Will a Secondary Surveillance Radar be affected by a 300m high wind turbine?
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.