Shou OH, Takeshi ISHIHARA
Transcript of Shou OH, Takeshi ISHIHARA
A Study on Structure Parameters of an Offshore Wind Turbine
by Excitation test Using Active Mass Damper
Shou OH, Takeshi ISHIHARA The University of Tokyo
PO.137
1. Japan Society of Civil Engineers, Design code for support structure of wind
turbine, 2010 (In Japanese)
2. AWEA/ASCE, Recommended Practice for Compliance of Large On-land Wind
Turbine Support Structures, 2011
3. Deutsches Institut für Bautechnik, Richtlinie für Windenergianlagen, 2012
4. Ishihara,T., Phuc,P.V., Fujino,Y., Takahara,K., and Mekaru,T., A field test
and full dynamic simulation on a stall regulated wind turbine, Proc. of
APCWE VI, 2005, pp. 599-612
Structure parameters are important in estimation of
loads on wind turbines. Especially the seismic load
depends strongly on the damping ratio of 1st and 2nd
mode. However, the values of damping ratio are
different among design codes. In Japanese design
code[1], 0.5~0.8% is recommended for 1st mode based
on a human-power excitation test on a 400kW wind
turbine. In the design code of United States[2], 1% is
suggested for seismic design, and 0.23% is adapted in
German code[3]. Moreover, there is little information
regarding 2nd mode damping, which is difficult to excite
by either nature wind or human power. In this study, an
excitation test using active mass damper is conducted
on a 2.4MW offshore wind turbine. Natural frequencies
and damping ratios of 1st and 2nd mode are evaluated
by sinusoidal vibration test and free vibration test. The
effect of blades to damping ratios is also evaluated by
comparing the excitation result under pitch-feathering
condition with that of pitch-fine condition.
Objectives
Free Vibration Test
Conclusions
References
EWEA OFFSHORE 2013
Outline of Excitation Test
0
10
20
30
40
50
60
70
80
0 0.2 0.4 0.6 0.8 1
FEM
Obs
He
ight
(m)
0
10
20
30
40
50
60
70
80
0 0.2 0.4 0.6 0.8 1
FEM
Obs
He
ight
(m)
-1
-0.5
0
0.5
1
0 50 100 150
Acceleration
damp=1.2%
Acceleration (m/s2)
time (s)
(c) y 1st mode
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 2 4 6 8 10
Acceleration
damp=3.2%
Accerelation (m/s2)
time (s)
(d) y 2nd mode
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 2 4 6 8 10
Acceleration
damp=2.4%
Acceleration (m/s2)
time (s)
(b) x 2nd mode
-1
-0.5
0
0.5
1
0 50 100 150
Acceleration
damp=0.19%
Acceleration (m/s2)
time (s)
(a) x 1st mode
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 50 100 150
Xfree1st
damp=1.0%
Acceleration (m/s2)
time (s)
(a) x 1st mode
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 50 100 150
Yfree1st
damp=0.3%
Acceleration (m/s2)
time (s)
(c) y 1st mode
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.3 0.32 0.34 0.36 0.38 0.4
Observation
Analytical
Normalized Amplitude
frequency (Hz)
(a)1st mode
amplitude
f=0.351Hz
=0.2%
0.0
0.1
0.1
0.2
0.2
0.3
2.6 2.8 3 3.2 3.4Normalized Amplitude
frequency(Hz)
(b)2nd mode
amplitude
f=2.98Hz
=2.4%
0.0
45.0
90.0
135.0
180.0
0.3 0.32 0.34 0.36 0.38 0.4
Phase (deg)
frequency (Hz)
(c)1st mode
phase
f=0.351Hz
=0.2%
0.0
45.0
90.0
135.0
180.0
2.6 2.8 3 3.2 3.4
Phase (deg)
frequency (Hz)
(d)2nd mode
phase
f=2.98Hz
=2.4%
X-1st X-2nd Y-1st Y-2nd
Ambient
vibration
Pitch-
Feather
0.357Hz
0.2%
3.00Hz
-
0.353Hz
0.3~1.5%
2.99Hz
-
Pitch-
Fine
0.375Hz
-
3.05Hz
-
0.363
-
-
-
Sinusoidal
vibration
Pitch-
Feather
0.351Hz
0.2%
2.94Hz
2.4%
0.357Hz
-
2.97Hz
-
Pitch-
Fine
0.350Hz
-
-
-
0.347Hz
-
3.01Hz
-
Free
vibration
Feather 0.2% 2.4% 1.2% 3.2%
Fine 1.0% - 0.2% 3.2%
Excitation test using active mass damper is performed
on an offshore wind turbine. The precise value of
resonant frequency of 1st and 2nd mode for fore-aft(X)
and side-side (Y) direction is obtained by sinusoidal test.
Structural damping ratio in the direction of blade edge-
wise is obtained as 0.2% for 1st mode and 2.4% for 2nd
mode. On the other hand, the damping ratio becomes
1.0%~1.2% for 1st mode and 2.4~3.2% for 2nd mode in
the direction of blade flap-wise. The difference is caused
by the aero-dynamic damping and the structure damping
of blade, which will be evaluated with numerical analysis
in future study. Mode shapes are obtained using
acceleration data at 5 heights and agreed with
calculation. Results obtained in this research will be
proposed in international standard for design of tower
and foundation.
X direction Y direction
Movable Mass 1700kg 1300kg
Effective Stroke 190mm 290mm
Maximum Speed 0.83m/s 0.83m/s
Frequency Range 0.1~4Hz 0.1~4Hz
Actuator power Maximum: 4400N
Continuous: 1900N
Maximum: 4400N
Continuous: 1900N
Acceleration Maximum: 2.58m/s2
Continuous: 1.11m/s2
Maximum: 3.38m/s2
Continuous: 1.46m/s2
The wind turbine targeted in this study is Mitsubishi
Heavy Industry 2.4MWturbine located 3.1km offshore
Choshi, Japan. The active mass damper(AMD) is
equipped at 55.5m height of tower for purpose of
vibration control. In this study, it is used to generate the
excitation force. During the test, the rotor and the yaw
were fixed in order to obtain pure structure parameters
without the effect of rotation. As the turbine is supported
with gravity foundation, the effect of tide and wave to
damping is neglected. Experiment date is chosen when
the wind speed is low to minimize the effect of aero-
dynamic damping.
Date 2014Feb,
21-22
2014Oct,
28-29
Mean Wind Speed 4~6m/s 5~8m/s
Mean Wind Direction 345~350
deg
20~25
deg
Yaw Angle 77.5deg 77.5deg
Sinusoidal Vibration Analysis
Mode Shapes
Sinusoidal vibration test is performed around the
approximate resonant frequency calculated from
ambient vibration monitoring to obtain more precise
value. Especially for excitation in fore-aft (X) direction
under pitch-feathering condition, the vibration can be
approximated as 1 degree of freedom, since the aero-
dynamic damping is small and the effect of nacell like
friction of gears can be neglected. Then, the analytical
solution of normalized amplitude and phase angle can
be written as
Mode shapes are
calculated from both
the acceleration
amplitude during
sinusoidal test at 5
height and from FEM
analysis [4]where
model is made from
design drawing. For
both 1st and 2nd
mode, the mode
shape shows good
agreement.
Detail of active mass damper
Left: Location of wind turbine; Right: Outline of AMD
96m
10.3m
Accelerometer
Accelerometer height Strain gage height
Strain gage
Detail of 2.4MW wind turbine and measurement devices
Active Mass Damper
Measurement Condition
Axis Definition
Excitation
X axis
Wind Direction 0°
(North)
(East)
22.5°
Excitation
Y axis Comparison of sinusoidal test with analytical solution
Left: X 1st mode; Right: X 2nd mode
Result of obtained structure parameters
Result of free vibration with pitch-feathering condition
Left: X&Y 1st mode; Right: X&Y 2nd mode
𝑎
𝐹= 𝛽2
𝜙𝑛(𝑉𝑖𝑏)𝜙𝑛(𝐴𝑐𝑐)
𝜙𝑛(𝑖)𝑖 𝑚𝑖𝜙𝑛(𝑖)
1
1 − 𝛽2 2 + 2𝜉𝑛𝛽2
𝜃 = tan−12𝜉𝛽
1 − 𝛽2
Free vibration test is conducted by stopping the
excitation force of natural frequency at the steady state.
Comparison of mode shapes of
observation and FEM
Left: 1st mode; Right: 2nd mode
Flow of excitation test
Outline of 2.4MW wind turbine and the equipment
Result of free vibration with pitch-fine condition
Left: X 1st mode; Right: Y 1st mode
Then the recorded free vibration is fitted with the
envelope equation below to obtain damping ratio.
a 𝑡 = Aexp −2𝜋𝑓𝜉𝑡
Fore-aft (X)
side-side (Y)