Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 116

Fundamental period of oscillation:

s

r

131

P i G i

y i

: 119

g P i G i

T 1 ¼ 2 p ¼ 2 p ¼ 3 : 46 s

ð y i Þ 9 : 81 44 : 192

Fundamental natural frequency:

f ¼ n 1 ; x ¼ 1 = T 1 ¼ 1 = 3 : 46 ¼ 0 : 289 Hz

4.3.3 Natural frequency analysis of loadbearing structure
There should be an adequate safety margin between the natural frequency of the total
system, consisting of foundation, tower, nacelle and rotor, and the excitation frequen-
cies. Therefore, the structural engineer should design the tower structure in such a way
that the desired natural frequency is reached but at the same time the structural safety is
guaranteed.

The excitation frequencies of a wind turbine support structure are (Figure 4.6): 3)
a) Periodic excitation with 1 rotational speed ( ¼ rotor frequency) ¼ 1P excitation
b) Periodic excitation with 3 rotational speed from blade passing frequency ¼ 3P
excitation
c) Whole-number multiples of the rotor frequency

The ranges of the permissible natural frequencies are shown in the Campbell diagram,
in this example for a 5MW turbine (Figure 4.7).

The closer the tower excitation frequencies are to the range of natural frequencies, the
higher are the action effects of the mechanical components and the tower itself.

Fig. 4.6 Natural frequency analysis of loadbearing structure

3) Always take into account the design of the wind turbine manufacturer!