Multi-rotor turbine arranged for coordinated rotational speeds

A wind turbine system is described comprising a plurality of wind turbine modules, each including a rotor, mounted to a support structure including a tower. In use, each rotor has an associated rotating unbalance that defines an unbalance vector. The wind turbine system includes control means configured to coordinate the rotational speeds of the plurality of rotors to attenuate oscillations of the support structure caused by the rotating unbalance of the rotors. Also described is a method of controlling such a wind turbine system. The method comprises coordinating the rotational speeds of the plurality of rotors to attenuate oscillations of the support structure caused by the rotating unbalance of the 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 may be 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. EP1483501B1 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 support structure, the support structure including a tower. Each of the plurality of wind turbine modules includes a rotor and, in use, each rotor has an associated rotating unbalance that defines an unbalance vector. The wind turbine system comprises control means configured to coordinate the rotational speeds of the plurality of rotors to attenuate oscillations of the support structure caused by the rotating unbalance of the rotors.

This aspect of the invention provides a wind turbine system wherein the forces exerted on the support structure as a result of the rotating unbalances of the rotors are controlled using the rotor speed control function of the wind turbine modules. This allows oscillations of the support structure to be attenuated without limiting the freedom of operation of the wind turbine system, advantageously enabling operation of the wind turbine modules at critical rotation frequencies, close to the resonant frequency of the support structure.

In embodiments of the invention, there is a general aim to avoid the unbalance vectors of the rotors of the wind turbine system from rotating in phase such that the resulting cyclic forces coincide.

For example, the control means may be configured to coordinate the rotational speeds of a first rotor and a second rotor such that the angle between the unbalance vector of the first rotor and the unbalance vector of the second rotor varies. To achieve this, the control means may be configured to control the rotational speed of the first rotor to be different from the rotational speed of a second rotor. In particular, the control means may be configured to control the rotational speed of the first rotor such that the rotational speed of the first rotor is less than 98 percent of the rotational speed of the second rotor. Alternatively or additionally, the control means may be configured to control the rotational speed of the first rotor such that the rotational speed of the first rotor is more than 90 percent of the rotational speed of the second rotor. The control means may also be configured to control the rotational speed of the second rotor. The control means may be configured to control the rotational speed of the first rotor to be different from the rotational speed of at least one further rotor.

According to other embodiments of this aspect of the invention, the control means may be configured to coordinate the rotational speeds of a first rotor and a second rotor to achieve a target angular difference between an azimuth position of the first rotor and an azimuth position of a second rotor. The wind turbine system may comprise a sensor configured to detect vibrations within the support structure. In such cases, the control means may be configured to vary the target angular difference in dependence on the magnitude of the detected vibrations. The control system may be configured to determine the unbalance vector of the first rotor and the unbalance vector of the second rotor and the target angular difference may correspond to a non-zero angle between the first and second unbalance vectors. In such cases, the target angular difference may correspond to an angle between the first and second unbalance vectors of between 90 degrees and 240 degrees. More particularly, the target angular difference may correspond to an angle between the first and second unbalance vectors of 180 degrees.

According to a second aspect of the invention, there is provided a method of controlling a wind turbine system. The wind turbine system comprises a plurality of wind turbine modules mounted to a support structure including a tower. Each of the plurality of wind turbine modules includes a rotor. In use, each rotor has an associated rotating unbalance that defines an unbalance vector. The method comprises coordinating the rotational speeds of the plurality of rotors to attenuate oscillations of the support structure caused by the rotating unbalance of the rotors.

The method may comprise coordinating the rotational speeds of a first rotor and second rotor such that the angle between the unbalance vector of the first rotor and the unbalance vector of the second rotor is continuously varying. The method may comprise controlling the rotational speed of the first rotor to be different from the rotational speed of the second rotor.

The method may comprise coordinating the rotational speeds of a first rotor and a second rotor to achieve a target angular difference between an azimuth position of the first rotor and an azimuth position of a second rotor. In such cases, the method may comprise detecting vibrations within the support structure and varying the target angular difference in dependence on the magnitude of the detected vibrations. Alternatively or additionally, the method may comprise determining the unbalance vector of the first rotor and the unbalance vector of the second rotor and setting the target angular difference to correspond to a 180 degree angle between the first and second unbalance vectors.

According to a further aspect of the invention, there is provided a controller for a wind turbine system comprising a plurality of wind turbine modules mounted to a support structure including a tower. Each of the plurality of wind turbine modules includes a rotor and, in use, each rotor has an associated rotating unbalance that defines an unbalance vector. 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 previously-described aspect.

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

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.

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 toFIG. 1, a multi-rotor wind turbine installation10is shown including a tower12on which is mounted a plurality of wind turbines, or wind turbine modules14. Note that the term ‘wind turbine’ is used here in the industry-accepted sense to refer mainly to the generating components of the wind turbine installation10and as being separate to the tower12.

The entire wind turbine installation10is supported on a foundation, as is usual. As examples, the foundation may be a large mass buried in the ground18, as shown here, or in the form of monopole or ‘jacket’ like structure.

The wind turbine modules14are mounted to the tower12by a support arm arrangement20. Together, the tower12and the support arm arrangement20can be considered to be a support structure22of the wind turbine installation10.

The support arm arrangement20comprises mutually opposed first and second support arms24extending generally horizontally from the tower12, each support arm24carrying a respective wind turbine module14at its distal end. The support arms24are secured to the tower12at their proximal ends by a coupling25. Alternative configurations are known, for example in which the turbine modules14are mounted centrally on the tower12, one above the other, and where the support arm structures20are mounted at a different angle with respect to the tower12.

The wind turbine modules14can be considered to be substantially identical, each including a rotor26comprising a set of blades28that is rotatably mounted to a nacelle30in the usual way. Thus, each of the wind turbine modules14is able to generate power from the flow of wind that passes through the area swept by the blades28, known as the ‘rotor disc’32. However, in general wind turbine modules14with different specifications may be used, such as different rotor diameter and different generators.

In this example, each wind turbine module14is a three-bladed upwind horizontal-axis wind turbine module14, in which the rotor26is at the front of the nacelle30and positioned to face the wind upstream of the support structure22. Other configurations are possible; for example, different numbers of blades may be provided.

In this example, there are two wind turbine modules14; 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 modules34is shown in dashed lines mounted to the tower12, although for the purposes of this description, reference will only be made to two wind turbine modules14.

Further explanation will now be provided on the system components of the wind turbine installation10with reference toFIG. 2.

Each wind turbine module14is provided with a gearbox40that is driven by the rotor26, and a power generation system including a generator42connected to the gearbox40and which feeds generated power to a converter system44. The power output of the converter system44of each wind turbine module14is fed to a distribution unit46which allows for onward power transmission. In this example, the distribution unit46is located inside the tower12, although it is envisaged that other locations would be acceptable. The precise configurations of these aspects of the wind turbine installation10are 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.

The wind turbine installation10also includes a control means configured to carry out a control process, described in more detail later, to ensure that excitation of the vibration modes of the support structure22is reduced, eliminated, or controlled to an acceptable level. In this embodiment, the control system includes a centralised control element and a localised control element.

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 module48in the form of a computing device incorporated in the distribution unit46, having a suitable processor, memory module and input/output system. The central control module48is configured to monitor the operation of the wind turbine installation10as a whole in order to achieve a supervisory control objective.

The localised control element is operable to monitor and control respective ones of the plurality of wind turbine modules14to achieve a set of local control objectives. In this embodiment, the localised control element is provided in the form of a plurality of local control modules50that are embodied as respective computing devices each of which is dedicated to an associated wind turbine module14and 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 tower12, for example.

Each local control module50incorporates a speed controller52which is configured to control the rotor speed of the associated turbine module14. To achieve this, the speed controller52is operable to control the converter system44to influence the torque exerted on the rotor26by the generator42, and also to control the pitch of the blades28through a pitch control system53which adjusts the angle of attack of the blades28relative to the wind. The skilled person will be familiar with such systems for controlling the rotor speed of a wind turbine module14, so a detailed explanation will not be provided here.

The input/output system allows the local control modules50to receive supervisory control commands from the central control module48. In addition, the local control modules50also receive inputs from various sensors within the wind turbine installation10. For example, each wind turbine module14includes a rotor speed sensor54which measures the rotational speed of the rotor26and provides this data to the associated local control module50. Each local control module50also receives an input from an azimuth angle sensor56which measures the position of the rotor26of the associated wind turbine module14in the circumferential direction of the rotation axis between 0 and 360 degrees. Typically, the ‘zero degree’ position for a given rotor will be considered to be when a selected one of the blades28is in a vertically upwards position and the azimuth position is measured in the direction of rotation.

The skilled person will appreciate that many more components may be included in a practical wind turbine control system, as appropriate.

Reference will be made in the following description to a single rotor26. It should be understood, however, that the description applies equally to both rotors26.

Due to manufacturing tolerances, the centre of mass of the rotor26may not exactly coincide with the rotation axis, that is to say, the rotor26may have an associated unbalance. In practice, all wind turbine rotors exhibit some unbalance due to deviations in the profile and mass properties of the rotor blades28, as well as differences in the blade pitch when the rotor is assembled. In addition, a change in rotor balance may occur over a period of use of the rotor26, for example due to ice accretion or contamination which is not evenly distributed on the blades28.

As the rotor26rotates, the centre of mass rotates around the rotation axis and the rotor26therefore exhibits a rotating unbalance.

The rotating unbalance can be described by an ‘unbalance vector’ which is defined to be the vector connecting the rotation axis of the rotor26to the centre of mass of the rotor26. Since the direction of the unbalance vector corresponds to the position of the centre of mass, any reference in the following description to the direction of the unbalance vector of the rotor26can equally be understood to refer to the position of the centre of mass of the rotor26.

The direction of an unbalance vector can conveniently be expressed as an angle between zero and 360 degrees, for example corresponding to the angle (in the direction of rotation of the rotor26) between the vertical line and the direction of the unbalance vector. Since the unbalance vector rotates with the rotor26, the angle of the unbalance vector is in fixed angular relation to the azimuth position of the rotor26.

As the rotor26rotates, the support structure22experiences a cyclic force in the direction of the unbalance vector. If the rotational speed of the rotor26corresponds to a critical frequency, for example close to a resonant frequency of the support structure22, then the cyclic force due to the rotating unbalance may excite a resonant vibration mode of the support structure22. This may cause oscillations of the support structure22to increase, leading to fatigue which may impact the service life of the installation as a whole. To avoid this, a single rotor installation is typically not operated at its critical rotation speeds. However, this limits the freedom of operation of the wind turbine and can cause loss of power generation efficiency.

As will be described, the present invention identifies a solution to this problem in the context of multi-rotor wind turbine installations that enables operation at critical rotor speeds.

In embodiments of the invention, the rotor speed control function of the wind turbine modules14may be used to control the forces exerted on the support structure22as a result of the rotating unbalances. Thus, excitations of structural resonances of the system are controlled and oscillations of the support structure22are prevented from increasing to unacceptable levels of magnitude. That is to say, oscillations of the support structure22caused by the rotating unbalances of the rotors26are attenuated.

There are at least two control strategies that may be employed to achieve this, as will now be described by way of example. In each strategy, there is a general aim to avoid the centres of mass of the rotors26of the installation10from rotating in phase such that the resulting cyclic forces coincide, thereby amplifying each other and increasing resonance around the critical frequency.

In one strategy, the rotor speeds are controlled to maintain the unbalance vectors out of phase with one another such that the rotating unbalances of the rotors26apply opposing, i.e. counteracting, forces on the support structure22, thereby neutralising one another to some extent and reducing the net cyclical excitation force on the support structure22.

In another strategy, the wind turbine modules14are controlled so as to have differing rotor speeds, such that the rotating unbalances of the rotors26only exert a force in the same direction momentarily when the rotor unbalances are briefly in phase; that is, when the unbalance vectors are in the same direction. The forces therefore do not generally reinforce one another. The skilled person will be able to envisage other rotor speed control strategies that may be employed in order to attenuate oscillations of the support structure22caused by the rotating unbalance of the rotors26.

An example of a rotor speed control process relating to the first strategy mentioned above will now be described with reference toFIGS. 3 and 4.

FIG. 3shows a schematic representation of the rotors26of the wind turbine installation10. The unbalance vectors58of the rotors26are shown, extending between the rotation axis60and centre of mass62of the respective rotors26. The turbine modules14are being controlled such that the rotor speeds of the two wind turbine modules14are the same, but the angle between the unbalance vectors58is 180 degrees.

In other embodiments, the wind turbine modules14may be controlled such that there is a different fixed angle between the unbalance vectors58. In such embodiments, the forces exerted by the rotating unbalances are not strictly opposing (that is to say, they are not diametrically opposed); however, the forces are not in the same direction. Provided the angle between the unbalance vectors58is at least 90 degrees, the forces advantageously cancel each other to some extent.

An example of a process100that may be performed by the control system of the wind turbine installation10in order to achieve this control strategy will now be described with reference toFIG. 4. The skilled person will appreciate that various steps of the process may be carried out within the local control modules50or the central control module48as is appropriate.

Firstly, the control system determines, at step102, if an initiation requirement that triggers the process to begin is satisfied. In some embodiments of the invention, the initiation requirement may be that the wind turbine installation10is operational; in such cases, the process100executes continuously during operation of the wind turbine installation10. In other embodiments, the process100may be initiated when a sensing means such as a vibration sensor, or force sensor provided to the wind turbine installation10detects that a magnitude of vibration within the support structure22exceeds a predetermined threshold, indicating that a vibration mode is being excited. Alternatively, the process100may be initiated when the speed controller52issues a command that requires the rotor speed to correspond to a critical frequency of the support structure22.

Following initiation of the process100, the control system makes two comparisons between the two turbine modules14: a comparison of the unbalance vectors58of the rotors26, and a comparison of the rotor speeds. The process100includes a sequence of steps relating to each comparison. The steps relating to the comparison of the unbalance vectors58will be described first.

The angle of the unbalance vector58of the rotor26of the first turbine module14, the ‘first unbalance vector’58, is determined at step104. In this example, the angle of the first unbalance vector58is calculated from the azimuth angle of the first rotor26, based on the fixed angular relationship between the azimuth angle of the first rotor26and the angle of the first unbalance vector58. The angle between the azimuth position of the first rotor26and the angle of the first unbalance vector58is typically determined during commissioning of the wind turbine installation10. Alternatively or additionally, the first rotor26may be designed, manufactured and installed with the centre of mass62positioned deliberately in a known angular relationship to the azimuth angle. In other embodiments, the angle of the first unbalance vector58is derived in operation through monitoring for cyclic loads in the support structure22. The angle of the first unbalance vector58in relation to the azimuth angle of the first rotor26may be periodically or continuously monitored to account for changes in the balance of the first rotor26over a period of use.

The angle of the unbalance vector58of the rotor26of the second turbine module14, the ‘second unbalance vector’58, is determined at step106. Since the turbine modules14are substantially identical, the description of step104above equally applies to step106. The skilled person will appreciate that either turbine module14may be designated the ‘first’ turbine module14and that steps104and106may be performed sequentially (in either order) or simultaneously.

The determined angles of the unbalance vectors58are compared at step108by calculating the angle from the first unbalance vector58to the second unbalance vector58. It is subsequently determined, at step110, whether the angle between the vectors is 180 degrees, completing the comparison between the unbalance vectors58of the two rotors26. For the purposes of this process100, the angle between the unbalance vectors58is also considered to be 180 degrees if the angle falls within a predefined tolerance region.

As mentioned above, the process100also includes a sequence of steps relating to the comparison of the rotor speeds of the two turbine modules14. The rotor speeds of the two turbine modules14are respectively determined at steps112and114. In this example, the rotor speeds are measured directly by the respective rotor speed sensors54. However, the skilled person will be aware of other ways of determining the rotor speed of a wind turbine module14, for example by calculation using various other operating parameters of the wind turbine module14such as the gearbox speed.

The rotor speeds are compared at step116and it is subsequently determined, at step118, if the rotor speeds are the same. For the purposes of this process100, the rotor speeds are also considered to be the same if they are within a predefined tolerance range of each other.

The turbine modules14are controlled on the basis of these comparisons. If the angle between the unbalance vectors58is 180 degrees and the rotor speeds are the same, then the rotating unbalances are counteracting one another appropriately, and so no additional rotor speed control is required and the process100is complete. The control system then returns to the initiation requirement step and the process100is reiterated continuously until the installation10is shut down.

However, if the angle between the unbalance vectors58is not 180 degrees, or if the rotor speeds are not the same (or both) then the control system issues, at step120, an appropriate command to control the rotor speed of the first turbine module14such that the rotor speeds are equal and the angle between the unbalance vectors58is 180 degrees.

To achieve this, the control system commands the rotor26of the first turbine module14to undergo a speed adjustment operation. The speed adjustment operation may comprise a single acceleration or deceleration or a series of accelerations and/or decelerations of the rotor26as appropriate. The speed adjustment operation is appropriately composed to ensure that the rotor speeds become equal at a moment at which the angle between the unbalance vectors58is 180 degrees.

In other examples, the control system may also control the rotor speed of the second turbine module14to achieve the control strategy. In such cases, the control system may command the rotor26of the second wind turbine module14to undergo an appropriate rotor speed adjustment operation.

Following conclusion of the rotor speed adjustment operation(s), the control system issues a command to maintain equal rotor speed for each wind turbine module14, ensuring that the forces exerted by the rotating unbalances remain opposed.

The control system then returns to the initiation requirement step102. Accordingly, the control system acts to control the rotor speeds whenever the initiation requirement is met. So, in this example, the process executes100continuously when the wind turbine installation10is in operation.

The description above relates to one example of a possible process100that may be performed by the control system to ensure that the rotating unbalances of the rotors26apply opposing forces on the support structure22which cancel each other out to some extent. The skilled person will be able to envisage other appropriate processes that may be carried out. For example, an adaptive algorithm may be employed in which the rotor speeds are controlled to achieve a target angular difference between the azimuth positions of the first and second wind turbine module rotors26, the target angular relationship being modified during operation until vibration detected within the support structure22is below a predetermined threshold.

As mentioned previously, an alternative strategy for using the rotor speed control functions of the turbine modules14to reduce or eliminate excitation of vibration modes of the installation10support structure22is to control the speeds of the rotors26such that they are different. The angle between the unbalance vectors58of the two rotors26is therefore continually changing and the forces due to the rotating unbalances will only be in phase momentarily. A process200that may be carried out by the control system in order to achieve this control strategy will now be described with reference toFIG. 5which shows a flow diagram illustrating steps of the process200.

The process200begins by checking at step202whether an initiation requirement is satisfied. This step generally corresponds to the initiation step102of the process100shown inFIG. 4, and the same considerations and alternatives apply. For example, in this embodiment the initiation requirement is that the wind turbine installation10is operational, but in other embodiments the method may be initiated when a vibration sensor provided to the wind turbine installation10detects that a magnitude of vibration within the support structure22exceeds a predetermined threshold, indicating that a vibration mode of the support structure22is being excited. Alternatively, the method may be initiated when the speed controller52issues a command that requires the rotor speed to correspond to a critical frequency of the support structure22.

Following initiation of the process200, the system determines, at step204, the rotor speed of the first turbine module14and, at step206, the rotor speed of the second turbine module14. In this example, the rotor speeds are measured directly by the rotor speed sensors54. However, the skilled person will be aware of other ways of determining the rotor speed of a wind turbine module14, for example by calculation using various other operating parameters of the wind turbine module14, for instance generator speed. The rotor speeds are compared at step208.

Following this comparison, the turbine modules14are controlled such that the angle between the unbalance vectors58of the two rotors26varies continually. That is to say, the rotors26are set at different speeds.

In order to achieve this, the system determines, at step208, a target rotor speed difference that defines the desired difference between the two rotor speeds in order to achieve the control strategy. In this example, the target rotor speed difference is a fixed percentage difference of four percent; that is, one rotor26rotating at 96 percent of the speed of the other rotor26. The value of the percentage difference may be stored within a memory module of the control system as appropriate.

In other examples, the target percentage difference may take any other suitable value. In some examples of the process100, the target percentage difference is more than two percent; that is, one rotor26rotating at less than 98 percent of the speed of the other rotor26. Additionally or alternatively, the target percentage difference may be less than ten percent; that is, one rotor26rotating at more than 90 percent of the speed of the other rotor26.

Generally speaking, a larger percentage difference advantageously reduces the period of time in any given rotation during which the forces due to the rotating unbalances are substantially reinforcing each other which minimises resonant excitation. However, there is a trade-off effect: the larger the percentage difference, the more the speed of one (or both) rotors26must deviate from the desired operating speed. This may affect the power generation efficiency of the wind turbine installation10.

The skilled person will be able to envisage many possible definitions of the target rotor difference, for example based on one or more operating parameters of the wind turbine installation10.

The system controls the rotor speeds, at step210, in order to achieve the determined target rotor speed difference. That is to say, the control system issues a command for the rotor26of the first turbine module14to accelerate or decelerate as appropriate such that the rotor26of the first turbine rotates at 96 percent of the speed of the rotor26of the second turbine module14. In other examples, the control system may also issue a command for the rotor of the second turbine module14to accelerate or decelerate as appropriate to achieve the target rotor speed difference.

Since the turbine modules14are substantially identical, either turbine module14may be designated to rotate at the faster speed. The control system determines target rotor speeds for each turbine module14based on the instantaneous local wind conditions. For example, the control system may determine that the rotor26of the turbine module14in the vicinity of which the wind speed is greater should rotate faster than the rotor26of the other turbine module14. In other examples, the target rotor speeds may depend on various operating parameters of the wind turbine installation10as appropriate.

The control system then issues a command to maintain the target rotor speed difference between the wind turbine modules14and returns to the initiation requirement step202.

The skilled person will appreciate that the description above relates to one example of a process200that may be carried out by the control system. Many modifications may be made to this example to provide alternative processes that ensure that the angle between the unbalance vectors58of the two rotors26varies. The angle varies continuously, but not necessarily smoothly, for example as would be achieved by a periodic step-change in rotor speed.

In addition, the control system may be operable to switch between control strategies, for example based on various operating parameters of the wind turbine installation10such as the wind speed. Thus, the most appropriate control strategy (and associated process) is implemented at any moment during operation of the wind turbine installation10.

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.

As mentioned above, the present invention is also applicable to multi-rotor wind turbine installations including three or more wind turbines 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 installation10comprising four wind turbine modules14,34, as shown partially in dashed lines inFIG. 1, the upper wind turbine modules14may be controlled as described above whilst the lower wind turbine modules34are controlled separately according to a similar process. Thus, the wind turbine modules14,34are controlled as two pairs in order to control the forces exerted on the support structure22as a result of the rotating unbalances of the rotors26. Other wind turbine module pairings are also envisaged, for example, the left- and right-hand modules14,34may each be controlled as a pair.

In other embodiments, the first control strategy described above can be adapted for the four-rotor wind turbine installation10. For example, the rotor speeds of the wind turbine modules14,34may be controlled such that the unbalance vectors58of the rotors26are equally spaced at 90 degree intervals. According to another control process, the target angular differences between the unbalance vectors58of the wind turbine modules may be determined based on the magnitudes of the unbalance vectors58. This allows the target angular differences to be determined appropriately such that the forces on the support structure22due to the unbalance vectors neutralise one another as much as possible in order to minimise the cyclic excitation force exerted on the support structure22.

The second above-described control strategy is also applicable to the four-rotor installation10. For example, the four wind turbine modules14,34may be controlled such that each module14,34rotates at a different speed.

As described above, these control processes are similar in that there is a general aim to avoid the centres of mass of the rotors26of the installation10from rotating in phase such that the resulting cyclic forces coincide. The skilled person will be able to envisage further alternative control processes that may be employed to achieve this aim in the context of wind turbine installations12having any appropriate number of wind turbine modules14.

In embodiments, the wind turbine modules are all configured to rotate in same direction, typically in the clockwise direction as is normal for single rotor turbines. However with multi-rotor turbine the is the possibility that a sub-group of the wind turbine modules are arranged for counter-clockwise rotation, so that one group of the wind turbine modules rotate in clockwise direction, whereas another group of the wind turbine modules are arranged to rotate in counter-clockwise rotation. In such a situation the embodiments of the present invention may take into account the rotational direction of the wind turbine modules so that the rotating unbalance vector includes the dimension of the rotating direction. Moreover, the control means is furthermore configured to coordinate the rotational speeds of the plurality of the rotors taking also the rotating direction of the unbalance vector into account.