Porterbrook Cl.769 'Flex' trains from 319s, initially for Northern

[Pete_uk]

Electrical fields can cause strange problems. A Formula 1 cars electronics shut down when it went over a tram tunnel with a tram running through it in Singapore one year.

 

[Greybeard33]

Not sure why a 321 couldn't be converted other than it would need a single phase supply from diesel sets

Single phase AC generators are best avoided. The shaft torque falls to zero twice per cycle, so the vibration tends to wreck the bearings. Whereas three phase generators have a constant torque for a given electrical load, as do DC generators.

 

[AM9]

Electrical fields can cause strange problems. A Formula 1 cars electronics shut down when it went over a tram tunnel with a tram running through it in Singapore one year.

Which indicates an EMC design shortfall.

 

[anthony263]

Well looks ike tfw going to put the class 231s onto the pensrth to rhymney line first to replace the class 769s according to the rumour mill

 

[adrock1976]

Well looks ike tfw going to put the class 231s onto the pensrth to rhymney line first to replace the class 769s according to the rumour mill

Are the 231s a further design of the former London Underground D78s, or is this something totally different?

I ask because the 230s are the above mentioned D78s.

 

[Domh245]

Are the 231s a further design of the former London Underground D78s, or is this something totally different?

I ask because the 230s are the above mentioned D78s.

Totally different, it's the DEMU Stadler FLIRTs (there's another fleet of tri-modal 756s - OLE/Diesel/Battery - but these ones are pure diesel), the first two of which were recently delivered as per this thread

 

[adrock1976]

Totally different, it's the DEMU Stadler FLIRTs (there's another fleet of tri-modal 756s - OLE/Diesel/Battery - but these ones are pure diesel), the first two of which were recently delivered as per this thread

Many thanks for that.

I did fall into the classic trap of using logic here, which TOPS classification and logic are not necessarily the same thing

 

[AM9]

Totally different, it's the DEMU Stadler FLIRTs (there's another fleet of tri-modal 756s - OLE/Diesel/Battery - but these ones are pure diesel), the first two of which were recently delivered as per this thread

But surely a DEMU isn't "pure diesel" in terms of through-life operation. Do they have pantograph roof bays?

 

[Domh245]

Many thanks for that.

I did fall into the classic trap of using logic here, which TOPS classification and logic are not necessarily the same thing

It's a strange one as there is a slight amount of logic to TfW's TOPS classifications. The Vivarail became a new subclass of 230, but all the other new fleets (197/231/756/398) are one up (or down) from the current highest number in each TOPS range! Whether the 197s deserve their own class or should have been 196 subclasses is still a matter for debate!

But surely a DEMU isn't "pure diesel" in terms of through-life operation. Do they have pantograph roof bays?

Hopefully they won't remain purely diesel! They're pure diesel in terms of current operation (in that of the three energy sources that their tri-mode sisters can use, these can only use the one), but there is provision for pantographs, in that the area where they're mounted on the other FLIRTs is clear of any other equipment and they have the slight cutout in the roofline. They don't appear to have the roof mounted bus-line, but nothing obviously in the way that would prevent it.

 

[Wyrleybart]

Interesting never considered that before but as 319's were dual voltage straight thyristor control wouldn't have worked on DC so hence the need to use a GTO chopper. Not sure why a 321 couldn't be converted other than it would need a single phase supply from diesel sets but that would need a bus line running down the train which the 319's already had due to the shoegear either end of the unit to feed the power equipment was on the driver trailer.

Found this video on IMeche YouTube site which gives some background to the conversion but doesn't answer any of your questions though

Class 769 FLEX: Re-purposing rolling stock to meet industry demands

I had read that the class 319 was supposed to be a fairly easy conversion exactly because of the handy 750V bus cable down the unit. But then I read that Wabtec have had to install another bus cable anyway. I wonder if the GWR units which need to operate on 750V DC on the routes to Guildford and Gatwick need to use the existing 750V bus for the third rail, and require the second bus for the use of the diesels,

It's a strange one as there is a slight amount of logic to TfW's TOPS classifications. The Vivarail became a new subclass of 230, but all the other new fleets (197/231/756/398) are one up (or down) from the current highest number in each TOPS range! Whether the 197s deserve their own class or should have been 196 subclasses is still a matter for debate!



Hopefully they won't remain purely diesel! They're pure diesel in terms of current operation (in that of the three energy sources that their tri-mode sisters can use, these can only use the one), but there is provision for pantographs, in that the area where they're mounted on the other FLIRTs is clear of any other equipment and they have the slight cutout in the roofline. They don't appear to have the roof mounted bus-line, but nothing obviously in the way that would prevent it.

There is the mountings for the underslung transformers and the HT cabling from the roof to the underframe etc to consider. I am guessing the 755s have a bus on the roof from one end car to the other.

Does anyone know the side numbers of 231002 and 231004 please ?
I am guessing the start 9xxxxx if they are in the same arrangement as the GA 755s.

 

[plugwash]

AIUI the old shoegear wiring was just a single wire, whereas the new generator wiring needs to have two wires, because relying on the track in non-electrification areas to carry return currents to the generators would not be acceptable.

I think the "relatively easy conversion" was more about the availiability of a conviniant point to feed the generator supply into the existing drive system (and the availibility of sufficient space to physically fit the generators).

 

[Nicholas Lewis]

AIUI the old shoegear wiring was just a single wire, whereas the new generator wiring needs to have two wires, because relying on the track in non-electrification areas to carry return currents to the generators would not be acceptable.

I think the "relatively easy conversion" was more about the availiability of a conviniant point to feed the generator supply into the existing drive system (and the availibility of sufficient space to physically fit the generators).

Yes that is a good point non electrified jointed track wouldn't be bonded for traction currents.

 

[edwin_m]

I had read that the class 319 was supposed to be a fairly easy conversion exactly because of the handy 750V bus cable down the unit. But then I read that Wabtec have had to install another bus cable anyway. I wonder if the GWR units which need to operate on 750V DC on the routes to Guildford and Gatwick need to use the existing 750V bus for the third rail, and require the second bus for the use of the diesels,


There is the mountings for the underslung transformers and the HT cabling from the roof to the underframe etc to consider. I am guessing the 755s have a bus on the roof from one end car to the other.

Does anyone know the side numbers of 231002 and 231004 please ?
I am guessing the start 9xxxxx if they are in the same arrangement as the GA 755s.

AIUI the old shoegear wiring was just a single wire, whereas the new generator wiring needs to have two wires, because relying on the track in non-electrification areas to carry return currents to the generators would not be acceptable.

I think the "relatively easy conversion" was more about the availiability of a conviniant point to feed the generator supply into the existing drive system (and the availibility of sufficient space to physically fit the generators).

Thanks for confirming something that has bothered me for a while - how the return current would find its way back to the gensets. Passing it through the rail certainly wouldn't be acceptable, especially when it was passing over an insulated rail joint...

Because the 319 was built as a dual-voltage unit, it does have the capability to accept 750Vdc from the gensets, whereas any of the other classes 317 to 322 would have needed either a 50Hz feed or replacement traction electronics. I suspect they underestimated the difficulty of keeping the traction package working with a supply that behaved in ways it wasn't designed to handle.

 

[Halish Railway]

Sorry if this has been asked before, but would have AC traction motors a la SWR 455 / Renatus 321s have made any difference to the slightly tragic diesel performance?

Or would have it have made no difference due to the limitations of the DC wire bus & would have added too much cost to the project?

 

[AM9]

Sorry if this has been asked before, but would have AC traction motors a la SWR 455 / Renatus 321s have made any difference to the slightly tragic diesel performance?

Or would have it have made no difference due to the limitations of the DC wire bus & would have added too much cost to the project?

AC traction motors are nothing to do with the power supply type or voltage. All new EMUs (I think) have three phase ac traction motors which are in power, space and weight requirements, more efficient than traditional DC series wounld motors or even single phase ac motors. When DC motors were normal (e.g. all the MKIII EMUs when built), a 750V supply was derived from a transformer through rectifiers. The traction current was controlled either by resistor banks or in latter days electronic chopper (class 319) or thyristor control from the transformer secondary.
As far as the 319/769s go, replacing the DC motors with three phase ac would require the chopper electronics replacement with a variable frequency inverter connected to the 750VDC bus. That would be a far more expensive change to these mature EMUs, and in practice, the power would still be limited on ac to the transformer rating and on diesel (769s only of course) the diesel engine is the limiting factor. The "slightly tragic" performance that you refer to is more a function of the power limitation of two low-emission engines. The adhesion issues which 319s already had at times, are slightly exacerbated by the reduction in adhesive weight brought about by the engines and fuel tanks located above non-driven axles.

 

[Gricer99]

Not a single 769 diagrammed on RTT to go through Hindley today. All substituted with 2 car 150s which will be rammed at rush hour.

 

[nightflyian]

769434 has joined 424 on platform 4 at Southport. 2K82 0716 SOP-ALD is running as shown a 150 on RTT.
NR Driver at Southport advises me no rescue trains available to recover the failed units.

 

[apk55]

AC traction motors are nothing to do with the power supply type or voltage. All new EMUs (I think) have three phase ac traction motors which are in power, space and weight requirements, more efficient than traditional DC series wounld motors or even single phase ac motors. When DC motors were normal (e.g. all the MKIII EMUs when built), a 750V supply was derived from a transformer through rectifiers. The traction current was controlled either by resistor banks or in latter days electronic chopper (class 319) or thyristor control from the transformer secondary.
As far as the 319/769s go, replacing the DC motors with three phase ac would require the chopper electronics replacement with a variable frequency inverter connected to the 750VDC bus. That would be a far more expensive change to these mature EMUs, and in practice, the power would still be limited on ac to the transformer rating and on diesel (769s only of course) the diesel engine is the limiting factor. The "slightly tragic" performance that you refer to is more a function of the power limitation of two low-emission engines. The adhesion issues which 319s already had at times, are slightly exacerbated by the reduction in adhesive weight brought about by the engines and fuel tanks located above non-driven axles.

If you wanted to replace the traction motors to 3 phase an obvious example would be the conversion kit used on Class 455. Might need to repackage the equipment to fit in the available space although the traction motors would probably do ok.
However the power limitations of the diesel engines would still apply so there would be negligible change to performance on diesel. Only improvement might be in starting in poor conditions although chopper controlled DC motors can be as good as AC motors if a good control system is fitted.

 

[nightflyian]

769434 has joined 424 on platform 4 at Southport. 2K82 0716 SOP-ALD is running as shown a 150 on RTT.
NR Driver at Southport advises me no rescue trains available to recover the failed units.

Looks like there is some movement 5Z85 1247 SOP-XAL. Nothing seen inbound so the rear end unit must be now serviceable !
Realtime Trains - 5Z85 1247 Southport to Allerton Depot

 

[Llama]

There may in theory be a couple more 769s available per day from December when the current round of training stops.

Re the comments for the power current return, the 769s have indeed been fitted with a negative return busline throughout the unit. The technology is quite clever, however the systems have to be able to fail safe so any problem in communication between the two diesel generator rafts will result in issues.

I'm no engineer and the training course doesn't explain the technical details to make this assumption safe: but perhaps I'm right in thinking that each alternator is capable of supplying either 750v DC or 375v DC depending if they're running alone or effectively together in series. The 769s can run on one engine but obviously not very well (they're not great even on two). Trying to make sure that the supply to the 750v busline is absolutely failsafe and can't ever end up with 1500v DC down it from the combined alternators, which I assume would result in a rather expensive bang and fireworks - and more importantly making such a system reliable yet absolutely failsafe - is probably a lot more technically challenging than people give credit for.

 

[AM9]

There may in theory be a couple more 769s available per day from December when the current round of training stops.

Re the comments for the power current return, the 769s have indeed been fitted with a negative return busline throughout the unit. The technology is quite clever, however the systems have to be able to fail safe so any problem in communication between the two diesel generator rafts will result in issues.

I'm no engineer and the training course doesn't explain the technical details to make this assumption safe: but perhaps I'm right in thinking that each alternator is capable of supplying either 750v DC or 375v DC depending if they're running alone or effectively together in series. The 769s can run on one engine but obviously not very well (they're not great even on two). Trying to make sure that the supply to the 750v busline is absolutely failsafe and can't ever end up with 1500v DC down it from the combined alternators, which I assume would result in a rather expensive bang and fireworks - and more importantly making such a system reliable yet absolutely failsafe - is probably a lot more technically challenging than people give credit for.

I have no knowledge about the actual generator configuration but I presume that the two generators produce a nominal 750VDC which they individually control at the point that they feed the DC bus. There should be an independant link between the two genset's control units to ensure that they do not compete or cycle when sensing the bus voltage. A problem like that was experienced during system testing in the development programme causing a design modification to prevent this happening. Under diesel, the demand from the traction electronics I imagine would be backed-off if the bus voltage falls outside it's operating range under diesel, meaning that the diesel engines effectively control the maximum power available to the traction electronics from their govenors.
I suspect that some of the problems that the units have had in service are related to the interaction between the two generators sometimes locking each other out.

 

[507020]

There may in theory be a couple more 769s available per day from December when the current round of training stops.

Re the comments for the power current return, the 769s have indeed been fitted with a negative return busline throughout the unit. The technology is quite clever, however the systems have to be able to fail safe so any problem in communication between the two diesel generator rafts will result in issues.

I'm no engineer and the training course doesn't explain the technical details to make this assumption safe: but perhaps I'm right in thinking that each alternator is capable of supplying either 750v DC or 375v DC depending if they're running alone or effectively together in series. The 769s can run on one engine but obviously not very well (they're not great even on two). Trying to make sure that the supply to the 750v busline is absolutely failsafe and can't ever end up with 1500v DC down it from the combined alternators, which I assume would result in a rather expensive bang and fireworks - and more importantly making such a system reliable yet absolutely failsafe - is probably a lot more technically challenging than people give credit for.

They could of course have put a 319 driving trailer with 750V DC generator and alternator on either end of one of the Woodhead electrics if they needed something that could accept 1500V DC, but I asked about this a while ago and I think the 2 generators have to be wired in parallel so current is split and provide a constant 750V DC, so when only one engine is running there is quite a significant current limitation and the unit can’t exceed about 35mph, but using the alternate method, if each generator provided a constant 375V and they were wired in series, if one engine failed I don’t think the unit would be able to operate at a voltage as low as 375V, especially when no additional current is available.

I have no knowledge about the actual generator configuration but I presume that the two generators produce a nominal 750VDC which they individually control at the point that they feed the DC bus. There should be an independant link between the two genset's control units to ensure that they do not compete or cycle when sensing the bus voltage. A problem like that was experienced during system testing in the development programme causing a design modification to prevent this happening. Under diesel, the demand from the traction electronics I imagine would be backed-off if the bus voltage falls outside it's operating range under diesel, meaning that the diesel engines effectively control the maximum power available to the traction electronics from their govenors.
I suspect that some of the problems that the units have had in service are related to the interaction between the two generators sometimes locking each other out.

I’m not sure even Porterbrook has any knowledge of the actual electronic configuration at this point. Both the generators and alternators, which are brand new and the existing 1980s DC traction package of the 319, as is evidenced by the 319/3s still running out of Lime Street, do work. The issues are entirely caused by the interactions between these fundamentally different technologies from different eras which are being required to do things which they were not designed for. If even the 2 identical diesel generators are having conflicts which are causing unit to be stranded then that is very unfortunate.

Anyone who has ever been on a 769 is well aware that the engines rev down to a very low level when the motors are not drawing any power i.e. when stopped at stations and I have always had a suspicion that some sort of hastily cobbled together environmental software is forcing this behaviour and that when it comes time to go again that this software refuses to allow the engines to supply power again.

I have heard drivers complaining that unlike the pure Diesel 15x units, where controlling the throttle of the engines filters down straight to the speed of the (driven) wheels, a power application on a 769 DEMU is electronically asking the motors to increase their speed, but on Diesel no power is available for any acceleration immediately before the engine speed has been increased in software (and simultaneously for both engines) allowing the unit to accelerate and once this electronic software process has been followed you have lost 30-60 seconds and can’t keep to 15x timings. It really is a very complicated proposition technologically to create a functioning DEMU in this way.

I don’t really understand why measures have to be taken to prevent the return current from being fed into unelectrified track when all of the electrical equipment is contained onboard the train and not drawing power from any external electrification system for any current to be returned to, so why would it be doing this without a 2nd negative bus line in the first place?

 

[Halish Railway]

Anyone who has ever been on a 769 is well aware that the engines rev down to a very low level when the motors are not drawing any power i.e. when stopped at stations and I have always had a suspicion that some sort of hastily cobbled together environmental software is forcing this behaviour and that when it comes time to go again that this software refuses to allow the engines to supply power again.

I have heard drivers complaining that unlike the pure Diesel 15x units, where controlling the throttle of the engines filters down straight to the speed of the (driven) wheels, a power application on a 769 DEMU is electronically asking the motors to increase their speed, but on Diesel no power is available for any acceleration immediately before the engine speed has been increased in software (and simultaneously for both engines) allowing the unit to accelerate and once this electronic software process has been followed you have lost 30-60 seconds and can’t keep to 15x timings. It really is a very complicated proposition technologically to create a functioning DEMU in this way.

Regarding the engine revs lowering when the reverser is in neutral/off, the Voyagers/Meridians & FLIRTS do the same thing and I wouldn’t describe them as being hastily assembled or poorly designed… actually wait the Voyagers have CET tanks on the roof, but that’s a discussion left for another thread. Either way it doesn’t seem to compromise their performance.

I would say that this engine behaviour is a result of them being DEMUs, but neither the 80Xs nor any diesel locomotive that I’m aware of does it.

 

[AM9]

They could of course have put a 319 driving trailer with 750V DC generator and alternator on either end of one of the Woodhead electrics if they needed something that could accept 1500V DC, but I asked about this a while ago and I think the 2 generators have to be wired in parallel so current is split and provide a constant 750V DC, so when only one engine is running there is quite a significant current limitation and the unit can’t exceed about 35mph, but using the alternate method, if each generator provided a constant 375V and they were wired in series, if one engine failed I don’t think the unit would be able to operate at a voltage as low as 375V, especially when no additional current is available.

I think they are both connected the the 750VDC bus, the control being autonomous to maintain that bus voltage within the output limits of the engines.

I’m not sure even Porterbrook has any knowledge of the actual electronic configuration at this point. Both the generators and alternators, which are brand new and the existing 1980s DC traction package of the 319, as is evidenced by the 319/3s still running out of Lime Street, do work. The issues are entirely caused by the interactions between these fundamentally different technologies from different eras which are being required to do things which they were not designed for. If even the 2 identical diesel generators are having conflicts which are causing unit to be stranded then that is very unfortuna

I don't think that the technology difference is a problem in itself, after all, the requirement is for a steady voltage on the bus. The traction electronics was originally designed to operate from a 750VDC bus, the only difference now being the maximum available current is less than a full power demand of the traction system so some means must be there to back that demand off when the train is in diesel mode.

Anyone who has ever been on a 769 is well aware that the engines rev down to a very low level when the motors are not drawing any power i.e. when stopped at stations and I have always had a suspicion that some sort of hastily cobbled together environmental software is forcing this behaviour and that when it comes time to go again that this software refuses to allow the engines to supply power again.

The engines spin down to a tickover as they can maintain the bus voltage at that speed when there is no traction demand. When the driver selects a power notch, the traction electronics places al load on the bus causing the gensets to spin up to meet that demand. The enghines are of current specification so are pretty greeen anyway, but no multiple unit should have diesel engines running faster than is necessary when dwelling at a station.

I have heard drivers complaining that unlike the pure Diesel 15x units, where controlling the throttle of the engines filters down straight to the speed of the (driven) wheels, a power application on a 769 DEMU is electronically asking the motors to increase their speed, but on Diesel no power is available for any acceleration immediately before the engine speed has been increased in software (and simultaneously for both engines) allowing the unit to accelerate and once this electronic software process has been followed you have lost 30-60 seconds and can’t keep to 15x timings. It really is a very complicated proposition technologically to create a functioning DEMU in this way.

When I travelled on a 769 there was a slight delays on pulling away but it was no more than 2-3 seconds before moving and acceleration commenced less than five seconds after that. Given that both Southport and Wallgate have speed restrictions on starting, the effect was no more a that would be expected from a reasonably cautious driver. The train made up over 5 minutes between WGW & SOP so it didn't have a problem keeping to the timetable.

I don’t really understand why measures have to be taken to prevent the return current from being fed into unelectrified track when all of the electrical equipment is contained onboard the train and not drawing power from any external electrification system for any current to be returned to, so why would it be doing this without a 2nd negative bus line in the first place?

Unelectrified track has bonding only to satisfy track circuiting requirements. A 769, when running on diesel, can be drawing up to 1000A. Therefore the track bonding at times could be carrying half of the maximum bus current i.e. 500A which would not only screw the track circuit operation, but also fuse the bonds!

 

[plugwash]

I don’t really understand why measures have to be taken to prevent the return current from being fed into unelectrified track when all of the electrical equipment is contained onboard the train and not drawing power from any external electrification system for any current to be returned to, so why would it be doing this without a 2nd negative bus line in the first place?

The 319 had all the high power electrics on one intermediate motor coach. The wire was simply a connection to the shoegear, so in third rail land current would be picked up by the shoes on the end coaches and returned through the wheels on the motor coach. Not a problem, the track bonding in third rail areas is designed to tolerate large DC currents.

With generators on the end coach, the current needs to be returned to the generators. Doing it through the track is not an option as in non-electrification areas it can't be relied on to carry high currents. So the return current needs to run via a wire.

 

[507020]

Regarding the engine revs lowering when the reverser is in neutral/off, the Voyagers/Meridians & FLIRTS do the same thing and I wouldn’t describe them as being hastily assembled or poorly designed… actually wait the Voyagers have CET tanks on the roof, but that’s a discussion left for another thread. Either way it doesn’t seem to compromise their performance.

I would say that this engine behaviour is a result of them being DEMUs, but neither the 80Xs nor any diesel locomotive that I’m aware of does it.

There is an aspect that engines simply supplying power to motors will not rev as much when the motors are not drawing any power, but Voyagers are the definition of hastily assembled and poorly designed. Why would a separate order of non-tilting units need to be assembled with a body profile designed for tilting that just reduces passenger space if not being done hastily to save redesigning it? Other problems that they sit down on Dawlish sea wall because of waves and there being no passive provision for conversion to bi-mode or electric operation for what is initially a DEMU constitutes poor design. The 80Xs are a slightly more modern bi-mode so their behaviour is more logical.

I think they are both connected the the 750VDC bus, the control being autonomous to maintain that bus voltage within the output limits of the engines.

I assume you mean both upper and lower limits.

I don't think that the technology difference is a problem in itself, after all, the requirement is for a steady voltage on the bus. The traction electronics was originally designed to operate from a 750VDC bus, the only difference now being the maximum available current is less than a full power demand of the traction system so some means must be there to back that demand off when the train is in diesel mode.

What is unusual about DEMU operation is that when there is a demand for power, the electricity has not been generated and is not latent in a wire or a 3rd rail, so while the maximum output of the engine limits performance to the level of DMUs of a similar age, there is quite a significant delay in accessing any power from the engines. During this interval is when I believe the conflicts between systems occur.

The engines spin down to a tickover as they can maintain the bus voltage at that speed when there is no traction demand. When the driver selects a power notch, the traction electronics places al load on the bus causing the gensets to spin up to meet that demand. The enghines are of current specification so are pretty greeen anyway, but no multiple unit should have diesel engines running faster than is necessary when dwelling at a station.

I 100% agree that engines shouldn’t be left idling in stations, not only of multiple units but also of locomotives as well as road vehicles on the road. Not only is it an unnecessary waste of Diesel, but it produces noise and more importantly particulate pollution immediately where passengers are, however I find the amount that they spin down by to be quite extreme and this may just be the most modern specification for environmental Diesel engines, but I’m convinced there’s some software artificially limiting the speed further to just marginally below the level that the same software will allow them to spin back up again from.

When I travelled on a 769 there was a slight delays on pulling away but it was no more than 2-3 seconds before moving and acceleration commenced less than five seconds after that. Given that both Southport and Wallgate have speed restrictions on starting, the effect was no more a that would be expected from a reasonably cautious driver. The train made up over 5 minutes between WGW & SOP so it didn't have a problem keeping to the timetable.

Of course acceleration has commenced immediately upon moving. I think 769s actually pull away from a stand slightly faster than 15Xs for the first few seconds of acceleration, but it is when you are already moving at a low speed around 20-30mph and attempting to accelerate to higher speeds around 60-75mph then you will lose time.

Unelectrified track has bonding only to satisfy track circuiting requirements. A 769, when running on diesel, can be drawing up to 1000A. Therefore the track bonding at times could be carrying half of the maximum bus current i.e. 500A which would not only screw the track circuit operation, but also fuse the bonds!

That does sound quite catastrophic! When track such as this is electrified and begins to have to deal with these return currents, how is this mitigated?

The 319 had all the high power electrics on one intermediate motor coach. The wire was simply a connection to the shoegear, so in third rail land current would be picked up by the shoes on the end coaches and returned through the wheels on the motor coach. Not a problem, the track bonding in third rail areas is designed to tolerate large DC currents.

With generators on the end coach, the current needs to be returned to the generators. Doing it through the track is not an option as in non-electrification areas it can't be relied on to carry high currents. So the return current needs to run via a wire.

I am aware that the 750V DC through bus line was simply a connection from the shoes on the driving trailers to the motors and it makes sense that to complete a DC circuit on a 769 the return current has to be fed back into the generators, but even just ignoring the fact that the track can’t take it, for this to be accomplished would need electrical pickups on the running rails, whether these are the wheels on end carriages or something else. The 3rd rail is not bonded to the running rails for obvious reasons and can’t be if there is to be any difference in electrical potential between them, so how would this work, or is the answer that it simply wouldn’t so much time and money has had to be spent resolving this specific issue?

 

[edwin_m]

What is unusual about DEMU operation is that when there is a demand for power, the electricity has not been generated and is not latent in a wire or a 3rd rail, so while the maximum output of the engine limits performance to the level of DMUs of a similar age, there is quite a significant delay in accessing any power from the engines. During this interval is when I believe the conflicts between systems occur.

The electricity isn't actually latent to any significant extent (there's a small amount of capacitance in the system but it can be ignored for most purposes).

The difference is that if an electric train increases its power demand, the substation or feeder that provides the power into the system can increase its supply quickly, because it is only a question of solid state components passing more current with no moving parts involved (there may be cooling fans etc but these will only need to speed up later on if the system gets hotter). Even the old rotary converters, which I believe no longer exist in the UK, could meet a sudden power demand because they had big flywheels that could slow down a bit without causing any problems. Even so, the voltage in the third rail will fluctuate when power demand changes, mainly due to voltage drop in the third rail itself which varies with current flowing and with distance from the substation. Many of us will remember how the compressor slowed down and sometimes the lights dimmed on the old Southern Region slam door units when they started away from a station. Traction packages have to be designed to tolerate these fluctuations.

In the case of the 769 on diesel power, when the driver increases the power demand the 319-vintage traction electronics will respond immediately and demand more current from the DC bus. This will cause the bus voltage to drop quite rapidly, and both gensets have to detect this and spin up the diesels to a higher RPM to produce more current and bring it back to 750V. I think it's possible this takes the traction package close to or beyond the size and duration of line voltage fluctuations that it was designed to cope with.

That does sound quite catastrophic! When track such as this is electrified and begins to have to deal with these return currents, how is this mitigated?

When track is electrified, joints are removed or electrically bonded so the return current has a good path through the rails back to the substation. This probably means all the track circuits need changing to different types that don't need bonds and won't be destroyed if the return current gets out of balance between the two rails.

I am aware that the 750V DC through bus line was simply a connection from the shoes on the driving trailers to the motors and it makes sense that to complete a DC circuit on a 769 the return current has to be fed back into the generators, but even just ignoring the fact that the track can’t take it, for this to be accomplished would need electrical pickups on the running rails, whether these are the wheels on end carriages or something else. The 3rd rail is not bonded to the running rails for obvious reasons and can’t be if there is to be any difference in electrical potential between them, so how would this work, or is the answer that it simply wouldn’t so much time and money has had to be spent resolving this specific issue?

In a third rail electric train, current comes from the pickup shoes (usually on each end bogie to prevent "gapping") to a positive DC bus running the length of the unit. The traction electronics, or the camshafts and resistors in older units, take their positive traction supply from this bus and the return is connected to the wheels on the car they are located in. So in a sense there are electrical pickups on the running rails, but they go through the wheels instead of being separate shoes. In fact I believe all trains, electric or not, have metal brushes on the axle to make sure there is good electrical contact between the body and the wheels. This ensures that the train is earthed and any stray currents don't go through the axle boxes where they could cause damage.

On the 769 we've already established that the running rails can't be used for return current, so a separate bus cable is needed to take it back to the generators.

 

[GC class B1]

The electricity isn't actually latent to any significant extent (there's a small amount of capacitance in the system but it can be ignored for most purposes).

The difference is that if an electric train increases its power demand, the substation or feeder that provides the power into the system can increase its supply quickly, because it is only a question of solid state components passing more current with no moving parts involved (there may be cooling fans etc but these will only need to speed up later on if the system gets hotter). Even the old rotary converters, which I believe no longer exist in the UK, could meet a sudden power demand because they had big flywheels that could slow down a bit without causing any problems. Even so, the voltage in the third rail will fluctuate when power demand changes, mainly due to voltage drop in the third rail itself which varies with current flowing and with distance from the substation. Many of us will remember how the compressor slowed down and sometimes the lights dimmed on the old Southern Region slam door units when they started away from a station. Traction packages have to be designed to tolerate these fluctuations.

In the case of the 769 on diesel power, when the driver increases the power demand the 319-vintage traction electronics will respond immediately and demand more current from the DC bus. This will cause the bus voltage to drop quite rapidly, and both gensets have to detect this and spin up the diesels to a higher RPM to produce more current and bring it back to 750V. I think it's possible this takes the traction package close to or beyond the size and duration of line voltage fluctuations that it was designed to cope with.

When track is electrified, joints are removed or electrically bonded so the return current has a good path through the rails back to the substation. This probably means all the track circuits need changing to different types that don't need bonds and won't be destroyed if the return current gets out of balance between the two rails.

In a third rail electric train, current comes from the pickup shoes (usually on each end bogie to prevent "gapping") to a positive DC bus running the length of the unit. The traction electronics, or the camshafts and resistors in older units, take their positive traction supply from this bus and the return is connected to the wheels on the car they are located in. So in a sense there are electrical pickups on the running rails, but they go through the wheels instead of being separate shoes. In fact I believe all trains, electric or not, have metal brushes on the axle to make sure there is good electrical contact between the body and the wheels. This ensures that the train is earthed and any stray currents don't go through the axle boxes where they could cause damage.

On the 769 we've already established that the running rails can't be used for return current, so a separate bus cable is needed to take it back to the generators.

On older non powered rolling stock (e.g. coaches) the earthing is achieved by bonds from body to bogie and from bogie frame to axle box but any fault current will pass through the bearings. Because of the risk of axle bearing damage rolling stock that operated regularly over third rail dc areas had only one earth bond from body to bogie and only one axle on each bogie had a bogie to axle box bone to prevent the return current circulating through the bearings and causing electrical pitting of the bearings.

 

[AM9]

The electricity isn't actually latent to any significant extent (there's a small amount of capacitance in the system but it can be ignored for most purposes).

The difference is that if an electric train increases its power demand, the substation or feeder that provides the power into the system can increase its supply quickly, because it is only a question of solid state components passing more current with no moving parts involved (there may be cooling fans etc but these will only need to speed up later on if the system gets hotter). Even the old rotary converters, which I believe no longer exist in the UK, could meet a sudden power demand because they had big flywheels that could slow down a bit without causing any problems. Even so, the voltage in the third rail will fluctuate when power demand changes, mainly due to voltage drop in the third rail itself which varies with current flowing and with distance from the substation. Many of us will remember how the compressor slowed down and sometimes the lights dimmed on the old Southern Region slam door units when they started away from a station. Traction packages have to be designed to tolerate these fluctuations.

It wasn't just the 3rd rail stock that did that, I remember some of the MKI GEML EMUs doing the same. The 306s also did that and they were ex 1500VDC units converted.

In the case of the 769 on diesel power, when the driver increases the power demand the 319-vintage traction electronics will respond immediately and demand more current from the DC bus. This will cause the bus voltage to drop quite rapidly, and both gensets have to detect this and spin up the diesels to a higher RPM to produce more current and bring it back to 750V. I think it's possible this takes the traction package close to or beyond the size and duration of line voltage fluctuations that it was designed to cope with.

I think some who have experienced the 769's acceleration performance are mistaking the relatively low engine speed required to generate sufficient power compared the the DMU's response to driver demands, where little torque is available from the transmission until there is a large difference between input and output speeds. That's the requirement for the torque converters to work. The 150s have to be thrashed before the brakes release.

 

[Llama]

A couple of points to note. 769s actually respond initially quicker, when the driver moves the traction power controller from off to a power notch when starting away from a stand quicker in diesel mode than in electric mode or than a 319 does. Engine revs rise with each notch taken however there is often no discernable rise in engine revs between power notches 3 and 4. As I understand it there is no field weakening in power notch 4 on a 319 when the traction power is supplied in 'DC mode', which is effectively what a 769 on diesel mode is.

One other thing that's odd is that the engine revs only really die down to idle when the unit actually comes to a stand. The driver will have shut off traction power long before this (and in any event braking in brake step 2 or more -normal braking on a 319/769- would disable any traction power still applied) but even when coasting with no traction power applied the engines still rev quite high until the train virtually comes to a stand.

I think some who have experienced the 769's acceleration performance are mistaking the relatively low engine speed required to generate sufficient power compared the the DMU's response to driver demands, where little torque is available from the transmission until there is a large difference between input and output speeds. That's the requirement for the torque converters to work. The 150s have to be thrashed before the brakes release.

150s etc don't have to be 'thrashed' before the brakes are released. Once the torque converter is actually filled (2-3 seconds) so long as sufficient traction power is applied to overcome any rising gradient, and even on the steepest gradients I drive of 1-in-40-odd, then the unit won't roll back even in just notch 3 power. Lesser rising gradients wouldn't need that much traction power.