Short circuit calculation for railway substation

[lz5pl]

With a current project I have some problem with customer engineers about SC current level on MV side of railway substation.
It is existing substation, where I have to calculate relay protections settings and to set-up relays.
Scheme is following:
Busbar 110 kV, 3-phase SC current 16.3 kA (3420 MVA)
Power Transformer, 110/27.5 kV, 12.5 MVA, Uk=7.61%, connected between phases B and C on 110 kV side.
MV Switchgear is 27.5 kV, one terminal of the transformer is connected to contact wire, second one - to the rail.

I made a model using ETAP and NEPLAN - in both cases SC current on 27.5 kV busbar is 5.8 kA (or 6.4 kA if factor 1.1 is applied according to IEC 909). But the client insists that SC current here, calculated from long time ago is 3.6 kA (my result, divided by 1.73).
I would like to present him calculation by hand, because he is an old man and doesn't rely on computers I am OK with calculations in 3-phase network, but this case is a bit non-standard for me. And of course I am affraid if I miss something in modeling.


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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

It would help if you could show a circuit diagram. There are several different topologies when it comes to railway systems. One phase, two phase, three phase systems exist. There are backbone systems that use two phases with 180 degrees and there are variations thereof.

It seems that the sqrt(3) discrepancy may be a result of a misunderstanding regarding topology. Or was the old calculation assuming the wrong phase configuration?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[lz5pl]

Thank you Gunnar,
Attached is SLD - actually some mix of 3-line and 2-line diagram. Important for me is SC level on 27.5 kV busbars - I would like to check my calculation for busbar's SC, next points along the grid are not a problem.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

Doing simple hand math produces 12500*100/(27.5*7.61) = 5.97 kA and that is very close to your result.

I think that someone someetime wrongly transformed the system from three-phase to single phase and arrived at 3.6 kA and that that number was accepted and is now 'difficult' to change.

Interesting diagram, by the way. Russian?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[lz5pl]

Thank you, I have a similar result by hand calculation, but was not sure also if I correctly transformed 3-phase to 1-phase systems. Now it seems that mistake is not mine.
This is typical system in Bulgara, but I work for a first time for railways. Our usual field are utilities -

http://www.triel.bg

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[pwrtran]

Addition to Skogsgurra's calculation, you may also want to include the 110kV line impedance. On primary it sees as a L-L fault, no zero sequence involved.

 

[TLCPA]

On the diagram, each transformer is 12.5 MVA, 110/27.5kV. The two transformers serve three busses labeled 52kV. I assume the two transformers are not paralleled on the 52kV bus - either the switch -Q11 in cubicle HA06 or HA09 is open. My hand calculation (ignoring the 110kV transmission) is 3.2kA for 52kV and 6kA for 27.5kV.

 

[lz5pl]

Thank you TLCPA.
MV bus is labeled 52 kV, but this have to be isolation level. I don't know why the switchgear is for 52 kV - I entered the project on the very last stage, just for relay protections settings.
Bus works on 27.5 kV and your calculation matches my results and Gunnar's.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant