Page 237 - Wind Energy Handbook

BASIS FOR DESIGN LOADS 211

these systems to the overall design process is emphasized by placing them at the
start of the document. Final sections deal with operation and maintenance, noise
and lightning protection.

5.1.4 Danish Standard DS 472

DS 472 bases the derivation of design-extreme wind speeds on four terrain classes,
ranging from the very smooth (expanses of water) to the very rough (e.g., built-up
areas). The base wind velocity is taken to be the same all over Denmark, so the
result is four alternative profiles of wind speed variation with height. The philoso-
phy behind the selection of design load cases in the Danish standard is similar to
that in IEC-1400 and the GL rules, although the number of load cases is fewer.
Similarly, the requirements for the control and safety systems are again clearly set
out. DS 472 is distinctive in that it includes detailed treatments of the derivation of
simplified fatigue load spectra for a three-bladed, stall-regulated machine of up to
25 m diameter and a method of calculating gust response factors for the blades and
the tower.

5.2 Basis for Design Loads

5.2.1 Sources of loading

The sources of loading to be taken into account may be catagorized as follows:

• aerodynamic loads,
• gravitational loads,

• inertia loads (including centrifugal and gyroscopic effects), and
• operational loads arising from actions of the control system (e.g., braking,
yawing, blade-pitch control, generator disconnection).

5.2.2 Ultimate loads

The load cases selected for ultimate load design must cover realistic combinations
of a wide range of external wind conditions and machine states. It is common
practice to distinguish between normal and extreme wind conditions on the one
hand, and between normal machine states and fault states on the other. The load
cases for design are then chosen from:

• normal wind conditions in combination with normal machine states,
• normal wind conditions in combination with machine fault states, or
• extreme wind conditions in combination with normal machine states.