Torque vectoring gears for smaller, more efficient wind turbines
Torque vectoring is a relatively new technology that has been employed in automobile differentials, most commonly all-wheel-drive vehicles, that allows the amount of power sent to each wheel to be varied. Scientists at the Technische Universitaet Muenchen (TUM) have now adapted this technology to wind turbines, to eliminate the need for converting the alternating current produced by the turbines into direct current and back again before it is fed into the grid.
As the rotational speed of the wind turbine, and thus the generator that is connected to the rotor via a gearbox, changes depending on the force of the wind, the alternating current it produces must first be rectified so that it can be fed into the grid with the correct frequency – usually 50 or 60 hertz. To accomplish this, the alternating current from the wind turbine's generators is transformed into direct current using giant rectifiers before being transformed back into alternating current of the right frequency. This twofold conversion process results in a loss of close to five percent.
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To attain the desired grid frequency of 50 hertz, a generator with the usual two poles pairs must operate at a synchronous speed of exactly 1500 revolutions per minute. The scientists at TUM developed an active torque-vectoring gear similar to a controlled differential in a motor vehicle, that could operate at this speed in spite of the variable input rotational speed of the rotor.
In the TUM system, as in conventional designs, planetary gears generate most of the transmission required, but these are supplemented by a torque-vectoring gear with a supplemental electric motor that can be used as both a drive and a generator. This motor allows the power from the rotor to be either boosted or diverted to ensure a constant rotational speed for the generator. The researchers say that an electric motor of about 80 kW is sufficient for a 1.5 MW wind turbine.
By doing away with the need for giant rectifiers, the TUM system results in a lighter power train that doesn't require as large a wind turbine nacelle. Also, the researchers say that because a robust, low maintenance synchronous generator can be used, there's no need for powered electronics for frequency adjustment, which results in a boost to the overall efficiency of the wind farm.