METHOD FOR OPERATING A WIND TURBINE AND A WIND TURBINE

Abstract

A method for operating a wind turbine is disclosed, wherein said wind turbine comprises a rotor having at least one rotor blade with a rotor blade surface and an icing detection device for detecting an icing condition for the rotor blade and/or for detecting the presence of icing on the rotor blade. Further, a controller configured for controlling a rotational speed of the rotor can be provided. The method comprises the steps of monitoring, via the controller and/or via the icing detection device, whether an icing condition for the rotor blade is present and/or if icing on the surface of the wind turbine is present, thus, that ice has been generated on the surface. If an icing condition is detected, or if it is detected that ice has generated on the surface of the rotor blade, the wind turbine is operated further according to a de-rated icing-mode having a reduced rotational speed, in particular while maintaining a generation of electrical energy by a generator of the wind turbine.

Claims

1-15. canceled.

16. A method for operating a wind turbine, wherein the wind turbine includes a rotor with at least one rotor blade having a rotor blade surface, and an icing detection device configured to detect an icing condition for the rotor blade or icing on the rotor blade, the method comprising monitoring, via the icing detection device, whether an icing condition for the rotor blade or icing on the rotor blade is present; and when an icing condition or icing on the rotor blade is detected, operating the wind turbine according to a de-rated icing-mode having a reduced rotor rotational speed.

17. The method according to claim 16, wherein the reduced rotational speed of the rotor during the de-rated icing-mode is determined such that generation of ice on the rotor blade surface is prevented or ice on the rotor blade surface detaches.

18. The method according to claim 16, wherein the de-rated icing-mode comprising operating the wind turbine at a reduced power output that is determined such that generation of ice on the rotor blade surface is prevented or ice on the rotor blade surface is detaches.

19. The method according to claim 16, wherein the wind turbine further includes an ice mitigation device configured to provide a de-icing heat flow to the rotor blade, wherein the method further comprises activating the ice mitigation device during the de-rated icing mode.

20. The method according to claim 16, wherein the icing condition or icing on the rotor blade is detected based on an analysis of operational data or an icing model.

21. The method according to claim 20, wherein the analysis declares a rotor blade status of “icing confirmed” when icing is present on the rotor blade surface, or declares a rotor blade status of “icing imminent” when no icing is detected but icing is imminent.

22. The method according to claim 21, wherein when the rotor bade status is “icing confirmed”, the method comprises determining the reduced rotor rotational speed or a power reduction of the wind turbine such that the rotor is rotated with a minimum de-rated rotational speed thereby generating a minimum electrical power that is entirely supplied to the wind turbine or to a different wind turbine in a wind part.

23. The method according to claim 22, wherein the wind turbine or the wind park are connected to a grid, and the minimum electrical power is sufficient to operate the wind turbine or the wind park without energy extraction from the grid while operating an ice mitigation device of the wind turbine or another wind turbine in the wind park at a maximum heating power output.

24. The method according to claim 21, wherein when the rotor blade status is “icing imminent”, the method comprises determining the reduced rotor rotational speed or a power reduction of the wind turbine such that the rotor is rotated with a maximum de-rated rotational speed thereby generating a maximum electrical power while a generation of ice on the rotor blade surface is not present.

25. The method according to claim 16, further comprising determining a temperature of the rotor blade, and determining the reduced rotor rotational speed or a power reduction of of the wind turbine such that: a temperature gradient of the temperature exceeds a temperature gradient threshold; the surface temperature of the rotor blade surface exceeds a surface temperature threshold; or dissipation of thermal energy of the rotor blade in an environment of the wind turbine does not exceed a heat flow threshold.

26. The method according to claim 25, wherein determining a temperature of the rotor blade comprises determining a surface temperature of the rotor blade with a temperature sensor configured on the rotor blade.

27. The method according to claim 25, wherein determining a temperature of the rotor blade comprises estimating a surface temperature of the rotor blade based on: (a) one or more variables including ambient temperature, a wind speed, the rotational speed of the rotor, a pitch angle of the rotor blade, a relative wind speed of the rotor blade, an air humidity, or an environmental heat input; or (b) an inner air temperature within an inner volume of the rotor blade.

28. The method according to claim 26, wherein the rotor blade includes a plurality of temperature sensors positioned at different locations of the rotor blade to provide a plurality of measured temperatures, wherein the step of determining the amount of reduction of the rotor or reduction of power production of the wind turbine is based on one or more of the lowest values of the measured temperatures.

29. The method according to claim 16, further comprising operating the wind turbine in a shutdown mode upon detection of icing of the rotor blade above a defined severe value.

30. A wind turbine, comprising: a rotor comprising at least one rotor blade having a rotor blade surface; an icing detection device; a wind turbine controller that controls a rotational speed of the rotor, the wind turbine controller in communication with the icing detection device and configured to perform the following operations: monitoring whether an icing condition of the rotor blade is present or icing on the rotor blade surface is present; and operating the wind turbine in a de-rated icing-mode having a reduced rotational speed of the rotor when an icing condition of the rotor blade or icing on the rotor blade surface is detected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0040] FIG. 1 illustrates a perspective view of one embodiment of a wind turbine according to the present disclosure;

[0041] FIG. 2 illustrates a simplified, internal view of one embodiment of a nacelle of a wind turbine according to the present disclosure, particularly illustrating the nacelle during standard operation;

[0042] FIG. 3 reflect an embodiment of a method for operating the wind turbine according to FIG. 1; and

[0043] FIG. 4 schematically indicates an exemplary control strategy applied in the method according to FIG. 3.

[0044] Single features depicted in the figures are shown relatively with regards to each other and therefore are not necessarily to scale. Similar or same elements in the figures, even if displayed in different embodiments, are represented with the same reference numbers

DETAILED DESCRIPTION OF THE INVENTION

[0045] Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0046] FIG. 1 is a perspective view of an exemplary wind turbine 10. In the exemplary embodiment, the wind turbine 10 is a horizontal-axis wind turbine. Alternatively, the wind turbine 10 may be a vertical-axis wind turbine. In the exemplary embodiment, the wind turbine 10 includes a tower 12 that extends from a support system 14, a nacelle 16 mounted on tower 12, and a rotor 18 that is coupled to nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 100 coupled to and extending outward from the hub 20. In the exemplary embodiment, the rotor 18 has three rotor blades 100. In an alternative embodiment, the rotor includes more or less than three rotor blades. In the exemplary embodiment, the tower 12 is fabricated from tubular steel to define a cavity (not shown in FIG. 1) between a support system 14 and the nacelle 16. In an alternative embodiment, the tower 12 is any suitable type of a tower having any suitable height.

[0047] The rotor blades 100 are spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. The rotor blades 100 are mated to the hub 20 by coupling a blade root portion 24 to the hub 20 at a plurality of load transfer regions 26. The load transfer regions 26 may have a hub load transfer region and a blade load transfer region (both not shown in FIG. 1). Loads induced to the rotor blades 100 are transferred to the hub 20 via the load transfer regions 26.

[0048] In one embodiment, the rotor blades 100 have a length ranging from about 15 meters (m) to about 91 m. Alternatively, rotor blades 100 may have any suitable length that enables the wind turbine 10 to function as described herein. For example, other non-limiting examples of blade lengths include 20 m or less, 37 m, 48.7 m, 50.2 m, 52.2 m or a length that is greater than 91 m. As wind strikes the rotor blades 100 from a wind direction 28, the rotor 18 is rotated about an axis of rotation 30. As the rotor blades 100 are rotated and subjected to centrifugal forces, the rotor blades 100 are also subjected to various forces and moments. As such, the rotor blades 100 may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position.

[0049] The rotor blade 100 extends in a longitudinal direction 102 and comprises a rotor blade surface 104, wherein said rotor blade surface 104 can be a pressure surface of the rotor blade 100, a suction surface of the rotor blade 100, a surface of a leading-edge of the rotor blade 100, or a surface of a trailing edge of the rotor blade 100. Furthermore, the rotor blade 100 includes an ice mitigation device 106 for providing thermal energy to the rotor blade surface 104 in order for prevent a generation of ice on the rotor blade surface 104 (icing), and/or for removing ice from the rotor blade surface 104 by at least partially melting said ice and thereby reducing an adhesion of ice and the rotor blade surface 104.

[0050] For example, the ice mitigation device 106 comprises a direct heating system for directly generating thermal energy in a rotor blade skin of the rotor blade surface, and/or a hot air system for increasing an inner temperature in an inner volume of the rotor blade. According to an embodiment, the ice mitigation device 106 comprises heating strips or heating mats which are arranged within a shell of the rotor blade 100 or on an inner surface of a shell of the rotor blade 100.

[0051] The wind turbine 10 further comprises an ice detection device configured for detecting an icing condition for the rotor blade 100 or for detecting icing on the rotor blade 100.

[0052] Moreover, a pitch angle of the rotor blades 100, i.e., an angle that determines a perspective of the rotor blades 100 with respect to the wind direction, may be changed by a pitch system 32 to control the load and power generated by the wind turbine 10 by adjusting an angular position of at least one rotor blade 100 relative to wind vectors. Pitch axes 34 of rotor blades 100 are shown. During operation of the wind turbine 10, the pitch system 32 may change a pitch angle of the rotor blades 100 such that the rotor blades 100 are moved to a feathered position, such that the perspective of at least one rotor blade 100 relative to wind vectors provides a minimal surface area of the rotor blade 100 to be oriented towards the wind vectors, which facilitates reducing a rotational speed and/or facilitates a stall of the rotor 18.

[0053] In the exemplary embodiment, a blade pitch of each rotor blade 100 is controlled individually by a wind turbine controller 36 or by a pitch control system 80. Alternatively, the blade pitch for all rotor blades 100 may be controlled simultaneously by said control systems.

[0054] Further, in the exemplary embodiment, as the wind direction 28 changes, a yaw direction of the nacelle 16 may be rotated about a yaw axis 38 to position the rotor blades 100 with respect to wind direction 28.

[0055] In the exemplary embodiment, the wind turbine controller 36 is shown as being centralized within the nacelle 16, however, the wind turbine controller 36 may be a distributed system throughout the wind turbine 10, on the support system 14, within a wind farm, and/or at a remote control center. The wind turbine controller 36 includes a processor 40 configured to perform the methods and/or steps described herein. According to an embodiment, the wind turbine controller 36 includes the icing detection device for detecting an icing condition or an icing on the rotor blade 100.

[0056] Further, many of the other components described herein include a processor. As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and/or a control system can also include memory, input channels, and/or output channels.

[0057] FIG. 2 is an enlarged sectional view of a portion of the wind turbine 10. In the exemplary embodiment, the wind turbine 10 includes the nacelle 16 and the rotor 18 that is rotatably coupled to the nacelle 16. More specifically, the hub 20 of the rotor 18 is rotatably coupled to an electric generator 42 positioned within the nacelle 16 by the main shaft 44, a gearbox 46, a high speed shaft 48, and a coupling 50. In the exemplary embodiment, the main shaft 44 is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle 16. A rotation of the main shaft 44 drives the gearbox 46 that subsequently drives the high speed shaft 48 by translating the relatively slow rotational movement of the rotor 18 and of the main shaft 44 into a relatively fast rotational movement of the high speed shaft 48. The latter is connected to the generator 42 for generating electrical energy with the help of a coupling 50.

[0058] The gearbox 46 and generator 42 may be supported by a main support structure frame of the nacelle 16, optionally embodied as a main frame 52. The gearbox 46 may include a gearbox housing that is connected to the main frame 52 by one or more torque arms 103. In the exemplary embodiment, the nacelle 16 also includes a main forward support bearing 60 and a main aft support bearing 62. Furthermore, the generator 42 can be mounted to the main frame 52 by decoupling support means 54, in particular in order to prevent vibrations of the generator 42 to be introduced into the main frame 52 and thereby causing a noise emission source.

[0059] Preferably, the main frame 52 is configured to carry the entire load caused by the weight of the rotor 18 and components of the nacelle 16 and by the wind and rotational loads, and furthermore, to introduce these loads into the tower 12 of the wind turbine 10. The rotor shaft 44, generator 42, gearbox 46, high speed shaft 48, coupling 50, and any associated fastening, support, and/or securing device including, but not limited to, support 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0060] The nacelle 16 also may include a yaw drive mechanism 56 that may be used to rotate the nacelle 16 and thereby also the rotor 18 about the yaw axis 38 to control the perspective of the rotor blades 100 with respect to the wind direction 28.

[0061] For positioning the nacelle appropriately with respect to the wind direction 28, the nacelle 16 may also include at least one meteorological mast 58 that may include a wind vane and anemometer (neither shown in FIG. 2). The mast 58 provides information to the wind turbine controller 36 that may include wind direction and/or wind speed.

[0062] In the exemplary embodiment, the pitch system 32 is at least partially arranged as a pitch assembly 66 in the hub 20. The pitch assembly 66 includes one or more pitch drive systems 68 and at least one sensor 70. Each pitch drive system 68 is coupled to a respective rotor blade 100 (shown in FIG. 1) for modulating the pitch angel of a rotor blade 100 along the pitch axis 34. Only one of three pitch drive systems 68 is shown in FIG. 2.

[0063] In the exemplary embodiment, the pitch assembly 66 includes at least one pitch bearing 72 coupled to hub 20 and to a respective rotor blade 100 (shown in FIG. 1) for rotating the respective rotor blade 100 about the pitch axis 34. The pitch drive system 68 includes a pitch drive motor 74, a pitch drive gearbox 76, and a pitch drive pinion 78. The pitch drive motor 74 is coupled to the pitch drive gearbox 76 such that the pitch drive motor 74 imparts mechanical force to the pitch drive gearbox 76. The pitch drive gearbox 76 is coupled to the pitch drive pinion 78 such that the pitch drive pinion 78 is rotated by the pitch drive gearbox 76. The pitch bearing 72 is coupled to pitch drive pinion 78 such that the rotation of the pitch drive pinion 78 causes a rotation of the pitch bearing 72.

[0064] Pitch drive system 68 is coupled to the wind turbine controller 36 for adjusting the pitch angle of a rotor blade 100 upon receipt of one or more signals from the wind turbine controller 36. In the exemplary embodiment, the pitch drive motor 74 is any suitable motor driven by electrical power and/or a hydraulic system that enables pitch assembly 66 to function as described herein. Alternatively, the pitch assembly 66 may include any suitable structure, configuration, arrangement, and/or components such as, but not limited to, hydraulic cylinders, springs, and/or servo-mechanisms. In certain embodiments, the pitch drive motor 74 is driven by energy extracted from a rotational inertia of hub 20 and/or a stored energy source (not shown) that supplies energy to components of the wind turbine 10.

[0065] The pitch assembly 66 also includes one or more pitch control systems 80 for controlling the pitch drive system 68 according to control signals from the wind turbine controller 36, in case of specific prioritized situations and/or during rotor 18 overspeed. In the exemplary embodiment, the pitch assembly 66 includes at least one pitch control system 80 communicatively coupled to a respective pitch drive system 68 for controlling pitch drive system 68 independently from the wind turbine controller 36. In the exemplary embodiment, the pitch control system 80 is coupled to the pitch drive system 68 and to a sensor 70. During standard operation of the wind turbine 10, the wind turbine controller 36 controls the pitch drive system 68 to adjust a pitch angle of rotor blades 100.

[0066] In one embodiment, in particular when the rotor 18 operates at rotor overspeed, the pitch control system 80 overrides the wind turbine controller 36, such that the wind turbine controller 36 no longer controls the pitch control system 80 and the pitch drive system 68. Thus, the pitch control system 80 is able to make the pitch drive system 68 to move the rotor blade 100 to a feathered position for reducing a rotational speed of the rotor 18.

[0067] According to an embodiment, a power generator 84, for example comprising a battery and/or electric capacitors, is arranged at or within the hub 20 and is coupled to the sensor 70, the pitch control system 80, and to the pitch drive system 68 to provide a source of power to these components. In the exemplary embodiment, the power generator 84 provides a continuing source of power to the pitch assembly 66 during operation of the wind turbine 10. In an alternative embodiment, power generator 84 provides power to the pitch assembly 66 only during an electrical power loss event of the wind turbine 10. The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine 10, and/or failure of the wind turbine controller 36. During the electrical power loss event, the power generator 84 operates to provide electrical power to the pitch assembly 66 such that pitch assembly 66 can operate during the electrical power loss event.

[0068] In the exemplary embodiment, the pitch drive system 68, the sensor 70, the pitch control system 80, cables, and the power generator 84 are each positioned in a cavity 86 defined by an inner surface 88 of hub 20. In an alternative embodiment, said components are positioned with respect to an outer surface 90 of hub 20 and may be coupled, directly or indirectly, to outer surface 90.

[0069] FIG. 3 shows an example of an embodiment of a method for operating the wind turbine according to the present invention, wherein FIG. 4 depicts a working principle of the present disclosure.

[0070] According to FIG. 3, operational data is collected in step 200 by monitoring an operation of the wind turbine 10. Initially, the wind turbine is operated in a standard mode according to a general operational standard. For example, if the wind speed at least reaches rated wind speed the wind turbine 10 is operated at rated power. Monitoring 200 may include a step 216 of determining a blade temperature of the rotor blade 100. In particular, the step 216 may comprise the determination of a surface temperature of the rotor blade surface 104.

[0071] In step 202 it is detected via the icing detection device and/or via the wind turbine controller 36 if an icing condition is present or if ice has developed on the rotor blade 100, in particular on the rotor blade surface 104. In case the icing condition is confirmed by the icing detection device, an ice mitigation device 106 can be activated in step 206. This ice mitigation device 106 is configured for providing a de-icing heat flow to the rotor blade surface 104, for example by activating heating elements being arranged in a shell of the rotor blade 100 or by blowing hot air into an interior of the rotor blade 100. Generally, the ice mitigation device 106 is configured for heating up the rotor blade surface 104 of the rotor blade 100, thus, that the icing condition is eliminated and/or that ice, which is already present on the rotor blade surface 104, is at least partially melted and subsequently detaching from the rotor blade surface 104.

[0072] In a step 222 the wind turbine controller 36 and/or the ice detection device may conduct a procedure for checking if severe icing is present on the rotor blade 100. If severe icing 222 is present, a specific icing situation is detected in which icing leads to an inacceptable increase of loads for the wind turbine 10, for the rotor 18 and/or for the rotor blade 100. In such case, a shutdown step 218 for shutting down the wind turbine 10 is initiated, wherein either the rotation of the rotor 18 can be entirely brought to a halt or wherein rotor 18 is put in an idling mode by moving the rotor blade 100 in a feathered position.

[0073] In step 204 a de-rate icing-mode is activated. According to the de-rate icing-mode the rotor 18 of the wind turbine 10 is operated with a reduced rotational speed compared to the standard operational mode, in which the wind turbine was operated prior to switching to the de-rate icing-mode. By reducing the rotational speed of rotor 18, a relative wind speed of the rotor blade 100 is reduced.

[0074] FIG. 4 describes the effect of reducing rotational speed of rotor 18 on de-icing of the rotor blade 100, while assuming one specific environmental condition. The dissipation of thermal energy of the rotor blade 100 in the surrounding and passing air increases with the relative wind speed. Hence, at a certain rotational speed, while assuming constant environmental conditions, a maximum heating capacity 226 of the ice mitigation device 106 is surpassed. This leads to a cooling down of the rotor blade 100 and of the rotor blade surface 104 which subsequently may result in a generation of ice on the rotor blade surface 104. Therefore, the reduction of rotational speed of the rotor 18 is chosen such that a generation of ice on the rotor blade surface 104 is prevented and/or that the detected icing condition is no longer present.

[0075] For example, in FIG. 4 the wind turbine 10 initially is operated in a standard operational mode having a standard rotational speed 224. Hence, the wind speed facing the wind turbine 10 is facilitating that the rotor 18 rotates faster than a cut-in rotational speed 228 and slower than a cut-out rotational speed 230 of the wind turbine 10. The wind turbine 10 may be operated at rated power including a rated rotational speed when rotating with the standard rotational speed 224. Regardless, the standard rotational speed 224 may also represent a rotational speed below rated rotational speed.

[0076] According to the example of FIG. 4, an icing condition is detected while operating the wind turbine 10 according to a standard operational mode with the standard rotational speed 224. Furthermore it is detected that the dissipation of thermal energy of the rotor blade 100 exceeds the maximum heating capacity 226 of the ice mitigation device 106 when the rotor 18 is rotated with standard rotational speed 224.

[0077] According to the specific example, but not limited to it, a status “icing imminent” is declared in step 210. Accordingly, it has not been confirmed that ice has already generated on the rotor blade surface 104 or any ice being already present on the rotor blade surface 104 is not considered to be critical to any wind turbine load. For that reason, in step 214 the desired rotational speed of rotor 18 is determined such, that dissipation of thermal energy of rotor blade 100 falls just below (or equals) the maximum heating capacity 226 in order to prevent ice being generated on the rotor blade surface 104 or in order for ice already generated being removed by melting an adhesive layer between the ice and the rotor blade surface 104. Hence, according to this type of de-rate icing-mode it is decided in step 214 to optimize power generation of the wind turbine 10.

[0078] In contrast to step 210, it may also be detected that ice has already been generated on the rotor blade surface and a status “icing confirmed” is declared. In particular, it is determined that the ice on the rotor blade surface 104 has a non-negligible impact on the load situation of the wind turbine 10. For that reason, the reduction of rotational speed of rotor 18 according to step 212 is also based on a desired load reduction effect, hence, the reduction of the rotational speed of rotor 18 is larger than the reduction of the rotational speed in step 214. If required, the rotational speed of rotor 18 is reduced in step 212 such that energy generated by the wind turbine is entirely consumed by the ice mitigation device 106 and by further components of the wind turbine 10, while no electrical energy is delivered to the electric grid to which the wind turbine 10 is connected.

[0079] Step 212 and step 214, respectively, operational situations 208 “icing confirmed” and 210 “icing imminent”, reflect extreme operational situations wherein either energy generation is maximized or icing-caused loads are minimized while maintaining operation. Therefore, it shall also be disclosed that there are a variety of interim operational situations between situation 208 “icing confirmed” and situation 210 “icing imminent”, wherein the rotational speed and possibly further operational details of the wind turbine 10 are carefully chosen in order to optimize the operation of the wind turbine 10 to the specific situation.

[0080] Furthermore, the respective determination of rotational speed and/or of generated power of wind turbine 10 may be achieved in an iterative manner, wherein a certain desired rotational speed/generated power is determined and a subsequent effect on the icing condition, on a temperature gradient, or on a temperature of the rotor blade 100 or of the rotor blade surface 104 is observed, and consequently a new desired rotational speed may be determined. For example, the amount of rotational reduction and/or determining the amount of power reduction is embodied such that a temperature gradient of the temperature of the rotor blade 100 or of the rotor blade surface 104 exceeds a specific temperature gradient threshold, for example, wherein the temperature gradient threshold exceeds 0° C./min, preferably exceeds 1.1° C./min, and/or in wherein the temperature gradient threshold is below 3° C./min, preferably is below 2° C./min. Additionally or alternatively, the temperature of the rotor blade 100 or the temperature of the rotor blade surface 104 may be used as an input factor for determining the reduction in rotational speed and/or of generated power. For example, the reduction of rotational speed of rotor 18 is determined such, that the temperature exceeds a temperature threshold, in particular wherein the temperature threshold exceeds 0° C., preferably exceeds 1° C., and/or in particular wherein the temperature threshold is below 5° C., preferably is below 3° C. Further additionally or alternatively, the reduction of rotational speed of rotor 18 is determined such, that a dissipation of thermal energy of the rotor blade 100 in an environment of the wind turbine 10 does not exceed a maximum heating capacity 226 of the ice mitigation device 106.

[0081] When it is determined in step 220 that wind turbine 10 may be operated in standard operation 200 without the threat that an icing condition is repeatedly detected, the de-rate icing-mode is terminated and wind turbine 10 continues to operate according to a standard operational mode.

[0082] The order of steps has exemplary shown in FIG. 3 shall not limit the claimed method for operating a wind turbine, for example, the step 222 for checking if severe icing is present may as well be conducted prior to step 206 or even after step 204, or at any other reasonable strategic position. The same applies for the step 206 of activating the ice mitigation system 106, wherein this step 206 may also be timely and/or hierarchy-wise positioned in an attractive manner. Even further, an embodiment may be possible, wherein the wind turbine either comprise an ice mitigation system, nor step 206 of activating the ice mitigation device.

[0083] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

[0084] The present invention is not limited to the above-described embodiments and modifications and may be embodied in various forms within the gist thereof, for example, that the wind turbine 10 does not comprise an ice mitigation device 106 but still can implement the method for operating a wind turbine as disclosed. The technical features may also be omitted as appropriate unless they are described as being essential in this specification.

REFERENCE NUMBERS

[0085] 10 wind turbine 100 rotor blade

[0086] 12 tower 102 longitudinal direction

[0087] 14 support system 104 rotor blade surface

[0088] 16 nacelle 106 ice mitigation device

[0089] 18 rotor 108 temperature sensor

[0090] 20 rotatable hub

[0091] 24 blade root portion 200 monitoring

[0092] 26 load transfer regions 202 detecting an icing condition

[0093] 28 wind direction 204 activating de-rate mode

[0094] 30 axis of rotation 206 activating ice mitigation system

[0095] 32 pitch system 208 declaring “icing confirmed”

[0096] 34 pitch axes 210 declaring “icing imminent”

[0097] 36 when turbine controller 212 determining

[0098] 38 yaw axis 214 determining

[0099] 40 processor 216 determining blade temperature

[0100] 42 electric generator 218 activating shutdown mode

[0101] 44 main shaft 220 return to standard operation

[0102] 46 gearbox 222 checking on severe icing

[0103] 48 high speed shaft 224 standard operational speed

[0104] 50 coupling 226 maximum heating capacity

[0105] 52 main frame 228 cut-in rotational speed

[0106] 54 decoupling support means 230 cut-out rotational speed

[0107] 56 yaw drive mechanism

[0108] 58 meteorological mast

[0109] 60 forward support bearing

[0110] 62 aft support bearing

[0111] 64 drive train

[0112] 66 pitch assembly

[0113] 68 pitch drive system

[0114] 70 sensor

[0115] 72 pitch bearing

[0116] 74 pitch drive motor

[0117] 76 pitch drive gearbox

[0118] 78 pitch drive pinion

[0119] 80 pitch control system

[0120] 84 power generator

[0121] 86 cavity

[0122] 88 inner surface

[0123] 90 outer surface