WO2017092762A1

WO2017092762A1 - Control system for wind turbine having multiple rotors

Info

Publication number: WO2017092762A1
Application number: PCT/DK2016/050399
Authority: WO
Inventors: Erik Carl Lehnskov Miranda; Jacob Deleuran GRUNNET
Original assignee: Vestas Wind Systems A/S
Priority date: 2015-11-30
Filing date: 2016-11-28
Publication date: 2017-06-08

Abstract

A wind turbine system comprising a plurality of wind turbine modules mounted to a common support structure is described. The wind turbine system comprises a control system configured to designate at least one wind turbine module as a reference wind turbine module and to designate at least one wind turbine module as a regulative wind turbine module. The control system is further configured to operate the reference wind turbine module according to a predetermined power output level; and control the power output of the wind turbine system by controlling the power output of the regulative wind turbine module based on the power output of the reference wind turbine module. Also described are: a method of controlling such a wind turbine system; and a controller suitable for, and computer program product comprising program code instructions for, implementing such a method.

Description

CONTROL SYSTEM FOR WIND TURBINE

HAVING MULTIPLE ROTORS

Technical field

The invention relates to a control system for a wind turbine system having multiple rotors and more particularly, but not exclusively, to an array-type, or multi-rotor, wind turbine system in which the separate rotors of the system are aligned generally in a common plane. Background to the invention

There exist a number of alternative wind turbine installation designs. One example is the multi-rotor array type wind turbine. For example, EP1483501 B1 discloses a multi-rotor array-type wind turbine installation in which several co-planar rotors are mounted to a common support structure. Such a configuration achieves economies of scale that can be obtained with a very large single rotor turbine, but avoids the associated drawbacks such as high blade mass, scaled up power electronic components and so on. However, although such a co-planer multi-rotor wind turbine has significant advantages, it presents challenges to implement the concept in practice, particularly in how to control the multiple rotors to achieve optimum power production. EP1483501 B1 approaches the control strategy by treating each wind turbine of the system as a separate item that is controlled individually. It is against this background that the invention has been devised.

Summary of the invention

According to a first aspect of the invention, there is provided a wind turbine system comprising: a plurality of wind turbine modules mounted to a common support structure; and a control system. The control system is configured to: designate at least one wind turbine module as a reference wind turbine module and to designate at least one wind turbine module as a regulative wind turbine module; operate the reference wind turbine module according to a predetermined power output level; and control the power output of the wind turbine system by controlling the power output of the regulative wind turbine module based on the power output of the reference wind turbine module. The control system may be configured to determine an available power output of the regulative wind turbine module based on the power output of the reference wind turbine module and controlling the output of the wind turbine system may comprise controlling the power output of the regulative wind turbine module based on the determined available power output of the regulative wind turbine module.

This aspect of the invention provides a wind turbine system that can be accurately de-rated in order to provide reserve power capacity. This may be required for example to provide support for a connected utility grid. Generally speaking, the reference wind turbine module(s) allow the available power output of the wind turbine system to be determined accurately whilst the regulative wind turbine module(s) provide the required reserve power capacity. Accordingly, the wind turbine system is de-rated accurately in accordance with the required reserve power capacity. This prevents excessive de-rating (which results in lost power) and inadequate de-rating (which results in a lack of support capacity). Thus, this aspect of the invention provides a multi-rotor wind turbine with improved profitability and viability.

The plurality of wind turbine modules are mounted to a common support structure in order to form a multi-rotor type turbine. This provides a system where each of the wind turbine modules to a large extent are seeing the same wind field, or at least are grouped together in a manner and proximity where the structural relationship provides the ability to take into account differences in the wind field in a manner which is more precise than it would be for individual turbines erected at large distances apart. Embodiments of the present invention provides a structurally unified wind turbine system which supports accurate derating in accordance with a required power reserve.

The control system may be configured to measure a power output of the reference wind turbine module.

The predetermined power output level of the reference wind turbine module may correspond to maximum power generation.

The control system may be configured to control the power output of the wind turbine system based on a de-rating target. The de-rating target may be expressed as a function of an available power output of the wind turbine system. The wind turbine system may be connected to a utility grid. The de-rating target may be set by a grid system operator. The control system may be configured to reduce the power output of the regulative wind turbine module based on a correction factor. The control system may be configured to determine the correction factor based on the number of regulative wind turbine modules. The control system may be configured to control the power output of the wind turbine system by adjusting a pitch angle of a set of blades of the regulative wind turbine module or by controlling a torque.

The control system may be configured to determine the available power output of the regulative wind turbine module based on a difference between a wind speed in the vicinity of the or each reference wind turbine module and a wind speed in the vicinity of the or each regulative wind turbine module.

The control system may designate at least one of the wind turbine modules as regulative wind turbine modules to reduce loads exerted on the support structure of the wind turbine installation.

The control system may designate one or more of the wind turbine modules as regulative wind turbine modules based on the thrust produced by the respective wind turbine modules.

The control system may designate one or more of the wind turbine modules as regulative wind turbine modules based on identified turbulent airflow in the vicinity of respective wind turbine modules. The control system may designate one or more of the wind turbine modules as regulative wind turbine modules based on the potential power recovery characteristic of respective wind turbine modules.

According to a second aspect of the invention, there is provided a method of controlling a wind turbine system comprising a plurality of wind turbine modules mounted to a common support structure. The method comprises: designating at least one wind turbine module as a reference wind turbine module and at least one wind turbine module as a regulative wind turbine module; operating the reference wind turbine module according to a predetermined power output level; and controlling the power output of the wind turbine system by controlling the power output of the regulative wind turbine module based on the power output of the reference wind turbine module. According to another aspect of the invention, there is provided a controller for a wind turbine installation comprising a plurality of wind turbine modules mounted to a common support structure. The controller comprises a processor, a memory module, and an input/output system, and the memory includes a set of program code instructions which when executed by the processor, implement a method according to the second aspect.

According to a further aspect of the invention, there is provided a computer program product downloadable from a communication network and/or stored on a machine readable medium, comprising program code instructions for implementing a method in accordance with the second aspect.

For the purposes of this disclosure, it is to be understood that the control system described herein can comprise a control unit or computational device having one or more electronic processors. Such a system may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. As used herein, the term "control system" will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein. The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. Brief description of the drawings

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a front view of a multi-rotor wind turbine installation;

Figure 2 is a schematic view of the wind turbine installation of Figure 1 , showing components of the wind turbine installation in more detail;

Figure 3 is a flow diagram showing steps of a process according to an embodiment of the invention that may be executed by a control system of the wind turbine installation of Figure 1 ; and

Figures 4a and 4b are graphs showing the relationship between the wind speed in the vicinity of a wind turbine module and the thrust generated by that wind turbine module. It should be noted that the accompanying figures are schematic representations to illustrate features of the invention and are not intended to be realistic representations or reflect the scale or relative proportions of the various components. The illustrated examples have been simplified for the purposes of clarity and to avoid unnecessary detail obscuring the principle form of the invention. The skilled person will appreciate that many more components may be included in a practical wind turbine system.

Detailed description of embodiments of the invention

With reference to Figure 1 , a multi-rotor wind turbine system or installation 10 is shown including a tower 12 on which is mounted a plurality of wind turbines, or wind turbine modules 14a, 14b. Note that the term 'wind turbine module' is used here in the industry- accepted sense to refer mainly to the generating components of the wind turbine installation 10 and as being separate to the tower 12. The entire wind turbine installation 10 is supported on a foundation, as is usual. The foundation may be a large mass buried in the ground 16, as shown here, or in the form of monopole or 'jacket' like structures used in offshore installations. The wind turbine modules 14 are mounted to the tower 12 by a support arm arrangement 18. Together, the tower 12 and the support arm arrangement 18 can be considered to be a support structure of the wind turbine installation 10.

The support arm arrangement 18 comprises mutually opposed first and second support arms 20a, 20b extending generally horizontally from the tower 12, each support arm 20a, 20b carrying a respective wind turbine module 14a, 14b at its distal end. The support arms 20a, 20b are secured to the tower 12 at their proximal ends by a coupling 22. Alternative configurations are known, for example in which the turbine modules are mounted centrally on the tower 12, one above the other, and where the support arms 20a, 20b are mounted at a different angle with respect to the tower.

The wind turbine modules 14a, 14b can be considered to be substantially identical, each including a rotor 24a, 24b comprising a set of blades 26a, 26b that is rotatably mounted to a nacelle 28a, 28b in the usual way. Thus, each of the wind turbine modules 14a, 14b is able to generate power from the flow of wind that passes through the area swept by the blades 30a, 30b, known as the 'rotor disc'. However, in general wind turbine modules with different specifications may be used, such as different rotor diameter and different generators, for example.

In this example, each wind turbine module 14a, 14b is a three-bladed upwind horizontal-axis wind turbine module 14a, 14b, in which the rotor 24a, 24b is at the front of the nacelle 28a, 28b and positioned to face the wind upstream of the support structure. Other configurations are possible; for example, different numbers of blades may be provided.

In this example, there are two wind turbine modules 14a, 14b; however, the invention is equally applicable to multi-rotor wind turbine installations including more wind turbine modules. By way of example, an additional pair of wind turbine modules 30 is shown in dashed lines mounted to the tower 12, although for the purposes of the following description, reference will only be made to two wind turbine modules 14a, 14b.

Having described the general arrangement of the main components of the wind turbine installation 10, further explanation will now be provided on the system components of the wind turbine installation 10 with reference to Figure 2. Each wind turbine module 14a, 14b is provided with a gearbox 40a, 40b that is driven by the rotor 24a, 24b, and a power generation system including a generator 42a, 42b connected to the gearbox 40a, 40b and which feeds generated power to a converter system 44a, 44b. The power output of the converter system 44 of each wind turbine module 14 is fed to a distribution unit 46 which allows for onward power transmission. In this example, the distribution unit 46 is located on the tower 12, although it is envisaged that other locations would be acceptable. The precise configurations of these aspects of the wind turbine installation 10 are not central to the invention and will not be described in detail. For present purposes, these aspects can be considered to be conventional and, in one embodiment, may be based on a full scale converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, although other architectures would be known to the skilled person.

It should be noted at this point that only a single wind turbine installation 10 is described here, but that several such systems may be grouped together to form a wind power plant, also referred to as a wind farm or 'park'. In this case, a wind power plant control and distribution facility (not shown) may be provided to coordinate and distribute the power outputs from the individual wind turbine installations to the wider grid. The wind turbine installation 10 also includes a control means configured to carry out a control process, described in more detail later, to ensure that the power output of the wind turbine installation 10 is appropriately controlled. In this embodiment, the control system includes a centralised control element and a localised control element. The skilled person will appreciate that many more components may be included in a practical wind turbine control system, as appropriate.

The centralised control element serves a supervisory function in order to provide a coordinated control strategy. In this example, the centralised control element is provided by a central control module 48 in the form of a computing device incorporated in the distribution unit 46, having a suitable processor, memory module and input/output system. The central control module 48 is configured to monitor and control the operation of the wind turbine installation 10 as a whole in order to achieve a supervisory control objective.

Since the wind turbine installation 10 includes a plurality of wind turbine modules 14a, 14b, each of which is operable to generate electrical power as the rotor 24a, 24b is driven by the wind, the control system includes a localised control element that is operable to monitor the operation of respective ones of the plurality of wind turbine modules 14a, 14b and to issue commands thereto to achieve a set of local control objectives, as will be explained. In this embodiment, the localised control element is provided in the form of a plurality of local control modules 50a, 50b that are embodied as respective computing devices each of which is dedicated to an associated wind turbine module 14a, 14b and comprises a suitable processor, memory module and input/output system. In other embodiments, the local control element of the control system may be provided as a single unit integrated with the centralised control element and may be located inside the tower 12, for example.

Each local control module 50a, 50b is configured to control the power output of the associated wind turbine module 14a, 14b. The skilled person will be familiar with systems and processes for controlling the power output of a wind turbine module 14a, 14b, and so a detailed explanation will not be provided here. Briefly, in this embodiment each local control module 50a, 50b controls the power output of the associated wind turbine module 14a, 14b by controlling the pitch of the blades 26a, 26b through a pitch control system 52a, 52b which adjusts the angle of attack of the blades 26a, 26b relative to the wind. In alternative embodiments, each local control module 50a, 50b may be operable to control the power output of the associated wind turbine module 14a, 14b by controlling the converter system 44a, 44b to influence the torque or power exerted on the rotor 24a, 24b by the generator 42 and/or by controlling the rotor speed.

The local control modules 50a, 50b receive supervisory control commands from the central control module 48. In addition, each local control module 50a, 50b also receives data signals from various components within the associated wind turbine module 14a, 14b. For example, each local control module 50a, 50b receives a data signal from the converter system 44a, 44b of the associated wind turbine module 14a, 14b indicating the power output of that wind turbine module 14a, 14b.

These data signals are utilised by the local control modules 50a, 50b to perform monitoring and control processes of the associated wind turbine modules 14a, 14b. In addition, the local control modules 50a, 50b output these data signals to the central control module 48 for monitoring and control of the wind turbine installation 10 as a whole. For example, the central control module 48 may calculate the overall power output of the wind turbine installation 10 based on the power output signals of each wind turbine module 14a, 14b. It is necessary to control the power output of the wind turbine installation 10 since the wind turbine installation 10 may be connected to a grid and therefore required to comply with grid system requirements set by a grid system operator. In particular, the wind turbine installation 10 may be required to maintain reserve power capacity in order to be able to support the grid when required by providing additional active power. That is to say, the wind turbine installation 10 may be required to operate in a 'de-rated' or 'curtailed' mode. This allows the wind turbine installation 10 to support the grid with additional active power when necessary, for example if the grid frequency drops below its nominal value.

The required reserve power capacity is typically expressed as a proportion, percentage or fraction of the 'available power output' of the wind turbine installation 10, the available power output being the maximum 'continuous' power output that the wind turbine installation 10 is capable of producing under the prevailing operating conditions, such as wind speed. Note that the available power output may be equal to the rated power output of the wind turbine, for example if the wind speed if above rated speed, but this is not necessarily the case, for example if the wind speed is lower than rated wind speed. Note also that the reserve power capacity is the proportion of the available power output that the wind turbine installation 10 must maintain in reserve to provide support for the grid. In other embodiments, the required reserve power capacity may be expressed as an absolute value.

To determine the appropriate de-rated power output of the wind turbine installation 10, the available power output of the wind turbine installation 10 must first be determined. It is important that this is done accurately, to avoid de-rating the wind turbine installation 10 too much (resulting in lost power), or too little (resulting in lack of support capacity).

In single rotor installations, the available power output of the installation is estimated from the measured wind speed or the calculated wind speed based on wind turbine parameters such as the coefficient of power value and the pitch angle of the blades and also the estimated losses and rpm of the rotor. However, it is almost impossible to determine the available output power of a wind turbine installation accurately in this way due to the indirect nature of the calculation, which is therefore sensitive to uncertainty in the measured wind speed and errors in the wind turbine parameters. However, the present invention recognises that a more accurate determination of the available power output is possible in the context of a multi-rotor wind turbine installation 10 such as that shown in Figures 1 and 2, and embodiments of the present invention provide systems and methods for accurately de-rating a wind turbine installation 10. An embodiment of the invention will now be described with continued reference to Figure 2. As already described, the wind turbine installation 10 comprises a central control module 48 and two wind turbine modules 14a, 14b, each including a local control module 50a, 50b. In this embodiment of the invention, one of the wind turbine modules is designated as a reference wind turbine module 14a and the other wind turbine module is designated as a regulative wind turbine module 14b. The reference wind turbine module 14a and the regulative wind turbine module 14b have respective local control modules referred to as the 'reference control module' 50a and the 'regulative control module' 50b. The skilled person will appreciate that since the wind turbine modules 14a, 14b are substantially identical, either module 14a, 14b can be designated the reference wind turbine module 14a or the regulative wind turbine module 14b.

The regulative wind turbine module 14b is operated in a de-rated mode to provide the reserve power capacity required by the grid system operator. The reference wind turbine module 14a is operated to produce a predetermined power output level; specifically, the maximum available power output. This enables accurate determination of the available power output of the regulative wind turbine module 14b.

An example of a process 100 according to an embodiment of the invention that may be performed by the control system of the wind turbine installation 10 in order to accurately derate the wind turbine installation 10 will now be described with reference to Figure 3. The skilled person will appreciate that various steps of the process may be carried out within the local control modules 50a, 50b or the central control module 48 as is appropriate.

The control system sets or determines, at step 102, the available power output, a₂ , of the regulative wind turbine module 14b based on the measured power output of the reference wind turbine module 14a. Specifically, the reference control module 50a receives a data signal from the associated converter system 44a that indicates the power output of the reference wind turbine module 14a. The reference control module 50a outputs this signal to the central control module 48. Based on the value of this signal, the central control module 48 determines the available power output, a₂ , of the regulative wind turbine module 14b.

In determining the available power output, a₂ , of the regulative wind turbine module 14b, the central control module 48 may implement various forms of calculation processes depending on the specific circumstances. According to this example, it is assumed that the available power output, a₂ , of the regulative wind turbine module 14b is equal to the measured power output of the reference wind turbine 14a. Tests have shown that this relatively simple model gives good results in practice and the determined available power output of the regulative wind turbine 14b is sufficiently accurate in many circumstances using this approach. In other embodiments, it is envisaged that other wind turbine installation parameters could be used to determine the available power output, a₂ , of the regulative wind turbine module 14b. For example, the wind speed difference between the rotors of the two wind turbine modules 14a, 14b may be taken into account. The difference between the wind speed at the reference wind turbine module 14a and the wind speed at the regulative wind turbine module 14b may be measured when the wind turbine installation 10 is not de-rated, for example during commissioning. This difference can be assumed to be constant or to vary according to a model during subsequent operation of the wind turbine installation 10 to enable accurate determination of the available power output, a₂ , of the regulative wind turbine 14b. Alternatively, the wind speed difference between the turbine modules 14a, 14b may be continuously monitored during operation of the wind turbine installation 10.

In further embodiments, the central control module 48 may determine the available power output, a₂ , additionally based on the effect that de-rating the regulative wind turbine module 14b has on the wind conditions in the vicinity of the reference wind turbine module 14a. This effect can be modelled using computational fluid dynamics calculation techniques. Alternatively, this effect can be determined based on measurements of the power output of the reference wind turbine module 14a that are taken over a relatively long period of operation of the wind turbine installation 10. Since they are taken over a period of operation, these measurements will correspond to periods of time when the regulative turbine module 14b is de-rated to various levels. Therefore the power output of the reference wind turbine 14a as a function of the de-rating of the regulative turbine module 14b can be determined. It will be appreciated that several other parameters, for example pitch angle and generator speed, also influence the available power output, a₂ , of the regulative wind turbine 14b. The skilled person will be able to envisage appropriate calculation methods to account for these parameters when determining the available power output of the regulative wind turbine module 14b.

The control system receives, at step 104, a de-rating or curtailment request signal, R, from the grid system operator. As described previously, the de-rating request signal, R, indicates the amount of reserve power capacity that the wind turbine installation 10 must maintain. As such, the re-rating request signal defines a de-rating target for the wind turbine installation 10. Typically, the de-rating request signal, R, is expressed as a percentage. The de-rating request signal indicates the percentage of the available power output of the wind turbine installation 10 at that moment in time that must be maintained in reserve. As mentioned previously, in other embodiments the grid system operator may provide an absolute de- rating command. That is, the de-rating request signal, R, indicates the required reduction in actual power output of the wind turbine installation 10 compared to available power output of the wind turbine installation 10. It is noted that the grid system operator may not know what the available power output is, in which case reduction targets are determined, for example, according to standard best practice, or with reference to an average performance of a range of generating systems connected to the grid.

If several wind turbine installations 10 are grouped together in a wind power plant, the grid system operator may provide the power plant control facility with a de-rating request for the wind power plant as a whole. The power plant control facility then issues the central control module 48 of each wind turbine installation 10 with an appropriate individual de-rating request signal for the associated wind turbine installation 10.

The control system determines, at step 106, a correction function, χ, that must be applied to the de-rating request signal, R, to account for the fact that the reference wind turbine module 14a is not de-rated. The correction function, χ, is based on the proportion of the available power output of the wind turbine installation 10 that is attributable to the regulative wind turbine module 14b. In this example where there are only two wind turbine modules 14a, 14b, half of the available power output of the wind turbine installation 10 is attributable to the regulative wind turbine 14b and so the correction function acts to double the power reduction in the regulative wind turbine 14b so that the overall power reduction is as requested. That is to say, the proportion of its available power output that the regulative wind turbine module 14b must maintain in reserve is double the proportion indicated by the de-rating request signal, R. Thus, the regulative wind turbine module 14b maintains the required reserve power capacity for the entire wind turbine installation 10.

For installations having more than two wind turbine modules, the correction function is calculated accordingly so that the overall reserve power of the installation is provided for by the de-rating of the regulative wind turbine modules. It is noted that the correction function may be calculated dynamically as part of the de-rating process 100, allowing the correction function to be responsive to instantaneous wind conditions, for example. Alternatively, the correction function may be defined outside of the process, for example during commissioning, if it is assumed to be generally constant and based entirely on the number of wind turbine modules present. The above-described steps 102, 104, 106 may be executed simultaneously or sequentially in any appropriate order.

The control system calculates, at step 108, the power output, p₂ , that the regulative wind turbine module 14b is required to produce in order to maintain the power reserve capacity demanded by the grid system operator. The required power output of the regulative wind turbine module 14b is calculated based on the determined available power, a₂ , of the regulative wind turbine module 14b the external de-rate request signal, R, and the correction function, χ. In this example, the required power output of the regulative wind turbine module 14b is given by: p₂ = a₂ (l - xR)

The control system issues, at step 100, a local de-rate command to the regulative control module 50b indicating the required regulative power output. The regulative control module 50b adjusts the power output of the regulative wind turbine module 14b based on the local de-rate command by adjusting the pitch of the blades 26b. In this example, the regulative control module 50b implements proportional-integral (PI) control of the blade pitch angle to comply with the local de-rate command. However, the skilled person will be aware of alternative control strategies that may be suitably employed.

The power output of the regulative wind turbine module 14b may be suitably controlled by adjusting the rotor speed, for example by altering the blade pitch angle. Alternatively, the regulative control module 50b may implement full load control of the regulative wind turbine module 14b. In this case, the power output of the regulative wind turbine module 14b may be controlled by adjusting the converter system 44b to influence the torque exerted on the rotor 24b by the generator 42b. In some embodiments, these two techniques may be used in combination in order to appropriately control the power output of the regulative wind turbine module 14b. As mentioned above, the present invention is also applicable to multi-rotor wind turbine installations including three or more wind turbine modules and the skilled person will appreciate that the above-described control strategies and processes can be adapted for such installations. For example, in the context of a wind turbine installation 10 comprising four wind turbine modules 14a, 14b, 30a, 30b, as shown partially in dashed lines in Figure 1 , two of the wind turbine modules 14a, 14b, 30a, 30b may be designated as reference wind turbine modules 14a, 30a and two as regulative wind turbine modules 14b, 30b. Similarly to the control process 100 described above, the reference wind turbine modules 14a, 30a operate at maximum power generation to enable accurate estimation of the available power outputs of the regulative wind turbine modules 14b, 30b. The regulative wind turbine modules 14b, 30b are de-rated to maintain the required reserve power capacity.

The skilled person will appreciate the various ways that the general control strategy may be applied to a four-rotor wind turbine installation 10. In an embodiment, each reference wind turbine module 14a, 30a provides an indication of the available power output of a corresponding regulative wind turbine module 14b, 30b. In another embodiment, the power outputs of the reference wind turbines 14a, 30a are averaged to determine the available power output of the regulative wind turbine modules 14b, 30b. In some embodiments, the regulative wind turbine modules 14b, 30b are de-rated equally, with identical local de-rate commands. In alternative embodiments, individual and specific local de-rate commands may be issued to each regulative wind turbine module 14b, 30b.

The selection of the wind turbine modules that are to be designated as regulative wind turbine modules can be can be influenced by various factors. For example, the action of derating a wind turbine module 14a, 14b, 30a, 30b reduces the thrust generated by its rotor 24a, 24b, 54a, 54b. Therefore, if the upper wind turbine modules 14a, 14b of an installation 10 such as the variant with four turbine modules shown in Figure 1 are designated as the regulative wind turbine modules 14a, 14b, the moment on the tower base due to the thrust generated by the rotors 24a, 24b, 54a, 54b can be minimised.

The difference in wind speed in the vicinity of the upper and lower rotors 14a, 14b, 30a, 30b of such an installation 10 can also be taken into account when selecting regulative wind turbine modules, as will now be described with reference to Figures 4a and 4b.

Figure 4a shows a graph 200 of wind speed against the thrust generated by a wind turbine module rotor operating at maximum power generation. It is noted that the graph is a curve, referred to hereafter as a 'thrust curve' 201 , and shows that the generated thrust peaks at a certain wind speed 202, above which the thrust reduces with increasing wind speed.

The graph 200 includes two vertical lines 204, 206 indicating local wind speeds: the line 204 shown to the left in Figure 4a represents wind speed 208 in the vicinity of the lower wind turbine modules 30a, 30b; and the line shown to the right 206 and generally coinciding with the peak of the thrust curve represents the wind speed 202 around the upper wind turbine modules 14a, 14b. As shown in Figure 4a, and as is typically the case as wind speed generally increases with height above ground, the wind speed 208 in the vicinity of the lower wind turbine modules 30a, 30b is lower than the wind speed 202 in the vicinity of the upper wind turbine modules 14a, 14b. As a result, the thrust 210 generated by the upper wind turbine modules 14a, 14b is greater than the thrust 212 generated by the lower wind turbine modules 30a, 30b.

As already noted, de-rating a wind turbine module reduces the generated thrust. Moreover, the reduction in thrust is proportional to the thrust generated by that wind turbine module operating at maximum power generation at the relevant wind speed as indicated on the graph 200 Therefore, in the circumstances shown in Figure 4a where the wind speed is higher around the upper wind turbines 14a, 14b, it is advantageous to designate the upper wind turbine modules 14a, 14b as the regulative wind turbines. Thus, the upper wind turbine modules 14a, 14b are de-rated and the loads on the installation support structure due to the thrusts generated by the rotors 24a, 24b, 54a, 54b of the wind turbine modules 14a, 14b, 30a, 30b are minimised.

However, due to the nature of the thrust curve 201 , in some circumstances it is advantageous to designate the lower wind turbine modules 30a, 30b as the regulative wind turbines, as will now be explained with reference to Figure 4b. Similarly to Figure 4a, Figure 4b shows a graph 300 of wind speed against the thrust generated by a wind turbine module rotor operating at maximum power generation. As in Figure 4a, Figure 4b includes two vertical lines 302, 304: one 302, shown to the right, indicating wind speed 306 in the vicinity of the upper wind turbine modules 14a, 14b; and a second 304, shown to the left and generally coinciding with the peak of the thrust curve 201 , representing wind speed 308 around the lower wind turbine modules 30a, 30b.

In Figure 4b, both wind speeds 306, 308 are higher than their counterparts 202, 208 shown in Figure 4a. As is usual, the wind speed 306 in the vicinity of the upper wind turbine modules 14a, 14b is higher than the wind speed 308 in the vicinity of the lower wind turbine modules 30a, 30b. However, due to the shape of the thrust curve 201 , in these circumstances the thrust 310 generated by the upper wind turbine modules 14a, 14b is less than the thrust 312 generated by the lower wind turbine modules 30a, 30b. Thus, it is advantageous to de-rate the lower wind turbine modules 30a, 30b; that is, to designate the lower wind turbine modules 30a, 30b as the regulative wind turbine modules, as this will yield a greater reduction in thrust than de-rating the upper wind turbine modules 14a, 14b. As a further alternative, it may be desirable to select the regulative wind turbine modules based on air turbulence intensity in the vicinity of each of the wind turbine modules 14a, 14b, 30a, 30b. If the turbulence intensity in the vicinity of the lower wind turbine modules 30a, 30b is greater than the turbulence in the vicinity of the upper wind turbine modules 14a, 14b then it may be advantageous to designate the lower wind turbine modules 30a, 30b as the regulative wind turbine modules in order to reduce extreme transient thrust loads experienced by the installation support structure.

Another consideration in selecting regulative wind turbine modules for de-rating is the ability of each wind turbine module to return to available power output rapidly when required. Wind turbine modules experiencing high wind speeds return more relatively quickly to available power output due to the fact that the power available from the wind is proportional to the third power of wind speed. So, it may be desirable Thus, it may be advantageous to designate the wind turbine modules experiencing the highest wind speeds (typically, the upper wind turbine modules 14a, 14b) as the regulative wind turbine modules to maximise the responsiveness of the wind turbine installation 10 to changing power output demands. Expressed another way, the regulative wind turbines may be selected as those having advantageous potential power recovery characteristics.

The above-described strategies for selecting regulative wind turbine modules may be used in isolation or in combination as appropriate. The control system may incorporate a fuzzy logic controller that is operable to switch between selection strategies, for example based on various parameters such as the instantaneous wind speed or grid requirements. Thus, the most appropriate selection strategy is implemented at any moment during operation of the wind turbine installation 10.

The skilled person will be able to envisage many other modifications that may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

Claims

A wind turbine system comprising: a plurality of wind turbine modules mounted to a common support structure; and a control system configured to:

designate at least one wind turbine module as a reference wind turbine module and to designate at least one wind turbine module as a regulative wind turbine module;

operate the reference wind turbine module according to a predetermined power output level; and

control the power output of the wind turbine system by controlling the power output of the regulative wind turbine module based on the power output of the reference wind turbine module.

The wind turbine system of Claim 1 , wherein the control system is configured to measure a power output of the reference wind turbine module.

The wind turbine system of Claim 1 or Claim 2, wherein the control system is configured to control the power output of the wind turbine system based on a derating target.

The wind turbine system of any preceding claim, wherein the control system is configured to reduce the power output of the regulative wind turbine module based on a correction factor.

The wind turbine system of Claim 4, wherein the control system is configured to determine the correction factor based on the number of regulative wind turbine modules.

The wind turbine system of any preceding claim, wherein the control system is configured to control the power output of the wind turbine system by adjusting a pitch angle of a set of blades of the regulative wind turbine module or by controlling a torque.

The wind turbine system of any preceding claim, wherein the control system is configured to determine the available power output of the regulative wind turbine module based on a difference between a wind speed in the vicinity of the or each reference wind turbine module and a wind speed in the vicinity of the or each regulative wind turbine module.

8. The wind turbine system of any preceding claim, wherein the control system designates at least one of the wind turbine modules as regulative wind turbine modules to reduce loads exerted on the support structure of the wind turbine installation.

9. The wind turbine system of Claim 8 wherein the control system designates one or more of the wind turbine modules as regulative wind turbine modules based on the thrust produced by the respective wind turbine modules.

10. The wind turbine system of Claim 8 wherein the control system designates one or more of the wind turbine modules as regulative wind turbine modules based on identified turbulent airflow in the vicinity of respective wind turbine modules.

11. The wind turbine system of Claim 8 wherein the control system designates one or more of the wind turbine modules as regulative wind turbine modules based on the potential power recovery characteristic of respective wind turbine modules.

12. A method of controlling a wind turbine system comprising a plurality of wind turbine modules mounted to a common support structure, the method comprising:

designating at least one wind turbine module as a reference wind turbine module and at least one wind turbine module as a regulative wind turbine module; operating the reference wind turbine module according to a predetermined power output level; and

controlling the power output of the wind turbine system by controlling the power output of the regulative wind turbine module based on the power output of the reference wind turbine module.

13. A controller for a wind turbine system comprising a plurality of wind turbine modules mounted to a common support structure, wherein the controller comprises a processor, a memory module, and an input/output system, and wherein the memory includes a set of program code instructions which when executed by the processor, implement a method according to Claim 12.

14. A computer program product downloadable from a communication network and/or stored on a machine readable medium, comprising program code instructions for implementing a method in accordance with Claim 12.