Will a Secondary Surveillance Radar be affected by a 300m high wind turbine? 60 wind turbines are located at a distance of about 30 km away from a Secondary Surveillance Radar installation; what will be the impact of this?

My less than stellar knowledge of radar systems tells me that there shouldn't be an issue, since its a secondary radar and not a primary radar (the blades are big enough to be mistaken for an aircraft) which relies on reflection.

But the 30 km distances makes me doubt if this will actually be the case, might a 300m high obstacle in a radar's direct line of sight might cause problems?
No they won't, since secondary surveillance radar (SSR), as you point out, does not rely on reflecting electronic pulses, but rather on digital communication with an aircraft transponder.

Transponder signals do still travel in a straight line (line of sight), so obviously putting a very large obstacle (such as a mountain) close to an SSR will limit the radar coverage in the area behind the obstacle.