Power boost for a wind turbine

Field of invention

The present invention relates to the control of a wind tur bine for power boosting or increasing of the operative power above the nominal power of the wind turbine.

Art Background

Wind turbines may provide power boost, i.e. the possibility to increase power over the nominal power value. The rotation al speed is increased together with the power. Boost is con figurable and may be set as default to a 5% of the nominal power. If the generator and converter capacity allow for it, the power boost can be increased to a higher number, for ex ample 10% of the nominal power. This overcapacity may be for example used to boost power at high winds.

According to a possible known implementation the bower boost is a step increase in power production when the pitch angle and the wind speed has reached above a predefined threshold. This is done to make sure that the power is not boosted at rated winds, where most extreme loads drive component design. By first boosting not at rated winds but just above where the loads have already lowered, the extreme loads will not be in creased above their design levels by the additional speed and power production. The discrete jump in the power output de termines two main problems: if the boost percentage changes, the pitch angle where boost is applied must be adapted as well. For example, if a power boost of 5% may be enabled at 6 degrees away from opti mum pitch, a boost of 10% may only by applied at a pitch an gle 12 degrees away from optimum pitch. By delaying the boost threshold, a lot of power is lost; when the boost is applied, the turbine blades pitch slightly into the wind to balance the increase in extracted electrical power to the extracted wind power. This extra pitch travel is not necessary and due to the high loading, it adds a lot to the cumulative structural damage in wind tur bine components, such as the pitch bearings.

Summary of the Invention

Scope of the present invention is to provide a control method and circuit for controlling a wind turbine, in order to opti mize the boosted power, resulting in an increased annual en ergy production (AEP), independently of wind distributions and without exceeding extreme structural capacity of blades, tower and other structural wind turbine components.

This scope is met by the subject matter according to the in dependent claims. Advantageous embodiments of the present in vention are described by the dependent claims.

According to a first aspect of the present invention a method of controlling a wind turbine is provided, which comprises the step of boosting the output power of the wind turbine above the nominal power of the wind turbine, according to a boost operational function representing a boost level for the wind turbine, the boost operational function being a crescent function of an operational variable at least between a first threshold value and a second threshold value of the opera tional variable.

According to a second aspect of the present invention, a con troller for a wind turbine is provided, which includes a boosting circuit for boosting the output power of the wind turbine above the nominal power of the wind turbine, accord ing to a boost operational function representing a boost lev el for the wind turbine. The boost operational function is a crescent function of an operational variable at least between a first threshold value and a second threshold value of the operational variable, the boosting circuit receiving as in put: an enabling criteria command, a desired pitch angle, and a desired power reference, the boosting circuit producing as output: a pitch reference, and a boosted power reference representing the boost level for the wind turbine.

In the context of the present invention, a "boosting circuit" can be implemented as a hardware circuit and/or a programma ble logic circuit configured and arranged for implementing the specified operations/activities. In possible embodiments, a programmable circuit may include one or more computer cir cuits programmed to execute a set (or sets) of instructions (and/or configuration data). The instructions (and/or config uration data) can be in the form of firmware or software stored in and accessible from a memory. The "boosting cir cuit" may be part of the controller of the wind turbine.

According to embodiments of the present invention, the boost operational function is a linear function of the operational variable at least between a first threshold value and a sec ond threshold value. The boost operational function may be constant between the second threshold value and a third threshold value of the operational variable. The boost opera tional function may be decrescent between the third threshold value and a fourth threshold value of the operational varia ble.

According to embodiments of the present invention, between the first threshold value and the second threshold value of the wind speed, the boost operational function is defined at constant pitch. Keeping constant the pitch angle when speed and power are boosted lowers the pitch bearing damage. Alternatively, the boost operational function is defined at crescent pitch angle ranging between the first threshold val ue and the second threshold value.

According to another embodiment of the present invention, the pitch angle is kept constant between the first threshold val ue and the second threshold value and increased, for example linearly, between the second threshold value and the third threshold value, i.e. the dynamic boost increases the power by keeping the pitch angle constant until the maximum power output is achieved and, when the maximum is achieved, then the blades pitch out further if winds increase. This keeps the power and speed constant, thereby following the wind.

Advantageously, by providing a dynamic boost instead of the discrete boost of the prior art, the present invention per mits to significantly increase the AEP.

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

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

Brief Description of the Drawings

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

Figure 1 shows a schematic section of a wind turbine to which the control method and circuit of the present inven tion can be applied for controlling power boost.

Figure 2 shows a graph illustrating showing how power boost according to the present invention is operated.

Figure 3 shows a schematic diagram representing a circuit for controlling power boost according to the present in vention.

Detailed Description

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

Figure 1 shows a wind turbine 1 according to the invention. The wind turbine 1 comprises a tower 2, which is mounted on a non-depicted foundation. A nacelle 3 is arranged on top of the tower 2. The wind turbine 1 further comprises a wind ro tor 5 having at least one blade 4 (in the embodiment of Fig ure 1, the wind rotor comprises three blades 4, of which only two blades 4 are visible). The wind rotor 5 is rotatable around a rotational axis Y. The blades 4 extend substantially radially with respect to the rotational axis Y and along a respective longitudinal axis X.

The wind turbine 1 comprises an electric generator 11, in cluding a stator 20 and a rotor 30. The rotor 30 is rotatable with respect to the stator 20 about the rotational axis Y.

The wind rotor 5 is rotationally coupled with the electric generator 11 either directly, e.g. direct drive or by means of a rotatable main shaft 9 and/or through a gear box (not shown in Figure 1). A schematically depicted bearing assembly 8 is provided in order to hold in place the main shaft 9 and the rotor 5. The rotatable main shaft 9 extends along the ro tational axis Y.

The wind rotor 5 comprises three flanges 15 for connecting a respective blade 4 to the wind rotor 5. A pitch bearing is interposed between each blade flange 15 and the respective blade 4. A hydraulic pitch actuation circuit is associated to the pitch bearings of the blades 4 for regulating the pitch angle of each blade, i.e. the angular position of each blade about the respective blade longitudinal axis X. The hydraulic pitch actuation circuit may adjust all pitch angles on all rotor blades 4 at the same time and/or individual pitching of the rotor blades 4 may be available. On a rear side rear side of the nacelle 3, opposite to the wind rotor 5, a converter 21 is provided. The transformer 21 is electrical connected to the electrical generator 11 to transform the electrical out put of the generator 11 to a certain predetermined power out put at a predetermined voltage level to be provided to an electrical network, which is electrically connected to the transformer 21.

The wind turbine 1 comprises a controller (not shown) with a processor and a memory. The processor executes computing tasks based on instructions stored in the memory. According to such tasks, the wind turbine in operation generates a re quested power output level. In particular, the power output level may be a boost power, i.e. having a value greater than a nominal output power Pw of the wind turbine 1. This may be obtained by adjusting the pitch angle by operating according ly the hydraulic pitch actuation circuit associated to the pitch bearings of the blades 4. Alternatively, the requested power output level may be obtained by adjusting the power output of the converter 21.

Figure 2 shows three boost operational functions 110, 120, 130, which may be generated by the controller for operating the wind turbine 1 above nominal output power Pw. The three boost operational functions 110, 120, 130 are shown on a car tesian graph having an abscissa axis representing values of an operational variable and an ordinate axis representing values of output power (as a percentage of the nominal power output Pw of the wind turbine 1). The operational variable may be different according to the different embodiments of the present invention, as better clarified in the following.

According to a first embodiment of the invention, a boost power is generated according to a first boost operational function 110. The first boost operational function 110 is a crescent function of the operational variable between a first threshold Tvl value and a second threshold value Tv2 of the operational variable. The operational variable is a wind speed, which may be a measured or an estimated value of the wind speed. In particular, the first boost operational func tion 110 is a crescent linear function of the wind speed. At the second threshold value Tv2 the power output reaches a predefined power boost level, which may be exceeding the nom inal power Pw of a 10% (power output is 110% of the nominal power Pw). The first boost operational function 110 is a con stant function of the wind speed between the second threshold Tv2 value and a third threshold value Tv3 of the wind speed and a decrescent function of the wind speed between the third threshold Tv3 value and a fourth threshold value Tv4 of the wind speed. In particular, the boost operational function 110 is a decrescent linear function of the wind speed between the third threshold Tv3 value and the fourth threshold value Tv4. The first boost operational function 110 is obtained at a constant pitch, by adjusting only the power output of the converter 21 according to a wind speed - dependent look-up table. According to another embodiment of the present inven tion, the first boost operational function 110 may be ob tained at a variable pitch.

According to other embodiments of the present invention, the operational variable may be a turbulence intensity or a pitch angle. According to a second embodiment of the invention, a boost power is generated according to a second boost operational function 120. The second boost operational function 120 is a crescent function of a pitch angle between a first threshold Tvl' value and a second threshold value Tv2' of the pitch an gle. In particular, the second boost operational function 120 is a crescent linear function of the pitch angle. At the sec ond threshold value Tv2' the power output reaches a prede fined power boost level, which may be exceeding the nominal power Pw of a 5% (power output is 105% of the nominal power Pw). The second boost operational function 120 is a constant function of the pitch angle between the second threshold Tv2' value and a third threshold value Tv3' of the pitch angle and a decrescent function of the pitch angle between the third threshold Tv3' value and a fourth threshold value Tv4' of the pitch angle. In particular, the second boost operational function 120 is a decrescent linear function of the pitch an gle between the third threshold Tv3' value and the fourth threshold value Tv4'. The second boost operational function 120 is obtained at variable pitch, by operating accordingly the hydraulic pitch actuation circuit to generate a prede fined pitch ranging between a pitch minimum, at the first threshold Tvl', and a pitch maximum, at the second threshold Tv2 .

According to a third embodiment of the invention, a boost power is generated according to a third boost operational function 130. The third boost operational function 130 is a crescent function of a pitch angle between a first threshold Tvl ^,f value and a second threshold value Tv2 of the pitch an gle. In particular, the third boost operational function 130 is a crescent linear function of the pitch angle. At the sec ond threshold value Tv2 the power output reaches a predefined power boost level, which may be exceeding the nominal power Pw of a 10% (power output is 110% of the nominal power Pw). The third boost operational function 130 is a constant func tion of the pitch angle between the second threshold Tv2 val- ue and a third threshold value Tv3 of the pitch angle and a decrescent function of the pitch angle between the third threshold Tv3 value and a fourth threshold value Tv4' of the pitch angle. In particular, the third boost operational func tion 130 is a decrescent linear function of the pitch angle between the third threshold Tv3 value and the fourth thresh old value Tv4'. Similarly to the second boost operational function 120, the third boost operational function 130 is ob tained at variable pitch, by operating accordingly the hy draulic pitch actuation circuit to generate a predefined pitch ranging between a pitch minimum, at the first threshold Tvl ^,f , and a pitch maximum, at the second threshold Tv2.

The first threshold Tvl, Tvl', Tvl ^,f is chosen sufficiently high to not start boosting at a peak thrust where highest ex treme loads and most fatigue loads occur.

The second and the third boost operational functions 120, 130 may be obtained through a boosting circuit 200, represented in Figure 3. The boosting circuit 200 can be implemented as a hardware circuit and/or a programmable logic circuit config ured and arranged for implementing the specified opera tions/activities. The boosting circuit 200 may be included in the controller of the wind turbine 1. The boosting circuit 200 receives as input anenabling criteria command 210 (for example depending from the wind speed, so that the wind speed during boost operations is chosen sufficiently high to not start boosting at a peak thrust where highest extreme loads and most fatigue loads occur), a desired pitch angle 220 and a desired power reference 230. The boosting circuit 200 gen erates as output a pitch reference 240 and a boosted power reference 250. The boosting circuit 200 comprises a dynamic power boost block 260 which receives as input the enabling criteria command 210 and the desired pitch angle 220 and gen erates as output a dynamic power boost signal 265, which rep resents the percentage of desired boost, and boosted speed reference signal 290. The boosting circuit 200 comprises a speed-pitch controller 270 which receives as input the boost- ed speed reference signal 290 and generates as output the pitch reference 240. The boosting circuit 200 further com prises a speed-power controller 280 which receives as input a sum of the desired power reference 230 and the dynamic power boost signal 265 and generates as output the boosted power reference 250. The outputs of the boosting circuit 200 are then an increased power reference sent to the converter 21 and a pitch angle that accounts for the increase in desired speed which is sent to the the hydraulic pitch actuation cir- cuit.