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

[whole diary]

[ellendune]

So it is like this.



Source Wikipedia

The primary on the main feed transformer is between two phases and the return is connected to the centre tap in the secondary (which is also earthed?).

Sorry images defeat me follow the link

Admin Note: Link to image fixed - bobm

[stuving]

There are 3 Grid supply points on the English side of the Seven, Kensel Rise, Didcot and Melksham; all will have 2 Grid transformers the 2 transformers at each Grid site will be split across phase pairs eg one will be R - Y the other will be B - Y.

So the 50kV is between two phases and the 25kV/0/25kV is phase - neutral - phase (e.g. Red - Neutral - Yellow or Blue - Neutral - Yellow)?

Polyphase terminology can be confusing. You might reason that three-phase has three voltages equally spaced over a whole cycle, so 120 degrees apart. So two-phase would be two voltages 180 degrees apart, right? No, that means they are 90 degrees apart. The transmission link from Melksham to Thingley does have two voltages 180 degrees apart, but that's still single phase.

The reason why is that each phase comes from a winding, in a transformer or generator. So each has two ends, and until you connect one end (or the middle if it's available) to something its phase is ambiguous - the ends are 180 degrees apart in phase. With three phases you can connect one end of each winding to a common point, almost always earthed to define the peak voltages and for protection purposes. Your transmission line can then convey all three phase voltages and, optionally, the common as well. But you can also connect the three windings in a ring, which gives three phases and no common point.

The point here is that every phase can also supply its antiphase, so the three phases are really 60 degrees apart by dividing 180 degrees by three. So two-phase is 90 degrees apart. In this case you can't connect them in a ring, you need four - because there is no two-sided polygon (think about it).

The idea of running phase and antiphase conductors plus a common is (or was) very common with DC^▸ (Direct Current). It's called a three-wire system (I think the AC one can be as well). Since you can't make a transformer for DC, doubling up is the most you can manage to raise the voltage for efficiency. London Underground used it a lot, and still may in some places.

It also happens in North America for domestic electricity supply, where the practice was set by Edison's DC system before Westinghouse and Tesla won that war. Most individual houses have a pole-mounted transformer, with a 120V/0/120V centre-tapped secondary. The two "hot" wires are red and black (from DC practice), and the centre is white and earthed at the pole (but not very well). Big fixed loads like heaters, aircon, cookers, are run off 240V between red and black. Socket circuits are run off one or other of the "hot" feeds and common, so should use red/white or black/white installed cable. And that's why US flex uses black/white - though why they chose that and not red/white I have no idea.

Installation practice seems to have been based on the idea that 120V is "safe" - so in our terms it is often scary. True it is safer, but a row of sockets above the bath, for example (I saw that in Canada) puts them in one place where 120V can certainly kill you. So more recently houses have to be protected by a green (or in cables bare) ground wire, usually going to a lump of buried metal. Sockets can also be protected by a GFCI (ground-fault circuit interrupter), though not (AFAIK^▸ (as far as I know)) whole houses (yet).

The laws of physics and of physiology are universal, so the diversity of electrical wiring rules and practices is pretty amazing.

[Oxonhutch]

Big fixed loads like heaters, aircon, cookers, are run off 240V between red and black. Socket circuits are run off one or other of the "hot" feeds and common, so should use red/white or black/white installed cable. And that's why US flex uses black/white - though why they chose that and not red/white I have no idea.

Certainly how my British kettle was wired during our five year sujourn in the States, plus all my workshop equipment that I took there with me.

Black is the normal live or 'hot' even on the anti-phase circuits - Red only appears in 3-wire+E cable. Black-white-green solves the colour blind problems associated with red. My challenge with each new house was to find the nearest source of (co-breakered for safety) 120V+120V=240V.  Sometimes it was a light fitting (!)*, sometimes I had to go all the way back to the DB^» (Deutsche Bahn - German State Railway - about).

*Americans do not differentiate between lighting and power circuits - that took some getting used to. 

[Electric train]

Big fixed loads like heaters, aircon, cookers, are run off 240V between red and black. Socket circuits are run off one or other of the "hot" feeds and common, so should use red/white or black/white installed cable. And that's why US flex uses black/white - though why they chose that and not red/white I have no idea.

Certainly how my British kettle was wired during our five year sujourn in the States, plus all my workshop equipment that I took there with me.

Black is the normal live or 'hot' even on the anti-phase circuits - Red only appears in 3-wire+E cable. Black-white-green solves the colour blind problems associated with red. My challenge with each new house was to find the nearest source of (co-breakered for safety) 120V+120V=240V.  Sometimes it was a light fitting (!)*, sometimes I had to go all the way back to the DB^» (Deutsche Bahn - German State Railway - about).

*Americans do not differentiate between lighting and power circuits - that took some getting used to. 

From my limited understanding of USA wiring, they retained the old dc colours ie + is red - black and 0 is white because they have 115-0-115 V system.

Back to real power stuff here in the UK^▸ (United Kingdom).
One of the requirements now is to keep touch potentials below 50V and in the case of services (like station domestic supplies from the DNO^▸ (Distribution Network Operator)) we are not allowed to export a rise in earth potential of 25V this is challenging for what is an "earthy" return system.   

[DidcotPunter]

Mid way between Grid sites the is a MPATS^▸ (Mid Point Auto Transformer Site - electrification) (Mid Point Auto Transformer Site) Maidenhead is the MPATS between Kensal Rise and Didcot so there will be Neutral Sections at Maidenhead MPATS.  It is also the boundary between 2 different protection schemes (by protection I refer to electrical over current etc) which need to interface with each other. Why different ones well the Crossrail scheme was developed and tendered a few years a head of GWEP^▸ (Great Western Electrification Program), Crossrail will a similar AT protection scheme to that used on the WCML^▸ (West Coast Main Line) and GWEP is "new and novel"

So presumably there will be another MPATS around Swindon between the Melksham and Didcot Grid sites?  Also, what do the switching stations do that are going in at places like Pangbourne, Wantage Road and Uffington? Do these simply break the line into electrical sections or do they also provide a feed from the -25kV line?

[Electric train]

Mid way between Grid sites the is a MPATS^▸ (Mid Point Auto Transformer Site - electrification) (Mid Point Auto Transformer Site) Maidenhead is the MPATS between Kensal Rise and Didcot so there will be Neutral Sections at Maidenhead MPATS.  It is also the boundary between 2 different protection schemes (by protection I refer to electrical over current etc) which need to interface with each other. Why different ones well the Crossrail scheme was developed and tendered a few years a head of GWEP^▸ (Great Western Electrification Program), Crossrail will a similar AT protection scheme to that used on the WCML^▸ (West Coast Main Line) and GWEP is "new and novel"

So presumably there will be another MPATS around Swindon between the Melksham and Didcot Grid sites?  Also, what do the switching stations do that are going in at places like Pangbourne, Wantage Road and Uffington? Do these simply break the line into electrical sections or do they also provide a feed from the -25kV line?

The classic way the track has been sectioned in the UK^▸ (United Kingdom) on 25kV railways is a circuit breaker ever 7 miles for each track at both ends.  Circuit breakers in this form have been deemed to expensive so a system based on the French Railways of sequential isolators is being used, basically the track breakers every 7 miles have been replaced with isolators, the event of a fault the main circuit breaker will trip the protection scheme will know where the fault is (because each isolator has fault detection) it open the isolator and recloses the main breaker................. all this in literally seconds from fault detection to clearing faulty section to reenergise healthy sections.

The French do theirs manually at the control room in the UK we will be using RATS which is a BS EN 61850 (Communication networks and systems for power utility automation) based system

[stuving]

Mid way between Grid sites the is a MPATS^▸ (Mid Point Auto Transformer Site - electrification) (Mid Point Auto Transformer Site) Maidenhead is the MPATS between Kensal Rise and Didcot so there will be Neutral Sections at Maidenhead MPATS.  It is also the boundary between 2 different protection schemes (by protection I refer to electrical over current etc) which need to interface with each other. Why different ones well the Crossrail scheme was developed and tendered a few years a head of GWEP^▸ (Great Western Electrification Program), Crossrail will a similar AT protection scheme to that used on the WCML^▸ (West Coast Main Line) and GWEP is "new and novel"

So presumably there will be another MPATS around Swindon between the Melksham and Didcot Grid sites?  Also, what do the switching stations do that are going in at places like Pangbourne, Wantage Road and Uffington? Do these simply break the line into electrical sections or do they also provide a feed from the -25kV line?

Well, sort of. This diagram from ABB, who are installing all this stuff, shows the plan. The English feeders are at Bramley, Reading, Didcot, Royal Wootton Basset, and Thingley Junction.

While ATS^▸ (Automatic Train Supervision) come in four flavours - plain (ATS), sectioning (SATS), feeder (ATFS), and mid-point (MPATS) - practice does seem to have evolved separately from terminology (again). So while an ATFS feeds one, two, or more sections from the end, and most sections have feeders at both ends, there exceptions to that. MPATS are now only for major boundaries, and within each section there are ATS and SATS. One SATS per section might be expected, but again that's not what happens. I suspect that sections are short enough that, in effect, the mid-point is replaced by the next feeder.

But Wales does seem to be different - three ATFS in a row, and a TSS (not sure what that does). Multiple feeders may be for security of supply, and happens elsewhere (e.g. the three on HS1^▸ (High Speed line 1 - St Pancras to Channel Tunnel) at St Pancras, Stratford, and Barking). But why are there MPATS at Filton and Westerleigh?

Obviously the actual design is based on many extra unknown (by me, anyway) factors. I'm still puzzling over how the AT feed arrangement allows you to replace the return conductor by a screen conductor, and how (or if) it acts to reduce earth return currents. I can see how a booster transformer does that - now there's another bit of fossil terminology; the last thing it does is "boost" anything!

[paul7575]

I'm still puzzling over how the AT feed arrangement allows you to replace the return conductor by a screen conductor, and how (or if) it acts to reduce earth return currents.

AIUI^▸ (as I understand it) the vast majority of the return current flows in the other (anti phase) 25 kV line.   

If for comparison you think of a 3 phase star connected supply with a balanced load, the neutral conductor is effectively carrying no current.

[onthecushions]

Very many thanks to stuving for the GW^▸ (Great Western) network circuit diagram and for ellendune's AT system diagram.

I'm surprised to see the extra feeder points as I understood these had to be taken from the 400kV supergrid at some expense or proximity to a substation with a spare transformer bay. Presumably there are some long buried cables. Where is the Reading feed to come from?

The weakness of the old system was that it couldn't cope with excessive traffic, with line volts dropping to 19kV at times, close to an undervoltage trip. Modern transformers (x4) allow for less regulation (i.e. voltage drop at the transformer). Ellendune's diagram shows how the imbalance of load being drawn across one side of the 25-0-25 system should be opposed by the AT, pushing up the OHL^▸ (Over-Head Line) voltage and balancing the earthed neutral track potential. The problem there is that a lot of electrons have to flow from point 7 to points 10 and 4, raising the track voltage away from the earthing points at the transformers. I've read of this being measured at above 100V at times. Perhaps that's why there are quite a number of close  AT's.

Can't wait...

OTC

[stuving]

I'm surprised to see the extra feeder points as I understood these had to be taken from the 400kV supergrid at some expense or proximity to a substation with a spare transformer bay. Presumably there are some long buried cables. Where is the Reading feed to come from?
OTC

When the environmental studies were done, Wiltshire were told this:

17.2.5 As part of the Great Western Main Line (GWML^▸ (Great Western Main Line)) electrification in Wiltshire, Network Rail is proposing a high level ^bare feed^ electricity cable (i.e. the electricity cable would not be insulated), running from the proposed feeder station at Thingley Junction, eastwards along the line via Chippenham to Wootton Bassett Junction. This high level feed (HLF) cable will allow electricity from the Thingley Junction feeder station to reach the South Wales Line when the circuits on the Bath line are isolated. The gantries for the HLF will be separate structures to the Overhead Line Equipment (OLE^▸ (Overhead Line Equipment, more often "OHLE")) catenary system masts, with an estimated height of 15 m for the HLF stanchions. The distance between stanchions would be dependent on wire tensions and sag requirements; and the distance will range from 50 m up to 200 m. With regards to structures that cross the track, it is proposed to have a higher stanchion that supports the HLF over structures such as bridges, although this will be assessed on a case by case basis.

I'm not sure if that's still the plan, or if they would have to bury a cable - it's about 22 km. Bramley and Didcot are both grid station sites, but as you say Reading is not. However Bramley is only about 14 km from Reading, depending on the exact feed station siting, so the same method should work here too.

[onthecushions]

I'm not sure if that's still the plan, or if they would have to bury a cable - it's about 22 km. Bramley and Didcot are both grid station sites, but as you say Reading is not. However Bramley is only about 14 km from Reading, depending on the exact feed station siting, so the same method should work here too.

This shows how much lower a Voltage drop/m a modern system achieves, even with higher current. The ECML^▸ (East Coast Main Line) feeders (at 25kV) could reach a maximum of 18 miles/29km, or 36/58 with a 45mph speed limit.

OTC

[Electric train]

My understanding is the 400kV Grid supply to Didcot ATFS (Auto Transformer Feeder Station) was commissioned this weekend (8/9 Aug 2015) not heard if it was fully successful yet.  All that is needed now is the knitting strung up 

[JayMac]

Good to know National Grid aren't behind schedule! 

[Chris from Nailsea]

They may be: we just haven't heard about it yet. 

[Oxman]

I noticed at Pangbourne a few days ago that some dangly bits had been fixed to some of the masts there. Only seen from the road under the Whitchurch Road bridge, so no idea how much has been achieved.