EMU power distribution (inter-car connections)

Rhydgaled

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I'm aware that distributed traction means not all motor coaches on AC EMUs have a pantograph, and even when there are two pantographs there may only be one transformer on the unit. Thus there must be inter-car connections to distribute the power from the pantograph(s) between all the motors. A while ago I read on Twitter that one complication of e-Voyager / Project Thor was that each vehicle is currently largely self-contained, with the diesel engine on that coach generating electricity for the motor(s) on that coach only. If an engine is out, the motor(s) on that coach cannot receive electricity from the engine on another vehicle. Thus e-Voyager would require the installation of inter-car connections, which I presumed would be 25KV and therefore might be difficult/expensive to make safe). However, when I pointed this out at a meeting yesterday I was told that only the class 390s have 25KV inter-car connections.

If only 390s have 25KV inter-car connections, how are other EMUs (eg. 395s, Electrostars, 331s) done? I'm particularly interested in how the class 331 is set up given the potential for 161 Civity DMUs to be in service by 2023 and the UK government's target of removing diesel-only trains by 2040. I'm thinking that giving the 19x DMUs the ability to make use of OHLE / 3rd rail is available would be extremely difficult. Am I wrong?
 
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37057

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It appears 390s aren't the only unit with 25kV connections between vehicles. 395/397/80x are the same and can be identified as the 'pigtail' that goes across the gangway.

Units such as Electrostars / Desiros / Civitys have a single transformer and the collector equipment is fitted to the same vehicle so no need for these intercar connectors. The lower voltage power cables from the transformer secondary windings for traction / auxiliary are distributed (usually) below solebar. To add, the only units to have a motored vehicle that carries the 25kV collector gear are 319/320/321/322/325, not sure of any more.

An observation of mine regarding the 19x to EMU idea highlights the need to either construct additional pantograph/transformer vehicles for them or modify the existing vehicles, notably rebuilding the roof section to house the collector equipment.
 
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Domh245

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I'm thinking that giving the 19x DMUs the ability to make use of OHLE / 3rd rail is available would be extremely difficult. Am I wrong?
It would be difficult, not least because the 19x are mechanical gearboxes, so if you wanted them to make use of OLE/3rd Rail you'd need to replace the gearbox with a generator, and the final drive units with motors.
 

37057

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It would be difficult, not least because the 19x are mechanical gearboxes, so if you wanted them to make use of OLE/3rd Rail you'd need to replace the gearbox with a generator, and the final drive units with motors.
Another option could be to swap the transmission with an AC motor drive system with similar characteristics and retain the cardian shaft and final drive arrangement.
 

Rhydgaled

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Thanks for the replies so far.

It appears 390s aren't the only unit with 25kV connections between vehicles. 395/397/80x are the same and can be identified as the 'pigtail' that goes across the gangway.
Ok, so it appears that typically 125mph+ units have 25kV connections and lower speed units do not. Is there a reason for that (which would mean e-Voyager would need the same) or just a co-incidence? And would obtaining a safety case etc. for adding 25kV connectors to (say) a Voyager be difficult or was the Twitter post I read a red herring?

Units such as Electrostars / Desiros / Civitys have a single transformer and the collector equipment is fitted to the same vehicle so no need for these intercar connectors. The lower voltage power cables from the transformer secondary windings for traction / auxiliary are distributed (usually) below solebar. To add, the only units to have a motored vehicle that carries the 25kV collector gear are 319/320/321/322/325, not sure of any more.
So none of those types have a pantograph more than one vehicle? If so, may answer my question above re. the 39x/80x stock having 25kV inter-connectors but not lower-speed stock; ie. it is due to them having multiple pantograph-equipped vehicles rather than speed. In which case a 2-car 19x would need 25kV connections (because they would need a pantograph on both vehicles otherwise running in multiple could result in a pantograph spacing of less than 45m which I believe would impose a speed restriction on the formation). Returning to e-Voyager, would installing a single pantograph vehicle along with the lower voltage cabling (such as found on Electrostars etc.) pose much of a problem?

It would be difficult, not least because the 19x are mechanical gearboxes, so if you wanted them to make use of OLE/3rd Rail you'd need to replace the gearbox with a generator, and the final drive units with motors.
Another option could be to swap the transmission with an AC motor drive system with similar characteristics and retain the cardian shaft and final drive arrangement.
Both sound complicated to me but I'm not a proper engineer so don't really understand how feasible these suggestions would be. However, I assume the mechanical transmission means that no thought has been given to conversion and that the designers intended for the units to remain forever self-powered with no ability to make use of electrification infrustructure.
 

swt_passenger

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Basically, the requirement for a through train 25kV bus line usually arises when the total traction demand requires more than one transformer. Without mentioning exact figures, the transformer physical size can be considered as being proportional to its power rating. So as a single formation gets longer, eventually there’s no room, and a second (or in the case of Pendolino 11 car trains a third) transformer is required.

As you probably realise, where a unit with a through bus line such as a 390 has two pantographs, only one is ever raised, this is because the train would be able to bridge neutral sections.

But even then, there’s an option to bring more than one pantograph into use, and this is effectively what’s going on in the Class 700, which is logically two half trains permanently joined in the middle.

When two 5 car 80x operate together, each unit will raise one of its two pans, by default those at the opposite ends of the train - using any other permutation that puts the pantographs nearer one another may result in speed limitations because the contact with the catenary will be degraded as pans become closer.

So going back to your original premise, it is distribution of the transformers that may require a 25 kV bus; rather than distribution of the motors.
 

37057

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Pendolinos are a good example for this thread. They naturally need more power because of their overall weight and the top speed capability.

On a 9 car set, there are six traction converters and four auxiliary converters, powered by two transformers. These items of equipment are split proportionally at opposite ends of the unit (imagine them as two units in one). The 25kV busline is there purely to feed the transformer that can't be fed by it's own pantograph for the reason stated in the post above. An 11-car just has an additional transformer, traction and auxiliary converter. Those pigtail connections are fairly sturdy from what I can make out.

A 4-car Desiro for example doesn't need that ample power as the overall weight of the unit and top speed capability doesn't require it. Two traction converters powered by a single transformer and collector equipment is sufficient. No 25kV busline required as it comes off the collector equipment, through the body and down to the transformer.

I don't really know anything about the e-Voyager project (or just Voyagers) to comment on but I don't think the 25kV intercar connections would be the worst engineering problem a project like that could face. Insulated 25kV components are pretty safe when fitted correctly and everything around it is earthed! I've seen an earth braid (partly) connected to the 25kV bushing on a transformer happily arcing away!
 
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AM9

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Pendolinos are a good example for this thread. They naturally need more power because of their overall weight and the top speed capability.

On a 9 car set, there are six traction converters and four auxiliary converters, powered by two transformers. These items of equipment are split proportionally at opposite ends of the unit (imagine them as two units in one). The 25kV busline is there purely to feed the transformer that can't be fed by it's own pantograph for the reason stated in the post above. An 11-car just has an additional transformer, traction and auxiliary converter. Those pigtail connections are fairly sturdy from what I can make out.

A 4-car Desiro for example doesn't need that ample power as the overall weight of the unit and top speed capability doesn't require it. Two traction converters powered by a single transformer and collector equipment is sufficient. No 25kV busline required as it comes off the collector equipment, through the body and down to the transformer.

I don't really know anything about the e-Voyager project (or just Voyagers) to comment on but I don't think the 25kV intercar connections would be the worst engineering problem a project like that could face. Insulated 25kV components are pretty safe when fitted correctly and everything around it is earthed! I've seen an earth braid (partly) connected to the 25kV bushing on a transformer happily arcing away!
Just adding to your comments, EMUs like Electrostars and Desiros seem to have low voltage DC bus lines (see supplementary question below), throughout the train length which has enabled them to be configured for third rail service as well as 25kV OLE. As mentioned, the class 700 12-car desiro cities are effectively two six-car EMUs that can share control and auxiliary supplies, but each traction bus is only required to feed motors on the 1st, 3rd and 4th cars (or 9th, 10th and 12th counting the other half) so the lines are kept to lengths typical of some 4-car EMUs. Those lines carry similar voltages and currents* whether power collection is from 3rd rail or OLE.

*Owing to the inadequacies of LV 3rd rail distribution, performance is reduced by limiting the currents drawn to about 60% of that required for full power.

Supplementary question:
Where trains are ac/DC compatible, is the LV bus fed from rectified ac when on OLE or does it carry raw ac, leaving the traction electronics to handle both ac and DC inputs?
 

edwin_m

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I don't really know anything about the e-Voyager project (or just Voyagers) to comment on but I don't think the 25kV intercar connections would be the worst engineering problem a project like that could face.
The e-Voyager wouldn't have had a 25kV intercar connector. As mentioned above these are only used when the pantograph and transformer aren't on the same car. Adding either to a body not designed for it would be very difficult so the logical thing to do would be to insert a new car with both (and probably no motors). The power from the transformer would then have to be fed through the train to the traction electronics on the other cars. These cables would have to be quite fat, and I believe the difficulty of getting them through the train was one reason the project was abandoned.
 

37057

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Supplementary question:
Where trains are ac/DC compatible, is the LV bus fed from rectified ac when on OLE or does it carry raw ac, leaving the traction electronics to handle both ac and DC inputs?
Class 350 on AC - each traction converter takes x2 single phase AC supplies from the transformer and uses two four-quadrant choppers to feed the DC link (which is only contained within the traction converter itself), the inverters in another section of the traction converter then manipulates the DC into the 'pretend' three-phase!

Class 350 on DC - the shoe gear feeds the same DC link as above but via the HSCB (high speed circuit breaker) and line filter, both external to the traction converter. Not sure if there's a 750vdc bus line for traction linking the two converters though (can't find 350/1 notes and I only dealt with AC only ones) but there certainly will be for the bodyside heaters (which can take 750 dc from third rail or 750 ac from the transformer/OLE).

More modern stuff like the 700s tend to be a little different in terms of power distribution. More to what you're saying, ie AC is rectified in a module adjacent to the transformer and feeds a 750vdc bus which is then distributed to multiple inverters elsewhere on the unit.
 
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AM9

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Class 350 on AC - each traction converter takes x2 single phase AC supplies from the transformer and uses two four-quadrant choppers to feed the DC link (which is only contained within the traction converter itself), the inverters in another section of the traction converter then manipulates the DC into the 'pretend' three-phase!

Class 350 on DC - the shoe gear feeds the same DC link as above but via the HSCB (high speed circuit breaker) and line filter, both external to the traction converter. Not sure if there's a 750vdc bus line for traction linking the two converters though (can't find 350/1 notes and I only dealt with AC only ones) but there certainly will be for the bodyside heaters (which can take 750 dc from third rail or 750 ac from the transformer/OLE).

More modern stuff like the 700s tend to be a little different in terms of power distribution. More to what you're saying, ie AC is rectified in a module adjacent to the transformer and feeds a 750vdc bus which is then distributed to multiple inverters elsewhere on the unit.
Thanks for that. It might also explain (to me) why the 700s have a continuous traction tone on ac which assumes that the bus has a 100Hz ripple, and it warbles when running on DC when a (virtually) smooth supply might require delibrate modulation of sampling frequencies to avoid major EMI issues.
 

aleggatta

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This is the standard Electrostar traction power distribution in a basic form, the secondary side of the transformer runs at about 900V. On single voltage units the AC kit and LCM's don't exist, and on units without air operated shoe arms the shoegear earthing switch becomes a shoegear isolation switch to prevent a fixed shoe earthing out the juice rail when coming off shed.

377 Power Circuit.JPG
 

AM9

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This is the standard Electrostar traction power distribution in a basic form, the secondary side of the transformer runs at about 900V. On single voltage units the AC kit and LCM's don't exist, and on units without air operated shoe arms the shoegear earthing switch becomes a shoegear isolation switch to prevent a fixed shoe earthing out the juice rail when coming off shed.

View attachment 78097
That's really interesting. So a dual voltage Electrostar has both an ac bus and a DC bus. Given that the ac feeds to the local DC traction boxes are each rated for 622A, and the DC bus must carry up to 1000A when traversing a section break, there's a lot of copper (or aluminium) connecting those subsystems.
 

swt_passenger

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That's really interesting. So a dual voltage Electrostar has both an ac bus and a DC bus. Given that the ac feeds to the local DC traction boxes are each rated for 622A, and the DC bus must carry up to 1000A when traversing a section break, there's a lot of copper (or aluminium) connecting those subsystems.
I’m not sure if you’d necessarily refer to the low voltage AC feeds as having a BUS architecture, as it’s three separate output pairs from the main transformer. It‘s still a very useful diagram to add to the increasing number I’ve got though...
 

aleggatta

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That's really interesting. So a dual voltage Electrostar has both an ac bus and a DC bus. Given that the ac feeds to the local DC traction boxes are each rated for 622A, and the DC bus must carry up to 1000A when traversing a section break, there's a lot of copper (or aluminium) connecting those subsystems.
I'm curious as to where your 622A figure has come from... each high speed circuit breaker has a trip rating of 1250A, but the MCM and ACM are self monitoring and would normally self isolate before drastic fault conditions occur.

It's also worth noting that the DC line capacitor is there to hold the bus line voltage up over some lengths of con rail gaps. each shoegear position can sustain the whole inrush of starting current on the unit, and IIRC each shoe position has two stranded conductors, I couldn't tell you the CSA or current rating but I think the copper diameter is between 18-20mm.

And yes, as mentioned above, the AC side is a distribution network as opposed to a bus-line.

Finally, as probably goes without saying, a three car has no MOSL, and a 5 car has exactly the same traction equipment on the MOS as the MOSL. I tried to find an answer as to which winding MOS2 comes off of on the AC side of things, but never found a definitive answer. If I was to speculate the two MOS would share one winding on the transformer.
 

AM9

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I’m not sure if you’d necessarily refer to the low voltage AC feeds as having a BUS architecture, as it’s three separate output pairs from the main transformer. It‘s still a very useful diagram to add to the increasing number I’ve got though...
Yes you are right, that did cross my mind when typing it. The ac cables are not architecturally connected as a bus structure, but they do follow the longtitudonal path down the train, the DMOS(A)-PTOSL cable is quite long.
I'm curious as to where your 622A figure has come from... each high speed circuit breaker has a trip rating of 1250A, but the MCM and ACM are self monitoring and would normally self isolate before drastic fault conditions occur. ...
My supposition was that each motored bogie is rated at 560kW, which means that its feed that you said was 900V from the transformer must deliver 560kW/900v= 622A. Maybe I understand it wrong. In fact, I guess that the feeds to the DMOSs might be slightly higher to accommodate the ACM (Auxiliary Converter Module?) requirements.
 

aleggatta

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My supposition was that each motored bogie is rated at 560kW, which means that its feed that you said was 900V from the transformer must deliver 560kW/900v= 622A. Maybe I understand it wrong. In fact, I guess that the feeds to the DMOSs might be slightly higher to accommodate the ACM (Auxiliary Converter Module?) requirements.
Ahh I get you! So I thought I would have a look through some documentation to refresh myself, the transformer windings actually kick out 444v ac, but I believe when you measure across the windings you get 888 as it is effectively two phases rather than a conventional phase/neutral winding(so plus444/minus444). The LCM takes this voltage and kicks out 875VDC. The 900v figure I was quoting fame from the DC link limit voltage (where the DC link voltage is dumped across the brake resistors to protect the electronic equipment if the dc link voltage is too high)

unfortunately I can’t find any power ratings in the documents at this time, but will keep looking and come back if I find anything.
 

Rhydgaled

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Thanks for the further replies everyone.

So going back to your original premise, it is distribution of the transformers that may require a 25 kV bus; rather than distribution of the motors.
Ok, so a 25kv bus is needed in the following cases, have I understood correctly?:
  • you have transformers in multiple vehicles but have chosen to design the train to run with only one pantograph raised at a time
  • you need a pantograph on a vehicle that has no transformer for any reason
The e-Voyager wouldn't have had a 25kV intercar connector. As mentioned above these are only used when the pantograph and transformer aren't on the same car. Adding either to a body not designed for it would be very difficult so the logical thing to do would be to insert a new car with both (and probably no motors). The power from the transformer would then have to be fed through the train to the traction electronics on the other cars. These cables would have to be quite fat, and I believe the difficulty of getting them through the train was one reason the project was abandoned.
So it would have been a lower-voltage intercar connector, not 25kv, but the need to add cables between (and through) vehicles was still a serious job that contributed to the project going nowhere? A 195 must be even harder to convert to bi-mode surely.
 

edwin_m

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Ok, so a 25kv bus is needed in the following cases, have I understood correctly?:
  • you have transformers in multiple vehicles but have chosen to design the train to run with only one pantograph raised at a time
Not so much a choice. At higher speeds there has to be a certain spacing between raised pantographs, and this affects the pantograph positions on high-speed units. For example 5-car 80x have a pan each end, and when coupled together have to run with the outer two raised or they are speed-restricted.
So it would have been a lower-voltage intercar connector, not 25kv, but the need to add cables between (and through) vehicles was still a serious job that contributed to the project going nowhere?
I believe the difficulty of adding the cables was one reason. Not sure whether it was lack of space or that a lot of other stuff would have had to be stripped out to get to where the cables were going, and put back afterwards.
A 195 must be even harder to convert to bi-mode surely.
In respect of the cables, not necessarily. The 195 shares much of its design with the 331 which does have similar cables, so it may be that the route for them also exists on the 195.
However there are other problems with making a 195 bi-mode. The main one is probably that it has a mechanical drive from the engine to the wheels, unlike a Voyager where the engine drives an alternator to generate electricity to power electric motors on the wheels. So the 195 would have to have most of its transmission replaced by a generator and motor. Another problem is that most of the space underneath a 195 is taken up with diesel engines and related equipment, so a bi-mode might have to have no engine on the car that carried the transformer and would therefore have poorer performance on diesel. This would probably have be a brand new car, as none of the existing ones has a pantograph well.
 
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Llama

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All the traction related cabling and equipment on a 331 is under the solebar, there wouldn't be room for the minimum amount of electric traction equipment from a 331 on a 195, even just the traction converters (basically boxes of rectifiers, inverters, capacitors and control electronics) if the traction current was supplied remotely, so cable routing is a moot point.
 

D7666

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Thanks for the replies so far.

Ok, so it appears that typically 125mph+ units have 25kV connections and lower speed units do not. Is there a reason for that (which would mean e-Voyager would need the same) or just a co-incidence? And would obtaining a safety case etc. for adding 25kV connectors to (say) a Voyager be difficult or was the Twitter post I read a red herring?

So none of those types have a pantograph more than one vehicle? If so, may answer my question above re. the 39x/80x stock having 25kV inter-connectors but not lower-speed stock; ie. it is due to them having multiple pantograph-equipped vehicles rather than speed. In which case a 2-car 19x would need 25kV connections (because they would need a pantograph on both vehicles otherwise running in multiple could result in a pantograph spacing of less than 45m which I believe would impose a speed restriction on the formation). Returning to e-Voyager, would installing a single pantograph vehicle along with the lower voltage cabling (such as found on Electrostars etc.) pose much of a problem?

Both sound complicated to me but I'm not a proper engineer so don't really understand how feasible these suggestions would be. However, I assume the mechanical transmission means that no thought has been given to conversion and that the designers intended for the units to remain forever self-powered with no ability to make use of electrification infrustructure.
16x Networker DMU were built with conversion to EMU in mind ... under Crossrail plans at that time NSE 16x and 46x were being introduced assumed that Crossrail would be NSE worked by Networker type stock and that the Thames Valley 16x would be adsorbed by conversion into the EMU fleet along with electrification towards Reading. The replacement of the 16x diesel engine by a traction motor and retention of the cardan shaft and final drive was a definite design possibility. As was simply de-motoring them as trailers and inserting new build motor coaches. There is nothing new in this world. It's all been thought of before. All this is in a technical paper somewhere from one of the Learned Institutions.
 
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D7666

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But even then, there’s an option to bring more than one pantograph into use, and this is effectively what’s going on in the Class 700, which is logically two half trains permanently joined in the middle.
No on a 700.

There is no traction power electrical connection between the two halves of a 700.

It is correct a 700 works as two half units permanently connected, but they are self contained, if one pan or transformer or other AC HV gubbins in either half unit fails, the unit is 50% disabled, there is no means to feed any traction power into the failed section at all from the working section.

There is of course auxiliary and control power. Traction power, no.
 

swt_passenger

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No on a 700.

There is no traction power electrical connection between the two halves of a 700.

It is correct a 700 works as two half units permanently connected, but they are self contained, if one pan or transformer or other AC HV gubbins in either half unit fails, the unit is 50% disabled, there is no means to feed any traction power into the failed section at all from the working section.
I was only trying to use the 700 as an example of when you’d see two pans up on one train, as opposed to something like a 390, where you wouldn‘t. I was implying electrical power separation even I didn’t make it clear...
 

D7666

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I was only trying to use the 700 as an example of when you’d see two pans up on one train, as opposed to something like a 390, where you wouldn‘t. I was implying electrical power separation even I didn’t make it clear...
ok, understood now
 

MikePJ

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It's worth noting that the issue of 25kV distribution was a major factor in the design of the APT-P units: these trains were two half-sets with two power cars in the middle, meaning that there was no way for passengers to walk through the entire train. This was because of the desire to use a single pantograph to avoid issues with the OLE at high speed, and there was a reluctance to run a 25kV bus line between passenger-carrying vehicles (as was done on TGV) because of concerns about safety. I suspect this concern remained for many years subsequently.
 

43096

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I believe so, yes.
The first series of Eurostars (373s) don't have 25kV connections, I don't believe. Each power car has a pantograph raised and the power car supplies power just for its own motors and the adjacent motor bogie under the first coach.
 

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