Advanced Electric Drives: Analysis, Control, and Modeling Using MATLAB/Simulink

7Dynamic Analysis of Doubly Fed Induction Generators and Their Vector Control

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 [1]. 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.

Fig. 7-1    Doubly fed induction generator (DFIG) where P and Q inputs are defined into the stator and the rotor.

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 N_s turns per phase that are assumed to be distributed sinusoidally in space. The rotor consists of a wye-connected three-phase windings, each having N_r 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.

Fig. 7-2    Cross-section of DFIG.

The benefits of using a DFIG in wind applications are as follows:

1. The speed can be control over a sufficiently wide range to make the turbine operate at its optimum coefficient of performance C_p.