Calculating 3 winding impedances to put into SKM model?

[Slarty]

So I am getting confused and feel like different sites/documents are saying different things here. I've got a 3-winding transformer and I've calculated my %R and %X, and therefore %Z values for each winding. However, in SKM you must input these values using the impedances from pri-sec, pri-ter, and sec-ter. I know that to get, say, %R(pri-sec) I should just be able to add my %Rpri and %Rsec together, except they are on different MVA bases because my primary winding has a higher MVA than my secondary and tertiary. So in order to refer all of the impedances to the primary base before I add them, what is the formula?? I know there is a difference between formulas for %Z and Zp.u., and I have %Z. Is it %Znew = %Zold * (Snew / Sold)? Or is it %Znew = %Zold * (Sold / Snew)? Or even %Znew = %Zold * (Snew / Sold) * (Vold / Vnew)^2. Because I have seen all three formulas being used for this same situation online and in books... any advice??

Thanks,
Slarty

 

[mparenteau]

Try this:

https://support.skm.com/KB/View/20316-modeling-3-winding-transformer

Mike

 

[Slarty]

It helps, but when they refer impedances to another base, they are not doing so across voltages so I don't know if I should be factoring voltages in or not since I am referring across voltages

 

[prc]

1) How you calculated % R and %X for each winding? Usually, these are derived from measured values between a pair of windings eg HV-LV1, HV-LV2 and LV1-LV2.

2) % Z varies in proportion to the base MVA.

 

[7anoter4]

Usually, we follow IEC 60909-0

 

[dpc]

Just send your transformer data to SKM tech support and ask them to model it. I think they will help you out.