AC Third Rail?

[MatthewRead]

This one was oddball all round

http://volkselectricrailway.co.uk/history/the-daddy-long-legs/


Related, but I've always wondered just what were the issues that afflicted the North London line 3rd rail electrification put in during the mid-1980s, only running 2-car sets, but which caused all sorts of electrical interference, including at Highbury apparently affecting the signalling and causing electrolysis on the Piccadilly and Victoria lines well below, whereas the GN suburban line there, also 3rd rail, was not doing this. It caused, after a few years of investigation, the further changeover to 25Kv overhead and the need to use dual voltage Class 313 units. It's not as if 3rd rail lines crossing the Underground are not uncommon in London, but I recall it being written that the BR engineers did not have a basic understanding of stray currents.

Do you know when 313's first started running on the North London Line?

 

[AndrewE]

I don't know of any technical reason why this couldn't be done. The question is, why? Once power electronics were sufficiently developed to use AC reliably on trains...

...except that A/C OLE worked very well before electronics were invented. Electricity obeyed the rules throughout the 20th century, and I think a variety of rectifiers were used before electronics were developed.
I'm not sure whether the class 87s were really electronic until the thyristor-controlled 87101 was built...

 

[AndrewE]

I occasionally muse on the combination of, say, 25kV supply at rail level, but with the transmission cabling encased in a form of insulating tube allowing shoe contact but preventing earthing and accidental contact. !

That sounds suspiciously like the sales pitch for Brunel's Atmospheric railway to me!

 

[Elecman]

I suppose it's like power line operatives, they actually connect themselves and the platform they are working on to the power line, it doesn't really matter what the voltage is so long as the voltage difference between your body and the power source is 0V and there is no path to earth/a different voltage (although that voltage would probably have to be at least ~100V different to do any harm).

The highest safe voltage difference for human contact as detailed in the IET wiring regs is just over 60 volts

 

[edwin_m]

...except that A/C OLE worked very well before electronics were invented. Electricity obeyed the rules throughout the 20th century, and I think a variety of rectifiers were used before electronics were developed.
I'm not sure whether the class 87s were really electronic until the thyristor-controlled 87101 was built...

The earliest AC electrification schemes just fed AC through what was basically a DC motor. This sort of worked provided the frequency of the AC wasn't too high, hence why the electrification on German/Swiss/Austrian railways is AC but at the odd frequency of 16 2/3 Hz. At higher frequencies the motors would overheat.

The French pioneered AC at the standard Grid frequency (50Hz in Europe) in the 1950s using mercury-arc rectifiers to convert it to DC on board the locomotive. These worked but weren't entirely satisfactory - basically a pool of mercury in a big glass bulb and not surprisingly it splashed around a bit.

The first semiconductor rectifiers came in early in the 60s and quite rapidly became universal on 50Hz traction units. These were just diodes to convert the current with no control function. 87101 and most AC EMUs of the 1980s used thyristors instead, which functioned as the diodes but could also control the amount of current getting to the motors, which were still DC.

Since the 1990s the technology has come full circle in that the motors are now fed with AC again. However this time they are a genuine AC motors which need a variable voltage and frequency depending on the speed and the power demanded. Modern power electronics can generate that from either a DC or an AC traction supply, and I think there is some confusion on this topic between AC supply and AC motors.

 

[AM9]

The highest safe voltage difference for human contact as detailed in the IET wiring regs is just over 60 volts

That is the highest safe voltage to be in contact whilst also at or near ground potential. HV transmission lines are sometimes maintained by crews in cages isolated from ground but in contact with HV. In the UK, the line voltage can be 132kV, 275kV or even 400kV. There is no problem for the operators as they aren't connected to anything else, so their bodies only see one voltage. Now if it was to rain heavily, allbets are off as there is a slight possibility of a path to water via heavy rain.

 

[KingDaveRa]

I like the idea of a three-phase railway. It sounds like an utter nightmare to cable and operate!

I also remember seeing the daddy-longlegs but for some reason I thought it got power from the track, which makes absolutely zero sense!

Of course the Blackpool tramway originally got it's power from a conduit in a trough, but it kept filling with sand and/or water, and either obstructing it or shorting it out. It also sounds like a very scary idea to me, as the access to the trough would need to be very small to avoid any potential shorts from people passing over it.

 

[L&Y Robert]

I occasionally muse on the combination of, say, 25kV supply at rail level, but with the transmission cabling encased in a form of insulating tube allowing shoe contact but preventing earthing and accidental contact. All the benefits of high voltage AC without the huge infrastructure required for OHLE. Wind damage disappears and snow/ice can't form on the shrouded contact surface. Now to design the interface detail to eliminate accidental contact...!

Manchester-Bury! SIDE contact conductor rail, the live rail boxed in with, I think, jarra hardwood. All the way, as well. I don't remember seeing any sections with 'naked' conductor rail, even in sidings and so-on/

 

[jopsuk]

conduit fed tramways were quite common, much of London had conduit sections using a detachable sledge.

The DLR uses bottom contact, with a plastic shroud over the top, for safety. The engineering for the shoes is rather more complex than for top contact. Some US railroads (eg Long Island) use top contact with a cover over the top throughout.

 

[Taunton]

Of course the Blackpool tramway originally got it's power from a conduit in a trough, but it kept filling with sand and/or water, and either obstructing it or shorting it out. It also sounds like a very scary idea to me, as the access to the trough would need to be very small to avoid any potential shorts from people passing over it.

Not too scary, much of the vast London tramway network had the same to the end (so did Washington DC, it seems to have been a thing of capitals). The conductor rails were actually well hidden inside, the collector was quite a twisted shape beneath the roadway. Centre slots were well understood from cable tramway days. However, they are a maintenance liability, they needed connecting to drainage, and regular cleaning out. You can still see what the London system looked like in the tunnel ramp that remains in the middle of Kingsway, just north of Holborn Underground station.

 

[AM9]

...except that A/C OLE worked very well before electronics were invented. Electricity obeyed the rules throughout the 20th century, and I think a variety of rectifiers were used before electronics were developed.
I'm not sure whether the class 87s were really electronic until the thyristor-controlled 87101 was built...

As edwin_m says, the power and control system on trains powered by ac OLE has evolved as power components have been developed. The original 25kV 50Hz ac trains in the UK were, the '50s conversions of the LNER/BR 1500VDC EMUs used on the GE Shenfield &Southend Victoria lines*. By interposing a transformer/rectifier set between the pantograph and the contactors controlling the existing DC traction circuits, control of motor power and speed was achieved by resistor banks as before.
New trains provided for the GEML, LT&S and West Anglia lines had tapped secondaries on the ac transformers allowing more efficient control of the traction current without power wastage through heat dissipation in resistors.
The next major development involved the ac/DC class 319s which had a DC bus to feed the conventional DC traction motors, but instead of using conventional camshaft switchgear and resistors, as used on the functionally similar class 455s, they had thyristor controllers which operated the same whether the DC bus was fed by 750VDC 3rd rail of 25kV OLE.
Having set the precedent of a common DC bus on DC 32rd rail/ac OLE EMUs, when more efficient synchronous ac motors became available, the thyristor controllers were replaced by electronic 3 phase inverters that provided controlled variable-frequency ac power and pole switching to control the speed. With various electronic inverter designs, this basic configuration has become standard for ac motored EMUs notably 323s, Networkers, Electrostars, Desiros etc., with features like electroregenerative braking becoming standard on new designs.

*Excepting the ex LMS Lancaster, Morecambe and Heysham EMUs which were converted from 6.5kV 25Hz toevaluate 25kV 50Hz in the UK.

 

[Taunton]

Excepting the ex LMS Lancaster, Morecambe and Heysham EMUs which were converted from 6.5kV 25Hz toevaluate 25kV 50Hz in the UK.

it was one of the four units on this section which was the first solid-state power control unit in Britain, from when it was modified for the line in 1954. It used Germanium instead of Silicon in the rectifier (they are related elements). It was shown at various technical exhibitions when "new", but unfortunately the 45-year old wooden body, even if nicely polished up for display, must have been a bit incongruous.

THE LMH, although changed to 50Hz for the trial, was still 6.6Kv. It was standard frequency being tried, voltage was by the by (although it mirrored the original reduced voltage sections in London and Glasgow). This also avoided having to change the original overhead and insulators.

The mercury arc rectifiers were not that strange, as this approach was used elsewhere in power controls. The takeoff point was dead centre over the tank, which had substantial baffles inside to minimise any sloshing with motion.

 

[AndrewE]

As edwin_m says, the power and control system on trains powered by ac OLE has evolved as power components have been developed. The original 25kV 50Hz ac trains in the UK were, the '50s conversions of the LNER/BR 1500VDC EMUs used on the GE Shenfield &Southend Victoria lines*. By interposing a transformer/rectifier set between the pantograph and the contactors controlling the existing DC traction circuits, control of motor power and speed was achieved by resistor banks as before.
New trains provided for the GEML, LT&S and West Anglia lines had tapped secondaries on the ac transformers allowing more efficient control of the traction current without power wastage through heat dissipation in resistors.
The next major development involved the ac/DC class 319s which had a DC bus to feed the conventional DC traction motors, but instead of using conventional camshaft switchgear and resistors, as used on the functionally similar class 455s, they had thyristor controllers which operated the same whether the DC bus was fed by 750VDC 3rd rail of 25kV OLE.
Having set the precedent of a common DC bus on DC 32rd rail/ac OLE EMUs, when more efficient synchronous ac motors became available, the thyristor controllers were replaced by electronic 3 phase inverters that provided controlled variable-frequency ac power and pole switching to control the speed. With various electronic inverter designs, this basic configuration has become standard for ac motored EMUs notably 323s, Networkers, Electrostars, Desiros etc., with features like electroregenerative braking becoming standard on new designs.

*Excepting the ex LMS Lancaster, Morecambe and Heysham EMUs which were converted from 6.5kV 25Hz toevaluate 25kV 50Hz in the UK.

Bull**** baffles brains, as they say. I suppose it comes down to the definition of electronics (which I think of as post-valve electrickery.) Unfortunately Wikipedia was as unhelpful in this context as these over-informative posts! I'm pleased to learn that

semiconductor rectifiers came in early in the 60s and quite rapidly became universal on 50Hz traction units. These were just diodes to convert the current with no control function

(Sorry, I can't get this multiple quote to work yet) so I suppose that if a simple diode is electronic, even though there was one in my crystal set in 1965 or so, then the electronic age goes back further than I realised.
A

 

[GRALISTAIR]

Bull**** baffles brains, as they say. I suppose it comes down to the definition of electronics (which I think of as post-valve electrickery.) Unfortunately Wikipedia was as unhelpful in this context as these over-informative posts! I'm pleased to learn that
(Sorry, I can't get this multiple quote to work yet) so I suppose that if a simple diode is electronic, even though there was one in my crystal set in 1965 or so, then the electronic age goes back further than I realised.
A

Since transistors have been around since the 40s (first patent even earlier IIRC) and I had one in my electronics set in the 1960s - yes.

 

[AM9]

Bull**** baffles brains, as they say. I suppose it comes down to the definition of electronics (which I think of as post-valve electrickery.) Unfortunately Wikipedia was as unhelpful in this context as these over-informative posts! I'm pleased to learn that
(Sorry, I can't get this multiple quote to work yet) so I suppose that if a simple diode is electronic, even though there was one in my crystal set in 1965 or so, then the electronic age goes back further than I realised.
A

Electronics goes back far earlier than your personal recollections of crystal sets. There is no formal differentiation between 'electric' and 'electronic', but if we consider a timeline of when engineers started to use devices to control by electrons (diodes, thermionic valves, thyratrons, transistors etc.) rather than bulk current (switches, resistors, capacitors etc.), it would start in 1904 when Alexander Fleming invented the thermionic diode and in 1906 when Lee de Forest, the triode. Thereafter, it became possible to control which way electrons (and hence current) flowed and to use a small quantity of electrons or current to control al larger current.
There were semiconductor devices as early as 1874 when the point contact diode was discovered creating the cat's whisker as used in crystal sets. The first semiconductor power device was the selenium rectifier, invented in 1906. Transistors were first created in 1948 when germanium became available for them.
So you can see that electronics is far older than me or you (I presume). If the soid state (electronic) diodes hadn't been invented, I doubt that ac via OLE would be in abundance today.
Electronics used in traction is no less reliable that the electric side of equipment provided both have been designed properly. It's absence would however put train design back to the '30s.

 

[DerekC]

Since transistors have been around since the 40s (first patent even earlier IIRC) and I had one in my electronics set in the 1960s - yes.

Valves (vacuum tubes) were quite definitely a form of electronics, although not much used in railways as far as I know. The first valve was patented (by a Brit) in 1904. I learned all about them, many years ago!

 

[billio]

Of course the Blackpool tramway originally got it's power from a conduit in a trough, but it kept filling with sand and/or water, and either obstructing it or shorting it out. It also sounds like a very scary idea to me, as the access to the trough would need to be very small to avoid any potential shorts from people passing over it.

The trams in the centre of Bordeaux pick up power from two narrow metal strips that run down the street in the middle of the track. The strips are segmented and only the segment under the tram is live. This type of pick-up may limit speed but that isn't an issue for pedestrian areas and street running. This approach removes the visual intrusion of catenary from a world heritage site. I am surprised this approach isn't more widely used.

 

[Taunton]

The trams in the centre of Bordeaux pick up power from two narrow metal strips that run down the street in the middle of the track. The strips are segmented and only the segment under the tram is live. This type of pick-up may limit speed but that isn't an issue for pedestrian areas and street running. This approach removes the visual intrusion of catenary from a world heritage site. I am surprised this approach isn't more widely used.

Such systems first turned up a century ago, patented by Dolter and by Lorain

https://en.wikipedia.org/wiki/Stud_contact_system

Their main issue is they are not fail safe, and a number of faults could leave the street pickup live after the vehicle has passed. I don't believe any of the systems have managed to fully overcome this. Absolute 100% fault free equipment is still somewhat illusory, which is why fail safe exists as a concept.

 

[CAF397]

No reason it couldn't be done but one of the reasons to use AC is that it allows you to use much higher voltages. You definitely wouldn't want to run 25kV at ground level!

And yet with the move to autotransformer feeders, where a 'spare' 25kV AC cable is strung next to the existing overheads, where structures have limited clearance the cable is wrapped thickly with insulation, and placed in new cable troughing through the structure, and then back up to the top of the OHL stanchions once clear.

 

[edwin_m]

Valves (vacuum tubes) were quite definitely a form of electronics, although not much used in railways as far as I know. The first valve was patented (by a Brit) in 1904. I learned all about them, many years ago!

At a stretch, the mercury arc rectifier could be considered to be a valve.

 

[edwin_m]

And yet with the move to autotransformer feeders, where a 'spare' 25kV AC cable is strung next to the existing overheads, where structures have limited clearance the cable is wrapped thickly with insulation, and placed in new cable troughing through the structure, and then back up to the top of the OHL stanchions once clear.

The contact and catenary wires have to be uninsulated to do their job, so necessary clearances must be provided around them. This isn't the case with the other cable, so when it gets to a bridge it is often easier to run it via a trough than to enlarge the bridge to provide clearance for that as well.

 

[CAF397]

I get that, just some of our wonderful contractors have a habit of chopping through 750 v Signalling cables, they'll get a shock when they chop through a 25k v cable!

 

[snowball]

Am I right in thinking that where AT cables are insulated, e.g. through a tunnel, they tend to remain at a similar height to the contact wire rather than running along the ground?

I think this is done both to minimise the EM field generated and to reduce the chance of people digging through them.

 

[edwin_m]

I get that, just some of our wonderful contractors have a habit of chopping through 750 v Signalling cables, they'll get a shock when they chop through a 25k v cable!

I've seen cable troughs in various places with "Danger 25kV" or similar signs on them. Not sure if they are ever buried on the railway, but the troughs sound a bit safer.

 

[rebmcr]

I've seen cable troughs in various places with "Danger 25kV" or similar signs on them. Not sure if they are ever buried on the railway, but the troughs sound a bit safer.

Those could well be untruths to deter cable thieves.

 

[Elecman]

Those could well be untruths to deter cable thieves.

I'm sure they are perfectly correct as most troughs carrying 11 or 33 KV traction supply cables are also marked as containing HV cables for safety reasons