Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 598

is destabilizing. Fortunately, the negative damping involved can be of about the same mag-
nitude as the positive damping available trom a typical yaw bearing. Depending on system
geometry, however, it may be necessary to provide additional mechanical damping to the
yaw drive system of a downwind-rotor HAWT.
Control System Interaction
While control system activity may not be perceived as a factor in aeroelastic stability,
control actions can have the same effects as those induced by elastic deflections. Thus,
controls can stabilize or destabilize any system dynamic mode which falls within the their
frequency range. If the control rate is designed for a response to changes sensed in the
rotor torque or speed without regard to thrust effects, it can create the following two
adverse results: First, it may create large transient overloads in thrust, because torque and
speed changes are very slow relative to the buildup of thrust and blade flatwise bending
loads. A control that appears to be sufficiently stable and successful as viewed through
torque time histories may be creating or permitting excessive periodic thrust loads.
Secondly, a control that is merely tasked to hold power constant in high winds will tend
to negatively damp the first longitudinal mode of tower bending. The good aerodynamic
damping that a fixed-pitch rotor would supply to this mode will tend to be overcome by
pitch control action. Thus, the control system should be provided with the means to sense
the offending tower motion and act to correct it.
Instability of the overall pitch control loop can also affect the system like an aeroelastic
instability. Unexpectedly high control gains can reduce stability. This may arise from
neglect of the time lag of induced flow changes behind sharp changes of rotor aerodynamic
loads. A fast-acting pitch control should be considered suspect in any situation where
sustained periodic thrust loads are observed.
The above is a general description of the instabilities explored in simulations during the
development of large-scale HAWTs. No instabilities within the rotor blades or in shaft
whirl have been encountered in practice, primarily because of high blade and control
stiffness.
Typical Dynamic Responses
The following discussion briefly covers the expected responses of a wind turbine
system to various sources of excitation and the parameters that influence the magnitude of
these responses.
Aeroelastic Effects
Aeroelastic effects that are usually encountered in wind turbines will be favorable. In
a teetered HAWT rotor, for example, both the rigid-body teeter motion and first flatwise
bending mode of the blades are highly-damped aerodynamically. Thrust load excitations
can create blade and tower deflections rather than stresses. Transient wind loadings involve
upwind-downwind velocities of the structure and accompanying aerodynamic damping.
Generally, when the control system of a HAWT is strong enough to handle gravity and
flatwise bending loads, blade torsional stiffness will be high enough to prevent flutter and
divergence.
As discussed previously, a d 3 angle that is too large can destabilize the first
unsymmetrical flatwise bending mode in a high-speed rotor, and a control system which
only controls power can negatively damp the first tower bending mode in high winds.