yooper,
In my opinion, you need some type of EGC conductor between the main switchgear and the transformers. This is needed to clear any faults on the primary side of the transformer. Each of these transformers would be separately derived systems and would need a grounding electrode system. If the secondary side has a neutral, they will also need a main bonding jumper.
Where I am currently working, we use a similar set up only with 4.16kV distribution. The 5kV cables have either wire or tape shield. These shields are not suitable for use as a grounding or neutral conductor. We pull in a bare or 600 volt insulated EGC in each raceway. This is bonded to the frame of the transformers. The size is based on the rating of the primary OCPD and Table 250.122.
Don
