The DFIG stator is directly connected to the grid and two bidirectional converters connect the rotor to the electrical network via a DC-link voltage, as shown in Fig. Examples of such advantages are the ability to use a partial sized converter in the rotor to control the power, reducing power losses and cost, reducing efforts on mechanical parts, noise reduction, the control of active power and reactive, and a controllable power factor ( Lamnadi, Trihi, Bossoufi, & Boulezhar, 2016 Ouassaid, Elyaalaoui, & Cherkaoui, 2016). Supersync alternatives generator#Several advantages make the DFIG the widely utilized generator in variable speed wind energy systems over any other configuration ( Li & Haskew, 2009). The wind turbine, with high power in the order of megawatts, can operate with different generators technologies, such as permanent magnets synchronous generator, squirrel cage induction generator (SCIG), and doubly fed induction generator (DFIG). Furthermore, the wind energy conversion system (WECS) has known significant technological developments that have improved the conversion efficiency and reduced the costs for wind energy production. Besides, wind energy is renewable, omnipresent, and inexhaustible green energy resource, which justifies the significant importance according to this energy. In the two last decades, the electrical power produced by employing wind energy sources has been progressively growing, due to the important number of installed wind turbines by several countries. On the other hand, DFIGs operate at supersynchronous speeds at higher wind speeds when P m is high and surplus power is diverted to the grid via the slip rings keeping P g at the required value, which is transferred to the grid via the stator. Since DFIGs are used predominantly for variable speed grid-fed wind turbines the previously mentioned operation is affected in lower wind speeds when P m is low and is augmented by slip power P r to get the required P g. In subsynchronous generating, power is extracted from the grid and fed to the rotor slip rings. In supersynchronous motoring power is extracted from the grid and fed to the rotor and thus slip power augments air-gap power to transfer to shaft. In subsynchronous motoring, power is extracted from the rotor and fed to the grid and thus slip power is recovered. A simplified model (equivalent circuit) of SEIG.īased on the earlier logic, variation of different power with slip under motoring and generation is shown in Figure 12.27a,b, respectively. P m is the mechanical power in the shaft transferred to an external mechanical port – load or prime mover to extract or feed power – which is equal to P g minus rotor copper loss ( P cur).įigure 12.30. P g is the air-gap power transferred from stator to rotor equal to P e minus stator copper loss ( P cus) and core loss. This facilitates suitable voltage transformation to interface the machine with the grid. Often a three-winding transformer is used with two primary windings connected respectively to stator and rotor (through a converter) and one secondary winding connected to the grid. A bidirectional power electronic converter transforms power P r at rotor frequency to mains frequency f via a DC link, which in turn is fed to the grid through a transformer. From rotor slip rings, power P r is extracted at rotor frequency f r equal to s f where s is the slip. Here the stator is directly connected to the grid at nominal voltage and frequency f, drawing power P e from the grid. A schematic of doubly fed induction machine (DFIM) is shown in Figure 12.20.
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