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Discussion in 'Infrastructure & Stations' started by GRALISTAIR, 15 Jun 2012.
Agreed and also I doubt equipotential bonding would be red.
You are right with +/- 25 KV as these are Auto Transformer supply cables, the 3rd cable is a return cable. The 2 main cables are single core not multi core
So, to clarify - the current in the 2 black cables comes from Heyrod, and the current in the single red cable goes back to Heyrod?
(I'm aware how simple this makes it sound - I'm sure it's anything but!)
Alternating current does not work like that. The electrons in all three cables will just shuttle to and fro, 50 times per second, with no net movement in either direction. The current in the "+25kV" and "-25kV" cables will be in antiphase, meaning that the electrons in one cable move one way while those in the other are moving the opposite way.
The "return" cable will be connected to the centre tap of the 50kV transformer winding at Heyrod and will be pretty much at earth potential. It will carry the difference in current between the other two cables, so that the sum of the current in all three cables will be zero.
The "+" and "-" designations are confusing shorthand to distinguish between the two antiphase feeders. Both carry an AC voltage with no DC component. 25kV with respect to earth, 50kV relative to each other.
Ahhh I see; there's a reason I hold a degree in Civil & not Electrical engineering and that is it!
I've always had trouble understanding the "current" part of ATF cables if I'm being quite honest.
I still struggle slightly myself. ATF is also different than 3 phase supply. In 3 phase each phase is 2/3 pi radians out of phase with the other 2phases. The voltage between any phase and neutral/earth is the voltage of the phase but the voltage between any two phases is square root 3 (because 60 degrees is square root 3 divided by 2) multiplied by the voltage of the phase.
I think I got that right.
Correct, in AT just imagine the 2 sine waves exactly 180 degrees apart rather than th3 120 degrees in a 3 phase supply
The way you have written it is a little confusing, but I think you got it right. 415 = 240 * root(3). Nominally that is correct but when you get to high voltages electricity is really really weird.
I understand the principle of the Autotransformer, however, I don't see how the return cable in such a situation can be so much thinner. Yes it is nominly only conducting imbalance in the currents between the two main conductors, but surely it also has to deal with the potential fault current, and thus would have to at least match the carrying capacity in the other two? Hence why I assumed Fibreoptics.
Yes the return cable must withstand fault currents, but only for a short time before a circuit breaker trips. More likely it is the continuous rating that determines the cable size, particularly with these high voltage cables, which have to have thick insulation that traps the heat. Also the troughing restricts the flow of cooling air.
The maximum continuous current in the return cable will be less than in the HV cables, but also it does not need such a high voltage rating, so the insulation can be thinner, further reducing the outside diameter.
However, considering the length of this extension lead, I suspect that voltage drop limitations might drive the use of larger cables than the current rating requirements. Again, the lower current in the return would allow a smaller diameter/higher resistance.
The auto transformer system is similar to the US 3 wire system which has 2 live (hot) conductors with 220V between them and a central return (ground) conductor which gives 110V between any hot conductor and ground. The 220v is used for high power appliances with the 110V for lighting etc. If only 220V is used or the two 110V loads are equal there is no current in the ground cable and it only needs to be sized for out of balance 110V loads.
For the railway electification auto transformer system read 50KV instead of 230V and 25KV instead of 110V.
If both +25KV and -25KV loads are equal as with the autotransformer in circuit there is no current in the return and the conductor can be much reduced being mainly to be a ground reference and cope with minor imbalances.
May be of interest.
Tried best to explain it all.
Okay, sorry, this is probably going to be the dumbest question on the thread.
There's only one cable, so I'm assuming the return goes through the tracks? Is the return current as strong as the input current and capable of causing electricution? E.g on a DC tram system for example.
If not, on an AC system like this, with it switching from negative to positive does that not make it dangerous to come into contact with the rails. Thank-you for answering this painfully "noob" question.
Each of the two fat cables is at 25kV relative to the thin cable and the tracks. There is AC current in both of them, which reverses 100 times every second, but any instant when the current in one cable is flowing in one direction the current in the other one is flowing in the other direction. This means that the voltage between the two cables is 50kV.
At the Ordsall end each of the two fat cables will be connected to different sections of OLE (call them A and B) and the thin cable will be connected to the rails. So the trains will "see" 25kV between the pantograph and the rails. The current in the thin cable will be the difference between the currents being used by all the trains fed by A and all the trains fed by B. The feed areas are designed so that most of the time A and B will need about the same amount of current, so much less will flow in the thin cable.
The above is a highly simplified explanation and one of the things that makes it more complicated is the "voltage drop" along a cable or a rail when current is flowing through it. In particular the current returning through the rails causes a voltage drop so the voltage between the rails and earth increases further away from the substation. Various measures are taken to prevent keep this voltage below hazardous levels, including limiting the distances between the substations on a DC railway. It's less of a problem on AC because the higher supply voltage means the return currents, and therefore the voltage drops, are much less.
Overhead Line Electrification for Railways, by Garry Keenor, is a free publication that can be downloaded from http://www.ocs4rail.com/. It is an excellent resource that describes all aspects of electrification systems in comprehensive detail (including DC OLE).
Regarding mitigation of the shock hazard from the traction return current through the rails, it explains that AC systems employ equipotential bonding of the rails to the OLE masts, trackside cabinets and other nearby metalwork, with each mast locally earthed.
In DC systems the rails are normally insulated from earth to minimise galvanic corrosion, with the only earth connections in the substations. But, as @edwin_m explained, distances between the substations are short enough to avoid hazardous voltages between the rails and local earth.
Thank-you for your responses! Very informative.
Not quite... The +/-25kV is used to feed a series of autotransformers every 5-8km creating high current stable 25KV supplies from the output from each autotransformer that is less susceptable to voltage drop /spikes than previous systems.
As we live next to this line, we received a letter earlier this week form Network Rail, thanking us for our patience during the last 6-7 years of 'major improvements'.
My wife (whose only interest in the topic is that she's had quite a lot of disturbed sleep over the last few years due to late-night floodlights and heavy machinery, and that I've been making much more use of the main family car), asked "so what's improved then?"
Her: "are the trains quicker?"
Me: "erm, nope, not yet, but they might at some point in the future, maybe"
Her: "do they have newer trains?"
Me: "not really, they've replaced 30-year old Northern trains with 30-year-old repainted Southern trains, but we might get some new ones next year"
Her: "do these trains have more coaches or seats?"
Me: "nope, we used to have 4 or 6-coach trains, but now they're either 4 or 2 coaches"
Her: "are they more frequent?"
Me: "nope, about the same, if not slightly less frequent"
Her: "and am I right that they go to fewer places?" (she's aware that our daughter used to get direct train to Uni at Glasgow, but this has now ceased)
Me: "yep, fewer destinations"
Her: "are they any more reliable?"
Me: "nope - if anything they've been much less reliable in recent years"
Me: "some of the trains are less polluting now"
Her: "but you travel by diesel car most trips now, cos the trains are so unreliable, and so does..." (she rhymes of 3 other friends/acquaintances who've completely given up on rail travel)
Me: "suppose so"
Her: "well that was a bit of a waste of time and money then"
I'd been quite positive about the work finally finishing, but put like that, I'm starting to agree with her.
Having traveled from Manchester to Lancaster at 1730 yesterday I can see why people might think the net benefits of the scheme are modest.
FYI the Blanket 75mph ESR will be withdrawn on Sunday
Is it too late for TPE to instate Bolton calls?
I believe this was the last thing they claimed to be waiting on.
If they do it'll be STP diagrams as the LTP diagrams are out with no trains calling at Bolton.
...à la 1S92 21:05 Manchester International Airport to Glasgow Central High Level
(Although this was added before the ESR removal)
I'm not massively familiar with the STP and LTP. What do they mean? Thanks again.
STP= Short Term Panned ie different from the daily published working Timetable
LTP = Long Term Plan ie the published Working Timetable for that period.
You can also have VSTP = Very Short Term Planned ie almost planned on the hoof due to disruption etc
Oh okay. Hopefully they will act and not be like "ohohoh it'll have to wait until December now" *chuckles in franchise breach*
As I understand it STP services are subdivided into "New" and "Overlay" classes, flagged as STP and VAR respectively in RTT. "Overlay" is a minor variation to the schedule of a service in the Working Timetable (WTT), keeping the same headcode and schedule Unique Identification code (UID). "New" can be a service that is additional to the WTT, or a replacement for a service in the WTT.
I guess the Bolton calls could be added to the TPE services by means of STP overlays that persist until the December timetable change.
Came south on the Trans Pennine Express from the Lakes and we were late from Preston, very fast to Bolton, slow through station and three minutes early at Deansgate where we then waited for Oxford Road to clear. So Bolton stops an easy quick win.
Looking at today's RTT almost all of the TPE Bolton services have been early or made up significant time.
Any more electric services running to Bolton after the new timetable change, or are we still waiting for the extension lead to be unwound and plugged in?
From what I could see around the evening peak it was a full electric service between Euxton and Lostock Junction. The northbound TPE became delayed as it was stuck behind the Man Vic - Preston stopper. I was a little surprised it didn't make use of the new platform at Bolton to overtake.
Perhaps it should be renamed the Grayling lead