Doubly Fed Induction Generators on wind turbines
In: Engineering
This isn’t my area of expertise, but here’s what I remember from a class that discussed DFIGs – hopefully this is directionally correct.
An AC (alternating current) electric generator produces power by spinning. The rate at which it spins is measured in Hertz (Hz), or cycles per second. Some countries’ AC power grids use a frequency of 60 Hz; others use 50 Hz. An induction generator produces power by spinning faster than the frequency of the power that’s going in to the generator. For instance, if there’s 60 Hz grid power going into an induction generator, it would only generate power if it is spinning faster than 60 Hz. However, this 60+ Hz AC power couldn’t be used by the grid, because it would be out of sync. The power would need to all be converted into DC, and then converted to 60 Hz AC. This approach is used for some smaller wind turbines, but it’s not very efficient.
Instead, most large modern wind turbines use a Doubly Fed Induction Generator. Instead of having 60 Hz grid power flow directly into the induction generator, the DFIG converts this grid input power to DC and then to AC power. The frequency of this AC power depends on how hard the wind is blowing. If the wind is blowing hard, low-frequency power will be supplied to the induction generator; because there’s a large difference between that frequency and the rate at which the turbine is spinning, there will be a large electromagnetic force that resists the turning of the generator (meaning that a lot of power is produced). If the wind speed is lower, higher-frequency power will be supplied to the induction generator; because there’s a small difference between that frequency and the rate at which the turbine is spinning, there will only be a small electromagnetic force to resist the turning of the generator (meaning that less power is produced). By constantly adjusting the frequency of the input power up and down, the output power is kept in sync with the grid.
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