Page 424 - Wind Energy Handbook

398 COMPONENT DESIGN

speed above rated that does not produce a blade pitch response can be estimated
using
v ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð
u 1
u
u S u (n)
u Ù=2 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ª
u max ¼ ó u u ð 1 2 ln(ÙT) þ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
t 2 ln(ÙT)
S u (n)
0
" #
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:5772
2 ln(ÙT) þ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (7:9)
¼ (ó u ) n . Ù=2
2 ln(ÙT)
where (ó u ) n . Ù=2 is the standard deviation of wind speed fluctuations above the
pitch response cut-off frequency (assumed to be half the rotational frequency) and
T is the total period of operation in the wind speed band centred on the rated speed
at the yaw angle under consideration. For a 12 m/s rated wind speed, ó u ¼
2:34 m=s from Equation (7.8) and (ó u ) n . Ù=2 ¼ 2:34(0:4) 0:5 ¼ 1:48 m=s. Taking a
wind speed band of 2 m/s and yaw angles between 208 and 408, the expression
in square brackets (i.e., the peak factor) comes to 5.5, so that the lifetime extreme
value of the wind speed increase without pitch response is about 8 m/s. If the wind
speed fluctuations over the outer 8 m of blade are treated as perfectly correlated,
this results in a maximum value of 12 m radius flapwise bending moment of
96 kNm (see Figure 7.11), which is over 50 percent greater than that occurring in a
steady 12 m/s wind. Thus the extreme flapwise bending moment during operation
occurs at winds around rated rather than around the upper cut-out speed – a
phenomenon which is a normal feature of pitch-regulated machines. Also, the
extreme flapwise bending moment is less than for the similarly rated stall-regulated
machine considered above.
The relative criticality of the load cases corresponding to extreme turbulent wind
speed fluctuations and the occurrence of the 50 year extreme operating gust during
operation at rated wind speed, which are exemplified by IEC load cases 1.1 and 1.6
respectively, will be determined by pitch control system performance.

Extreme loading at standstill

The derivation of stationary blade loads is described in Section 5.6. Figure 7.12
shows the out-of-plane bending moment distribution for a TR blade under the
action of a 60 m/s wind, corresponding to the 50 year return extreme 3 s gust
specified for a Class II wind turbine in IEC 61400-1. A uniform lift coefficient of 1.5
is assumed. Two curves are shown: the lower curve is the quasistatic bending
moment while the upper one incorporates dynamic magnification due to excitation
of resonant oscillations (see Section 5.6.3).
For comparison purposes, the extreme operational out-of-plane bending moment
(excluding dynamics) for the example stall-regulated machine (load case 1.1, Table
7.1) is also plotted. It is clear that the operational load case is critical over most of
the blade length, though only by a small margin. The non-operational bending