a dozen times this year. There was one this week. This is usually reported as a broken down train, as that's what it is. Similarly, many failures of the 750v DC system manifest themselves as signalling failures - eg the major delays on the Southern network on 31 March were down to a traction power problem (negative return bonding at London Bridge to be precise), but reported as signalling problems. There are also a handful of 'displaced conductor rail' incidents each year - one a couple of weeks ago - which have the same impact as when the wires are down.

I've been responsible for building, operating and maintaining lines with both types of electrification, and without doubt 25kV AC OLE results in a more reliable rail system; one that is easier, safer and thus cheaper to maintain; and uses about 20% less electricity from the grid for a given power at the traction motor.

--- old post above --- --- new post below ---

How much rebuilding of overbridges is required with 25kv, given that the UK loading gauge isn't the most generous on the planet.

How long is a piece of string? Some need rebuilding, some you can get away with lowering the track, some need alterations rather than a full rebuild, some need no work.

 

[snowball]

Not sure what you're driving at here. There are places where there is both 3rd rail and OHLE on the same lines and yet there aren't any issues.

Dual electrified track is kept to a minimum, isn't it, because the two systems have different earthing requirements? I gather the heavier the traffic, the bigger the problem. Somebody on another thread within the last year said that on the dual-electrified stretch of Thameslink there's a lot of sophisticated and expensive equipment to switch the earthing system between the passage of one train and the next.

 

[Class377/5]

I appreciate the arguments in favour of 25kV from a transmission efficiency, conversion efficiency, infrastructure cost and track safety point of view) but as a passenger OHLE failure seems to cause far more delays than 3rd rail.

I can recall just one occasion of a 3rd rail specific failure (a shoe ripped off by obstruction on line back in the mid-90's) but even though I live and mostly travel in 3rd rail land I've experienced many more delays due to OHLE failure (high winds (ECML/WCML) , saggy wires (GEML), + pans ripping the wires the down for unspecified reasons).

Is this just down to how delays are reported to passengers or poor implementation of OHLE in the UK (or is it something else)?

Just one? You must not travel in winter then. Well yesterday there was an hour with nothing between Lewes and Hasting due to an issue unique to 3rd rail. And this is hardly an isolated incident.

 

[jopsuk]

How much rebuilding of overbridges is required with 25kv, given that the UK loading gauge isn't the most generous on the planet.

25kV can't actually jump that big a gap in most circumstances, especially if the structure isn't very conductive (eg concrete bridges are not something you'd use for power distribution!). Clearences can be quite small- eg at Whittlesford Parkway or Tottenham Hale- at Whittlesford it is noticable quite how compressed the pantograph is

 

[sarahj]

Over the years I've lost shoes twice. Once between Victoria and Clapham (they just fell off, why? well errr) and between London Rd and Mouslecombe, rock fall ripped off two. I've also been on trains that have splutted and jerked along due to ice on the rail. Plus delays die to electrical supply issues. Oh and a naughty shoe on another train ripped up about 2 miles of rail from their pots.

From what I've read train speeds etc are even resticted by power through the over head. So high speed trains running under 1500dc (NL) and 3000dc (B) hace much lower speeds and total power. Even German ICE's and French TGV's have a lower top speed on the German 15kv AC lines than the 25kv lines on the LGV's in F,B,NL etc.

As for wind, I notice that German overhead has a tiny wire running from the registration arm to the main bracket which helps with wind. Could work here, esp on the ECML Vale of York section, but would require a redesign of the overhead.
http://commons.wikimedia.org/wiki/File:Oberleitung_Mannheim-Stuttgart.jpg You can just see the tiny wires here on this picture.

Anyway, included is a picture of a shoe that came off my train between Victoria and Clapham. After the nice men on all over high viz came along they dumped the naughty part in the rear cab. Delay 1.5 hours.

 

[Bald Rick]

Dual electrified track is kept to a minimum, isn't it, because the two systems have different earthing requirements? I gather the heavier the traffic, the bigger the problem. Somebody on another thread within the last year said that on the dual-electrified stretch of Thameslink there's a lot of sophisticated and expensive equipment to switch the earthing system between the passage of one train and the next.

The TL core is an unusual case, as it is built for 30tph in a recovery situation, all 12 car, 5MW units. And the feeder station for the third rail is between Blackfriars and City Thameslink (and it's a whopper), so there is a lot of return current flowing around along with the potential for stray currents. The presence of the LU traction system at Farringdon doesn't help either. So there is a contractor system that switches between the earthing systems, literally following the trains using track circuit indications.
--- old post above --- --- new post below ---

From what I've read train speeds etc are even resticted by power through the over head. So high speed trains running under 1500dc (NL) and 3000dc (B) hace much lower speeds and total power. Even German ICE's and French TGV's have a lower top speed on the German 15kv AC lines than the 25kv lines on the LGV's in F,B,NL etc.

Almost every DC compatible unit delivered here in the past decade or so has had the power artificially limited on the DC as it would overload the traction supply system. Compare a 377/5 at full acceleration on the Brighton main line vs the MML.