transformer sequence impedance

[tonyflair]

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

 

[dpc]

tony,

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.

 

[MWGMIKE]

The impedances are based on the percent impedance. Convert the percent impedance to ohms. For a transformer this size, the positive, negative, and zero sequence impedances are the same. Zero sequence impedances only differ when you get to bigger MVA transformers and autotransformers.

If you are concerned about the impedance diagrams, the zero sequence diagram will only be conductive on the wye side of your transformer.

 

[tonyflair]

So what your saying is....
positive sequence impedance (Z1) = 0 + j0.0342 and
zero sequence impedance (Z0) = 0 + j0.0342

Is this correct ?

 

[DanDel]

tonyflair, what they are saying is that:
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.

 

[dpc]

The impedance is 0.0342 pu on a 250 kVA base. You are assuming it is all reactance with no resistance. This is a reasonable approximation, but in reality there will be some resistance. As long as you are only interested in symmetrical current magnitudes, you can neglect resistance. If you are interested in asymmetrical current, then you will need to make an estimate of transformer resistance to get an accurate result.

 

[alehman]

Don't forget that the impedance will include a significant resistive component. R may be in the range of 20% of X. You will have to get that information from the manufacturer (or possibly the nameplate).

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.

 

[JJHorak]

Maybe going off on a tangent, but zero sequence excitation impedance is very low on 3 legged core transformers, which have no zero sequence flux path that stays within the core. To prevent the buildup of zero sequence voltage on 3 legged core xfmrs, one winding is always delta, which effectively shorts circuits any zero sequence voltage applied to a wye widing. Wye-Wye xfmrs are made 4 or 5 legged so that there is a zero sequence flux path.

 

[zelvica]

TONYFLAIR:

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.