Method of operating a wind turbine

Abstract

Method of operating a wind turbine in response to a wind speed, the wind turbine having at least a rotor with a plurality of blades and a generator comprising a generator rotor and a generator stator, the method comprising, at wind speeds above a first wind speed, increasing the pitch angle of the blades and reducing the rotor speed with increasing wind speed, said first wind speed being superior to the nominal wind speed; wherein at a second wind speed, the speed of the generator rotor is equal to the synchronous generator rotor speed, said second wind speed being superior to said first wind speed; and wherein at wind speeds superior to said second wind speed, the speed of the generator rotor is lower than the synchronous generator rotor speed.

Claims

1. A method of operating a wind turbine in response to a wind speed greater than nominal wind speed, the wind turbine having at least a rotor with a plurality of blades and a generator comprising a generator rotor and a generator stator, the method comprising at wind speeds above a first wind speed, increasing a pitch angle of the blades so as to reduce a rotor speed with increasing wind speed, the first wind speed being greater to a nominal wind speed, wherein at a second wind speed, a speed of the generator rotor is equal to a synchronous generator rotor speed, the second wind speed being greater to the first wind speed, and wherein at wind speeds greater to the second wind speed, the speed of the generator rotor is lower than the synchronous generator rotor speed, and as wind speed increases beyond the first wind speed and at least to an intermediate wind speed less than the second wind speed, maintaining generator torque constant at a value of the generator torque at nominal wind speed value while reducing the speed of the generator rotor.

2. The method according to claim 1, wherein the generator is a doubly fed induction generator, the method further comprising at the intermediate wind speed, reducing the generator torque.

3. The method according to claim 2, wherein the generator torque is reduced up to second wind speed, and further comprising increasing the generator torque at wind speeds greater to the second wind speed.

4. The method according to claim 2, wherein the generator torque is reduced up to second wind speed, and further comprising maintaining the generator torque at a constant reduced level at wind speeds greater to the second wind speed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1 illustrates a wind turbine comprising a doubly-fed induction generator;

(3) FIGS. 2a and 2b illustrate a prior art method of operating a wind turbine;

(4) FIGS. 3a-3c illustrate a potential problem related to a method of operating a wind turbine in accordance with embodiments of the present invention; and

(5) FIGS. 4a-4f illustrate further methods of operation of a wind turbine in accordance with various embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(6) In FIG. 1, a schematic representation of a wind turbine comprising a rotor 10 having a plurality of blades is shown. The wind sets the rotor 10 into rotation, such that the low speed shaft (rotor shaft) 12 starts rotating. A gearbox 14 may convert the relatively slow rotation of the rotor shaft 12 into a fast rotation of the high speed shaft 16. This high speed shaft 16 may be operatively connected with the generator rotor 24.

(7) The generator 20 further comprises a stator 22. As illustrated, the generator stator may be directly connected to electricity grid 40. Generally, such an electricity grid may have three electric phases as illustrated in FIG. 1. The generator rotor 24 is also connected to grid 40 through a back-to-back frequency converter 30. The frequency converter 30 may comprise a machine side converter 32 and a grid side converter 36 converting AC current to DC current and vice versa. The frequency converter 30 may further comprise a DC link 34. Through the back-to-back converter, the generator torque may be regulated. In addition to the pitch control system which regulates the angle of attack of the blades of rotor 10, a further operational control is thus provided, specifically for variable speed operation.

(8) FIG. 2a illustrates a prior art control method and shows respectively the pitch angle (β), rotor speed (ω.sub.rotor), to electrical power (P) and aerodynamic torque (M) at varying wind speeds.

(9) As previously described, the pitch angle is generally not changed until nominal wind speed is reached, e.g. at 11 m/s. At a slightly lower wind speed, e.g. around 8.5 m/s, nominal rotor speed may be reached. At wind speeds above nominal wind speed, the pitch angle may be varied such as to maintain the aerodynamic torque substantially constant. The rotor speed, generator torque and electrical power generated may also be maintained substantially constant. This may be maintained from nominal wind speed to cut-out wind speed. Alternatively, and such as is shown in FIG. 2a, close to cut-out wind speed, e.g. in the range 22 m/s-25 m/s, both rotor speed and torque may be slightly reduced.

(10) FIG. 2b illustrates the generator torque (M.sub.gen) versus the generator speed ω.sub.gen corresponding substantially to the wind turbine operation shown in FIG. 2a. With increasing wind speed (below nominal wind speed), the speed of rotation of the generator rotor increases until nominal rotor speed is reached. At the same time, the generator torque is gradually increased. At wind speeds above a wind speed corresponding to a nominal rotor speed, but below the nominal wind speed (i.e. using the same numbers as before, between 8.5 m/s and 11 m/s), the pitch angle of the blades may be kept the same, and control is carried out by increasing the generator torque. Once nominal wind speed is reached, the pitch of the blades is changed such as to keep the speed of rotation constant at the nominal rotor speed. This corresponds to a speed of rotation of ω.sub.N, the nominal generator rotor speed.

(11) At the very top right corner of the diagram, at nominal rotor speed, and maximum torque, nominal power is produced by the wind turbine generator.

(12) Optionally, in accordance with FIG. 2a, at wind speeds close to the cut-out wind speed, pitch and/or torque may be controlled in such a way as to slightly reduce the rotor speed and, consequently, the generator rotor speed. As a result, structural loads, and generated power are also reduced in this region.

(13) FIGS. 3a - 3c illustrate a method of operating a wind turbine in accordance with embodiments of the present invention and a potential problem that may arise. FIG. 3a illustrates a control method and shows, similarly to what was shown in FIG. 2a, respectively the pitch angle (β), rotor speed (ω.sub.rotor), electrical power (P) and aerodynamic torque (M) at varying wind speeds.

(14) A first notable difference between this implementation and the prior art control method is that above nominal wind speed, e.g. from approximately 16 m/s the pitch angle of the blades is changed to a further extent than in the previous example, and the rotor speed is reduced at a substantially lower wind speed than in the prior art method of FIG. 2a. Furthermore, whereas in FIG. 2a, the electrical power generated may be kept at a maximum value for a large share of the operational range, this may not be the case with this method of operation. In the method of FIG. 3a, the loads may be reduced to a further extent than in the prior art method.

(15) With increasing wind speed, the generator rotor speed may be further reduced, beyond the point of synchronous speed of the generator rotor, see also FIG. 3b. Synchronous speed, ω.sub.s, refers to the speed at which the frequency of the magnetic field of the generator rotor corresponds to the frequency of the magnetic field of the stator (corresponding to the frequency of the grid, e.g. 50 Hz or 60 Hz). At 60 Hz, synchronous generator rotor speed may be 1800 rpm for a generator comprising two pole pairs. For a 50 Hz grid, with the same generator the synchronous generator rotor speed may be 1500 rpm.

(16) At this point, DC current is produced instead of AC current at the Machine-Side-Converter (MSC). FIG. 3c illustrates the maximum load capabilities of a typical Machine-Side-Converter (MSC). On one axis, the magnitude of currents is shown and on the other axis, the time the MSC can endure such currents. It may be seen that the currents the MSC can withstand during a prolonged period of time are low at f=0 Hz (which corresponds to a situation wherein the MSC is subjected to DC current). There is thus a risk that the MSC is overloaded and it may thus happen that the MSC (and the wind turbine as a whole) shuts down.

(17) In principle, a similar situation could potentially also arise at a lower wind speed, as may be seen in the same FIG. 3b. As the wind turbine is speeding up with increasing wind speed, the generator rotor also passes by the synchronous generator rotor speed. However, at this point, the generator torque is much smaller and therefore generally does not cause any problem.

(18) However, when synchronous generator rotor speed is reached at high wind speeds, the generator torque (and rotor currents) may be relatively high and it is possible that the wind turbine would need to operate at or close to that point for a prolonged period of time.

(19) In order to avoid the aforementioned problem, several options are available: on the one hand, the power converter may be adapted to account for this situation. However, oversizing of the converter may lead to a higher cost. As this region of operation may not be a very common one, it may not always be a preferred choice.

(20) Various alternative methods are available that are able to follow the method of operating a wind turbine and reduce the speed beyond the synchronous speed. Several of these methods are discussed with reference to FIGS. 4a-4f.

(21) One way to avoid the aforementioned problem of overloading the MSC is illustrated in FIGS. 4a and 4b. FIG. 4a shows respectively the pitch angle (β), rotor speed (ω.sub.rotor), electrical power (P) and aerodynamic torque (M) at varying wind speeds, and FIG. 4b shows the generator torque (M.sub.gen) versus the generator speed ω.sub.gen corresponding substantially to the wind turbine operation shown in FIG. 4a.

(22) From a first wind speed, the rotor speed (and inherently also the generator rotor speed) is reduced. With further increasing wind speed, the generator torque is also reduced in such a way that at a second wind speed, at which the generator rotor reaches the synchronous speed, the currents are smaller than what can be sustained by the MSC. The MSC does not need to be specifically adapted for this situation, the operation range may be maintained, and the Cost-of-Energy may be controlled. At the same time, due to the relatively large blades, more electrical power may be generated at lower wind speeds.

(23) A further alternative is illustrated in FIGS. 4c and 4d. In this example, at wind speeds above said second wind speed, generator torque is increased again.

(24) Merely in the area close to the point at which the generator rotor speed may equal synchronous speed (DC operation of the MSC), the generator torque is reduced. With increasing torque, the power generated may also be slightly increased at wind speeds superior to the second wind speed.

(25) Yet a further alternative is illustrated in FIGS. 4e and 4f. In this example, at wind speeds above said second wind speed, the generator torque is maintained substantially constant. Also in this example, the generator torque is reduced in the area close to the point at which the generator rotor speed may equal synchronous speed.

(26) Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.