An induction motor where both rotor and the stator windings are fed with 3 phase AC signal is called a Doubly Fed Induction Motor or a 3 phase induction motor. There are multiphase windings on the rotor and the stator bodies. There is also a multiphase slip ring assembly for transferring power. For generating electricity in wind turbine generators, this device is used.
Let us see what is power generation using wind energy before knowing about Double Fed Induction generator.
One of the most renewable sources of energy is wind energy. With the blow of the winds, large turbines are made to rotate and generate electricity. The wind turbine work in a cut in speed (minimum wind speed required for the generator to connect to the power grid) and cut off speed (maximum wind speed required for the generator to disconnect from the power grid).
4 TYPES OF WIND TURBINE GENERATORS:
Type 1: Squirrel cage induction generator, used for smaller wind ranges, is directly connected to the power grid.
Type 2: An AC-DC-AC converter is used an extra fitting for the induction generator before getting connected to the power grid.
Type 3: A wound rotor induction generator which is directly connected to the power grid and rheostat is used for adjusting the rotors speed.
Type 4: It includes a Doubly Fed Induction Generator linked straight to the lines, where the blades rate is modified using consecutive converters.
Doubly Fed Induction Generator.
There us a 3 phase wound rotor and a stator in it and the rotor has an input of 3 phase AC signal which causes an AC current in the rotor windings. With the help of rotation of wind turbines induce the mechanical force on the rotor as result causing it to rotate. The produced magnetic field due to the AC current also rotates at a speed corresponding to the AC signal frequency applied to the rotor windings.
For producing AC signal of constant frequency irrespective of the wind speed, the basic need for electricity generation is wind speed. On the contrary the frequency of the AC signal produced across the stator should be static and it doesn’t matter about the wind speed.
The frequency of the AC signal in the rotor increases as the rotor speed decreases and it is of a positive polarity and on the contrary the rotor speed increases and frequency increases and is of the negative polarity.
There are two back to back converters- a machine side converter and a grid side converter. The machine side converter is for controlling the active and the reactive powers by taking power of the d-q components of the rotor and also torque and speed of the machine. The grid side converter is for maintaining a dc link voltage by making the reactive power drawn to the utility grid to null it ensures the unity power factor. For storage of energy a capacitor is connected between two converters and this provides a fixed voltage frequency and it doesn’t matter about the variable voltage output of the generator.
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