DK201470170A1 - Damping of lateral acceleration by adjusting the yaw angle - Google Patents

Abstract

The present invention discloses a method for operating a wind turbine wherein a yaw angle of the rotor is adjustable via a yaw drive, the method comprising detecting at least one of a lateral acceleration, a lateral velocity and a lateral deflection of the tower, which upon detection is decrease by adjusting the yaw angle.

Description

DAMPING OF LATERAL ACCELERATION BY ADJUSTING THE YAW ANGLE BACKGROUND OF THE INVENTION

The present invention is directed to a method for operating a wind turbine in order to dampen oscillations of a tower of the wind turbine and a control device performing the method.

DESCRIPTION OF THE RELATED ART A wind turbine known in the art comprises a wind turbine tower and a rotor. The rotor is positioned on top of the tower and comprises a hub holding a number of rotor blades. The rotor is adapted to drive a generator. As an example, the wind turbine may have a horizontal axis rotor configuration. Such wind turbines are commonly referred to as horizontal axis wind turbines. In most cases, the hub is always oriented to the side of the tower which side is exposed to wind, the upwind side. To this end, a yaw controller operates a yaw drive orienting the rotor hub accordingly.

Both, the tower and the rotor blades are built slim and elastic. Thus, they are prone to all kinds of oscillations. Such oscillations may be due to cyclic rotor forces and to inhomogeneities in a wind field. Particularly, imbalances in a rotor lead to an excitation and forces perpendicular to the rotor axis. The wind field may be inhomogenous, as obstacles in front of the wind turbine partially slow down the wind. When the rotor blades sweep the area behind such an obstacle, the rotor is exposed to lower forces than the other rotor blades. Further, due to frictional forces on the ground, the wind in higher areas usually has higher wind speeds than the wind closer to ground. A higher wind speed leads to additional forces on the rotor blade.

Essentially, the rotor blade experiences two forces from the wind. A first force is due to the air drag of the rotor blade and is directed in parallel to the wind. This drag force leads to tilting and yawing moments on the rotor and thus, if not balanced by the other rotor blades, to a sideways force on the tower. A second force is perpendicular to a blade axis and to the rotor axis and is due to the aerodynamic properties of the rotor blade. This aerodynamic force provides momentum about the rotor axis and conventionally causes the rotor to rotate. If the aerodynamic force is paired symmetrically to the rotor axis by the aerodynamic forces of the other rotor blades, their directions and magnitudes compensate each other such that there is no resulting sideway force on the rotor axis. However, if the aerodynamic forces are not balanced, a resulting force on the rotor axis acts perpendicularly on the rotor axis. Such a resultant force may lead to a lateral oscillation of the tower, especially if it is variable over time.

For example, every time a wind turbine blade sweeps the highest area of the rotor area, where the highest wind speeds prevail, a resulting aerodynamic force on the rotor axis will occur with a rotational frequency of the rotor multiplied by the number of rotor blades, for example, three times the rotational frequency of the rotor. The sideway forces due to mechanical imbalances of the rotor will particularly have a frequency equal to the rotational frequency of the rotor. Thus, the tower will particularly experience sideways oscillations with a rotational frequency of the rotor and with a frequency corresponding to the rotational frequency of the rotor multiplied by the number of its rotor blades.

SUMMARY OF THE INVENTION

The invention is defined in the independent claims, and further aspects of the invention are set forth in the dependent claims. Embodiments of the present invention are explained by example with respect to the accompanying drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a wind turbine in accordance with embodiments of the invention, and FIG. 2 illustrates a cut-through sketch of the nacelle.

FIG. 1 shows a wind turbine 102 with a nacelle 104, and a rotor hub 106 pivotally mounted to the nacelle 104 via a rotor shaft. The rotor shaft extends from the rotor hub 106 facing away from the viewer and is therefore not shown. The nacelle 104 is mounted on a tower 108 of the wind turbine 102 via a nacelle bearing (see further below). The rotor hub 106 of the wind turbine includes three rotor blades 110 attached to the rotor hub 106. The rotor hub 106 is adapted to rotate about its axis of rotation, which is aligned to an axis of rotation of the rotor shaft, such that the rotor blades 110 sweep a rotational plane substantially perpendicular to the axes of rotation. The rotor hub 106 and the rotor blades 110 form a rotor 106, 110. The nacelle bearing is a rotary joint and is adapted to turn the rotor 106, 110 about a vertical axis 112 of the tower 108. The wind turbine is equipped with a yaw drive for rotating the nacelle 104 in a horizontal plane, this rotation may be defined in accordance with a yaw angle 114.

FIG. 2 shows the rotor hub 106, the nacelle 104 and the tower 108 in a side sectional view. The nacelle 104 contains a rotor shaft bearing 201, a generator 202. The rotor shaft bearing supports the rotor shaft 204. The rotor shaft is part of a drive train 203 connecting the rotor hub 106 to the generator 202.

The nacelle bearing 205 supports the nacelle 104 on the tower 108. The nacelle bearing 205 comprises a yaw drive 206 adapted to orient the rotor 106, 110 about the vertical axis 112 of the tower 108. The yaw drive 206 operates according to a signal from a yaw controller 207. The yaw controller 207 enables the yaw drive 206 to adjust the nacelle 104 and the rotor 106, 110 according to several parameters. In particular, the yaw controller 207 enables the yaw drive 206 to orient the rotor 106, 110 in an upwind direction.

The yaw controller 207 according to the invention is also adapted to counteract a lateral oscillation. The lateral oscillation is indicated by a signal corresponding to a state parameter comprising at least one of a lateral acceleration of a top portion of tower 108, a lateral velocity of the top portion of tower 108 and a lateral deflection of the top portion of The tower parameter 108. The state parameter is provided by a state indicator 208. The state indicator 208 comprises a sensor to detect any of a lateral acceleration, a lateral velocity and a lateral deflection.

In some embodiments, the yaw controller 207 receives a filtered version of the state parameter from a filter in the state indicator 208. For example, the state parameter is low pass filtered to deduce a constant deviation. In further embodiments the filter indicates an oscillation magnitude.

The yaw controller 207 adjusts the yaw angle of the tower to decrease any lateral acceleration, lateral velocity and lateral deflection. In some embodiments, the yaw controller 207 adjusts the yaw angle in a countercyclical movement tracking a lateral movement of the center of gravity. This way the yaw controller 207 decreases a movement of the center of gravity of the rotor 106, 110 with respect to ground, and the oscillations with rotational frequency of the rotor are damped.

In further embodiments, the yaw controller 207 adjusts the yaw angle in order to reduce force on the rotor 106, 110 which initially leads to the lateral oscillation. For example, the yaw controller 207 adjusts the yaw angle to decrease an air drag of the rotor blade which is the respective top rotor blade. This way an oscillation with a multiple of a rotational frequency, such as a 3P oscillation, should be damped, where 3P oscillation corresponds to an oscillation with a rotational frequency of the rotor multiplied by the number of rotor blades 110, which is 3 in the present embodiments.

Claims (7)

1. A method for operating a wind turbine which comprises a tower and a rotor with at least one rotor blade, the rotor being connected to the tower and being adapted to drive a generator, wherein a yaw angle of the rotor is adjustable via a yaw drive, the method comprising detecting at least one of a lateral acceleration, a lateral velocity and a lateral deflection of the tower, characterised by adjusting the yaw angle to decrease the corresponding one of the lateral acceleration, the lateral velocity and the lateral deflection of the tower.

2. The method according to claim 1, wherein the at least one of the lateral acceleration, the lateral velocity and the lateral deflection of the tower originate from a lateral oscillation of the tower, the lateral oscillation having a frequency component corresponding to a rotational frequency of the rotor.

3. The method according to any of the claims 1 or 2, wherein the rotor comprises a number of rotor blades, and wherein the at least one of the lateral acceleration, the lateral velocity and the lateral deflection of the tower originate from a lateral oscillation of the tower, the lateral oscillation having a frequency component corresponding to a rotational frequency of the rotor multiplied by the number of rotor blades of the rotor.

4. The method according to any of the preceding claims, wherein the rotor comprises a number of rotor blades, and wherein detecting the at least one of the lateral acceleration, the lateral velocity and the lateral deflection of the tower comprises filtering a signal corresponding to the lateral acceleration, the lateral velocity or the lateral deflection to obtain a frequency component corresponding to at least one of a constant signal component, a magnitude of a signal with a rotational frequency of the rotor, and a magnitude of a signal with a rotational frequency of the rotor multiplied by the number of rotor blades of the rotor.

5. The method according to any of the preceding claims, wherein the yaw angle is adjusted to track a lateral movement of the centre of gravity of the rotor such that a lateral movement of the centre of gravity of the rotor is minimised.

6. The method according to any of the preceding claims, wherein the yaw angle is adjusted to decrease an air drag of the rotor blade that is the respective top rotor blade of the rotating rotor at a given point in time.

7. The method according to any of the preceding claims, wherein detecting at least one of a lateral acceleration a top portion of the tower, a lateral velocity the top portion of the tower and a lateral deflection the top portion of the tower comprises detecting a lateral acceleration of a top portion of the tower, a lateral velocity of the top portion of the tower and a lateral deflection of the top portion of the tower, respectively.