A Doubly Fed Induction generator as its name suggests is a 3 phase induction generator where both the rotor and stator windings are fed with 3 phase AC signal. It consists of multi phase windings placed on both the rotor and stator bodies. It also consists of a multiphase slip ring assembly to transfer power to the rotor. It is typically used to generate electricity in wind turbine generators.
Before going to further details about a Double Fed Induction Generator used in wind turbine generators, let us have a brief idea about power generation using wind energy.
As we are already familiar, wind energy is recently one of the most widely used renewable sources of energy. Large turbines are made to rotate according to the blowing of the wind and accordingly electricity is generated. Generally the wind turbine generators work in a range of wind speed between the 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: It consists of a squirrel cage induction generator connected directly to the power grid. It is used for a small range of wind speed.
- Type 2: It consists of an AC-DC-AC converter in addition to the induction generator before being connected to the power grid.
- Type 3: It consists of a wound rotor induction generator connected directly to the grid, where the rotors speed is adjusted using a rheostat.
- Type 4: It consists of a Double Fed Induction Generator connected directly to the grid, where the rotor speed is adjusted using back to back converters.
Basic introduction to the electricity generation from the wind energy using Double Fed Induction Generator.
The DFIG consists of a 3 phase wound rotor and a 3 phase wound stator. The rotor is fed with a 3 phase AC signal which induces an ac current in the rotor windings. As the wind turbines rotate, they exert mechanical force on the rotor, causing it to rotate. As the rotor rotates the magnetic field produced due to the ac current also rotates at a speed proportional to the frequency of the ac signal applied to the rotor windings. As a result a constantly rotating magnetic flux passes through the stator windings which cause induction of ac current in the stator winding. Thus the speed of rotation of the stator magnetic field depends on the rotor speed as well as the frequency of the ac current fed to the rotor windings.
The basic requirement for the electricity generation using wind energy is to produce ac signal of constant frequency irrespective of the wind speed. In other words the frequency of the ac signal generated across the stator should be constant irrespective of the rotor speed variations. To achieve this, the frequency of ac signal applied to the rotor windings need to be adjusted.
The frequency of the rotor ac signal increases as the rotor speed decreases and is of positive polarity and vice versa. Thus the frequency of rotor signal should be adjusted such the stator signal frequency is equal to the network line frequency. This is done by adjusting the phase sequence of the rotor windings such that the rotor magnetic field is in the same direction as the generator rotor (in case of decreasing rotor speed) or in opposite direction as the generator rotor (in case of increasing rotor speed).
The whole system consists of two back to back converters – a machine side converter and a grid side converter, connected in the feedback loop of the system. The machine side converter is used to control the active and reactive powers by controlling the d-q components of the rotor and also torque and speed of the machine. The grid side converter is used to maintain a constant dc link voltage and ensures the unity power factor operation by making the reactive power drawn from the utility grid to zero. A capacitor is connected between the two converters such that it acts as an energy storage unit. This back to back arrangement provides a fixed voltage fixed frequency output irrespective of the variable frequency, variable voltage output of the generator. Other applications of the induction generators are fly-wheel energy storage systems, pumped storage power plants, power converters feeding a railway power grid from public grid where the frequency is fixed.