345kV/34.5kV Transformer Impedance & X/R 1

[cyriousn]

Sorry to start another one of these threads but I've gone through some of the old threads where Cuky2000 has responded and have a few questions. For a 345kv/34.5kv transformer is there a reasonable way to estimate the impedance and X/R? I've been looking at the Cuky2000 table named "Standard Impedance based on 55C Rating of Largest Capacity Winding" but for 345kV my understanding is that the BIL would be higher than 900kV. The only other reference I know of is the old Westinghouse book but that doesn't help over 230kv.

The transformer size that I'm looking at is rated 76/101/126MVA with two stages of forced air which I've assumed would output roughly 133% / 166% of the base rating.

Also when it comes to the X/R values, does anyone know how to read the ANSI C37.010 document that I've attached? There are two tables, is it safe to assume that the bottom table is completely separate from the one above? The one above says for FOA (Forced Oil & Forced Air) which I don't have but right below it in the same font it talks about OA (self cooled) and FA (Forced Air Cooled) transformers. Are these adjustment factors meant for the table above?

For the 76/101/126MVA transformer example would the bottom chart be used by using the self cooled rating or would I use the 2nd stage cooling rating multiply by 1.25 and use the upper chart and find a value somewhere in the middle of the road?

 

[JJ_Roy]

Here's a pretty good technical guide on transformers that was produced for the AESO.

AESO Transformer Modelling Guide

I believe there's a section on X/R ratios

 

[cyriousn]

Thanks for sharing that link, lots of good info in there to read up on! I was able to get some data points for similar sized 345/34.5 utility transformers and the impedance ranged between 9-12.5% and X/R ranged between 40-49