Field of the Invention

The present invention relates to a wind turbine comprising a nacelle with a mainframe, a wind turbine tower with a mounting flange at a top end for mounting to a yaw bear- ing of a yaw system. The yaw system further comprises a brake calliper unit arranged relative to a brake disc connected to the wind turbine tower, wherein the brake calliper unit is connected to a drive unit for operating a number of brake pads in the brake calliper unit between an engaged position and a disengaged position. The present invention also relates to a method of operating a yaw system of a wind turbine as described above, wherein the method comprises the steps of positioning the brake calliper unit relative to the brake disc, coupling the brake calliper unit to the drive unit, and yawing the nacelle relative to the wind turbine tower via the drive unit.

Background of the Invention

It is well known that the capacity of wind turbines has increased from a kilowatt [kW] range into a megawatt [MW] range which, in turn, means that the size of the wind turbine as well as the size of the rotor has increased. It is also known when increasing the diameter of the swept area, the rotor will increase more in size relative to the available nacelle space. By increasing the size of the wind turbine rotor, the weight thereof also increases, which in turn also increases the required strength and capacity of the yaw system. It thus becomes more and more difficult to position yaw motors and yaw gears relative to the yaw bearing of a sufficient size in order to meet these requirements. It also becomes more and more difficult to access the yaw system for service due to the limited space.

Conventional yaw system comprises a number of yaw gears driven by a corresponding number of electrical yaw motors where the teeth of the pinion shaped yaw gear engage the teeth of the large ring gear connected to the wind turbine tower. A known problem with such yaw systems is that the teeth on the large ring gear are not evenly worn as the wind turbine is normally yawed back and forth within a relatively small yawing range. Another known problem is that the aerodynamic yaw movement often leads to damages in the yaw gear and also worn or even broken yaw teeth. Such repairs are often very complex and time consuming, thus increasing the repair costs. Some conventional yaw systems also comprise friction elements in the form of brake pads which are passively pushed against a sliding surface of the yaw ring or brake disc by pretensioned springs in order to apply a constant friction torque. Other conventional yaw systems also comprise actively operated brake pads contacting a brake disc wherein a hydraulic system operates the brake pads in order to apply a high friction torque when not yawing and a low friction torque when yawing. Such passive or active systems suffer from stick slip vibrations which generate a loud squealing noise when the mainframe is sliding along the brake pads. This noise leads to customer dissatisfaction and optionally also complaints from people living next to the wind tur- bine. Used grease and other unwanted particles may contaminate the lubrication of the sliding pads and teeth, and also contaminate the interior of the wind turbine tower.

One way to solve the above-mentioned problems is disclosed in the article "Analysis of load reduction possibilities using a hydraulic soft yaw system for a 5-MW turbine and its sensitivity to yaw-bearing friction" by S. Stubkier et al. The article discloses a tree-bladed 5MW wind turbine with a hydraulic operated yaw system comprising six hydraulic motors each connected to a pinion gear which, in turn, engages the ring gear on the wind turbine tower. A hydraulic operated accumulator is arranged on either side of the motor and in fluid communication with the motor. The two hydraulic ac- cumulators are interconnected via a cross coupling so that when the pressure raises in one accumulator the pressure in the other accumulator falls. The nacelle is locked hy- draulically by increasing the pressure inside the motor which is achieved by operating the motors as pump units. The accumulators act as shock absorbers allowing the nacelle to yaw slightly in either direction even when locking in a desired yaw angle. It is stated in the article that this above-mentioned system significantly reduces the fatigue loads and the maximum loads on the wind turbine by providing a shock absorbing effect.

The article further discloses that the yaw bearing in the above-mentioned wind turbine can have a ball bearing or a slide bearing with upper sliding pads where the mainframe slides along these upper pads. The article states that the friction force generated by this yaw bearing has a minimal effect on the yaw movement at high wind speed since the majority of wind gusts generate a yaw torque exceeding this friction force. At low wind speed, the majority of wind gusts do not generate a yaw torque exceeding the friction force and, thus, the yaw movement will be at a minimal. It is stated that the best effect is achieved by a yaw bearing generating a friction force of about 1 mega Newton metre [MNm] . It is further stated that the best result in relation to reduction of the fatigue loads and of the maximum loads and in view of the yaw torque is achieved by the ball bearing.

However, the above-mentioned system in combination with a ball bearing causes a very jerky movement of the nacelle and increased yaw movement in either direction due to the use of hydraulic operated accumulators to provide the shock absorbing effect. This movement will, in turn, lead to increased wear on the engaging teeth, partic- ular on the pinion gear, and potentially one or more broken teeth.

Patent application US2013/014946 by Sassen discloses a yaw actuation and braking system, a method for controlling a yaw position of a wind turbine nacelle with respect to a tower. The system relies on a first and second linear actuator, each at one end fixed in an anchor portion of the nacelle, and each at the other end having a brake calliper interacting with a brake rotor connected to the tower.

Thus, there is a need for an improved design of the yaw system that takes into account the limited space for the yaw system and solves the problem of damaged yaw gears and broken teeth.

Object of the Invention

An object of the invention is to provide a wind turbine yaw system that solves the above-mentioned problems. Another object of the invention is to provide a wind turbine yaw system that reduces the noise generated by the yaw system.

Yet another object of the invention is to provide a wind turbine yaw system that has a reduced risk of a mechanical failure.

A further object of the invention is also to provide a method of operating and repairing a wind turbine yaw system that provides easy access to the yaw system and allows for a quick and simple replacement of a damaged or worn part. Description of the Invention

An object of the invention is achieved by a wind turbine comprising:

a rotor arranged relative to a nacelle, the rotor comprising at least two wind turbine blades mounted to a hub configured to be rotatably connected to a drivetrain in the wind turbine,

the nacelle being rotatably connected to the wind turbine tower via a yaw system, the nacelle comprising a mainframe configured to support the rotor during rotation, the mainframe having a bottom end facing a top end of the wind turbine tower when installed,

the wind turbine tower comprising a flange arranged at said top end, the flange being configured to be connected to the yaw system,

the yaw system comprising a yaw bearing unit with a first bearing part and a second bearing part rotatably arranged relative to the first bearing part, the first bearing part is connected to the mainframe and the second bearing part is connected to the flange, the yaw system further comprises a brake disc and at least one brake calliper unit arranged relative to the brake disc, the at least one brake calliper unit comprising at least one brake pad configured to move between an engaged position and a disengaged position, the at least one brake calliper unit further being connected to a drive unit configured to operate said at least brake calliper unit, characterised in that, at least one actuator unit is connected to said at least one brake calliper unit via at least one movable element, the at least one actuator unit being configured to move the at least one brake calliper between a retracted position and an extended position via said at least one element, wherein the yaw system is configured to yaw the nacelle relative to the tower by moving the at least one actuator unit in one direction when said at least one brake calliper unit is in the engaged position.

This provides a simple yaw system that does not require electrical or hydraulic motors, pinion gears or a large rim gear, thus eliminating the need for replacing damaged motors and worn or broken teeth due to aerodynamic yawing. This saves space inside the nacelle and, thus, allowing for easier access to the individual components of the yaw system. The present configuration also provides a relatively clean and dry system compared to other conventional yaw system as it does not have any engaging teeth or loaded sliding pads requiring lubrication which otherwise could contaminate the surrounding components and reduce the function of the brake pads or drip onto the sides of the wind turbine tower and contaminate the wind turbine tower.

The present yaw system can be adapted to the particular configuration and capacity of the wind turbine. The present yaw system can also be adapted to wind turbines outfitted with a larger rotor and, thus, a greater capacity. The wind turbine may be any type of wind turbine, such as a variable speed wind turbine, in which a yawing capability is desired. The rotor may comprise, but not limited to, two or three wind turbine blades.

The wind turbine tower comprises at least a top end facing the nacelle when installed. The top end comprises a mounting flange which acts as a mounting interface for the yaw system. The brake disc, e.g. a ring shaped brake disc, may be a separate element configured to be mounted to the flange or be integrated into the flange so that the brake disc and flange form a single element. The brake disc allows the braking torque to be transferred to the wind turbine tower. According to one embodiment, the at least one actuator unit comprises a first actuator unit and at least a second actuator unit, and the at least one brake calliper unit comprises a first brake calliper unit and at least a second brake calliper unit, wherein the first actuator unit is connected to the first brake calliper unit via a first movable element, and the at least second actuator unit is connected to the at least second brake calliper unit via at least a second moveable element.

The present yaw system comprises at least one brake calliper unit, i.e. a moveable brake calliper unit, connected to at least one actuator unit via at least one moveable element. Each brake calliper unit and its adjacent actuator unit may be arranged in a pair where said actuator unit faces in a clockwise or anticlockwise direction. A first pair comprising a first brake calliper unit and a first actuator unit may be arranged relative to a second pair comprising a second brake calliper unit and a second actuator unit along a circumference of the brake disc. This allows the first and second pairs to be operated independently or synchronically depending on the desired yaw movement. The brake calliper unit comprises at least one brake pad configured to contact the brake disc when engaged. The brake pad may be a movable brake pad unit arranged in a calliper structure, e.g. an L-shaped or U-shaped yoke, configured to move said brake pad into and out of engagement with a contact surface of the brake disc. At least one brake pad, e.g. movable brake pad unit, may be arranged relative to a top side and/or a bottom side of the brake disc. The brake calliper unit may comprise a number of individual brake pads, e.g. between 1 and 5 brake pads. The brake pads are unloaded when the brake calliper unit is moved relative to the brake disc and loaded when the brake calliper unit is used to yaw the nacelle, thus reducing the wear on the individual brake pads to a minimum.

The brake calliper unit may further be coupled to a drive unit configured to operate the brake calliper unit. The drive unit may be a hydraulic, pneumatic or electrical drive unit configured to drive the brake calliper unit via a suitable hydraulic or pneumatic fluid or an electrical connection. The fluid may be hydraulic oil, compressed air, or another suitable fluid, which can be supplied to the brake calliper unit via a suitable compressor and/or pump system in the drive unit. The pump system may further be connected to at least one directional control valve for controlling the actuator units and at least one directional control valve for controlling the brake calliper units. The respective control valves may be selected depending on the configuration of the actuator unit and brake calliper unit. The drive unit may further be connected to a suitable control unit configured to control the operation of the present yaw system. The control unit and/or the drive unit may be arranged in the nacelle, in the wind turbine tower, or in a combination thereof. This enables the drive unit to control the holding torque supplied by the individual brake pads of the brake calliper unit.

The actuator unit is configured to move the brake calliper unit in a substantially tangential direction relative to a radial direction and may also be coupled to the drive unit or another drive unit connected to the control unit. The actuator unit may be a hydraulic, pneumatic or electrical operated actuator. The movable element may be a piston rod of the actuator unit or an intermediate rod connected to the piston rod. The actuator unit may in example, but not limited to, be a linear actuator. This enables the drive unit to activate the individual actuator unit and, thus, control the drive torque supplied to the nacelle.

The drive unit may in example, but not limited to, generate an operating pressure be- tween 100 bar to 500 bar. Each brake calliper unit may in example, but not limited to, generate a maximum holding torque between 200 kilo Newton metre [kNm] and 1600 kNm. The drive unit may in example, but not limited to, generate a maximum drive torque for each brake calliper unit between 200 kNm and 1600 kNm. This allows the drive capacity and/or the holding capacity of the yaw system to be selected depending on the upscaling of the rotor as well as the capacity (e.g. rated MW) of the wind turbine.

Each actuator unit may be selected so that it has a holding torque greater than the above-mentioned maximum holding torque of the brake calliper unit. In the event that the yaw system is subjected to a very high external moment, the brake calliper will thus start to slide relative to the brake disc before the actuator unit is unintentionally moved in one direction. There is thus no excessive loading on the mechanical components and, thus, a minimal risk of a mechanical failure. According to a special embodiment, the first actuator unit faces in one rotational direction and the at least second actuator unit faces in an opposite rotational direction.

The individual actuator units in the first and second pairs may all face in the same rotational direction, e.g. in the clockwise or anticlockwise direction. The rotational direction defines the direction in which the nacelle and, thus, the rotor are yawed. Alternatively, the first actuator unit in the first pair may face in the clockwise rotational direction, and the second actuator unit in the second pair may face in the anticlockwise rotational direction, or vice versa. This allows the actuator units to be operated in at least two groups where one group is operated differently from another group during the yaw movement.

In example, but not limited to, one half of all actuator units may face in the clockwise rotational direction while the other half of actuator units may face in the anticlockwise rotational direction. This enables one half to extend while the other half retracts, and vice versa.

The brake calliper units in the first and second pairs may also be operated in at least two groups where one group is operated differently from another group during the yaw movement. In example, but not limited to, one half of all brake calliper units may be in a loaded state, i.e. brought into engagement with the brake disc, while the other half of the brake calliper units may be in a non-loaded state, i.e. brought out of engagement with the brake disc.

According to one embodiment, the first and second actuator units are configured to be operated independently during yawing where one of said first and second actuator unit is extended in the one direction, while the other of said first and second actuator units is retracted in an opposite direction.

During relatively small yaw movements, i.e. yawing equal to or below an angular threshold in either rotational direction, and/or during relative large yaw movements, i.e. yawing above the angular threshold in either rotational direction, the individual actuator units may be operated in groups as described above.

In one example, the first actuator unit may be actively extended from a retracted position towards an extended position via the drive unit while the second actuator may be actively retracted from the extended position towards the retracted position, or vice versa. Alternatively, one of said first and second actuator units may be actively moved as mentioned above while the other actuator unit is being passively moved due to the active movement of the one actuator unit. This allows the yaw system to apply at least a minimum of drive torque or a maximum drive torque dependent on the aerodynamic forces acting on the rotor.

In another example, the first and second actuator units may both be actively extended from the retracted position towards the extended position or actively retracted from the extended position towards the retracted position via the drive unit. Alternatively, one of said first and second actuator units may be actively moved as mentioned above while the other actuator unit is being passively moved due to the active movement of the one actuator unit. This allows the yaw system to apply the required drive torque needed to yaw the nacelle and the rotor. This allows some actuator units to be used to reposition the brake calliper units which are not engaged. The number of actively moved actuator units may be varied, e.g. as function of the wind speed, wind direction or loadings on the wind turbine blades. This allows the yaw system to save energy and only activate the number of actuator units needed to perform the yaw movement. The passively moved actuator units may optionally be used as brakes to control the speed of the yaw movement by adjusting the internal pressure in these actuator units.

According to a special embodiment, the first and second brake calliper units are further configured to be operated independently during yawing where one of said first and second brake calliper units is engaging the brake disc while the other of said first and second actuator units is disengaged from the brake disc.

During relative large yaw movements the individual brake calliper units may also be operated in groups as described above. In a first operating mode, the first brake calliper unit may be engaged via the drive unit while the second brake calliper unit may be disengaged via the drive unit, or vice versa. The first brake calliper unit may thus be used to perform the yaw movement while the second brake calliper unit may be repositioned relative to the brake disc via the actuator units as described above. After repositioning the brake calliper units, the brake calliper units may be operated in a second mode where the first brake calliper unit may be disengaged via the drive unit and the second brake calliper unit may be engaged with the brake disc via the drive unit. If further yaw movement is required, the brake calliper units may alternately switched between the first and second operating modes. When the nacelle is yawed into the desired yaw angle, the first and second brake calliper units may both be engaged with the brake disc via the drive unit in order to lock the nacelle and thus the rotor in the desired yaw angle. The nacelle can thus be yawed into the desired yaw angle by using a crawling function. This allows the yaw system to apply at least a minimum of holding torque in order to yaw the nacelle and thus the rotor. The number of engaged brake calliper units may also be varied, e.g. as function of the wind speed, wind direction or loadings on the wind turbine blades. The disengaged brake calliper units can then be repositioned and subsequently take over from the all or some of the engaged brake calliper units. This allows the yaw system to save energy and only activate the number of brake calliper units needed to perform the yaw movement. This also allows the yaw system to apply at least a minimum of holding torque at all time. According to another special embodiment, the first and second brake calliper units are configured to be operated synchronously during yawing where all brake calliper units is engaging the brake disc.

During relative large small movements the individual brake calliper units may be op- erated together in the same operating mode as described above. The first and second actuator units may then be moved between the retracted and extended positions as described above. This allows the yaw system to apply a maximal holding torque and at least a minimum of drive torque or a maximum drive torque dependent on the aerodynamic forces acting on the rotor in order to yaw the nacelle.

According to one embodiment, the brake disc extends towards a centre of the wind turbine tower, and at least the at least one brake calliper unit or the at least one actuator unit is connected to the mainframe. The yaw system may be arranged relative an inner surface of the wind turbine tower where the brake disc may extend inwards towards the centre of the wind turbine tower. This enables the brake calliper unit and actuator units to be located on the inside of the wind turbine tower, thus providing easy access to the individual components and allows for a simple and easy replacement of any damaged components.

The actuator unit may at one end be firmly connected directly to the bottom end of the mainframe, e.g. via bolts, screws or other fasteners. The actuator unit may instead be connected, e.g. rotatably connected, to a first mounting pin firmly connected to the bottom end of the mainframe. The brake calliper unit may be movable arranged rela- tive to the bottom end of the mainframe. The brake calliper unit may be firmly connected to a second mounting pin at one end which at the other end may be slidable arranged in a groove formed in the bottom end of the mainframe. This prevents the brake calliper unit from moving radially away from the brake disc when the actuator unit is activated. Other types of radial fixation means may be used. The actuator unit may at the other end be connected, e.g. rotatably connected, to the second mounting pin. This allows the actuator unit to move the brake calliper unit relative to the brake disc. Alternatively, the actuator unit may at one end be connected, e.g. rotatably connected, to the wind turbine tower, e.g. via a mounting bracket. The actuator unit may at the other end be connected, e.g. rotatably connected, to another mounting pin firmly connected to the brake calliper unit. This mounting pin may face the bottom of the wind turbine tower. This also allows the actuator unit to move the brake calliper unit rela- tive to the brake disc.

According to one embodiment, the yaw bearing is one of a ball bearing, a roller bearing or a friction bearing. The mainframe of the nacelle is connected to a first bearing part and the wind turbine tower, e.g. the flange thereof, is connected to a second bearing part. Aerodynamic and structural loads from the nacelle and thus rotor are transferred via the yaw bearing to wind turbine tower. The yaw bearing may be a friction bearing wherein the first bearing part may be rotatably arranged relative to the second bearing part. The second bearing part may comprise a number of sliding pads, e.g. partly arranged in a cavity, facing the first bearing part. The bottom end of the mainframe may define the first bearing part and may slide along the sliding pads. This generates a substantially constant friction torque which in turn reduces the relative yaw movement at low wind speeds. A lubricant, e.g. a lubrication oil or grease, supplied by an external lubrication system may be introduced between the sliding surfaces of the first and second bearing parts for reducing the friction and wear of the sliding pads.

The yaw bearing may alternatively be a ball or roller bearing wherein the first and second bearing parts may be rotatably connected to each other via a plurality of balls or rollers. The gap between the two bearing parts may be sealed off to prevent dust, moisture, rain water or other particles from entering the gap and contaminating the balls or rollers. The gap may be partly or fully filled to a lubricant, e.g. a lubrication oil or grease, optionally supplied from an external lubrication system. This reduces the friction generated in the ball or roller bearing and thus less torque is required to yaw the nacelle and the rotor. This also eliminates the need for any loaded sliding pads between the nacelle and the yaw bearing which could generate loud squealing noises due to stick- slip vibrations. Further, no lubrication of the sliding pads are required thus reducing the risk of the lubrication grease contaminating the surrounding parts.

This yaw bearing in combination with the drive unit provides a silent system compared to conventional yaw systems using yaw motors and sliding pads. Any noise may substantially derive from the drive unit which generates a more tolerable sound at a much lower decibel level compared to the conventional yaw system for the workers inside the wind turbine as well as any people living near the wind turbine.

A further known problem with conventional yaw system is that the friction torque varies significantly depending on the lubrication and wear conditions of the sliding pads as well as the aerodynamic and structural loads from the nacelle and rotor. This reduces the available drive capacity of the yaw motors. The present yaw system has the advantage of distributing the loads evenly over the number of actuator units and increasing the available drive capacity of the drive unit. This also allows for an easy upscaling or upgrade of the present yaw system compare to conventional yaw system. According to one embodiment, the yaw system is configured to yaw the nacelle in one rotational direction within a multiple of angular segments, where each angular segment is defined by said retracted position and said extended position of the at least one actuator unit. The relative movement between the retracted and extended positions of each actuator unit defines an absolute angular segment in which the nacelle can be yawed without having to reposition the brake calliper units. The relative movement of each actuator unit and thus the absolute angular segment may be selected dependent on the desired yaw misalignment, i.e. yaw error, relative to a measured wind direction. In example, but not limited to, the actuator unit may be selected to have a relative movement between 100 millimetres [mm] and 500mm or define an absolute angular segment between 2 degree and 20 degrees. This angular segment may also define an angular movement between +1 degree and +10 degrees in either rotational direction relative to a reference yaw angle. This allows the present yaw system to correct most yaw errors of the nacelle without having to reposition the brake calliper units. The control unit may be configured to activate the yaw system when the yaw misalignment or yaw error between the current yaw angle of the nacelle and the wind direction ex- ceeds a predetermined threshold.

The control unit may be configured to yaw the nacelle and thus the rotor into a selected yaw angle relative to the wind direction, e.g. so that the rotor is facing into the wind or out of the wind, via the actuator units and the brake calliper units. The actua- tor units and thus brake calliper units may be stopped, e.g. by closing the control valves, at any positions between the retracted and extended positions thereby allowing the nacelle to be locked into any desired yaw angle, e.g. between 0 degree and 360 degrees. When the nacelle is yawed into the desired yaw angle, e.g. aligned with the wind direction, the actuator units and thus the brake calliper units may optionally be repositioned into a final position so that the nacelle can be equally yawed in either rotational direction relative to this yaw angle without having to reposition the brake calliper units.

According to one embodiment, a number of actuator units and a corresponding num- ber of brake calliper units are distributed along the circumference of the brake disc, the number being between 4 and 16.

The actuator units and brake calliper units may be arranged in a number of pairs, as described earlier, distributed along the circumference of the brake disc. The actuator units and brake calliper units may be arranged in an alternate order if all actuator units face in the same rotational direction. Alternatively, the first actuator units of the first and second pairs or the first or third brake calliper units of the first and second pairs may be positioned adjacent each other. The number of actuator units and corresponding number of brake calliper units may be selected depending on the configuration of the wind turbine and the capacity of wind turbine. In example, but not limited to, said number may be between 4 and 16, e.g. between 8 and 12, or any numbers therein between.

According to one embodiment, the yaw system further comprises at least a third brake calliper unit arranged relative to the at least one brake disc, wherein the at least third brake calliper unit is firmly connecting to the mainframe and further connected to the at least one actuator unit.

The actuator unit may alternatively be connected to another brake calliper unit, i.e. a stationary brake calliper unit, instead of connecting the actuator unit directly to the mainframe. This brake calliper unit may be firmly connected to the bottom end of the mainframe, e.g. via bolts, screws or another fastener, and fixated in the radial direc- tion. Alternatively, the brake calliper unit may be connected to one end of a mounting pin or spacer element which at the other end may be connected to the bottom end of the mainframe. This brake calliper unit may also be coupled to the drive unit and operated via the control unit. The actuator unit may thus be positioned between the moveable brake calliper unit as described earlier and this stationary brake calliper unit. These two brake calliper units may have the same configuration or different configurations. This allows the above- mentioned number of actuator units to be reduced to half. In this configuration, the drive torque of each actuator unit may be doubled in order to maintain the maximum drive torque.

During a relatively small yaw movement, the third brake calliper units of the first and second pairs may be disengaged via the drive unit, while the first and/or second brake calliper unit may be engaged via the drive unit. Any first or second brake calliper units which are not engaged may thus be repositioned via the first or second actuator unit as described above. This allows the drive unit to apply at least a minimum of drive torque and a minimum of holding torque. During relative large yaw movement, the third brake calliper units of the first and second pairs may be disengaged via the drive unit while the first and/or second brake calliper units may be engaged via the drive unit. The first and second actuator units may then be moved in opposite rotational directions. The third brake calliper units of the first and second pairs may afterwards be engaged via the drive unit. Further, the first and/or second brake calliper units may be disengaged via the drive unit and repositioned as described above. The third brake calliper units of the first and second pairs may then be disengaged via the drive unit while the first and/or second brake calliper units may be engaged via the drive unit. The above-mentioned process may be repeat- ed if needed. The nacelle can thus be yawed into the desired yaw angle by using an alternative crawling function.

An object of the invention is also achieved by a method of operating a yaw system of a wind turbine, comprising:

- providing a wind turbine with a rotor arranged relative to a nacelle, the rotor comprising at least two wind turbine blades mounted to a hub configured to be rotata- bly connected to a drivetrain in the wind turbine,

the nacelle being rotatably connected to the wind turbine tower via a yaw system, the nacelle comprising a mainframe configured to support the rotor during rota- tion, the mainframe having a bottom end facing a top end of the wind turbine tower when installed,

the wind turbine tower comprising a flange arranged at said top end, the flange being configured to be connected to the yaw system,

the yaw system further comprises a brake disc and at least one brake calliper unit arranged relative to the brake disc, the at least one brake calliper unit comprising at least one brake pad configured to move between an engaged position and a disengaged position, the at least one brake calliper unit further being connected to a drive unit configured to operate said at least brake calliper unit,

positioning a first of said at least one brake calliper unit relative to the brake disc; - arranging a first of said at least one actuator unit relative to said at least one brake calliper unit, connecting the first actuator unit to the second brake calliper unit via at least one movable element,

coupling at least the first actuator unit or the first brake calliper unit to the at least one drive unit, yawing the nacelle relative to the wind turbine tower by bringing the first brake calliper unit into engagement with the brake disc and moving the first actuator unit in one direction, and

optionally, holding the nacelle in a yaw angle by stopping the first actuator unit in a predetermined position and maintaining the first brake calliper unit in engagement with the brake disc.

This provides a simple and easy method of installing a yaw system as described above. The present method enables the actuator units and the brake calliper units to be accessed from the inside of the wind turbine tower. Whereas some conventional yaw systems require the yaw motors and the pinion gear to be installed at the outer side surface of the yaw bearing. This allows for easy access to the individual components during servicing and repairing.

The brake calliper unit is positioned relative to the brake disc so that the brake pads are facing a contact surface on the brake disc. The brake calliper unit is then movably connected to the mainframe, e.g. by connecting the brake calliper structure to a second mounting pin and inserting this mounting pin into a groove in the bottom end of the mainframe. The actuator unit is positioned relative to the mainframe and connected to the mainframe, e.g. via a first mounting pin. Alternatively, the actuator unit may be connected to a stationary brake calliper unit which is firmly connected to the mainframe. The actuator unit may be connected to the movable brake calliper unit and/or the stationary brake calliper unit via the moveable element described above before or after installing the brake calliper unit(s). This process is repeated for each pair of actuator units and brake calliper units distributed along the circumference of the brake disc. Afterwards, the actuator units and/or the brake calliper units are coupled to the drive unit via at least one suitable pipe or hose arrangement. This enables the worker to install the present yaw system without having to manoeuvre multiple yaw motors into position in the limited available nacelle space. Furthermore, the worker does not have to correctly align any pinion gears with the rim gear as the present yaw system does not use a teethed coupling.

The individual brake calliper units and actuator units are operated using a suitable control unit, such as the wind turbine control system. When the nacelle is yawed into the desired yaw angle, the movement of the actuator units is stopped and the brake calliper units engage with the brake disc, i.e. are placed in their loaded state. This allows the yaw system to lock the nacelle and, thus, the rotor relative to the wind turbine by applying a maximum holding torque. The present yaw system can suitably be installed in any wind turbine having a brake disc, thus the present yaw system can be installed in a new wind turbine or retrofitted into an existing wind turbine. The present yaw system can advantageously be used in combination with a ball or roller bearing, however, it may also be used in wind turbines having another type of yaw bearing.

According to one embodiment, said step of yawing the nacelle relative to the wind turbine tower comprises:

further moving at least a second of said at least one actuator unit in an opposite direction relative to said one direction or in the same direction.

Depending on the orientation of the individual actuator units, they may either retract or extend in a substantially tangential direction. The individual actuator units may be arranged in at least two groups where one group extends while at least another group retracts. Alternatively, both groups may extend or retract depending on the desired yaw direction. Likewise, the individual brake calliper units may be arranged in at least two groups, where one group is engaged while at least another group is disengaged. Alternatively, both groups may be engaged at the same time. The activation of each group may be controlled by the control unit. If a relatively small yaw movement is required, then the first and second brake calliper units may remain in engagement with the brake disc. At least the first actuator unit may be actively moved, e.g. extended, in the clockwise or anticlockwise rotational direction via the drive unit. The second actuator unit may also be actively moved, e.g. retracted, in same rotational direction, thus enabling the drive unit to apply a maxi- mum drive torque. Alternatively, second actuator unit in the second pair may be passively moved in the same rotational direction due to the movement of the first actuator unit, the drive unit may thus apply a minimum drive torque. The amount of drive torque applied may be adjusted by varying the number of actuator unit actively moved. According to one embodiment, the method further comprises the steps of:

bringing at least a second of said at least one brake calliper unit out of engagement with the brake disc before being moved,

- moving the at least second brake calliper unit relative to said brake disc, and

bringing the at least second brake calliper unit into engagement with the brake disc again after being moved.

If relative large yaw movements are required, then the actuator units and the brake calliper units may alternate between different operating modes. The first brake calliper unit may remain in engagement with the brake disc, and the first actuator unit may be moved, e.g. extended, in one rotational direction. The second brake calliper unit may move out of engagement with the brake disc, and the second actuator unit may be moved, e.g. extended, in the opposite rotational direction. Then, the second brake cal- liper unit may move into engagement with the brake disc, and the first brake calliper unit may move out of engagement with the brake disc. Afterwards, the first actuator unit may be moved, e.g. retracted, in said opposite rotational direction, and the second actuator unit may be moved, e.g. retracted, in said one rotational direction. The first brake calliper unit may then move into engagement with the brake disc, and the sec- ond brake calliper unit may move out of engagement with the brake disc. The above- mentioned process may be repeated until the nacelle is placed in a desired yaw angle.

According to one embodiment, the method further comprises the steps of:

further positioning a third of said at least one brake calliper unit relative to the brake disc, firmly connecting said third brake calliper unit to the mainframe, further connecting the first actuator unit to the said third brake calliper unit, and moving the first brake calliper unit relative to the third brake calliper unit when yawing the nacelle. One or more of the above-mentioned first and second pairs may comprise two brake calliper units where the actuator unit is arranged between these two brake calliper units. During relatively small yaw movements, the third brake calliper units in the first and second pairs may move out of engagement with the brake disc. The first and second brake calliper units may then remain in engagement with the brake disc. The first actuator unit may be moved, e.g. extended, in one rotational direction, and the second actuator unit may also be moved, e.g. retracted, in the same rotational direction. When the nacelle is yawed into the desired yaw angle, the third brake calliper units in the first and second pairs may be moved into engagement with the brake disc again. This allows the drive unit to apply at least a minimum drive torque. During relatively large yaw movements, the third brake calliper units in the first and second pairs may be moved out of engagement with the brake disc. The first and second brake calliper units may then remain in engagement with the brake disc. The first actuator unit may be moved, e.g. extended, in one rotational direction, and the second actuator unit may also be moved, e.g. retracted, in said one rotational direction. The third brake calliper units in the first and second pairs may then be moved into engagement with the brake disc. The first and second brake calliper units may then move out of engagement with the brake disc. The first actuator unit pair may be moved, e.g. retracted, in the opposite rotational direction, and the second actuator unit in the second pair may also be moved, e.g. extended, in said opposite rotational direction. The first and second brake calliper units may then move into engagement with the brake disc again. The third brake calliper units in the first and second pairs may then move out of engagement with the brake disc. The process may be repeated if further yaw movement is needed. When the nacelle is yawed into the desired yaw angle, the third brake calliper units and the first and second brake calliper units may all move into engagement with the brake disc again. This allows the nacelle to be yawed using an alternative crawling function.

According to one embodiment, the method further comprises at least one of the following steps:

- disconnecting an old of said at least one actuator unit from an adjacent brake calliper unit of said at least one brake calliper unit, removing said old actuator unit, positioning a new actuator unit relative to said adjacent brake calliper unit, and reconnecting the new actuator unit to the adjacent brake calliper unit, or disconnecting an old of said at least one brake calliper unit from an adjacent actuator unit of said at least one actuator unit, removing said old brake calliper unit, positioning a new brake calliper unit relative to said adjacent actuator unit and the brake disc, and reconnecting the new brake calliper unit to the adjacent actuator unit.

The present method also provides a simple and easy way of servicing or repairing the yaw system as described above. By placing the components of the yaw system on inside of the wind turbine, the worker is not restricted by the limited space between the mainframe and the nacelle cover. The worker thus has easy access to the individual components and the servicing can be done in a fast and easy manner.

If a component gets damaged or otherwise requires replacement, the worker is able to access the damaged component from the wind turbine tower and decouple it from the drive unit. The damaged component can thus be removed with ease and a new component can be positioned with ease. Once the new component is mounted in its place, the new component can be recoupled to the drive unit.

In the following, specific objectives and implementations of the above-mentioned principles are disclosed.

An object of the invention is achieved by a wind turbine with a nacelle rotatably connected to a tower and configured with a yaw system for positioning and locking the nacelle in a yaw position relative to the tower during operation of the wind turbine. The wind turbine comprising at least one brake disc fixed to the tower, at least one brake calliper unit comprising at least one brake pad configured with an engaged position and a disengaged position of the brake disc and further being connected to a drive unit configured to operate the at least one brake calliper unit. There may be at least one actuator unit fixed to the nacelle and configured to move at least one brake calli- per between a retracted position and an extended position via at least one movable element.

The actuator unit may be connected to two brake calliper units, wherein one brake calliper unit is configured as a moveable brake calliper unit relative to the nacelle and the other brake calliper unit is fixed relative to the nacelle as a stationary brake calliper unit.

It is hereby achieved that the nacelle can be locked relative to the tower whilst the actuator moves the movable brake calliper unit. It is further achieved that the locking can be performed withholding a larger holding torque.

A further advantage is that the configuration may save space and/or increasing the holding torque and/or decrease the wear and tear of individual brake calliper units.

A further advantage is that the configuration may allow for brake calliper units to be arranged with multiple arrangements of brake calliper units, stationary and movable, and actuators arranged facing in the same direction, say clockwise or anticlockwise, so that extending the actuators result in say a clockwise movement of the nacelle when the brake pads of the movable brake calliper unit are in the engaged position and vice versa when retracting.

In special situations, the actuators may be in a fully extracted position and the respective brake pads engaged thereby providing a holding torque independent of the actua- tor torque in an intermediate position in the direction of extending the actuator.

Likewise, when the actuators are in a fully retracted position, the holding torque may be independent of the actuator torque in an intermediate position in the direction of retracting the actuator.

At the same time one or more stationary brake units may have respective brake pads engaged and thereby providing a substantially stiff locking of the nacelle relative to the tower. In an aspect, the movable brake calliper unit is rotatably connected to the movable element by a first mounting pin. In an aspect, the actuator unit is rotatably connected to a mainframe of the nacelle by a second mounting pin further connected to the stationary brake calliper unit. In an aspect, the first mounting pin is identical to the second mounting pin. This allows for the stationary and movable brake calliper units to be identical and/or interchangeable. In an aspect, the movable brake calliper units are configured with a guiding pin arranged to interact in a groove in the mainframe. The groove may prevent the brake calliper unit to move in a radial direction. A further advantage may be that the actuator can extend longer than otherwise. In an aspect, the movable brake calliper unit is interchangeable with a stationary brake calliper unit. The movable calliper unit may be identical to the stationary brake unit. Having similar or identical mounting pins and the mainframe, or a mounting flange, prepared to receive the mounting pins, the brake calliper units can easily be arranged or rearranged using the same elements. There may be a plurality of receiving holes allowing for a plurality of configurations and rearrangements.

In an aspect, the yaw system comprises at least one pair of actuator units, wherein a first actuator unit is configured to retract and extend in the opposite direction of a second actuator unit.

This allows for the respective first movable brake calliper unit to operate in one direction and the respective second movable brake calliper unit to operate in either the same direction or the opposite direction. The operation may be simultaneous or independent of each other, so that one is positioned whilst disengaged when the other is engaged and moves. At the same time, the stationary brake calliper units are always able to engage and lock.

Thus, in an aspect, the yaw system comprises at least one pair of actuator units each having a movable brake calliper unit and a stationary calliper unit, wherein a first ac- tuator unit is configured to retract and extend the first movable brake calliper unit in opposite directions to the second actuator unit configured to retract and extend of the second movable calliper unit. In an aspect, the yaw system is configured with fixtures such that the tower is the nacelle and vice versa. A person skilled in the art will appreciate that the arrangements disclosed can be interchanged relative to each other. An object is achieved by a method of operating a wind turbine as disclosed. The method comprising acts of positioning the nacelle relative to the tower by providing all stationary brake calliper units having brake discs being disengaged and engaging one, more, or all movable brake calliper units by having respective brake discs being engaged.

As required, the disclosed structural elements may be used to achieve the following.

There may be an act of moving the one, more, or all movable brake calliper units having brake discs engaged by activating the respective one, more, or all actuators in the same direction.

In an aspect, the method may further comprise an act of disengaging one, more, or all movable brake calliper units by having respective brake discs being disengaged and moving the one, more, or all movable brake calliper units having brake discs disen- gaged by activating the respective one, more, or all actuators in the opposite direction.

In an aspect, there method may further comprise an act of locking the nacelle relative to the tower by activating one, more or all stationary brake calliper units in their engaged positions.

In an aspect, one or more, but not all movable brake calliper units may have brake discs being engaged and the respective actuators being locked, thus, providing the option of grading the holding and allowing the non-engaged movable brake calliper units to be positioned or reduce wear and tear.

There may be acts of repeating of one or more acts of the before outlined actions.

An object is achieved by a method of operating a wind turbine as outlined and wherein the yaw system comprises at least one pair of actuator units each having a movable brake calliper unit and a stationary calliper unit. A first actuator unit may be configured to retract and extend the first movable brake calliper unit in opposite directions to the second actuator unit configured to retract and extend the second movable calliper unit.

The method may comprise an act of positioning the nacelle relative to the tower by the following actions.

There may be an act of providing all stationary brake calliper units having brake discs being disengaged.

There may be an act of providing one, more, or all first movable brake calliper units in a retracted position. There may be an act of engaging the one, more or all first movable brake calliper units having respective brake discs being engaged.

There may be an act of extending the one, more or all first movable brake calliper units by respective first actuator units.

Similarly, there may be a method of operating a wind turbine where the method comprises an act of positioning the nacelle relative to the tower by the following actions.

The method may thus allow for one or more movable brake calliper units to yaw the nacelle, whilst others are being positioned. At the same time, the stationary brake calliper units are available for locking the nacelle.

There may be an act of providing all stationary brake calliper units having brake discs being disengaged.

There may be an act of providing one, more, or all second movable brake calliper units in an extended position. There may be an act of engaging the one, more or all second movable brake calliper units having respective brake discs being engaged.

There may be an act of retracting the one, more or all second movable brake calliper units by respective second actuator units.

In continuation of the before mentioned acts of positioning, there may be acts of providing first and second movable brake calliper units in respective retracted and extended positions and extending and retracting the respective first and second brake calliper units are performed substantially synchronized.

There may be a further act of disengaging the one, more or all first movable brake calliper units and retracting the one, more or all first movable brake calliper units. There may be a further act of disengaging the one, more, or all second movable brake calliper units and extending the one, more or all second movable brake calliper units.

There may be an even further act of locking the nacelle relative to the tower by activating one, more or all stationary brake calliper units in their engaged positions whilst retracting the respective first movable brake calliper units or extending the respective second movable calliper units.

The before mentioned acts may be repeated. A person skilled in the art will appreciate that an act of extending may be an act of retracting and that an act of retracting may be an act of extending. That is that the directions or order of events may be reversed.

An object may be achieved by a method of operating a wind turbine as disclosed by an act of locking the nacelle relative to the tower by activating one or more brake calliper units in the engaged position.

In an aspect, an act of locking may involve locking two brake calliper units in the engaged position substantially simultaneously. In an aspect, the act of locking may involve locking one or more movable brake calliper units in the engaged position whilst one, more or all stationary brake calliper units are in the disengaged position.

In an aspect, one or more actuators move respective movable brake calliper units either actively or passively.

In an aspect, one, more, or all stationary brake calliper units are engaged when the movement is below a threshold value.

This further allows for the movable brake calliper units to dampen yaw movement to below a certain movement before a "hard" lock is achieved by activating the stationary brake calliper units.

The damping may be balanced between active and passive movement.

The invention is not limited to the embodiments described herein, and thus the described embodiments can be combined in any manner without deviating from the ob- jections of the invention.

Description of the Drawing

The invention is described by example only and with reference to the drawings, wherein: Fig. 1 shows an exemplary embodiment of a wind turbine,

Fig. 2 shows an exemplary embodiment of the yaw system according to the invention,

Fig. 3 shows a cross sectional view of the yaw system shown in fig. 2,

Fig. 4 shows a first arrangement of the brake calliper unit and the actuator unit,

Fig. 5 shows a second arrangement of the brake calliper unit and the actuator unit, Fig. 6a-c show a first exemplary embodiment of a method of yawing the nacelle relative to the wind turbine tower according to the invention,

Fig. 6ai-ci show an embodiment using stationary and movable brake calliper units,

Fig. 7a-f show a second exemplary embodiment of the method of yawing the nacelle,

Fig. 7ai-fi show an embodiment using stationary and movable brake calliper units, and

Fig. 8a-b,bi show an exemplary embodiment of the drive unit and the corre- sponding arrangement of brake calliper unit and actuator units; which may be applied to a configuration using also stationary brake calliper units

In the following text, the figures will be described one by one, and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

Position number list

1. Wind turbine

2. Tower

3. Nacelle

4. Hub

5. Wind turbine blades

6. Yaw system

7. Brake calliper units (movable)

8. Actuator units

9. Moveable element

10. Mainframe

11. Brake disc

12. Mounting flange 13. Yaw bearing unit

14. First bearing part

15. Second bearing part

16. First mounting pin

17. Groove

18. Second mounting pin

19. Calliper structure

20. Brake pads

21. Brake calliper unit (stationary)

22. First brake calliper unit

22.7. First movable brake calliper unit

22.21. Fist stationary brake calliper unit

23. First actuator unit

24. Rotational direction

25. Second brake calliper unit

25.7. Second movable brake calliper unit

25.21. Second stationary brake calliper unit

26. Second actuator unit

27. Reference yaw angle

28. Desired yaw angle

29. Drive unit

30. Pipe or tube system

31. Pump system

Detailed Description of the Invention

Fig. 1 shows an exemplary embodiment of a wind turbine 1. The wind turbine 1 comprises a wind turbine tower 2 provided on a foundation (not shown). A nacelle 3 is arranged on top of the wind turbine tower 2 and configured to yaw relative to the wind turbine tower 2 via a yaw system (shown in fig. 2). A hub 4 is rotatably arranged relative to the nacelle 3, wherein at least two wind turbine blades 5 are mounted to the hub 5. The wind turbine 1 is here outfitted with three wind turbine blades 5. The rotor, i.e. the hub 4 with wind turbine blades 5, is connected to an electrical generator (not shown) arranged in the nacelle 3 via a drive shaft (not shown) for producing an electrical power output.

Fig. 2 shows exemplary embodiment of the yaw system 6 comprising a number of brake calliper units 7 and another number of actuator units 8. The actuator units 8 and the brake calliper units 7 are arranged in pairs and are operated individually or synchronously via a control unit (not shown). The actuator unit 8 is here shown as a linear actuator having a moveable element 9 in the form of an extendable piston rod. The brake calliper unit 7 is here shown as a moveable brake calliper unit. Each actuator unit 8 is at one end firmly connected to a mainframe 10 located in the nacelle 3 via a mounting pin (shown in figs. 3 and 4). The actuator unit 8 is at the other end connected to an adjacent brake calliper unit 7 via another mounting pin (shown in figs. 3 and 4). The brake calliper unit 7 is moveable arranged relative to a brake disc 11 which is mounted to the wind turbine tower 2, e.g. to a mounting flange 12 provided at the top end of the wind turbine tower 2.

The actuator unit 8 is configured to move the brake calliper unit 7 relative to the ac- tuator unit 8 and, thus, the brake disc 11 by extending or retracting the moveable element 9.

Fig. 3 shows a cross sectional view of the yaw system 6 shown in fig. 2, where the yaw system further comprises a yaw bearing unit 13 in the form of a ball bearing. The yaw bearing unit 13 has a first bearing part 14 and a second bearing part 15 rotatably arranged relative to the first bearing part 14. The first bearing 14 is firmly connected to the mainframe 10, and the second bearing part 15 is firmly connected to the mounting flange 12 of the wind turbine tower 2. The brake calliper unit 7 is connected to a first mounting pin 16 in the form of a slida- ble guide pin extending towards the mainframe 10. The mainframe 10 has a groove 17 arranged in the bottom end configured to receive the free end of the first mounting pin 16. The first mounting pin 16 is able to slide along the groove 17 when the brake cal- liper unit 7 is moved. The first mounting pin 16 and the groove 17 prevent the brake calliper unit 7 from moving in a radial direction.

Fig. 4 shows a first arrangement of the brake calliper unit 7 and the actuator unit 8. The actuator unit 8 is connected to the mainframe 10 via a second mounting pin 18, as shown in fig. 3, which is firmly connected to a bottom end of the mainframe 10 via suitable fastening means, such as bolts, screws, or other fasteners.

The free end of the moveable element 9 is rotatably connected to the first mounting pin 16. Alternatively or additionally, the actuator unit 8 is at the other end rotatably connected to the second mounting pin 18. This enables the actuator unit 8 to substantially follow the rotational movement of the brake calliper unit 7.

The brake calliper unit 7 comprises a calliper structure 19 in which a number of brake pads 20 are arranged. The brake pads 20 are configured as moveable brake pads which can be moved between an engaged position where they apply pressure onto the brake disc 11 and a disengaged position where they do not apply pressure onto the brake disc 11. Here the brake calliper unit 7 is outfitted with two brake pads, but it can be outfitted with fewer or additional brake pads. Fig. 5 shows a second arrangement of the brake calliper unit 7 and the actuator unit 8. This arrangement differs from the arrangement shown in fig. 4 by the actuator unit 8' being connected to two brake calliper units 7, 21. Here, one brake calliper unit 7 is configured as a moveable brake calliper unit as described above, while the other brake calliper unit 21 is configured as a stationary brake calliper unit.

The brake calliper unit 21 is further connected to the second mounting pin 18 which is firmly connected to the mainframe 10, thereby preventing the brake calliper unit 21 from moving in the radial direction. The brake calliper unit 21 has the same configuration as the brake calliper unit 7 as shown in fig. 4. In this embodiment, the actuator unit 8' has a greater moveable element 9' and, thus, a greater drive torque than the actuator unit 8.

Fig. 6a-c show a first exemplary embodiment of a method of yawing the nacelle 3 relative to the wind turbine tower 2 according to the invention. This embodiment is suitable when only relatively small yaw movements are required.

A first pair comprising a first brake calliper unit 22 and a first actuator unit 23 is arranged so that the first actuator unit 23 is facing in one rotational direction (indicated by arrow 24). A second pair comprising a second brake calliper unit 25 and a second actuator unit 26 is arranged so that the second actuator unit 26 is facing in an opposite rotational direction (indicated by arrow 24').

The nacelle 3 is initially positioned in a reference yaw angle (indicated by arrow 27) as shown in fig. 6a. The first and second brake calliper units 22, 25 are both placed in their engaged positions so that the brake pads 20 of each brake calliper unit 22, 25 apply pressure to the brake disc 11. Optionally, the first and second brake calliper units 22, 25 are further positioned in a reference position. If yaw movement in the anticlockwise direction is required as shown in fig. 6b, the first actuator unit 23 is activated and extends the moveable element 9 and, thus, the first brake calliper unit 22 towards an extended position. The second actuator unit 26 is activated and retracts the moveable element 9 and, thus, the second brake calliper unit 25 towards a retracted position.

When the nacelle 3 is yawed into the desired yaw angle (indicated by arrow 28'), e.g. realigned with the wind direction, the movement of the first and second actuator units 23, 26 is stopped. The first and second actuator units 23, 26 are then maintained in their current positions via the drive unit (shown in fig. 8), thereby locking the nacelle 3 in a desired yaw angle.

If yaw movement in the clockwise direction is required as shown in fig. 6c, the first actuator unit 23 is activated and retracts the moveable element 9 and, thus, the first brake calliper unit 22 towards the retracted position. The second actuator unit 26 is activated and extends the moveable element 9 and, thus, the second brake calliper unit 25 towards the extended position.

When the nacelle 3 is yawed into the desired yaw angle (indicated by arrow 28"), the movement of the first and second actuator units 23, 26 is stopped and the nacelle 3 is locked in the desired yaw angle as described above.

The relative movement of the first and second actuator units 23, 26 defines an angular segment (end points indicated by arrows 28' and 28") in which the yaw system 6 is able to yaw the nacelle 3 without having to reposition the brake calliper units 7.

Figs. 6ai-ci show an embodiment where the arrangement of fig. 5 is implemented. The figures are only illustrative and a person skilled in the art will arrange the brake calliper units and actuators with required spacing according to the number of arrangements installed and/or the dimensions of actuators and arms.

In the i-labelled figures, one brake calliper unit 7 is configured as a moveable brake calliper unit as described above, while the other brake calliper unit 21 is configured as a stationary brake calliper unit. The embodiment is one example of an implementation and describes one particular mode of operation.

Fig. 6ai illustrates pairs of brake calliper units and actuator units arranged around the periphery. Referring to the naming from Figs. 6a-c, there is a first arrangement having a first and movable brake calliper unit 22.7 and a first and stationary brake calliper unit 22.21 connected to a first actuator unit 23. There is second arrangement having second and movable brake calliper unit 25.7 and a second and stationary brake calliper unit 26.21 connected to a second actuator unit 26.

The nacelle 3 is positioned in a reference yaw angle 27. The first and second stationary brake calliper units 22.21, 25.21 may be in the engaged position. Alternatively or additionally, the first and second movable brake calliper units 22.7, 25.21 may be in the engaged position locking the nacelle 3 relative to the tower 2. When only the stationary brake calliper units 21 are engaged, the respective movable calliper unit 7 may be positioned and prepared to perform movements. The respective first and second movable brake calliper units 22.7, 25.7 may even be positioned and prepare to perform movements independently of each other.

If a yaw movement is to be in the anticlockwise direction as indicated in fig. 6bi, there are several options. The first movable brake calliper unit 22.7 may be prepared to extend and to be engaged. The stationary brake calliper units 21 may be disengaged.

The first actuator unit 23 may be extended and result in anticlockwise yawing. The second movable brake calliper unit 25.7 may be prepared to retract and to be engaged. The second actuator unit 26 may be retracted and result in anticlockwise yawing.

The activation of the respective first and second actuator units 23, 26 may be per- formed simultaneously. The result is yawing in the anticlockwise rotational direction 24.

The yawing may also be performed by first moving the first movable brake calliper unit 22.7 in a first yawing in the anticlockwise direction 24a. During this, the second movable brake calliper unit 25.7 either has been positioned or is being positioned. The first movable brake calliper unit 22.7 may be disengaged and the second movable brake calliper brake calliper unit 25.7 may be engaged and retracted and continue the first yawing in a second yawing in the anticlockwise direction 24b. At all times, the stationary brake calliper units 21 are able to engage.

When the desired yaw angle 28' is achieved, the stationary brake calliper units 21 may be engaged and thereby locking the nacelle 3 in the position. The movable brake calliper units 7 may either support the locking or prepare (i.e. positioned) for retracting and/or extending. The movable brake calliper units 7 may also be engaged and the actuators locked.

Reversely, yawing in the clockwise direction to a desired yaw angle 28" is illustrated in fig. 6ci.

Fig. 7a-f show a second exemplary embodiment of a method of yawing the nacelle 3 relative to the wind turbine tower 2 according to the invention. This embodiment is suitable when relative large yaw movements are required.

The nacelle 3 is initially positioned in a reference yaw angle (indicated by arrow 27) as shown in fig. 7a. The first and second brake calliper units 22, 25 are both placed in their engaged positions so that the brake pads 20 of each brake calliper unit 22, 25 apply pressure to the brake disc 11. Optionally, the first and second brake calliper units 22, 25 are further positioned in a reference position.

If yaw movement in the anticlockwise direction is required as shown in figs. 7b-f, the first brake calliper unit 22 remain in the engaged position so that the brake pads 20 remain in contact with the brake disc 11. The first actuator unit 23 is activated and extends the moveable element 9 and, thus, the first brake calliper unit 22 towards the extended position as shown in fig. 7b. The nacelle 3 is thus yawed into a first intermediate yaw angle.

The second brake calliper unit 25 is moved to a disengaged position where the brake pads 20 do not apply pressure to the brake disc 11, thereby enabling the second brake calliper unit 25 to slide relative to the brake disc 11. The second actuator unit 26 is activated and extends the moveable element 9 and, thus, slides the second brake calliper unit 25 towards the extended position as shown in fig. 7b. The second brake calliper unit 25s thus repositioned while the first brake calliper unit 22 is used to yaw the nacelle 3.

Afterwards, the second brake calliper unit 25 is moved to the engaged position where the brake pads 20 apply pressure to the brake disc 11. The second actuator unit 26 is activated and retracts the moveable element 9 and, thus, the second brake calliper unit 24 towards the retracted position as shown in fig. 7c. The nacelle 3 is thus yawed into a second intermediate yaw angle.

The first brake calliper unit 2 is moved to the disengaged position so that the brake pads 20 do not apply pressure to the brake disc 11, thereby enabling the first brake calliper unit 25 to slide relative to the brake disc 11. The first actuator unit 23 is activated and retracts the moveable element 9 and, thus, slides the first brake calliper unit

25 towards the retracted position as shown in fig. 7c. The first brake calliper unit 22 is thus repositioned while the second brake calliper unit 25 is used to yaw the nacelle 3.

If further yaw movement in the same direction is required, then the process described above in relation to figs. 7b and 7c is repeated as shown in fig. 7d and fig. 7e respectively. This enables the nacelle 3 to yaw into further intermediate yaw angles by using a crawling function controlled by a control unit (not shown).

When the nacelle 3 is yawed into the desired yaw angle (indicated by arrow 28' "), e.g. realigned with the wind direction, the movement of the first and second actuator units 23, 26 is stopped and the first and second brake calliper units 22, 25 are moved to their engaged positions. The first and second actuator units 23, 26 are then main- tained in their current positions via the drive unit (shown in fig. 8), thereby locking the nacelle 3 in a desired yaw angle as shown in fig. 7f. Optionally, the first and second brake calliper units 22, 25 are further positioned in the reference position before locking the nacelle 3 in the desired yaw angle. As mentioned above, the relative movement of the first and second actuator units 23,

26 defines a multiple of angular segments in which the yaw system 6 is able to yaw the nacelle 3 until it reaches the desired yaw angle.

Figs. 7ai-fi illustrates an embodiment of a method of yawing the nacelle 3 relative to the wind turbine tower 2. The figures illustrate pairs of brake calliper units and actuator units arranged around the periphery.

Referring to the naming from Figs. 7a-f, there is a first arrangement having a first and movable brake calliper unit 22.7 and a first and stationary brake calliper unit 22.21 connected to a first actuator unit 23. There is a second arrangement having a second and movable brake calliper unit 25.7 and a second and stationary brake calliper unit 26.21 connected to a second actuator unit 26. Fig. 7ai illustrates the nacelle 3 positioned in a reference yaw angle 27. The stationary brake calliper units 21 may be engaged to lock the nacelle 3 in the reference position. The movable brake calliper units 7 may be positioned, be positioning, or engaged to provide additional locking. If yaw movement in the anticlockwise direction to a desired yaw angle 28' " (fig. 7fi) is required, the following acts may be performed.

The first movable brake calliper unit 22.7 may be prepared to be in retracted position and then engaged. The stationary brake calliper units 21 are disengaged. If the second movable brake calliper units 25.7 are engaged, they are also disengaged.

The first actuators 23 may be activated to extending the first movable brake calliper units 22.7 in the anticlockwise direction and being engaged, the nacelle 3 is moved to a first intermediate yaw angle. At the same time or prior to, the second movable brake calliper unit 25.7 is moved or was moved in the clockwise direction 24' by extending the actuator unit. This is illustrated in fig. 7bi.

In continuation and as illustrated in fig. 7ci, the second movable calliper unit 25.7 is engaged to the disc. The first movable calliper unit 22.7 is disengaged. The second movable calliper unit 25.7 is then moved in anticlockwise direction 24 by retracting the second actuator unit 26. Thereby, the nacelle is moved to a second intermediate yaw angle. Whilst doing this, the first movable brake calliper unit 22.7 may be prepared by moving the clockwise direction 24 by retracting the first actuator 23. At all times the stationary brake calliper units 21 may engage and lock the nacelle. In one aspect, the stationary brake calliper units 21 may be engaged just before and just after disengaging and engaging respective first and second movable calliper units 22.7, 25.7, and vice versa. In a further aspect, engagement and disengagement are performed of only the respective first or stationary brake calliper units 22.21, 25.21 of the first and second movable calliper 22.7, 25.7 to be engaged.

Figs 7di-fi illustrates repetitive acts with intermediate yaw angle positions before the desired yaw angle 28' " is achieved. At this time, the stationary brake calliper units 21 may be engaged locking the nacelle 3 in the desired yaw angle 28" ' . The first and second movable brake calliper units 22.7, 25.7 may then be positioned or prepared. Optionally, the movable brake calliper units 7 may be engaged and further contributing to locking the nacelle 3.

In a further aspect, if the second arrangement shown in fig. 5 is operated according to the methods shown in figs. 6a-c or in figs. 7a-f, each of the pairs described above further comprises a third calliper unit, i.e. the brake calliper unit 21. In both methods the brake calliper unit 21 is operated opposite of the first and second brake calliper unit 22, 25. When the nacelle 3 is yawing, e.g. the first and/or second brake calliper unit 22, 25 is retracted or extended, the brake calliper unit 21 is in the disengaged position. When the first and/or second brake calliper unit 22, 25 is repositioned, e.g. sliding along the brake disc 11, the brake calliper unit 21 is in the engaged position. Fig. 8a shows an exemplary embodiment of the drive unit 27 while fig. 8b shows the corresponding arrangement of brake calliper units 7 and actuator units 8. The drive unit 29 is connected to the respective brake calliper units 7 and actuator units 8 via a suitable fluid pipe or tube system 30. The drive unit 29 comprises a suitable pump system 31 configured to circulate a suitable fluid between the respective brake calliper units 7 and the actuator units 8. Here, the circulated fluid is a hydraulic oil, but other fluids may be used.

The drive unit 29 is configured to generate a maximum holding torque and a maximum drive torque for each brake calliper unit 7 dependent on the particular configura- tion and rated capacity of the wind turbine 1. Here, the drive unit 29 is configured to generate a maximum holding torque between 200 kNm and 1600 kNm and/or a maximum drive torque between 200 kNm and 1600 kNm. The pump system 31 is connected to at least one first directional control valve, e.g. having three positions, which in turn is connected to the actuator units 8 via a first set of pipe or tubes. The pump system 31 is further connected to at least one second directional control valve, e.g. having two positions, which in turn is connected to the brake calliper units 7 via a second set of pipe or tubes. The operation of the directional control valves is controlled by the control unit.

Fig. 8bi illustrates an arrangement of brake calliper units of the stationary and movable types. The drive system shown in figure 8a is easily modified by adding additional stationary brake calliper units. The stationary brake calliper units may be identical to the movable brake calliper units 7 shown in fig. 8a.

The present invention may comprise one or more of the following aspects. Aspect 1. A wind turbine comprising:

a rotor arranged relative to a nacelle, the rotor comprising at least two wind turbine blades mounted to a hub configured to be rotatably connected to a drivetrain in the wind turbine,

the nacelle being rotatably connected to the wind turbine tower via a yaw system, the nacelle comprising a mainframe configured to support the rotor during rotation, the mainframe having a bottom end facing a top end of the wind turbine tower when installed,

the wind turbine tower comprising a flange arranged at said top end, the flange being configured to be connected to the yaw system,

- the yaw system comprising a yaw bearing unit with a first bearing part and a second bearing part rotatably arranged relative to the first bearing part, the first bearing part is connected to the mainframe and the second bearing part is connected to the flange, the yaw system further comprises a brake disc and at least one brake calliper unit arranged relative to the brake disc, the at least one brake calliper unit comprising at least one brake pad configured to move between an engaged position and a disengaged position, the at least one brake calliper unit further being connected to a drive unit configured to operate said at least brake calliper unit, wherein, at least one actuator unit is connected to said at least one brake calliper unit via at least one movable element, the at least one actuator unit being config- ured to move the at least one brake calliper between a retracted position and an extended position via said at least one element, wherein the yaw system is configured to yaw the nacelle relative to the tower by moving the at least one actuator unit in one direction when said at least one brake calliper unit is in the engaged position.

Aspect 2. A wind turbine according to aspect 1, wherein, the at least one actuator unit comprises a first actuator unit and at least a second actuator unit, and the at least one brake calliper unit comprises a first brake calliper unit and at least a second brake cal- liper unit, wherein the first actuator unit is connected to the first brake calliper unit via a first movable element, and the at least second actuator unit is connected to the at least second brake calliper unit via at least a second moveable element.

Aspect 3. A wind turbine according to aspect 2, wherein, the first actuator unit faces in one rotational direction and the at least second actuator unit faces in an opposite rotational direction.

Aspect . A wind turbine according to aspect 2 or 3, wherein, the first and second actuator units are configured to be operated independently during yawing, where one of said first and second actuator unit is extended in the one direction while the other of said first and second actuator units is retracted in an opposite direction.

Aspect 5. A wind turbine according to aspect 4, wherein, the first and second brake calliper units are further configured to be operated independently during yawing, where one of said first and second brake calliper units is engaging the brake disc while the other of said first and second actuator units is disengaged from the brake disc.

Aspect 6. A wind turbine according to aspect 4, wherein, the first and second brake calliper units are configured to be operated synchronously during yawing where all brake calliper units are engaging the brake disc.

Aspect 7. A wind turbine according to any one of aspects 1 to 6, wherein, the brake disc extends towards a centre of the wind turbine tower, and at least the at least one brake calliper unit or the at least one actuator unit is connected to the mainframe. Aspect 8. A wind turbine according to any one of aspects 1 to 7, wherein, the yaw bearing is one of a ball bearing, a roller bearing, or a friction bearing.

Aspect 9. A wind turbine according to any one of aspects 1 to 8, wherein, the yaw system is configured to yaw the nacelle in one rotational direction within a multiple of angular segments, where each angular segment is defined by said retracted position and said extended position of the at least one actuator unit.

Aspect 10. A wind turbine according to any one of aspects 1 to 9, wherein, a number of actuator units and a corresponding number of brake calliper units are distributed along the circumference of the brake disc, the number being between 4 and 16.

Aspect 11. A wind turbine according to any one of aspects 1 to 10, wherein, the yaw system further comprises at least a third brake calliper unit arranged relative to the at least one brake disc, wherein the at least third brake calliper unit is firmly connected to the mainframe and further connected to the at least one actuator unit.

Aspect 12. A method of operating a yaw system of a wind turbine, comprising:

providing a wind turbine with a rotor arranged relative to a nacelle, the rotor com- prising at least two wind turbine blades mounted to a hub configured to be rotata- bly connected to a drivetrain in the wind turbine,

the nacelle being rotatably connected to the wind turbine tower via a yaw system, the nacelle comprising a mainframe configured to support the rotor during rotation, the mainframe having a bottom end facing a top end of the wind turbine tow- er when installed,

the wind turbine tower comprising a flange arranged at said top end, the flange being configured to be connected to the yaw system,

the yaw system further comprises a brake disc and at least one brake calliper unit arranged relative to the brake disc, the at least one brake calliper unit comprising at least one brake pad configured to move between an engaged position and a disengaged position, the at least one brake calliper unit further being connected to a drive unit configured to operate said at least brake calliper unit,

positioning a first of said at least one brake calliper unit relative to the brake disc, arranging a first of said at least one actuator unit relative to said at least one brake calliper unit, connecting the first actuator unit to the second brake calliper unit via at least one movable element,

coupling at least the first actuator unit or the first brake calliper unit to the at least one drive unit,

yawing the nacelle relative to the wind turbine tower by bringing the first brake calliper unit into engagement with the brake disc and moving the first actuator unit in one direction, and

optionally, holding the nacelle in a yaw angle by stopping the first actuator unit in a predetermined position and maintaining the first brake calliper unit in engagement with the brake disc.

Aspect 13. A wind turbine according to aspect 12, wherein, said step of yawing the nacelle relative to the wind turbine tower comprises:

further moving at least a second of said at least one actuator unit in an opposite direction relative to said one direction or in the same direction.

Aspect 14. A wind turbine according to aspect 13, wherein, the method further comprises the steps of:

- bringing at least a second of said at least one brake calliper unit out of engagement with the brake disc before being moved,

moving the at least second brake calliper unit relative to said brake disc, and bringing the at least second brake calliper unit into engagement with the brake disc again after being moved.

Aspect 15. A wind turbine according to any one of aspects 12 to 14, wherein, the method further comprises the steps of:

further positioning a third of said at least one brake calliper unit relative to the brake disc, firmly connecting said third brake calliper unit to the mainframe, - further connecting the first actuator unit to the said third brake calliper unit, and moving the first brake calliper unit relative to the third brake calliper unit when yawing the nacelle. Aspect 16. A wind turbine according to any one of aspects 12 to 15, wherein, the method further comprises at least one of the following steps:

disconnecting an old of said at least one actuator unit from an adjacent brake calliper unit of said at least one brake calliper unit, removing said old actuator unit, po- sitioning a new actuator unit relative to said adjacent brake calliper unit, and reconnecting the new actuator unit to the adjacent brake calliper unit, or

disconnecting an old of said at least one brake calliper unit from an adjacent actuator unit of said at least one actuator unit, removing said old brake calliper unit, positioning a new brake calliper unit relative to said adjacent actuator unit and the brake disc, and reconnecting the new brake calliper unit to the adjacent actuator unit.