DFIG wind turbines use wound-rotor induction generators,
where the rotor winding is fed through a back-to-back
variable-frequency PWM converter as shown in Fig. 2
[13–15]. The converter system permits the two-way transfer
of power. Converter 2 (C2) is fed from the generator-stator
terminals via a reactive link, and provides a DC supply to
converter 1 (C1) that produces a variable-frequency threephase
supply to the generator rotor via slip rings. The
frequency of the rotor supply is controlled so that, under
steady conditions, the combined speed of the rotor plus
the rotational speed of the rotor-flux vector with respect to
the rotor matches that of the synchronously rotating statorflux
vector fixed by the network frequency. Voltage limits
and an overcurrent ‘crowbar’ circuit protect the machine
converters. For the model used, if the DFIG rotor current
exceeds a value of twice the rated current, then the rotor is
short-circuited. Whenever the rotor current is lower than
this value, normal control is exercised.
The favoured way of representing a DFIG for the
purpose of analysis, simulation and control is in terms of
direct and quadrature axes (dq), which form a reference
frame that rotates synchronously with the stator-flux vector.
In terms of this form of representation, adjustment of the
dq-axis components of the rotor voltage provides the
capability of independent control over two generator
variables. A conventional third-order model using rotor
speed or and the dq-axis components of the internal
generator voltage eDfig as state variables was employed.
Inertial dynamics are expressed employing a single-mass
model of the turbine and generator-rotor system. A full
description of a DFIG wind-farm model is provided in
[8, 13–15], and the basic equations are presented in
Appendix 1 (Section 10.1).Figure 3a shows the vector-diagram representation of the
operating characteristics of a DFIG where eDfig represents
the internally generated voltage vector in the stator (often
referred to as the voltage behind transient reactance). The
magnitude of eDfig depends on the magnitude of the rotorflux
vector wr. This flux is dependent on the generator
stator and rotor currents, iis and iir, but can be manipulated
by adjustment of the rotor-voltage vector vr.