The windmill's output is connected to the grid. The variable speed of the air does not make any change in the frequency output because a clever circuit is installed between the generator and the grid.
First of all the direct output of the induction generator is connected to an AC-DC converter and then this converter is connected to a filtering circuit to smooth its DC output. Then that DC is connected to a controlled DC-AC converter (Inverter) and then this AC is connected to the grid. We can control the firing angle of the inverter so we can now control the frequency.
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