How to choose the short circuit power of a transformer to calculate the max.short circuit current ?

[Roleci]

I want to calculate the max short circuit current of a power transformer.But I'm confused about something.The rating power on the nameplate is 100 MVA but the transformer can be operated up to 110 MVA for short periods.In this case,what power should I choose to calculate the max. short circuit current ? 100 MVA which is the nominal power or the max.loadability power which is 110 MVA ?

 

[cuky2000]

Use the transformer self cooling rating (ONAN).

I_SC(kA)= ( MVA_ONAN/[1.73xkV_RMS)x(%Z_transformer + %Z_source)]x10².

^{Notice that in most applications, satisfactory results for SC is obtained for Z_source =0 (Infinite bus)}

 

[Roleci]

ONAF POWER rating of the transformer is 100 MVA.But it still can be operated up to 11O MVA for a a short time in case the second transformer is out of service.I want to know the maximum short circuit current the transformer is able to deliver.

 

[jghrist]

I think that impedance of IEC transformers is based on the highest rating, not the lowest. ANSI (North America) transformer impedances are based on the lowest (ONAN) rating. The base should be noted on the nameplate.

 

[davidbeach]

Right, and in either case, ANSI or IEC, the transformer is the transformer. Impedance is a physical property, although the resistive component of the impedance is temperature adjusted, and the impedance doesn't change based on loading. Max fault current is max fault current. If you want to change from a 100MVA base on that transformer to a 110MVA base, the impedance will be 90.909% as much; but the fault current will be the same.

 

[Roleci]

Let me make this clear.So impedance of a transformer is constant if we ignore temperature.In this case when I change the power from 100MVA to 110MVA, the fault current must increase according to the formula since other values are constant.ISC(kA)= ( MVA/[1.73xkVRMS)x(%Ztransformer + %Zsource)].

 

[Parchie]

@Roleci (Electrical),
In IEC land, I know they use a C factor for correcting the impedance and the voltage changes! I also knew C factor = 1.1 when the system voltage is greater than 1kV. The same thing that you were worried about when you are faced with a choice between 100MVA and 110MVA!

We usually use the ONAN rating of a transformer when doing fault calculations (100MVA). Hope that helps.

 

[argotier]

Transformer impedance is constant, as david said, when you talk about its absolute physical magnitude (in ohms).

BUT with the formula cuky gave you, you are working with a % impedance(relative to the power base). Reread what david wrote taking this into account.

So you need to use the same transformer power that corresponds to the % impedance. It should be clear in the nameplate but as already said, the usual is an ONAF base impedance for IEC and a ONAN base for ANSI. Is not a mather of choice, the MVA should match the base.

Hope it helps.

 

[7anoter4]

In order to calculate short-circuit current one needs constant c, indeed. However, in order to calculate short-circuit impedance no need of this constant [See IEC 60909-0 or IEC 60076-1]:
IEC 60909-0 3.3.1 Two-winding transformers:
The positive-sequence short-circuit impedances of two-winding transformers ZT= RT + jXT, with and
without on-load tap-changer can be calculated from the rated transformer data as follows:
ZT=ukr%/100*UrT^2/SrT RT=PkrT/3IrT^2
where:
UrT is the rated voltage of the transformer on the high-voltage or low-voltage side;
IrT is the rated current of the transformer on the high-voltage or low-voltage side;
SrT is the rated apparent power of the transformer;
PkrT is the total loss of the transformer in the windings at rated current;
ukr is the short-circuit voltage at rated current in per cent;
IEC 60076-1/2000 4.1 Rated power
If different values of apparent power are assigned under different circumstances, for example,
with different methods of cooling, the highest of these values is the rated power.

 

[7anoter4]

About the tapped windings:
IEC 60076-1 3.7.1 short-circuit impedance of a pair of windings
In a transformer having a tapped winding, the short-circuit impedance is referred to a particular tapping. Unless otherwise specified the principal tapping applies.

 

[cuky2000]

No, the % impedance is not constant. %Z_110MVA > %Z_100MVA. However, the MVA_SC is virtually constant: 100MVA/%Z_100MVA = 110MVA/%Z_110MVA. The same rational apply to the SC current and for simplification let's assume that %Z_Source=0.
. >> I_sc = I_FLC@100MVA/%Z_100MVA = I_FLC@110MVA/%Z_110MVA.
_{Converting Impedance: %Z110MVA≅ %Z100MVA x(110MVA/100MVA)}

This may be a good opportunity to review how the %Z of a transformer is determine:
_{1) One winding of the transformer is short circuited,
2) Gradually voltage is applied to the other winding to create full load current to flow in the short circuited winding.
3) This voltage is known as the impedance voltage.
4) The transformer nameplate impedance value is given in percent of the MVA base.}

We are hoping that the below illustration and the following Video help to clarify what impedance or MVA should be used to calculate the transformer SC current.