Structural Dynamic Behavior of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 607

by past experience and frequently involves trade-offs with factors outside the realm of
structural dynamics ( e.g. , cost, manufacturing, and energy capture). The remaining key
objectives are accomplished by straightforward (though sometimes complex) mathematical
modelling of the selected configuration. Of course, the likelihood of a good detail design
solution should be evident at the completion of the concept definition stage.
Wind Turbine Substructures and Subsystems
General background material on wind turbine nomenclature, configurations, and major
substructures may be found in Chapters 2 and 4. It is its rotor which sets a wind turbine
apart from other structural systems. Therefore, the thrust of this chapter is the understand-
ing and analysis of wind turbine rotors. The structural dynamic behavior of the power
train and support structures of a wind turbine can usually be analyzed by well-established
methods, so those subsystems will not be discussed here.
Mathematical Models
While certain analyses can be accomplished with simple formulas, experience has shown
that the analyst will need separate, specialized computer models to determine the following:
-- system structural modes and natural frequencies which incorporate rotational effects
-- system loads , both steady-state and transient, incorporating all important degrees of
freedom; includes deflections, vibrations, and power output quality
-- aeroelastic stability , including the effects of structure/control system interaction
In theory, one comprehensive computer code could be used for all three of these tasks, but
it might not be as efficient to develop or to run as separate specialized codes.
Modal Analysis Models
Modal analysis is the determination of the set of discrete patterns or mode shapes , each
with at its own modal frequency and modal damping , which a vibrating structure describes.
These patterns are also known as natural modes. Modal analysis of a wind turbine is
generally based on well-developed finite element procedures ( e.g. NASTRAN ) . Each
substructure in the turbine system is modeled separately to determine its own mode shapes
and frequencies. With special care, substructures can be coupled together to produce system
modes. Modal models can be derived directly or spawned from more-detailed stress
analysis models using standard modal reduction techniques. For rotors, centrifugal
stiffening and gyroscopic effects can be incorporated directly into the finite element code,
or they can be computed externally and applied later.
System Loads Model
The system loads code should be tailored to suit a specific wind turbine configuration,
since a code that attempts to analyze all possible wind turbine types can become unwieldy.
For example, the important dynamic loads usually result from motion in only a few
dominant modes that are characteristic of a particular configuration, so the degrees of
freedom used in the system loads model are generally a selected subset of the natural modes
computed with the finite element models described above. While it is possible to use all
of the finite-element degrees of freedom in the loads model, a modal approach is commonly
used for analyzing system loads.