Patent Description:
Wake losses in a wind farm may cause power production losses.

<CIT> discloses a method for increasing the power production of a wind park during conditions when the wake created by an upstream turbine effects the power production of a downstream turbine. Minimizing the wake effects created by an upstream turbine on a downstream turbine increases the net power produced by both the upstream and downstream turbines. Also disclosed is an implementation of an algorithm to determine the controller settings of one or more upstream turbines to increase total energy capture of the turbines in the wind park. The algorithm also reduces the fatigue loads on the downstream turbines by reducing the turbulence created by the wake effects of the upstream turbine.

<CIT> discloses a wake steering control scheme where activating a wake steering control for the wind turbine is based on a wind direction impinging on a turbine.

There is still a need for improving the capability of wake control methods to reliably improve the compensation of wake losses.

It is an object of the invention to provide improved methods for limiting power production losses due to wake effects.

In a first aspect of the invention there is provided a method for controlling a yaw offset of a plurality of wind turbines in accordance with claim <NUM>, wherein the method comprises.

Advantageously, the decision function determines which of the wind turbines should have the yaw offset control enabled or disabled. Since the decision is based on the wind variation range, there is a higher likelihood that a given energy production is obtainable as long as the actual wind variations are within the wind variation range. That is, since there is a given likelihood that certain wind conditions such as wind directions will be within the wind variation range, there is a corresponding (high) likelihood that a given energy production is obtainable as long as the wind condition is within the wind variation range.

Accordingly, the method may increase the likelihood that the control action actually improves production and does not worsen the power production in an attempt to partially compensate wake induce power production losses.

The decision function provides information, e.g. via the activation parameter, that enables determination of whether the yaw offset control of the specific wind turbine should be enabled or disabled based on the variables of the function.

The wind direction variable indicates the direction of the wind in the vicinity of the specific wind turbine. The wind direction could be a mean value of the wind direction measured within a given time window such as the mean absolute wind direction.

The wind variation variable could define a range of the variable where the offset control is disabled, such as a range of wind directions where the offset control is disabled irrespective of the values of the other variables. Additionally, the wind variation variable defines ranges of the variable where the offset control will be disabled or enabled dependent on the other variables of the decision function.

The wind variation variable may be determined as a standard deviation of the wind variable such as the standard deviation of the wind direction or the wind turbulence. Thus, the wind variation range can be defined as a level of the standard deviation.

The yaw offset control provides an offset yaw angle to the default yaw setting, i.e. the yaw setting normally applied to maximize the wind intake of the rotor plane.

According to an embodiment, the wind variation variable comprises one or both of: a wind direction variation variable indicative of a level of variation of a wind direction over time, and a wind turbulence variation variable indicative of a level of variation of a wind turbulence.

Advantageously, by determining the activation parameter dependent on the level of variation of the wind direction, the activation of the yaw offset control may only be invoked if the variation in the wind direction is sufficiently low to make it more likely that introduction of the yaw offset control improves the power production.

According to an embodiment, the activation parameter provides information defining if the yaw offset control should be disabled and/or enabled.

According to an embodiment, the decision function has been determined based on comparing a simulated energy production of the plurality of wind turbines when the yaw offset control is enabled, respectively disabled, for one or more of the plurality of wind turbines.

According to an embodiment, the decision function has been determined and/or is updated based on a comparison of an actual energy production of the plurality of wind turbines obtained when the yaw offset control is enabled respectively disabled for one or more of the plurality of wind turbines.

Advantageously, the decision function is updated based on actual data so that the decision to enable or disable yaw offset control is based on experience and therefore more accurate with respect to the obtained energy production.

According to an embodiment, the yaw offset is scaled according to a weight provided by the decision function or by the activation parameter.

Advantageously, instead of simply enabling or disabling the yaw offset, the level of the yaw offset may be scaled dependent on the values of the variables of the decision function.

According to an embodiment, the variables of the decision function further comprises: a wind shear variable, a nacelle direction variable for the specific wind turbine, a nacelle direction variable for a neighbor wind turbine to the specific wind turbine, and a wind turbulence variable for a neighbor wind turbine to the specific wind turbine.

According to an embodiment, the method comprises providing a plurality of decision functions arranged for providing a plurality activation parameters for a plurality of specific wind turbines dependent on the two or more variables.

Advantageously, a decision function is provided for each wind turbine.

A second aspect of the invention relates to a wind turbine controller unit arranged for controlling a yaw offset of one or more wind turbines in accordance with claim <NUM>, the wind turbine controller unit comprises.

wherein the activation parameter is provided for enabling or disabling the yaw offset control of the specific wind turbine, where the enabled yaw offset control provides a yaw offset relative to a default yaw setting.

A third aspect of the invention relates to a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

<FIG> shows a wind farm <NUM> which comprises a plurality of wind turbines <NUM>.

The wind farm is connectable with an electrical power grid (not shown) for supplying power generated by the wind turbines <NUM> to the electrical power grid.

In the illustrated example, the wind turbines of the wind farm <NUM> is controlled by a central wind farm controller <NUM>. The wind farm controller <NUM> is arranged to control power generation from the wind turbines <NUM> according to a wind farm reference Pref which defines the desired power to be supplied to the grid from the wind farm <NUM>. Furthermore, the central controller is arranged to dispatch power set-points Pset to the wind turbines, i.e. individual power set-points to each wind turbine <NUM> which set the desired power productions of individual wind turbines. The power set-points Pset may be determined by the wind farm controller <NUM> dependent on the wind farm reference Pref so that the sum of power set-points Pset corresponds to the wind farm reference Pref.

The wind turbine <NUM> comprises a tower and a rotor with at least one rotor blade, such as three blades. The rotor is connected to a nacelle which is mounted on top of the tower and being adapted to drive a generator situated inside the nacelle. The rotor is rotatable by action of the wind. The wind induced rotational energy of the rotor blades is transferred via a shaft to the generator. Thus, the wind turbine is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. The wind turbine may further include a power converter for converting the generator AC power into a DC power and a power inverter for converting the DC power into an AC power to be injected into the electrical power grid.

The generator of the wind turbine <NUM> is controllable to produce power corresponding to the power set-point Pset provided by the wind farm controller <NUM>. For wind turbines, the output power may be adjusted according to the power set-point by adjusting the pitch of the rotor blades or by controlling the power converter to adjust the power production.

<FIG> shows relative locations of wind turbines <NUM> in a principal sketch of a wind farm <NUM> seen from above. In the configuration of the wind farm <NUM> shown to the left, the rotor plane <NUM> of all wind turbines are perpendicular to the wind direction <NUM> in order to maximize the wind power intake. However, the rotor of the upstream wind turbine 101a generates a wake in the form of a wind field <NUM> with reduced wind speed which affects the downstream wind turbines 101b and 101c. This reduction of the wind speed in the shadow of the upstream wind turbine is known as the wake effect.

In the configuration of the wind farm <NUM> shown to the right, the rotor plane <NUM> of the upstream wind turbine 101d has been rotated, i.e. yawed out of the wind, so that the rotor plane normal deviates from the wind direction <NUM> by a certain angle θ which depends on or corresponds to the nacelle direction <NUM>.

The orientation of the nacelle or equivalently the rotor plane normal relative to a fixed direction such as the geographical north is referred to as the nacelle direction <NUM> which is indicative of the yaw angle of a specific wind turbine <NUM>.

Due to the rotation of the rotor plane <NUM> of the upstream wind turbine 101d out of the wind, the wind field <NUM> downstream to the wind turbine is directed away from the wind direction. Thus, by providing yaw offset control to the wind turbines <NUM>, the downstream wind field <NUM> may be directed according to a yaw offset θ up an upstream wind turbine so that the downstream wind turbines 101e, 101f are less affected by the wake effect. The angle of the direction of the downstream wind field <NUM> depends obviously on the wind direction <NUM>, but also on the wind speed, wind shear (i.e. distribution of different wind speeds over the rotor plane), wind turbulence as well as other wind characteristics.

Thus, in this ideal situation with a fixed wind direction <NUM> and a fixed wind speed it may be possible to determine the nacelle direction <NUM> of all wind turbines <NUM> so that the wake compensation is optimized for the wind farm <NUM>.

Therefore, with predictable wind conditions including predictable wind speeds and wind directions and with known nacelle directions <NUM>, it may be possible to determine yaw offset angles θ for each turbine which optimizes the power production, at least with respect to the wake effects.

However, such wind conditions are only predictable with a certain probability and, therefore, wake control of the wind farm <NUM> performed in order to optimize power production risk being unsuccessful when the statistical variations of the wind parameters are not taken into account.

For example, with the wind direction <NUM>, the redirected wind field <NUM> of the upstream wind turbine 101a may pass through the rows of the downstream wind turbines <NUM>. However, if the wind direction changes from direction <NUM> to wind direction 122a, the redirected wind field <NUM> may change into another redirected wind field 123a which may impact one of the downstream wind turbines 101e with reduced wind speed and possibly wind turbulence. This shows that in one wind condition, the yaw offset control, i.e. the wake control, may provide a power production optimized wake compensation, but in another likely wind condition, the yaw offset control may even worsen the power production as compared to a situation where no yaw offset control is performed.

The yaw offset θ may be defined as a deviation of the nacelle direction <NUM> due to the yaw offset compared to the normal, not wake compensated, nacelle direction <NUM>, e.g. where the rotor plane <NUM> is perpendicular to the wind direction <NUM>.

<FIG> shows a wind turbine controller such as a yaw control system <NUM> arranged for controlling the yaw angle of a wind turbine <NUM>. The yaw control system <NUM> comprises a yaw offset controller <NUM> arranged to provide the yaw offset control in terms of a yaw offset angle θ, and a yaw controller <NUM> arranged to control a yaw mechanism <NUM> according to a desired nacelle direction <NUM>, e.g. the optimal nacelle direction <NUM> which ensures that the rotor plane is perpendicular to the wind direction <NUM>. The optimal nacelle direction <NUM> is obtained by the default yaw setting provided by the yaw controller <NUM>.

The yaw offset controller <NUM> is configured to be enabled to provide a yaw offset θ or disabled, i.e. to not provide a yaw offset θ or to provide a zero yaw offset θ dependent on the activation parameter Px, e.g. a binary parameter Px provided by a binary activation signal. Thus, the activation parameter Px provides information defining if the yaw offset controller <NUM> should generate yaw offset control or not.

In case the yaw offset controller <NUM> is enabled, the yaw offset θ is determined and added to the output of the yaw controller <NUM> so that the default yaw setting is modified by the yaw offset θ.

Alternatively, instead of a separate yaw offset controller <NUM>, the yaw offset controller <NUM> may be combined with the yaw controller <NUM> so that the yaw offset θ could be combined with other control signals of the yaw controller than the output control signal.

<FIG> further shows a wind turbine controller unit <NUM> arranged for controlling a yaw offset θ of one or more wind turbines <NUM>. The wind turbine controller unit <NUM> comprises a decision function <NUM> arranged for providing an activation parameter Px for a specific wind turbine 101a of the plurality of wind turbines <NUM> dependent on two or more variables <NUM>.

The variables <NUM> includes a wind direction variable Wx indicative of the wind direction of the specific wind turbine. The wind direction variable may be based on the wind direction <NUM> obtained from a central wind sensor such as a met mast, from decentral wind sensors such as wind sensors of individual wind turbines <NUM>, estimated wind directions, etc..

The variables <NUM> further includes a wind variation variable ΔWx which defines or relates to at least a wind variation range where the yaw offset control is disabled and/or a wind variation range where yaw offset control is enabled, either independent on other variables in some ranges or dependent on other variables in other ranges. The range where the yaw offset is disabled is the complement of the range where the yaw offset is enabled.

The wind variation variable may comprise a wind direction variation ΔWDx variable indicative of a level of variation of a wind direction over time.

Alternatively or additionally, the wind variation variable may comprise a wind turbulence variation variable ΔWTx indicative of the level of variation of a wind turbulence.

The wind direction variation variable ΔWDx may be determined based on wind direction measurements of individual wind turbines or via central wind direction measurements. The measurements used for determining the wind direction variation variable ΔWDx may be the most recent measurements obtained in a period of time prior to the enabling or disabling the yaw offset control. The period of time may be a predetermined period of time, e.g. the past <NUM> minutes prior to the enabling or disabling the yaw offset control. In this manner, it can be ensured that the yaw offset control is only enabled if the wind direction is sufficiently stable, and has been sufficiently stable in the period prior to the invoking the yaw offset control.

The wind direction variation variable ΔWDx may be determined as the standard deviation of the wind direction measurements, as a range covering the maximal and minimal wind directions obtained within a given period, as a range covering the wind directions occurring with a frequency above a given threshold, or by other methods which reflects the variations of the wind direction <NUM>.

Similarly, the wind turbulence variation variable ΔWTx may be determined based on wind turbulence measurements of individual wind turbines or via central wind turbulence measurements. The measurements used for determining the wind turbulence variation variable ΔWTx may be the most recent measurements obtained in a period of time, in a similar manner as for the wind direction variation variable. The period of time used for the wind direction variation variable and the wind turbulence variation variable may be of the same length or of different length.

The wind turbulence variation variable ΔWDx may be determined as the standard deviation of the wind turbulence measurements, as a range covering the maximal and minimal turbulence values or such values occurring with a frequency above a given threshold, or by other methods which reflects the variations of the wind turbulence.

Other optional variables <NUM> that may be used for determining or providing the activation parameter includes a wind shear variable indicating a variation of wind speeds over the rotor plane <NUM>, a nacelle direction variable for the specific wind turbine such as the nacelle direction <NUM>, a nacelle direction variable for a neighbor wind turbine to the specific wind turbine such as nacelle directions <NUM> of wind turbines other than the specific wind turbine 101a, a wind turbulence variable indicative of the actual wind turbulence of the specific wind turbine 101a, a wind turbulence variable indicative of the actual wind turbulence of a neighbor wind turbine or one more other wind turbines <NUM> than the specific wind turbine 101a.

The activation parameter Px is provided for enabling or disabling the yaw offset control of the specific wind turbine, i.e. for enabling or disabling the generation of a yaw offset θ relative to the default yaw setting.

For example, the wind turbine controller unit <NUM> may comprise a control function <NUM> arranged to enable or disable the yaw offset control of the specific wind turbine 101a based on the activation parameter Px. Alternatively, the control function <NUM> may be comprised by the yaw offset controller <NUM> or provided otherwise to invoke the yaw offset θ.

The yaw offset θ may be scaled according to a weight, e.g. a weight between <NUM> and <NUM>, provided by the decision function <NUM> or by the activation parameter Px. For example, the activation parameter Px may be a value between <NUM> and <NUM>, where <NUM> disables the yaw offset control and where values above <NUM> and up to <NUM> scales the yaw offset θ determined by the yaw offset controller <NUM>. The weight or value of the activation parameter Px is determined dependent on the variables <NUM>.

<FIG> shows an example of the configuration of the yaw offset controller <NUM> where a look up table is used for determining the yaw offset θ of a specific wind turbine 101a dependent on the wind direction Wx and the wind speed for the wind turbine. for wind speeds in the <NUM>-<NUM>/s range, and wind directions in the intervals <NUM>-<NUM>; <NUM>-<NUM>; <NUM>-<NUM> and <NUM>-<NUM>, the yaw offset values θ are <NUM>°, <NUM>°, <NUM>° and <NUM>°, respectively. Accordingly, each wind turbine <NUM> may have its own yaw offset controller <NUM> configured individually with look up tables which may have been determined for each wind turbine based on wake analyses as shown in <FIG>.

<FIG> illustrates a possible configuration of the decision function <NUM> where the function, represented as a piecewise linear curve <NUM>, is dependent on two variables <NUM>: The wind direction variable Wx and the wind direction variation variable ΔWDx. That is, based on the two variables <NUM> (in general n variables) it can be determined if the coordinate point is inside or outside the envelope of the curve <NUM>, where the curve or curves are generally defined in the n-dimensional space of the variables <NUM>.

<FIG> illustrates an example where the decision function <NUM> is dependent on three variables: The wind direction variable Wx, the wind direction variation variable ΔWDx and the wind turbulence variation variable ΔWTx. In this case the function is represented by the 3D surface <NUM>.

In these examples, and other configurations of the decision function <NUM> which is based on curves <NUM>, the activation parameter Px is dependent on where the variables <NUM> are within the inside space of the function <NUM> or within the outside space of the function <NUM>. Thus, if the variables <NUM> are within the outside space of the function <NUM>, the activation parameter Px is determined, e.g. set to zero, so as to disable the yaw offset control.

In other examples, the decision function may be configured as a look-up table or a mathematical function or algorithm which provides the activation parameter Px.

<FIG> shows how the yaw offset can be scaled by a weight provided by the decision function. For example, as illustrated the decision function <NUM> may define different regions <NUM>, <NUM> within the function <NUM>, or may define two or more functions <NUM>,301a so that the value of the activation parameter Px depends on the magnitude of the variables <NUM>. In this example, Px may be provided by a binary activation signal, and the yaw offset values θ may then be scaled according to the value of the activation parameter Px or other weight values provided by the decision function <NUM>. Although not illustrated in <FIG>, the decision function <NUM> of three or higher dimensions, as well as look-up table and other configurations, may be configured similarly to provide weight values for scaling the yaw offset θ.

The decision function may be determined from simulations of the energy production from the wind turbine park. For different wind directions Wx and wind speeds, the yaw offset θ is given by the yaw offset controller <NUM>, e.g. according to look-up tables as in <FIG>. Based on the yaw offset θ for each individual wind turbine, the wind farm energy productions, with the yaw offset control enabled, respectively disabled, for one or more of the plurality of wind turbines, are compared. The comparison is determined for different values of the variables <NUM>, and permutations of enabled and disabled wind turbines, so as to define the boundaries of the decision function <NUM> with respect to enabling and disabling the yaw offset control and possibly with respect to different scaling weights of the enabled yaw offset θ.

Alternatively, the decision function may be determined based on an optimization function which determines which pattern of enabled and disabled yaw offset control settings that optimizes the energy production of the plurality of wind turbines dependent the variables <NUM> and the wind speed.

Alternatively or additionally, the decision function may be determined and/or updated based on measurements of the energy production, e.g. based on comparing the actual energy production of the plurality of wind turbines when the yaw offset control is enabled respectively disabled for one or more of the plurality of wind turbines and in different ranges of the variables <NUM> and the wind speed. For example, during normal operation of the wind park, the pattern of enabled and disabled yaw offset control settings may be permuted in order to search for a possible more advantageous pattern of the enabled and disables yaw offset states of the plurality of wind turbines for the actual values of the variables <NUM> and the wind speed.

The wind turbine controller unit <NUM> may be comprised by a wind turbine controller which is associated with each wind turbine <NUM> and arranged for controlling a specific wind turbine. The wind turbine controller includes the yaw control system <NUM> and the wind turbine controller unit <NUM>.

Alternatively, the wind turbine controller unit <NUM> may be comprised by the wind farm controller <NUM>. In this case the wind turbine controller unit <NUM> is arranged for controlling a plurality of wind turbines, i.e. arranged to provide the activation parameter Px to individual wind turbines or to individual yaw offset controllers <NUM>. In principle the yaw offset controllers <NUM> may be located de-centrally, e.g. in the wind farm controller. However, advantageously, yaw offset controllers <NUM> are comprised by individual wind turbines <NUM>. In this case, the wind farm controller <NUM> is configured with a communication function for sending the activation parameters Ps, e.g. as yaw-offset activation signal, to one or more of the plurality of wind turbines to enable or disable the yaw offset control, individually.

Claim 1:
A method for controlling a yaw offset (θ) of a plurality of wind turbines (<NUM>), the method comprises
- providing a decision function (<NUM>) arranged for providing an activation parameter (Px) for a specific wind turbine (101a) of the plurality of wind turbines dependent on two or more variables (<NUM>), where the two or more variables (<NUM>) comprise:
- a wind direction variable (Wx) indicative of the wind direction of the specific wind turbine, and
- a wind variation variable (ΔWx) defining at least a wind variation range where yaw offset control is disabled and a wind variation range where yaw offset control is enabled,
- enabling or disabling the yaw offset control of the specific wind turbine based on the activation parameter, wherein the enabled yaw offset control provides a yaw offset (θ) relative to a default yaw setting, and wherein the wind variation variable is determined from the most recent measurements obtained in a period of time prior to the enabling or disabling the yaw offset control.