The Puget Sound Energy project, installed in Garfield County, south-east Washington, was completed in 2012 and comprises 149 Siemens SWT-2.3-101 turbines.
Siemens will continue to service the projects, as it has done since it was commissioned.
It will also install a power boost upgrade and a high-wind retrofit to all of the turbines to avoid complete shutdown of the machines in wind speeds over 25m/s.
The manufacturer expects the upgrade could increase yield by 4% in certain site conditions, it said.
The extension comes a week after Siemens announced an operating and maintenance North American partnership with developer Duke Energy.
