Doubly Fed Induction Generator in Wind
Energy Conversion System (WECS)
-The "Danish Concept", as
shown in fig.1, is the first technology design for low-power wind turbine for
power energy conversion. This conventional fixed speed generator (FSG) design
uses squirrel cage induction generator directly connected to the electricity
grid via a step up transformer. The Danish Concept turbine scheme delivers its
rated output power at constant wind speed with a fixed ratio gearbox. At wind
gusts (high wind speed), a great variation to the electrical output power is
observed, that affects the system stability from grid view. Also it affects the
wind turbine performance due to the high mechanical stresses.
Fig.1 - The
Danish Concept scheme
-Adjustable speed generators (ASG)
design offer more advantages over the Danish Concept turbine such as improving
power quality, improving efficiency, reducing the mechanical stresses at high
wind speed, and dynamic compensation for power and torque. Two different designs
of Adjustable speed generators are commonly used for wind energy conversion
system;
(i) Direct-in-line system, as shown
in fig.2, it uses synchronous generator that its stator directly connected into
the grid through a full capacity power converter. With the use of power
converter, the generator can operate within full speed range. The only
disadvantage of this scheme is the high cost of the full rated power converter
whose rating is 1 p.u. to system power as it is connected directly in series
with the generator.
Fig.2 - Direct
in line system scheme
(ii) Doubly Fed Induction Generator
(DFIG), as shown in fig.3, it uses wound rotor induction generator that its
rotor is connected through a power converter and its stator is directly
connected into the grid. With the use of power converter in this scheme, it
allows a bidirectional power flow in the rotor circuit and increases the wind
speed ranges of the generator.
Fig.3 – Doubly
fed Induction Generator scheme
-Among various alternatives to
construct grid-connected wind turbines, the ones based on doubly fed induction
generators (DFIGs) are dominating the energy market due to their outstanding
merits ;
(i) Reduced mechanical stress,
(ii) Improved Power Quality,
(iii) Improved system efficiency,
(iiii) reduced acoustic noise,
(v) Reduced cost of inverter set as
it is connected in rotor circuit.
