Battery boost for grid feeders?

[Nottingham59]

Many grid feeders are in continuous use - supplying relativel steady levels of power to a succession of trains, such as around London or on the WCML north of Weaver.

But grid feeders on less intensively used parts of the network have a much peakier demand curve. They may need to supply tens of Megawatts for a few minutes to neet the demands of several EMUs accelerating uphill simultaneously, or to recharge a battery BEMU during a layover. But then demand could be neglible for the next hour until the next peak of activity.

So my question is: As electrification and battery trains extend out into more provincial parts of the network, is there scope to use batteries to provide the peak power needed by occasional passing trains?

The main advantage would be to reduce the cost of grid connections. The grid feeder at Tweedbank was specified as 2 x 10MW connections, each a single phase from different 132kV circuits at Galashiels substation. That was hugely expensive for a single track railway that can only ever deliver one train an hour to the extremity of the network to be recharged. The average load on Tweedbank grid feeder will be tiny: well under 1MW.

So should future grid feeders like Tweedbank be designed as 1MW balanced three-phase supply that was used to trickle charge a bank of grid-scale batteries? These could then supply the 10MW load for a few minutes when required.

My view is yes. And I believe such a set up could be self-financing: a 5-10 MWh battery could be charged overnight when electricity is cheap, and discharged back to the grid the following evening at much higher prices on those days when the full capacity of the battery has not been used up by trains.

What do you think?

 

[GraphitProjekt]

I imagine that could work quite nicely, but that is a lot of expensive and complicated equipment for the railway to fund, operate and maintain. SFCs are already bad enough since they can be very big sites! With a mind for cost, the Siemens 11kV system is relatively tried and tested and could work well for rural installations since it opens up so many more options to connect at. The 11kV switchboard of a primary substation (33kV or 66kV) being a good bet.
Perhaps worth noting that the Galashiels grid connection has transformers sized/rated for 10MVA but that's no indication on what the firm supply capacity is, which I think refers to the top wattage that is actually agreed to be pulled.

 

[Nicholas Lewis]

Many grid feeders are in continuous use - supplying relativel steady levels of power to a succession of trains, such as around London or on the WCML north of Weaver.

But grid feeders on less intensively used parts of the network have a much peakier demand curve. They may need to supply tens of Megawatts for a few minutes to neet the demands of several EMUs accelerating uphill simultaneously, or to recharge a battery BEMU during a layover. But then demand could be neglible for the next hour until the next peak of activity.

So my question is: As electrification and battery trains extend out into more provincial parts of the network, is there scope to use batteries to provide the peak power needed by occasional passing trains?

The main advantage would be to reduce the cost of grid connections. The grid feeder at Tweedbank was specified as 2 x 10MW connections, each a single phase from different 132kV circuits at Galashiels substation. That was hugely expensive for a single track railway that can only ever deliver one train an hour to the extremity of the network to be recharged. The average load on Tweedbank grid feeder will be tiny: well under 1MW.

So should future grid feeders like Tweedbank be designed as 1MW balanced three-phase supply that was used to trickle charge a bank of grid-scale batteries? These could then supply the 10MW load for a few minutes when required.

My view is yes. And I believe such a set up could be self-financing: a 5-10 MWh battery could be charged overnight when electricity is cheap, and discharged back to the grid the following evening at much higher prices on those days when the full capacity of the battery has not been used up by trains.

What do you think?

At Tweedbank you have to oversize the transformers for regulation purposes as even a 4 car train will take c1.5x its rated power when accelerating. You also need a high short circuit level to ensure protection operates satisfactorily. then of course you need to future proof the fact they may see sense at some point and just realise electrifying throughout is more sensible.

Where the potential lies in your proposal is lowering grid charges as you have to state you firm demand however short a period it is so I would imagine here they will be going for 6-8MVA so if you were only taking 1MVA and using batts that would lower the fixed charges and it might also avoid further upstream grid costs.

 

[Nottingham59]

SFCs are already bad enough since they can be very big sites! With a mind for cost, the Siemens 11kV system is relatively tried and tested and could work well for rural installations since it opens up so many more options to connect at. The 11kV switchboard of a primary substation (33kV or 66kV) being a good bet.

This idea would have to use SFCs (Static Frequency Converters) which convert three-phase grid supply to DC and then back to 25kV AC. The battery would fit naturally into the DC part of the circuitry.

Perhaps worth noting that the Galashiels grid connection has transformers sized/rated for 10MVA but that's no indication on what the firm supply capacity is, which I think refers to the top wattage that is actually agreed to be pulled.

I don't know what the firm supply capacity is, but the grid feeder cost £14.4 million. That's nearly half the £32 million cost of the whole current electrification project.

 

[Nicholas Lewis]

This idea would have to use SFCs (Static Frequency Converters) which convert three-phase grid supply to DC and then back to 25kV AC. The battery would fit naturally into the DC part of the circuitry.


I don't know what the firm supply capacity is, but the grid feeder cost £14.4 million. That's nearly half the £32 million cost of the whole current electrification project.

Firm Supply capacity is an annual charge but am surprised at the capex cost here for a couple of transformers and extension to busbars as there being banked off the existing 132/33kV transformers.

 

[Nottingham59]

At Tweedbank you have to oversize the transformers for regulation purposes as even a 4 car train will take c1.5x its rated power when accelerating. You also need a high short circuit level to ensure protection operates satisfactorily.

Thank you, that's informative and helpful.

then of course you need to future proof the fact they may see sense at some point and just realise electrifying throughout is more sensible.

Apologies, but I think we may have to differ on this. (For what it's worth, my view is that they should seen sense at some point and realise that 30-mile branch lines can be served by Battery trains with a worst-case operating range of 61 miles or more, and any electrification at the terminus is a complete waste of money.)

 

[hwl]

any electrification at the terminus is a complete waste of money.)

[As some one who has actually done quite a bit of rail battery modelling] Depending on the route, electrification at Termini can actually be very useful for battery health (you also don't power the hotel and aux loads from battery during that time period). The nominal dwell time at Tweedbank is 8 minutes plus the time / distance on the electrification section while moving of ~9minutes in each direction give 26minutes for charging and not using the battery for aux/ hotel loads in one go.
With battery modelling you have to focus on both distance and time otherwise you reach the wrong conclusions very easily. When you start thinking about time and total number and depth of cycles termini become more interesting.
In this case there is reasonable easy supply option at the southern end which helps tip the balance.
It also covers about a third of the steep climb northbound.

 

[Nottingham59]

[As some one who has actually done quite a bit of rail battery modelling] Depending on the route, electrification at Termini can actually be very useful for battery health (you also don't power the hotel and aux loads from battery during that time period). The nominal dwell time at Tweedbank is 8 minutes plus the time / distance on the electrification section while moving of ~9minutes in each direction give 26minutes for charging and not using the battery for aux/ hotel loads in one go.
With battery modelling you have to focus on both distance and time otherwise you reach the wrong conclusions very easily. When you start thinking about time and total number and depth of cycles termini become more interesting.
In this case there is reasonable easy supply option at the southern end which helps tip the balance.
It also covers about a third of the steep climb northbound.

Interesting thanks. I can see the argument for electrifying the platforms at Tweedbank to provide hotel power and battery top up for trains stranded there in times of disruption. But that doesn't need a £14m grid feeder and £16m of other works.

And I don't agree that anyone would need 26 minutes charging under the wires, when BEMUs like the Stadley Flirt Akku can fully recharge in just 15 minutes. (And that gets them 100 miles of range, compared to Tweedbank, which is just 30 miles from the wires at Newcraighall.)

 

[Zomboid]

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[hwl]

Interesting thanks. I can see the argument for electrifying the platforms at Tweedbank to provide hotel power and battery top up for trains stranded there in times of disruption. But that doesn't need a £14m grid feeder and £16m of other works.

And I don't agree that anyone would need 26 minutes charging under the wires, when BEMUs like the Stadley Flirt Akku can fully recharge in just 15 minutes. (And that gets them 100 miles of range, compared to Tweedbank, which is just 30 miles from the wires at Newcraighall.)

Just because you can fast charge doesn't mean you should especially if you want the batteries to last a high number of cycles.
This is why you haven't seen other OEMs claiming good numbers for range or charging time, they talk about total cost of ownership, battery life and reslience during disruption.

There is a convienient feed location near Tweedbank so is was chosen, there aren't any sensible feed option in the middle of this route. Grid feeds are expensive and this will have been less expensive than alternatives that were further away.

I haven't been involved in this project but i can see and understand all the thinking behind the decisions they have made.

If there had been sensible feed options in the middle of the route then different decisions would probably have been made.

== Doublepost prevention - post automatically merged: 15 Aug 2026 ==

A follow up comment relevant to much of the above at the top of the page about the "grid connection being over sized":

The normal minimum size of connection at 132kV is circa 10MVA due to how the protection devices and isolation on the higher voltage side of 132kV step down transformers work (including minimum conductor sizing on the primary winding need to make the protection function).

Hence they have applied for the smallest connection they can get. If you want 33KV for SFC you would still need a minimum 10MVA 132 kV feed upstream as there is sufficient spare 33kV available (a reasonable amount has already been committed for other uses).

All the other supply options would require either a completely new 132kV DNO substation (more expensive than Galashields feed option) or new connections to 275kV/400kV close to Millerhill which they are doing to improve the power supply around Edinburgh especially to the south.


Battery boost for supplies does make sense in some circumstances, Borders isn't one of them though!

 

[GraphitProjekt]

Interesting thanks. I can see the argument for electrifying the platforms at Tweedbank to provide hotel power and battery top up for trains stranded there in times of disruption. But that doesn't need a £14m grid feeder and £16m of other works.

Scotland's idea, I'm pretty sure, is to use discontinuous as a stepping stone to full electrification for most lines south of Inverness. So it makes sense they would install a normal 132/25kV GSP for a future conventional system. Furthermore, if controlling costs is a real aim, it does seem prudent to not try and experiment with what could be a good idea but definitely risks being costly to develop and trial. Instead they're dealing with a traction power system for which they have decades of experience. Scotland is already being the guinea pig for discontinuous operation in general.

And I don't agree that anyone would need 26 minutes charging under the wires, when BEMUs like the Stadley Flirt Akku can fully recharge in just 15 minutes. (And that gets them 100 miles of range, compared to Tweedbank, which is just 30 miles from the wires at Newcraighall.)

Well, unlikely to be 100 miles of charge by the end of the batteries working life which I think is still only maybe a 3rd of the rolling stock's lifespan

Just because you can fast charge doesn't mean you should especially if you want the batteries to last a high number of cycles

This is a good point actually, and in some ways counter to the thinking around discontinuous electrification in that you're supposed to be minimising STKs of OLE. But if your want to charge in motion (which is surely better suited to normal 25kV OLE compared to static fast charging off a pantograph) then even a few minutes of charging could be quite a few miles of wires, even more so if you want to preserve battery life by slow charging as you say. And then it's likely you might end up having to electrify through some difficult clearances that you would surely have wanted to be pan down. And since you don't have the economies of scale from wiring all the easy bits, the actual STK price would probably suffer.

This is why you haven't seen other OEMs claiming good numbers for range or charging time, they talk about total cost of ownership, battery life and reslience during disruption.

Yeah always worth thinking about a conceivable scenario. Something like the performance at the end of the batteries operational life, on a cold day so it's operating less efficiently and has higher hotel load, and disruption on the line means you're stranded for hours running heaters and lights.

The normal minimum size of connection at 132kV is circa 10MVA due to how the protection devices and isolation on the higher voltage side of 132kV step down transformers work (including minimum conductor sizing on the primary winding need to make the protection function).

Hence they have applied for the smallest connection they can get. If you want 33KV for SFC you would still need a minimum 10MVA 132 kV feed upstream as there is sufficient spare 33kV available (a reasonable amount has already been committed for other uses).

Interesting, this is not a consideration I've heard people discuss before. So at Galashiels they might have had to install a third 132/33kV grid transformer to allow a 33kV SFC connection just because of required minimum draw on the Grid voltage side?

The normal minimum size of connection at 132kV is circa 10MVA

The first generation 1960s schemes did use 7.5 and 5 MVA supplies. I know that Port Glasgow FS is 2x 5MVA from that era. And for 6.25kV you could have 3, 5 or 7.5 MVA

 

[HSTEd]

I suppose if you want to get more out of the Tweedbank feeder, you could contract with SP Energy Networks to build and maintain a 50km long wooden pole power line to Marshall Meadows.

With autotransformers at both ends at 25kV-0-25kV, you could probably significantly reinforce the ECML feeders! Although that would itself cost several million pounds and it might be cheaper to just pay to reinforce Marshall Meadows more.

EDIT: Of course we could ask the question about what the cost of completing the proposed static frequency converter at Marshall Meadows and then feeding Tweedbank with a single phase power line would have been, compared to the 132kV grid feeder?

 

[GraphitProjekt]

I suppose if you want to get more out of the Tweedbank feeder, you could contract with SP Energy Networks to build and maintain a 50km long wooden pole power line to Marshall Meadows.

With autotransformers at both ends at 25kV-0-25kV, you could probably significantly reinforce the ECML feeders! Although that would itself cost several million pounds and it might be cheaper to just pay to reinforce Marshall Meadows more.

EDIT: Of course we could ask the question about what the cost of completing the proposed static frequency converter at Marshall Meadows and then feeding Tweedbank with a single phase power line would have been, compared to the 132kV grid feeder?

This may be one of the strangest traction power ideas I've yet come across, but definitely very original. At risk of misinterpreting a joke by taking it seriously, I can imagine a couple of issues here. The supply at Galashiels is 10 MVA, which is the same as Marshall Meadows so the only actual benefit would be the availability of 2 extra back up supplies. There's no reason to assume it's possible to run them in-phase with each other. Also you'd have to heavily modify the brand new transformers at Galashiels to have totally different secondary windings, and then install further AT transformers at Marshall Meadows.

Considering the SFC solution to Marshall Meadows sits around halfway built, they should really just do that instead. There isn't much of a point building a powerline even to share power the other way, the money is better spent electrifying the rest of the Borders Line which not only means the trains can run off that power but it also acts as a power line between Portobello and Tweedbank. Such is the original point of the 25kV standard after all - compared to LV DC systems which had to supply power to trains and supply HV feeding using different parallel systems.

The old idea for the ECML upgrade was to install 400/50kV ATFSs at Innerwick and Stannington (just north of Newcastle) and do away with the old feeders either entirely, or as N-2 only.

On the wider subject of powerlines that could allow helpful sharing of nearby 25kV supplies: I've always thought they should install something like that in Southend between the Victoria Branch and the LT&S. And also between at Hertford East and North which was actually planned to be the case in the 1960s but it never came about.

 

[HSTEd]

This may be one of the strangest traction power ideas I've yet come across, but definitely very original. At risk of misinterpreting a joke by taking it seriously, I can imagine a couple of issues here. The supply at Galashiels is 10 MVA, which is the same as Marshall Meadows so the only actual benefit would be the availability of 2 extra back up supplies. There's no reason to assume it's possible to run them in-phase with each other. Also you'd have to heavily modify the brand new transformers at Galashiels to have totally different secondary windings, and then install further AT transformers at Marshall Meadows.

Well yes, the chance to do such things has probably passed now, but I suppose if you were doing this the correct transformers would have been ordered for Galashields

Considering the SFC solution to Marshall Meadows sits around halfway built, they should really just do that instead. There isn't much of a point building a powerline even to share power the other way, the money is better spent electrifying the rest of the Borders Line which not only means the trains can run off that power but it also acts as a power line between Portobello and Tweedbank. Such is the original point of the 25kV standard after all - compared to LV DC systems which had to supply power to trains and supply HV feeding using different parallel systems.

You wouldn't get very much 25kV electrification at all for the cost of 50km wooden pole line (making the assumption that cost is similar to a 33kV transmission line), which probably has a cost significantly under £300k or so per kilometre. I've seen suggestions that they can run as low as £125k per kilometre.

EDIT: Of course, if the Marshall Meadows SFC was finished, it could be configured to operate in parallel with the supply from the existing transformers at Galashiels if you wanted! Although you'd still need suitable autotransformers.

On the wider subject of powerlines that could allow helpful sharing of nearby 25kV supplies: I've always thought they should install something like that in Southend between the Victoria Branch and the LT&S. And also between at Hertford East and North which was actually planned to be the case in the 1960s but it never came about.

I think the promise of Static Frequency Converter deployment is that, eventually, we'd be able to operate the entire 25kV system in parallel. Such power lines would be invaluable in that situation.

But we'd need a lot more SFCs to get to that point!

 

[GraphitProjekt]

Well yes, the chance to do such things has probably passed now, but I suppose if you were doing this the correct transformers would have been ordered for Galashields

I think if you were to do things in this way, it'd be better to have 400/50kV transformers at Eccles and then run the 25-0-25kV powerlines 25 miles in each direction.

But at these distances it doesn't really seem sensible to me when you paralleling so much transmission grid infrastructure.

You wouldn't get very much 25kV electrification at all for the cost of 50km wooden pole line (making the assumption that cost is similar to a 33kV transmission line), which probably has a cost significantly under £300k or so per kilometre. I've seen suggestions that they can run as low as £125k per kilometre.

More like dual circuit 66kV which I think are the standards used for 25-0-25kV feeders on the grid side. While there are a handful of 25kV wood pole lines, I don't think there has ever been an overhead powerline for a AT system - would be interesting to see. The examples of this kind of thing on GWML all used underground cables even when they aren't lineside. (Melksham - Thingley) I wonder if building new overhead distribution infrastructure is kinda frowned upon nowadays.

EDIT: Of course, if the Marshall Meadows SFC was finished, it could be configured to operate in parallel with the supply from the existing transformers at Galashiels if you wanted! Although you'd still need suitable autotransformers.

With the firm supply capacity you can get from an SFC connection there'd be no point surely.

I think the promise of Static Frequency Converter deployment is that, eventually, we'd be able to operate the entire 25kV system in parallel. Such power lines would be invaluable in that situation.

But we'd need a lot more SFCs to get to that point!

Yeah it'd be interesting to see if anymore crop up. All the current Scottish projects seem to be using classic feeding perhaps because these are all plans made 10ish years ago. TRUP will bring in an SFC, but we frankly have no electrification projects actually planned so idk

 

[hwl]

The first generation 1960s schemes did use 7.5 and 5 MVA supplies. I know that Port Glasgow FS is 2x 5MVA from that era. And for 6.25kV you could have 3, 5 or 7.5 MVA

I should have said normal modern practice!

== Doublepost prevention - post automatically merged: 15 Aug 2026 ==

Yeah it'd be interesting to see if anymore crop up. All the current Scottish projects seem to be using classic feeding perhaps because these are all plans made 10ish years ago. TRUP will bring in an SFC, but we frankly have no electrification projects actually planned so idk

The "SFC fan club" included lots of savings from SFC that weren't actually from SFC and it wasn't long before this emerged.
About the time of the ECML SFCs, that was simplification in HV switch gear in the standards (effectively reducing redundancy with no longer having quite so much extra switch gear to be able to isolate the normal switchgear for maintenance and they booked this as an SFC saving.

The conventionally fed Scottish electrification benefits from this cost saving too.

If you don't have enough spare 33kV or 11kV capacity then you are going to need more transformer anyway (e.g. Galashields) so the main benefit of SFC becomes phases balancing but is more expensive.