hi haribo
I also had a 30-min phd interview. They allotted only 30 min per applicant (their technician told me!).
Dont worry about it. During the 30 min, try to answer their questions to the best of your ability.
For example, they might ask u, "why do u want to do a phd"
answer it confidently and IMMEDIATELY, dont wait to mull or think because you only have 30 minutes.
Make the best of it.
for my interview, I was asked why I wanted to work on the project, what would my contributions be, a bit about my background and what further recommendations to that project (if it happened, after that etc.).
all in 30 minutes.
you can do it too.
good luck
satchi