Transformer Type & Arrangement at ATS & ATFS in 2x25kV systems

[OLEngineer]

Hello,

I've been trying to wrap my head around our 2x25kV autotransformer system, and I would love some clarification! (Please poke holes in my knowledge!!!!) What types of auto transformer are in use in an ATFS and an ATS? As per the major feeding diagram, 400kV down to 25-0-25 at an ATFS (KAGA in my local case) through a centre tapped auto transformer. +25kV to OLE, 0v centre tap to traction return, -25kV to ATF. Then at a ATS, is there also a centre tapped auto transformer? As per Garry Keenor's diagram:
What is physically present at the ATS? Is it one centre tapped auto transformer? Are there multiple regular transformers? From his explaination of the diagram, he says that the transformers primary winding has twice the number of windings as the secondary, therefore the transformer action drives twice the current. This makes sense, but the drawing is confusing me. Why is there a transformer across +25kV OLE and the 0v Traction return, and one across the 0v Traction Return and the -25kV ATF? What is the purpose of the latter auto transformer? Or is this how he is trying to prove the transformer action driving twice the current by having half the number of windings as the primary?

Also, what kind of core is used in these auto transformers, both at the ATS and the ATFS? Is it toroidal, EI type? It would certainly help knowing what!


Is this image correct? (Im much less of an artist than I am an electrical engineer, which I am a complete layman!) Does the regular autotransformer cause the primary current to phase shift 180 degrees or is that only caused by the centre tapped autotransformer at the ATFS? How do they interact here in the primary winding?

Also, can you have centre taps across the primary and the secondary? Can you connect these centre taps together?

That should be all of my questions, I hope this is the right place to ask!

 

[buz33]

Also, what kind of core is used in these auto transformers, both at the ATS and the ATFS? Is it toroidal, EI type? It would certainly help knowing what!

I can't say as to the specific arrangement on the railway but may be able to help with auto-transformers.

What is physically present at the ATS? Is it one centre tapped auto transformer?

It is one centre tapped autotransformer.

Are there multiple regular transformers?

No.

Why is there a transformer across +25kV OLE and the 0v Traction return, and one across the 0v Traction Return and the -25kV ATF? What is the purpose of the latter auto transformer?

It is the same autotransformer with the centre tap connected to the traction return rail. What is not shown on the diagram is that the two coils are wound on the same core, they are coupled magnetically. Any current passing through the coil connected to -25kV ATF will induce a magnetic field in the core which can increase the current in the coil connected to the +25kV overhead line.

Is this image correct?

You have shown a conventional transformer, connected to provide isolation between primary and secondary, which is not the arrangement used in the ATF system.

A centre tapped autotransformer can be made from a standard 1:1 transformer (equal number of turns on the primary and secondary). If the end of one winding is connected to the beginning of the other winding then that will form the centre tap. It is important to connect it the right way, connecting the end of one winding to the end of the other winding (or the beginning to the beginning) will not work.

 

[Agricola]

The ATS sketch

shows a double wound transformer - that is one where the primary and secondary are galvanically isolated (there is no physical connection between the primary and secondary windings). An autotransformer however is different in that the primary and secondary are physically connected. A more appropriate sketch would be


There are two windings, each with same number of turns,which are connected together and brought out to a terminal connected to the earthed running rails. Hence the description

the transformers primary winding has twice the number of windings as the secondary

Also, what kind of core is used in these auto transformers, both at the ATS and the ATFS? Is it toroidal, EI type? It would certainly help knowing what!

These transformers are generally a bit big for EI core, but they do use stacked sheets of core plate. Unlike the EI core the top core plates have a 45 degree joint between plates.

Does the regular autotransformer cause the primary current to phase shift 180 degrees or is that only caused by the centre tapped autotransformer at the ATFS

I believe the feeder station transformer (which for the examples I have seen is a double wound transformer and not an autotransformer) has two separate secondary windings. One secondary is connected to give the 180 degree phase shift.

 

[GraphitProjekt]

Why is there a transformer across +25kV OLE and the 0v Traction return, and one across the 0v Traction Return and the -25kV ATF? What is the purpose of the latter auto transformer?

I can't claim any real expertise here but I've seen other explanatory diagrams that better show that each transformer at an ATS will be making 3 connections (contact wire, ATF, traction return).

More specifically, this diagram says that the ATs sit between the +25kV busbar and -25kV busbar.
And on the only picture I could find of a relevant SCADA screen, you can see that each of the ATs connects between both live busbars at Grahame Park SATS on the MML. The top two lines being the ATFs and at the bottom of the screen 8 OLE circuits. In terms of other equipment at an ATS, you can also see various LV transformers here, but they aren't relevant to the traction supply. What is sometimes left out of textbook diagrams is that most ATS substations will have at least 2x duplicate autotransformers, sometimes more depending on how many circuits happen to be installed at a particular location. Although, I have heard of a few ATSs on GWML and HS1 that just have 1 transformer but I imagine it's less common.

As per the major feeding diagram, 400kV down to 25-0-25 at an ATFS (KAGA in my local case)

Just wondering what you mean by "KAGA"?

 

[59CosG95]

I can't claim any real expertise here but I've seen other explanatory diagrams that better show that each transformer at an ATS will be making 3 connections (contact wire, ATF, traction return).
View attachment 207172
More specifically, this diagram says that the ATs sit between the +25kV busbar and -25kV busbar.
And on the only picture I could find of a relevant SCADA screen, you can see that each of the ATs connects between both live busbars at Grahame Park SATS on the MML. The top two lines being the ATFs and at the bottom of the screen 8 OLE circuits. In terms of other equipment at an ATS, you can also see various LV transformers here, but they aren't relevant to the traction supply. What is sometimes left out of textbook diagrams is that most ATS substations will have at least 2x duplicate autotransformers, sometimes more depending on how many circuits happen to be installed at a particular location. Although, I have heard of a few ATSs on GWML and HS1 that just have 1 transformer but I imagine it's less common.
View attachment 207178

Just wondering what you mean by "KAGA"?

I believe "KAGA" is the four-letter code for a specific site, as OP did refer to using a Major Feeding Diagram for details.

 

[GraphitProjekt]

ah i see, Kensal Green would be my guess

 

[OLEngineer]

ah i see, Kensal Green would be my guess

Yes it is Kensal Green @GraphitProjekt , should've been more specific!

@Agricola @buz33 See my below attached picture, I understand the arrangement as the one on the left, but is it really the one on the right?


Im also trying to wrap my head around the nodal analysis side, see below attachment. I know I've got it wrong but I would love to see a correct nodal analysis just to wrap my head around things.

View attachment 206995

The nodal analysis is confusing me as surely you cannot have more current in at a node than you get when leaving it, however how does this work with the transformer action? Garry Keenors diagram shows them as current loops, is that a better way of thinking about things? Do you know of any publicly available sources I can use to understand these particular arrangements and to understand how the current around the circuit works?