GWML Electrification - getting the electric into the wires above the trains

[whole diary]

[Henry]

 'Dangly Bits'.

 Now that's the technical jargon I can understand !   

[Electric train]

Good to know National Grid aren't behind schedule! 

Actually I think they are, quietly relived that NR^» (Network Rail - home page) don't want to take power for a while

[gwr2006]

These may seem like daft questions but with so much now appearing at the trackside I hope some of the experts here will be able to answer them.

1) There are four tall posts either side of the tracks just west of the old line into Didcot A Power Station. Are they part of the neutral section or is that nearer to Foxhall Junction?
2) Maybe these masts that will eventually supply power from Didcot ATFS to the OLE^▸ (Overhead Line Equipment, more often "OHLE") west of Didcot as far as Wootton Bassett Junction? There is a cable trough back to the Didcot ATFS, and another one to a similar set of posts near Didcot North Junction for the Oxford lines.
3) Does this mean Didcot ATFS provides a power source for three sections - 1) Maidenhead MPATS^▸ (Mid Point Auto Transformer Site - electrification) to Milton, 2) Milton to Wootton Bassett and 3) Didcot to Oxford?
4) Am I right in thinking there are only neutral sections where the source of power changes from one ASTF to another?
5) Does anyone have a drawing showing the general arrangement of a GWEP^▸ (Great Western Electrification Program) neutral section?
6) Finally, with an ATF system, does +25Kv phase flow through the OLE and -25Kv phase flow through the ATF cable and they come together at an ATS^▸ (Automatic Train Supervision), get boosted, and then repeat that to the next ATS and so on?

[paul7575]

These may seem like daft questions but with so much now appearing at the trackside I hope some of the experts here will be able to answer them.

5) Does anyone have a drawing showing the general arrangement of a GWEP^▸ (Great Western Electrification Program) neutral section?
6) Finally, with an ATF system, does +25Kv phase flow through the OLE^▸ (Overhead Line Equipment, more often "OHLE") and -25Kv phase flow through the ATF cable and they come together at an ATS^▸ (Automatic Train Supervision), get boosted, and then repeat that to the next ATS and so on?

5.   There was a pdf guide quoted in the other thread the other day,  http://www.railwaysarchive.co.uk/OLE/
It describes the GW^▸ (Great Western) series 1 OHLE as using a "carrier wire neutral section" on pages 88/90.  I think because there are overlapping electrically isolated sections of contact wire over a significant distance this explains why they look so complex compared to inline insulators.

6.  AIUI^▸ (as I understand it) there is no boosting as such.  What the auto transformers do is to share the current 50/50 between the contact wire and the autotransformer feeder, (ATF).   Again, if you look at the section of the guide around pages 46/47 you can get an idea.   Because the current flowing in the contact wire and ATF are halved, then the transmission losses are significantly reduced because they are proportional to I².

Paul
 

[stuving]

1). Those sets of tall posts in pairs appear at all lineside sites so, even without ever seeing how one is wired, they obviously do make the electrical connection(s) to the OLE^▸ (Overhead Line Equipment, more often "OHLE"). At an ATS^▸ (Automatic Train Supervision), the autotransformer(s) just connect to the catenaries and ATFs. At an MTAPS (e.g. Maidenhead) there is a section break, left open in normal operation, needing switchgear and AT connections both sides of it. I can see four post pairs on the London side and two on the Reading side.

An SATS is in theory the same as an MPATS^▸ (Mid Point Auto Transformer Site - electrification), but with the break normally closed and only opened during fault/maintenance operation. An ATFS does not need a section break, or even ATs (e.g. Kensal Green). But for either of those at a junction, extra section breaks (and perhaps ATs) are needed. I have no idea where they are being put at Didcot or Reading, though there is more than enough ironmongery to make one around Kennet Bridge.

4). As implied above, you need breaks to separate power from separate feeds in all feeding modes that may arise. Only a few breaks are open normally, and how many others are provided will be the result of a "nice to have" versus "not on my budget" discussion.

5). When a pantograph transfers from one contact wire to the next, it connects them. You can't allow that at a section break, so there has to be an extra insulated wire run in between. How much length this takes is subject to a lot of variation.

6). There's no + and - involved, just two feeds of power opposite in phase (antiphase). That's like two pistons 180^o apart on the crankshaft, or the directions your arms point if you stick them out sideways and twirl round until you fall over. I think it's simplest to see the ATF as a second wire that can carry a second, equal, reserve supply of power to top up the line feeds. Between ATs, trains draw power and the line voltage drops. By the magic of magnetic induction, the autotransformer responds to that voltage drop by taking power from the ATF, inverting its phase, and topping up the line catenary wires.

This inversion of phase results in similar currents in the two overhead feeds, but in antiphase (they would cancel if added) so only a small current flows to earth via the rails. That's useful, as too much earth/rail return current is a nuisance. (I think I² and V² are red herrings here.)

[Electric train]

These may seem like daft questions but with so much now appearing at the trackside I hope some of the experts here will be able to answer them.

1) There are four tall posts either side of the tracks just west of the old line into Didcot A Power Station. Are they part of the neutral section or is that nearer to Foxhall Junction?

The feeds to the OLE^▸ (Overhead Line Equipment, more often "OHLE") and ATF go on "across track" span wires from the OLE lineside switches to "drapes" that connect to the OLE  there are insulators segregate it all.  These are place well above the OLE to allow for a cross track span wire being alive when part of the OLE below it is isolated & earthed to work on

2) Maybe these masts that will eventually supply power from Didcot ATFS to the OLE west of Didcot as far as Wootton Bassett Junction? There is a cable trough back to the Didcot ATFS, and another one to a similar set of posts near Didcot North Junction for the Oxford lines.

To allow for parts of the Didcot triangle to be isolated yet maintain a feed north to Oxford cables are run at ground level, in BR^▸ (British Rail(ways)) days "bare feeders" were used on the side of the OLE structures however this causes problems in "emergency" isolations because the bare feeders have to be switch off as well.

3) Does this mean Didcot ATFS provides a power source for three sections - 1) Maidenhead MPATS^▸ (Mid Point Auto Transformer Site - electrification) to Milton, 2) Milton to Wootton Bassett and 3) Didcot to Oxford?

Yes that's the plan
It would be possible to feed Oxford from Wooton Basset in the event of a total loss at Didcot, however this would be second emergency feeding and would impose major timetable restrictions

4) Am I right in thinking there are only neutral sections where the source of power changes from one ASTF to another?

There are Neutral sections ate MPATS eg Maidenhead and ATFS eg Didcot.  When Bramley is built there will be Neutral sections at Reading

The neutral sections are needed for two reasons, 1) often the supply to the Grid transformers are taken from different phase pairs to allow for some balancing of the loading across the Grid, 2) to prevent power flow between Grid sites via the Railway OLE system.

5) Does anyone have a drawing showing the general arrangement of a GWEP^▸ (Great Western Electrification Program) neutral section?

The GWEP neutral sections are what are called carrier wire neutral sections, basically 3 over lap section of OLE, a floating section , an earthed section, a floating section.  The pan transitions form the live wire to
the floating section, then the earthed section to the floating section and onto the live section.

There were trials done to see if the earthed section could be done away with, not sure of the results of this.  The carrier wire neutral section does have by pass switches in the event a train gets stranded in a floating or earthed section

6) Finally, with an ATF system, does +25Kv phase flow through the OLE and -25Kv phase flow through the ATF cable and they come together at an ATS^▸ (Automatic Train Supervision), get boosted, and then repeat that to the next ATS and so on? [/list]

The + 25kV and - 25kV terms were used in the early days of the WCML^▸ (West Coast Main Line) conversion to AT to make it easy to explain to the lay railway(wo)man.   The system is 25 - 0 - 25kV the train only ever see 25kV but the power is distributed at 50kV there by reducing the I²R losses

[Western Pathfinder]

Thanks chaps my head hurts less now.

[rogerw]

Mine hurts more 

[gwr2006]

Thanks to Electric train, stuving and paul7755 - I knew you guys would come up trumps!  Thanks for explaining it all - its slowly sinking in.

[TonyK]

Somebody had to invent all this stuff from nothing but a few equations, maybe developing a few more equations on the way!