I have a transformer test report in which the zero sequence impedance has measured according to IEEE standard.
YNyn0 Transformer has rating of 132/11kV, 45MVA, 28% impedance with no delta winding.
In the test report there is no single value for zero sequence impedance but have three derived impedance values. They have measured Z1no, Z1ns and Z2no.
That is impedance measured from primary side while secondary open and short, and impedance measured from secondary side while primary opened. Eventually calculated Z1, Z2, and Z3.
These are the steps of deriving.
I assume this Z3 is phantom/ tank delta impedance. My question is what should I used as zero sequence impedance? is it Z3 or any combination of Z1,Z2 & Z3?
Again in DIgSILENT, this winding configuration requires following data.
Zero sequence impedance and zero sequence magnetizing impedance.
Are those values respectively Z3 and Z1+Z2?
quite similar question!
What is the zero-sequence equivalent of a YNy0 transformer?
Best Answer
If the Z3 component is very large, like would be found in a 5-leg core or shell-type unit, use Z12 and ignore Z3. If Z3 is low enough to affect your calculations, like in a 3-leg core design, implement it as a Ynyn0d 3-winding transformer in DIGSILENT PowerFactory. Use the standard pi-to-tee equations to calculate the effective zero sequence impedances Z0ps, Z0pt, Z0st for entry in PF.