Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 600

Response to Yawed Flow and Nacelle Yaw Motion
This response is the most distinguishing difference between a hingeless and a teetered
HAWT rotor. In the hingeless system, yawed flow and yaw motion are sources of
relatively large cyclic loads. On the other hand, they create no design-driving loads in a
teetered system, even at yaw angles and yawing rates not usually believed feasible.
Response to Control Inputs
The rapid application of pitch angle control can be lethal to the fatigue life of a wind
turbine. Control systems must be designed to assure limited blade pitching rates.
Differences in lag times between the thrust and torque responses to a wind transient must
be carefully considered in the control design, to avoid a damaging increase in thrust loads.
Response to Loss of Electrical Load
The initial response to a loss of electrical load is benign, without any significant
dynamic loads. However, the rise in rotor speed will be rapid and require control actions
to prevent overspeed. These actions can result in a thrust reversal that is a very damaging
cycle in the fatigue load spectrum. Thus, the control system response to loss of electrical
load must be carefully shaped to assure a safe shutdown without structural damage.
Response to Gravity
Gravity applies large cyclic lead-lag or in-plane bending moments to a rotating HAWT
blade that are its primary design-driving loads. The strength required for adequate fatigue
life under this 1 P excitation results in a high blade stiffness and a high lead-lag (in-plane)
natural frequency. As a result, the gravity bending response is not amplified significantly,
nor can anything be done to attenuate it. A hand-calculation of the gravity-induced dynamic
response is adequate for preliminary design. In a coned rotor, components of the cyclic
gravity load can enter the control linkage, and other components can act on the rotor shaft
as a 2 P load unless the rotor is teetered.
Response to Rotor Speed Fluctuations
Fluctuation in rotor speed is desirable as both a source and a sink for the energy in
wind gusts. While a torsionally-soft power train permits some fluctuation, a constant-torque
generator is the design feature that takes full advantage of this energy benefit. Power
quality, in terms of steadiness of the power output of the generator, can be better with rotor
speed fluctuations than with pitch control and a constant rotor speed. Energy capture is
much enhanced when rotor speed is free to follow the wind.
Response to Resonance Crossings
The dynamic response of the wind turbine system when a natural frequency coincides
with an integer multiple of the rotor speed can vary from “critical” to “no problem,”
depending on the design philosophy employed, the mode in question, and the damping
features of that mode. Potential resonances are displayed graphically by means of a
Campbell diagram , which is discussed in detail in Chapter 11. In a variable-speed HAWT
with a broad speed range there will be one or more poorly-damped “tower” modes that are