Transformer

[RAVWARD]

Hi,

IF case of a 20/26 MVA Oil natural fan cooled transformer, if the base impedance is %z =7 on 20 MVA, and when the loading is 26 MVA, will the percenatage impedance change and so will the fault contribution of Transformer. I mean, will a transformer generate different fault current depending upon its loading. Please advise.

 

[bacon4life]

The fault current is independent of loading. The physical impedance will not change with loading either.

 

[RAVWARD]

The fault current depends upon percentage impedance and that depends upon loading. If I am right, then fault current should change. Physical impedance is in ohms. You are right but then why percenatge impedance comes in to play when fault current needs to be calculated.
Thanks

 

[stevenal]

% impedance is based on the rating of the transformer, not it's loading. The rating remains constant and so does the impedance and fault current.

 

[rbulsara]

To rephrase what bacon4life said, the %Z is based on the base rating only, that is 7% at 20MVA in your case. The actual ohmic values of internal impedance does not change with loading. The fault current would not change just because you choose a different base for fault calculaitons.

It is correct that if you choose 26MVA as your base then the %Z will change numerically. For that matter you can choose even 100MVA as your base too. The fault current will not change.

 

[waross]

If the percent impedance is 7% at 20 MVA it will be 9.1% at 26 MVA. 7% x 26/20 = 9.1%
Another way, full load current at 20 MVA is 7% of the available fault current.
Full load current at 26 MVA is 9.1% of the available fault current.
respectfully

 

[cuky2000]

The nameplate impedance is based in the self cooling rating and the nominal voltage at tap 0.

Conversion of the impedance is required to reflect the new base values. Should not be significant difference in the SC calculation with difference base values.