Page 330 - Wind Energy Handbook

304 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

Size reduction factor for quasistatic or background response, K SMB – see Step (5)
above 0.837
Ratio of standard deviation of tower base moment quasistatic response to mean
value
p ffiffiffiffiffiffiffiffiffiffiffi p ffiffiffiffiffiffiffiffiffiffiffi
ó MB ó u
¼ 2 K SMB ¼ 2 3 0:1225 3 0:837 0:2241
M U
Ratio of standard deviation of total tower base moment response to mean value
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2 2 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ó M ó MB ó M1 2 2
¼ þ ¼ 0:2241 þ 0:1432 0:2660
M M M
Zero up-crossing frequency of quasistatic response, n 0 – see Step (6) above 0.31 Hz
Zero up-crossing frequency of total tower base moment response, í – Equation
(A5.54) 0.68 Hz
Peak factor, g, based on í – Equation (A5.42) 3.63
Ratio of extreme tower base moment to mean value

M max ó M
¼ 1 þ g ¼ 1 þ 3:63(0:2660) 1:966
M M

Peak factor, g 0 , based on n 0 – Equation (A5.42) 3.41
Ratio of quasistatic component of extreme tower base moment to mean value

ó M
¼ 1 þ g 0 ¼ 1 þ 3:41(0:2241) 1:764
M

Dynamic factor, Q D ¼ 1:966=1:764 – Equation (5.17) 1.115
It is apparent from Equation (5.17) that a key parameter determining the dynamic
factor is the damping value. If the rotor contribution to aerodynamic damping were
not available, the damping in the above example would be reduced by a factor of
four, which would increase the dynamic factor to 1.41. This is of relevance to non-
operational pitch-regulated machines facing into the wind, because if the lift
loading on a blade is near the maximum, the aerodynamic damping may be near
zero or even negative.

5.12.3 Operational loads due to steady wind (deterministic
component)

Tower fore-and-aft bending moments result from rotor thrust loading and rotor
moments. The moments acting on the nacelle due to deterministic rotor loads have
already been described in Section 5.11.1. Although the thrust loads on individual
blades vary considerably with azimuth as a result of yaw misalignment, shaft tilt or
wind shear, the fluctuations on different blades balance each other, so that the total