Transformer impedance indexed to k-rating

[Lucas_Sterquino]

Hi everyone,

I have specified a transformer to have 7.5% impedance and k=8. I received a datasheet from the manufacturer with following information:

Transformer impedance - 6.6% @ K=1; 7.5% @ K=8

It is the first time I see a transformer impedance indexed to the k-rating factor. Have some of you seen this before? If so, could you explain how the transformer impedance is measured/calculated based on the k-rating? Furthermore, how would be the test procedure during FAT to confirm this specification?

Thank you,

Lucas Sterquino.

 

[Gr8blu]

As I understand it, the k-factor is a measure of the ability of a transformer to handle non-linear loads (aka harmonic content) with higher k values corresponding to higher harmonic content. Since one method to determine the per-unit impedance of a transformer is ratio the output voltage to the input voltage, the presence of harmonic content may make one or both larger than they might be on a system with no harmonic content (i.e. k = 1).

 

[Lucas_Sterquino]

As I understand it, the k-factor is a measure of the ability of a transformer to handle non-linear loads (aka harmonic content) with higher k values corresponding to higher harmonic content. Since one method to determine the per-unit impedance of a transformer is ratio the output voltage to the input voltage, the presence of harmonic content may make one or both larger than they might be on a system with no harmonic content (i.e. k = 1).

Thank you for the reply, @Gr8blu.

The change in the impedance from 7.5% to 6.6% is really impacting the fault levels downstream. Despite theoretically possible as you explained, when I look at the specification (6.6% @ K=1, 7.5% @ K=8), constructively speaking, I see two different transformers - or it is k=1 rated, or k=8 rated. Therefore, how come the same transformer has two different impedances when performing the short circuit/open voltage tests? Are they introducing harmonics in these tests? Wouldn't be the K-rating test to ensure the temperature build-up is within the limits when the transformer is exposed to a certain level of harmonics? I am interested to see if someone has had this kind of specification before and what that means.

 

[waross]

If those impedance numbers are for the same transformer than I would assume that they are for different base KVA ratings (ONAN vs ONAF).
When you change the base KVA several other parameters also change.

 

[Lucas_Sterquino]

If those impedance numbers are for the same transformer than I would assume that they are for different base KVA ratings (ONAN vs ONAF).
When you change the base KVA several other parameters also change.

Hi Waross,

You are right, but in this case the impedance is specified to be on the AN mode (from right to left - second column is the project requirement, first column is the manufacturer answer).



I honestly don't understand this answer from the manufacturer. Constructively speaking, if the transformer is set based on the k-rating, expected to find a specific impedance when doing the short-circuit/open voltage tests. Please, let me know if I am missing something.

Thank you,

Lucas.

 

[dpc]

I'm not sure how these numbers are derived - I've never seen this impedance difference listed in transformer data. But you absolutely must use the lower of the two impedances (K=1) for any short circuit calculations. I suppose I could see that a transformer with significant non-linear loads could have a higher leakage reactance under normal operation due to the high frequency harmonics. This might be useful in power flow studies to better predict the voltage drop across the transformer. But in the event of a fault, the short circuit circuit will effectively collapse the voltage and normal loads would be shunted, rendering this higher impedance irrelevant. The temperature rise in the transformer has little effect on the impedance, since the Z is nearly entirely based on the reactance.

 

[waross]

Don't use % impedance for load studies, use % regulation.