Inverter output filters and EMI filters are rated for 1
p.u. output power, making filter design difficult and
costly.
Converter efficiency plays an important factor in total
system efficiency over the entire operating range.
Doubly Fed Induction Generator ASG System
Recent developments seek to avoid most disadvantages of
direct-in-line converter based ASGs. Fig. 5 shows an alternative
ASG concept that consists of a doubly fed induction
generator (DFIG) with a four-quadrant ac-to-ac converter
based on insulated gate bipolar transistors (IGBTs) connected
to the rotor windings. Compared to direct-in-line
systems, this DFIG offers the following advantages:
Reduced inverter cost, because inverter rating is typically
25% of total system power, while the speed
range of the ASG is ±33% around the synchronus
speed (Fig. 6).
Reduced cost of the inverter filters and EMI filters,
becuase filters are rated for 0.25 p.u. total system
power, and inverter harmonics represent a smaller
fraction of total system harmonics.
Improved system efficiency; Table 2 shows the system
losses for different windmill concepts. The losses
are shown separately for the generator and for the
IGBT inverters. Approximately 2-3% efficiency improvement
can be obtained.
Power-factor control can be implemented at lower
cost, because the DFIG system (four-quadrant converter
and induction machine) basically
operates similar to a
synchronous generator. The converter
has to provide only excitation
energy.
In addition, compared to silicon-controlled
rectifier (SCR) based Kramer
drives [3], the DFIG with a four-quadrant
converter in the rotor circuit enables decoupled
control of active and reactive
power of the generator.
Dynamic Model
of a Doubly Fed Induction
Generator
To develop decoupled control of active
and reactive power, a DFIG dynamic
model is needed. The construction of a
DFIG is similar to a wound rotor induction
machine (IM) and comprises a
three-phase stator winding and a
three-phase rotor winding. The latter is
fed via slip rings. The voltage and torque
equations of the DFIG in a stationary reference
frame are: