Curiosities: Why, when I see wind generators on a windy day, are some rotating and others not?

There are several possible reasons why a turbine may be still even when the wind is blowing, says Giri Venkataramanan, a professor of electrical and computer engineering at UW–Madison. The turbine may be down for reasons such as maintenance or repair. 

Because large amounts of electricity cannot be stored at reasonable cost, power must be used as soon as it is generated. Therefore, the turbine rotor may be stationary, with the “parking brake” set, during a period of low demand. “The blades are feathered, so the wind escapes safely even though the blades are not turning,” says Venkataramanan.

Finally, it is possible that the stationary turbines were just installed and are being held stationary until they can be hooked up to the electric grid.