THRUST BALANCE CONTROL OF A MULTIROTOR WIND TURBINE BASED ON CONSTRAINTS

Abstract

A method for controlling a multirotor wind turbine is disclosed. A first operational state of each of the energy generating units of the wind turbine is obtained. A difference in thrust acting on at least two of the energy generating units is detected. At least one constraint parameter of the set of operational constraints is adjusted in accordance with prevailing operating conditions and in accordance with the detected difference in thrust, and a new operational state for at least one of the energy generating units is derived, based on the at least one adjusted constraint parameter, the new operational state(s) counteracting the detected difference in thrust. Finally, the wind turbine is controlled in accordance with the new operational states for the energy generating units.

Claims

1. A method for controlling a wind turbine, the wind turbine comprising a support structure and at least two energy generating units mounted on the support structure, each energy generating unit comprising a rotor comprising a set of wind turbine blades, the energy generating units being operable within a set of operational constraints, the method comprising: obtaining a first operational state of each of the energy generating units, detecting that a difference in thrust acting on at least two of the energy generating units is present, adjusting at least one constraint parameter of the set of operational constraints in accordance with prevailing operating conditions and in accordance with the detected difference in thrust, deriving a new operational state for at least one of the energy generating units (5), based on the at least one adjusted constraint parameter, the new operational state(s) counteracting the detected difference in thrust, and controlling the wind turbine in accordance with the new operational states for the energy generating units.

2. The method of claim 1, wherein the adjusting at least one constraint parameter comprises adjusting a load constraint relating to at least one of the energy generating units.

3. The method of claim 1, wherein the adjusting at least one constraint parameter comprises adjusting a load constraint relating to the support structure.

4. The method of claim 1, wherein the adjusting at least one constraint parameter comprises adjusting a power production constraint relating to at least one of the energy generating units.

5. The method of claim 1, wherein the adjusting at least one constraint parameter comprises adjusting a range and/or a setpoint for at least one constraint.

6. The method of claim 1, wherein the step of detecting that a difference in thrust is present comprises detecting that operation of at least one of the energy generating units has stopped.

7. The method of claim 1, wherein the detecting that a difference in thrust is present comprises detecting a difference in wind conditions at the at least two energy generating units.

8. The method of claim 1, where in the detecting that a difference in thrust is present comprising detecting a difference in operational state of each of the energy generating units.

9. The method of claim 8, wherein detecting the difference in operational state includes detecting a difference in at least pitch angle and rotor speed.

10. The method of claim 1, wherein the support structure comprises a main tower part extending along a substantially vertical direction and at least two arms, each arm extending away from the main tower part along a direction having a horizontal component, each arm being connected to the main tower part via a yawing mechanism, and wherein the step of detecting that a difference in thrust is present comprises detecting a torque on the yawing mechanism.

11. The method of claim 1, wherein the deriving a new operational state for each of the energy generating units comprises specifying that at least one of the energy generating units shall operate in motor mode.

12. The method of claim 1, wherein the deriving a new operational state for at least one of the energy generating units comprises adjusting at least one yaw setting of the wind turbine.

13. The method of claim 1, wherein the new operational state for at least one of the energy generating units is a shutdown of the energy generating units.

14. The method of claim 1, further comprising performing an optimization calculation, optimizing total power production of the wind turbine, taking the at least one adjusted constraint parameter and the detected difference in thrust into account, and wherein the step of deriving a new operational state for at least one of the energy generating units is further based on the optimization calculation.

15. The method of claim 14, wherein the step of performing an optimization calculation is performed using a model predictive control (MPC) algorithm.

16. The method of claim 14, wherein the performing an optimization calculation comprises consulting a database.

17. A wind turbine, comprising: a support structure; at least two energy generating units mounted on the support structure, each energy generating unit comprising: a rotor; and a set of wind turbine blades disposed on the rotor; wherein the energy generating units are operable within a set of operational constraints; and a control system confirmed to perform an operation, comprising: obtaining a first operational state of each of the energy generating units; detecting that a difference in thrust acting on at least two of the energy generating units is present; adjusting at least one constraint parameter of the set of operational constraints in accordance with prevailing operating conditions and in accordance with the detected difference in thrust; deriving a new operational state for at least one of the energy generating units, based on the at least one adjusted constraint parameter, the new operational state(s) counteracting the detected difference in thrust, and controlling the wind turbine in accordance with the new operational states for the energy generating units.

18. The wind turbine of claim 17, wherein the adjusting at least one constraint parameter comprises adjusting a load constraint relating to at least one of the energy generating units.

19. The wind turbine of claim 17, wherein the adjusting at least one constraint parameter comprises adjusting a load constraint relating to the support structure.

20. The wind turbine of claim 17, wherein the adjusting at least one constraint parameter comprises adjusting a power production constraint relating to at least one of the energy generating units.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The invention will now be described in further detail with reference to the accompanying drawings in which

[0061] FIG. 1 is a top view of a wind turbine being controlled in accordance with a method according to an embodiment of the invention,

[0062] FIG. 2 is a diagrammatic view of a wind turbine being controlled in accordance with a method according to an embodiment of the invention, and

[0063] FIGS. 3-5 illustrate adjustment of various constraint parameters.

DETAILED DESCRIPTION OF THE DRAWINGS

[0064] FIG. 1 is a top view of a wind turbine 1 being controlled in accordance with a method according to an embodiment of the invention. The wind turbine 1 comprises a support structure comprising a main tower part 2 and two arms 3 connected to the main tower part 2 via a yawing mechanism 4. The two arms 3 form part of a common transversal structure, and thereby both arms 3 are moved together relatively to the main tower part 2, via the yawing mechanism 4.

[0065] Each of the arms 3 carries an energy generating unit 5 comprising a rotor 6 with a set of wind turbine blades 7. The thrust acting on one of the energy generating units 5a is significantly higher than the thrust acting on the other energy generating unit 5b. Accordingly, a difference in thrust acting on the two energy generating units 5a, 5b is present. This results in a torque on the yawing mechanism 4 as illustrated by arrow 8. Thereby the presence of the difference in thrust can be detected by detecting the torque on the yawing mechanism 4.

[0066] The difference in thrust acting on the two energy generating units 5a, 5b may, e.g., be caused by a difference in ambient conditions, such as wind conditions. This could, e.g., include a difference in wind speed at the positions of the energy generating units 5a, 5b, e.g. due to a difference in wake effects or horizontal wind shear, etc. As an alternative, the difference in thrust acting on the two energy generating units 5a, 5b may be caused by internal control actions, such as one of the energy generating units 5b being stopped, idling, or operating with reduced power production.

[0067] In any event, when a difference in thrust acting on the two energy generating units 5a, 5b is detected, a new operational state for at least one of the energy generating units 5a, 5b is derived. The new operational states are selected in such a manner that the difference in thrust is counteracted.

[0068] Furthermore, the new operational states are derived with due consideration to a set of operational constraints, where at least one constraint parameter has been adjusted. Normally, the wind turbine 1 is operated within a set of operational constraints which may be selected in such a manner that the wind turbine 1 can be operated without causing damage to the wind turbine 1 and in a manner which limit fatigue in the wind turbine 1 to a level which allows a design lifetime of the wind turbine 1 to be obtained. The operational constraints may, e.g., relate to loads on various parts of the wind turbine 1, energy production, etc. The operational constraints may be defined during design of the wind turbine 1. When adjusting at least one constraint parameter of the set of operational constraints, the wind turbine 1 is allowed to temporarily operate outside the normal constraints, e.g. with higher loads on some parts of the wind turbine 1. The adjusted constraint parameter could, e.g., include an adjusted range or an adjusted setpoint value for a constraint. This will be described in further detail below with reference to FIGS. 3-5.

[0069] The wind turbine 1 is then controlled in accordance with the new operational states, and thereby the difference in thrust acting on the energy generating units 5a, 5b is counteracted. Furthermore, since the new operational states are also derived on the basis of the adjusted constraint parameters, the new operational settings may require that the wind turbine 1 is temporarily operated outside the normal operational constraints. However, this is considered acceptable as long as the new operational settings cause the difference in thrust to be counteracted, and as long as ultimate wind turbine design loads are not exceeded.

[0070] FIG. 2 is a diagrammatic view of a wind turbine 1 being controlled in accordance with a method according to an embodiment of the invention. The wind turbine 1 could, e.g., be the wind turbine 1 of FIG. 1. Thus, similar to the wind turbine 1 of FIG. 1, the wind turbine 1 of FIG. 2 comprises a support structure comprising a main tower part 2 and two arms 3 being connected to the main tower part 2 via a yawing mechanism 4. Each arm 3 carries an energy generating unit 5 comprising a rotor 6 with a set of wind turbine blades 7.

[0071] Each of the energy generating units 5 comprises a production controller 9 arranged to control the operation of the corresponding energy generating unit 5. It should be noted that the production controllers 9 need not be physically arranged in the energy generating units 5, but each of the production controllers 9 is dedicated for controlling a given energy generating unit 5. Furthermore, the wind turbine 1 comprises a multirotor wind turbine controller 10 arranged to control the operation of the entire wind turbine 1, including coordinating the operation of the individual energy generating units 5.

[0072] The wind turbine 1 of FIG. 2 may be operated in the following manner. During normal operation, the production controllers 9 of each of the energy generating units 5 forward their operational states to the multirotor wind turbine controller 10. The operational states include pitch angle, , rotational speed of the rotor, , power production, P, and wind speed, v.

[0073] At a certain point in time, a torque, M.sub.yaw, as indicated by arrow 8, on the yawing mechanism 4 is detected and reported to the multirotor wind turbine controller 10. This is considered as an indication that a difference in thrust acting on the energy generating units 5 is present. Accordingly, the multirotor wind turbine controller 10 adjusts at least one constraint parameter of the set of operational constraints which are normally applied, and derives a new operational state for at least one of the energy generating units 5. The new operational states are derived in such a manner that the detected difference in thrust acting on the energy generating units 5 is counteracted, and in such a manner that the at least one adjusted constraint parameter is taken into account, i.e. an operational state which causes the wind turbine 1 to be operated outside the normal set of operational constraints but within the adjusted set of constraints, can be selected. The new operational states may further be derived in such a manner that an optimal power production of the wind turbine 1 is obtained, given the prevailing operating conditions and the adjusted set of operational constraints.

[0074] In the embodiment illustrated in FIG. 2, a new operational state in the form of new settings for the rotational speed of the rotor, , and the pitch angle, , is forwarded to the production controller 9 of each of the energy generating units 5.

[0075] Finally, the energy generating units 5 are controlled in accordance with the new operational states received from the multirotor wind turbine controller 10.

[0076] FIG. 3 is a graph showing a torque, M.sub.yaw, on a yawing mechanism of a multirotor wind turbine as a function of time, t. An operational constraint in the form of a range within which the torque, M.sub.yaw, is allowed to be, is illustrated as dashed lines, indicating upper and lower limits of the range.

[0077] Constraint parameters in the form of the upper limit of the range and the lower limit of the range are adjusted for a limited time interval. This is done in such a manner that the upper limit is temporarily increased and the lower limit is temporarily decreased. Thereby the torque, M.sub.yaw, is allowed to fluctuate more than normally, but the mean value of the torque, M.sub.yaw, is not changed.

[0078] FIG. 4 is also a graph showing a torque, M.sub.yaw, on a yawing mechanism of a multirotor wind turbine as a function of time, t. Also in FIG. 4, an operational constraint in the form of a range within which the torque, M.sub.yaw, is allowed to be, is illustrated as dashed lines, indicating upper and lower limits of the range.

[0079] However, in FIG. 4 the adjustment of the constraint parameters is performed in such a manner that the upper limit of the range as well as the lower limit of the range is increased. Thereby the torque, M.sub.yaw, is not allowed to fluctuate more than normally, but the torque, M.sub.yaw, is generally allowed to be at a higher level than normally.

[0080] FIG. 5 is also a graph showing a torque, M.sub.yaw, on a yawing mechanism of a multirotor wind turbine as a function of time, t. In FIG. 5 a constraint in the form of a setpoint value for the torque, M.sub.yaw, is illustrated as a dashed line.

[0081] A constraint parameter in the form of the setpoint value is adjusted for a limited time interval, by temporarily increasing the setpoint value. Accordingly, the wind turbine is temporarily controlled in accordance with a higher torque, M.sub.yaw, on the yawing mechanism.