Page 308 - Wind Energy Handbook

282 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

5.8.10 Wind turbine dynamic analysis codes

A large modern turbine is a complex structure. Relatively sophisticated methods
are required in order to predict the detailed performance and loading of a wind
turbine. These methods should take into account:

• the aerodynamics of the rotating blade, including induced flows (i.e., the mod-
ification of the flow field caused by the turbine itself), three-dimensional flow
effects, and dynamic stall effects when appropriate;

• dynamic analysis of the blades, drive train, generator and tower, including the
modification of the aerodynamic forces due to the vibrational velocities of the
structure;

• dynamic response of subsystems such as the yaw system and blade pitch control
system;

• control algorithms used during normal operation, start-up and shut-down of the
turbine;

• temporal and spatial variations of the wind field impinging on the turbine,
including the three-dimensional structure of the turbulence itself.

Starting from a wind turbulence spectrum, it is possible to develop techniques in
the frequency domain which account for many of these aspects, including rotational
sampling of the turbulence by the blades, the response of the structure, and the
control system. These techniques are set out in Sections 5.7.5, 5.8.6, 5.12.4 and
elsewhere. However, although frequency domain methods are elegant and compu-
tationally efficient, they can only be applied to linear time-invariant systems, and
therefore cannot deal with some important aspects of wind turbine behaviour, such
as:

• stall hysteresis;
• non-linearities in subsystems such as bearing friction, pitch rate limits, and non-
linear aspects of control algorithms;
• variable speed operation;
• start-up and shut-down.

As a result, time-domain methods are now used almost exclusively for wind
turbine design calculations. The ready availability of computing power means that
the greater computational efficiency of frequency domain methods is no longer
such an important consideration.
A number of codes are available commercially for the calculation of wind turbine
performance and loads using time-domain simulations. These simulations use
numerical techniques to integrate the equations of motion over time, by subdividing