I found something, but additional informations are welcome...
The bushing is mounted on a turret which is in the power transformer. The turret is contained in the transformer tank and it, as well as the bottom part of the bushing, is immersed in oil. The bottom part of the turret contains a current transformer which measures the amount of current flowing in or out of the power transformer. The current transformer is directly connected to protection relays in the substation.