How the variable speed of the wind is connected to the stable frequency of the grid?

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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