I have an assignment question that I just can't find the answer to - Why would it not be possible to connect a star star transformer to a star delta transformer? - any help appreciated.
Have you tried figuring out the answer rather than finding the answer? Sit down and draw it out. I got a feeling it produces a short-circuit somewhere or something like that.
For stability of supply, and to provide additional protection via a neutral CT, normal practise is, to use Delta - Star configuration for distributiontransformers.