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I am using a 2MVA transformer (KNAF rating will be 30% more at 2.6MVA)

The question i have is whether we calculate the short circuit rating based on 2MVA or 2.6MVA?

I'm using pu method to calculate, Isc.pu = V.pu/Z.pu

So Isc = Isc.pu * Ibase

Should Ibase be 2886A or 3752A? Assuming base voltage of 400V

Would appreciate any advice on this. Thanks!

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  • \$\begingroup\$ What precisely do you mean by "short circuit rating" and what does KNAF mean? \$\endgroup\$
    – Andy aka
    Commented Sep 14, 2018 at 13:49
  • \$\begingroup\$ Its the short circuit fault level. KNAF means the transformer is forced cooled by fans which increases its capacity, basically overloading it and bringing down temp \$\endgroup\$
    – Joga78
    Commented Sep 15, 2018 at 12:03
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    \$\begingroup\$ I thought "K" NAF was associated with the type of oil used (high flash temp). N=natural circulation of the oil , A=Air, F=forced \$\endgroup\$
    – user16222
    Commented Sep 16, 2018 at 20:04
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    \$\begingroup\$ @Andy aka, I believe that Joga78 is trying to calculate the maximum through-fault current of the transformer, for design of the downstream equipment. KNAF is identification symbol for a cooling method, which is defined in IEC 60076.2. \$\endgroup\$
    – Li-aung Yip
    Commented Sep 20, 2018 at 5:18

2 Answers 2

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This is an excellent question, which I also had to deal with when I was a graduate electrical engineer.

The base values for the impedance should be given on the nameplate.

You should go to site, if possible, and take a photo of the transformer nameplate. Failing that, you can ask someone to take a photo for you, or look up your company's records for a previous photo of the nameplate, drawing of the nameplate, or a copy of the factory acceptance test (routine test) results.

Part of a typical nameplate is shown below (identifying information has been removed.) Note that the transformer is rated 36/45/60 MVA ONAN/ONAF/ODAF, but all impedances have been given on the basis of 60 MVA at 75 deg. C.

enter image description here

Another example:

enter image description here

If the base for the impedance value has not been specified, do not assume that the impedance is based on the highest MVA rating of the transformer. This is not necessarily the case. The transformer below is rated 7.5/10MVA, but the impedance is given on 7.5 MVA base.

enter image description here

If the basis for the impedance is not listed on the transformer's nameplate, you can look at the transformer's factory acceptance test (routine test) results:

enter image description here

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It depends on why you are doing the calculation.

If the transformer has three ratings like some of the larger transformers do (i.e. "40/50/60 MVA - ON/FA/OF") then you typically chose the mid-rating of 50MVA for any calculations using transformer capacity.

However, for calculating the short circuit current for fault protection you should always use parameters which will maximize the short circuit fault currents to determine the highest possible ratings your equipment will need. More information on these calculations here: Isc fault Calcs

If you are just doing a calculation to determine the Isc at the transformer terminals for a textbook question, I would use the lower rating because we design transformers for use under "normal" conditions not the conditions of maximum capacity. From an engineering standpoint additional stages of cooling are designed for system fluctuations and not typical use. Although in practice they may use additional cooling cycles.

@Andy aka these are the transformer types Joga78 is asking about.

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