The 3,300 tonne topside of the transformer has now been installed on its foundation in water at a depth of 23 metres.
Seaway's crane vessel Oleg Stashnov completed the work at the project in the North Sea, 23 kilometres from the German coast.
Now that the transformer is in place, all work on the wind farm is complete, with the final turbine installed earlier this month. However, The HelWin1 grid connection to shore, which will serve the project, is not due to be completed until early autumn.
The project, which is being developed by WindMW, is made up of 80 Siemens 3.6MW turbines.