Doubly fed induction generators

Doubly fed induction generator         Since new wind turbines operate at variable speed the generator must also be able to cope with variations in rpm. One of the ways to achieve this is through a Doubly Fed Induction Generator (DFIG) generator.

Variable speed design choices in wind turbines
        Most modern wind turbines are nowadays equipped with variable speed generators. The most common application is the Doubly Fed Induction Generator.The stator of such a generator is directly connected to the utility grid. The electrical rotor, equipped with windings, is connected through a set of sliprings to a converter. The converter is able to feed the rotor with variable frequency AC. This rotor AC frequency is chosen such that on the stators the grid frequency is obtained despite variation s in the rotational frequency. A range from roughly 60% to 110 % of the rated speed can be obtained which is sufficient for a good energy yield. If the gearbox ratio is chosen such that the synchronous speed of the generator just falls in the middle of the speed range, in this case at 85% of rated speed, then the lowest converter power rating is obtained.
        A converter rating of roughly 35% of the rated turbine power is sufficient. At wind speeds above the rated wind speed, the power is reduced by pitching the blades.
        Especially for these DFIG generators it is difficult to stay connected to the grid in case of grid faults.

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- Doubly fed induction generator: general description, advantages-disadvantages