Patent Description:
It is known that wind turbines may move due to the force of wind incident upon the wind turbine and that the movement may take the form of bending of a tower of the wind turbine. The movement may result in an oscillation in a resonant mode of the wind turbine. Such resonant oscillations may potentially be damaging for a wind turbine.

When the wind turbine is a traditional wind turbine, having a single rotor at the top of a tower, the wind turbine may be considered as an inverted pendulum and the number of oscillation modes and the complexity of those oscillation modes may be relatively small. However, multi-rotor turbines may have more complex oscillation modes. In particular, there may be significant oscillation modes in which a nacelle at the top of the tower remains substantially stationary and measurement of the movement of the top nacelle therefore may not provide meaningful data for determining some oscillation modes.

Some existing systems for determining wind turbine movement use strain sensors arranged on the structure of the tower. However, strain sensors may have slow response times and may thereby provide inadequate data for a determination to be made for characteristics of complex or high frequency oscillations. Documents <CIT> and <CIT> disclose prior art examples pertinent to the invention.

A first aspect of the invention provides a wind turbine comprising: a tower; a first arm extending from the tower; a first rotor-nacelle-assembly disposed on the first arm; a first movement sensor disposed on the first arm or on the first rotor-nacelle-assembly and arranged to generate first movement data based on movement of the first arm or of the first rotor-nacelle-assembly; a second arm extending from the tower; a second rotor-nacelle-assembly disposed on the second arm; a second movement sensor disposed on the second arm or on the second rotor-nacelle-assembly and arranged to generate second movement data based on movement of the second arm or of the second rotor-nacelle-assembly; and a control system coupled to the first and the second movement sensors and arranged to receive and to process the first and second movement data; wherein the control system is arranged to determine an oscillation characteristic of the wind turbine from the first and the second movement data.

With such an arrangement, data from two separate movement sensors may be combined in order to determine characteristics of a greater range of oscillation modes. The oscillation characteristic determined may provide an indication of the structural integrity of the tower and/or the likelihood of any damage to the tower occurring due to the movement of the tower.

The first and second movement sensors are configured to be time synchronous with each other and/or with the control system. The first and the second movement sensors and the control system may form part of a time triggered network, optionally a time triggered Ethernet network.

The first and the second movement sensors may each comprise an accelerometer and/or a gyroscope.

The first and the second movement sensors may each comprise a gyroscope and an accelerometer and the first and the second movement data may each comprise linear acceleration data and angular velocity data and/or angular acceleration data.

The control system may comprise a first processor disposed on the first arm or on the first rotor-nacelle-assembly and/or a second processor which may be disposed on the second arm or on the second rotor-nacelle-assembly, wherein the first and the second processors may be arranged to process the first and the second movement data respectively.

The control system may comprise a processing unit disposed on or within the tower, the processor arranged to determine the oscillation characteristic.

The oscillation characteristic may be an oscillation mode or an oscillation amplitude.

The control system may be arranged to control a wind turbine operation parameter to modify an oscillation of the wind turbine on the basis of the oscillation characteristic.

The wind turbine may comprise an actuator and each rotor-nacelle-assembly may comprise a respective rotor, and controlling the wind turbine operation parameter may comprise: driving the actuator to alter a pitch of the blades of at least one of the rotors, or driving the actuator to yaw at least one of the rotor-nacelle-assemblies.

The wind turbine may further comprise a generator, which may be situated within a nacelle of one of the rotor-nacelle-assemblies, and controlling the wind turbine operation parameter may comprise modifying a power set point and/or a rotational speed of the generator.

The wind turbine may further comprise a memory storing possible oscillation modes of the wind turbine, and a control system may be arranged to determine an amplitude, frequency, or phase of at least one of the stored oscillation modes.

The first and/or second arms may extend non-vertically, optionally substantially horizontally, away from the tower.

According to a second aspect of the invention, there is provided a method of monitoring a wind turbine, the wind turbine comprising a tower and a first and a second arm extending from the tower, each arm carrying a respective rotor-nacelle-assembly, the method comprising: generating first movement data based on movement of the first arm or of the first rotor-nacelle-assembly of the wind turbine by a first movement sensor; generating second movement data based on movement of the second arm or of the second rotor-nacelle-assembly of the wind turbine by a second movement sensor; and determining an oscillation characteristic of the wind turbine based on the first and the second movement data.

The method may further comprise: identifying a first oscillation in the first movement data, identifying a second oscillation in the second movement data, and determining a phase difference between the first and the second oscillations, and the determination of the oscillation characteristic may be based at least partially on the phase difference.

The determining may comprise interrogating a database comprising possible oscillation modes.

According to a third aspect of the invention, there is provided a method of controlling a wind turbine, comprising: monitoring the wind turbine according to the second aspect of the invention; and controlling a wind turbine operation parameter based on the oscillation characteristic in order to modify an oscillation of the wind turbine.

Controlling the wind turbine parameter may comprise altering the pitch of a blade of the wind turbine or yawing at least one rotor-nacelle-assembly.

Controlling the wind turbine oscillation parameter may comprise modifying a power set point or a rotational speed of a generator.

According to a fourth aspect of the invention, there is provided a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the second or third aspects.

The computer program product may be a processor arranged to carry out the method when coupled to a suitable wind turbine or may be a non-transitory computer readable medium including instructions, which, when carried out on a processor, cause the processor to carry out the method according to the second or third aspects.

<FIG> shows a multi-rotor wind turbine <NUM>. A multi-rotor wind turbine is any wind turbine with at least two rotor-nacelle-assemblies, which distinguishes the wind turbine <NUM> from a traditional, single-rotor wind turbine. Multi-rotor wind turbines may have significantly different oscillation modes from single-rotor wind turbines and therefore oscillation detection systems of single-rotor wind turbines may not be suitable or adaptable for use on multi-rotor wind turbines.

The wind turbine <NUM> shown in <FIG> has a central tower <NUM> extending upwardly from a foundation <NUM> along a central axis A1 and first and second arms <NUM> extending from the tower <NUM>. While the first and second arms <NUM> are shown as extending substantially horizontally from the tower, it will be understood that the arms may extend non-horizontally and may extend diagonally upwardly or diagonally downwardly from the tower <NUM>.

The first and second arms <NUM> may be coupled to the tower <NUM> via a rotatable collar <NUM>. The collar <NUM> may be arranged to rotate about the tower <NUM> in order that the rotors <NUM> of the wind turbine <NUM> face more directly into incident wind and the wind turbine may thereby achieve improved energy production.

At the ends of each of the arms <NUM>, there may be disposed respective rotor-nacelle-assemblies <NUM>, comprising respective nacelles <NUM> and respective rotors <NUM>. As shown in <FIG> there may be first and second monitoring units <NUM> disposed on the nacelles <NUM>. Alternatively, the monitoring units <NUM> may be disposed on the arms <NUM>. Generally, the monitoring units <NUM> may be arranged to measure or detect movement of the arms <NUM> and the respective nacelles <NUM> mounted on the arms <NUM>. The monitoring units <NUM> may be disposed on a surface of the arms <NUM> or nacelles <NUM> or may be embedded or otherwise placed inside the arms <NUM> or nacelles <NUM>.

Rotatably coupled to each nacelle <NUM> is a respective rotor <NUM>, which is arranged to rotate in order to drive a generator and thereby to generate electricity. Each rotor <NUM> has a plurality of wind turbine blades <NUM>, which are arranged to create lift due to incident wind and may thereby cause the rotors <NUM> to rotate. While rotors with three blades are shown, it will be understood that rotors having more or fewer blades may equally be used.

The wind turbine may also have a central processing unit <NUM>. The central processing unit <NUM> may be disposed at a base of the tower <NUM>, proximate the foundation <NUM>, or may be disposed elsewhere on or in the wind turbine <NUM>. The central processing unit <NUM>, in conjunction with the first and second monitoring units <NUM> may form at least a part of a control system, which is described in more detail below.

<FIG> shows an example of an oscillation mode which may be encountered by a multi-rotor wind turbine. Due to the thrust caused by incident wind on the rotors <NUM> and the weight of the rotor-nacelle-assemblies <NUM>, as well as the length of the arms <NUM>, a significant torque may be created about the central axis A1 of the wind turbine <NUM>. This may cause bending of the arms <NUM> and/or torsion of the central tower <NUM>. As the bending may be opposed by the structure of the wind turbine <NUM>, and the bending force may change over time due to changing wind speed or direction and/or vibration of the rotor-nacelle-assemblies due to rotation of the rotors <NUM>, the bending of the arms <NUM> and torsion of the tower <NUM> may develop into an oscillation.

In <FIG>, it can be seen that the oscillation of the wind turbine <NUM> is related to the movement of the arms <NUM> and of the rotor-nacelle-assemblies <NUM>. The monitoring units <NUM> may measure this movement and may provide movement data to a control system. Such movement data may provide useful information for determining the characteristics of the oscillation of the wind turbine <NUM>. For example, the predominant oscillation mode may be determined, and may be characterised by, the frequency, the amplitude, and/or the phase of the oscillation.

The oscillation mode may involve torsion of the tower <NUM>. Such torsion may be more reliably detected by using movement data from both of the monitoring units <NUM> on the two different rotor-nacelle-assemblies <NUM>. Moreover, detection of torsion in the tower <NUM> is assisted by a time-synchronous network, which may provide more accurate information regarding the phase of the movement of the arms <NUM>. This may allow determination of whether the movement is symmetrical or asymmetrical about the tower <NUM>.

<FIG> shows a second multi-rotor wind turbine <NUM>, having a third rotor-nacelle-assembly <NUM> at the top of a tower <NUM>, in addition to two rotor-nacelle-assemblies <NUM> on the ends of arms <NUM>.

Parts of the wind turbine <NUM> shown in <FIG> may be substantially similar to parts of the wind turbine <NUM> shown in <FIG> and so substantially unchanged parts are not described again here for brevity.

The top rotor-nacelle-assembly <NUM> may be substantially similar to the rotor-nacelle-assemblies <NUM> of the wind turbine <NUM> of <FIG> and to the rotor-nacelle-assemblies <NUM> on the arms <NUM> of the wind turbine <NUM> of <FIG>.

The arms <NUM> may extend substantially horizontally, or diagonally upwardly or downwardly, from a mid-point on the tower <NUM>. The arms <NUM> may be coupled to a rotatable collar <NUM> arranged to yaw the arms <NUM> and the rotor-nacelle-assemblies <NUM> coupled to the arms <NUM>.

Each rotor-nacelle-assembly <NUM> may have a monitoring unit <NUM> (not shown in <FIG>), arranged to measure movement of the rotor-nacelle-assembly <NUM>. The monitoring units <NUM> may be substantially similar to the monitoring units <NUM> described in conjunction with the wind turbine <NUM> of <FIG>. The monitoring units <NUM> may be coupled to a central processing unit <NUM> and may be coupled via a time-synchronous network. The monitoring units <NUM>, the network and the central processing unit <NUM> may form a control system.

<FIG> shows a wind turbine <NUM> in an oscillation mode known colloquially as a "belly dancer" mode. In this oscillation mode, the wind turbine may oscillate with a wavelength twice the height of the wind turbine <NUM>, meaning that the top rotor-nacelle-assembly <NUM> may sit at a node and may remain substantially stationary while the central tower <NUM> oscillates. Therefore, if a traditional wind turbine monitoring system were used, incorporating only a measurement sensor at the top rotor-nacelle-assembly <NUM>, this oscillation mode may not be detected. However, by using measurements from monitoring units <NUM> mounted to each of the rotor-nacelle-assemblies <NUM> and/or on the arms <NUM>, this bending mode may be detected.

A further alternative multi-rotor wind turbine <NUM> is shown in <FIG>. The wind turbine <NUM> has a main tower <NUM> extending upwardly from a foundation <NUM>. The wind turbine has four rotor-nacelle-assemblies <NUM>, which are arranged in two separate, vertically spaced rows. An upper row has a rotatable collar <NUM> coupled to the main tower <NUM> and two arms <NUM> extend outwardly from the collar <NUM>. Each arm <NUM> supports a respective rotor-nacelle-assembly. The wind turbine <NUM> also has a lower row, which may be substantially similar to the upper row, with two rotor-nacelle assemblies <NUM> supported by two respective arms <NUM>, which extend from a rotatable collar <NUM>, the rotatable collar <NUM> being coupled to the main tower <NUM>.

Each rotor nacelle assembly <NUM> may have a nacelle <NUM> mounted on an arm <NUM> and a rotor <NUM> rotatably coupled to the nacelle <NUM>. The rotor <NUM> may have blades <NUM> arranged to generate lift when wind is incident upon the blades <NUM> so as to rotate the rotor <NUM> and to generate electricity by transferring rotational kinetic energy to a generator inside the nacelle <NUM>.

The wind turbine <NUM> of <FIG> may have movement sensors on or in the nacelles <NUM> and/or on or in the arms <NUM> and arranged to detect movement of the nacelles <NUM> and/or the arms <NUM>. The movement sensors may output movement data, which may be analysed locally or by a central processing unit <NUM> in the main tower <NUM>. The wind turbine <NUM> may have four movement sensors, each sensor being arranged to detect movement of a respective arm <NUM> or nacelle <NUM>.

The movement sensors and central processing unit <NUM> may form a control system similar to those of the first- and second-described wind turbines <NUM>, <NUM>. Such a control system may be arranged to detect oscillations in the tower, which may be those shown in either of <FIG> or <FIG>. Further, more complex oscillations may also occur within the wind turbine <NUM> due to the number of nacelles and their distance from the main tower <NUM>. The oscillations may be detected by combining data from the movement sensors.

The wind turbine <NUM> may further comprise a further rotor-nacelle-assembly disposed at the top of the main tower <NUM>. Such a top rotor-nacelle-assembly may be substantially similar to the other rotor-nacelle-assemblies <NUM> and may have a further movement sensor, which may be part of the control system described above.

<FIG> shows a schematic diagram of a control system <NUM>, comprising monitoring units <NUM> of the wind turbine. It will be understood that control systems may have more than two monitoring units <NUM>, for example with wind turbines having a greater number of rotor-nacelle-assemblies. The control system <NUM> may also comprise more than one monitoring unit <NUM> per rotor-nacelle-assembly and/or may comprise one or more monitoring units <NUM> within a central tower <NUM>, <NUM>, <NUM>.

Each monitoring unit <NUM> may comprise a movement sensor <NUM> and a processor <NUM>. The movement sensor <NUM> may be an accelerometer, a gyroscope, or may comprise multiple accelerometers and/or gyroscopes.

The processors <NUM> disposed within the monitoring units <NUM> may perform preliminary data processing of the data from the movement sensors <NUM>, such as compression and/or time-stamping of the data. The data transmission along the network of the control system <NUM> may therefore be improved.

The control system <NUM> also comprises a central processing unit <NUM>, which has a processor 206a and a memory 206b. The memory 206b may store instructions, which when carried out by the processor 206a cause the control system <NUM> to operate.

The control system <NUM> may operate to drive one or more actuators <NUM>. The driving of the actuators <NUM> may alter the oscillation of the wind turbine.

The actuators <NUM> may be blade pitch actuators, arranged to alter the blade pitch such that the angle of attack relative to incident wind on the blade may change, or may be yaw actuators arranged to yaw the collar <NUM>, <NUM>, <NUM> of the wind turbine, in order to move the rotors <NUM>, <NUM> out of incident wind, or in the case of a multi-rotor wind turbine <NUM> with a top rotor <NUM>, to yaw the top rotor <NUM> on the main tower <NUM> out of the wind.

Alternatively or additionally, the control system <NUM> may be coupled to a generator <NUM> and may be arranged to change a generator set point, a generator rotational speed or a generator power output.

Optionally, the control system <NUM> may be arranged to communicate with an adjacent wind turbine and may alter the properties of wind incident on the wind turbine <NUM> by changing operating parameters of an upwind wind turbine.

The control system <NUM> comprises a time synchronous network, which may be a deterministic time-synchronous network and may be a time-triggered Ethernet network, and which calculates latency within the network and may thereby make adjustments for time differences of signals received at the central processing unit <NUM> from the monitoring unit <NUM>. This may improve determination of oscillation characteristics such as phase differences between movements of different rotor-nacelle-assemblies <NUM> and/or different wind turbine arms <NUM>, <NUM>, <NUM>.

Movement data may be transferred between monitoring units <NUM> via a network and not necessarily directly to a central processing unit. The central processing unit <NUM> may be omitted in control systems with distributed processing within the monitoring units <NUM>.

<FIG> shows a schematic diagram of an example movement sensor <NUM> comprising three accelerometers <NUM>, each accelerometer <NUM> being arranged to detect acceleration in a different, orthogonal direction and three gyroscopes <NUM>, each gyroscope <NUM> being arranged to detect an angular velocity and/or an angular acceleration about a different orthogonal axis.

While the illustrated embodiments show wind turbines having only two, three, and four rotor-nacelle-assemblies, it will be understood that wind turbine assemblies having <NUM>, <NUM>, <NUM> or <NUM> arms may be used and any of these numbers of arms may be combined with central rotor-nacelle-assemblies on the top of main towers of the wind turbines. It will also be understood that such wind turbines may have rotor-nacelle-assemblies arranged at different vertical locations, providing multiple rotatable collars when necessary. Overall, the above teaching is applicable to any configuration of wind turbine having at least two rotor-nacelle-assemblies.

Claim 1:
A wind turbine (<NUM>) comprising:
a tower (<NUM>);
a first arm (<NUM>, <NUM>, <NUM>) extending from the tower;
a first rotor-nacelle assembly (<NUM>, <NUM>, <NUM>) disposed on the first arm;
a first movement sensor (<NUM>) disposed on the first arm or on the first rotor-nacelle assembly and arranged to generate first movement data based on movement of the first arm or of the first rotor-nacelle assembly;
a second arm (<NUM>), <NUM>, <NUM> extending from the tower;
a second rotor-nacelle assembly (<NUM>, <NUM>, <NUM>) disposed on the second arm;
a second movement sensor (<NUM>) disposed on the second arm or on the second rotor-nacelle assembly and arranged to generate second movement data based on movement of the second arm or of the second rotor-nacelle assembly; and
a control system (<NUM>) coupled to the first and the second movement sensors and arranged to receive and to process the first and second movement data;
characterized in that the control system comprises a time synchronous network;
wherein the first and the second movement sensors are configured to be time synchronous with each other and/or with the control system;
wherein the control system is arranged to calculate latency within the network to make adjustments for time differences of signals received to determine an oscillation characteristic of the wind turbine from the first and the second movement data.