A few comments that may be of use:
I work for a company called Pager Power that assists wind farm developers that have radar issues.
www.pagerpower.co.uk
CAA Guidance - There is CAA, MOD and wind industry guidance (somewhat dated) which gives a useful overview
http://www.bwea.com/pdf/Wind-Energy-...guidelines.pdf. There is also a more recent CAA policy and guidance document on windfarms CAP764
http://www.caa.co.uk/docs/33/Cap764.pdf
Operational Impact - On a more practical note I received the following ATC message whilst receiving a RIS (Radar Information Service) in Cumbria "Possible slow moving traffic, unknown altitude, x miles ahead - could be a wind farm". We kept our eyes peeled and flew on.
SSR - There appears to be no real impact on SSR although SSR radar are normally safeguarded for wind turbines to 5km or more in the United Kingdom.
Overseas - There are more than 40 large wind turbines within 10km of Kastrup airport in Copenhagen and there is a similar number of turbines in the industrial area of Amsterdam, to the north of Schipol. The UK generally takes a more cautious approach to wind turbine radar safeguarding than other countries.
Lighting - Almost all onshore turbines in Britain are unlit. Where they are lit there are two medium intensity red lights mounted on top of the nacelle.