Tuesday, February 4, 2014

Doubly Fed Induction Generator in Wind Energy Conversion System (WECS)

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.