Why single phase AC?

[stanley T]

Excuse me if this is a silly and ignorant question.

As I understand it, the national grid is three phase AC and that in a 25kV AC electrification with modern traction
- it is converted to single phase AC
- rectified to DC
-put through an inverter to convert back to 3 phase AC for the motors

Why not directly supply three phase AC through the OHLE?

 

[Kali]

Because the motors would try and run at one speed - AC motor speed is controlled by changing the frequency of the power supply ( basically! ) and it appears the easiest way to do that is as you said.

 

[Cliveblackpool]

Excuse me if this is a silly and ignorant question.

As I understand it, the national grid is three phase AC and that in a 25kV AC electrification with modern traction
- it is converted to single phase AC
- rectified to DC
-put through an inverter to convert back to 3 phase AC for the motors

Why not directly supply three phase AC through the OHLE?

I am not an expert on traction electricity.
The mains 3 phase is converted to DC by a solid state 3 phase rectifier.
You would need 3 OH wires, each with trolley poles/ triple pantographs for the train to receive the the 3 phase current direct!

 

[Railsigns]

I am not an expert on traction electricity.
The mains 3 phase is converted to DC by a solid state 3 phase rectifier.
You would need 3 OH wires, each with trolley poles/ triple pantographs for the train to receive the the 3 phase current direct!

Three-phase AC electrification does exist - and it only requires two overhead wires. The third phase is through the rails.

http://en.wikipedia.org/wiki/Three-phase_AC_railway_electrification

 

[HSTEd]

It would be a nightmare to engineer, which is why three phase electrification is not really used on large sections of railway at the moment.

 

[Cliveblackpool]

Three-phase AC electrification does exist - and it only requires two overhead wires. The third phase is through the rails.

http://en.wikipedia.org/wiki/Three-phase_AC_railway_electrification

Thanks for that. Would the rail be live, eg like third rail?

 

[HSTEd]

Thanks for that. Would the rail be live, eg like third rail?

Nope, you can keep one phase at ground if you rig your transformers correctly.

A Scott-T Transformer allows you to interconvert between three wire three phase and four wire two phase fairly easily, and since two wires will always be at ground they can be shorted and you end up with three wire two phase with common ground.

 

[John55]

Excuse me if this is a silly and ignorant question.

As I understand it, the national grid is three phase AC and that in a 25kV AC electrification with modern traction
- it is converted to single phase AC
- rectified to DC
-put through an inverter to convert back to 3 phase AC for the motors

Why not directly supply three phase AC through the OHLE?

The grid is a 3 phase distribution network but there is no conversion to single phase, the railway uses just one of the phases to supply each section. This does have consequences as the load on each phase in a perfect world should be the same. Quite a lot of engineering goes into ensuring any imbalance is minimised and accommodated in the distribution system.

Unusually the German Railways generate and distribute single phase through their own private distribution system largely for historical reasons to avoid problems like this.

 

[MattRobinson]

An induction motor's rotational speed is proportional to the frequency of the supply, meaning that the only way to vary its speed is to vary the supply frequency. That's the reasoning behind converting to DC and then converting back again.

In terms of 3-phase motors, I believe there was a team in the 90's whose aim was to implement 3-phase supplies to trains, which may merit research.

 

[Jonny]

An easy way around is to "tap across" two of the three phases, using a suitable transformer; many power supplies to farms do this with a transformer between two phases (of a three phase) on the primary and a two phase output with the middle of the secondary winding tied to earth/neutral and the phases at "180 degrees" to one another (my Dad's words, and he worked for the electricity board (as was) when many of these were set up), and IIRC some substations feeding into the ECML have a similar arrangement*.

* having cycled past the "Durham rail" substation (https://maps.google.co.uk/maps/place?ftid=0x487e87535f0e754b:0x33925aaa4dec4bef&q=54.768962,-1.606864&ved=0CA4Q-gswAA&sa=X&ei=pxZeUKi3HciZ8gPCqoC4BQ&sig2=kXHHd0hQUM8C27HWZYEi5g) (on the adjacent greenway, which for any pedants is separated from the railway by the boundary fence) and seen the wiring arrangement close-up.

 

[Old Hill Bank]

And this would lot would cause even more headaches to the S & T people.

Immunising the lineside kit against the effects of the traction supply and the associated return currents through the rail has been a significant cost of electrification schemes through years. Copper cables, Track Circuits, Axle Counter Heads etc......... all need to be assessed for EMC compatability anytime you introduce a new traction supply system or indeed train to any given route.

 

[apk55]

The problems of twin overhead wires and 10s of KV between them (to supply the power demands of modern trains) would be a task in itself. Trains would require a panagraphs for each wire and to get clearance between them would require wide clearances on each side so there would be problems with bridge arches etc. Then there is a problem a problem with points where one pantagraph would cross the other wire, so this would demand a a neutral section. Therefore all trains would have to have at least two pantographs on each wire. A look at the overhead on the the remaining 3 phase overhead electrified railways (such as the Swiss Gornagrat railway) shows the problem.
Could you imagine the the complexity of overhead wiring on the throut of a complex station? I would be interested to know how the Italians coped with their three phase electrification. I would also be interested in knowinig how they coped with triagular junctions or reversing loops, which would require long neutral section (remember twin pantographs) and a phase sequence relay - or did they not have any.

We nearly had a 3 phase electrified line in this coultry, the North Wales narrow guage line in Snowdonia.

 

[MattRobinson]

You could get around the whole issue of having two overhead wires by electrifying each rail to a different phase. But every wheelset in operation over those lines would have to be altered so that they were non-conductive, and it would be quite dangerous for people that were on or about the line (might solve the trespasser issue though...)

Sent from my HTC Sensation Z710e using Tapatalk 2

 

[JGR]

You could get around the whole issue of having two overhead wires by electrifying each rail to a different phase. But every wheelset in operation over those lines would have to be altered so that they were non-conductive, and it would be quite dangerous for people that were on or about the line (might solve the trespasser issue though...)

Sent from my HTC Sensation Z710e using Tapatalk 2

What happens when you get to a set of points? Also: track circuits, isolation, safety, etc?
Considering that just about every axle/wheel set ever made is a continuous metal block, this would never really work. A discontinuity in the axle would be a massive weak spot and potential failure mode.

Transmitting 3 phase AC is theoretically "better", but you'd still have to rectify it and run it through a VFD to match the current motor speed/phase, so there's really no point.
The only real problem with single-phase AC distribution is the phase-unbalanced load on the overall network, but there are ways to deal with this for the most part.

 

[jopsuk]

Whilst I quite agree that three-phase-via-the-rails is insane, the axle problem can be got around using a system such as the Spanish Talgo- these don't have axles across the train; indeed, the passenger corridor passes through where the axle would be.

 

[Teaboy1]

My understanding is that the rail OHL system is 3 -phase ... but in a very lengthy manner! Every 4-5 km or so there will be a phase 'splitter' or a section of copper where the phases change. Its an insulated bit naturally and allows the next phase to be utilies, hence when watching a train pass at speed there will be a nice 'flash' visible as the panto pickes up the next phase. Yes its 3-phase but with 3 single (1)-phase sections each about 4-5 km long. Put them all together and you have a modern railway layout. From an electrical point-of-view, engineers like to keep the phases balanced (equal load) so as not to disturb the balance.

 

[ffm]

That is not 3 phase, that is three single phases. I don't know but I see no rationale behind each section being a different phase, there is no engineering reason why this would be, nor why it would make a damned bit of difference which phase was used.

I am amused by the idea of 3 phase power distribution to a moving train, it would need 3 wires, three pantographs and sufficient conductive gaps - maybe up to a metre between phases at 25kV. Also the idea of the rail being one phase? this would need to an electric power supply voltage as the other phases, so a 25kV rail? ARGHH!!!!!! The rail can be the neutral return, yes. Remember that a 3 phase supply requires 4 cables - just look at the power cables on pole supplies, 3 live phases (thicker cable) and one return, the bottom thinner cable. Just accept that a moving vehicle is far simpler with a single phase, but motors like the constant of 3 live inputs (so 3 phase) by having 3 points of current input on their rotors so they only have a third of a turn between more power input.

 

[JGR]

That is not 3 phase, that is three single phases. I don't know but I see no rationale behind each section being a different phase, there is no engineering reason why this would be, nor why it would make a damned bit of difference which phase was used.

I am amused by the idea of 3 phase power distribution to a moving train, it would need 3 wires, three pantographs and sufficient conductive gaps - maybe up to a metre between phases at 25kV. Also the idea of the rail being one phase? this would need to an electric power supply voltage as the other phases, so a 25kV rail? ARGHH!!!!!! The rail can be the neutral return, yes. Remember that a 3 phase supply requires 4 cables - just look at the power cables on pole supplies, 3 live phases (thicker cable) and one return, the bottom thinner cable. Just accept that a moving vehicle is far simpler with a single phase, but motors like the constant of 3 live inputs (so 3 phase) by having 3 points of current input on their rotors so they only have a third of a turn between more power input.

Running separate sections on separate phases would be done to try to keep the overall load on the (3-phase) power network near to balanced.
Unbalanced loads can result in significant currents through the ground conductor and/or a non-zero ground voltage which are generally a pain and preferably avoided.

The main reason why just about all serious motors use 3 phase is that it very conveniently allows you to create a consistent smoothly rotating magnetic field, without needing any brushes/commutation/chunky permanent magnets/etc.
DC or single phase motors need to resort to commutation or clumsy side-coil arrangements, neither of which are particularly good for cheap, reliable and smooth traction.
Also, you'd need to alter the AC frequency to match the desired motor speed anyway, so you might as well just have three inverters and do it properly.

 

[HSTEd]

Ofcourse, with the advent of modern power electronics....
You could probably implement at 25kV DC supply if you really wanted to and simply use stacked chopper modules to produce a lower voltage.

Would have some benefits too, but too much legacy equipment at this point.

 

[Kali]

I did wonder why - other than convenience of supply - HV electric railway supply is AC anyway.

 

[HSTEd]

! The rail can be the neutral return, yes. Remember that a 3 phase supply requires 4 cables - just look at the power cables on pole supplies, 3 live phases (thicker cable) and one return, the bottom thinner cable.

But they don't require 4 cables, they require 3.

The idea with three phase is that the currents in the fourth conductor sum to zero and thus it is unneccesary.

And the reason three phase power is preferred for motors is because most motors these days are induction motors for reasons of cost and they cannot be started using a single phase supply, as they require a rotating magnetic field, and an alternating single phase field is the sum of two contrarotating fields.

This means that a) they won't generate torque when stopped and b) if something starts them moving they will generate rated torque in either direction.

 

[MattRobinson]

The reason polyphase supplies are useful is because, in a balanced system, the neutral current is zero. This means that you can distribute polyphase power with n phases with n wires, as opposed to single phase systems where you need 2 (n+1) wires for distribution. A lot of phase balancing goes on in real life: houses are on different phases because that means that no neutral is required (neutrals are tied together and to ground at the substation); different sections of OHLE are on consecutive phases, etc. It should be pointed out that the more phases in a system, the more efficient it becomes, but after 3, the efficiency loses out to cost of installing extra windings and cabling on the supply side of the power network.

Sent from my HTC Sensation Z710e using Tapatalk 2

 

[HSTEd]

Also with three phases there are only two ways to wire an induction motor, both of which will result in the motor running, and reversing any two of the connections will result in the motor changing direction.

This is not true for any other polyphase system.

 

[dosxuk]

Remember that a 3 phase supply requires 4 cables -

Three-phase delta supplies only use three conductors with no neutral. In a three phase system with a neutral (star), when the phases are properly balanced, the neutral will carry no current at any time.

 

[Cliveblackpool]

3 phase motors do not require a neutral.
A large motor is started on 'star', which gives the coils 240v, and when up to a reasonable speed switched to 'delta', the coils then get the full 415v.
The start current is very heavy, until the motor spins, hence this arrangement.
http://www.wikiplc.com/index.php?option=com_content&task=view&id=42&Itemid=34
3 phase Rectifiers often require a neutral connection as well.
DC rectified from 3 phase is a lot smoother than from single phase.

 

[Nym]

3 phase Rectifiers often require a neutral connection as well.

Not really...

Most good rectifiers include galvoelectronic isolation and/or star delta delta star transformers at 1:3^0.5 where a neutral tap would be taken from if required. (For 12 pole transformers)

Or 24 pole ones that I think are too complex to explain on here.

 

[ffm]

I didn't say that 3 phase motors, or supplies to a user have 4 cables, but the distribution network has.....if you really don't believe this just take a quick look at ANY overhead, pole supply, within towns...4 cables. 3 phases and a return.
Yes the cross of the 3 phases gives 0v but each phase requires a return to the grid, within a premises on one phase (220v) this is the neutral return, which connects to the 4th cable. Each one of the three phases uses this return. A good time can be had watching the arcing across the three live phase cables during heavy ice/snow, far less dramatic when the lower one touches the return.....

AC is used because of the greater transmission distances, DC supplies require more frequent supply inputs.

 

[Nym]

I didn't say that 3 phase motors, or supplies to a user have 4 cables, but the distribution network has.....if you really don't believe this just take a quick look at ANY overhead, pole supply, within towns...4 cables. 3 phases and a return.

No, they don't, the transmission grid does not have a 0v line, it has two three phase circuits and the line on the top is an earth.

Within towns this is put through delta star transformers to provide the neutral feed at a very local level, but that is not part of the transmission grid, this is only in local areas.

 

[MattRobinson]

No, when you look at an electricity pylon, the wire at the top is a protective earth (used primarily to draw lightning toward it rather than the phase connectors).

We're working on quite old technology: it wasn't efficient to convert between DC voltage levels, but with new DCC converters, it is now possible to transmit DC efficiently. In fact, this map shows some links where DC transmission lines are in use. There are some advantages to this, for example, the skin effect (where electric charges repel each other, resulting in the charge travelling through the outside of the wire) is minimised; the effects of capacitive and inductive losses are minimised; and, there is no synchronisation issue between different electrical networks.

I did a project on whether we should replace the current AC transmission/distribution system with a DC version. If anyone is interested, we came to the conclusion that we shouldn't, but we should install large AC-DC converters in strategic locations (such as one per house, street or office block) because one large AC-DC converter is more efficient than lots of little ones (think about computers in an office- you'd only have one power supply for all of them, which would be more efficient than each one having its own power supply unit); and we also came up with a new plug design so that devices could use both AC and DC without having two separate plugs, which could get quite confusing (think about things like washing machines, which require a motor to spin the drum/pump the water and DC to run the spin-cycle computer(s)).

 

[JGR]

No, when you look at an electricity pylon, the wire at the top is a protective earth (used primarily to draw lightning toward it rather than the phase connectors).

We're working on quite old technology: it wasn't efficient to convert between DC voltage levels, but with new DCC converters, it is now possible to transmit DC efficiently. In fact, this map shows some links where DC transmission lines are in use. There are some advantages to this, for example, the skin effect (where electric charges repel each other, resulting in the charge travelling through the outside of the wire) is minimised; the effects of capacitive and inductive losses are minimised; and, there is no synchronisation issue between different electrical networks.

I did a project on whether we should replace the current AC transmission/distribution system with a DC version. If anyone is interested, we came to the conclusion that we shouldn't, but we should install large AC-DC converters in strategic locations (such as one per house, street or office block) because one large AC-DC converter is more efficient than lots of little ones (think about computers in an office- you'd only have one power supply for all of them, which would be more efficient than each one having its own power supply unit); and we also came up with a new plug design so that devices could use both AC and DC without having two separate plugs, which could get quite confusing (think about things like washing machines, which require a motor to spin the drum/pump the water and DC to run the spin-cycle computer(s)).

The skin effect is almost irrelevant at main frequencies.
Whilst HVDC links are efficient, the equipment at each end is still more expensive than just using an HVAC link (within a single synchronised area).
Capacitative/inductive losses are not such a big problem (or did you mean power factor issues?). Most electricity is generated (and much of it is also consumed by) grid-frequency synchronous machines, and an intermediary DC stage is just overhead in such cases.

I'm a bit sceptical about the plug, but I'm sort of curious as to what sort of thing you've come up with.
That said, if you make the two sets of plugs different shapes, I'm sure that even the most unwashed of masses could work out which goes where.

Having a single DC supply to a house/office sounds good at first, however, modern devices require a plethora of different DC voltages over a significant range, at which points it becomes simpler/cheaper to just generate them on the spot. Also, the resistive losses start of LV distribution to become significant again, which is effectively what killed DC off as the preferred distribution method the first time round.
Unless you meant just supplying a single mains-peak level DC voltage, which would be a bit redundant as rectification is trivial, whereas the reverse is much less so.