There is a need for very large wind turbines in order to meet the increasing demands from renewable energy sources. A directly coupled synchronous generator with a variable transmission is one of the options for very large wind turbines. This wind turbine topology benefits from harmonic free, transformer free, better fault current contribution and greater reliability. However, there are challenges associated with the topology, such as low voltage ride through performance and complex gearbox arrangements.
Wind turbine size and technologies have been developed rapidly over the last decade.
Until 2000, most wind turbines were based on cage induction generators that are directly connected to the grid, as shown in Figure 2. The rotational speed of the rotor is essentially fixed and the rotational speed varies only by a few percentage points. These fixed speed induction generator (FSIG) wind turbines have the advantages of being simple, reliable and well proven. However, FSIG wind turbines suffer from a number of disadvantages. Firstly, fixing the rotor speed generates high mechanical loading on the structure. Secondly, FSIG wind turbines cannot maintain maximum aerodynamic efficiency. Finally, FSIG turbines are not grid friendly in terms of reactive power, low voltage ride through and flicker. For these reasons an FSIG drive train is no longer favourable for large wind turbines.
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