Zero-Sequence estimate: use Reactance or Impedance?
Zero-Sequence estimate: use Reactance or Impedance?
(OP)
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.
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.






RE: Zero-Sequence estimate: use Reactance or Impedance?
If you use only X, the calculated X/R ratio at the fault can be misleadingly high.
RE: Zero-Sequence estimate: use Reactance or Impedance?
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...
RE: Zero-Sequence estimate: use Reactance or Impedance?
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.
RE: Zero-Sequence estimate: use Reactance or Impedance?
RE: Zero-Sequence estimate: use Reactance or Impedance?
RE: Zero-Sequence estimate: use Reactance or Impedance?
thank you both for your follow-ups...