Zero-Sequence estimate: use Reactance or Impedance?

[siberhusky]

We have formulas that estimate the zero-sequence impedance for CORE transformers based on the positive sequence values as follows:

2-winding: Z0 = Z1 x 0.90

3-winding: ZHL0 = 0.85 x ZHL1
ZHT0 = 0.75 x ZHT1
ZLT0 = 0.90 x ZLT1

Should it be the impedance that is used for the estimate in these equations, or should it actually be the reactance that is used? My understanding is that the resistance values don't change.

 

[dpc]

I would use whatever X/R ratio you have for the positive sequence as an approximation.

If you use only X, the calculated X/R ratio at the fault can be misleadingly high.

 

[siberhusky]

Thx for the reply, dpc...

That's not what I meant, 'tho. We do have calculations for the resistance... what I mean is, does the derating of the positive sequence values apply to the X or the Z. That is, should the equations instead be:

2-winding: X0 = X1 x 0.90

3-winding: XHL0 = 0.85 x XHL1
XHT0 = 0.75 x XHT1
XLT0 = 0.90 x XLT1

It may seem insignificant, but we have some tight breaker duties..

thx...

 

[dpc]

It's not really a "de-rating" -it's just a multiplier. If you use the same X/R ratio, as I suggested, then you are scaling both the X and the R by the same amount. I would not just scale the X.

BTW - if your breaker duties are so close to the rating that this will make a difference, you've got problems. Also, the three-phase fault duties are generally the worst-case for breaker ratings, at least in the ANSI C37 world. C37 allows for more line-to-ground fault current than three-phase. I think it's 15% more for the 5 cycle value.

 

[siberhusky]

Thx!!

 

[JensenDrive]

I believe that for core form xfmrs, if you apply Vo to the wye side, the core tends toward saturation, and what you get what I have heard referred to as a "phantom" tertiary current path. The phantom teriary is actually an increase in excitation current flowing in just the wye winding. The R in this phantom tertiary is the R of just the secondary wye winding, as compared to R of normal through current and fault current that includes both the primary and secondary winding resistances. I do not know how X/R of the phantom tertiary, but it there is some resistance resistance, and is not a pure reactance. I think you should use the mulitplying factors on Z rather than X, unless of course I have missed something.

 

[siberhusky]

thank you both for your follow-ups...