%Z When Using Full Load kVA 1

[recs]

I know (as a rule of thumb) that the %Z of a transformer with multiple kVA ratings changes according to the kVA rating. For example if a transformer has a 2,000 kVA Nominal rating and a 2.500 Full Load kVA rating and the published %Z is 5.5% (at Nominal kVA rating) the %Z at Full Load kVA rating is 5.5 x 2,500/2000 = 6.875%.

Two questions:

1. Is it true?
2. If it is, how or where can I get a good reputable reference that proves this rule of thumb is correct?

Thanks.

RECS

 

[davidbeach]

It's not a "rule of thumb", it just is. Basic definition of what that %Z number is has it tied to a specific kVA rating (in ANSI land that is the self cooled rating and in IEC land it is the top rating). If you want the %Z at a different rating you have to convert the impedance base.

 

[waross]

Another way to prove it is to calculate the impedance in ohms. That value will not change even though the transformer is up-rated for fan cooling or in the case of an IEC transformer for the case of natural air cooling.
Now if the transformer is re-rated to any other KVA base, the impedance in ohms will not change and may be used to calculate the percent impedance voltage at the new KVA base.
Why am I referring to percent impedance voltage instead of just impedance?
To determine the impedance, a transformer at normal full load temperature has the secondary short circuited. A primary voltage is applied and ramped up until full load current is flowing in the shorted secondary winding. That voltage is divided by the rated voltage and the result is the percent impedance voltage, percent impedance or just impedance.
If the KVA rating is increased then the rated secondary current will be increased. If the rated secondary current is increased then the voltage required to drive the current through the shorted winding will also be increased. It will then be a lower percentage of rated voltage.

If you want the %Z at a different rating you have to convert the impedance base.

This works both ways. When a small cash short utility has to restore service after a transformer failure, the only option may be to parallel the two available transformers. If these transformers have different percent impedance voltages then the KVA bases must be adjusted to match the percent impedance voltages to determine the KVA capacity of the pair of transformers. Transformers with equal percent impedance voltages will share a load in proportion to their KVA ratings. It is found that the KVA capacity of two transformers in parallel is less than the sum of their original KVA ratings when the percent impedance voltages are not equal. There are instances when, after re-rating, it is found that the maximum safe capacity of the pair of transformers is less than the capacity of the largest transformer by itself.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Sorry folks. I mis-spoke
If the rated secondary current is increased then the voltage required to drive the current through the shorted winding will also be increased.
It will then be a lower higher percentage of the rated voltage.


Bill
--------------------
"Why not the best?"
Jimmy Carter