State Premiums are only awarded in Germany - the UK equivalent for Hanoverians is Elite mare.
To be awarded St Pr St in Lower Saxony, mares have to gain marks over 7.0 on their ridden performance test & be awarded 1A at a mare show as a 3 or 4 year old - they can only go to one mare show a year.
If they are awarded a 1A they are then become a State Premium candidate - St Pr A.
When they foal within a specified period - 3 years I think? they are then St Pr St.
I am not sure if this is similar in all German studbooks?