Autotransformer positive sequence impedance 2

[usercan]

Hello,
I've been working on symetrical components of transformers. I found a paper in which there is a table shows the impedance values of a 275/132/33 kV autotransformer. Is this a correct representation of impedance values ? Since the unit is in PU, isn't it supposed to be the same impedance value for both HV and LV side ? Why are there different positive sequence values for HV and LV side ? Is tranformer %Z impedance is the sum of positive sequence impedances of LV and HV sides ?

 

[cranky108]

There are several good books on the subject, but what does the nameplate say?
Also this can't be a test report, as there are no R values.

 

[jghrist]

From Westinghouse T&D Book:

It looks like your table has the values that can be used in the positive and zero sequence diagrams, based on test valuse of Z45, Z46, etc.

 

[bacon4life]

The individual values in the table have no direct physical meaning; the negative values for TV do not indicate capacitance. Instead, the values only make sense when used as part of the entire sequence network.

There are two common sequence models for autotransformers. The 'T' model for autotransformers has a fictitious middle point. Rather than HV, LV and TV, the values in your table are HV-middle, LV-Middle and TV-Middle. The image from jghrist shows how the 'T' model values can be derived from values measured in the actual short circuit tests.