Method for starting up a wind turbine

Field of invention

The present invention relates to the technical field of starting up a wind turbine with ice accretion on a rotor blade . In particular , the pre sent invention relate s to a method for starting up a wind turbine . Further , the pre sent invention relates to a wind turbine control system and to a wind turbine which are all adapted for executing the method for starting up a wind turbine .

Art Background

Ice accretion or ice formation on the rotor blades remarkably reduces the aerodynamic performance of wind turbines . The shapes of the aerodynamic profiles are modif ied due to the ice accreted on them and , as a result , their aerodynamic behavior is severely altered . Furthermore , the wind turbine ' s operation can be threatened due to the aerodynamic stall phenomena .

By now, the main obj ective of the operation with ice control functionality is to avoid stop of the wind turbine during icing conditions and maximize its power production . To that end, adaptive pitch and torque functionalities can be implemented in order to try to avoid the wind turbine from suf fering aerodynamic stall during icing periods .

There are several methods for the operation with ice or the ice removal from the wind turbine blades .

CA 2508316 C discloses a generator which is used as a motor to remove ice from the blades . CN 103821665 A, DE 20014238 Ul , WO 1998001340 Al disclose removal of ice using a de-icing system .

CN 101124402 B dis closes ice prevention in off shore wind turbines using sea water .

RU 2567616 C2 discloses operation in ice situations based on stopping the wind turbine .

US 20170058871 Al discloses operation in ice situations based on yawing the wind turbines .

Normally, control strategies designed for the operation with ice are focused on maximizing the power production of the turbines during icing conditions . However , these strategies might re sult usele s s if the wind turbine is unable to complete the start-up procedure due to the ice accretion .

Therefore , there may be a need for improving the start-up procedure of wind turbine s in cold climates .

Summary of the Invention

Thi s need may be met by the subj ect matter according to the independent claims . Advantageous embodiments of the pre sent invention are described by the dependent claims .

According to a first aspect of the invention there is provided a method for starting up a wind turbine for generating electrical power , wherein ice accretion is on at lea st one blade of a rotor of the wind turbine and wherein the at least one blade has a variable pitch angle , the method comprising

- mea suring the rotational speed of the electromechanical transducer ,

- calculating a pitch angle adj ustment with the goal of maximizing the rotational speed, - modifying the pitch angle in a positive or negative step according to the pitch angle adjustment,

- iterate the preceding steps.

The described method is based on the idea to provide an adaptive pitch strategy for a wind turbine start-up procedure in cold climates. The described method serves to improve the start-up capability of wind turbines affected by ice accretion on the blades. The pitch angle is modified in positive or negative steps according to the temporal development of the rotational speed and with the objective of maximizing it.

The proposed adaptive pitch strategy improves the start-up process of the wind turbines during icing periods. As the ice accretes on the blades, the aerodynamic profile is changed and the optimal pitch angle changes to an unknown value. Even due to this worsened and unknown aerodynamic behavior of the blades, the proposed method enables the wind turbine to accelerate and reach the rotational speed necessary to connect the wind turbine to the grid.

The optimization algorithm utilizes the rotational speed as an input. This allows operation even when the wind turbine is not connected to the grid. In that case there is no electrical power to measure.

The described method may have the advantage of an Annual Energy Production (AEP) gain in cold climate sites. The method might enable the wind turbine to start-up and connect to the grid even with ice accretion on the blades. Without this functionality, the wind turbine might suffer important energy production losses.

Further, the described method does not require to install or calibrate any sensor or de-icing system, which is usually not included in a wind turbine. Thus, the method is robust, cost competitive and easy to implement. According to a further embodiment of the invention the pitch angle adj ustment i s calculated with a cost function , the cost function receiving the rotational speed of the electromechanical transducer as an input with the goal of maximizing the rotational speed . Having ice accretion on the blades , the pitch angle behavior i s dynamic and not static anymore . Such s ituation can be covered well by a cost function . The cost function aims at maximizing the rotational speed by comparing it before and after each adj ustment to the pitch angle . A cost function may be designed by a numerical optimization procedure for example . Simulations can be performed to reach value s for implementing a cost function . Such cost function may be a scalar cost function .

According to a further embodiment of the invention as a first step , ice accretion is detected . Such detection may be achieved by identifying non-plausible or not expected sensor readings , the wind turbine not producing enough energy, the startup procedure not working in normal parameters and/or by weather information .

According to a further embodiment of the invention as a la st step , the method i s terminated after sufficient rotational speed is measured to connect the electromechanical transducer to the grid, a predetermined time limit is reached without being able to start-up the wind turbine and/or no ice is detected .

According to a further embodiment of the invention the mea sured rotational speed of the electromechanical transducer is filtered with a low pas s and/or a band pa s s . Filters can be implemented as first order or second order . Filtering may enhance robustnes s of the method .

According to a further embodiment of the invention a f irst pitch angle adj ustment i s a pitch step towards feather . Feathering means that the blade pitch is increased to the point that the chord line of the blade is approximately parallel to the on-coming airflow . Thi s approach minimizes the unwanted effect of stalling the wind turbine .

According to a further embodiment of the invention if the pitch angle adj ustment towards feather does not lead to an increase of the speed of the electromechanical transducer a pitch angle adj ustment towards fine is introduced . The next or second iteration step could be directed to the opposite direction i . e . towards fine . Such approach may deliver better information of the ice s ituation and could lead to the optimized pitch angle .

According to a further embodiment of the invention amplitudes of the pitch angle adj ustment s are adaptive with respect to the number of iterations . As the optimal pitch angle for the current ice accretion situation should be closer with an increasing number of iterations the amplitude of the adj ustments may be lowered . Further , the pitch movement according to each adaptation or iteration may have a maximum and a minimum . This may prevent that the algorithm runs to far in a wrong direction . Adaptation of the amplitude of pitch angle adj ustments may help to better adapt to the unknown ice conditions .

According to a further aspect of the invention there is provided a wind turbine control system for controlling a wind turbine having an electromechanical transducer . The control system comprises a computation unit adapted for executing the method as des cribed above , an input adapted for receiving a rotational speed of the electromechanical transducer , and an output adapted for outputting a pitch angle adj ustment . The same advantages and modifications as de scribed above apply .

According to a further embodiment of the invention the computation unit comprises a low pas s and/or a band pa s s filter adapted for filtering the measured rotational speed of the electromechanical transducer . The filter can be implemented as first order or second order. Implementing a filtering stage allows for more reliable and sturdy data handling .

According to a further aspect of the invention there is provided a wind turbine for generating electrical power. The provided wind turbine comprises a tower, a rotor, which is arranged at a top portion of the tower and which comprises at least one blade, wherein the at least one blade has a variable pitch angle, an electromechanical transducer which is mechanically coupled with the rotor, and a wind turbine control system as described above. The same advantages and modifications as described above apply.

According to a further embodiment of the invention the wind turbine comprises a rotational speed sensor adapted for measuring the rotational speed of the electromechanical transducer and/or a pitch sensor adapted for measuring a pitch angle of the at least one blade. According to the described wind turbine it is not required to install or calibrate any further sensors or a de-icing system, which is usually not included in a wind turbine. Thus, the improvement is being robust, cost competitive and easy to implement.

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 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 subject matter also any combination between features relating to different subject 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 document. The aspects defined above and further a spect s of the present invention are apparent f rom the example s of embodiment to be des cribed hereinafter and are explained with reference to the examples of embodiment . The invention will be des cribed in more detail hereinafter with reference to example s of embodiment but to which the invention is not limited .

Brief De scription of the Drawings

Figure 1 shows a wind turbine according to an embodiment of the present invention .

Figure 2 shows a schematic illustration of a control system of the wind turbine .

Figure 3 shows a flow diagram of a method for starting up a wind turbine having ice accretion on its blade .

Figure 4 shows diagrams of the pitch , the generator speed and the cost according to an implementation of the method depicted in Figure 3 .

Detailed Description

The illustration in the drawing is schematically . It i s noted that in different figure s , similar or identical elements are provided with the same reference signs .

Figure 1 shows a wind turbine 100 according to an embodiment of the invention . The wind turbine 100 compri ses a tower 120 , which is mounted on a non-depicted fundament . On top of the tower 120 there is arranged a nacelle 122 . In between the tower 120 and the nacelle 122 there is provided a yaw angle adj ustment device 121 , which i s capable of rotating the nacelle 122 around a non depicted vertical axi s , which i s aligned with the longitudinal extension of the tower 120 . By controlling the yaw angle adj ustment device 121 in an appropriate manner it can be made sure , that during a normal operation of the wind turbine 100 the nacelle 122 is always properly aligned with the current wind direction. However, the yaw angle adjustment device 121 can also be used to adjust the yaw angle to a position, wherein the nacelle 122 is intentionally not perfectly aligned with the current wind direction .

The wind turbine 100 further comprises a rotor 110 having three blades 114. In the perspective of Figure 1 only two blades 114 are visible. The rotor 110 is rotatable around a rotational axis 110a. The blades 114, which are mounted at a hub 112, extend radially with respect to the rotational axis 110a.

In between the hub 112 and a blade 114 there is respectively provided a blade adjustment device 116 in order to adjust the blade pitch angle of each blade 114 by rotating the respective blade 114 around a non depicted axis being aligned substantially parallel with the longitudinal extension of the blade 114. By controlling the blade adjustment device 116 the blade pitch angle of the respective blade 114 can be adjusted in such a manner that at least when the wind is not so strong a maximum wind power can be retrieved from the available wind power. However, the blade pitch angle can also be intentionally adjusted to a position, in which only a reduced wind power can be captured.

As can be seen from Figure 1, the rotor 110 is directly coupled with a shaft 125, which is coupled in a known manner to an electromechanical transducer 140. The electromechanical transducer is a generator 140.

Accordingly, the turbine is a direct drive type, in which the hub is directly connected to the generator 140, i.e. no gearbox is present. Alternatively, a gearbox may be arranged between the shaft 125 and the generator 140. Further, a brake 126 is provided in order to stop the operation of the wind turbine 100 or to reduce the rotational speed of the rotor 110 for instance (a) in case of an emergency, (b) in case of too strong wind conditions, which might harm the wind turbine 100, and/or (c) in case of an intentional saving of the consumed fatigue life time and/or the fatigue life time consumption rate of at least one structural component of the wind turbine 100.

In accordance with basic principles of electrical engineering the generator 140 comprises a stator assembly 145 and a rotor assembly 150. The generator 140 may include an external rotor 150 which is arranged outside the stator 145.

The stator assembly 145 comprises a plurality of coils for generating electrical current in response to a time alternating magnetic flux. The rotor assembly comprises a plurality of permanent magnets, which are arranged in rows being aligned with a longitudinal axis of the rotor assembly 150. The permanent magnets may be skewed to reduce torque ripple .

The wind turbine 100 further comprises a control system 153 for operating the wind turbine 100 in a highly efficient manner. Apart from controlling for instance the yaw angle adjustment device 121 the depicted control system 153 is also used for adjusting the blade pitch angle of the rotor blades 114 in an optimized manner.

The control system 153 is connected with a rotational speed sensor 154 which is adapted for measuring the rotational speed of the generator 140. The control system 153 is further connected with a pitch sensor 155 which is adapted for measuring a pitch angle of the rotor blades 114. For ease of understanding power lines and signal lines between the control system 153 and the sensors are not depicted. Details of the control system 153 are discussed in conjunction with the following figures. In cold climates , ice accretion 156 aggregates on the rotor blade s 114 which severely reduces the aerodynamic performance of the wind turbine 100 . The shape s of the aerodynamic profiles of the rotor blades 114 are modified due to the ice accreted on them so that their aerodynamic behavior is altered .

Cold climates can be defined a s continental climate and polar climate . Regions in a continental climate tend to have warm to cool summers and very cold winters . In the winter , this climate zone can experience snowstorms , strong winds , and very cold temperatures— sometimes falling below -30 ° C . For regions in a polar climate zone , the temperatures tend to never increase higher than 10 ° C even during summer .

Figure 2 shows a s chematic view of the control system 153 which is adapted to execute or calculate an adaptive pitch strategy for a start-up procedure of the wind turbine 100 in cold climates .

The control system 153 includes a computation unit 157 like a microproces sor or the like . The control system 153 or the computation unit 157 , re spectively is adapted for executing a method for starting up a wind turbine 100 when ice accretion is on a blade 114 of the rotor 110 of the wind turbine 100 . The method is described in conj unction with the following figures .

The control system 153 i s connected with the rotational speed sensor 154 via a first input 158 . The first input 158 i s connected with the computation unit 157 . A value or signal provided by the rotational speed sensor 154 is treated as an input by the control system 153 and is utilized for calculations such a s the method or algorithm for starting up a wind turbine 100 with ice accretion . The control system 153 i s further connected with a pitch sensor 155 via a second input 159 . The second input 159 is connected with the computation unit 157 . A s ingle pitch sensor 155 can be implemented or one pitch sensor 155 can be provided for each of the rotor blades 114 . The number of second inputs 159 may equal to the number of pitch sensors 155 .

A value or signal provided by the pitch sensor 155 is treated as an input by the control system 153 and is utili zed for calculations such a s the method or algorithm for starting up a wind turbine 100 with ice accretion .

The control system 153 has at least one output 160 for outputting control or steering values or signals which can among others be used for adj usting the blade pitch angle of the rotor blades 114 . The control signals are derived from calculations based on the input signal or signals .

Figure 3 shows a flow diagram of a method for starting up a wind turbine 100 having ice accretion on its blade 114 and/or being located in a cold climate . The method can be executed by or implemented in the control system 153 as des cribed before .

In a first step 300 , the rotational speed of the electromechanical transducer here in form of the generator 140 is mea sured . The mea sured rotational speed may be filtered for example with a low or band pas s before further proces sing it .

Optionally, a s a primary step or as a starting condition , ice accretion is detected . Alternatively, the adaptive pitch strategy of the described method can be set a s default for a wind turbine 100 start-up procedure in cold climates .

In a second step 310 , a pitch angle adj ustment is calculated with the goal of maximi zing the rotational speed of the wind turbine 100 . The rotational speed of the wind turbine 100 corresponds to the measured rotational speed of the generator 140 . With the goal of maximiz ing the rotational speed of the wind turbine 100 a quick start of the wind turbine 100 can be achieved as the generator needs a certain rotational speed for connecting it to the grid .

The calculation of the pitch angle adj ustment may be achieved by a cost function . Such cost function receives the rotational speed of the generator 140 a s an input with the goal of maximizing the rotational speed by maximizing the cost . The cost function is explained in conj unction with Figure 4 .

In a third step 320 , the pitch angle is modif ied in a pos itive or negative step or amount according to the calculated pitch angle adj ustment .

The preceding steps are iterated in a loop so that effect of the calculated pitch angle adj ustment can be evaluated in the calculation step . Depending on the change in rotational speed the pitch angle adj ustment of the next iteration will have the same or opposite sign . The same sign may be applied when the rotational speed had increased while the opposite may be applied when the rotational speed had decreased .

As an optional last step or as a termination condition , the method i s terminated after sufficient rotational speed is mea sured to connect the generator 140 to the grid, a predetermined time limit is reached without being able to start-up the wind turbine 100 and/or no ice is detected, e . g . when implausible sensor readings have stopped .

Figure 4 shows a diagram 410 of the pitch , a diagram 420 of the generator speed and a diagram 430 of the cost according to an implementation of the method depicted in Figure 3 . All diagrams are shown over time in seconds . The described optimization algorithm or method cyclically looks for the optimal pitch angle according to the evolution of the rotational speed. This evolution of the pitch angle is shown in diagram 420 in revolutions per minute (rpm) .

Diagram 410 shows the pitch angle in degrees. Changes in the pitch angle originate from the calculated pitch angle adjustments .

Diagram 430 shows the cost in rpm which corresponds to the incremental difference between the rotational speed before and after the pitch angle change, this change due to a calculated and applied pitch angle adjustment.

In this example, the cost function starts with a pitch angle adjustment towards feather. This calculated pitch angle adjustment is applied as a pitch step towards feather. Feathering means that the blade pitch angle is increased to the point that the chord line of the blade is approximately parallel to the on-coming airflow. This approach minimizes the unwanted effect of stalling the wind turbine.

In diagram 410, this first pitch angle adjustment is depicted as the first increase of the pitch angle. As a consequence, the rotational speed decreases as is shown in diagram 420. This decrease in speed is unwanted. Hence, the cost decreases as well as is depicted in diagram 430.

As described, the first pitch angle adjustment towards feather does not lead to an increase of the speed of the generator 140 and thus decreases cost. As the cost function aims to maximize cost a pitch angle adjustment towards fine i.e. to the opposite is introduced.

As the rotational speed as well as the cost decreases further a further step towards fine having a smaller amplitude is introduced. As a consequence, the rotational speed increases slowly over time. Hence, the cost function has found a way to maximize the generator speed and issues some increasing pitch angle adjustments with the result of increasing rotational speed and cost.

The amplitudes of the pitch angle adjustment and pitch steps may be adaptive with respect to the number of search iterations, as the optimal pitch angle should be closer with the increasing number of iterations .

As a termination condition of the iteration, the method may be terminated after sufficient rotational speed is measured to connect the generator 140 to the grid, a predetermined time limit is reached without being able to start-up the wind turbine 100 and/or no ice is detected anymore.

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 different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.