Fluid reaction surfaces (i.e. – impellers) – With weight-balancing means
Reexamination Certificate
2001-06-13
2004-01-06
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With weight-balancing means
C416S24400R, C416S500000
Reexamination Certificate
active
06672837
ABSTRACT:
The present invention relates to a wind turbine with oscillation damping means provided at the nacelle and being designed for damping edgewise oscillations of the rotor blades in the rotational plane of the rotor.
In particular, the invention relates to a wind turbine in which the oscillation damping means are provided at the end of the nacelle being opposite to the end from which the rotor extends and are designed for damping oscillations of the first eigenfrequency of the rotor blades in the rotational plane, especially oscillations being perpendicular to the rotational axis of the rotor.
BACKGROUND
Undesired oscillations may occur in wind turbines for power generation. These oscillations may include the entire turbine, i.e. several parts are oscillating in a combined mode, or the oscillations may occur locally in single part. Of particular severity are oscillations in the rotor blades either edgewise (in the rotor plane), flapwise (perpendicular to the rotor plane), or in a combined edge- and flapwise mode. Whether these oscillations do occur is dependent on the wind turbine design and the meteorological conditions.
Blade oscillations may be dampened by building a damping device into the blades, such as described in WO-A-95/21327, but it is difficult to produce a feasible design that is sufficiently compact and flat in order to satisfy the severe spatial restrictions. Furthermore, to build in a damper into existing blades is difficult and expensive.
The oscillation phenomena may cause dangerously high loads on the blade and other parts of the wind turbine, which may lead to a sudden collapse or alternatively may be the cause of fatigue damage and lifetime reduction, as cracks in the components slowly grow ultimately leading to failure. The occurrence of oscillations adds an uncertainty factor to predictions of lifetime loads on the various parts of a wind turbine, making it necessary to make the design stronger and heavier than would otherwise be the case.
DESCRIPTION OF THE INVENTION
Oscillations of the blades in the rotational plane of the rotor, the edgewise oscillations, are particular severe and may cause sudden fracture of the base part of the blades and may under unfortunate circumstances cause blades to break off the rotor.
The edgewise oscillations of the blades cause the centre of gravity of the rotor to oscillate, and the oscillations are transmitted to the nacelle on which the rotor is arranged. An alternative to dampening of the oscillations by arranging damping means in the blades have been found to be dampening of the oscillations of the nacelle. By arranging oscillation damping means at the nacelle for damping oscillations of an appropriate frequency the oscillations of the blades may be dampened to a harmless level since the nacelle and the rotor oscillates together.
This technical solution of the problems of edgewise blade oscillations is advantageous because it may be installed in existing wind turbines contrary to dampers in the blades, it is an inexpensive solution and an oscillation damping means in the nacelle may easily be accessed for adjustment and maintenance.
It is an object of the present invention to provide dampening of edgewise oscillations of the blades of a wind turbine by oscillation damping means arranged at the nacelle of the wind turbine.
Thus, the present invention relates to a wind turbine comprising
a foundation,
a tower extending substantially vertically and being mounted at a lower end on the foundation,
a nacelle supported by an upper end of the tower,
a wind rotor having at least one blade arranged on a main shaft having a substantially horizontal rotation axis and being arranged at the nacelle with the wind rotor extending from one end of the nacelle, the at least one blade defining a rotational plane being perpendicular to the rotation axis, and
oscillation damping means provided at the nacelle and being designed for damping oscillations of the at least one blade in the rotational plane.
The oscillation damping means may be arranged inside the nacelle or on the outside of the nacelle, mainly depending on the spatial restrictions inside the nacelle. However, it is for most wind turbines advantageous that the oscillation damping means are arranged at the end of the nacelle being opposite to the end from which the rotor extends because the nacelle is arranged pivotally on the tower about a central vertical yawing axis and the amplitude of the horizontal oscillations of the nacelle increases with the horizontal distance to the yawing axis for which reason oscillation damping means are more efficient at this position. Alternatively, the oscillation damping means could be arranged anywhere near the periphery of the nacelle, even at the end at which the rotor extends from the nacelle.
The oscillation damping means should be capable of at least damping oscillations being substantially horizontal and substantially perpendicular to the rotation axis since the oscillations of the rotor caused by the edgewise oscillations of the blade(s) are perpendicular to the rotation axis. The vertical oscillations are normally dampened sufficiently by the stiffness of the vertical connection between the nacelle and the tower, whereas the horizontal oscillations are less dampened because the yawing arrangement commonly has a certain clearance.
The oscillation damping means are preferably designed for damping oscillations of a frequency being substantially equal to the first eigenfrequency in the rotational plane of the at least one blade minus the frequency of rotation of the rotor. This mode has in most cases by experiment and simulations proven to be the frequency having the largest amplitude. However, for some construction it has be found to be advantageous to, alternatively or additionally to this frequency, to design the oscillation damping means for damping oscillations of a frequency being substantially equal to the first eigenfrequency in the rotational plane of the at least one blade plus the frequency of rotation of the rotor.
In case both of the aforementioned frequencies should be dampened, a favourable embodiment of the oscillation damping means comprises first damping means designed for damping oscillations of a frequency being substantially equal to the first eigenfrequency in the rotational plane of the at least one blade minus the frequency of rotation of the rotor and second damping means connected to an oscillating mass element of the first damping means and being designed for damping oscillations of a frequency being substantially equal to the first eigenfrequency in the rotational plane of the at least one blade plus the frequency of rotation of the rotor The second damping means may with such an arrangement be design much smaller that the first damping means, the oscillating mass of the second damping means being typically of about a tenth of the oscillating mass of the first damping means.
The oscillation damping means are in preferred embodiments designed for damping oscillations of a frequency deviating less than 0.2 Hz, preferably less than 0.1 Hz and most preferred less than 0.03 Hz from the first eigenfrequency in the rotational plane of the at least one blade minus, respectively plus, the frequency of rotation of the rotor.
The frequency of the oscillations that the oscillation damping means are designed for damping may be of variable frequency and comprise means for varying said frequency at least according to variations of the frequency of rotation of the rotor. Such means could be means for varying the mass of a mass element by pumping a liquid such a water to and from a liquid container arranged on or constituting the mass element.
The oscillation damping means are suitably designed for damping oscillations of a frequency in the range of 0.5-4 Hz, preferably in the range of 1-3 Hz.
The oscillation damping means should be designed for damping oscillations of the at least one blade in the rotational plane so that the oscillations at all operating conditions are dampened to a harmless amplitude level. This may be obtained
Rasmussen Flemming
Risager Lars
Veldkamp Herman F.
Birch & Stewart Kolasch & Birch, LLP
Look Edward K.
NEG Micon A/S
White Dwayne
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