Dear all,
I am starting to study/model the 2x25 kV AC railway system.
When developing the AT short circuit test equations, I come to the equation (Zs+Zc)'=Ucc*U1n^2/S1n.
For a typical 10 MVA AT with U1n/U2n=55/27.5 kV and Ucc(pu)=0.01, this would yield 0.01*55^2/10=3.025 Ohms.
However, I have seen a couple of references where the formula used to calculate the AT impedance in Ohms is: Ucc*U2n^2/S1n, which yields around 0.7562 Ω.
I have added an attachment in an attempt to show my understanding of the AT short circuit test and the equations development as well as a couple of references showing the different results.
Could you please give me your opinion?
Many thanks in advance.
Ricardo
I have to refresh my memory before being able to give you any good answers unfortunately.
However, I would recommend going into the details of the work done by György Varju regarding both 2x25 kV 50 Hz and 2x15 kV 16 2/3 Hz AT railways.
https://www.researchgate.net/profile/Gyorgy_Varju?_iepl%5BgeneralViewId%5D=50mcOhCV7bJpsyJojaTlHBeIHd1T2zQggrwh&_iepl%5Bcontexts%5D%5B0%5D=searchReact&_iepl%5BviewId%5D=rFVNK6cIkPtW0LAaoWLag6jpGDfeXsLGKZF0&_iepl%5BsearchType%5D=researcher&_iepl%5Bdata%5D%5BcountLessEqual20%5D=1&_iepl%5Bdata%5D%5BinteractedWithPosition1%5D=1&_iepl%5Bdata%5D%5BwithoutEnrichment%5D=1&_iepl%5Bposition%5D=1&_iepl%5BrgKey%5D=AC%3A19329487&_iepl%5BinteractionType%5D=profileView
Dear Lars,
Many thanks for your reply. I will be out of office for the next 10 days, but I will definitely check György Varju´s work on my return.
Kind regards
Ricardo
If you have problems finding something particular check if I have it on paper. I might not have the time to scan it, but we'll find a solution.
The condition which you have assumed like I1=I2 as E1=E2 is applicable only for an ideal tranformer which is an imaginary T/F. So this condition cannot applied for normal transformer
Hi Ricardo
As far as I understand, your question is mainly related to the bases used for the machine (the AT). When you consider the AT as a transformer with a ratio 2:1, the impedance can be expressed in ohms of the 55kV side or in ohms of the 25kV side, which will give your different values depending on the side you are considering the impedance is. Both are ok if used in the right side.
I am not currently in the office but in a few days I think I will be able to expand a little bit my answer.
Merry christmas!
Thank you all for your answers
Eduardo Pilo,
- That is right, that was one of my doubts. My understanding is that to use the values of Zs and Zc as they are represented in Figure 1 (see attachment in the original question), then I need to use the primary side (e.g. 55 kV in this case) as the base voltage. However, in some references, the secondary voltage is used, while the drawings refer to the impedances as in Figure 1 (in the primary).
- Another doubt that came up is weather the impedance voltage Ecc stated by the manufacturer of the AT refers to a SC test carried out with an auto-transformer connection (the test described by Figure 1) or, if by any chance the test is carried out as a regular transformer, feeding the primary and shorting the secondary. In the latter case the Ecc would differ by a factor of 2.
Krishna Manchem
Indeed, I am deliberately ignoring the magnetizing branch and other non-idealities to highlight the question.
Lars Abrahamsson
I am taking a look at his work. Will come back to you if I cannot find any article.
My best wishes for 2019!!
Ricardo
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