The output from the wind turbines is converted to DC, then back to AC again to so the frequency can be matched to that of the power grid. The voltage of the wind turbines I know of are 400-500ish volts, usually to optimise the converters. If the voltage is much lower, you start talking about some huge cables connecting it. There is a few reasons, but thats the basic one. If the generator voltage is higher then construction can be harder as well. After the inverters it is stepped by a transformer to whatever voltage is present where they are connected. 690 might be a common voltage in europe for industrial equipment anyway, and so the wind turbines will be able to use common hardware, thus making it cost effective.