Method for controlling a multirotor wind turbine

Abstract

A method for controlling a multi-rotor wind turbine (1) comprising two or more energy generation units (5) is disclosed. At least one load-bearing structure (3) is connected to a base or tower (2) via a yaw device (4), the load-bearing structure (3) carrying at least two energy generation units (5). A need for an operational change in at least a first energy generation unit (5) is detected. Control commands are generated for the first energy generation unit (5) and at least a second energy generation unit (5) mounted on the same load-bearing structure (3). The control commands result in the desired operational change and coordinated operation of at least the first energy generation unit (5) and the second energy generation unit (5). The control commands are generated under the constraint that the yaw moment (9) of the yaw device (4) is maintained below a predefined threshold level.

Description

Technical Field

[0001] This invention relates to a method for controlling a multi-rotor wind turbine, i.e., a wind turbine comprising two or more energy generation units. More particularly, the method according to the invention reduces uneven loads on components of the wind turbine, especially the yaw mechanism. Background Technology

[0002] When wind acts on the wind turbine blades of the energy generation unit of a wind turbine, causing the rotor of the energy generation unit to rotate, for example, due to the thrust acting on the energy generation unit, a load is introduced onto the wind turbine.

[0003] In the case of a multi-rotor wind turbine, i.e., a wind turbine comprising two or more energy generation units, the loads on the energy generation units may differ. This could be due to differences in the wind acting on the energy generation units, such as due to wind shear, partial wake, etc. Alternatively or additionally, the load differences on the energy generation units may stem from operational differences within the energy generation units.

[0004] When at least two energy-generating units are mounted on the load-bearing structure on opposite sides of the yaw's rotation axis, away from the yaw's extension, the load differences described above can introduce uneven loads in wind turbines, particularly in the yaw mechanism, and potentially in the tower and / or base. If such loads are considered during the wind turbine's design phase, the manufacturing cost of some components can be high, for example, due to the addition of extra material to withstand the anticipated loads. If such loads are not considered during the wind turbine's design phase, there is a risk that the design load of the wind turbine may be exceeded, thereby posing a risk of damage to some of the wind turbine's components. Summary of the Invention

[0005] The purpose of embodiments of the present invention is to provide a method for controlling a multi-rotor wind turbine, wherein excessive uneven loads on the wind turbine are avoided.

[0006] This invention provides a method for controlling a multi-rotor wind turbine, the multi-rotor wind turbine comprising: two or more energy generation units, each energy generation unit including a rotor, the rotor including one or more wind turbine blades; and at least one load-bearing structure connected to a base or tower via a yaw device, the load-bearing structure carrying the at least two energy generation units, the method comprising the following steps:

[0007] - Detect the need for operational changes to at least the first energy generation unit among the energy generation units.

[0008] - Generate control commands for the first energy generation unit and at least the second energy generation unit, which are mounted on the same load-bearing structure. The control commands cause a desired operational change and result in coordinated operation of at least the first and second energy generation units.

[0009] - Control the energy generation unit according to the generated control commands.

[0010] The control command is generated under the constraint that the yaw moment of the yaw device is maintained below a predefined threshold level.

[0011] Therefore, the method according to the invention is for controlling a multi-rotor wind turbine, i.e., a wind turbine comprising two or more energy generation units. Each energy generation unit includes a rotor with one or more wind turbine blades. Thus, for each energy generation unit, the wind turbine blades are able to capture wind, thereby causing the rotor to rotate. The rotational movement of the rotor can then be converted into electrical energy. Thus, each of the energy generation units is able to independently produce electrical energy by extracting energy from the wind.

[0012] The multi-rotor wind turbine also includes at least one load-bearing structure. The load-bearing structure is connected to the base or tower of the wind turbine via a yaw device. Thus, the load-bearing structure can perform yaw movement relative to the base or tower. Furthermore, the load-bearing structure carries at least two energy generation units. Therefore, when the load-bearing structure performs yaw movement relative to the base or tower, the energy generation units carried by the load-bearing structure move accordingly. Consequently, the rotors of the energy generation units are oriented synchronously according to the direction of the incoming wind.

[0013] In the method according to the invention, the need for an operational change in at least a first energy generation unit is initially detected. The required operational change may be caused by changes in environmental conditions (such as wind speed, wind direction, wake conditions, turbulence conditions, etc.). Alternatively or additionally, the required operational change may be caused by events related to the energy generation unit and / or by events related to the entire multi-rotor wind turbine. This will be described in more detail below.

[0014] Next, control commands are generated for the first energy generation unit and at least the second energy generation unit. The first and second energy generation units are mounted on the same load-bearing structure, meaning they are connected to the base or tower via the same yaw mechanism. The generated control commands result in the required operational change, ensuring that the first energy generation unit actually undergoes the desired operational change.

[0015] Furthermore, the generated control commands result in coordinated operation of at least the first energy generation unit and the second energy generation unit. Therefore, control commands are generated for the second energy generation unit based on the control commands generated for the first energy generation unit. Moreover, while ensuring that the control commands for the first energy generation unit achieve the desired operational changes, it is also ensured that the control commands for the first energy generation unit do not conflict with the needs of the second energy generation unit.

[0016] Finally, the energy generation unit is controlled according to the generated control commands. Thus, the first energy generation unit is controlled in a manner that results in the desired operational change, and the first and second energy generation units are controlled in a coordinated manner.

[0017] The control commands are also generated under the constraint that the yaw moment of the yaw device is maintained below a predefined threshold level. In this context, the term "yaw moment" should be interpreted as the moment about an axis coinciding with the rotation axis of the yaw device.

[0018] When two energy generation units supported by the same load-bearing structure operate independently, there is a risk that combined operation of the energy generation units may result in different forces acting on opposite sides of the yaw unit's rotation axis. This could introduce undesirable yaw moments into the yaw unit, as well as potentially undesirable torsional tower loads. If such yaw moments become too high, there is a risk of introducing uneven loads into the wind turbine, particularly in the yaw unit. By generating control commands for the energy generation units while maintaining the yaw moment of the yaw unit below a predefined threshold, it is ensured that operating the energy generation units according to the generated control commands will not lead to undesirable uneven loads.

[0019] A predefined threshold can be a fixed value that defines the level of risk that the presence of a yaw moment in any environment would result in undesirable high uneven loads. Alternatively, the predefined threshold can be selected based on a given environment, such as wind speed, wind direction including associated wake conditions, gust conditions, turbulence conditions, the operating state of the energy-generating unit and / or multi-rotor wind turbine, etc. For example, the predefined threshold can be selected in a way that keeps the yaw moment within a set of design conditions for the energy-generating unit and / or multi-rotor wind turbine. Furthermore, a predefined threshold can be selected when the load-bearing structure is not actively performing yaw movement, while another threshold can be selected during yaw. In this case, the threshold can also be selected based on yaw speed and / or yaw acceleration.

[0020] The load-bearing structure may include two arm structures extending away from the yaw device on opposite sides of the rotation axis of the yaw device, and a first energy generation unit may be installed on the first arm structure and a second energy generation unit may be installed on the second arm structure.

[0021] According to this embodiment, the first energy generation unit and the second energy generation unit are mounted on opposite sides of the rotation axis of the yaw device. Furthermore, since the energy generation units are mounted on an arm extending away from the yaw device, they are also positioned offset from the rotation axis. The farther the energy generation units are positioned from the rotation axis, the higher the risk they may cause high yaw moments in the yaw device. Therefore, in this case, it is particularly relevant to operate the energy generation units in a coordinated manner to ensure that the yaw moment is maintained below a predefined threshold.

[0022] The step of detecting the need for operational changes to at least the first energy generation unit may include detecting changes in the operating conditions of the first energy generation unit. For example, this may include the first energy generation unit detecting events such as extreme changes in wind direction, changes in turbulence conditions, changes in wind shear, changes in the thrust effect on the energy generation unit, etc. Alternatively, it may include the first energy generation unit changing its operating state from one velocity region to another, such as changing to an idle state, normal production state, shutdown state, startup state, defensive operating state, or degraded state, for example, to avoid resonance.

[0023] Therefore, according to this embodiment, the need for operational changes is caused by things that are detectable at the first energy generation unit.

[0024] The steps of generating control commands may include setting the same thrust limit for both the first and second energy generation units. The thrust acting on the energy generation units has a significant impact on the yaw moment of the yaw mechanism, especially when the energy generation units are positioned on an arm far from the yaw mechanism's extension. Therefore, setting the same thrust limit for both the first and second energy generation units is an effective way to ensure that the difference in thrust acting on the two energy generation units is minimized, and thus the introduced yaw moment is also minimized.

[0025] Alternatively, the step of detecting the need for operational changes to at least the first energy generation unit can be performed based on a command requesting an operational change to the multi-rotor wind turbine. This could be caused, for example, by changes in operating conditions, such as a need to degrade power generation, degrade rotor rotation speed, start the wind turbine, or stop the wind turbine. Therefore, according to this embodiment, the need for operational changes is related to the overall operation of the entire wind turbine. The wind turbine then generates a control command to each of the energy generation units, ensuring that the overall operational requirements of the wind turbine are met while ensuring that the yaw moment of the yaw device is maintained below a predefined threshold level.

[0026] The step of generating control commands may include generating the same control commands for both the first and second energy generation units. According to this embodiment, it is ensured that the first and second energy generation units are controlled in the same manner. For example, if the first energy generation unit receives a control command that causes it to shut down, then the second energy generation unit also receives a control command that causes it to shut down.

[0027] Alternatively, the step of generating control commands may include generating a first control command for a first energy generation unit and generating a second control command for a second energy generation unit, the second control command being different from the first control command, and the step of controlling the energy generation units may include controlling the first energy generation unit and the second energy generation unit in a coordinated manner according to the different control commands.

[0028] According to this embodiment, the first energy generation unit and the second energy generation unit are not controlled in the same way. However, the control of the energy generation units is still coordinated so as to maintain the yaw torque of the yaw device below a predefined threshold.

[0029] For example, the steps to generate control commands may include:

[0030] - Generate a first control command for the first energy generation unit, the first control command causing the first energy generation unit to shut down, and

[0031] - Generate a second control command for the second energy generation unit, which causes the second energy generation unit to continue power production at a reduced load level.

[0032] According to this embodiment, the need for operational change is to shut down the first energy generation unit. Therefore, the control command generated for the first energy generation unit must cause it to shut down. However, shutting down one energy generation unit mounted on the load-bearing structure on the first side of the yaw's rotation axis while allowing another energy generation unit mounted on the load-bearing structure on the opposite side of the yaw's rotation axis to continue full operation can generate a significant yaw moment in the yaw, because the thrust acting on the operating energy generation unit is much greater than the thrust acting on the shut-down energy generation unit (e.g., due to differences in rotor rotation speed and / or pitch angle between the energy generation units). To avoid this, according to this embodiment, the control command generated for the second energy generation unit causes it to continue operating, but at a reduced load level, for example, by reducing power production below a predefined threshold level. This reduces the difference in thrust acting on the two energy generation units to a level that ensures the yaw moment in the yaw remains below the predefined threshold level. However, the second energy generation unit does not shut down. Therefore, the wind turbine continues to produce energy while the first energy generation unit is shut down. Furthermore, when the first energy generation unit is ready to start again, there is no need to start the second energy unit.

[0033] Control commands for reducing power production in the second energy generation unit may include commands for reducing rotor rotation speed, reducing thrust limit of the energy generation unit, and degrading the energy generation unit.

[0034] In situations requiring the shutdown of the first energy generation unit during high-load operating conditions on the wind turbine (e.g., at high wind speeds), control commands for the second energy generation unit may introduce a delay before they are executed. According to this embodiment, the first energy generation unit can be shut down immediately when it is determined that this is necessary. However, in cases where the first energy generation unit is the one that contributes the most to the yaw moment, it is desirable to allow the first energy generation unit to decelerate before the second energy generation unit also begins to shut down, as an increase in yaw moment might otherwise occur. Therefore, a delay is introduced before allowing the second energy generation unit to execute its control commands.

[0035] According to another embodiment, the control command for the first energy generation unit can cause the first energy generation unit to move to an idle state, and the control command for the second energy generation unit can cause the second energy generation unit to reduce power production, reduce thrust limit, reduce rotor rotation speed, and / or degrade the energy generation unit, etc.

[0036] The coordinated operation of the first energy generation unit and the second energy generation unit can be a time-coordinated operation. According to this embodiment, control commands for the first energy generation unit and control commands for the second energy generation unit are executed simultaneously, or with a clearly defined timing sequence between them.

[0037] The method may further include the step of monitoring the yaw moment of the yaw device, and the step of generating control commands based on the yaw moment, and the control commands for the second energy generation unit may provide instructions to control the second energy generation unit according to the control commands when the yaw moment is below a predefined threshold level.

[0038] According to this embodiment, the yaw moment is monitored, for example, to ensure that it is actually maintained below a predefined threshold level. In this case, the second energy generation unit is allowed to operate according to the generated control commands. On the other hand, when the yaw moment increases to exceed the predefined threshold level, it may not be feasible to immediately execute the control commands generated for the second energy generation unit. For example, as described above, instead, a delay can be introduced before executing the control commands for the second energy generation unit. For example, if the second energy generation unit is the energy generation unit that makes the smallest contribution to the yaw moment, then shutting down the energy generation unit may cause the yaw moment to increase, so it may be desirable to delay the shutdown of the energy generation unit until shutting down the first energy generation unit has reduced the yaw moment to an acceptable level.

[0039] The method may also include the following steps:

[0040] - Monitor the operation of the energy generation unit during operation based on the generated control commands, and

[0041] - Adjust the operation of at least one of the energy generation units in the event that the operations of the first energy generation unit and the second energy generation unit lose time coordination.

[0042] According to this embodiment, once a control command is generated for the first energy generation unit and the second energy generation unit in response to the need for operational change, the first energy generation unit and the second energy generation unit are controlled in a coordinated manner according to the generated control command, and the actual operation of the energy generation unit is monitored to ensure that the energy generation unit actually continues to operate in a coordinated manner.

[0043] If it is found that the operation of the first energy generation unit and the second energy generation unit has lost time coordination, the operation of at least one of the energy generation units is adjusted to ensure that time coordination is restored, and the yaw torque on the yaw device can be maintained below a predefined threshold level.

[0044] Loss of timing coordination in the operation of energy generation units may be caused, for example, by differences in how the energy generation units execute control commands. For instance, even if both the first and second energy generation units are requested to shut down, and even if the shutdown process is initiated simultaneously, the actual shutdown process may not be executed by the two energy generation units in the same way or at the same pace. In this case, it may be necessary to adjust the operation of at least one of the energy generation units, such as requesting the energy generation unit that is ahead of the other to slow down its shutdown process, to allow the other energy generation unit to catch up.

[0045] The step of detecting the need for operational changes can be performed by one of the energy generation units. According to this embodiment, the need for operational changes is detected locally by one of the energy generation units, preferably the one requiring the operational change. For example, this can include the energy generation unit detecting changes in environmental conditions (such as wind speed, wind direction, gust conditions, turbulence conditions, etc.). Alternatively or additionally, it can include the energy generation unit detecting internal conditions, for example, based on outputs from sensors (such as temperature sensors, load sensors, etc.).

[0046] As an alternative, the step of detecting the need for operational changes can be performed by the multi-rotor wind turbine. According to this embodiment, the need for operational changes is related to the entire wind turbine, and therefore detection is performed at the wind turbine level. For example, the wind turbine itself may need to be degraded, have its power production increased, be shut down, be started up, etc., and this needs to be done in a manner that ensures coordination between energy generation units, as described above.

[0047] The steps for detecting the need for operational changes may include detecting that the first energy generation unit will shut down. This has already been described above.

[0048] In this scenario, the step of generating control commands may include generating control commands that cause both the first and second energy generation units to shut down. This effectively prevents one energy generation unit from shutting down while the other continues normal operation. As mentioned above, this scenario could result in high yaw moments, and therefore it is desirable to avoid this situation.

[0049] Similarly, the step of detecting the need for operational changes may include detecting that the first energy generation unit will be activated. In this case, the step of generating control commands may include generating control commands that cause both the first and second energy generation units to activate. This ensures that the energy generation units activate simultaneously, or at least in a coordinated manner, thereby effectively ensuring that the yaw torque on the yaw device remains below a predefined threshold level during the activation process.

[0050] The step of generating control commands may include generating control commands that cause both the first and second energy generation units to start in a reduced-load operating mode. This ensures that the load on the wind turbine remains at a low level during the startup of the energy generation units, even if the energy generation units do not start in exactly the same manner.

[0051] As an alternative, the step of detecting the need for operational changes may include detecting that the first energy generation unit will degrade. In this case, the step of generating control commands may include generating control commands that cause both the first and second energy generation units to degrade.

[0052] It should be noted that coordinated control of the energy generation unit can be initiated based on predictions of conditions expected to occur at the wind turbine's location during a future time interval. This prediction can, for example, be based on a set of monitoring parameters. If the predicted conditions indicate that changes to the operation of at least one of the elements in the energy generation unit will be necessary when the future time interval arrives, measures can be taken in advance to avoid undesirable incidents. Attached Figure Description

[0053] The invention will now be described in more detail with reference to the accompanying drawings, in which:

[0054] Figure 1 This is a schematic diagram of a multi-rotor wind turbine controlled according to a method according to an embodiment of the present invention.

[0055] Figure 2 and Figure 3 The figures illustrate the control of a multi-rotor wind turbine during downtime using both existing techniques and methods according to embodiments of the present invention.

[0056] Figure 4 and Figure 5 The illustrations respectively depict the control of a multi-rotor wind turbine after event detection according to existing technology methods and methods according to embodiments of the present invention, and...

[0057] Figure 6 and Figure 7 The illustrations respectively depict the control of a multi-rotor wind turbine during startup using methods based on existing techniques and methods according to embodiments of the present invention. Detailed Implementation

[0058] Figure 1 This is a schematic diagram of a multi-rotor wind turbine 1 controlled according to a method according to an embodiment of the present invention. The wind turbine 1 includes a tower 2 and a load-bearing structure including two arms 3. The load-bearing structure 3 is connected to the tower 2 via a yaw device 4, thereby enabling the load-bearing structure 3 to perform yaw movement relative to the tower 2.

[0059] Each of the arms carries an energy generation unit 5, and each energy generation unit 5 includes a rotor 6 and multiple wind turbine blades 7. The arm 3 extends away from the yaw device 4 on the opposite side of the rotation axis 8 of the yaw device 4. Thus, the operational differences between the two energy generation units 5 may result in different forces applied to the load-bearing structure 3 on the opposite sides of the rotation axis 8 of the yaw device 4. This may introduce a yaw moment 9 in the yaw device 4.

[0060] Figure 1 The multi-rotor wind turbine 1 can be controlled in the following manner. During operation, the energy generation unit 5 provides operational data to the multi-rotor wind turbine controller 10. Upon detecting a need for an operational change in at least one of the energy generation units 5, the multi-rotor wind turbine controller 10 generates control commands for both energy generation units and provides the generated control commands to the respective energy generation unit 5.

[0061] Operational changes may be required due to events or conditions occurring or detected at the energy generation unit 5. In this case, the multi-rotor wind turbine controller 10 can react based on operational data received from the energy generation unit 5. Alternatively or additionally, operational changes may be required due to events or conditions applied horizontally at the wind turbine level. In this case, the multi-rotor wind turbine controller 10 can react based on other inputs and / or measurements performed by the multi-rotor wind turbine controller 10 itself.

[0062] The control commands are generated in a manner that ensures the desired operational changes are actually achieved. Furthermore, the control commands are generated in a coordinated manner, that is, in a manner that causes the two energy generation units 5 to operate in a coordinated manner, to ensure that the yaw torque 9 of the yaw device 4 is maintained below a predefined threshold level.

[0063] Figure 2 and Figure 3 The illustrations respectively depict the control of a multi-rotor wind turbine during shutdown using methods based on existing techniques and methods according to embodiments of the present invention. The controlled multi-rotor wind turbine can be, for example, a... Figure 1 A multi-rotor wind turbine. In any case, a multi-rotor wind turbine comprises two energy-generating units, illustrated by solid and dashed lines, respectively.

[0064] The graph above illustrates the operational state of the energy generation unit as a function of time. It can be seen that the energy generation unit can be in a "running" state (where the energy generation unit is operating) or a "stopped" state (where the energy generation unit has been shut down).

[0065] The lower graph illustrates the yaw moment of the yaw device of the multi-rotor wind turbine as a function of time, and during the operation of the energy generation unit shown in the upper graph.

[0066] At time t0, it is detected that the first energy generation unit in the energy generation unit needs to be shut down. In the prior art method shown in Figure 2, the first energy generation unit, represented by the solid line, switches from an "operating" state to a "stopped" state to shut down the energy generation unit, thereby fulfilling the requirement. However, the second energy generation unit, represented by the dashed line, remains in the "operating" state, thus continuing to operate as before the requirement to shut down the first energy generation unit was detected. It can be seen that this results in a large yaw moment on the yaw device, and a large variation in the yaw moment.

[0067] According to Figure 3 In the method of the embodiment of the present invention shown, the first energy generation unit also switches from a "running" state to a "stopped" state so as to shut down the first energy generation unit as needed. However, in this case, the control commands generated for the two energy generation units are coordinated, so the second energy generation unit also switches from a "running" state to a "stopped" state simultaneously with the first energy generation unit. Figure 3 As can be seen in the lower curve, this results in a much smaller yaw moment and a much smaller change in the yaw moment. Therefore, compared to the prior art situation shown in Figure 2, the load on the wind turbine, especially on the yaw mechanism, is significantly reduced.

[0068] Figure 4 and Figure 5 The figures illustrate control of a multi-rotor wind turbine after event detection using both existing techniques and methods according to embodiments of the present invention. See Figure 2 for similar details. Figure 3 The described scenario involves a multi-rotor wind turbine comprising two energy generation units: a first energy generation unit shown by a solid line and a second energy generation unit shown by a dashed line.

[0069] The graph above illustrates the operating state of the energy generation unit as a function of time. The energy generation unit can be in a "normal" state where it operates normally, or in a "reduced load" state where it operates cautiously, limiting the load introduced on the energy generation unit and thus on the multi-rotor wind turbine.

[0070] The intermediate curve illustrates the average pitch angle of the wind turbine blades of the energy generation unit as a function of time, and during the operation of the energy generation unit shown in the upper curve.

[0071] The lower graph illustrates the yaw moment of the yaw device of the multi-rotor wind turbine as a function of time, and during the operation of the energy generation unit shown in the upper graph.

[0072] At time t0, an event requiring the first energy generation unit to operate in a reduced-load mode is detected. In the prior art method, as shown in Figure 4, the first energy generation unit switches from a "normal" state to a "reduced-load" state as needed. As a result, the average pitch angle of the first energy generation unit increases. However, the second energy generation unit remains in the "normal" state and continues to operate regardless of the operation of the first energy generation unit. Therefore, the average pitch angle of the second energy generation unit remains at its level before t0. As can be seen from the lower curve of Figure 4, this leads to the introduction of a high yaw moment in the yaw device, and a large change in yaw moment due to the rapid change in yaw moment.

[0073] exist Figure 5 In the method shown according to an embodiment of the present invention, the first energy generation unit also switches from a "normal" state to a "reduced load" state at time t0. However, in this case, the second energy generation unit also switches from a "normal" state to a "reduced load" state at time t0. Therefore, the average pitch angle of the second energy generation unit increases to a level comparable to that of the first energy generation unit.

[0074] from Figure 5 As can be seen in the lower curve, this results in a much smaller yaw moment and a much smaller yaw moment variation. Therefore, compared to the prior art shown in Figure 4, the load on the wind turbine, especially on the yaw mechanism, is significantly reduced.

[0075] Figure 6 and Figure 7 The figures illustrate the control of a multi-rotor wind turbine during startup using both existing techniques and methods according to embodiments of the present invention. See Figure 2 for similar details. Figure 3 The described scenario involves a multi-rotor wind turbine comprising two energy generation units: a first energy generation unit shown by a solid line and a second energy generation unit shown by a dashed line.

[0076] The graph above illustrates the operating state of the energy generation unit as a function of time. The energy generation unit can be in the following states: "Reduced Load," where the energy generation unit operates cautiously; "Normal," where the energy generation unit operates normally; "Start-up," where the energy generation unit is starting up; and "Idle," where the energy generation unit does not produce electricity.

[0077] The lower graph illustrates the yaw moment of the yaw device of the multi-rotor wind turbine as a function of time, and during the operation of the energy generation unit shown in the upper graph.

[0078] Initially, both energy generation units are in an "idle" state, meaning neither is producing electricity. At time t0, it is detected that an energy generation unit should start. Therefore, both energy generation units switch from the "idle" state to the "start" state. At time t1, the first energy generation unit has completed its startup process and is ready to begin normal operation. However, the second energy generation unit has not yet completed its startup process.

[0079] In the existing technical method, as shown in Figure 6, the first energy generation unit switches from the "startup" state to the "normal" state at time t1, while the second energy generation unit simply remains in the "startup" state until it completes its startup process. This is obtained at time t2, where the second energy generation unit switches from the "startup" state to the "normal" state, after which both energy generation units operate normally.

[0080] As can be seen from the lower curve of Figure 6, this results in a larger yaw torque in the yaw system during the period when the first energy generation unit is operating normally while the second energy generation unit is still in the startup process, i.e., from time t1 to time t2.

[0081] exist Figure 7 In the method shown according to an embodiment of the invention, the operation of the two energy generation units is coordinated. Therefore, it is also taken into account that when the first energy generation unit completes its startup process at time t1, the second energy generation unit has not yet completed its startup process. Therefore, instead of switching the first energy generation unit to a "normal" state, it is switched to a "reduced load" state, where the load acting on the energy generation unit is lower. This reduces the difference in forces acting on the two energy generation units.

[0082] When the second energy generation unit completes its startup process at time t3, it also switches from the "startup" state to the "load reduction" state. At this time, both energy generation units operate in a cautious manner.

[0083] When the operation of the two energy generation units is stable, they both switch from the "reduced load" state to the "normal" state in a coordinated manner at time t4.

[0084] from Figure 7 As can be seen from the lower curve, this results in a lower yaw torque on the yaw device throughout the entire startup process of a multi-rotor wind turbine.

Claims

1. A method for controlling a multi-rotor wind turbine (1), the multi-rotor wind turbine (1) comprising: two or more energy generating units (5), each energy generating unit (5) comprising a rotor (6) comprising one or more wind turbine blades (7), and at least one load carrying structure (3) connected to a foundation or tower (2) via a yaw device (4) and carrying the at least two energy generating units (5), the method comprising the steps of: - detecting a need for a change in operation of at least a first one of the energy generating units (5), wherein the step of detecting a need for a change in operation of at least the first energy generating unit (5) comprises detecting a change in an operating condition of the first energy generating unit (5), the operating condition comprising wind speed, wind direction, wake conditions or turbulence conditions, - generating control commands for the first energy generating unit (5) and at least a second energy generating unit (5) mounted to the same load carrying structure (3), the control commands resulting in the required change in operation and the control commands resulting in a coordinated operation of at least the first energy generating unit (5) and the second energy generating unit (5), and - controlling the energy generating units (5) in accordance with the generated control commands, wherein the control commands are generated under the constraint that a yaw moment (9) of the yaw device (4) is maintained below a predefined threshold level.

2. The method of claim 1, wherein, The load carrying structure comprises two arm structures extending away from the yaw device (4) on opposite sides of an axis of rotation (8) of the yaw device (4), and wherein the first energy generating unit (5) is mounted to a first arm structure and the second energy generating unit (5) is mounted to a second arm structure.

3. The method of claim 1, wherein, The step of generating control commands comprises setting the same thrust limit for the first energy generating unit (5) and the second energy generating unit (5).

4. The method of any one of claims 1-3, wherein, The step of generating control commands comprises generating the same control commands for the first energy generating unit (5) and the second energy generating unit (5).

5. The method of any one of claims 1-3, wherein, The step of generating control commands comprises generating a first control command for the first energy generating unit (5) and a second control command for the second energy generating unit (5), the second control command being different from the first control command, and wherein the step of controlling the energy generating units (5) comprises controlling the first energy generating unit (5) and the second energy generating unit (5) in a coordinated manner in accordance with the different control commands.

6. The method of claim 5, wherein, The step of generating control commands comprises: - generating a first control command for the first energy generating unit (5), the first control command resulting in the first energy generating unit (5) being shut down, and - generating a second control command for the second energy generating unit (5), the second control command resulting in the second energy generating unit (5) continuing power production at a reduced load level.

7. The method of claim 1, wherein, The coordinated operation of the first energy generating unit (5) and the second energy generating unit (5) is a temporal coordinated operation.

8. The method according to claim 1, further comprising the step of monitoring a yaw moment (9) of the yaw device (4), and wherein the step of generating a control command is performed based on the yaw moment (9), and the control command for the second energy generating unit (5) provides instructions to control the second energy generating unit (5) in dependence of the control command when the yaw moment (9) is below the predefined threshold level.

9. The method according to claim 1, further comprising the steps of: - monitoring the operation of the energy generating units (5) during operation in dependence of the generated control command, and - adjusting the operation of at least one of the energy generating units (5) in case the operation of the first energy generating unit (5) and the second energy generating unit (5) loses time coordination.

10. The method of claim 1, wherein, The step of detecting a need for an operation change is performed by one of the energy generating units (5).

11. The method of claim 1, wherein, The step of detecting a need for an operation change is performed by the multi-rotor wind turbine (1).

12. The method of claim 1, wherein, The step of detecting a need for an operation change comprises detecting that the first energy generating unit (5) is to be shut down.

13. The method of claim 12, wherein, The step of generating a control command comprises generating a control command resulting in the first energy generating unit (5) and the second energy generating unit (5) being shut down.

14. The method of claim 1, wherein, The step of detecting a need for an operation change comprises detecting that the first energy generating unit (5) is to be started.

15. The method of claim 14, wherein, The step of generating a control command comprises generating a control command resulting in the first energy generating unit (5) and the second energy generating unit (5) being started.

16. The method of claim 14 or 15, wherein, The step of generating a control command comprises generating a control command resulting in the first energy generating unit (5) and the second energy generating unit (5) being started. The step of generating a control command comprises generating a control command resulting in the first energy generating unit (5) and the second energy generating unit (5) being started.

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