The Coronation of Queen Victoria is an event happening to 
United Kingdom early in the game usually within the first 4 years. It removes 
Hannover as a Puppet of Great Britain as well as removing all Influence and setting their attitude to Opposed. On the other hand, it gives 5 prestige.
It can happen after the year 1838 if Great Britain is either an absolute monarchy, Prussian Constitionalism or HM's Government and Hannover is a puppet of United Kingdom. It has a mean time to happen of 6 months.
When attempting to form the 
North German Federation, it's advisable to wait until this event happens before attempting to raise influence in Hannover beyond Friendly.