Impedance in 3W xfmr

[lume7006]

Hello fellows,

I have one question related to the way impedances for 3W xfmrs are given in a text protocol.
We have the following:
HV-MV = 4.23%
HV-LV = 2.51%
MV-LV = 6.55%

However, the same test protocol has some other values refered to the HV rated power:
HV-MV = 6.77%
HV-LV = 6.70%
MV-LV = 17.45%

My question is: what is the meaning of these impedance values refered to HV rated power?

This curiosity is because we are modelling this 3w xfmr for some studies and according to software manual
the values to be taken are the first ones, however, we are in doubt about the physical meaning of the second ones.

Any idea/comments will be welcome!

 

[waross]

The impedances of each winding pair are the same in ohmic value. % impedances or P.U. impedances are referred to a base KVA rating. If the secondary or tertiary winding has less capacity then the primary winding then the % impedance may be referred to either primary, secondary or tertiary KVA. You may find some correlation between the differing % impedance ratings and the differing primary KVA and secondary KVAs. The available short circuit currents should be the same for each winding when the % impedance values are combined with the proper KVA base.

Bill
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