Page 326 - Wind Energy Handbook

300 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

5.12 Tower Loading

5.12.1 Extreme loads

As noted in Section 5.4.1, it is customary to base the calculation of extreme loads on
a non-operational turbine on the 50 year return 3 s gust. Several loading configura-
tions may need to be considered, and the critical load case for the tower base will
generally differ from that for the tower top. In addition, it is necessary to investigate
the extreme operational load cases, as these can sometimes govern instead if the tip
speed is high in relation to the design gust speed.
In the case of non-operational, stall-regulated machines, the critical case for the
tower base occurs when the wind is blowing from the front and inducing maximum
drag loading on the blades. By contrast, sideways wind loading to produce maxi-
mum lift on a blade pointing vertically upwards or rear wind loading on the rotor
with one blade shielded by the tower will produce the maximum tower top bending
moment.
One of the benefits of pitch-regulation of three-bladed machines is that blade
feathering at shut-down considerably reduces non-operational rotor loading. The
critical configuration as far as tower base bending moment is concerned is sideways
wind loading, with two of the blades inclined at 308 to the vertical. The horizontal
3
component of the loading on these blades is cos 308 of the loading on a vertical
ffiffiffi
p
blade, so that the total rotor loading is only 43.3 percent (¼ 100: 3=4%) of the
maximum experienced by a stall-regulated machine.
The cases of sideways wind loading on a wind turbine referred to above can only
arise if the yaw drive is disabled by grid loss for sufficient time for a 908 wind
direction change to take place. In many areas, the level of grid security will be high
enough for this possibility to be ruled out, so that sideways wind loading need not
be considered if the yaw drive is programmed to remain operational in high winds.
The critical non-operational load case for a three-bladed, pitch-regulated machine
then occurs when the wind is from the front with a 158 to 208 yaw error. The rotor
load is maximum when one of the blades is vertical, and is similar in magnitude to
loading produced by a sideways wind. However, as the load results from blade lift
rather than drag, it is at right angles to the loading on the tower, so the total
moment at the tower base is significantly less.
Information on the drag factors appropriate for cylindrical and lattice towers is to
be found in Eurocode 1, part 2–4 (1997), and in national codes such as BS 8100
(1986) or DS 410 (1983). The drag factor for a cylindrical tower is typically 0.6–0.7.
Rotor loading is generally the dominating component of tower base moment for
stall-regulated machines, but with pitch-regulated machines the contributions of
tower loading and rotor loading are often of similar magnitude.

5.12.2 Dynamic response to extreme loads

Just as in the case of the single, stationary cantilevered blade considered in Section
5.6.3, the quasistatic bending moments in the tower calculated for the extreme gust