IEC 60909 Transformer Impedance Correction Factor

[bdfig]

Hi, I was wondering what reason is behind IEC 60909's transformer impedance correction factor. Is this replace transformer impedance tolerance, or should they both be applied?

 

[EddyWirbelstrom]

The attached spreadsheet calculates the corrected transformer and generator impedances to IEC60909-0 Section 3.3.3 and 3.6.1 resp.
The same form of formula for both transformer and generator suggests that the power factor before the fault influences the impedance.
Flux before and after fault ?
Can anyone comment ?

 

[juleselec]

IEC 60909 uses a simplifying assumption where the pre-fault voltage at the fault location is not calculated (e.g. by a load flow), but rather an equivalent voltage source is used (assumed to be the nominal voltage multiplied by some voltage factor c). Furthermore, the pre-fault load current is not known (i.e. not calculated) and transformers can operate at a wide range of off-nominal tap settings. Therefore, the simplified IEC calculations did not correspond to the calculations based on the more accurate superposition method. An impedance correction factor was thus introduced to improve the accuracy of the calculated fault currents.

In the older versions of IEC 60909 (pre-2001), there was no explicit impedance correction factor for transformers, instead just a suggestion that considerations need to be made. IEC found this unsatisfactory and introduced the correction factor in the 2001 edition. The correction factor was essentially defined empirically through an statistical analysis of a few hundred transformers. Most of this is described in some detail in IEC 60909-1.

 

[juleselec]

PS. I'm sorry for the font colour, I didn't mean to make the text yellow!

 

[bdfig]

Should the impedance correction factor be applied to the minimum fault calculations aswell or only the maximum fault calculations? I have seen a similar question to this where there was debate since the formula only mentions cmax, but not cmin.

 

[juleselec]

That's a good question because the aim of the impedance correction factor was to correct for the maximum fault current (i.e. when the voltage at the fault location is high). Anyway if you look at Figure 21a of IEC 60909-1, it shows that with a correction factor of Kt = 1, the error is not particularly high at the lower voltage range (i.e. min short circuit). I suppose that by the letter of the law, you should apply the correction factor for min short circuits, but your results would probably be more accurate if you didn't (although possibly less conservative).