Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 596

An induction generator with its slip behavior can be readily modeled as a dynamic
element. It should be noted again that the apparent softness of an induction generator
cannot provide the degree of reduced impedance of the power train that is necessary to
attenuate periodic rotor/power train coupled loads.
Variable-Speed Generators
When a variable-speed generator provides the opportunity to control generator air-gap
torque , it becomes very important to include the mass, stiffness, and damping characteristics
of the generator in the power train model. Except for minor inertial effects, such a drive
train subsystem presents a zero torsional impedance to the wind turbine rotor. The lowest
mode of rotor vibration in such a system will act as though the turbine rotor was totally
uncoupled from the power train. However, the mass of the generator rotor becomes free
to “ring” in torsion, with the wind turbine rotor acting as a clamped end in the mode shape.
Generator air-gap torque damping can and should be included in the model, since this
damping can be readily and usefully supplied by the actual generator.
Support Structures
The response dynamics of the coupled tower, yaw drive, nacelle, and drive shaft
support structures must be included in the system simulation model. In a VAWT,
simulation of the dynamic behavior of the support cables is critical. In a HAWT, it will
be generally necessary to construct detailed finite element models (FEMs) of all of the
structural elements between the rotor shaft and the ground, and to give close attention to
nacelle mass distribution.
Aeroelastic Instability
Rotor Aeroelastic Instability
Aeroelastic instability can occur whenever any modal response creates an accompanying
periodic aerodynamic force that feeds a buildup of that mode. The induced periodic
aerodynamic force, which acts as a negative damping mechanism , is created by some form
of feedback from the modal motion to the angle of attack of the airfoil. This instability can
occur with airfoils unstalled (as with classical flutter ) or as a result of stall (as with stall
flutter ) .
Because the prime mechanism of aeroelastic instability is a cyclic disturbance of blade
angles of attack, it is beneficial to retain high torsional stiffness in the blade and all the
elements of the pitch control system. It is also beneficial to provide the favorable blade
details of chordwise mass balance and shear center location that will reduce or eliminate
torsion deflections from periodic twist loads. In typical HAWTs, however, the robust blade
design needed to carry cyclic gravity loadings results in such a high torsional stiffness that
the blade need not have full chordwise balance to preclude flutter. The same is true for
bending modes within a rotor blade that might also be made unstable by pitch disturbances.
Deflection of control surfaces is a different matter. With either full- or partial-span
pitch control ( i.e. , control through an outboard movable portion of the blade) any loss or
inadequacy of control stiffness can enable flutter. Therefore, attention to chordwise balance
can be valuable protection against flutter in the event control stiffness deteriorates.