METHOD AND DEVICE FOR INDIVIDUALLY PITCHING BLADES OF A WIND TURBINE

Field of invention

The present invention relates to a method and a device for individually pitching blades of a wind turbine . A conventional wind turbine comprises a rotor having a plurality of rotor blades , wherein each blade is configured to be pitched by a pitch angle about a pitch axis of the blade , the rotor is mounted to a nacelle to rotate about a rotation axis with a rotor speed to drive a generator for producing electrical energy, and the nacelle is mounted to a tower . Some wind turbines are configured to collectively pitch all blades by a collective pitching degree and to individually pitch each blade by an individual pitching degree .

The pitch position shall be equal to ( or very close to ) the pitch reference . This is a crucial operation as loads on the turbine are dependent on the pitch position . Under certain conditions , an individual pitch control ( IPC ) is used to lower the loads due to e . g . , high yaw misalignment , high shear, wake situations . This control adds typically a IP component ( a sinusoid with the period time of a single full rotation) to the collective pitch reference . This IP component is dependent on the azimuth angle of the rotor, and the phase of this sinusoid is shi fted by 120 degrees between each of the blades ( in a wind turbine having three blades ) . By doing so , it increases thrust on one side of the rotor and relieves thrust on the other side . This way unbalanced load from yaw errors or high shear can be balanced out or lower the impact on the mechanical components . It keeps the average of the collective pitch position and keeps the desired power production . I f in these situations the pitch reference is not adhered to the blade positions , the loads from these sources will increase , with a risk of overloading the yaw system or hub bending .

There are certain situations where the actual pitch position cannot reach the pitch reference , for example , when there are large forces on the blade due to the wind conditions that the pitch system cannot overcome , when the pitch force is low due to low hydraulic pressure , and/or when the pitch in speed is lower than the pitch speed demanded by the individual pitching . These conditions can hardly be handled by conventional controllers , and there are risks of overloading the mechanical components .

Summary of the Invention

There may be a need for a method and a device for individually pitching blades of a wind turbine , which can reduce the loads on the blades . This need may be met by the subj ect matters according to the independent claims . The present invention is further developed as set forth in the dependent claims .

According to a first aspect of the invention, a method of pitching blades of a wind turbine is provided . The wind turbine comprises a rotor having a plurality of rotor blades , each blade being configured to be pitched by a pitch angle about a pitch axis of the blade , the rotor being mounted to a nacelle to rotate about a rotation axis with a rotor speed to drive a generator for producing electrical energy, the nacelle being mounted tower to rotate about a yaw axis , wherein the wind turbine is configured to collectively pitch all blades by a collective pitching degree and to individually pitch each blade by an individual pitching degree . The method comprising the following steps : determining a collective pitch reference for all blades ; determining an individual pitch reference for each blade ; determining whether at least one blade is unable to reach the individual pitch reference . I f it is determined that at least one blade is unable to reach its individual pitch reference , the individual pitch reference for this blade is prioriti zed against the collective pitch reference .

The loads , which are lowered using the individual pitch control , largely depend on the amplitude of the individual pitch control , the phase and to a small extent on the collective pitch reference . This novel control strategy prioriti zes the pitch reference of the individual pitch control over the collective pitch reference in case the pitch system cannot provide both . By this way, excessive loads on the turbine components can be avoided .

In an embodiment , each blade is configured to be pitched towards a production direction, in which the power for driving the generator is increased, and towards a stop direction, in which the power for driving the generator is decreased . I f it is determined that the at least one blade is unable to reach its individual pitch reference , the individual pitch reference for this blade is prioriti zed against the collective pitch reference by shi fting the collective pitch reference towards the stop direction . The inventors found out that , when the pitch force is not suf ficient for individual pitch control , this usually happens in the direction towards a production direction . Therefore , this embodiment shi fts the collective pitch reference towards the stop direction . Although the present invention generally works in both directions , there is a particular advantage when pitching of the blades will be limited towards the production direction, wherein a corrective reference of fset , for example that of the following embodiment , is added to the collective pitch reference which is then shi fted towards the stop direction .

In an embodiment , the step of prioriti zing the individual pitch reference for this blade against the collective pitch reference comprises a step of calculating a tracking error of each blade , which is a di f ference between the individual pitch reference and the actual blade pitch position of each blade ; a step of determining a maximum of the calculated tracking errors of the blades ; a step of converting the maximum to a corrective reference of fset ; a step of adding the corrective reference of fset to the collective pitch reference to obtain a corrected collective pitch reference ; and a step of pitching the blades with the individual pitch references and the corrected collective pitch reference . In this embodiment , the maximum from all tracking errors is taken and not the summation, as the blade with the highest tracking error will provide a suf ficient corrective reference of fset to the collective pitch reference for all blades that are impacted . I f the at least one blade is unable to reach the individual pitch reference , for example i f the at least one blade is being unable to be pitched towards the production direction of power production to provide the desired amplitude of the individual pitch control around the collective pitch reference , the collective pitch reference can be moved to stop by a portion of ( or the full amplitude of ) the (pitch) tracking error to preserve the amplitude of the individual pitch control .

In an embodiment , the step of converting the maximum to the corrective reference of fset is made by use of a look-up-table ( LUT ) , where the tracking error caused by an expected system inertia is tolerated without modi fying the collective pitch reference by the corrective reference of fset . Thereby, the step of prioriti zing the individual pitch reference is not performed in case of tracking errors , which are caused by inertia e . g . , of the blades and are part of a normal operation .

In an embodiment , the step of prioriti zing the individual pitch reference for this blade against the collective pitch reference comprises a step of weighting the individual pitch reference for this blade with a first weight ; and a step of weighting the collective pitch reference with a second weight . The first weight is larger than the second weight so that the individual pitch reference for this blade is priori- ti zed against the collective pitch reference . However, at least one of the first and second weights can have the value 1 by default (which means a default weight of 1 ) .

Preferably, at least one of the first weight and the second weight is a function of a tracking error of the blade , which is a di f ference between the individual pitch reference and the actual blade pitch position of the blade , wherein, e . g . , the larger the tracking error is , the larger a ratio between the first weight and the second weight is . However, it may also be a constant weight above a certain tracking error, where it does not increase further .

In an embodiment , it is determined that the at least one blade is unable to reach its individual pitch reference , i f a tracking error of each blade , which is a di f ference between the individual pitch reference and the actual blade pitch position of each blade , or a di f ference between an individual pitching speed reference of the blade and an actual pitching speed of the blade , exceeds a predetermined threshold value .

According to a second aspect of the invention, a device for pitching blades of a wind turbine is provided . The wind turbine comprises a rotor having a plurality of rotor blades , each blade being configured to be pitched by a pitch angle about a pitch axis of the blade , the rotor being mounted to a nacelle to rotate about a rotation axis with a rotor speed to drive a generator for producing electrical energy, the nacelle being mounted tower to rotate about a yaw axis . The device is configured to collectively pitch all blades by a collective pitching degree and to individually pitch each blade by an individual pitching degree . The device comprises a first determining unit configured to determine a collective pitch reference for all blades ; a second determining unit configured to determine an individual pitch reference for each blade ; a third determining unit configured to determine whether at least one blade is unable to reach the individual pitch reference ; and a prioriti zing unit configured to prior- iti ze the individual pitch reference for this blade against the collective pitch reference , i f it is determined by the third determining unit that at least one blade is unable to reach its individual pitch reference .

It has to be noted that embodiments of the invention have been described with reference to di f ferent subj ect matters . In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims . However, a person skilled in the art will gather from the above and the following description that , unless other notified, in addition to any combination of features belonging to one type of subj ect matter also any combination between features relating to di f ferent subj ect matters , in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application .

Brief Description of the Drawings

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

Fig . 1 shows a wind turbine and the di f ferent elements thereof ;

Fig . 2 shows a flowchart of a step of prioriti zing an individual pitch reference against a collective pitch reference according to an embodiment ;

Fig . 3 shows pitching amplitudes in an individual pitch control according to the prior art ; and Fig . 4 shows pitching amplitudes in an individual pitch control according to an embodiment of the present invention .

Detailed Description

The illustrations in the drawings are schematically . It is noted that in di f ferent figures , similar or identical elements are provided with the same reference signs .

Fig . 1 shows a wind turbine 1 . The wind turbine 1 comprises a nacelle 3 and a tower 2 . The nacelle 3 is mounted at the top of the tower 2 . The nacelle 3 is mounted rotatable with regard to the tower 2 by means of a yaw bearing . The axis of rotation of the nacelle 3 with regard to the tower 2 is referred to as the yaw axis .

The wind turbine 1 also comprises a rotor 4 with three rotor blades 6 ( of which two rotor blades 6 are depicted in Fig . 1 ) . The rotor 4 is mounted rotatable with regard to the nacelle 3 by means of a main bearing 7 . The rotor 4 is mounted rotatable about a rotation axis 8 ( rotor axis ) .

The wind turbine 1 furthermore comprises a generator 5 . The generator 5 in turn comprises a rotor connecting the generator 5 with the rotor 4 . I f the rotor 4 is connected directly to the generator 5 , the wind turbine 1 is referred to as a gearless , direct-driven wind turbine . Such a generator 5 is referred as direct drive generator 5 . As an alternative , the rotor 4 may also be connected to the generator 5 via a gear box . This type of wind turbine 1 is referred to as a geared wind turbine . The present invention is suitable for both types of wind turbines 1 .

The generator 5 is accommodated within the nacelle 3 . The generator 5 is arranged and prepared for converting the rotational energy from the rotor 4 into electrical energy in the shape of an AC power . The blades 6 can be pitched, i . e . , rotated about a longitudinal axis of the blade 6 . The wind turbine 1 is configured to collectively pitch all blades 6 by a collective pitching degree and to individually pitch each blade 6 by an individual pitching degree . A method of pitching blades of the wind turbine 1 comprises steps of determining a collective pitch reference for all blades 6 , determining an individual pitch reference for each blade 6 and determining whether at least one blade 6 is unable to reach the individual pitch reference . I f it is determined that at least one blade 6 is unable to reach its individual pitch reference , the individual pitch reference for this blade 6 is prioriti zed against the collective pitch reference .

Prioriti zing the individual pitch reference against the collective pitch reference can be made in di f ferent ways . In an embodiment , each blade 6 is configured to be pitched towards a production direction, in which the power for driving the generator is increased, and towards a stop direction, in which the power for driving the generator is decreased . I f it is determined that the at least one blade 6 is unable to reach its individual pitch reference , the collective pitch reference is shi fted towards the stop direction .

Fig . 2 shows a flowchart of a step of prioriti zing an individual pitch reference against a collective pitch reference according to another embodiment . In this embodiment , the wind turbine 1 comprises three blades 6 . In a block 11 , a position, i . e . , a pitch angle , of a first blade 6 is detected and input into a block 41 . An individual pitch reference for the first blade 6 is input from a block 12 into the block 41 . In the block 41 , a tracking error of the first blade 6 is calculated as a di f ference between the individual pitch reference for the first blade 6 and the actual position of the first blade 6 . The same is done for the second and third blades 6 . In a block 21 , a position, i . e . , a pitch angle , of a second blade 6 is detected and input into a block 42 . An individual pitch reference for the second blade 6 is input from a block 22 into the block 42 . In the block 42 , a tracking error of the second blade 6 is calculated as a di f ference between the individual pitch reference for the second blade 6 and the actual position of the second blade 6 . In a block 31 , a position, i . e . , a pitch angle , of a third blade 6 is detected and input into a block 43 . An individual pitch reference for the third blade 6 is input from a block 32 into the block 43 . In the block 43 , a tracking error of the third blade 6 is calculated as a di f ference between the individual pitch reference for the third blade 6 and the actual position of the third blade 6 .

The three tracking errors of the three blades 6 are input into a block 50 , where a maximum of the three tracking errors is determined . The determined maximum tracking error is input from the block 50 into a block 60 , where the determined maximum tracking error is converted to a corrective reference of fset .

The corrective reference of fset is input from the block 60 into a block 80 . In addition, a collective pitch reference for all blades 6 is input from a block 70 into the block 80 . In the block 80 , the corrective reference of fset and the collective pitch reference are added to obtain a corrected collective pitch reference in a block 90 . Eventually, the blades 6 are pitched with the individual pitch references and the thus corrected collective pitch reference .

In another embodiment , the step of prioriti zing the individual pitch reference for the respective blade 6 against the collective pitch reference comprises a step of weighting the individual pitch reference for this blade 6 with a first weight , and a step of weighting the collective pitch reference with a second weight . The first weight is larger than the second weight . One of the first and second weights can be 1 by default . In a modi fied embodiment , at least one of the first weight and the second weight is a function of a tracking error of the blade 6 , which is a di f ference between the individual pitch reference and the actual blade pitch position of the blade 6 .

There are many ways to determine that the at least one blade 6 is unable to reach its individual pitch reference . For example , it can be determined that the at least one blade 6 is unable to reach its individual pitch reference , i f a tracking error of each blade 6 , which is a di f ference between the individual pitch reference and the actual blade pitch position of each blade 6 , or a di f ference between an individual pitching speed reference of the blade 6 and an actual pitching speed of the blade 6 , exceeds a predetermined threshold value .

Fig . 3 shows pitching amplitudes in an individual pitch control according to the prior art . The solid lines indicate the amplitudes of the individual pitch control for each of the three blades 6 . The dotted line shows the collective pitch reference which in this example is constant/unchanged . The amplitudes in the individual pitch control in the middle area are limited by lack of a pitch force . All three blades 6 are unable to reach their individual pitch references .

Fig . 4 shows pitching amplitudes in an individual pitch control according to an embodiment of the present invention . The solid lines indicate the amplitudes of the individual pitch control for each of the three blades 6 . The dotted line shows the collective pitch reference . The control strategy of the present invention shows the solution where the collective pitch reference is lowered to preserve the amplitudes of the individual pitch control . The amplitudes of the individual pitch control are reached/maintained, and therefore the load targets in the individual pitch control in this situation are kept under control , whereas in prior art of Fig . 3 , the ef- fectiveness of the individual pitch control is reduced, and loads are increased . It should be noted that the term "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality . Also elements described in association with di f ferent embodiments may be combined . It should also be noted that reference signs in the claims should not be con- strued as limiting the scope of the claims .