Wind Farms do cause returns on primary radar because whilst the turbines themselves are stationary the blade tips have speeds of over 100 knots meaning that radar returns have enough doppler shift to pass through MTI filters.
These effects can be readily observed on a variety of radar screens in the UK on a regular basis. For some radar wind turbines can be in radar line of sight but may not appear on radar screens if they have been removed with blanking or some other form of filtering.
Whilst wind turbines can theoretically affect SSR, VORs and AGA radio as far as I know there are no records or experience of this happening in practice.
The best single publication on this subject is the CAA's CAP764 "CAA Policy and Guidelines on Wind Turbines". Just put CAP 764 in a search engine of your choice and you should be able to find it. There are many others.
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