Siemens will supply 54 3.2MW turbines for the Clyde extension, which will bring the wind project from 350MW to 522.8MW.
The new turbines will be delivered to the site in June 2016, with completion expected in June 2017. They will have a 101-metre diameter rotor.
Siemens supplied its 2.3MW turbines to the original 350MW project completed in 2012 – also with a 101-metre diameter rotor.
The current project comprises three sections covering 47 square kilometres in southern Scotland.