WO2021180291A1

WO2021180291A1 - A yaw system for a multiple rotor wind turbine

Info

Publication number: WO2021180291A1
Application number: PCT/DK2021/050081
Authority: WO
Inventors: Torben Ladegaard Baun; Julio Xavier Vianna NETO; Jesper Lykkegaard NEUBAUER; Peter FYNBO; Søren DALSGAARD; Anders Yde Wollesen; Brian Jørgensen
Original assignee: Vestas Wind Systems A/S
Priority date: 2020-03-13
Filing date: 2021-03-12
Publication date: 2021-09-16
Also published as: EP4118324A1

Abstract

A multirotor (MR) wind turbine (1) comprising a plurality of energy generating units (5) attached to a load carrying structure (3) carried by a tower via a main yaw assembly. The energy generating units (5) are attached to the load carrying structure via a local yaw assembly which allows yawing of the energy generating unit relative to the load carrying structure. To allow individual orientation of each energy generating unit, the MR wind turbine is configured to define an operational angle of each energy generating unit relative to an prevailing wind direction. This operational angle corresponds to a normal operational situation where the energy generating unit converts wind energy. The MR wind turbine is further configured to yaw each energy generating units to and from the operational angle individually by use of the local yaw assembly.

Description

A YAW SYSTEM FOR A MULTIPLE ROTOR WIND TURBINE

INTRODUCTION

The present disclosure relates to a multirotor (MR) wind turbine comprising a plurality of energy generating units configured to convert wind energy by rotation of a rotor about a rotor axis. The MR wind turbine comprises a tower extending in an upwards direction, and a load carrying structure is carried by the tower via a main yaw assembly located between a first section and a second section of the load carrying structure. The main yaw assembly allows yawing of the load carrying structure relative to the tower. The MR wind turbine may have two or more energy generating units, e.g. four placed in two sets of two units on different load carrying structures in different heights.

BACKGROUND

Wind turbines normally comprise one or more energy generating units. The energy generating units comprise a load carrying hub carrying one or more wind turbine blades. The wind acts on the wind turbine blades, thereby converting the wind energy into rotation of the hub. The rotational movement of the hub is transferred e.g. to a generator, either via a gear arrangement or, in the case that the wind turbine is of a so-called direct drive type, the rotation is transferred directly without a gear. In the generator, electrical energy is generated, which may be supplied to a power grid.

Some wind turbines are provided with two or more energy generating units to increase the total power produced by the wind turbine, without having to provide the wind turbine with one very large, and therefore heavy, energy generating unit. Such wind turbines are sometimes referred to as 'multirotor wind turbines', in short MR wind turbines.

SUMMARY

It is an object to improve an MR wind turbine in terms of production loss occurring if one or more of the rotors are not producing.

According to this and other objects, the disclosure provides an MR wind turbine wherein each energy generating unit is attached to one of the first section and the second section via a local yaw assembly allowing yawing of the energy generating unit relative to the load carrying structure.

The MR wind turbine is configured to convert energy by at least one energy generating unit which is in its operational angle while at least one energy generating unit is in its parked angle.

The MR wind turbine thereby facilitates that the energy generating units can be moved to a parked angle individually and one energy generating unit may therefore be serviced while other energy generating units are maintained in operation.

The operational angle may e.g. be directly against the prevailing wind. In some wind turbines, the operational angle is 5-8 degrees, such as 6 degrees to the prevailing wind. This angle is sometimes referred to as "toe-angle" and it is typically provided to avoid collision between blades and the load carrying structure.

Typically, the MR wind turbine is configured to convert the wind energy into electrical energy. When an angle relative to the prevailing wind is mentioned herein, it is the angle of the rotor axis relative to the prevailing wind when both directions are projected onto a horizontal plane. A zero-degree angle therefore corresponds to the rotor axis pointing directly towards the wind.

Herein, reference is made to "a parked one of the energy generating units" when they are moved to a position where they do not convert energy, i.e. they may be idling or stopped.

Herein, at least one of x and y, e.g. at least one of the main yaw assembly and the local yaw assembly, means x or y or a combination of x and y.

The operational angle defines an angle which is desired in an operational situation.

Conversion of energy may be possible also for other angles away from the operational angle. The MR wind turbine may be configured to maintain conversion of wind energy by energy generating units during yawing in an operation angle sector. The operation angle sector may e.g. be a sector of plus 1-10 degrees from the operational angle, and depending on the specific MR design, it may also include a range from the operational angle minus a few degrees depending on the location of the rotor plane relative to the load carrying structure.

In the operation angle sector, the MR wind turbine may still convert wind energy but typically in a less optimal way as in the operational angle. Even though the MR wind turbine may be able to convert wind energy in the operation angle sector, it may also be deactivated and make no conversion when not being in the operational angle or in the operational angle sector.

The MR wind turbine may be configured to define a shut-down angle sector which includes the parked angle. The shut-down angle sector may be in a range of 10-90 degrees to the prevailing wind, i.e. e.g. to an angle where the rotor axis is perpendicular to the prevailing wind. When an energy generating unit moves within this sector, it shuts down.

The MR wind turbine may be configured for sequential operation of the main yaw assembly and at least one local yaw assembly. This means that the main yaw assembly and the local yaw assemblies are operated one after the other. In the case of a 4 rotor MR wind turbine with two sets of two rotors on two different load carrying structures, the main yaw assembly may operate both load carrying structures synchronized, e.g. simultaneously, and the yawing of each energy generating unit by its local yaw assembly may be synchronized, e.g. such that the energy generating units on the first sections are yawed simultaneously by the local yawing assemblies.

The MR wind turbine may be configured for simultaneous operation of the main yaw assembly and at least one local yaw assembly, or it may be configured to switch between simultaneous and sequential operation of the main yaw assembly and at least one local yaw assembly.

The switching between simultaneous and sequential operation may be automatic, e.g. based on energy consumption in the main yaw assembly and the at least one local yaw assembly. This could prevent overloading of a power system feeding the yaw assemblies with power.

The automatic switching may e.g. form an electrical power overload protection.

The main and local yaw structures may be controlled by a processing unit in the MR wind turbine and depending on different control strategies and input, e.g. input defining prevailing wind direction, wind speed, power production, and defining a desired operational status, e.g. on/off for each energy generating unit. Different operational scenarios may be considered and implemented as control strategies in the processing unit:

Scenario I: The main assembly may, initially, bring the load carrying structure to a yaw position considered optimal for the energy generating units considering operation of all energy generating units. This position may typically be a position, where all energy generating units are at the same distance to the wind, i.e. where the load carrying structure is transverse or perpendicular to the prevailing wind. Subsequently, one or more of the local yaw assemblies may be used for bringing individual energy generating units to a specific orientation relative to the prevailing wind, e.g. to the operational angle which could be 6 degrees away from the prevailing wind, or they could be moved to positions away from the operational angle, e.g. to obtain reduced loading and reduced power production or to obtain complete stopping of individual energy generating units.

Scenario II: The main assembly may, initially, bring the load carrying structure to a yaw position considered optimal for reducing loading on the load carrying structure or the loading on selected energy generating units which are to be parked e.g. for service, repair, or replacement. This position may typically be a position, where the energy generating unit which is to be parked is further away from the wind, i.e. placed downstream considering the prevailing wind direction and the other energy generating unit(s). The MR wind turbine may be configured, by use of the main yaw assembly, to arrange the load carrying structure in a position where the first section is upwind and the second section is downwind. In this configuration, the load carrying structure or at least a part thereof may be essentially parallel to the prevailing wind. Subsequently, one or more of the local yaw assemblies may be used for bringing individual energy generating units to a specific orientation relative to the prevailing wind, e.g. to the operational angle for those energy generating units which are not to be parked. Again, this operational angle could be 6 degrees away from the prevailing wind, and it allows selected energy generating units to continue operational while other energy generating units are parked. The energy generating units could generally be yawed by the local yaw assembly to that angle being suitable for the situation.

Scenario III: This scenario is similar to scenario II but whereas the parked energy generating unit in scenario II was a position where the energy generating unit which is to be parked is further away from the wind, it is opposite in scenario III, i.e. the parked energy generating unit is placed upstream considering the prevailing wind direction and the other energy generating unit(s). The energy generating units which are not to be parked are placed downwind and they will have to be operated as downwind turbines with the wind direction being from the nacelle towards the rotor.

The MR wind turbine may comprise a control system configured to control yaw functions by use of the main yaw assembly and the local yaw assembly of each energy generating unit and based on the operational angle and a desired status of each energy generating unit. The desired status may e.g. be: a) keep energy generating unit no x operational at optimum energy conversion; b) keep energy generating unit no x operational at reduced energy conversion; c) shut down energy generating unit no x in an arbitrary position relative to the load carrying structure; or d) shut down energy generating unit no x in a specific position relative to the load carrying structure;

The MR wind turbine may be configured to receive an input signal defining at least one of wind speed, wind direction, air density, air temperature, wind speed in gusts, and wind direction in gusts.

In a second aspect, the disclosure provides a method of operating an MR wind turbine according to the above description, wherein one energy generating unit is yawed away from the operational angle by operation of the local yaw assembly. Particularly, the method may include carrying out service or repair work on the energy generating unit which is yawed away from the operational angle. This energy generating unit may be yawed, by use of its local yaw assembly or by use of the main yaw assembly, to an angle where at least one blade of the energy generating unit can be reached from the load carrying structure. In that way, the load carrying structure may be used as a working platform e.g. for blade inspection.

Service may be carried out on the energy generating unit which is yawed to the parked angle and/or on the other energy generating unit.

An azimuth angle of the rotor may be locked by a rotor braking structure while operating the local yaw assembly, or the azimuth angle of the rotor may be changed while operating the local yaw assembly. In that case, the azimuth angle change is coordinated with the yawing to prevent collision between a blade and the load carrying structure.

One of the energy generating units may be yawed by the local yaw assembly to a position where the hub is accessible from the load carrying structure and objects may be transported between the hub and the load carrying structure.

One of the energy generating units may be yawed by the local yaw assembly to a position where a rear part of a nacelle of the energy generating unit is accessible from the load carrying structure and objects may be transported between the rear part and the load carrying structure.

A hatch in the hub or in the rear part of the nacelle may be provided and access through the hatch may be prevented if the energy generating unit is not in the position where the rear part or the hub is accessible from the load carrying structure.

A wind direction may be determined by a wind direction sensor attached to a first one of the energy generating units and/or to the load carrying structure, and that first energy generating unit may be placed in the operational position based on the determined wind direction.

Alternatively, a wind direction may be determined by a wind direction sensor which is not attached to a first one of the energy generating units. The wind direction sensor may e.g. be placed on another one of the energy generating units, on the load carrying structure, on the tower, on another load carrying structure, or elsewhere. In this situation, the first energy generating unit may be placed in the operational position based on the determined wind direction after it is corrected by an angle of the local yaw assembly of the first energy generating unit or an angle of the main yaw assembly.

A wind direction may also be determined by using at least two wind direction sensors. According to this method, the wind direction at a first one of the energy generating units may then be determined by combining a signal from the at least two wind direction sensors, e.g. by correcting the combined signal by an angle of the local yaw assemblies of the first energy generating unit and/or an angle of the main yaw assembly. One of the sensors could be placed on the load carrying structure carrying the first energy generating unit, on another energy generating unit, at the tower, or on another load carrying structure.

LIST OF DRAWINGS

The disclosure will now be described in further detail with reference to the accompanying drawings in which:

Fig. 1 illustrates an example of a front view of a MR wind turbine;

Fig. 2 is a top view of the multirotor wind turbine of Figs. 1;

Fig. 3 shows a detail of the main yaw assembly;

Fig. 4 illustrates the local yaw assembly of one of the energy generating units;

Fig. 5 illustrates schematically a top view of the MR wind turbine;

Figs. 6 and 7 illustrate schematically two different configurations of the MR wind turbine according to two different operational scenarios; Figs. 8-12 illustrate a scenario wherein an energy generating unit which is to be serviced is shut down and then turned with the local yaw assembly, so that a rear-end or a front end of the nacelle is above the load carrying structure; and

Fig. 13 illustrates a wind turbine with two anemometers.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a front view of a multirotor wind turbine 1 comprising a tower 2 carrying two load carrying structures 3 according to an embodiment of the invention. The load carrying structures 3 are arranged one above the other along the length of the tower 2.

Each load carrying structure 3 comprises two sections 4, extending away from the tower 2 on opposite sides of the tower 2, as seen from the viewing angle of Fig. 1. Each section 4 carries an energy generating unit 5. The energy generating units 5 comprise a nacelle 6 and a rotor 7 carrying three wind turbine blades 8.

Each section 4 comprises a primary structure 9, in the form of a tube, and two secondary structures 10, in the form of double wires. In Fig. 1, only one of the secondary structures 10 for each section 4 is visible.

The primary structures 9 extend away from the tower 2 along a direction which forms an acute angle with respect to a substantially vertical longitudinal axis defined by the tower 2. The primary structures 9 extend away from the tower 2 in an inclined upwards direction.

The secondary structures 10 extend away from the tower 2 along a direction which is substantially perpendicular to the substantially vertical longitudinal axis defined by the tower 2. Thereby, the secondary structures 10 extend away from the tower 2 along a substantially horizontal direction. Accordingly, an angle is defined between the direction in which primary structure 9 of a given section 4 extends, and the plane in which the secondary structures 10 of the section 4 extend.

The primary structure 9 and the secondary structures 10 are attached to the tower 2 via a main yaw assembly 11, allowing the entire load carrying structure 3 to perform yawing movements with respect to the tower 2 to direct the rotors 7 into the incoming wind.

When gravity acts on the energy generating units 5, the mutual positions of the primary structures 9, and the secondary structures 10 cause push in the primary structures 9 and pull in the secondary structures 10, and a preload is thereby introduced in the secondary structures 10, due to the gravity acting on the energy generating units 5.

The MR wind turbine comprises a microprocessor-based wind turbine control system not illustrated. The control system is configured to control yaw functions of the MR wind turbine and handles different input signals defining at least one of wind speed, wind direction, air density and/or temperature, and wind speed and direction in gusts. Further, the control system receives a desired operational state e.g. full operation, reduced power, or deactivated. The control system may, depending on the implementation of the yaw assemblies, also receive error signals, loading signals, speed signals and other servo system related signals from servo drives included in each yaw assembly.

Based on the inputs and various settings and rules, the control system is configured to output control signals for the main yaw assembly and the local yaw assembly.

The control system may be configured to dynamically redefine a controller parameter e.g. a parameter defining structural properties of the MR wind turbine. The redefining may be based on a main yaw angle defined by the main yaw assembly and/or at least one local yaw angle defined by a local yaw assembly. The structural properties may e.g. relate to control features for structural dampening etc.

The disclosed MR wind turbine is an MR 2 turbine meaning that it carries 2 energy generating units. It may just as well carry more units, e.g. 3 or 4 units, e.g. two rows of two units in different altitude and carried by different load carrying structures.

Fig. 2 is a top view of the multirotor wind turbine 1 of Figs. 1 and illustrates that the rotor axes 15 forms an angle to the prevailing wind direction illustrated by the arrow 16. In the illustrated MR wind turbine, this angle is not parallel to the rotor axes. The offset from parallel is chosen to prevent collision between the blades and the load carrying structure. The difference between the parallel direction and the direction of the rotor axes is sometimes referred to as toe-angle, in the illustrated embodiment, it is 6 degrees. This angle may, in one embodiment, be considered as the operational angle to the prevailing wind. In other embodiments, other angles, e.g. zero degrees, may be considered as the operational angle, i.e. the operational angle is an angle desired for the rotor axes relative to the prevailing wind direction during normal operation.

Fig. 3 shows a detail of the multirotor wind turbine 1 of Figs. 1-2, illustrating the main yaw assembly attaching the load carrying structure 3 to the tower 2. One of the secondary structures 10 of each section 4 is attached to the spacer arrangement 12. The spacer arrangement 12 is, in turn, attached to a movable part of the main yaw assembly 11. The other secondary structure 10 of each section 4 is attached directly to the movable part of the main yaw assembly 11.

For each section 4, one of the secondary structures 10 is attached to the tower 2 via an attachment point arranged on a spacer arrangement 12, the attachment point thereby being arranged behind the tower 2 and at a distance from the tower 2. The other secondary structure 10 is attached to the tower 2 at an attachment point which is arranged in front of the tower 2 and close to the tower 2. As described above with reference to Fig. 2, the primary structure 9 extends from a position behind the tower 2 to a position in front of the tower 2. This allows the rotor 7 of each of the energy generating units 5 to be arranged in front of the tower 2, and in front of the primary structure 9 and both secondary structures 10. Thereby the wind turbine blades 8 are kept clear from these structures, and the risk of collision is minimized.

Fig. 4 illustrates further details of the end of one section 4 of the load carrying structure 3. The end section comprises an interface 17 forming a local yaw assembly 18 for holding the energy generating unit 5 and allowing the energy generating unit to yaw relative to the load carrying structure 3.

Fig. 5 illustrates schematically a top view of the MR wind turbine and illustrates the option of turning the load carrying structure 3 by use of the main yaw assembly 11 and turning each energy generating unit 5 by use of the local yaw assemblies 18. The three individual degrees of freedom are indicated by the arrows 19, 20, 21.

Figs. 6 and 7 illustrate schematically two different configurations of the MR wind turbine with prevailing wind direction indicated by the arrow 16, i.e. from the same direction.

In relation to Figs. 6 and 7, the energy generating unit 22 is an energy generating unit which is to maintain operation and in Fig. 7, the energy generating unit 23 is a parked energy generating unit. The energy generating unit 22 is maintained operational.

Minimize torsion load control:

In one control setting for the control system, the incentive is to minimize the wind turbine yaw load by reducing tower torsion while maintaining the above-mentioned status of the two energy generating units. This control setting implies a transition from the previously defined scenario I to scenario II. The starting situation corresponds to said scenario I illustrated in Fig. 6. In this scenario, the distance from each energy generating unit to the wind, illustrated by the dotted line 24 is the same. Flerein, we refer to the load carrying structure being perpendicular to the prevailing wind direction since the dotted line 4 is perpendicular to the prevailing wind direction. The specific structure of the sections 4 of the load carrying structure 3 is not necessarily perpendicular to the wind direction.

Fig. 7 illustrates the MR wind turbine after the transition to the scenario II. The load carrying structure 3 is yawed by use of the main yaw assembly and now has an orientation less transverse to the prevailing wind direction. In this situation, one section of the load carrying structure is upwind and the other section of the load carrying structure is downwind. The energy generating unit 23 becomes parked and unable to convert wind energy. Service and repair may be carried out on this unit. In the following, different control situations are described for the operational energy generating unit 22 and the parked energy generating unit 23.

The energy generating unit 22 could be operating during the transition, e.g. by simultaneous operation of the main yaw assembly and the local yaw assembly of the energy generating unit 22. The energy generating unit 22 may, alternatively, be shut down during the yawing, and it could be restarted once the MR wind turbine is in the configuration in Fig. 7, i.e. the yawing by the main yaw assembly and local yaw assembly of the energy generating unit 22 could be operated sequentially. This has, particularly, the advantage that energy consumption in the yaw motors is limited. The sequential operation may be carried out stepwise, e.g. 5 degrees with the main yaw assembly, then 5 degrees with the local yaw assembly, then again 5 degrees with the main yaw assembly etc. until reaching the situation in Fig. 7. This will limit yaw error and reduce energy consumption in the yaw motors.

In the operating state of energy generating unit 22, this unit is kept at a toe angle greater than the normal toe angle and this unit continuous to produce power in this state, e.g. in a completely normal power mode or in a de-rated power production mode.

The yaw angle of the energy generating unite 22 may differ from the angle which is optimal seen from an energy conversion point of view. This may e.g. be to avoid wake to hit the parked energy generating unit 23.

In the control system, controller parameters could be tuned differently. This could e.g. be damping parameters or wind correction parameters.

In this operating state of the energy generating unit 22, the main yaw assembly may be used for keeping the energy generating unit 22 upwind. Alternatively, the main yaw assembly is halted, and the local yaw assembly for the energy generating unit 22 is used. This is particularly feasible for smaller angle corrections e.g. below 10 degrees. The energy generating unit 23 is parked, e.g. for service. In the transition from scenario I (Fig. 6) to scenario II (Fig. 7), the energy generating unit 23 may be stopped. The local yaw assembly of this unit may be stopped leaving the reorientation of the parked energy generating unit to the main yaw assembly, or the local yaw assembly may be used e.g. for positioning the energy generating unit 23 in a particularly suitable orientation considering the service work.

The local yaw assembly may, as an example, yaw the energy generating unit 23 such that a particular part of the energy generating unit 23 is accessible from load carrying structure, e.g. such that the rear part or a front part, e.g. the hub part of the energy generating unit 23 is vertically over the load carrying structure.

If the hub part is to be moved over the load carrying structure, an azimuth control may be used to avoid blade collision with the load carrying structure. In one embodiment, the rotor is rotated simultaneously with the yawing by use of the local yaw assembly.

The coordinated control of the rotor rotation and the yawing of the energy generating unit could be carried out step-by-step or continuously.

The rotor rotation may be obtained e.g. by a turner gear, different rotor lock positions, or in a generator motor mode where the generator is used as a motor for rotating the rotor.

Once yawed over the load carrying structure, both the rotor and the local yaw assembly must be braked to prevent the blades from rotating or moving into the load carrying structure. A yaw lock and a yaw assembly lock may be provided and controlled by the control system.

Figs. 6 and 7 are referred to with the assumption that the energy generating units 22, 23 are mounted on top of the load carrying structure. It could also be a "top hinged" MR wind turbine in which the energy generating units are hanging below the load carrying structure.

In Fig. 6, the load carrying structure 3 is placed transverse to the prevailing wind direction and both energy generating units 22, 23 are placed in the operational angle relative to a prevailing wind direction. In this situation, the energy generating unit converts wind energy e.g. into electrical power. The operational angle is 6 degrees of the rotor axis to the prevailing wind direction. The 6 degrees toe angle is selected to avoid blade collision with the load carrying structure 3.

Figs. 8-10 illustrate a scenario wherein the energy generating unit which is to be serviced is shut down and then turned with the local yaw assembly, so that the rear-end of the nacelle or the front end of the nacelle is above the load carrying structure. Referring to Fig. 8 and 9, a hole in the load carrying structure is vertically below an access hatch in the nacelle, and objects can be transported this way.

Objects could be wind turbine components, spare parts, or personnel etc. In one embodiment, the access to the load carrying structure may form a rescue path.

In Figs. 8 and 9, access for objects is provided from the load carrying structure to a rear end of the energy generating unit.

In Fig. 10 access for objects are provided from the load carrying structure to the front end of the nacelle, or directly to the hub. Movement of the parked energy generating unit 23 to this position may include coordinated azimuth rotation and yawing of the local yaw assembly to avoid blade collision with the load carrying structure.

A hatch may be provided in the rear part of the nacelle, and a locking mechanism may be provided for preventing access through the hatch if the energy generating unit is not in the position where the rear part is accessible from the load carrying structure.

A hatch may be provided in the hub, and a locking mechanism may be provided for preventing access through the hatch if the energy generating unit is not in the position where the hub is accessible from the load carrying structure. The MR wind turbine may e.g. comprise an electronic sensor, or a mechanical structure, which is triggered by correct positioning of the hatch relative to the load carrying structure, e.g. for safe passage from the nacelle to the load carrying structure, and once correct position is achieved, the hatch is released for opening.

If blades are inspected or repaired by use of the load carrying structure as a working platform, particularly the secondary structures 10, c.f. previous Figs, may be used to carry a separate platform for carrying out work on the blades.

Figs. 11-12 illustrate a scenario wherein the energy generating unit which is to be serviced is shut down and then turned away from the wind by use of the main yaw assembly. The other energy generating units are turned to the operational angle by use of the local yaw assembly, in this example they are turned to an angle where the rear-end of the nacelle is above the load carrying structure while the energy generating unit is in operation. Also, in this example, an entrance hole in the load carrying structure may be located vertically below an access hatch in the rear end of the nacelle, and objects can be transported this way even during operation. Wind sensors measuring wind direction often measure relative to a line where they are mounted. For a conventional wind turbine with one rotor, the wind direction sensor is often mounted on top of the nacelle such that when direction measures 0° the incoming wind is perpendicular to the rotor plane yielding optimal power production. The three yaw degree of freedom obtained by the main yaw assembly and the two local yaw assemblies are indicated by arrows in Fig. 13. Wind direction sensors 25, 26, and 27 are mounted on the energy generating units and on the arm structure.

The sensor 27 on the arm structure measures wind speed and direction relative to the orientation of the arm and the sensors 25, 26 on the energy generating units, measure wind direction and speed relative to the nacelle.

One, two, or all anemometers may be utilized for obtaining wind direction information for controlling use of the main yaw assembly or the local yaw assembly. When combining the measurement from several sources the difference in orientation of the sensor needs to be considered. This may include adjusting the wind angle measurement of the sensor 27 based on the main yaw angle and adjusting the wind angle measurement of the sensors 25, 26 based on the main yaw angle and the local yaw angle of the energy generating unit on which it is attached.

Claims

1. A multirotor (MR) wind turbine (1) comprising:

- a plurality of energy generating units (5) configured to convert wind energy by rotation of a rotor about a rotor axis;

- a tower (2);

- at least one load carrying structure (3) carried by the tower via a main yaw assembly (11) located between a first section (4) and a second section (4) of the load carrying structure (3) and allowing yawing of the load carrying structure relative to the tower, wherein each energy generating unit (5) is attached to one of the first section and the second section via a local yaw assembly (18) allowing yawing of the energy generating unit relative to the load carrying structure, and each energy generating unit (5) is yawable between an operational angle for converting energy and a parked angle by use of at least one of the main yaw assembly and the local yaw assembly, and the MR wind turbine is configured to convert energy by at least one energy generating unit which is in its operational angle while at least one energy generating unit is in its parked angle.

2. The MR wind turbine according to claim 1, configured to maintain conversion of wind energy during yawing of one of the energy generating units (5) in an operation angle sector.

3. The MR wind turbine according to claim 1 or 2, configured to stop conversion of wind energy during yawing of one of the energy generating units (5) in a shut-down angle sector.

4. The MR wind turbine according to claim 3, wherein the shut-down angle sector extends to a position where the rotor axis is perpendicular to the prevailing wind direction and includes the parked angle.

5. The MR wind turbine according to any of the preceding claims, configured to place one energy generating unit in the parked angle by use of the main yaw assembly and to place the other energy generating unit in the operational angle by use of its local yaw assembly.

6. The MR wind turbine according to claim 5, wherein the energy generating unit which is placed in the operational angle by use of its local yaw assembly, is placed with the rotor axis extending along the load carrying structure within 10 degrees from parallel with the load carrying structure.

7. The MR wind turbine according to any of the preceding claims, configured to place one energy generating unit in the operational angle by use of the main yaw assembly, and the other energy generating unit in the parked angle by use of its local yaw assembly.

8. The MR wind turbine according to any of the preceding claims, configured for sequential operation of the main yaw assembly and at least one local yaw assembly.

9. The MR wind turbine according to any of the preceding claims, configured for simultaneous operation of the main yaw assembly and at least one local yaw assembly.

10. The MR wind turbine according to any of the preceding claims, configured to switch between simultaneous and sequential operation of the main yaw assembly and at least one local yaw assembly.

11. The MR wind turbine according to claim 10, configured to switch automatically between simultaneous and sequential operation based on energy consumption in the main yaw assembly and the at least one local yaw assembly.

12. The MR wind turbine according to any of the preceding claims, configured, by use of the main yaw assembly, to arrange the load carrying structure (9, 10) in a position where the first section is upwind and the second section is downwind, and to arrange an energy generating unit on the first section in the operational angle by use of its local yaw assembly, and to arrange an energy generating unit on the second section in the parked angle.

13. The MR wind turbine according to any of claims 1-11, configured, by use of the main yaw assembly, to arrange the load carrying structure (9, 10) in a position where the first section is upwind and the second section is downwind, and to arrange an energy generating unit on the second section in the operational angle by use of its local yaw assembly, and to arrange an energy generating unit on the first section in the parked angle.

14. The MR wind turbine according to any of the preceding claims, comprising a control system configured to control yawing by use of the main yaw assembly and the local yaw assembly of each energy generating unit and based on the operational angle and a desired status of each energy generating unit as either operational or parked.

15. The MR wind turbine according to claim 14, wherein the control system is configured to receive an input signal defining at least one of wind speed, wind direction, air density, air temperature, wind speed in gusts, and wind direction in gusts, and to control yawing based on the input.

16. The MR wind turbine according to claim 14 or 15, wherein the control system is configured to dynamically redefine a controller parameter based on a yaw angle or based on the input signal.

17. The MR wind turbine according to any of claims 14-16, wherein the control system is configured to coordinate simultaneous yawing by use of a local yaw assembly and rotation of the rotor to change an azimuth angle while yawing the energy generating unit.

18. A method of operating an MR wind turbine according to any of the preceding claims, wherein one energy generating unit (5) is placed in the parked angle by operation of the local yaw assembly or by operation of the main yaw assembly while at least one other energy generating unit is placed in the operational angle.

19. The method according to claim 18, comprising carrying out service on the energy generating unit (5) which is yawed to the parked angle.

20. The method according to claim 18 or 19, wherein the energy generating unit (5) which is yawed to the parked angle is positioned to enable at least one blade of the energy generating unit to be reached from the load carrying structure.

21. The method according to any of claims 18-20, wherein an azimuth angle of the rotor is locked by a rotor braking structure while operating the local yaw assembly.

22. The method according to any of claims 18-20, wherein an azimuth angle of the rotor is changed while operating the local yaw assembly, and wherein the azimuth angle change is coordinated with the yawing to prevent collision between a blade and the load carrying structure.

23. The method according to any of claims 18-22, comprising the step of yawing one of the energy generating units by the local yaw assembly to a position where the hub is accessible from the load carrying structure and transporting objects between the hub and the load carrying structure.

24. The method according to any of claims 18-23, comprising the step of yawing one of the energy generating units by the local yaw assembly to a position where a rear part of a nacelle of the energy generating unit is accessible from the load carrying structure and transporting objects between the rear part and the load carrying structure.

25. The method according to any of claims 23-24, comprising the step of providing a hatch in the hub or in the rear part of the nacelle, and preventing access through the hatch if the energy generating unit is not in the position where the rear part or the hub is accessible from the load carrying structure.

26. The method according to any of claims 18-25, comprising determining a wind direction by a wind direction sensor attached to a first one of the energy generating units and placing the first energy generating unit in the operational position based on the determined wind direction.

27. The method according to any of claims 18-25, comprising determining a wind direction by a wind direction sensor not attached to a first one of the energy generating units and placing the first energy generating unit in the operational position based on the determined wind direction corrected by an angle of the local yaw assembly of the first energy generating unit or an angle of the main yaw assembly.

28. The method according to any of claims 18-25, comprising determining a wind direction by using at least two wind direction sensors and determining the wind direction at a first one of the energy generating units by combining a signal from the at least two wind direction sensors.

29. The method according to claim 28, comprising correcting the combined signal by an angle of one of the local yaw assemblies and/or an angle of the main yaw assembly.