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transformer zero sequence impedance

transformer zero sequence impedance

RE: transformer zero sequence impedance

Z0 is always <= Z1; but there may be an open between Z0 and the outside world, or there may be a high impedance path across that open giving the practical appearance that Z0>>Z1.  That high impedance path may be a from a magnetic path the involves the tank of the transformer.  Core form and Shell form transformers have different relationships between Z0 and Z1.  There is an extensive table in St. Pierre's Short Circuit Calculations book.

RE: transformer zero sequence impedance

The simple answer for delta-wye xfmrs is that Zo=Z1, but it is a bit more complicated. If you have a three legged core xfmr, (i.e., core form) if you induce zero sequence flux, the flux is in the same direction on all legs. Assume the xfmr legs are verticle. When the flux gets to the top of the xfmr, how does it go in a circle, from top of the xfmr back to the bottom? Through air in the space, and in part through the xfmr tank. The xfmr zero sequence excitation is to some extent similar to an air core reactor. It takes lots of excitation current to induce zero sequence flux in a three legged core xfmr. This high excitation current is in parallel with the positive seqenece impedance branch, and effectively reduces the zero seqeuence impedance, typically stated to be about 90% of the positive sequence impedanc. However, if the xfmr is "core form" it implies the xfmr internally has a path for zero sequence flux to pass from the top to the bottom of the xfmr while staying within a steel path, and in this case, the exicitation current is small, and Z0 ~ Z1, which gives some meaning to what David mentions.  

RE: transformer zero sequence impedance

Oops, besides all my mispellings, in the last sentence above, I should have said "shell form" not "core form". Should have reread it before I posted, not after I posted. Oh well.

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