transformer sequence impedance
transformer sequence impedance
(OP)
Can anyone give me some info on determine positive and zero sequence impedances for a transformer ? The one that I'm considering is a 3 phase, delta-wye (wye solidly grounded), 250 kVA, 600 V - 25 kV, Z = 3.42 %.
Thanks in advance
Thanks in advance






RE: transformer sequence impedance
The positive sequence impedance (Z1) is just the standard impedance voltage of 3.42% that you gave. Zero sequence impedance (Z0) looking into the wye winding for a solidly grounded wye will be roughly the same as the positive sequence impedance, or even slightly less, depending on the transformer construction. For core-type transformers, we generally use about 85% of the transformer impedance for Z0, and for shell type you can assume Z0 roughly equal to Z1. The transformer test report may have this info.
You didn't ask, but negative sequence impedance Z2 will be equal to Z1.
RE: transformer sequence impedance
If you are concerned about the impedance diagrams, the zero sequence diagram will only be conductive on the wye side of your transformer.
RE: transformer sequence impedance
positive sequence impedance (Z1) = 0 + j0.0342 and
zero sequence impedance (Z0) = 0 + j0.0342
Is this correct ?
RE: transformer sequence impedance
positive sequence impedance (Z1) = 0 + j0.0342pu and
zero sequence impedance (Z0) = 0 + j0.0342pu; though this is an estimate for Z0.
These values are in pu (per unit) based on the transformer rated kVA and voltage.
RE: transformer sequence impedance
RE: transformer sequence impedance
So you actually have something like 0.007 + j0.0335 pu
Z = sqrt (R^2 + X^2)
Z0 looking in to the Y side is approx. Z1, if the neutral is solidly grounded. Z0 looking into the delta side is infinite.
RE: transformer sequence impedance
RE: transformer sequence impedance
You should find some equivalent circuits of transformers - they depend very much on the connection type. For example if the transformer is connected in Dyn like in your problem the delta winding has infinite impedance (as BrokenHandle mentioned it short circuits the zero sequence currents). Try to find some docs about that in google! And also if the wye winding is grounded by impedance (not in your case) in the equivalent circuit you have to consider the 3*Zgrounding. Afterwars you have the right equivalent circuit, you can do calculations.