Type-3 Wind Turbine Generators

The type-3 wind turbine generator model can be built with the doubly fed induction generators (DFIG) and is shown in Fig. 1. The DFIG is a machine that comes with AC excitation. The AC excitation (variable magnitude and frequency) is applied to the rotor, and it is provided by the power converters. The power converters are rated for 33% of the rated power of wind turbines and it provides the facility of variable speed in the range of ±33% with synchronous speed (Ns).

Fig. 1. Type-3 wind turbine generator.

The real power flow between the rotor and converter is bidirectional and it depends on the generator speed in comparison with Ns. If the generator is running above Ns, the power can be generated in both the stator and rotor. The power generated from the rotor is added with the power generated in the stator through back-to-back power converters. If the generator is running below Ns, the real power flows from the stator to the rotor. If the generator is running exactly at Ns, the power flows from the stator to the rotor to meet the winding resistance losses of the rotor.

The real component of the current is controlled by the phase difference of the rotor flux wave and the reactive component of the current is controlled by the magnitude of the rotor flux wave. By controlling the real and reactive currents from the control of power converters, the real power and terminal voltage of wind turbine generators can be controlled. Due to the fast control of back-to-back power electronic converters, type-3 wind turbine generators can provide constant real power and regulated terminal voltage response which is faster than the synchronous generator response.


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