Apparatus for estimating a resonant frequency of a wind turbine tower
The invention regards an apparatus for estimating a resonant frequency of a wind turbine tower, comprising a measuring module adapted for measuring acceleration values of the wind turbine tower wherein the acceleration values represent acceleration of the wind turbine tower, a filter module adapted for receiving the measured acceleration values, the filter module comprises a variable filter wherein the variable filter is adapted to attenuate frequencies in a band hereby attenuating frequencies for an output of the filter module, an adaptive algorithm module comprising an adaptive algorithm wherein the adaptive algorithm module is adapted for communicating with the filter module and wherein the adaptive algorithm is adapted to minimize the energy of the output of the filter module by adjusting the band of attenuated frequencies, a resonant frequency estimating module adapted for estimating the resonant frequency of a wind turbine tower based on the attenuated frequencies.
1. Apparatus for estimating a resonant frequency of a wind turbine tower, comprising
a measuring module adapted for measuring acceleration values of the wind turbine tower wherein the acceleration values represent acceleration of the wind turbine tower,
a filter module adapted for receiving the measured acceleration values, the filter module comprises a variable filter wherein the variable filter is adapted to attenuate frequencies in a band hereby attenuating frequencies for an output of the filter module,
an adaptive algorithm module comprising an adaptive algorithm wherein the adaptive algorithm module is adapted for communicating with the filter module and wherein the adaptive algorithm is adapted to minimise the energy of the output of the filter module during operation of the wind turbine by adjusting the band of attenuated frequencies of the variable filter,
a resonant frequency estimating module adapted for estimating the resonant frequency of a wind turbine tower based on the attenuated frequencies.
2. Apparatus according to claim 1 , wherein acceleration values are measured in a plane perpendicular to the tower.
3. Apparatus according to claim 1 , wherein the measuring module comprises an accelerometer.
4. Apparatus according to claim 1 , wherein the variable filter is a band-stop filter.
5. Apparatus according to claim 4 , wherein the range of the stop-band is less that 0.2 Hz within a −3 dB bandwidth.
6. Apparatus according to claim 1 , wherein the variable filter comprises an IIR filter, preferably the filter is based on the expression in the z-domain,
H
(
z
)
=
1
-
2
·
cos
(
ϕ
)
·
z
-
1
+
z
-
2
1
-
2
·
r
·
cos
(
ϕ
)
·
z
-
1
+
r
2
·
z
-
2
,
where φ is the center frequency of the attenuated frequency.
7. Apparatus according to claim 1 , wherein the adaptive algorithm module receives an input from the output of the variable filter and/or an input of the acceleration values of the wind turbine tower.
8. Apparatus according to claim 1 , wherein the adaptive algorithm is an adaptive LMS algorithm or an adaptive RLS algorithm.
9. Apparatus according to claim 1 , wherein the adaptive algorithm is an adaptive LMS algorithm based on the minimising of a cost function, expressed as
J
(
n
)
=
1
2
e
2
(
n
)
=
1
2
(
d
(
n
)
-
y
(
n
)
)
2
,
wherein d(n) is the desired response, y(n) is the actual response and e(n) is the error signal.
10. Apparatus according to claim 9 wherein the adaptive algorithm determines a center frequency, by use of the updating equation,
ϕ
(
n
+
1
)
=
ϕ
(
n
)
-
μ
·
∂
J
(
n
)
∂
ϕ
(
n
)
,
wherein φ is the center frequency of the attenuating band, μ is the step size and J(n) is the cost function.
11. Apparatus according to claim 1 , wherein the resonant frequency is estimated as the center frequency of the attenuated band.
12. Apparatus according to claim 1 , wherein the resonant frequency estimating module is communicating with the adaptive algorithm module.
13. Apparatus according to claim 1 , further comprising a sensor that is able to establish the frequency of the rotor rotation.
14. Apparatus according claim 13 , wherein the filter module further comprises a 1P filter, that attenuates the 1P-oscillation frequency based on the established frequency of the rotor rotation.
15. Apparatus according claim 13 , wherein the filter module further comprises a 3P filter, that attenuates the 3P-oscillation frequency based on the established frequency of the rotor rotation.
16. Method of determining a resonant frequency of a wind turbine tower, comprising
receiving acceleration measurement values of an acceleration of a wind turbine tower, by means of a measuring module,
forwarding the acceleration measurement values to a filter module which comprises a variable filter which attenuates a frequency band by use of an adaptive algorithm,
adapting the adaptive algorithm to minimise the energy of the output of the variable filter during operation of the wind turbine by adjusting the band of attenuated frequencies of the variable filter,
determining the resonant frequency on the basis of the attenuated frequencies by means of a resonant frequency estimating module.
17. Method according to claim 16 , wherein the variable filter is a band-stop filter.
18. Method according to claim 16 , wherein the algorithm is, at least partly, based on values of the acceleration after the variable filter has been applied.
19. Method according to claim 16 , wherein the resonant frequency is determined to be the center frequency of the band of attenuated frequencies.