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Questions about Neutral Sections

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kc_

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On EMUs with more than one pantograph and a 25 kV bus (e.g. 390, 397), does the VCB (vacuum circuit breaker) isolate the pantograph from the bus, or the bus from the transformer? If it's the latter, I guess the APMS (automatic power management system) magnets trip all the VCBs at the same time?

I've noticed that on 397s, at-seat power and aircon continued operating whilst going though the OHNS (overhead neutral section). Is that possible? Surely, they're not running those off the battery? I'd have thought at-seat power was non-essential so shouldn't be a drain on the battery.

I've also noticed the sound of the traction motors kick-in a few seconds earlier than aircon and seat power coming back on 350s. What's likely happening here?
 
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PudseyBearHST

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The APC (automatic power control) receiver opens the VCB on the pantograph (transformer) vehicle. This then forces the other VCBs in the bus line to open. The VCB on the other pantograph (transformer) vehicle on the same bus line will be open throughout as this pantograph will be down. (You can’t have more than one pantograph up in the same bus line)

When the VCB closes again after the neutral section, the traction packs for the vehicles will energise shortly after. About 15/20 seconds later when the auxiliary converters kick in, the fans will turn on.

This is my understanding, apologies if I’ve made any mistakes.
 

37057

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It's possible that capacitors in the auxiliary DC link keep power going through neutral sections.

Also when units are in multiple, VCBs synchronise to open but there is a delay on each closing.

On 350s the traction converters control the main transformer so the hierarchy appears to be traction first, auxiliaries second! I'd have to go through the schemes to understand it better myself.

Here's an APC demonstration showing the auxiliary converter on a 350...

https://www.flickr.com/photos/24784661@N05/47936722603/in/album-72157712317156882/
 
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driver9000

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I've noticed that on 397s, at-seat power and aircon continued operating whilst going though the OHNS (overhead neutral section). Is that possible? Surely, they're not running those off the battery? I'd have thought at-seat power was non-essential so shouldn't be a drain on the battery.

I've also noticed the sound of the traction motors kick-in a few seconds earlier than aircon and seat power coming back on 350s. What's likely happening here?

On the 397 when the VCB opens 1 motor keeps turning to retain the power to the sockets and HVAC, listen when the train goes through a neutral section slowly and you'll hear the motor whine and it provides a slight braking effect at low speeds. It doesn't last too long and will shut down if the VCB doesn't close within a set time. The reason it all kicks back in earlier than a 350 is because the 397 doesn't have the 20 second time out after a neutral section while the computer self checks, traction power is restored virtually instantly too.
 

PudseyBearHST

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Thanks for the explanation.


Looks like I've just made-up an acronym.

No problem, not sure if it’s a traction specific thing, etc...

Some more ‘fun’ facts (again traction specific): Secondary windings of the transformer are for traction packs (475
V AC); tertiary windings of the transformer are for auxiliary converters (900V AC). If a VCB fails to close/transformer isolated then for all the vehicles associated with that transformer vehicle (For example. transformer vehicle may supply 3 vehicles), traction packs for those vehicles will obviously not energise. However, I believe power can be supplied from a neighbouring transformer in the bus line just for essentials such as lighting and air conditioning.
 

driver9000

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It's possible that capacitors in the auxiliary DC link keep power going through neutral sections.

Also when units are in multiple, VCBs synchronise to open but there is a delay on each closing.

On 350s the traction converters control the main transformer so the hierarchy appears to be traction first, auxiliaries second! I'd have to go through the schemes to understand it better myself.

Here's an APC demonstration showing the auxiliary converter on a 350...

https://www.flickr.com/photos/24784661@N05/47936722603/in/album-72157712317156882/

The Auxiliary Power Supply (ACU on the 350) is fed from the TCU on the 397.
 

twpsaesneg

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Note that units with a common bus only run with a single pantograph up at any time, otherwise you could bridge a neutral with some lovely firework type effects!
 

supervc-10

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How long are those neutral sections? And I presume there's a minimum speed required to coast through them?
 

PudseyBearHST

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How long are those neutral sections? And I presume there's a minimum speed required to coast through them?

Must confess, I can’t remember the exact lengths but they are not very long- I think roughly about 15feet long but then there is also some distance from the APC magnets to the actual neutral section part.

Trains can roll a long way so unless you are doing a low speed and on a (steep) rising gradient, you should be ok. Some neutral sections are very close to red signals but even if you’re doing 5/10mph through the neutral section (no noticeable gradient), you should be easily ok. There is one example I can think of with a bit of an up hill gradient and a red signal after the neutral section so you can build up a bit of speed pass the signal yellow (e.g. 25mph) so that you won’t get stuck in the neutral.

If you get stuck, depending on traction, you may able to switch pantographs. E.g. use the rear pantograph instead of the front pantograph. Or you may able to switch mode of power such as Diesel engines in bimodes. Otherwise the train needs to be rescued.

As an additional point regarding minimum speeds for coasting, during (high speed) coasting when there is a problem with the overheads, Network Rail/signallers would ensure that they stop you at a far enough distance from the problem area such that you are able to build up enough speed to get through the coasting section. Obviously they would take into account that you’re not due to stop in the area, gradient, visibility, ensuring signals are all cleared, winds, etc...
 

Llama

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Fifteen feet sounds about right for the earthed section. I seem to remember that 155 feet was the standard length of the whole thing but that probably varies nowadays.
 

causton

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How long are those neutral sections? And I presume there's a minimum speed required to coast through them?

The other day, a trio of 319s lost power at Leighton Buzzard neutral section and coasted all the way to Tring. Also a while back a train coasted from Bushey(?) area all the way into Euston!

Trains can coast a long way, so that is not really an issue :)
 

Llama

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It's possible that capacitors in the auxiliary DC link keep power going through neutral sections.

Also when units are in multiple, VCBs synchronise to open but there is a delay on each closing.

On 350s the traction converters control the main transformer so the hierarchy appears to be traction first, auxiliaries second! I'd have to go through the schemes to understand it better myself.

Here's an APC demonstration showing the auxiliary converter on a 350...

https://www.flickr.com/photos/24784661@N05/47936722603/in/album-72157712317156882/
Interesting about VCBs synchronising on 350s and it is easy to see why, but I suspect that's not the norm and whether they're synchronised or not it's good driving practice to wait for all units to pass through and VCBs close before applying traction power. 331s open their VCBs individually, and 319s are a bit more 'analogue' so they open theirs individually too.
 

37057

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Interesting about VCBs synchronising on 350s and it is easy to see why, but I suspect that's not the norm and whether they're synchronised or not it's good driving practice to wait for all units to pass through and VCBs close before applying traction power. 331s open their VCBs individually, and 319s are a bit more 'analogue' so they open theirs individually too.

I'll have to check my notes to be sure about the correct operation or next time I have a pair multi on depot I'll have a play!
 

37057

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I take it then that the 230 V comes from an inverter rather than just using transformers to step down the incoming 25 kV?

The 230v equipment (plug sockets and toilet hand dryers etc) work off the 400v three phase system. Two phases are used via an isolating transformer.
 
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twpsaesneg

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How long are those neutral sections? And I presume there's a minimum speed required to coast through them?
The inline Neutral Sections are around 6m long (Don't have the drawing to hand to check) but the APC magnets are set a greater distance before and after based on linespeed, so the area in which the VCB is dropped out is longer than the actual earthed section.
The locations are sometimes problematic to design, as they have to be located far enough from a stopping location (signal or station) for the heaviest train to be able to start from a stand and coast through without stopping in the dead area.
In addition, there needs to be enough standage beyond for the longest train to stop at the next stopping location without the pan coming to a stand in the dead area. There are a set of design rules to enable signalling and OLE designers to determine the optimum location, these cover traction types, linespeed and gradient amongst other factors.
Carrier wire neutrals as introduced on GWEP have even longer dead areas but are supposed to be more robust and take higher speeds than Inline units.
 

supervc-10

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Thanks everyone! Does 3rd rail land have them too? Unlike with a long pantographed units (think Pendolino) where only one panto would be up, don't most (all?) 3rd rail units have shoes on the first and last bogies?
 

driver9000

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I'll have to check my notes to be sure about the correct operation or next time I have a pair multi on depot I'll have a play!

In practice the VCB on 350s in multiple operate separately when passing through a neutral section. This is seen by the VCB lamp illuminating twice when passing through the section. If the CPBC is opened before the time out then power goes to half then full when the rear set has its power restored. Synchronisation of the VCB is when raising and lowering the pantographs which is why we isolate the VCB synchronisation switch when hauling a dead set. I'm not a maintenance technician but that is how they work from a driving perspective.
 

swt_passenger

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Thanks everyone! Does 3rd rail land have them too? Unlike with a long pantographed units (think Pendolino) where only one panto would be up, don't most (all?) 3rd rail units have shoes on the first and last bogies?
Not in the same way. In principle the DC side of the 3rd rail network operates fully cross-connected normally, unlike on an AC Network there’s not the same electrical requirement to prevent connecting different supplies.
 

najaB

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Trains can coast a long way, so that is not really an issue
Indeed. I remember a recent ECML de-wiring that saw trains coasting through a section well over a mile long. I think it was near Retford.
 

Alfie1014

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Also happened a few years ago on the GE during planned engineering works on the DM between Chadwell Heath and Gidea Park neutral sections I think. Could only use cl321s as the more sophisticated control equipment on the cl360s didn’t like loss of current!
 

edwin_m

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Thanks everyone! Does 3rd rail land have them too? Unlike with a long pantographed units (think Pendolino) where only one panto would be up, don't most (all?) 3rd rail units have shoes on the first and last bogies?

Not in the same way. In principle the DC side of the 3rd rail network operates fully cross-connected normally, unlike on an AC Network there’s not the same electrical requirement to prevent connecting different supplies.
Indeed. The reason for only one pantograph on AC is that the neutral sections separate different "phases" where (to simplify considerably) when the alternating voltage is at maximum on one side of the neutral section it won't be on the other. So large currents could flow between the phases if they were electrically connected.

DC doesn't alternate so doesn't have this problem and doesn't need neutral sections except as below. Third rail units on Network Rail have 750V cables connecting the shoes each end with the traction equipment, so only one shoe needs to be in contact for the whole unit to have power.

There are however "neutral sections" where London Underground trans run onto Network Rail infrastructure, near Queens Park and on the Richmond and Wimbledon branches. These Network Rail sections have a fourth rail like LU, but it is at the same voltage as the running rails so third rail stock can also operate. On LU the fourth rail is at a couple of hundred volts below that of the running rails, and the voltage between the third rail and the running rails is also different, so interconnecting these through a train would cause a short circuit. Hence there has to be a section with no supply at the transition. However as LU trains do not have through bus cables, the section only has to be long enough to cope with the inter-linked shoes on one coach.
 
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KT530

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Not sure about other Neutral Sections, but certainly the one at Maidenhead can be energised if an EMU becomes stranded in the section.

Would be unlucky, but quite possible if a driver receives a REC on GSM-R.
 

37057

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In practice the VCB on 350s in multiple operate separately when passing through a neutral section. This is seen by the VCB lamp illuminating twice when passing through the section. If the CPBC is opened before the time out then power goes to half then full when the rear set has its power restored. Synchronisation of the VCB is when raising and lowering the pantographs which is why we isolate the VCB synchronisation switch when hauling a dead set. I'm not a maintenance technician but that is how they work from a driving perspective.

Cheers for that insite. The dark arts of 350 operation can't be replicated on shed and it's not often we need to deal with that kind of issue as they're quite reliable in that respect!
 

Rob F

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What is going on on the Underground when all the lights go out and then come back on again?
 

rebmcr

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What is going on on the Underground when all the lights go out and then come back on again?

That's when the train passes over a gap in the conductor rails. Such gaps are more common than on AC neutral sections — e.g. there are several for every set of points.

Older trains (Piccadilly and Bakerloo at this stage) don't have battery backups for the lighting, but they do have each side (left/right) of the train supplied from different pickup shoes (front/back) to try and keep at least some lighting on unless the gap is long.
 

apk55

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Regulations for deep level underground lines prohibit the use of power jumpers between cars. Therefore each power car must pick up its own power from shoes only on that car. In addition the fact that return path goes via shoes which can also be gapped means that through complex junctions cars are regularly gapped. In addition they also liked to keep power sections totally isolated from each other so they would have a power rail gap longer than a car length between sections. This is not normally a problem in operation as each train had several power cars and efforts would be made to avoid sections where all power cars were gapped at once.
For lighting to avoid cars being blacked out when gapped there would be two power cars permanently coupled together with a MA set on each, which supplied lighting for one side of both power cars, hence the lights going out on on only one side (normally followed by the other side going out.
 

Steve Eames

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The inline Neutral Sections are around 6m long (Don't have the drawing to hand to check) but the APC magnets are set a greater distance before and after based on linespeed, so the area in which the VCB is dropped out is longer than the actual earthed section.
The locations are sometimes problematic to design, as they have to be located far enough from a stopping location (signal or station) for the heaviest train to be able to start from a stand and coast through without stopping in the dead area.
In addition, there needs to be enough standage beyond for the longest train to stop at the next stopping location without the pan coming to a stand in the dead area. There are a set of design rules to enable signalling and OLE designers to determine the optimum location, these cover traction types, linespeed and gradient amongst other factors.
Carrier wire neutrals as introduced on GWEP have even longer dead areas but are supposed to be more robust and take higher speeds than Inline units.
The Arthur Flury Single Rod Neutral Section which is the most common installed on Network Rail Infrastructure with the exception of GWEP is 8202mm long on the contact and 9050mm on the catenary with 3280mm of earth section in the catenary at the centre of the Neutral section.
The APC magnets are 30m on the approach to and 30m beyond on lines of speeds 100 mph and above.
On lower speed lines , i.e. 20 mph the distance is 14m , 40 mph distance is 18m , 60 mph distance is 22m , 80 mph distance is 26m.
Hope this is of help.
 
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