Control device and method of controlling a wind turbine

Field of invention

The present invention relates to a control device and to a method of controlling a wind turbine.

When a wind turbine is not producing power to the grid, the wind turbine is normally put into either idling, i.e. an idling condition where a rotor is spinning with a controlled rotational speed and power is generated to supply the turbine alone (self-sustained turbine, SST), or completely into a stop mode (without rotor rotation). These mode are also re ferred to as off-grid modes.

In general, securing a stable and safe wind turbine in off- grid situations and/or faulty turbine events is becoming more and more important because offshore wind turbines will be in stalled further from the shore where an access to the wind turbine is reduced and/or very costly due to the price of offshore service vessels. This makes it important that the wind turbine alone can keep and maintain a stable and safe off-grid condition.

EP 2146 095 A2 discloses a method of controlling a wind tur bine, where a normal pitch strategy is continuously used to control the rotational speed in situations different from normal power production to the grid, i.e. in off-grid situa tions where the rotational speed is controlled for charging a pitch-drive power-supply backup (e.g. a battery). However, changing the pitch angle in the pitch strategy might induce stress to the system resulting in tear and wear of both pitch actuators and pitch bearings. Summary of the Invention

There may be a need for a control device and to a method of controlling a wind turbine which enable in the idling mode or in the SST mode an improved control of the rotor rotational speed to ensure that loads, wear, tear, power production and structural stability are kept within structural design capac ity of the wind turbine. This need may be met by the subject matters according to the independent claims. The present in vention is further developed as set forth in the dependent claims.

According to a first aspect of the invention, a control de vice is configured to control a wind turbine, the wind tur bine comprising a rotor being rotatable about a rotational axis, wherein the rotor has at least one blade, the blade comprising at least one add-on member which is actuated by a corresponding trim actuator to alter aerodynamic properties of the blade and a pitch actuator to alter a pitch angle of the blade. The control device is configured to determine an idling mode or a self-sustaining mode of the wind turbine; to control the pitch actuator to set a predetermined pitch angle of the blade, if the idling mode or the self-sustaining mode is determined; and to cause a target rotational speed of the rotor by variably controlling the trim actuator of the blade, if the idling mode or the self-sustaining mode is deter mined.

The active blade add-on member is used to secure a stable ro tational speed of the rotor and consequently a stable loading on the turbine and stable power production. For example, the rotational speed can be feedback-controlled (under a closed loop control) by actuating the add-on member of the blade. As a result, pitch loads on the pitch actuator and a blade bear ing can be reduced so that the design is more robust and maintenance costs are reduced. In an embodiment, the control device is configured to hold the predetermined pitch angle of the blade, while the trim actuator of the blade is variably controlled. During the giv en blade pitch angle, only the active blade add-on member is activated. In this operation, stress to the system is reduced resulting in less tear and wear of both pitch actuators and pitch bearings.

In an embodiment, the control device is configured to select the predetermined pitch angle of the blade out of a plurality of predetermined pitch angles of the blade. Thereby, a fine tuning can be achieved without unduly stressing the pitch ac tuators and pitch bearings.

In an embodiment, the control device is further configured to cause a trim stall at the blade by controlling the trim actu ator of the add-on member. Some wind turbines are already equipped with such active add-on members which are used to cause a trim stall at the blade. Thus, the present invention can be implemented in those existing wind turbines by retro fitting without expensive modifications. For example, the software of the control device can be updated, while the hardware is substantially maintained.

In an embodiment, the add-on member is a flap or spoiler at a trailing end or at a leading edge of the blade.

According to a second aspect of the invention, a wind turbine comprises a tower; a rotor, the rotor being mounted at the top of the tower to rotate about a rotational axis, wherein the rotor has at least one blade; and the control device.

According to a third aspect of the invention, a method of controlling a wind turbine is disclosed, wherein the wind turbine comprises a rotor being rotatable about a rotational axis, wherein the rotor has at least one blade, the blade comprising at least one add-on member which is actuated by a corresponding trim actuator to alter aerodynamic properties of the blade and a pitch actuator to alter a pitch angle of the blade. The method comprises steps of determining an idling mode or a self-sustaining mode of the wind turbine; controlling the pitch actuator to set a predetermined pitch angle of the blade, if the idling mode or the self-sustaining mode is determined; and causing a target rotational speed of the rotor by variably controlling the trim actuator of the blade, if the idling mode or the self-sustaining mode is de termined.

In an embodiment, the method further comprises a step of holding the predetermined pitch angle of the blade, while the trim actuator of the blade is variably controlled.

In an embodiment, the method further comprises a step of se lecting the predetermined pitch angle of the blade out of a plurality of predetermined pitch angles of the blade. The blade pitch angle can have multiple fixed positions while the blade active add-on members are working so that the blade pitch angle is not continuously changed to secure a stable rotor speed, power production and loads.

In an embodiment, the method further comprises a step of causing a trim stall at the blade by controlling the trim ac tuator of the add-on member.

In an embodiment, the target rotational speed of the rotor is determined for charging a battery backup of the wind turbine. For example, the rotor torque can be influenced by a State of charge (SoC) which is the level of charge of the battery backup relative to its capacity. The present invention can compensate for such a change of the rotor torque to maintain a stable rotor speed.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Fig. 1 shows an example of a wind turbine and different ele ments thereof;

Fig. 2 shows a wind turbine blade having an add-on member; and

Fig. 3 shows the add-on member in an activated position, where the add-on member is turned to maximum stalling effect.

Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ments are provided with the same reference signs.

Fig . 1 shows an example of a wind turbine 1. The wind turbine 1 comprises a nacelle 3 and a tower 2. The nacelle 3 is mounted at the top of the tower 2. The nacelle 3 is mounted rotatable with regard to the tower 2 by means of a yaw bear ing (not shown). The yaw bearing may belong to an active yaw system. The axis of rotation of the nacelle 3 with regard to the tower 2 is referred to as the yaw axis.

The wind turbine 1 also comprises a rotor (hub) 4 with three rotor blades 6 (of which two rotor blades 6 are depicted in Fig. 1). The blades 6 are connected to the rotor 4 via a joint section (not shown). Each blade 6 of the wind turbine 1 comprises a pitch actuator (not shown) to alter a pitch angle of the blade 6. The pitch angle usually refers to a rotation of the blade 6 about a longitudinal axis of the blade 6. The rotational speed of the rotor 4 and thus the output power of the generator 5 depend -amongst others- on the pitch angle of the blades 6.

The rotor 4 is mounted rotatable with regard to the nacelle 3 by means of a main bearing 7. The rotor 4 is mounted rotata ble about a rotational axis 8.

The wind turbine 1 furthermore comprises a generator 5. The rotor 4 is connected directly to the generator 5, thus the wind turbine 1 is referred to as a gearless, direct-driven wind turbine. Such a generator 5 is referred as direct drive generator 5. As an alternative, the rotor 4 may also be con nected to the generator 5 via a gear box. This type of wind turbine 1 is referred to as a geared wind turbine. The pre sent invention is suitable for both types of wind turbines 1.

The generator 5 is accommodated within the nacelle 3. The generator 5 is arranged and prepared for converting the rota tional energy from the rotor 4 into electrical energy in the shape of an AC power.

Fig . 2 shows a wind turbine blade 6 of the wind turbine 1. Each blade 6 has at least one active add-on member 17 which is actuated by a corresponding trim actuator (not shown) to alter aerodynamic properties of the blade 6. The cross- section of Fig. 2 shows only one active add-on member 17; however, additional active add-on members 17 of the blade 6 can be arranged in front of or behind this active add-on mem ber 17.

The add-on member 17 is designed as a spoiler. The spoiler 17 is here arranged near the leading (front) edge of the blade 6, but it can also be arranged near the trailing (back) edge of the blade 6. The add-on member 17 can be accommodated in a recess 16 in the blade 6 and can turn about a hinge 18 by ac tivation of the trim actuator. In Fig. 2, the spoiler 17 is shown in its normal deactivated position, where no spoiler effect and no stall are desired. This position is referred to as a predetermined first position. The predetermined first positions of the add-on members 17 of different blades 6 can be identical to ensure load symmetry when activating a plu rality of add-on members 17. This also reduces complexity as one trim actuator (for example a pressure supply or an elec tro motor) can drive the corresponding add-on members 17 on all blades 6.

Fig . 3 shows the same add-on member 17 in an activated posi tion, where the add-on member 17 is turned to a maximum by the trim actuator so that the stalling effect is maximum.

This position is referred as a predetermined second position. The predetermined second positions of add-on members 17 of different blades 6 can be identical. This ensures load sym metry when activating the add-on members 17. This also reduc es complexity as one trim actuator (for example a pressure supply or an electro motor) can drive the corresponding add on members 17 on all blades 6.

According to the present invention, the add-on member 17 is not necessarily to be formed as spoiler. The add-on member 17 can have any other configuration which is able to alter the aerodynamic properties of the blade 6, for example a flap. The wind turbine 1 comprises a control device (not shown) which is configured to determine an idling mode or a self- sustaining, SST, mode of the wind turbine 1. The idling mode and the self-sustaining mode are also referred to as off-grid modes where the wind turbine 1 is not connected to the grid or does substantially not supply power to the grid. However, the wind turbine 1 itself needs energy, for example for main taining an operation of the control device and for charging a battery backup of the wind turbine 1.

The control device is further configured to control the pitch actuator to set a predetermined pitch angle of the blade 6, if the idling mode or the self-sustaining mode is determined, and to cause a target rotational speed of the rotor 4 by var iably controlling the trim actuator of the blade 6, if the idling mode or the self-sustaining mode is determined. Thus, the pitch angle of the blade 6 can be locked at a pre determined pitch angle (predetermined position) after an off- grid event has been detected, and the wind turbine 1 can con tinue in the idling mode or the self-sustaining mode by only using the add-on members 17 of the blade 6 to control a rota tional speed of the rotor 4, i.e. to supply electric energy only to the wind turbine 1 itself.

The active blade add-on member 17 is used to secure a stable rotational speed of the rotor 4 and consequently a stable loading on the wind turbine 1 and stable power production.

For example, the rotational speed of the rotor 4 can be feed back-controlled (closed loop control) by actuating the add-on member 17 of the blade 6.

Preferably, the control device is configured to hold the pre determined pitch angle of the blade 6, while the trim actua tor of the blade 6 is variably controlled. In this operation, stress to the system is reduced resulting in less tear and wear of the pitch actuators and also of pitch bearings. Preferably, the control device is configured to select the predetermined pitch angle of the blade 6 out of a plurality of predetermined pitch angles of the blade 6. Thereby, a fine tuning can be achieved without unduly stressing the pitch ac tuators and pitch bearings.

Preferably, the control device is further configured to cause a trim stall at the blade 6 by controlling the trim actuator of the add-on member 17. Some wind turbines 1 are already equipped with the active add-on members 17 which are used to cause a trim stall at the blade 6. Thus, the present inven tion can be implemented in those existing wind turbines 1 just by retrofitting without expensive modifications. For ex ample, the software of the control device can be updated while the hardware is substantially maintained.

In a modified embodiment, the above-described control of the control device can be combined with an active yaw control for controlling a rotational speed of the rotor 4.

An active yaw system of wind turbine 1 is responsible for an orientation of the rotor 4 towards the wind, i.e. the yaw system usually aligns the rotational axis 8 to a detected wind direction in order to obtain the maximum power output. The active yaw system is equipped with a yaw actuator to ro tate the nacelle 3 of the wind turbine 1 against the station ary tower 2 based on signals from wind direction sensors or manual actuation (control system override).

In the modified embodiment, the active yaw control is com bined with the control of the trim actuator of the active add-on member 17 to control the rotational speed of the rotor 4, the output power of the wind turbine 1 and/or loads.

It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.