IEC 61400-1:2019 pdf download – Wind energy generation systems – Part 1: Design requirements.
6 External conditions 6.1 General The external conditions described in Clause 6 shall be considered in the design of a wind turbine. Wind turbines are subjected to environmental and electrical conditions that may affect their loading, durability and operation. To ensure the appropriate level of safety and reliability, environmental, electrical and soil parameters shall be taken into account in the design and shall be explicitly stated in the design documentation. The environmental conditions are further divided into wind conditions and other environmental conditions. The electrical conditions refer to the electrical power network conditions. Soil properties are relevant to the design of wind turbine foundations. The external conditions are subdivided into normal and extreme categories. The normal external conditions generally concern recurrent structural loading conditions, while the extreme external conditions represent rare external design conditions. The design load cases shall consist of potentially critical combinations of these external conditions with wind turbine operational modes and other design situations. Wind conditions are the primary external conditions affecting structural integrity. Other environmental conditions also affect design features such as control system function, durability, corrosion. The normal and extreme conditions, which are to be considered for design according to wind turbine classes, are prescribed in 6.2 to 6.4.
An optional wind turbine class CC, which may be used for areas with cold climate, is defined in Clause 1 4. The optional wind turbine class specifies additional requirements and parameters to what is given in Clause 6. A further wind turbine class, class S, is defined for use when special wind or other external conditions or a special safety class, see 5.3, are required by the designer and/or the customer. The design values for the wind turbine class S shall be chosen by the designer and specified in the design documentation. For such special designs, the values chosen for the design conditions shall reflect an environment at least as severe as is anticipated for the use of the wind turbine. Wind turbines to be designed for site-specific or very extreme conditions may require wind turbine class S design. In addition to the basic parameters showed in Table 1 , several other important parameters are required to completely specify the external conditions to be used in wind turbine design. In the case of the wind turbine classes I A+ through III C , later referred to as the standard wind turbine classes, the values of these additional parameters are specified in 6.3, 6.4 and 6.5. The design lifetime for wind turbine classes I to III shall be at least 20 years.
6.3 Wind conditions 6.3.1 General A wind turbine shall be designed to safely withstand the wind conditions defined by the selected wind turbine class. The design values of the wind conditions shall be clearly specified in the design documentation. The wind regime for load and safety considerations is divided into the normal wind conditions, which will occur frequently during normal operation of a wind turbine, and the extreme wind conditions that are defined as having a 1 -year or 50-year return period. The wind conditions include a constant mean flow combined, in many cases, with either a varying deterministic gust profile or with turbulence. In all cases, the influence of an inclination of the mean flow with respect to a horizontal plane of 8° shall be considered. This flow inclination angle shall be assumed to be invariant with height. The expression “turbulence” denotes random variations in the wind velocity from 1 0 min averages. The turbulence model, when used, shall include the effects of varying wind speed, shears and direction and allow rotational sampling through varying shears. The three vector components of the turbulent wind velocity are defined as • longitudinal – along the direction of the mean wind velocity, • lateral – horizontal and normal to the longitudinal direction, and • upward – normal to both the longitudinal and lateral directions, i.e. tilted from the vertical by the mean flow inclination angle.