We have purchasing standards which are particular to our system, though specs may match others' by coincidence. Most of these are electrical, e.g. how the primary and secondary windings are configured (delta-wye in most cases) but also what gets mounted to the outside (bushing potential devices, current transformers, gas monitoring units, etc. These are not unique to each site which is why there can be so much work to replace one.

Older stations were actually designed around the transformers in some cases and the physical configuration of a 50+ year old failed unit is rarely like those produced more recently. We might have a unit on hand that is electrically equivalent but impossible to install without essentially rebuilding the entire station.

To give a rough overview of the range of transformers on our system, the first thing we care about is the voltage and MVA rating, then whether the bushings are open air or gas insulated, what the winding configuration of the primary and secondary are (delta-wye being most common), whether the unit it is replacing has a tertiary winding (sometimes at transmission voltage, other times distribution or for light and power to the station), and then maybe what accessories it comes with such as current transformers for protective relaying.