Doubly fed induction generators (DFIGs) are used in harnessing wind energy. The principle of operation of doubly wound induction machines was described in steady state in Reference . In this chapter, we will mathematically describe doubly wound induction machines in order to apply vector control. As an introduction, Fig. 7-1 shows a doubly fed induction generator.
The cross-section of a DFIG is shown in Fig. 7-2. It consists of a stator, similar to the squirrel-cage induction machines, with a three-phase winding, each having Ns turns per phase that are assumed to be distributed sinusoidally in space. The rotor consists of a wye-connected three-phase windings, each having Nr turns per phase that are assumed to be distributed sinusoidally in space. Its terminals, A, B, and C, are supplied appropriate currents through slip-rings and brushes, as shown in Fig. 7-1b.
The benefits of using a DFIG in wind applications are as follows: