Transformer classifications

[wroggent]

IEEE 141 mentions a few classifications for transformers and I'm trying to understand the differences.

The first classifications mentioned are distribution (3-500kVA) and power (>500kVA). The next classifications mentioned are substation and unit substation. It says the difference between substation and unit substation is that substation has termination facilities for "direct cable or overhead line termination", and unit substation is designed for "integral connection to primary or secondary switch-gear, or both, through enclosed bus connections".

I don't understand the explanation given for the difference between substation and unit substation. Can someone please elaborate on the difference mentioned? Doing a google image search for these classifications returns the same images. (not very useful)

When would one be used versus the other?

IEEE 141 goes on to say the substation class (including unit substation???) is further divided into primary and secondary depending on the voltage rating load side.

Can both distribution and power transformers be substation or unit substation and primary or secondary?

Do the above mentioned classifications have anything to do with pad-mount and pole-mount classifications? If yes, how?

Thank you

 

[Marmite]

A unit substation typically comprises a transformer, MV switchgear and possibly an LV distribution board all in one packaged unit. The unit substation transformer has flanges and bushings suitable for direct connection to the MV/LV switchgear via a metal enclosed trunk unit. Essentially the switchgear is mounted directly on the transformer tank flange.
A substation transformer either has outdoor bushings for overhead outdoor exposed connections, or a metal enclosed cable box for the termination of an underground cable direct onto the transformer. Usually the cooling fin arrangement is different between the two types as for a unit substation you need room to accommodate the switchgear on the transformer tank side without the fins getting in the way.
Regards
Marmite