potential transformers

Well, I don't mean they have the chance of growing up and becoming real 

I have seen that Richard Hull and the TCBOR use something they refer to as 
only: potential transformers . They seem to be rather more compact than 
pole transformers and can handle maybe 3KVA or so at 15000 volts...

Anybody know what these are normally used for and where one might be able 
to obtain one and how much. Talk about run on sentences!

-Ed Harris (to avoid confusion)