WO2012139584A1

WO2012139584A1 - A method for adapting wind turbine power production to a power demand

Info

Publication number: WO2012139584A1
Application number: PCT/DK2012/050115
Authority: WO
Inventors: Robert Stevens
Original assignee: Vestas Wind Systems A/S
Priority date: 2011-04-15
Filing date: 2012-04-10
Publication date: 2012-10-18

Abstract

A method of reducing the power output of a wind turbine, the wind turbine having a rotor and a generator for producing power, the rotor operating at a first rotational speed, the method comprising the steps of: receiving a power request signal from a source external to the wind turbine, the power request signal indicating by how much the power output of the wind turbine should be reduced;determining a second rotational speed of the rotor based on the power request signal, the second rotational speed being lower than the first rotational speed; and reducing the power output of the turbine by reducing the rotational speed of the rotor to the second rotational speed.

Description

A METHOD FOR ADAPTING WIND TURBINE POWER PRODUCTION TO A POWER

DEMAND

The invention provides a method and a system for power regulation of a generator driven by a rotor of a wind turbine.

Wind turbines are designed for a nominal load and nominal power production. Traditionally, the rotor speed of the wind turbine, "the rotational speed" has been controlled by rotating the blades into or out of the wind, i.e. by blade pitching.

In some wind turbines, the rotational speed is maintained at a fixed speed, often called nominal speed, and in other turbines, the rotational speed is variable and a frequency converter is used to match the frequency of the connected grid. To reduce wear and damage, known wind turbines are generally controlled based on one or more loads acting on the wind turbine, e.g. expressed by a wind speed, a blade bending moment, or expressed by similar measurable condition. Various methods have been used to cut-off the wind turbine from the grid or to completely stop rotor rotation at a predefined wind speed, e.g. at 25 m/s.

As is well known in the art, a wind turbine is defined with a power curve which gives the power output of the wind turbine as a function of wind speed. The wind turbine starts to generate power at a cut in wind speed. The turbine then operates under part load (also known as partial load) conditions until the rated wind speed is reached. At the rated wind speed the rated (or nominal) generator power is reached and the turbine is operating under full load conditions. The rated power of a wind turbine is defined in IEC 61400 as the maximum continuous electrical power output which a wind turbine is designed to achieve under normal operating and external conditions. Large commercial wind turbines are generally designed for a lifetime of 20 years and their rated power output takes into account that lifespan. However, it is sometimes necessary to reduce the power output of a wind turbine.

The term "reducing the power output" means, in the context of this invention, that the wind turbine outputs less than the usual amount of power than would be expected at a certain wind speed. For, example, during part load operation, the output power is regulated down from the power curve of the wind turbine. Or, during full load operation, the power output is regulated down from the rated power of the turbine. An operator may want to reduce the power output of a wind turbine so that it can operate as a "spinning reserve", i.e. the turbine produces a smaller amount of power than would be expected for a given wind speed, but when needed, it can deliver more power to a grid in a short time frame by running at its regular power output as defined by the turbine's power curve. A utility power grid is configured so that it responds to instantaneous load variation on the grid. Wind turbines are well suited to act as peak load power supplies for compensating for short-term variations in the grid load because their power output can be regulated very quickly.

Existing power derate modes typically assume a constant rotational speed of the turbine, which has a tendency of causing torque reversals, or gear backlashing in a turbine's gearbox, due to the low main driving moment. A torque reversal is a zero down/up crossing of the main driving moment, i.e. a shift from a positive to a negative moment or vice versa on the drive train. Such a situation is highly undesirable since it can damage the drive train and particularly the gearbox.

The power output of the turbine is determined by the rotational speed of the rotor and the driving moment of the rotor:

Ρ=Μ χ ω

Where:

P is the power output

M is the driving moment on the rotor, torque [Nm], and

ω is the rotational speed of the rotor [rad/s]

In the prior art, if the turbine is derated so that it operates as a spinning reserve, the power output is reduced but the rotor keeps spinning at a given rotational speed. This has the effect that the torque on the drive train is reduced and this is detrimental to the turbine because with no moment on the gearbox, the gearbox may experience gear torque reversals. It is an object of the present invention to provide a method and a system by which a wind turbine can be operated as a spinning reserve.

According to a first aspect of the present invention there is provided a method of reducing the power output of a wind turbine, the wind turbine having a rotor and a generator for producing power, the rotor operating at a first rotational speed, the method comprising the steps of:

receiving a power request signal from a source external to the wind turbine, the power request signal indicating by how much the power output of the wind turbine should be reduced; determining a second rotational speed of the rotor based on the power request signal, the second rotational speed being lower than the first rotational speed; and

reducing the power output of the turbine by reducing the rotational speed of the rotor to the second rotational speed.

Since the output power of the turbine is determined based on an external request, the turbine may remain in operation with a reduced power output. The adaptation of the power output to the external request allows a more continuous power production which can be changed swiftly, and compared with a turbine which is completely stopped, the amendment of the power output facilitates a much quicker control of the turbine.

By controlling both the power output and the rotational speed, the wind turbine can continue in production in periods with a low power demand without introducing an increased wear on the transmission due to torque reversals which may occur when the turbine is operating with a reduced power output.

The advantage of being able to power-derate the turbine is that the owner of the turbine can keep the turbine running as a spinning reserve, which can be brought to full power production in a short period of time, which may be within seconds. Further, the turbine may contain elements which are maintained in better shape when always being active. This is the case e.g. with regards to bearings which may suffer from being held in a fixed position of the rotor- bearing-ring relative to the stator-bearing-ring.

Herein, we shortly refer to the rotor speed as being the rotational speed irrespective on which side of a gearbox the rotational speed is measured.

"Power request" defined herein may specify a signal which is generated externally and received by the wind turbine. The power request specifies a desired level of electrical power. The power request may for example express a number of MW a connected grid is willing to absorb from the turbine.

The rotor may be operated at a first driving moment and the method may further comprise the steps of:

determining a second driving moment of the rotor based on the power request signal; reducing the power output of the turbine by operating the rotor at the second driving moment. In one embodiment of the system, the speed reduction can be delayed such that the power is reduced first, and the speed is reduced second. As an example, the power is reduced sequentially, followed by a speed reduction and then again a power reduction. In yet another embodiment, the reduction in power and speed is carried out simultaneously but with different durations. Generally, the power can be changed instantly while the change in rotational speed typically takes longer due to the inertia of the heavy drive train and rotor system.

The steps of reducing the rotational speed to the second rotational speed and operating at the second driving moment may be carried out simultaneously.

The second rotational speed of the rotor may be determined from a lookup table. The lookup table may be stored in memory in the wind turbine.

The power request signal may indicate that the power output of the wind turbine should be reduced to a value of between 20% of a nominal power and the nominal power.

Preferably certain rotational speeds of the rotor are avoided such that resonant frequencies in the wind turbine are avoided. The rotational speeds to be avoided may be predetermined and stored in memory in the wind turbine.

The rotational speeds to be avoided may be determined online by: detecting a signal indicative of vibration and/or sound of a component of a drive train of the wind turbine at each rotational speed; and if the detected signal is greater than a predetermined value, preventing the rotor from operating at that rotational speed.

According to a second aspect of the present invention there is provided a wind turbine comprising a control unit; the control unit being adapted to carry out the method as described above. The invention will now be described by way of example only with reference to the following figures in which:

Figure 1 illustrates schematically a wind turbine;

Figure 2 illustrates schematically a wind turbine drive train and control system;

Figure 3 illustrates a typical power curve of a prior art wind turbine;

Figure 4 illustrates graphically a derate function with rotational speed and torque as a function of power;

Figure 5 illustrates graphically a further derate function with rotational speed and torque as a function of power; and Figure 6 illustrates minimum power setpoint (MPS) as a function of mean wind speed.

Further scope of applicability of the present invention will become apparent from the following detailed description and specific examples. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Figure 1 shows a typical horizontal axis wind turbine 10. The turbine comprises a tower 1 1 which supports a nacelle 12. The wind turbine 10 comprises a rotor 13 made up of three blades 14 each having a root end 15 mounted on a hub 16. Each blade 14 can pitch about its own pitch axis which extends longitudinally along the span of the blade, and the nacelle 12, together with the rotor 13, can yaw about a vertical axis aligned with the tower 1 1 , as is well known in the art. The blades 14 may be pitched using hydraulic or electric actuators as is known in the art.

Figure 2 shows the drive train of a typical wind turbine 10. The rotor 13 is connected to a gearbox 21 through a low speed shaft 20. A high speed shaft 22 couples the gearbox 21 to the electrical drive train 23. The output of the electrical drive train is to the grid. The electrical drive train comprises a generator which is coupled to an AC-AC converter for connection to the grid.

A control unit 24 provides control signals to components of the wind turbine to regulate the power output from the electrical drive train 23. The control unit 24 comprises a power control unit 25 which is connected to the generator and the converter of the electrical drive train 23. The power control unit 25 provides a power reference to the electrical drive train 23.

The control unit 24 also comprises a pitch control unit 26. The pitch control unit provides a pitch reference to pitch actuators 27 so that the blades are pitched to regulate the power produced by the turbine as is well known in the art.

The power control unit 25 and the pitch control unit 26 control the rotational speed and torque to given set points by adjusting the power reference and the pitch reference. The power produced by the turbine is a result of this control method.

As is well known in the art, a wind turbine is operated in a partial load range (also known as part load) until a rated wind speed is reached, at which point the wind turbine is then operated at rated power output in what is known as the full load region. Figure 3 illustrates a power curve of a typical wind turbine plotting wind speed on the x axis against power on the y axis. The power curve for the wind turbine defines the power output of the wind turbine generator as a function of wind speed. The wind turbine starts to generate power at a cut in wind speed Vmin. The turbine then operates under partial load conditions until the rated wind speed is reached at point Vr. At the rated wind speed at point Vr the rated generator power is reached (also known as the nominal power). The cut in wind speed in a typical wind turbine is 4 m/s and the rated wind speed is 12 m/s. At point Vmax is the cut out wind speed; this is the highest wind speed at which the wind turbine may be operated while delivering power. At wind speeds equal to and above the cut out wind speed the wind turbine is shutdown for safety reasons, in particular to reduce the loads acting on the wind turbine. When the wind turbine 10 is operating under partial load conditions, the blades 14 are pitched at a pitch setpoint angle about their longitudinal axis in order to maximise the energy capture from the oncoming wind. At the same time the rotational speed or torque is controlled by the power control unit 25 by adjusting the power reference. When the wind turbine is operating in the full load region, the pitch of the blades 14 is controlled so that the rotational speed or torque is kept at a desired reference. At the same time the power control unit 25 keeps the power reference at the nominal set point. In the invention, it is desirable to operate the wind turbine 10 as a spinning reserve. When the wind turbine is operating as a spinning reserve, the power produced by the turbine is less than that shown by the power curve in Figure 3, i.e. for a given wind speed, when operating as a spinning reserve, the power output of the turbine is less than the nominal power curve of Figure 3, in both the partial load conditions and above the rated wind speed.

When a grid operator 28 wishes the turbine 10 to act as a spinning reserve, a signal is sent from the grid operator to the control unit 24 with a derate request, that is a request to reduce the power output of the turbine from the nominal power curve. The derate request from the grid operator 28 may be transmitted as a radio signal, or there may be a cable to transmit the signal from the operator to the turbine.

When a turbine is derated so that it acts as a spinning reserve, the rotor keeps spinning and this results in a very low loading on the turbine.

Figure 4 shows a graph of power set points versus the rotational speed of the drive train on the left hand y axis and torque on the drive train on the right hand axis. In a conventional turbine operating at the nominal power P_nom, the rotational speed is given as co_nom; when a request comes in from the grid operator at 28 that the power should be derated to P_derate so that the turbine acts as a spinning reserve, the rotational speed is kept constant and thus the torque in the drive train is decreased. This is illustrated by line 40a which shows that for power setpoints lower than P_nom, the rotational speed is kept constant. As the rotational speed is kept constant, the power output is controlled by the torque setpoint, and as can be seen from dashed line 40b the torque set point is simply reduced from T_nom to T_min

In contrast, the relationship between the power and the rotational speed of the drive train according to the invention is illustrated by line 41 a. According to the invention, when a request comes in from the grid operator at 28 that the power should be derated to P_derate so that the turbine acts as a spinning reserve, the rotational speed is reduced to a setpoint as defined by the line 41 a. As the rotational speed of the drive train is reduced, the torque does not have to be reduced by as much as that compared to a conventional turbine and hence the risk of damage to the drive rain is reduced. The torque reduction according to the invention is illustrated by the dashed line 41 b.

As shown in Figure 4, the power is not reduced below a minimum power P_min which corresponds to a minimum rotational speed co_min. This is a constraint imposed by the electrical generator.

Furthermore, it is undesirable to reduce the drive train speed below co_min because with a decreased rotor speed there is an increased likelihood of aerodynamic stall due to the angle of attack of the blades of the rotor. The setpoints as shown by line 41 a are stored within the control unit 24 in a lookup table, for example. These setpoints are prestored in the control unit before the turbine is erected, or they could be uploaded at a later date if a more desirable range of setpoints becomes known. The line 41 a can be calculated based on a relationship between the blade tip speed and the mean wind speed

Certain rotational speeds of the drive train may cause unwanted vibrations due to resonant frequencies in the turbine, and to avoid such vibrations, the system may include means for avoiding specific rotational speeds. These means include introducing locked speed ranges in order to avoid rotational speeds where the resonance may be expected to occur.

As shown in Figure 5, resonance ranges 43 and 44 are illustrated and the turbine should be prevented from operating within these rotational speed bands. As can be seen, the rotational speed setpoints 42a are chosen such that the ranges 43 and 44 are avoided, thus reducing the risk of damaging the components of the drive train. When the control unit 24 determines a rotational speed which is desired considering the actual power of the generator, and it appears that the determined rotational speed is within a resonance band 43, 44, the control unit is adapted to determine the rotational speed reduction as the closest one of either a higher allowable rotational speed or a lower allowable rotational speed, or the control unit 24 may be adapted to postpone any amendment of the rotational speed until the actual power changes and corresponds to an allowable rotational speed. The torque setpoints are shown by the line 42b in Figure 5. The ranges 43 and 44 may be predetermined and stored within the control unit 24. Alternatively, the means for avoiding a specific rotational speed can comprise vibration and/or sound detection means for measuring online a vibration or noise pattern such that the control unit 24 may determine an unwanted rotational speed based on an actual operating condition for the wind turbine. In this case the control unit 24 may automatically adjust the rotational speed up and down relative to that rotational speed which is desired based on the actual power until an acceptable vibration level and/or noise level is achieved.

Figure 6 is a graph illustrating the Minimum Power Setpoint (MPS) of the turbine versus the mean oncoming wind speed. The MPS is the lowest power output (expressed as a percentage of the nominal power) that the turbine is allowed to generate.

The line 50 illustrates the MPS for a conventional wind turbine. In a conventional wind turbine which operates according to line 50, the MPS is dependent on the mean wind speed. At Vmax, the cut-out wind speed, the MPS is 40% of the nominal power for example. This means that if the grid operator wishes to derate the turbine when the wind speed is at Vmax, the power can only be reduced to 40% of the nominal power and it cannot be reduced further.

In the conventional wind turbine, at point Vx, the MPS is 25% of the nominal power for example, but at wind speeds lower than this the power cannot be reduced further because of the risk of damaging components in the drive train, for example though gear torque reversals.

According to the invention, as both the rotational speed and the torque of the drive train are reduced, it is possible to have a lower MPS (as compared to a conventional turbine) at every wind speed, without increasing the risk of damage to the drive train as a higher torque is maintained on the drive train. This is illustrated by line 51 where the MPS is at 20% of the nominal power. Although the invention has been described with reference to a single turbine being derated, it is also possible for a wind power plant, comprising multiple wind turbines to be derated so that it can act as a spinning reserve. In this instance, a single derate request may be sent to all the turbines in the wind power plant or a derate request may be sent to a substation, which then determines which individual turbines should be derated. It may be useful to derate some turbines over others as some turbines may have experienced greater loading during their lifetime, and so to derate these higher loaded turbines will increase their life.

Claims

1. A method of reducing the power output of a wind turbine, the wind turbine having a rotor and a generator for producing power, the rotor operating at a first rotational speed, the method comprising the steps of:

receiving a power request signal from a source external to the wind turbine, the power request signal indicating by how much the power output of the wind turbine should be reduced;

determining a second rotational speed of the rotor based on the power request signal, the second rotational speed being lower than the first rotational speed; and

2. A method according to claim 1 , wherein the rotor is operated at a first driving moment and the method further comprises the steps of:

determining a second driving moment of the rotor based on the power request signal; reducing the power output of the turbine by operating the rotor at the second driving moment.

3. A method according to claim 2, wherein the steps of reducing the rotational speed to the second rotational speed and operating at the second driving moment are carried out simultaneously.

4. A method according to any one of the preceding claims, wherein the second rotational speed of the rotor is determined from a lookup table.

5. A method according to claim 4, wherein the lookup table is stored in memory in the wind turbine.

6. A method according to any one of the preceding claims, wherein the power request signal indicates that the power output of the wind turbine should be reduced to a value of between 20% of a nominal power and the nominal power.

7. A method according to any one of the preceding claims, wherein certain rotational speeds of the rotor are avoided such that resonant frequencies in the wind turbine are avoided.

8. A method according to claim 7, wherein the rotational speeds to be avoided are predetermined and stored in memory in the wind turbine.

9. A method according to claim 7, wherein the rotational speeds to be avoided are determined online by:

detecting a signal indicative of vibration and/or sound of a component of a drive train of the wind turbine at each rotational speed;

if the detected signal is greater than a predetermined value, preventing the rotor from operating at that rotational speed.

10. A wind turbine comprising a control unit;

the control unit being adapted to carry out the method of any one of claims 1 to 9.