Y-Y 1:1 Transformer zero sequence impedance measurements

[waxcomb]

For a YNyn0 1:1 isolating power transformer, I've been given a test report which has open circuit, and short circuit tests of zero sequence impedance, done separately for each winding (see attached .png ).

There are slight differences in results for either open or short circuit tests from the same winding (0.1mΩ), but for the same test taken from the opposite winding there is a difference of just over 40mΩ (??).

I find these results a bit confusing. I would have expected very similar measurements for the short circuit test especially, taken from either side. My guess is that it might be due to a difference in magnetizing impedance seen by each side due to characteristics in the transformer construction, but I'm really not sure.

 

[Kiribanda]

waxcomb,
I think you are correct. When the secondary side terminals are floating (open circuited) the impedance should be higher than when the same terminals are shorted. Because there is no path for zero seq
currents when the terminals are floating. Therefore, we need a higher voltage to push the currents during floating. May be you have to redo the test and witness the test.