Static Frequency Converters for AC Feeding - can they be used alongside autotransformers?

[59CosG95]

On the second phase of the ECML Power Supply Upgrade, I have noticed that Static Frequency Converters (SFCs) are planned to be used for new grid connections. One is already installed at Potteric Carr (Doncaster) as an independent feeder for the IEP Depot & the ECML, utilising a 33kV grid connection from the DNO, rather than the more traditional 132kV DNO connection, or a 400kV National Grid one. More detail on this is given below:

The rapid withdrawal of LNER’s HST fleet was made possible by the commissioning of Potteric Carr OLE feeder station, which feeds both Hitachi’s new Azuma depot at Doncaster and provides additional clean power for the East Coast main line. Prior to then, HSTs had to run through Doncaster as there was insufficient power for a full electric service.

This 25kV traction feeder station is fed from the local 33kV distribution network operator (DNO) network. It is the first in the UK for Network Rail to use a power electronics based ‘PCS 6000 Rail’ 50MW Static Frequency Converter (SFC), supplied by ABB Power Grids under a £14 million contract.

The Potteric Carr SFC feeder station was successfully commissioned into commercial operation by ABB on the ECML end of October 2019, following the completion of Network Rail product and system acceptance tests and the issuance of trial certification.

One key advantage of this technology is that it enables connection to the utility grid at lower voltages (compared to normal 132kV and above for the range of power required). It also potentially eliminates neutral sections on the railways and enables the power utility to see a ‘balanced’ supply into the railway power infrastructure. Based on an earlier engineering and option selection scheme by Network Rail, it is estimated that the chosen SFC solution, which eliminates the need for a costly high-voltage grid connection, reduces the cost of a new feeder station by 60 per cent. High-voltage grid connections were previously needed, as a single-phase 25kV OLE supply would otherwise result in an unacceptable phase imbalance in the grid supply.

This is not a problem with an SFC, which takes a three-phase supply, converts it to DC and then into a clean single phase 25kV supply for traction OLE and depot use. As a result, it is possible to take a three-phase balanced load from a local distribution network 33kV supply to power the single-phase 25kV OLE system.

ABB Power Grids has already installed SFC feeder stations in Brisbane and Queensland, Australia, as well as several other European countries.

SFCs were developed to interconnect power networks operating at incompatible frequencies. A typical example of this is a cruise ship, with a 60Hz on-board network that may have to be connected to a 50Hz shore supply. Other examples in the UK for 50Hz to 50Hz connection include interconnectors within the Western Power Distribution network area and the Maygen Renewable Generation project in Scotland.

Further power supply enhancements are required to maximise electric running on the ECML, and these are being delivered by phase two of Network Rail’s ECML power supply upgrade. This includes the commissioning of a new feeder station and 132kV connection at Hambleton Junction, near Selby, to provide resilience of supply, as well as two static frequency converter compounds at 132 kV supply points and the upgrade of the feeder station at Marshall Meadows, immediately south of the Scottish border.

It is the Marshall Meadows feeder station won’t be upgraded until at least 2021. Until then, some LNER services and all the new TransPennine Express services (30 trains a day) will have to operate on diesel power for the 64 miles between Longniddry and Chathill.

Given that it simply converts 2-phase supplies into single phase 25kV, and that AT-fed railways split the power supply again into +25kV for the catenary & contact and -25kV for the ATF, would the use of SFCs mean that autotransformers can't be used as well?

 

[InOban]

I thought that the point of an SFC was that its used all 3 phases and so didn't unbalance the supply.

 

[swt_passenger]

I can’t see a reason why the output of an SFC can’t be compatible with an autotransformer system, if they can produce a 25 kV output from the 3 phase DNO input then surely they can also be arranged to directly produce the centre tapped 50 kV output?

An SFC is basically a 3 phase rectifier and single phase inverter pair in a box. (A large fairly complicated box in this case.)

But otherwise, if you can only get a 25 kV single phase output from the SFC, why not connect it to a 1:2 step up transformer?

 

[59CosG95]

I can’t see a reason why the output of an SFC can’t be compatible with an autotransformer system, if they can produce a 25 kV output from the 3 phase DNO input then surely they can also be arranged to directly produce the centre tapped 50 kV output?

An SFC is basically a 3 phase rectifier and single phase inverter pair in a box. (A large fairly complicated box in this case.)

But otherwise, if you can only get a 25 kV single phase output from the SFC, why not connect it to a 1:2 step up transformer?

My background is much more in Civils than Electrical Engineering, so do forgive my ignorance on this!
Now you put it like that, it does seem to make much more sense. I guess my query also stems from the fact that most of the places where SFCs are currently used (Germany, for example) tends to use these instead of ATFs AFAIK. (I've certainly seen no mention of a 15-0-15kV AT system on German Wikipedia for starters)

 

[HSTEd]

Nice thing about these devices is they can do whatever you want.

 

[GRALISTAIR]

I assume biggest thing with these is COST REDUCTION?

 

[59CosG95]

I assume biggest thing with these is COST REDUCTION?

Yes - they can reduce the cost of a new feeder station by up to 60%. Whether that's for equivalent power output I don't quite know.
So, AFAIK, assuming that it does, and being grossly over-simplistic with the ratios here, it gives a 33kV supply comparable output to a 132kV supply without one, and a 132kV supply comparable output to a 275kV or even 400kV supply without one.

 

[hwl]

On the second phase of the ECML Power Supply Upgrade, I have noticed that Static Frequency Converters (SFCs) are planned to be used for new grid connections. One is already installed at Potteric Carr (Doncaster) as an independent feeder for the IEP Depot & the ECML, utilising a 33kV grid connection from the DNO, rather than the more traditional 132kV DNO connection, or a 400kV National Grid one. More detail on this is given below:

Given that it simply converts 2-phase supplies into single phase 25kV, and that AT-fed railways split the power supply again into +25kV for the catenary & contact and -25kV for the ATF, would the use of SFCs mean that autotransformers can't be used as well?

Theoretically they could but:
1) so far SFCs (in UK) have only been used where the best supply available is 33kV
2) so far they have also been used as additional infill for relatively short sections when more power is needed (short distance= less need for autotransformer to reduce distance based losses) not for supplying long sections.
3) with a 33kV supply + SFC set up in an autotransformer type mode with no step up transformer the max (no losses!) voltage difference is ~49.5kV and less in reality. Hence you would need a 66kV or 120kV supply for the SFC for a step transformer free balanced supply (step ups have losses hence these need to be less than the distance based transmission losses to make it worth while) . At which point you could just go for a nearly balanced star-delta autotransformer type setup if you are going to have a transformer anyway... (that A***** were very good at destroying in London when connecting up wrongly in the recent past)

Yes - they can reduce the cost of a new feeder station by up to 60%. Whether that's for equivalent power output I don't quite know.
So, AFAIK, assuming that it does, and being grossly over-simplistic with the ratios here, it gives a 33kV supply comparable output to a 132kV supply without one, and a 132kV supply comparable output to a 275kV or even 400kV supply without one.

I think the cost saving is in those specific circumstances which wouldn't always be the case.

Plenty of comparable cases in rural 3rd rail land where only 11kV is available but with very little current too so limited power available.

Hence SFC is good where there is plentiful spare current on a 33kV line, which isn't the case in many places. More electric vehicles and heating may reduce these opportunities.

 

[GRALISTAIR]

All these cost savings do add up which is a good thing.

 

[hwl]

All these cost savings do add up which is a good thing.

If you have a 400kV substation near a busy line there may be no cost savings, it all depends on circumstances.

 

[InOban]

They're may however a time saving. Getting the Grid to work out how to accommodate an intermittent load on one phase is not their priority.

 

[59CosG95]

Theoretically they could but:
1) so far SFCs (in UK) have only been used where the best supply available is 33kV
2) so far they have also been used as additional infill for relatively short sections when more power is needed (short distance= less need for autotransformer to reduce distance based losses) not for supplying long sections.
3) with a 33kV supply + SFC set up in an autotransformer type mode with no step up transformer the max (no losses!) voltage difference is ~49.5kV and less in reality. Hence you would need a 66kV or 120kV supply for the SFC for a step transformer free balanced supply (step ups have losses hence these need to be less than the distance based transmission losses to make it worth while) . At which point you could just go for a nearly balanced star-delta autotransformer type setup if you are going to have a transformer anyway... (that A***** were very good at destroying in London when connecting up wrongly in the recent past)


I think the cost saving is in those specific circumstances which wouldn't always be the case.

Plenty of comparable cases in rural 3rd rail land where only 11kV is available but with very little current too so limited power available.

Hence SFC is good where there is plentiful spare current on a 33kV line, which isn't the case in many places. More electric vehicles and heating may reduce these opportunities.

On the ECML, I believe Hambleton Jn (currently an MPTSC) is due to become an SFC & FS, although I don't recall what the power supply will be - 33kV or 132kV. Marshall Meadows will also have an SFC (in fact, it'll have two), and that is already a 132kV supply AIUI.

 

[HSTEd]

If you have a 400kV substation near a busy line there may be no cost savings, it all depends on circumstances.

Although with HSLs coming along, even 400kV might start to feel the pain.

 

[apk55]

I do not see any reason why a SFC could not provide a 50KV output for auto transformer feed instead of a 25KV output. They will need to include transformers as much for isolation between the grid supply and the railway supply so it just a matter of picking appropriate transformer ratios.

 

[Philip Phlopp]

Not much to add really - you can use SFCs to provide an ATF supply if you desperately wanted to do so, but it's the opposite of what they're really designed for - at the moment.

I think, as others have alluded to, you're using an SFC currently to top up the power that's available for traction by using lower voltage grid connection points, which by their nature will tend to limit the total power you can pull through those connection points. It's power rather than voltage that's the crucial point - how many kW/MW you can pull rather than whether you're able to get 25kV or 50kV for an ATF supply.

 

[DerekC]

Part of the point of a an SFC is to balance the load on the grid. That results in significantly reduced costs on the supply side. There is a bit about them in RIA's Electrification Cost Challenge:

https://www.riagb.org.uk/RIA/Newsroom/Stories/Electrification_Cost_Challenge_Report.aspx

Another approach is the Static Frequency Converter (SFC) which addresses the limits that the electricity companies place on power quality issues (e.g. loading unbalance, load fluctuations, etc.). Although they have been used on some European railways for some time, the first application of SFC technology in the UK is part of the East Coast Power Supply Upgrade – PSU2 Alliance. The selected option for this project, which is currently nearing the end of GRIP 4, is the installation of two SFCs at Hambleton and Marshall Meadows (both 132kV connections) which is expected to reduce costs by c60% and the programme duration by a year compared to an AT system. The actual benefits achieved will need to be carefully monitored. There is other potentially beneficial new technology for example the combination of energy storage with converter technology.

 

[Class 170101]

Does the Hambleton installation have advantages for electrification between Leeds and York / Selby and Temple Hirst Jn and Selby (and to Hull in future) in that it will be able to supply this route as well?

 

[59CosG95]

Does the Hambleton installation have advantages for electrification between Leeds and York / Selby and Temple Hirst Jn and Selby (and to Hull in future) in that it will be able to supply this route as well?

I believe it is indeed supposed to improve supplies west and east of Hambleton as well as north & south.