They were shipped from the Belgian port of Antwerp and will be installed at the site, 85-kilometres off the Dutch coast, by 19 August, according to a Gemini spokesperson.
Belgian consortium FICG won the contract to design, build and install the substations for the project in June 2014. The FICG consortium is made up of engineering firms CG and Cofely Fabricom, and steel construction company Iemants.
The two substations will serve the 600MW project, which is due for completion by 2017.
FICG has previously supplied the electricity converter station for the 129MW Luchterduinen offshore wind project, also in the Netherlands.
Installation of both jacket foundations for the substations was completed on 8 August at the project site in the Dutch North Sea.
Each jacket foundation measures 58 metres high and 32 metres wide.
The project will use Siemens' 4MW turbine with a 130-metre rotor, with monopile foundations. Earlier this month, EPC contractor Van Oord installed the first of the 150 monopiles from its Aeolus vessel.
Gemini is owned by Northland Power (60%), Siemens (20%), Van Oord (10%) and HVC (10%).