Environmental Engineering Reference
In-Depth Information
only marginally stable which explains why stall regulation has been substituted by
pitch regulation in the design of multi MW machines.
Pitch regulation limits the onset of stall by keeping the mean angles of attack
along the blade up to the level of maximum lift. However light stall will occur
close to maximum lift fl ow conditions which correspond to wind speeds around
rated power production. This is shown in Fig. 15. Even at fi xed RPM, the range of
wind speeds with negative damping is limited around the rated wind speed. When
the generator is added the damping increases substantially and attains higher positive
values as compared to the stall regulated situation.
So in conclusion, theory indicates that wind turbines are subjected to low damp-
ing in certain modes. In order to assess the validity of this result comparison with
full scale measurements is needed. This is done in Figs 16-18 in terms of aeroelas-
tic frequency and damping for the most critical modes of a
3 MW pitch regulated
commercial machine. The measured data were obtained through Operational
Modal Analysis (OMA) [41] on a collection of normal operation time series of
∼
40
1st symmetric lag (fixed RPM)
1st regressive lag (fixed RPM)
1st symmetric lag (generator)
1st regressive lag (generator)
1st progressive lag (generator)
1st
tower
l
atera
l
(
f
ixed
RPM
)
1st tower longit. (fixed RPM)
1st tower lateral (generator)
1st tower longit. (generator)
30
1s
t
progress
i
ve
l
ag
(
fixed
RPM
)
30
20
20
10
10
0
0
-10
-10
-20
6
8
10
12
14
16
18
20
22
24
26
6
8
10
12
14
16
18
20
22
24
26
Wind speed (m/s)
Wind speed (m/s)
Figure 14: Damping variation for the tower bending modes (left) and the blade lag
modes (right) of a commercial 500 kW stall regulated wind turbine [38].
100
1st
t
ower
l
a
t
era
l
(
f
ixed
RPM)
1st tower longit. (fixed RPM)
1st tower lateral (generator)
1st tower longit. (generator)
80
80
60
60
1st symmetric lag (fixed RPM)
1st regress. lag (fixed RPM)
1st progress. lag (fixed RPM)
1st symmetric lag (generator)
1st regress. lag (genertor)
1st
progress
.
lag (gener
t
or)
40
40
20
20
0
0
6
8
10
12
14
16
18
20
6
8
10
12
14
16
18
20
Wind speed (m/s)
Wind speed (m/s)
Figure 15: Damping variation for the tower bending modes (left) and the blade lag
modes (right) of a commercial 3 MW pitch regulated wind turbine [38].
Search WWH ::
Custom Search