Method of operating a wind turbine

Abstract

The invention relates to a method of operating a wind turbine. The wind turbine (1) comprises a rotor (4) comprising at least one blade (2) with an adjustable pitch angle (β). The method comprises adjusting the rotor speed (Ω) of the rotor (4) and / or the pitch angle (β) along a transonic safe trajectory. The transonic safe trajectory is based on an operational map comprising transonic flow risk information. The transonic flow risk information provides an indication of the risk of transonic air flows occurring around at least one blade (2) for a plurality of combinations of rotor speed (Ω) and pitch angle (β).

Description

[0001] Method of operating a wind turbine

[0002] TECHNICAL FIELD

[0003] The invention relates to a method of operating a wind turbine. In addition, the invention relates to a wind turbine and a computer program product.

[0004] BACKGROUND

[0005] The wind energy industry expects significant growth in the next decades to meet the future energy demand, driven by the growing global population, increasing overall demand, and the need for clean, affordable, and renewable energy.

[0006] Wind turbines have several parameters to adjust their operation according to the speed and direction of the wind and a desired power output. The main parameters are 1) the speed of the rotor (Q), 2) the pitch angle of the blades (P), 3) the torque in the drive train and 4) the yaw angle of the rotor. The speed of the rotor, the pitch angle and the torque in the drivetrain are the main parameters used to adjust to variable wind speeds.

[0007] Fig. 1 schematically shows a wind turbine 1. The wind turbine comprises a rotor 4, the rotor 4 comprising a central element, known as the hub 3, and at least one blade 2, the at least one blade 2 being connected to the hub 3. Usually, the wind turbine 1 comprises more than one blade, typically three blades 2 as shown in Fig. 1. The wind turbine 1 shown in Fig. 1 is a horizontal-axis wind turbine, i.e. the axis of rotation of the rotor being substantially horizontal. The blades 2 comprise a root or proximal end and a tip or distal end, the proximal end being connected to the hub 3 and the distal end pointing away from the hub 3, the one or more blades being substantially positioned in a direction perpendicular to the axis of rotation of the rotor. The term substantially is used here as the blades may be at a small angle and / or may deform as a result of the force exerted by the wind on the blades. The wind turbine 1 is configured to convert kinetic energy of the wind into mechanical energy (rotation of the rotor), which may then for instance be converted into electric energy by an electric generator.

[0008] The blades may have an elongated, slender, and aerodynamic shape. The cross- sectional profile of the blades is often similar to that of an aircraft wing, with a curved shape known as an airfoil. The blades have a leading edge and a trailing edge, where the leading edge is the forward edge, the first part of the blade that encounters the oncoming wind, while the trailing edge is the rear edge, the last part of the blade that the wind interacts with before moving past the blade.

[0009] Fig. 1 schematically shows the axes associated with pitch and yaw. The pitch angle p is the angle over which the blades are turned about the longitudinal axis of the blade with respect to a start or reference position. By adjusting the pitch angle p, the angle of attack, i.e. the angle between the chord line (an imaginary line connecting the leading and trailing edges of the blade) and the oncoming wind — is carefully controlled to control and maximize energy extraction within the constraints of the wind turbine.

[0010] Fig. 2 shows a power curve, showing how much power is extracted from the incoming wind. The power curve has multiple operational regions. At very low wind speeds below a so-called cut-in speed, no power is generated. At low wind speeds above the cut-in speed, the wind turbine is run at the maximum efficiency to extract as much power as possible given the wind speed. In this operation region, primarily the speed of the rotor and the torque in the drive train are adjusted to maximize the amount of power extracted. The pitch angle is only adjusted to finetune the operation, i.e. to compensate for the fact that the turbine cannot always be run at the ideal rotational speed. The operation of the wind turbine has a primary adjustment mechanism which adjusts the speed of the rotor and the torque, and a secondary adjustment mechanism which adjusts the pitch angle to fine tune the primary adjustment mechanism.

[0011] Above a manufacturer defined rated wind speed, the wind turbine is operated at rated power output or rated turbine power to keep the amount of power extracted at a manufacturer defined maximum value. The rated wind speed of the wind turbine is the wind turbine at which the turbine achieves its maximum power output. It is a crucial design parameter set by the manufacturer. This rated wind speed is typically specified in meters per second. Below the rated wind speed, the turbine may not reach its maximum power output, and above the rated wind speed, various control mechanisms engage to prevent excessive power production and safeguard the turbine from potential damage, unwanted noise, and excessive wear.

[0012] In the rated operation region, primarily the pitch angle p of the blades is adjusted to control the rotational speed to stay at a predefined, rated rotational speed, despite changes in the wind speed. Due to a change of wind speed, the rotational speed will change (because generator torque is set / controlled at a fixed value with another controller). This change in rotational speed is measured and the pitch angle is adjusted to get rid of the deviation from the desired rotational speed. This adjustment mechanism is applied to prevent the extracted power from overshooting the rated power.

[0013] The size of wind turbines is increasing. The future generation of such giant wind turbines will bring new aerodynamic challenges.

[0014] Larger wind turbines result in a higher velocity at the outboard section of the wind turbine blades. In addition, the flow is accelerated around the blade airfoils in order to generate power, which leads to even higher local velocities.

[0015] Recently, it has been shown in a publication of De Tavernier & von Terzi (2022), that in some conditions the flow over the airfoil can become transonic, i.e. reach (and exceed) the speed of sound. This occurs, for example at, but not limited to, high wind speeds where the turbine operates at rated power and blades are pitched to negative angles of attack in order to shed power. If transonic flow occurs, shocks, flow separation, buffeting and other unsteady phenomena might occur that severely impact the performance and life of the wind turbine.

[0016] Currently, wind turbine operators do not know if and when transonic flows may occur. If transonic flows occur, this could result in damage to the blades and / or blade bearings. With the use of De Tavernier & von Terzi (2022), operators may be aware of the issue, but still lack a method to avoid transonic flows without losing significant power and associated revenue losses or warranty claims regarding violating the sold power curve.

[0017] SUMMARY

[0018] The objective is to provide a method for operating a wind turbine that prevents or at least reduces the risk of transonic flows from occurring and thereby prevents or at least reduces the negative effects associated with transonic flows, while minimizing the loss of extracted power, in particular for a wind turbine operating at rated power.

[0019] The objective is met by a method of operating a wind turbine (1), the wind turbine (1) comprising a rotor (4), the rotor (4) comprising at least one blade (2) with an adjustable pitch angle (P), the method comprising adjusting the rotor speed (Q) of the rotor (4) and / or the pitch angle ( ) along a transonic safe trajectory, wherein the transonic safe trajectory is based on an operational map comprising transonic flow risk information, the transonic flow risk information providing an indication of the risk of transonic air flows occurring along at least one blade (2) for a plurality of combinations of rotor speed (Q) and pitch angle (P).

[0020] It will be understood by a skilled person that instead of the rotor speed (Q) and / or the pitch angle ( ) other parameters may be used that provide an unambiguous indication of the rotor speed (Q) and / or the pitch angle (P), such as derivate parameters, e.g. power.

[0021] The phrase adjusting the rotor speed (Q) and / or the pitch angle (P) is used to indicate that both the rotor speed and the pitch angle are adjusted (actively or passively) in a dependent manner. The rotor speed (Q) and the pitch angle (P) may be adjusted simultaneously to keep the power output at the desired, e.g. rated, power output while reducing the risk of transonic air flows from occurring.

[0022] By adjusting the rotor speed (Q) and / or the pitch angle (P) it is possible to control the wind turbine to an operating point at which the risk of transonic air flows is reduced while the wind turbine continues to operate at the desired power output, e.g. rated power output, and at which further operating preferences can be taken into account.

[0023] The rotor speed (Q) and pitch angle (P) are adjusted in response to a change in wind speed (not direction).

[0024] The operational map comprises conditions from simulations and / or measurements or conditions inferred from such measurements and simulations. The operational map may be stored and represented in different manners, including a table and / or a visual representation showing (an indication of) the rotor speed on a first axis and (an indication) of the pitch on a second axis and showing an indication of the risk (transonic flow risk) for all or a subset of the combinations. The transonic flow risk may simply be a binary indication (high-low; yes-no; 1-0) indicating whether or not there is a risk of transonic air flow from occurring around the blades (high / yes / 1) or not (low, no, 0). The transonic flow risks may be included in a visual representation by a contour line separating areas of low risk from areas of high risk. An example of this will be provided with reference to Fig. 3 below.

[0025] The transonic flow risk may also be a risk number selected from a predefined scale, for instance running from 0 - 1 or from 0 - 100, where a low number represents a low transonic flow risk and a high number represents a high transonic flow risk. This allows users, programmers or controllers of wind turbines more freedom to make choices on how much risk is and isn’t acceptable when defining transonic safe trajectories.

[0026] The transonic flow risk may also be determined by an algorithm or artificial intelligence, based on actual or forecast conditions. When using an algorithm or artificial intelligence, the risks don’t have to be explicitly mapped in advance for all possible conditions.

[0027] The transonic flow risks may be obtained by performing simulations or obtaining measurements, such as from models in wind tunnels or from real wind turbines.

[0028] The term transonic flow risk is used here in a broad manner, i.e. taking into account the probability of occurrence and / or the potential consequence of such occurrences.

[0029] The transonic safe trajectory may be a predefined transonic safe trajectory. The predefined transonic safe trajectory may be provided by the manufacturer and may be embedded in the control program of the wind turbine. The transonic safe trajectory may also be computed on an ad-hoc basis depending on several parameters, as will be explained in more detail below.

[0030] A transonic safe trajectory is a series of combinations of rotor speeds and pitch angles for different wind speeds that allow for a smooth control in response to a change of the wind speed and all having a low or acceptable transonic flow risk. A manufacturer or controller has freedom to define what a low or acceptable transonic flow risk is.

[0031] The transonic safe trajectory may be represented as a curve in the operational map, where the curve represents the combinations of rotor speed (Q) and pitch angle (P) for different wind speeds together forming a transonic safe trajectory. All combinations of rotor speed (Q) and pitch angle ( ) having a transonic flow risk that is considered too high are to be excluded from the transonic safe trajectory. The transonic safe trajectory only comprises combinations of rotor speed (Q) and pitch angle (P) having a transonic flow risk that is considered safe, preferably not introducing any risk of transonic flows.

[0032] The term trajectory in this context may also be referred to as control trajectory.

[0033] The transonic safe trajectory and transonic flow risk information may be based on computations, simulations, measurements obtained at the wind turbine and / or one or more remote wind turbines and / or measurements obtained from wind tunnel testing, which include scaled wind tunnel testing. Computations and simulations may for instance be done as explained in De Tavernier & von Terzi (2022).

[0034] According to an embodiment the wind turbine is operated at rated power output.

[0035] The term rated power output will readily be understood by a skilled person and refers to a maximum electrical or equivalent mechanical power output for which the wind turbine is designed. The wind turbine is to be controlled such that no more power is generated than the rated power output, to prevent damage to the generator, power electronics and structures. In practice this means that above the rated wind speed the power output of the wind turbine will be kept constant even when more wind energy is available.

[0036] It is common practice to control the wind turbine operating above the rated wind speed by adjusting the pitch angle (P) to keep the rotor speed (Q) nearly constant. Reducing the risk of transonic air flows from occurring by only adjusting the pitch angle ( ) relative to this practice would however change the power being generated. This leads to either an undesirable reduction in power, or an unacceptable exceedance of the rated power. When the risk of transonic air flows from occurring is reduced by only adjusting the rotor speed (Q) relative to the common practice, the power will also change, with the same consequences.

[0037] According to the method and contrary to common practice at rated power output, the rotor speed is not kept at a fixed rated rotational speed. The rotational speed is made dependent on the conditions and is based on the operational map comprising transonic flow risk information. Hereby, the method provides a way to reduce or even eliminate the risk of transonic flows from occurring while continuing to operate at rated power output.

[0038] According to an embodiment adjusting the rotor speed (Q) of the rotor (4) and / or the pitch angle (P) along a transonic safe trajectory comprises targeting a combination of a target rotor speed (Q) and a target pitch angle (P).

[0039] Preferably, a combination is targeted that achieves the rated power output in response to a changing wind speed above the rated wind speed of the wind turbine (1). The rotor speed (Q) and the pitch angle (P) are adjusted simultaneously from a current rotor speed and a current pitch angle (P) to a targeted rotor speed and a targeted pitch angle (P) in response to a changing wind speed. By adjusting the rotor speed (Q) and the pitch angle (P) in combination, the power extracted from the wind can be kept at the rated power output, while reducing the risk of transonic air flows from occurring. This would (in almost all cases) not be the case when only one of the parameters (rotor speed, pitch angle) would be adjusted to reduce this risk.

[0040] The combination targeted is a combination that has a low, reduced risk of transonic air flows from occurring or preferably has no risk of transonic air flows from occurring.

[0041] So, the present method provides for a control mechanism that reduces the risk of transonic air flows from occurring, minimizing the loss of power and avoiding exceedance of the rated power.

[0042] According to an embodiment the method comprises updating the operational map.

[0043] By updating the operational map, i.e. updating the transonic flow risk information, it is to be ensured that the control of the wind turbine is in line with the latest learnings with respect to transonic flow risks.

[0044] Updating the transonic flow risk information may comprise receiving new transonic flow risk information and replacing current transonic flow risk information with the new transonic flow risk information.

[0045] The operational map may be updated using a self-learning algorithm, based on measurements performed on the wind turbine.

[0046] According to an embodiment updating the operational map comprises updating the transonic flow risk information, based on one or more of: vibration measurements obtained at the wind turbine (1), flow speed measurements obtained at least at one or more locations along the one or more blades (2) of the wind turbine (1), computations, computer simulations,

[0047] - wind tunnel testing, scaled wind tunnel testing, acoustic measurements and any other measurements that allow to derive or relate to any of the above techniques, including a virtual sensor.

[0048] The measurements, computations, simulations and / or testing may be part of the method.

[0049] The method may comprise performing one or more of: vibration measurements obtained at the wind turbine (1), flow speed measurements obtained at least at one or more locations along the one or more blades (2) of the wind turbine (1), computations, computer simulations,

[0050] - wind tunnel testing, scaled wind tunnel testing, acoustic measurements and any other measurements that allow to derive or relate to any of the above techniques, including a virtual sensor.

[0051] The vibration measurements may be obtained from vibration sensors mounted on the wind turbine, e.g. on or in the blades. Measuring vibrations of a certain frequency or within a certain frequency range and / or on or above a certain amplitude may be regarded an indication of transonic flows occurring. The operational map, i.e. the transonic flow risk information may be updated based on such measurements.

[0052] The flow speed measurements may be obtained from flow speed sensors mounted on one or more blades, configured to measure the relative flow speed of the air with respect to the blade. Measuring flow speed at a speed above the transonic speed may be regarded as an indication of transonic flows occurring. The operational map, i.e. the transonic flow risk information may be updated based on such measurements.

[0053] Updating the operational map may also be done based on computations or computer simulations, for instance as described in De Tavernier & von Terzi (2022).

[0054] Any other suitable measurement may be used, including a virtual sensor. A virtual sensor combines information from other sensors with (smart) algorithms that allows to construct a sensor that is physically not there, hence the term virtual sensor. For example, for wind turbines, a Lidar or a met mast may traditionally measure the wind speed at different heights ahead of the wind turbine, hence providing the wind shear. Alternatively, as a virtual sensor, by knowing that the blade bends according to the wind shear one can construct the wind shear profiles from other data, e.g. by using rotor speed, pitch angle, measurements of atmospheric conditions, main shaft bending etc. and knowledge about the wind turbine.

[0055] According to an embodiment updating the transonic flow risk information is based on: vibration measurements obtained at one or more further wind turbines, flow speed measurements obtained at least at one or more locations along one or more blades (2) of one or more further wind turbines (1), acoustic measurements obtained at one or more further wind turbines, and any other measurements that allow to derive or relate to any of the above techniques (virtual sensor).

[0056] The operational map may also be updated based on learnings from other wind turbines, logically typically of a similar size or type.

[0057] According to an embodiment adjusting a rotor speed (Q) of the rotor (4) and / or the pitch angle (P) along a transonic safe trajectory comprises targeting a combination of a rotor speed (Q) and pitch angle ( ) for a particular wind speed from a plurality of optional combinations, each optional combination being associated with the same power coefficient (Cp).

[0058] The term Cp refers to the power coefficient, being a dimensionless parameter that characterizes the efficiency of a wind turbine in converting the kinetic energy of the wind into mechanical power. It is an indicator used to assess how effectively a wind turbine captures the available wind energy. The power coefficient (Cp) is calculated using the formula: wherein:

[0059] • P is the mechanical power extracted by the turbine,

[0060] • p is the air density,

[0061] • A is the rotor swept area (= KR2, with R being the length of the blades), and

[0062] • V is the undisturbed wind speed.

[0063] To operate at a particular wind speed at the rated power output, the combination of rotor speed (Q) and pitch angle (P) are to be selected such that the power coefficient is achieved that corresponds to rated power at that wind speed, in order not to extract more power than the rated power output or to extract less power than possible.

[0064] The combinations of rotor speed (Q) and pitch angle ( ) that are eligible for selection for transonic safe operation at a particular wind speed therefore correspond with a Cp-isoline in the operational map of the wind turbine, where the Cp-isoline is a contour connecting operating points with the same power coefficient Cpin the operational map that yields the rated power output at that particular wind speed.

[0065] The method may advantageously comprise selecting a trajectory from a plurality of control trajectories to select a trajectory which is optimal for a specific situation, wherein the optional transonic safe trajectories comprise operating points for a given wind speed that are on the same Cp-isoline.

[0066] According to an embodiment the method comprises selecting a transonic safe trajectory from a plurality of optional transonic safe trajectories.

[0067] Different transonic safe trajectories are possible. The method may comprise selecting a transonic safe trajectory that is optimal depending on the circumstances. The selected transonic safe trajectory may be selected based on weather forecast, consequences for the loads that the wind turbine is subjected to and condition of the wind turbine, which may include the age and the wear of the wind turbine.

[0068] According to an embodiment the plurality of optional transonic safe trajectories comprise: a minimum-safety trajectory, a moderate-adjustment trajectory, a pitch-reversal trajectory.

[0069] The minimum-safety trajectory is a trajectory which selects the highest possible tip speed ratio ( ) given the wind speed, while keeping the risk of transonic air flows occurring at an acceptable level. This results in a transonic safe trajectory that is closest as possible to the boundary between acceptable and unacceptable transonic flow risk for all wind speeds. For all control trajectories without pitch reversal, that have a monotonously increasing pitch angle (P) for increasing wind speeds, this trajectory results in the highest rotor speed (Q). The pitch-reversal trajectory is a trajectory which selects such a low tip speed ratio for at least a small range of operational wind speeds that the pitch angle (P) needs to reduce for increasing wind speed. This leads to at least one point on the trajectory where the derivative of the targeted pitch angle ( ) with respect to the wind speed changes sign.

[0070] The moderate-adjustment trajectory selects a tip-speed-ratio ( ) that is lower than that of the minimum-safety trajectory, but that avoids pitch reversal at all wind speeds. This trajectory provides a bigger margin of safety than the minimum-safety trajectory, while avoiding the reversal from pitch angle (P) increase to pitch angle (P) decrease for increasing wind speed. This trajectory has a lower rotor speed (Q) than the minimumsafety trajectory at all wind speeds.

[0071] According to an embodiment the method comprises

[0072] • obtaining forecasted operational conditions,

[0073] • selecting a transonic safe trajectory based on the forecasted operational conditions.

[0074] The forecasted operational conditions may comprise forecasted weather conditions, in particular a forecasted undisturbed wind speed, and / or the forecasted operational conditions may comprise predicted power demand.

[0075] Based on the forecasted operation conditions, a transonic safe trajectory may be selected that is optimal for the forecasted operational conditions. For instance, if the forecasted operational conditions include an increasing undisturbed wind speed, possibly with strong variations in the undisturbed wind speed (gusts) for the coming hours, to reach a high maximum wind speed (e.g. 24 m / s) , a transonic safe trajectory may be selected that is safer than in a situation where the forecasted operational conditions include a steady, undisturbed wind speed that increases for a short period of time (e.g. 1 hour) to a moderate maximum wind speed (e.g. 17 m / s), followed by a decreasing wind speed, in which case a transonic safe trajectory which is less safe (e.g. minimum-safety trajectory) maybe selected.

[0076] According to an embodiment the method comprises

[0077] • obtaining a forecasted transonic flow risk, and

[0078] • selecting a trajectory based on the forecasted transonic flow risk. According to this embodiment, the transonic flow risk may be forecasted based on a currently applied transonic safe trajectory or may be forecasted as a transonic boundary in the operational map. Forecasting may be based on forecasted operational conditions, and / or may be done directly using artifical intelligence techniques (e.g. by using past data and forecasting the state of the wind turbine).

[0079] Based on the forecasted transonic flow risk, a different transonic safe trajectory may be selected to replace the currently applied transonic safe trajectory.

[0080] According to an embodiment the method comprises operating the wind turbine in a transonic safe mode wherein the risk of transonic air flows from occurring at the at least one blade (2) is below a predefined transonic risk threshold.

[0081] The operational map may comprise transonic flow risk information in the form of transonic risk levels. These risk levels may be updated as described. The method of operating the wind turbine may comprise selecting a combination of a rotor speed (Q) and pitch angle (P) that has a transonic risk level below a predefined transonic risk threshold.

[0082] According to a further aspect there is provided a wind turbine comprising a rotor (4), the rotor (4) comprising at least one blade (2), wherein the wind turbine is configured to adjust a pitch angle ( ) of the at least one blade (2) and the wind turbine is configured to adjust a rotor speed (Q) of the rotor (4), wherein the wind turbine comprises a control unit configured to perform any one of the methods described.

[0083] As will be understood by a skilled person, adjusting the rotor speed may be done in several manners, such as 'directly' by setting the frequency of an electrical (three-phase) connection to the generator. It may also be adjusted ‘indirectly’, by setting the torque of the generator, measuring the rotor speed and changing the pitch angle until the measured rotor speed is the desired rotor speed.

[0084] As will also be understood by a skilled person, adjusting the pitch angle may also be done in different ways, e.g. by means of hydraulics or an electric motor.

[0085] Additionally, an advantageous embodiment is a computer program product comprising instructions which, when the instructions are executed by a computer or control unit, cause the computer or control unit to carry out the method as described. Furthermore, a data carrier is provided on which the computer program is stored and / or data carrier signal is provided which transmits the computer program. A wind turbine can comprise a computer or control unit in order to execute the aforementioned program. Additionally or alternatively a remote computer or control unit may be provided that is configured to control the wind turbine remotely.

[0086] BRIEF DESCRIPTION OF THE DRAWINGS

[0087] In the figures, the subject-matter of the invention is schematically shown, wherein identical or similarly acting elements are usually provided with the same reference signs.

[0088] Figure 1 schematically shows a wind turbine,

[0089] Figure 2 schematically shows a power curve of a wind turbine,

[0090] Figure 3a-v show a schematic representation of an operational map of a wind turbine according to an embodiment,

[0091] DESCRIPTION OF EMBODIMENTS

[0092] Fig.’s 3a - 3v together form an operational map of a wind turbine according to an embodiment, which comprise three optional transonic safe trajectories (TSM1 , TSM2, TSM3). The operational map further comprises a trajectory that is not a transonic safe trajectory (No TSM). The term TSM used means transonic safe mode.

[0093] Fig. 3a is relevant for an incoming wind speed Vm= 4 m / s. On the left-hand side is a graph, wherein the horizontal axis represents the pitch angle of the blades (P) in degrees and the horizontal axis represents the tip speed ratio ( ), where

[0094] QHR

[0095] A — - ,

[0096] Voo with fl = rotational speed of the blades and R = the radius of the blades. The tip speed ratio is an unambiguous indication of the rotor speed for a given incoming wind speed and for a given radius.

[0097] The left-hand side graph further comprises a number of Cp-isolines, with Cpas defined above. Further shown is a Cp,max line, representing the maximum Cp-value for each pitch angle.

[0098] The left-hand side graph further comprises a transonic boundary line, in this case shown at the right-hand side upper corner. This transonic boundary line defines the border between combinations of pitch angle and tip speed ratio which are considered safe, i.e. have no or a neglectable risk of transonic flows and combinations which are not considered safe, i.e. have a non-neglectable risk of transonic flows. The transonic boundary line is based on transonic flow risk information.

[0099] The left-hand side graph further comprises an operating point (A), shown at the top in between the 0.2 and 0.3 Cp-isolines.

[0100] The graphs on the right-hand side in Fig. 3a show the operating point (A) in various graphs against the incoming wind speed Vm. The right-hand top graph shows the current Cp, the middle graph shows the pitch angle ( ) and the bottom graph shows the rotational speed o [rad / s]. It is noted that co as used in the Figures is equal to Q used in this text.

[0101] Fig. 3b shows the same graphs as Fig. 3a, but now for an incoming wind speed Vm= 5 m / s. As can be noted, the transonic boundary line has moved, as well as the operating point, which is now shown as operating point (B). For clarity, operating point (A) is still shown.

[0102] The right-hand top graph shows that at operating point (B), the Cp has gone up, which is preferred as at this low wind speed (i.e. below the rated wind speed) the wind turbine 1 is operated at maximum efficiency. The middle graph shows that the pitch angle is lowered, and bottom graph shows that the rotational speed has risen.

[0103] The further Fig’s 3c-3v show the same information, but with increasing wind speed, with Fig. 3v having a wind speed of 25 m / s. As can be seen, at some point the different trajectories diverge as will be described in more detail below.

[0104] Fig.’s 3a-3v comprise a transonic unsafe trajectory (No TSM), which is a trajectory according to the prior art, not taking into account the transonic flow risk information or transonic boundary line. As shown in the Figures, for higher wind speed, the transonic unsafe trajectory (No TSM) crosses the transonic boundary line, resulting in a combination of rotor speed (Q) and pitch angle (P) which have a risk of transonic flows resulting along the blades 2.

[0105] The Figures further show three optional transonic safe trajectories (TSM1 , TSM2, TSM3). It is noted that each of the transonic safe trajectories comprise operating points that are on the safe side of the transonic boundary line and at a Cp-isoline that ensures extracting the right amount of power output, i.e. as much power as possible below the rated wind speed and the rated power output above the rated wind speed.

[0106] One optional transonic safe trajectory is a minimum-safety trajectory (TSM1), one optional transonic safe trajectory is a pitch-reversal trajectory (TSM3) and one optional transonic safe trajectory is a moderate-adjustment trajectory (TSM2).

Claims

CLAIMS1. Method of operating a wind turbine (1), the wind turbine (1) comprising a rotor (4), the rotor (4) comprising at least one blade (2) with an adjustable pitch angle (P), the method comprising adjusting the rotor speed (Q) of the rotor (4) and / or the pitch angle (P) along a transonic safe trajectory, wherein the transonic safe trajectory is based on an operational map comprising transonic flow risk information, the transonic flow risk information providing an indication of the risk of transonic air flows occurring along at least one blade (2) for a plurality of combinations of rotor speed (Q) and pitch angle (P).

2. Method of operating a wind turbine (1) according to claim 1 , wherein the wind turbine (1) is operated at rated power output.

3. Method of operating a wind turbine (1) according to any one of the preceding claims, wherein adjusting the rotor speed (Q) of the rotor (4) and / or the pitch angle (P) along a transonic safe trajectory comprises targeting a combination of a target rotor speed (Q) and a target pitch angle (P).

4. Method of operating a wind turbine (1) according to any one of the preceding claims, wherein the method comprises updating the operational map.

5. Method of operating a wind turbine (1) according to claim 4, wherein updating the operational map comprises updating the transonic flow risk information, based on one or more of: vibration measurements obtained at the wind turbine (1), flow speed measurements obtained at least at one or more locations along the one or more blades (2) of the wind turbine (1), computations, computer simulations,- wind tunnel testing, scaled wind tunnel testing, acoustic measurements and any other measurements that allow to derive or relate to any of the above techniques, including a virtual sensor.

6. Method of operating a wind turbine according to any one of the claims 4 - 5, wherein updating the transonic flow risk information is based on:vibration measurements obtained at one or more further wind turbines, flow speed measurements obtained at least at one or more locations along one or more blades (2) of one or more further wind turbines (1), acoustic measurements obtained at one or more further wind turbines, and any other measurements that allow to derive or relate to any of the above techniques (virtual sensor).

7. Method of operating a wind turbine according to any one of the preceding claims, wherein adjusting a rotor speed (Q) of the rotor (4) and / or the pitch angle (P) along a transonic safe trajectory comprises targeting a combination of a rotor speed (Q) and pitch angle ( ) for a particular wind speed from a plurality of optional combinations, each optional combination being associated with the same power coefficient (Cp).

8. Method of operating a wind turbine (1) according to any one of the preceding claims, wherein the method comprises selecting a transonic safe trajectory from a plurality of optional transonic safe trajectories.

9. Method of operating a wind turbine (1) according to claim 8, wherein the plurality of optional transonic safe trajectories comprise: a minimum-safety trajectory, a moderate-adjustment trajectory, a pitch-reversal trajectory.

10. Method of operating a wind turbine according to any one of the preceding claims, wherein the method comprises• obtaining forecasted operational conditions,• selecting a trajectory based on the forecasted operational conditions.11 . Method of operating a wind turbine according to any one of the preceding claims, wherein the method comprises• obtaining a forecasted transonic flow risk, and• selecting a transonic safe trajectory based on the forecasted transonic flow risk.

12. Method of operating a wind turbine according to any one of the preceding claims, wherein the method comprises operating the wind turbine in a transonic safe modewherein the risk of transonic air flows from occurring at the at least one blade (2) is below a predefined transonic risk threshold.

13. Wind turbine (1) comprising a rotor (4), the rotor (4) comprising at least one blade (2), wherein the wind turbine (1) is configured to adjust a pitch angle (P) of the at least one blade (2) and the wind turbine (1) is configured to adjust a rotor speed (Q) of the rotor (4), wherein the wind turbine comprises a control unit configured to perform the method according to any one of the claims 1 - 12.

14. Computer program product comprising instructions which, when the instructions are executed by a computer or control unit, cause the computer or the control unit to carry out the method according to one of the claims 1 to 13.

15. Data carrier comprising a computer program product according to claim 14.

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