EP2776711A1

EP2776711A1 - Method of operating wind turbine and controller thereof

Info

Publication number: EP2776711A1
Application number: EP11869709.3A
Authority: EP
Inventors: Zhaosui ZHANG; Yuanzhang SUN; Guojie Li; Jin Lin
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2011-07-21
Filing date: 2011-07-21
Publication date: 2014-09-17
Also published as: WO2013010332A1; CN103732914B; CN103732914A; EP2776711A4

Abstract

A method of operating a wind turbine (100) is provided. The wind turbine (100) comprises a rotor (130) having a plurality of blades (140), wherein the pitch of each blade (140) is variable. The method comprises decreasing the power output of the wind turbine (100) by a predefined amount by increasing the rotational speed of the rotor (130) to a predetermined rotor speed, determining if the predetermined rotor speed exceeds a maximum rotor speed, and if the predetermined rotor speed exceeds the maximum rotor speed, limiting the rotational speed of the rotor (130) to the maximum rotor speed and decreasing the power output of the wind turbine (100) by varying the pitch of at least one of the plurality of blades (140).

Description

METHOD OF OPERATING WIND TURBINE AND CONTROLLER THEREOF

Field of the Invention

The present invention relates generally to a method for operating a wind turbine, and in particular, to a method of decreasing the power output of the wind turbine based on wind speed mode.

Background of the Invention

A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind incident on blades of the wind turbine generator (WTG) causes a rotor of the WTG to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. One type of wind turbine that provides constant frequency electrical power is a fixed-speed wind turbine. This type of wind turbine requires a generator rotor that rotates at a constant speed. Another type of wind turbine is a variable speed wind turbine. This type of wind turbine allows the generator to rotate at variable speeds to accommodate for fluctuating wind speeds.

Wind turbines usually operate on a MPPT (Maximum Power Point Tracking) curve in order to extract as much wind power as possible. However the wind turbines operating at maximum power do not have any frequency regulation margin. Therefore some grid codes require wind power plants to operate with a reserve margin (down-regulation percentage) in order to support frequency regulation. For example in the Spanish grid code, a reserve margin of 1.5% is required. One common way of operating with a reserve margin in a wind power plant is to cut off a part of wind turbines from the power grid. However this operation results in frequent starting up and shutting down of wind turbines and thus reduces the lifetime of the wind turbine components.

Another way of operating the wind power plant with a reserve margin is to operate one or more wind turbines in the wind power plant at a reduced power (down regulation of the wind turbines). In other words, the wind turbines are shifted from the MPPT to a sub- optimal operating point, resulting in a reduced power output generated by the turbines. A wind turbine may be down regulated by controlling the rotational speed of the rotor, or by controlling the pitch of the blades.

Summary of the Invention

According to a first aspect of the invention, a method of operating a wind turbine is provided. The wind turbine comprises a rotor having a plurality of blades, wherein the pitch of each blade is variable. The method comprises decreasing the power output of the wind turbine by a predefined amount by increasing the rotational speed of the rotor to a predetermined rotor speed, determining if the predetermined rotor speed exceeds a maximum rotor speed, and if the predetermined rotor speed exceeds the maximum rotor speed, limiting the rotational speed of the rotor to the maximum rotor speed and decreasing the power output of the wind turbine by varying the pitch of at least one of the plurality of blades.

According to a second aspect of the invention, a method of operating a wind turbine is provided. The wind turbine comprises a rotor having a plurality of blades, wherein the pitch of each blade is variable. The method comprises determining a wind speed, determining a wind speed mode based on the wind speed, the wind speed mode comprises a first wind speed mode and a second wind speed mode, decreasing the power output of the wind turbine by a predefined amount by increasing the rotational speed of the rotor to a predetermined rotor speed if the first wind speed mode is determined, and decreasing the power output of the wind turbine by the predefined amount by varying the pitch of at least one of the plurality of blades if the second wind speed mode is determined.

According to a third aspect of the invention, a controller for controlling the operation of a wind turbine is provided. The wind turbine comprises a plurality of blades, wherein the pitch of each blade is variable. The controller comprises a wind speed mode determination unit adapted to determine a wind speed mode based on wind speed, an overspeed control unit adapted to determine a reference rotor speed based on the determined wind speed mode, a power controller adapted to generate a power reference based on the reference rotor speed for controlling the rotational speed of the rotor, and a pitch controller adapted to generate a pitch reference based on the reference rotor speed for controlling the pitch of at least one of the plurality of blades. Brief Description of the Drawings

The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings.

Figure 1 shows a general structure of a wind turbine.

Figure 2 shows an electrical system layout of the wind turbine according to an embodiment.

Figure 3 shows a power-rotor speed curve of the wind turbine for different wind speed.

Figure 4 illustrates the different wind speed mode according to an embodiment.

Figure 5 shows a controller according to an embodiment.

Figure 6 - 8 show the simulation results of operating the wind turbine in the different wind speed mode according to an embodiment.

Detailed Description of the Invention

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.

Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to "the invention" shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s). The method according to the first aspect of the invention is suitable to be used in a wind turbine having a rotor. The rotor is rotatable and has one or more blades thereon. The pitch of each blade can be varied. In other words, the blade can be rotated along its longitudinal axis so that its angle with respect to wind is changed. The predefined amount of power output to be decreased may be a percentage of down regulation specified by a specific grid codes. It may also be specified by an operator or owner of the wind farm. The power output of the wind turbine is decreased by the predefined amount by increasing the rotational speed of the rotor to the predetermined rotor speed. The predetermined rotor speed corresponds to the decreased power output of the wind turbine. It is then determined whether the increased rotor speed exceeds the maximum rotor speed. The maximum rotor speed may be set as an upper rotor speed safety limit, beyond which there is a high risk of damaging the wind turbine components.

When it is determined that the predetermined rotor speed exceeds the maximum rotor speed, the predetermined rotor speed is limited to the maximum rotor speed. Specifically, the rotational speed of the rotor is set to the maximum rotor speed even though the predetermined rotor speed is higher than the maximum rotor speed. As the power output of the wind turbine may not be decreased by the predefined amount since the rotational speed of the rotor cannot be increased to the predetermined rotor speed, the power output is decreased by varying the pitch of one or more blades.

The method according to the invention is a coordinated rotor speed control and pitch control to decrease the power output of the wind turbine or to down regulate its operation. In this coordinated control method, priority is given to rotor speed control for decreasing the power output. When the rotor speed is increased, the risks of damaging wind turbine components are also increased. Therefore, a maximum rotor speed is normally set to prevent the rotor speed from exceeding the maximum rotor speed, and hence the risks of components damage are kept within acceptable levels. When the power output can be decreased by the predetermined amount by increasing the rotor speed without exceeding the maximum rotor speed, the power output is decreased or down regulated using rotor speed control. However when the increase in rotor speed cannot fully decrease the power output by the predefined amount (due to the maximum rotor speed being exceeded), pitch control is used to decrease the power output by the predefined amount. The advantage of decreasing the power output using rotor speed control is that the power output can be decreased very fast compared to pitch control because rotor speed control is power electronic device-based. In addition, frequent pitching of the blades in pitch control results in mechanical wear of the wind turbine components, especially the pitch bearings. By the coordinated control method according to the invention, the advantages of rotor speed control is maintained, and the disadvantages of the pitch control is kept to a minimum as pitch control is only used when rotor speed control is unable to fully decrease the power output by the predefined amount.

According to an embodiment, varying the pitch of one or more blades to decrease the power output of the wind turbine comprises increasing a pitch angle of the one or more blades. As the pitch angle increases, less energy is captured from the wind. Accordingly, power output is also decreased.

According to an embodiment, the method further comprises detecting a frequency deviation in a load connected to the wind turbine, and increasing the power output of the wind turbine corresponding to the frequency deviation. The load connected to the wind turbine may be a power transmission grid. The frequency at the load or grid is monitored. If the frequency deviates from a nominal or reference frequency, the power output of the wind turbine is increased. The increase of the power output is dependent on the frequency deviation. Accordingly, this embodiment supports frequency regulation. In a further embodiment, the frequency deviation comprises a frequency drop from a nominal frequency.

According to an embodiment, increasing the power output of the wind turbine comprises determining whether the rotational speed of the rotor is less than the maximum rotor speed, if the rotational speed of the rotor is less than the maximum rotor speed, increasing the power output of the wind turbine by decreasing the rotational speed of the rotor, and if the rotational speed of the rotor is at the maximum rotor speed, increasing the power output of the wind turbine by varying the pitch of at least one of the plurality of blades, or by both decreasing the rotational speed of the rotor and varying the pitch of at least one of the plurality of blades.

If the rotational speed of the rotor is less than the maximum rotor speed, the wind turbine is operating at the decreased power output using rotor speed control. Therefore, the power output can be increased also by rotor speed control, that is, decreasing the rotational speed of the rotor. If the rotational speed of the rotor is at the maximum rotor speed, the wind turbine may be operating at the decreased power output using pitch control, or a combination of rotor speed control and pitch control. Therefore, the power output is increased by either pitch control or a combination of rotor speed control and pitch control.

According to an embodiment, varying the pitch of the one or more blades to increase the power output of the wind turbine comprises decreasing the pitch angle of the one or more blades. As the pitch angle decreases, more energy is captured from the wind. Accordingly, power output is also increased. The method according to the second aspect of the invention determines the wind speed, and based on the wind speed, determines whether to operate in a first wind speed mode or a second wind speed mode. Based on the wind speed mode, a control strategy is adopted. Specifically in the first wind speed mode, the power output of the wind turbine is decreased by the predefined amount using rotor speed control. In the second wind speed mode, the power output of the wind turbine is decreased by the predefined amount using pitch control.

According to an embodiment, the wind speed mode further comprises a third wind speed mode, and if the third wind speed mode is determined based on the wind speed, the power output of the wind turbine is decreased by increasing the rotational speed of the rotor to the maximum rotor speed and varying the pitch of at least one of the plurality of blades. Specifically, in the third wind speed mode, the power output of the wind turbine is decreased by the predefined amount using both rotor speed control and pitch control. The rotor speed is limited to the maximum rotor speed.

According to an embodiment, the first wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to the power output of the wind turbine decreased by the predefined amount is equal or less than the maximum rotor speed. In other words, the increased rotor speed which results in the power output of the wind turbine to be decreased by the predefined amount does not exceed the maximum rotor speed. In this region, the wind turbine is down regulated (to the level specified, for example, by grid codes) using rotor speed control. According to an embodiment, the second wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to a maximum power output of the wind turbine is more than the maximum rotor speed. The rotational speed of the rotor for producing the maximum power has exceeded the maximum rotor speed. Therefore in this region, the power output cannot be decreased by further increasing the rotor speed, and pitch control is used to decrease the power output or down regulate the wind turbine by the predefined amount.

According to an embodiment, the third wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to the power output of the wind turbine decreased by the predefined amount is more than the maximum rotor speed and the rotational speed of the rotor corresponding to a maximum power output of the wind turbine is equal or less than the maximum rotor speed. The third wind speed mode corresponds to a region between the first wind speed mode and the second wind speed mode. In this region, both rotor speed control and pitch control are used to down regulate the wind turbine by the predefined amount.

According to an embodiment, varying the pitch of one or more blades to decrease the power output of the wind turbine comprises increasing a pitch angle of the one or more blades.

According to an embodiment, the method further comprises detecting a frequency deviation in a load connected to the wind turbine, and increasing the power output of the wind turbine corresponding to the frequency deviation.

According to an embodiment, increasing the power output of the wind turbine comprises decreasing the rotational speed of the rotor in the first wind speed mode, varying the pitch of at least one of the plurality of blades in the second wind speed mode, and decreasing the rotational speed of the rotor and varying the pitch of at least one of the plurality of blades in the third wind speed mode.

According to an embodiment, varying the pitch of the one or more blades to increase the power output of the wind turbine comprises decreasing the pitch angle of the one or more blades.

As mentioned earlier, in the first wind speed mode, the wind turbine was down regulated by increasing the rotor speed. Accordingly, to increase the power output of the wind turbine, the rotor speed is decreased. In the second wind speed mode, the wind turbine was down regulated by varying the pitch of the one or more blades. Accordingly, to increase the power output of the wind turbine, the pitch control is used. In the third wind speed mode, the wind turbine was down regulated by both increasing the rotor speed to the maximum rotor speed and varying the pitch of the one or more blades. Accordingly, to increase the power output of the wind turbine, the rotor speed is decreased from the maximum rotor speed and the pitch of the one or more blades is varied.

As can be seen from the above embodiments, rotor speed control is given a higher priority than pitch control. In scenarios where it is possible to down regulate the power output of the wind turbine using rotor speed control, the power output is decreased by increasing the rotor speed. When the maximum rotor speed is reached, then pitch control is used. This minimizes the use of pitch control, and hence, reduces mechanical wear of the bearings and other components in the wind turbines, resulting in extended lifetime of the wind turbine.

According to the third aspect of the invention, a controller for controlling the operation of the wind turbine in accordance to the methods described in the first and second aspect of the invention is provided. The controller comprises a wind speed mode determination unit adapted to determine a wind speed mode based on wind speed, an overspeed control unit adapted to determine a reference rotor speed based on the determined wind speed mode, a power controller adapted to generate a power reference based on the reference rotor speed for controlling the rotational speed of the rotor, and a pitch controller adapted to generate a pitch reference based on the reference rotor speed for controlling the pitch of at least one of the plurality of blades.

The wind speed mode determination unit, the overspeed control unit, the power controller and the pitch controller only refer to logical units of the controller. These logical units may reside in one physical controller unit or in separate controller units physically located in different parts of the wind turbine. The controller units may be implemented using computers, microprocessors, PLCs (programmable logic arrays), etc. It should be noted that the pitch controller is only activated in the second wind speed mode and the third wind speed mode. The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

Fig.l illustrates an exemplary wind turbine 100 according to an embodiment. As illustrated in Fig.l, the wind turbine 100 includes a tower 110, a nacelle 120, and a rotor 130. In one embodiment, the wind turbine 100 may be an onshore wind turbine. However, embodiments of the invention are not limited only to onshore wind turbines. In alternative embodiments, the wind turbine 100 may be an offshore wind turbine located over a water body such as, for example, a lake, an ocean, or the like. The tower 110 of such an offshore wind turbine is installed on either the sea floor or on platforms stabilized on or above the sea level.

The tower 110 of the wind turbine 100 may be configured to raise the nacelle 120 and the rotor 130 to a height where strong, less turbulent, and generally unobstructed flow of air may be received by the rotor 130. The height of the tower 110 may be any reasonable height, and should consider the length of wind turbine blades extending from the rotor 130. The tower 1 10 may be made from any type of material, for example, steel, concrete, or the like. In some embodiments the tower 110 may be made from a monolithic material. However, in alternative embodiments, the tower 1 10 may include a plurality of sections, for example, two or more tubular steel sections 111 and 112, as illustrated in Fig.l . In some embodiments of the invention, the tower 1 10 may be a lattice tower. Accordingly, the tower 110 may include welded steel profiles.

The rotor 130 may include a rotor hub (hereinafter referred to simply as the "hub") 132 and at least one blade 140 (three such blades 140 are shown in Fig.l). The rotor hub 132 may be configured to couple the at least one blade 140 to a shaft (not shown). In one embodiment, the blades 140 may have an aerodynamic profile such that, at predefined wind speeds, the blades 140 experience lift, thereby causing the blades to radially rotate around the hub. The hub 140 further comprises mechanisms (not shown) for adjusting the pitch of the blade 140 to increase or reduce the amount of wind energy captured by the blade 140.

Pitching adjusts the angle at which the wind strikes the blade 140. It is also possible that the pitch of the blades 140 cannot be adjusted.

The hub 132 typically rotates about a substantially horizontal axis along a drive shaft (not shown) extending from the hub 132 to the nacelle 120. The drive shaft is usually coupled to one or more components in the nacelle 120, which are configured to convert the rotational energy of the shaft into electrical energy.

Although the wind turbine 100 shown in Fig.l has three blades 140, it should be noted that a wind turbine may have different number of blades. It is common to find wind turbines having two to four blades. The wind turbine 100 shown in Fig.l is a Horizontal

Axis Wind Turbine (HAWT) as the rotor 130 rotates about a horizontal axis. It should be noted that the rotor 130 may rotate about a vertical axis. Such a wind turbine having its rotor rotates about the vertical axis is known as a Vertical Axis Wind Turbine (VAWT). The embodiments described henceforth are not limited to HAWT having 3 blades. They may be implemented in both HAWT and VAWT, and having any number of blades 140 in the rotor

130.

Fig.2 shows an electrical system of the wind turbine according to an embodiment. The electrical system includes a doubly-fed induction generator 201 , a power converter 202 and a main transformer 203. The stator windings 210 of the generator 201 are connected to the power grid 207 through the transformer 203, and the rotor windings 21 1 are connected to the power converter 202, which is in turn connected to the power grid 207 through the transformer 203. The generator 201 converts mechanical energy to electrical energy or power. The electrical energy or power is supplied on the stator windings 210 which is then supplied to the power grid 207 through the transformer 203. The power converter 202 controls the operation of the generator 201 through the rotor windings 21 1.

The wind turbine also includes a power controller 220 for controlling the operation of the power converter 202, a pitch controller 221 for controlling the pitch of the blades 140, and a turbine controller 222 for controlling the operation of the power controller 220 and the pitch controller 221. As will be seen in the description later, the turbine controller 222 may send instructions to the power controller 220 to control the rotational speed of the rotor 130 via the power converter 202. The turbine controller 222 may also send instructions to the pitch controller 221 to control the pitch angle of the blades 140.

It should be noted that Fig.2 is only an illustration of an electrical system in a wind turbine where only common components are shown. The electrical system may include other components such as generator-side filters, sensors, pre-charge circuit, etc. In another embodiment, a permanent magnet generator may be used wherein the power output on the stator windings of the generator are converted by a power converter before being supplied to the grid via the turbine transformer. In this embodiment, there are no rotor windings from the generator.

Fig.3 shows a power-rotor speed curve of a wind turbine for different wind speed.

For each curve 310, there is a maximum point which corresponds to the maximum power at a rotor speed, also know as MPPT (Maximum Power Point Tracking). The bold curve 300 shows the MPPT for all the wind speeds. The bold line 301, 302 on the left and right of the MPPT curve 300 are the 90% sub-optimal curves based on under-speed operation. However the left curve 301 is unstable and undesirable, and hence the sub-optimal curve 302 on the right is used.

In the example of rotor speed control at 90% sub-optimal operation with wind speed at 9 m/s, the wind turbine initially operates at point A and generates an active power of Po. When the frequency at the power grid drops by a certain amount, an increase of power ΔΡ is desired. Accordingly, the operating point moves from point A to point D. At point D, the rotor speed is decreased and the power output is now Po+ ΔΡ. The maximum power that can be generated at wind speed 9 m/s is at point C.

The determining of the different wind speed modes will now be illustrated with reference to Fig.4. Three wind speed modes are defined, namely low wind speed mode, medium wind speed mode and high wind speed mode. Fig.4 shows the power-rotor speed curve of the wind turbine for different wind speed. Similarly, the MPPT curve 300 and the 90% sub-optimal curve 302 are shown. The maximum rotor speed of the wind turbine is defined to be 1.2 p.u. For down regulation of the wind turbine to 90%, the low wind speed mode is defined to be the region where rotor speed control is able to achieve the 90% sub- optimal operation without exceeding the maximum rotor speed. This corresponds to point C in Fig.4. In other words, if the wind speed is 9.6 m/s or below, the low wind speed mode is determined.

In the low wind speed mode, down regulation is achieved by rotor speed control, specifically by increasing the speed of the rotor speed. Pitch control need not be activated to down regulate the wind turbine. Therefore, the pitch angle can be fixed to zero or at a minimal angle. In the event when the frequency of the grid drops, the wind turbine can support the frequency drop by increasing its power output by rotor speed control, specifically by decreasing the rotor speed.

The medium wind speed mode is defined to be the region where the rotor speed control is unable to fully achieve the 90% sub-optimal operation and rotor speed at the MPPT does not exceed the maximum rotor speed. In Fig.4, this corresponds to wind speed higher than 9.6 m/s and lower or equal to 11.8 m/s. In the medium speed mode, down regulation cannot be achieve solely by rotor speed control due to the maximum rotor speed limit. In an example of wind speed of 10.7 m/s (shown as curve 330 in Fig.4), the 90% sub-optimal operation point is shown as point D. It can be seen that point D has a rotor speed which is higher than the maximum rotor speed of 1.2 p.u. Therefore, the rotor speed cannot be increased to point D, but is capped at the maximum rotor speed at point F. In order to further decrease the power output to the required level at point A, pitch control is used. Specifically, the pitch angle is increased such that power is decreased from point F to point A.

When there is a frequency drop in the power grid, an increase in power output ΔΡ is requested. The additional power output ΔΡ is achieved by both decreasing the rotor speed and the pitch angle. As both rotor speed control and pitch control are used, the control path follows the dashed line from point A to point E, (or to point B if more power is needed). At point E, the rotor speed is decreased from 1.2 p.u. to CO_E. CO_E can be estimated using triangle theorem:

where CO_B (=(Q_MPPT) is the optimal speed at point B, P_B (=P_MPPT) is the optimal power at point B, P_A is 90% of P_MPPT, P_E = P_A + ΔΡ= 0.9P + ΔΡ, and co_B = 1.2 p.u. Accordingly, the following is obtained: AP

ω^ = ω_Ε = \ 2 + 'MPPT 1.2) (2)

OAR MPPT

Therefore in order to provide an additional power of ΔΡ, the rotor speed is increased to COE, together with decreasing the pitch angle accordingly.

The high wind speed mode is defined to be the region where rotor speed at the MPPT exceeds the maximum rotor speed. This corresponds to wind speed higher than 11.8 m/s. In the high wind speed mode, it is not possible to down regulate the turbine by increasing the rotor speed, as the maximum rotor speed has already been reached. Accordingly, the rotor speed is capped at the maximum rotor speed of 1.2 p.u., and the turbine is down regulated by pitch control. Specifically, the pitch angle is increased to decrease the power output of the wind turbine. . In the event when the frequency of the grid drops, the wind turbine can support the frequency drop by increasing its power output by pitch control, specifically by decreasing the pitch angle.

It can be seen from the above that the regions of wind speed modes are determined by the power-rotor speed curves at different wind speed and reserve margin (the amount of down regulation) of the wind turbines. The power-rotor speed curves are normally provided by turbine manufacturers or in the general specifications. The reserve margin is usually set by grid operators or specified in the grid codes. Therefore when the power-rotor speed curves and reserve margin are obtained, the different regions can be determined. Accordingly, based on the wind speed, the different wind speed mode for supporting frequency regulation of the power grid can be determined.

Fig.5 shows a controller for implementing the method according to an embodiment. The controller comprises a wind speed mode determination unit 501, an overspeed control unit 502, a pitch controller 503 and a rotor speed controller 504. Wind speed data and reserve margin are provided as inputs to the wind speed mode determination unit 501 and the overspeed control unit 502. The pitch controller 503 generates a pitch angle reference β for regulating the pitch angle of the blades of the wind turbine. The power controller 504 generates a power reference P_ref to regulate the rotor speed and hence the power output of the wind turbine.

Based on the wind speed and reserve margin, the wind speed mode determination unit

501 determines the wind speed mode for down regulation of the wind turbine. The overspeed control unit 502 determines a reference rotor speed co_ref for regulating the rotor speed through power controller 504. During the medium and high wind speed mode, the reference rotor speed co_ref is at the maximum rotor speed, and the pitch controller 503 is activated to adjust the power output according to co_ref. During the low wind speed mode (i.e. when the current rotor speed is less than the maximum rotor speed), the pitch controller 503 is not activated.

In an embodiment, the rotor speed controller 504 comprises a PI (Proportional Integral) controller and generates the power reference P_ref for controlling the operation of the power converter. When there is a frequency deviation in the grid, a corresponding change in power output ΔΡ may be requested. This power output ΔΡ may be provided as input to the power controller 504 according to an embodiment.

Fig.6-8 show the simulation results of operating the wind turbine in the different wind speed modes according to an embodiment. Fig.6 shows the simulation results when operating the wind turbine in the first wind speed mode. In (a) of Fig.6, a frequency drop is shown to occur at about t=l Is. In response to that, the rotor speed is decreased as shown in (d) and the power output is increased from its initial power and reached a steady state as shown in (b). The pitch angle remains unchanged in this first wind speed mode as shown in (c), as pitch control is not activated.

Fig.7 shows the simulation results when operating the wind turbine in the second wind speed mode. In (a) of Fig.7, a frequency drop is shown to occur at about t=10s. In response to that, both the pitch angle and rotor speed are decreased (see (c) and (d) respectively). Accordingly, the power output is increased as shown in (b).

Fig.8 shows the shows the simulation results when operating the wind turbine in the third wind speed mode. In (a) of Fig.8, a frequency drop is shown to occur at about t=10.5s. In response to that, the pitch angle is decreased as shown in (c) and the power output is increased as shown in (b). The rotor speed remains unchanged at the maximum rotor speed of 1.2 p.u. in this third wind speed mode as shown in (d).

It should be emphasized that the embodiments described above are possible examples of implementations which are merely set forth for a clear understanding of the principles of the invention. The person skilled in the art may make many variations and modifications to the embodiment(s) described above, said variations and modifications are intended to be included herein within the scope of the following claims.

Claims

Claim:

1. A method of operating a wind turbine, the wind turbine comprises a rotor having a plurality of blades, wherein the pitch of each blade is variable, the method comprising:

- decreasing the power output of the wind turbine by a predefined amount by increasing the rotational speed of the rotor to a predetermined rotor speed;

- determining if the predetermined rotor speed exceeds a maximum rotor speed; and

- if the predetermined rotor speed exceeds the maximum rotor speed, limiting the rotational speed of the rotor to the maximum rotor speed and decreasing the power output of the wind turbine by varying the pitch of at least one of the plurality of blades.

2. The method according to claim 1, wherein varying the pitch of the at least one of the plurality of blades to decrease the power output of the wind turbine comprises increasing a pitch angle of the at least one of the plurality of blades.

3. The method according to any of claim 1 or 2, further comprising:

- detecting a frequency deviation in a load connected to the wind turbine; and

- increasing the power output of the wind turbine corresponding to the frequency deviation.

4. The method according to claim 3, wherein increasing the power output of the wind turbine comprises:

- determining whether the rotational speed of the rotor is less than the maximum rotor speed;

- if the rotational speed of the rotor is less than the maximum rotor speed, increasing the power output of the wind turbine by decreasing the rotational speed of the rotor; and

- if the rotational speed of the rotor is at the maximum rotor speed, increasing the power output of the wind turbine by varying the pitch of the at least one of the plurality of blades, or by both decreasing the rotational speed of the rotor and varying the pitch of the at least one of the plurality of blades.

5. The method according to claim 4, wherein varying the pitch of the at least one of the plurality of blades to increase the power output of the wind turbine comprises decreasing a pitch angle of the blades.

6. A method of operating a wind turbine, the wind turbine comprises a rotor having a plurality of blades, wherein the pitch of each blade is variable, the method comprising:

- determining a wind speed;

- determining a wind speed mode based on the wind speed, the wind speed mode comprises a first wind speed mode and a second wind speed mode;

- decreasing the power output of the wind turbine by a predefined amount by increasing the rotational speed of the rotor to a predetermined rotor speed if the first wind speed mode is determined; and

- decreasing the power output of the wind turbine by the predefined amount by varying the pitch of at least one of the plurality of blades if the second wind speed mode is determined.

7. The method according to claim 6, wherein the wind speed mode further comprises a third wind speed mode, and wherein if the third wind speed mode is determined, the power output of the wind turbine is decreased by increasing the rotational speed of the rotor to a maximum rotor speed and varying the pitch of at least one of the plurality of blades.

8. The method according to any one of claims 6 or 7, wherein the first wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to the power output of the wind turbine decreased by the predefined amount is equal or less than the maximum rotor speed.

9. The method according to any one of claims 6 or 7, wherein the second wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to a maximum power output of the wind turbine is more than the maximum rotor speed.

10. The method according to claim 7, wherein the third wind speed mode comprises a region wherein the rotational speed of the rotor corresponding to the power output of the wind turbine decreased by the predefined amount is more than the maximum rotor speed and the rotational speed of the rotor corresponding to a maximum power output of the wind turbine is equal or less than the maximum rotor speed.

11. The method according to any of claims 6 to 10, wherein varying the pitch of the at least one of the plurality of blades to decrease the power output of the wind turbine comprises increasing a pitch angle of one of the plurality of the blades.

12. The method according to any of claims 6 to 11, further comprising:

- detecting a frequency deviation in a load connected to the wind turbine; and

13. The method according to claim 12, wherein increasing the power output of the wind turbine comprises:

- decreasing the rotational speed of the rotor in the first wind speed mode;

- varying the pitch of at least one of the plurality of blades in the second wind speed mode; and

- decreasing the rotational speed of the rotor and varying the pitch of at least one of the plurality of blades in the third wind speed mode.

14. The method according to claim 13, wherein varying the pitch of the at least one of the plurality of blades to increase the power output of the turbine comprises decreasing a pitch angle of the blades.

15. A controller for controlling the operation of a wind turbine, the wind turbine comprises a plurality of blades, wherein the pitch of each blade is variable, the controller comprises: a wind speed mode determination unit adapted to determine a wind speed mode based on wind speed;

an overspeed control unit adapted to determine a reference rotor speed based on the determined wind speed mode;

a power controller adapted to generate a power reference based on the reference rotor speed for controlling the rotational speed of the rotor; and

a pitch controller adapted to generate a pitch reference based on the reference rotor speed for controlling the pitch of at least one of the plurality of blades.