In general i would say that if a ground foult occurs in the secundary of the transformer the ground protection won't "see" the zero sequence component of the fault. A severe fault in the secondary will probably motivate de fuse in the primary of the transformer to blow/trip.
Tale a look at this site:
http://www.engineering.schneider-electric.se/Attachments/ed/cat/catalog_easergy_flite_110_sa_datasheet.pdf.pdf
schneider flite's are easy to mount (live) and are reliable. They work either by the effect of magnetic or electric field so i think that would work in your case.
I'm trying to determine how other utilities identify the faulty medium voltage cables after a switchgear trip in a main substation. Assuming that a network is entirely underground, do you measure each segment of the network and a progressive feed oh healthy sections or you do live connection of...
