Static Frequency Converters (SFCs) - capabilities and scope for battery train operation

[Nottingham59]

Can anyone answer a couple of questions on static frequency converters? I'm asking as I would like to understand the scope for using SFCs for isolated islands of electrification at places like Exeter or the Highlands. Note this thread is only for actual SFC capabilities - not for speculation about possible locations.

1. Can an SFC be fed from a 33kV power line? The 33kV is the phase-to-phase voltage; from each phase to earth is just 19kV, which is less than the 25kV needed for OHLE. I don't think this would be sufficient to deliver +25kv/0/-25kV for AT lines without a transformer, but could an SFC deliver a single phase 25kV output from 33kV? I note that both Hambleton and Potters Carr feed from 132kV supplies.
   
   
2. How much does an SFC cost? Say 10MVA capacity. (Potters Carr is 30MVA, I understand.) How does this compare to conventional grid feeders like at Braybrooke (2x80MVA from 400kV supply).
   
   
3. What capacity would be needed for a BEMU line in a remote part of the rail network? Recharging, say, a 1MWh traction battery in 15mins would require 4MVA - so I'm thinking that 2 x 5MVA SFCs would be enough - one at each end of an isolated segment of OHLE to recharge two BEMUs at the same time, with some redundancy built in.
   
   
4. In places where the local supply is not adequate for Megawatt recharging loads, is it feasible to use static grid batteries in conjunction with a SFC which can be trickle charged in advance of the BEMU arrival? Do any SFCs in the world do this already?

Thanks in advance.

 

[Zomboid]

1. AIUI SFCs are essentially rectifier-inverter sets (no doubt a gross oversimplification), so could be connected at 11kV and output 27kV. You'd be unlikely to get big power outputs though such a connection though as 11kV doesn't tend to be the strongest supply. SFCs are an alternative to AT feeding, which is usually connected to 400 or 275kV. I suppose you could have one output 50kV with a centre tap, but it wouldn't behave like a transformer if you did.

Don't know about 2 & 3. I expect it depends on the application, probably no single answer.

4. Technically, yes. Practically I don't know. I guess you could supply the inverter part from batteries or the rectifier or both. But for isolated static charging it might be simpler to just use DC rather than invert it.

 

[Nottingham59]

Thanks for the reply.

For info, I've found more information to address question 1.

Static Frequency Converters improving the 25kV - Rail Engineer

Listen to this article Getting a reliable 25kV power supply is important in the thrust to obtain more railway electrification commitment from the government. All too often, reports emerge of train services being disrupted because of problems with the power supply. Over the years, the design of...

www.railengineer.co.uk

SFCs work by taking a three-phase balanced load from a high-voltage transmission line, converting it to DC and then to a single phase 25kV OLE supply. As a result, their use does not affect the balance between the transmission line’s three phases. Hence, they can be fed from a local distribution network’s 33kV supply.

The benefits of feeding the 25kV OLE from a 33 kV supply are considerable as this significantly reduces the cost of substation connections and offers greater flexibility in the location of substations.

 

[GRALISTAIR]

Fantastic read. Thanks for posting. Very informative and not too technical.

 

[m0ffy]

Very interesting article. I’d be interested to see how this would play out in practice - I’ve been involved in removing 132/25kV connections over the past decade. I can, however, see this being used in previously industrial areas which now have lightly loaded 33kV systems.

 

[ac6000cw]

Can an SFC be fed from a 33kV power line? The 33kV is the phase-to-phase voltage; from each phase to earth is just 19kV, which is less than the 25kV needed for OHLE. I don't think this would be sufficient to deliver +25kv/0/-25kV for AT lines without a transformer, but could an SFC deliver a single phase 25kV output from 33kV? I note that both Hambleton and Potters Carr feed from 132kV supplies.

The 19kV phase to earth is the RMS (root mean square) voltage i.e. providing the equivalent energy over time as a DC supply of that voltage. But if you are rectifying it to DC, the average DC voltage out of the rectifier (the input to the inverter) is higher, depending on the circuit configuration - see https://en.wikipedia.org/wiki/Rectifier#Three-phase_rectifiers for more info.

 

[ABB125]

Very interesting article. I’d be interested to see how this would play out in practice - I’ve been involved in removing 132/25kV connections over the past decade. I can, however, see this being used in previously industrial areas which now have lightly loaded 33kV systems.

Presumably those removed connections have been replaced with something else? 275/400 kV supplies instead?

 

[Zomboid]

The 19kV phase to earth is the RMS (root mean square) voltage i.e. providing the equivalent energy over time as a DC supply of that voltage. But if you are rectifying it to DC, the average DC voltage out of the rectifier (the input to the inverter) is higher, depending on the circuit configuration - see https://en.wikipedia.org/wiki/Rectifier#Three-phase_rectifiers for more info.

It is 33kV RMS phase to phase though, which is the relevant voltage in most cases.

 

[m0ffy]

Presumably those removed connections have been replaced with something else? 275/400 kV supplies instead?

That’s right. Moving to supergrid connections seemed to be a strategic move for Network Rail (although there is a lot still connected at 132kV). It’s not a move I was upset about, my interactions with NR and their ways of working could be tricky.

 

[zwk500]

1. What capacity would be needed for a BEMU line in a remote part of the rail network? Recharging, say, a 1MWh traction battery in 15mins would require 4MVA - so I'm thinking that 2 x 5MVA SFCs would be enough - one at each end of an isolated segment of OHLE to recharge two BEMUs at the same time, with some redundancy built in.
   
   
2. In places where the local supply is not adequate for Megawatt recharging loads, is it feasible to use static grid batteries in conjunction with a SFC which can be trickle charged in advance of the BEMU arrival? Do any SFCs in the world do this already?

Thanks in advance.

For these options would you not use an alternative fast-charge system at ground level at a static location rather than feeding isolated lengths of OLE? I appreciate you may have just wanted to know either way.

 

[Nottingham59]

For these options would you not use an alternative fast-charge system at ground level at a static location rather than feeding isolated lengths of OLE? I appreciate you may have just wanted to know either way.

Well yes, but my thinking is that BEMUs will already have a 25kV pantograph and the electrical gubbins to use it. And they can recharge on the move, rather than having to wait a stations. So a recharging solution that uses the existing OHLE standard is likely to be simpler and cheaper in the long run.

 

[edwin_m]

Well yes, but my thinking is that BEMUs will already have a 25kV pantograph and the electrical gubbins to use it. And they can recharge on the move, rather than having to wait a stations. So a recharging solution that uses the existing OHLE standard is likely to be simpler and cheaper in the long run.

It would also allow the OLE to be put wherever along the line is most suitable, for example somewhere that doesn't need major work to bridges and tunnels. And unlike a fixed ground level supply it contributes to ultimately achieving continuous electrification by joining them all together.

 

[poffle]

I'd expect a lot of the battery chargers at terminus stations will have built in battery storage so that they can charge up slowly from the grid and then do a high speed discharge to charge a train once an hour or so.

I don't think the electricity distribution system operators would be too enamoured of something that sucks a huge load for 5 minutes or so every hour.

The battery charger could probably connect into the local 11KV network whereas an instantaneous charger would probably need a dedicated connection back to a higher voltage transformer site.

I know this is how charging is going to work for the battery electric trains being deployed in Ireland.

 

[swt_passenger]

I know this is how charging is going to work for the battery electric trains being deployed in Ireland.

…and on the West Ealing to Greenford branch here.

 

[Zomboid]

I'd expect a lot of the battery chargers at terminus stations will have built in battery storage so that they can charge up slowly from the grid and then do a high speed discharge to charge a train once an hour or so.

I don't think the electricity distribution system operators would be too enamoured of something that sucks a huge load for 5 minutes or so every hour.

The battery charger could probably connect into the local 11KV network whereas an instantaneous charger would probably need a dedicated connection back to a higher voltage transformer site.

I know this is how charging is going to work for the battery electric trains being deployed in Ireland.

On a wider grid level, energy storage (battery out otherwise) is going to be a big thing to allow less predictable renewables to take a larger chunk of the load.

But for spike loads I'm not sure it's going to be the answer everywhere, batteries using current technology at least aren't that well suited to spike loads - I think we may see supercapacitor and flywheel storage used in some instances, and I wouldn't be surprised if they all find a niche of some sort.

 

[gingertom]

an SFC also takes in 3-phase, rectifies each and the inverter stage gives out with singe phase. This makes for a balanced load to the grid or DNO, and can vary the power take between phases to even out imbalances in the grid- a useful feature.

 

[HSTEd]

A combination of SFCs and Autotransformer feeding (2x25kV with phases 180 degrees apart) would allow us to move towards a world where the entire traction supply is in parallel, and all feeder stations can cooperate.

A wooden pole 2x25kV line over open fields is almost certainly much cheaper than railway electrification per kilometre covered (likely comparable to a 33kV power line). So you might end up seeing segments joined to wherever the nearest 25kV supply is, allowing any SFCs to cooperate with the main system. It is often practice to provide a duplicated supply to 25kV systems, so you might join two SFCs together or join an SFC to a normal traction supply.

Either way, a very interesting period ahead for railway electrification.

 

[GraphitProjekt]

A combination of SFCs and Autotransformer feeding (2x25kV with phases 180 degrees apart) would allow us to move towards a world where the entire traction supply is in parallel, and all feeder stations can cooperate.

A wooden pole 2x25kV line over open fields is almost certainly much cheaper than railway electrification per kilometre covered (likely comparable to a 33kV power line). So you might end up seeing segments joined to wherever the nearest 25kV supply is, allowing any SFCs to cooperate with the main system. It is often practice to provide a duplicated supply to 25kV systems, so you might join two SFCs together or join an SFC to a normal traction supply.

Either way, a very interesting period ahead for railway electrification.

I'm not sure I completely follow what you're suggesting. But that does sounds a lot like how DC electrification works. As far as I understand it all the conductor rails in southern England are all theoretically sharing the same pool of DC power (provided nothing's been switched off) that is topped up at substations every 3–5kms. What I'm not getting about your idea is what you're doing with lots of 25kV distribution lines separate to railway OLE.

But what that does remind me of is how AC electrification works in Germany/Switzerland/Austria. If I'm understanding it correctly, Southern Germany/Switzerland/Austria share a multiple nation-spanning 16.7Hz single phase distribution grid at 110 or 132kV that is fed by railway power-stations and large scale frequency converter stations connected to the 3 phase public utility. Even though the transmission voltage is 110kV, it's actually transmitted in an auto-transformer-esque way with two wires and a centre tapped earth point at +55kV, 0V and -55kV (or +66kV and -66kV if it's 132kV). The two main wikipedia articles I've found this out on are for the 15kV AC system generally, and a list of German/Austrian/Swiss systems. In Northeast Germany every OLE substation is connected directly to the 3 phase grid using things like SFCs and rotary converters instead of being supported by a railway grid

 

[HSTEd]

I'm not sure I completely follow what you're suggesting. But that does sounds a lot like how DC electrification works. As far as I understand it all the conductor rails in southern England are all theoretically sharing the same pool of DC power (provided nothing's been switched off) that is topped up at substations every 3–5kms. What I'm not getting about your idea is what you're doing with lots of 25kV distribution lines separate to railway OLE.

Distribution lines will cost far less than railway electrification per kilometre. They can also travel in nearly straight lines when the railway does not, ascending or descending slopes with ease.

Distribution lines could be used to interconnect 25kV electrification segments into a system that can share capacity as required. This would increase the average utilisation of 25kV feeders and reduce costs per train operated. When coupled with disconnected charging systems it would allow feed of power to be arranged however convenient, independent of the number of actual electrification segments on the railway. It would also eliminate giant spikes in load on an SFC from charging in a small area, by allowing all the charging points on the line to share power.

 

[Zomboid]

In theory you could do away with neutral sections by having lots of SFCs, but that approach introduces plenty of complexity of its own, I'd be surprised if that caught on as an approach.

 

[GraphitProjekt]

Something like that has already been done in small ways. For instance two 25kV grid connections were acquired at Corey's Mill on the ECML in 2001 but connected to the OLE at two FSs at Hitchin and Langley Junction. There are also examples of two separate feeder stations on the same 132kV single phase line Acton Lane A (Euston to Wembley) and North Pole, and also Wood Lane and Ferme Park on ECML. Indeed Acton Lane B (NLL: Acton Central to Camden + Gospel Oak to South Tottenham) used to be on hot-standby to power the GWML on the Heathrow Express scheme by way of a cable to a substation at Old Oak Common (the normal feed being at a "Hayes" FS from the now infamous North Hyde substation).
So i guess you mean arrangements like this but at large. The one thing I'd consider would be if it'd be more worthwhile to do it at a high voltage than OLE.

Distribution lines will cost far less than railway electrification per kilometre. They can also travel in nearly straight lines when the railway does not, ascending or descending slopes with ease.

Distribution lines could be used to interconnect 25kV electrification segments into a system that can share capacity as required. This would increase the average utilisation of 25kV feeders and reduce costs per train operated. When coupled with disconnected charging systems it would allow feed of power to be arranged however convenient, independent of the number of actual electrification segments on the railway. It would also eliminate giant spikes in load on an SFC from charging in a small area, by allowing all the charging points on the line to share power.