Question

Can anyone provide useful source or information about Doubly Fed Induction generator ? I can able the basic information on the operating principles of DFIG, however, I want more information about the innovations, statistics , just any additional information about DFIG would be appreciated.

Answer 1

Doubly fed induction generators(DFIG) are similar to AC electrical generator , but have additional features that allow them to run at speeds slightly above or below their synchronous speed. This is greatly useful for large variable speed wind turbines , because wind speed can change abruptly. When a gust of wind hits a wind turbine, the blades of turbines try to speed up, but a synchronous generator is locked to the speed of the grid and can't gain speed. So large forces are developed in the hub of turbine , gearbox, and the generator as the power grid pushes back. This causes damage to the mechanism also causes it's wear . If the turbine is allowed to gain speed immediately when hit by a strong wind gust, the stresses are lesser and the power from the wind gust is converted to useful electrical energy.

One approach to allow changes in wind turbine speed is to accept whatever the frequency the generator develops , convert it to DC, and then again convert it to AC at the desired output frequency using an inverter. This is common for all small house and farm wind turbines. But the inverters required for megawatt range wind turbines are huge and too expensive.

Doubly fed generators are one solution to this. Instead of feeding with DC, and an armature winding where the generated electricity is taken out, there are two three-phase windings, one stationary and one rotating, both separately connected to equipment outside the generator. Thus the term given "doubly fed".

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 and when 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. Hence 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.

One winding is directly connected to the output, and produces 3-phase AC power at the desired grid frequency. The other winding (traditionally called the field, but here both windings can be outputs) is connected to 3-phase AC power at variable frequency. This input power is adjusted in frequency and phase so as to compensate for speed variations of the turbine. Adjusting the frequency and phase requires an AC to DC to AC converter. This is usually constructed from very large semiconductors. The converter is bidirectional, and can pass power in either direction. Power can flow from this winding as well as from the output winding.

Many advanced controllers have been developed so as to harvest maximum power out of DFIG. It is seen as a potential equipment that can help to revolutionise the wind power generation in near future.

Characteristics of a DFIG

Construction of a wind energy conversion system employing DFIG

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