Around 1,500 of the 1,800 traffic signals in Germany's second biggest city suddenly blacked out and lights at shopping centres also failed.
An incident at a transformer of the Kruemmel power station triggered the blackout, said a spokesman from Swedish firm Vattenfall, which runs the plant.
There were also power cuts on Wednesday thought to have been caused by a transformer from the plant, which is close to the northern port city.
The plant only reopened within the past fortnight after a two-year shutdown triggered by a fire.
The fire in 2007, and incidents at the Vattenfall-run nuclear plant Brunsbuettel in the same year, drew criticism in Germany at the time.