The "dual fed" generator is new to me (no surprise there), at first glance it doesn't sound possible for a variable speed generator to have any of its windings directly grid connected. My reading of the fairly sketchy information on Enercon's web site suggested that they rectified the generator output then used an invertor to handle synchronisation with the grid.
More reading needed I suspect, as my knowledge of power generation dates back to the middle '80s and was only second hand at that time in any case.
What I was really getting at, is what happens to Wind generation if there's an over-supply to the grid? Do they back off in the way that traditional rotating machinery would, or do they continue to generate and expect other generators to back off?
There used to be a sort of analogy with Hydros since they could change output quickly without thermal problems, so my understanding was that their controls were set more sensitive to grid frequency.