Interbus Transformer Scheme

The winding configuration of a transformer really depends on the application. There is not just one scheme that we can point to for an EHV transformer. I think your "Interbus Transformer" is just terminology used for a specific application of a transformer.

The nameplate info you give is for a three-winding transformer: A Wye-connected primary winding, wye-connected secondary, and delta-connected tertiary. A third (tertiary) winding is often included in a EHV transformer to give a lower voltage source for auxiliary power (usually medium voltage, maybe 11kV or 13.8kV, depending on where you are). the primary and secondary do your main transformation, and have the highest MVA rating, while the tertiary is usually lower rating.

Your IBT may be a single-phase transformer, but the application uses three of them together to do the job? I'm only guessing here on that. Very large transformers sometimes are made up of 3 single-phase units. However, to make a delta connection would have to be done externally by conductors between the 3 units.

I assume that IBT is Inter-Bus-Tie. If so, please stand corrected that it is not a 'single-phase' transformer but a "Single-Winding" transformer. Yes! Generally, IBTs are Auto-Transformers, particularly when they are tying two EHV Yards like 400kV & 220KV. This is to reduce the insulation cost between the primary and secondary.

Even this IBT Auto transformer is three phase and it does have a primary and secondary (HV and LV to be more precise) and, in many cases, a Tertiary (MV to be more precise) too. And, when you have three sets each of the HV, LV and MV, you can connect them to any Vector Connection. Can't you?