Patent Description:
The main shaft of a wind turbine is rotatably supported by a rolling element bearing within a main bearing housing situated in the nacelle of the wind turbine. A gearbox may support the main shaft at the rear end of the main shaft. The rolling element bearing is usually monitored by temperature and/or vibration sensors. Both of these sensors can only detect a potential failure with the rolling element bearing after the rolling element bearing has already been damaged.

It is therefore desirable to be able to monitor the rolling element bearing and/or grease level within the main bearing housing so that action may be taken to prevent rolling element bearing damage.

<CIT> discloses the features of the preamble of claim <NUM>, a failure detection device for a blade bearing which a sensor for detecting the content condition of metal powder in lubricant which lubricates the rolling elements of the blade bearing, and a failure detecting part for detecting a failure of the blade bearing in accordance with a change in the output of the sensor. Preferably, the lubricant is grease, and the sensor includes a sensor for detecting the permeability of light into the grease. The failure detecting part determines a degree of an increase of the metal powder in accordance with a change in the permeability of light into the grease.

<CIT> discloses a component arrangement for a wind turbine including an outer component, an inner component arranged within the outer component, and a rolling bearing pair, which has a first rolling bearing and a second rolling bearing arranged in a manner adjusted relative to one another and which is preloaded by means of a clamping force. The inner component and the outer component are mounted so as to be rotatable relative to one another about an axis of rotation by means of the rolling bearing pair. The component arrangement also includes a pressure sensor for determining a preload of the rolling bearing pair, which is arranged in a flow of the clamping force.

A first aspect of the invention provides a wind turbine, the wind turbine comprising: a bearing housing comprising a rolling element bearing within the bearing housing and a space within the bearing housing for containing grease for lubricating the rolling element bearing; a shaft rotatably supported by the rolling element bearing; wherein the rolling element bearing comprises a plurality of rolling elements spaced by a cage, characterised in that the wind turbine further comprises a pressure sensor, wherein the pressure sensor is in fluid communication with the space and configured to measure the pressure of grease in the space within the bearing housing.

An advantage of at least the first aspect is that the use of a pressure sensor can avoid a time consuming inspection of the rolling element bearing which may otherwise be done by periodically monitoring the rolling element bearing by removing the bearing housing cover and/or by disassembling part of the bearing arrangement. The pressure sensor may be used to generate important information for service planning to save on resources. The pressure sensor may be used to generate an early alert for grease losses, which may be caused by seal damage or wear. The pressure sensor may be used to monitor the fluid pressure within the bearing housing to ensure correct breather and seal function. The wind turbine may be used to back up the lubrication system function, e.g. to deliver grease from a grease reservoir into the space within the bearing housing to top up the grease level. The wind turbine may be used to cold trigger the lubrication system as emergency mediation, e.g. to deliver all of the grease from the grease reservoir into the space within the bearing housing until service can be organised.

The bearing housing may further comprise a grease outlet for adding and/or removing grease from the space within the bearing housing. The pressure sensor may be coupled to the grease outlet and may be suitable for containing grease inside the bearing housing during operation of the wind turbine.

The pressure sensor may be a first pressure sensor. The wind turbine may further comprise a second pressure sensor in fluid communication with the space and may be configured to measure the pressure of the grease in the space within the bearing housing. The first pressure sensor may be in fluid communication with the space at a first measurement point, and the second pressure sensor may be in fluid communication with the space at a second measurement point.

The rolling element bearing may further comprise a second plurality of rolling elements spaced by a second cage. The first measurement point may be positioned to measure the pressure of grease in the space adjacent the first plurality of rolling elements, and the second measurement point may be further positioned to measure the pressure of grease in the space adjacent the second plurality of rolling elements.

The bearing housing may be a main bearing housing of the wind turbine. The shaft may be a main shaft of the wind turbine.

A second aspect of the invention provides a method of monitoring a wind turbine of the first aspect, the method comprising:.

Determining the cage slip ratio of the rolling element bearing may comprise: determining a cage frequency from the pressure signal, wherein the cage frequency may correspond to an angular rotation speed of the plurality of rolling elements of the rolling element bearing; measuring the shaft angular rotation speed; and calculating the cage slip ratio of the rolling element bearing may be based on the determined cage frequency, and an ideal cage frequency at the measured shaft angular rotation speed.

Determining the cage frequency from the pressure signal may comprise decomposing the pressure signal into its constituent frequencies and identifying the frequency corresponding to the angular rotation speed of the plurality of rolling elements of the rolling element bearing.

Determining the amount of grease in the space within the bearing housing may comprise: decomposing the pressure signal into its constituent frequencies and identifying the frequency corresponding to the angular rotation speed of the plurality of rolling elements of the rolling element bearing; measuring the amplitude of the decomposed pressure signal at the frequency corresponding to the angular rotation speed of the plurality of rolling elements of the rolling bearing element; and, estimating the amount of grease in the space within the bearing housing based on the measured amplitude.

Transmitting a maintenance request for wind turbine maintenance may be based on the determined cage slip ratio, and/or, amount of grease.

The method may comprise controlling the speed of the shaft or halting operation of the wind turbine based on the determined cage slip ratio, and/or, amount of grease.

The method may comprise performing maintenance on the wind turbine based on the determined cage slip ratio, and/or, amount of grease.

A third aspect of the invention provides a method of assembling or retro-fitting a wind turbine comprising a bearing housing, a rolling element bearing within the bearing housing and a space within the bearing housing for containing grease for lubricating the rolling element bearing, and a shaft rotatably supported by the rolling element bearing, the method comprising:
fitting a pressure sensor such that it is in fluid communication with the space and configured to measure the pressure of grease in the space within the bearing housing.

The method may comprise removing a plug from the grease outlet. The plug may be suitable for containing grease inside the bearing housing. The method may comprise fitting the pressure sensor to the grease outlet while the shaft of the wind turbine is idle.

The pressure sensor may be fitted so as to be suitable for containing grease inside the bearing housing during operation of the wind turbine.

In any of the above aspects a rolling element bearing may be a bearing which carries a load by placing rolling elements (such as balls or rollers) between two bearing rings called races. The relative motion of the races may cause the rolling elements to roll with very little rolling resistance and with little sliding.

In any of the above aspects a cage slip ratio directly reflects the sliding state between the rolling elements and the raceway of the rolling element bearing. Cage slip ratio may be a ratio of the angular rotational speed of the cage with respect to the idealised angular rotational speed of the cage for a particular angular speed of the inner race.

In any of the above aspects a maintenance request may be any transmitted signal which may result in maintenance being caused to be performed on the wind turbine. This may be a specific value (such as the amount of grease or the cage slip ratio), which may be analysed at a control centre distinct from the wind turbine, which may indicate that the wind turbine requires maintenance. Alternatively, this may be a direct request for maintenance of the wind turbine which may comprise details or reasons for the request. For example, this may be formatted as: maintenance requested; wind turbine address; and/or further details (e.g. low grease volume).

<FIG> illustrates, in a schematic perspective view, an example of a wind turbine <NUM>. The wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> disposed at the apex of the tower, and, a rotor <NUM> operatively coupled to a generator housed inside the nacelle <NUM>. In addition to the generator, the nacelle houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine <NUM>.

The rotor <NUM> of the wind turbine <NUM> includes a central hub <NUM> and a plurality of blades <NUM> that project outwardly from the central hub <NUM>. In the illustrated wind turbine <NUM>, the rotor <NUM> includes three blades <NUM>, but the number may vary. Moreover, the wind turbine comprises a control system. The control system may be placed inside the nacelle or distributed at a number of locations inside (or externally to) the turbine and communicatively connected.

When wind blows against the wind turbine <NUM>, the blades <NUM> generate a lift force which causes the rotor <NUM> to rotate, which in turn causes the generator within the nacelle <NUM> to generate electrical energy.

<FIG> schematically illustrates the inside of the nacelle <NUM> of the wind turbine <NUM>. The nacelle <NUM> comprises a nacelle frame <NUM> which structurally supports the nacelle <NUM> and the components within the nacelle <NUM>. The wind turbine <NUM> comprises rotor blades <NUM> which are mechanically connected to an electrical generator <NUM> via the gearbox <NUM>. In direct drive systems, and other systems, the gearbox <NUM> may not be present. The electrical power generated by the generator <NUM> is injected into a power grid via an electrical converter (not shown). A main shaft <NUM> is mechanically attached to the hub <NUM> at a front end. A bearing housing <NUM> is mechanically attached to the nacelle frame <NUM> and is configured to rotatably support the main shaft <NUM> such that the bearing housing <NUM> supports the hub <NUM> and the plurality of blades <NUM> to allow them to rotate relative to the nacelle <NUM>. The main shaft <NUM> extends through the bearing housing <NUM> and into the gearbox <NUM> (or electrical power generator <NUM> in a direct drive system) at a rear end.

<FIG> illustrates a cross section of the main shaft <NUM> and main bearing housing <NUM> of the wind turbine <NUM>. The main bearing housing <NUM> contains the rolling element bearing <NUM>. The main bearing housing <NUM> rotatably supports the main shaft <NUM> with the rolling element bearing <NUM>. The rolling element bearing comprises a plurality of rolling elements <NUM> spaced by a cage (not shown in <FIG>) configured to guide the plurality of rolling elements <NUM>.

The bearing housing <NUM> further contains grease <NUM> or other lubricant for lubrication of the rolling element bearing <NUM>, in particular the plurality of rolling elements <NUM>, in a space within the bearing housing. The grease <NUM> preferably partially fills, e.g. approximately half, the space with the reminder as air. The space may be vented. The space may be fluidly connected to a grease reservoir which may supply grease to the space, e.g. by a fluid pump. The space may include a sump, or other low point, within the bearing housing <NUM> where excess grease collects under gravity and for delivery onto the rolling element bearing <NUM> as the bearing rotates.

This grease <NUM> may be monitored with a pressure sensor <NUM> to gain an insight into the grease level and/or the present condition of the rolling element bearing <NUM> and/or may be used as an early warning system which can notify the wind turbine <NUM> (via a control system) and/or an operator of potential adverse events which may further be avoided (e.g. by maintenance and/or by control of the wind turbine <NUM>).

The main shaft <NUM> is fixed to the inner race <NUM> of the rolling element bearing <NUM>. The outer race <NUM> may be fixed such that is does not rotate with respect to the main bearing housing <NUM> during operation of the wind turbine <NUM>. The outer race <NUM> may be fixed to the structure of the main bearing housing <NUM>, the nacelle <NUM>, or the nacelle frame <NUM>. This allows the inner race to rotate with the main shaft <NUM> during operation of the wind turbine <NUM>, with respect to the fixed outer race <NUM>.

There are many types of rolling element bearings <NUM>, for example the rolling element bearing <NUM> may be cylindrical roller bearings as shown in <FIG>. Alternatively, many other types of rolling element bearings <NUM> may be used, such as: single row deep-groove ball bearings; single row angular contact bearings; double row angular contact ball bearings; self-aligning ball bearings; needle roller bearings; taper roller bearings; and/or, spherical roller bearings, among others.

<FIG> illustrates an example of a rolling element bearing <NUM>. The rolling element bearing <NUM> is shown comprising the inner race <NUM>, the outer race <NUM>, and the plurality of rolling elements <NUM> spaced by the cage <NUM>. The cage <NUM> rotates with the speed of the rotation of the plurality of rolling elements about the centre of the rolling element bearing within the inner race <NUM> and outer race <NUM>. The cage <NUM> rotation speed may be less than the inner race <NUM> rotation speed.

<FIG> illustrates a magnified view of the rolling element bearing of <FIG>. It can be seen that the wind turbine <NUM> further comprises the pressure sensor <NUM>. A pressure sensor (including the pressure sensor <NUM>) in general is an easy to work with and low-cost sensor to implement in order to monitor the grease <NUM> of the rolling element bearing <NUM>. The pressure sensor <NUM> may dependably supervise the grease <NUM> without field intervention. Additional information regarding the operational condition of the rolling element bearing <NUM> and/or the grease <NUM> may be extracted from the raw pressure sensor <NUM> data, either on-request via a control system (not shown) or by reading the pressure sensor <NUM>.

The pressure sensor <NUM> is in fluid communication with the grease <NUM> and configured to measure the pressure of the grease within the bearing housing <NUM>. The pressure sensor <NUM> is preferably located in the sump or other low point of the bearing housing <NUM> where the grease <NUM> collects under gravity.

The bearing housing <NUM> comprise a grease outlet <NUM> for the addition and/or removal of grease <NUM> from the bearing housing <NUM>, for example, during a grease replacement. The grease outlet <NUM> may be located at the sump or other low point of the bearing housing <NUM>. Grease outlets such as grease outlet <NUM> further typically comprise a plug (not shown) suitable for containing the grease <NUM> inside the bearing housing <NUM>. The plug prevents the grease <NUM> from escaping from the bearing housing <NUM> during normal operation of the wind turbine <NUM>.

As shown in <FIG> the pressure sensor <NUM> additionally performs the same role as plug <NUM>. The pressure sensor <NUM> is coupled to the grease outlet <NUM> such that it may be suitable for containing the grease <NUM> inside the rolling element bearing <NUM> at least during operation of the wind turbine <NUM>. Therefore, use of a plug may no longer be required. Alternatively, a plug adaptor (not shown) may be used to couple the pressure sensor <NUM> to the grease outlet <NUM> such that it may suitable for containing the grease <NUM> inside the bearing housing <NUM> at least during operation of the wind turbine <NUM>.

<FIG> illustrates the same features as <FIG>, except it shows two pressure sensors 20a, 20b instead of one pressure sensor <NUM>. The first pressure sensor 20a is associated with a first plurality of rolling elements 17a. The first pressure sensor 20a is in fluid communication with the grease <NUM> in the space within the bearing housing <NUM> and is positioned such that it can measure the pressure of the grease <NUM> associated with the first plurality of rolling elements 17a of the rolling element bearing <NUM>. The position of the first pressure sensor 20a to the first plurality of roller bearings 17a is therefore proximal. The benefit of a proximal pressure sensor (such as <NUM>, 20a and/or 20b) is a larger received pressure signal. This is due the attenuation of the pressure signal as it travels through the grease <NUM>, and/or any air in the grease. In practice this may mean that the first pressure sensor 20a is positioned closer to the first plurality of roller bearings 17a than the second plurality of roller bearings 17b. The position of the first pressure sensor 20a may be defined as a first measurement point. The first measurement point is positioned to measure the pressure of the grease <NUM> corresponding to the first plurality of rolling elements 17a, such that the pressure signal caused by the first plurality of rolling elements 17a can be identified by the first pressure sensor 20a and the subsequent analysis of the output signal of the first pressure sensor 20a.

Similarly, the second pressure sensor 20b corresponds to the second plurality of rolling elements 17b in much the same way as described above. It is of note that the rolling element bearing <NUM> comprises a first plurality of rolling elements 17a and a second plurality of rolling elements 17b, each of which is spaced by a respective cage (not shown in <FIG>). Since each of the plurality of rolling elements 17a and 17b are part of the same rolling element bearing, both of the plurality of rolling elements 17a and 17b share the same grease <NUM> in the same space within the bearing housing <NUM>.

<FIG> illustrates that the main shaft <NUM> may be supported by the main bearing housing <NUM> which rotatably supports the main shaft <NUM> with first rolling element bearing 15a and second rolling element bearing 15b. Each rolling element bearing 15a, 15b comprises: a plurality of rolling elements 17a, 17b; and respective cages (not shown in <FIG>); and, grease 18a, 18b for lubrication (from respective spaces within the bearing housing <NUM>). Each rolling element bearing 15a, 15b also comprises a pressure sensor 20a, 20b which is in fluid communication with the respective grease 18a, 18b in the respective space. Each pressure sensor 20a, 20b is configured to measure the pressure of the respective grease 18a, 18b within its respective space associated with the respective rolling element bearing 15a, 15b in the common bearing housing <NUM>.

By using pressure sensor <NUM> in fluid communication with the grease <NUM> associated with the rolling element bearing <NUM>, it is possible to monitor the rolling element bearing <NUM> and/or the grease level within the bearing housing of wind turbine <NUM>. The output of the pressure sensor <NUM> may be analysed to decide whether maintenance or other action is required, and/or whether maintenance or other action may be required in the future. This monitoring may allow the control system (not shown) of the wind turbine <NUM> to halt operation of the wind turbine <NUM> if a major fault with the rolling element bearing <NUM> occurred. For example, if there was a sudden loss of grease <NUM>, then the rolling element bearing <NUM> would frictionally heat and may become damaged if the wind turbine <NUM> continued to operate.

Typically, temperature sensors (not shown) in the wind turbine <NUM> may be used to indirectly sense that the bearing housing <NUM> is exceeding normal thermal limits and then halt operation of the wind turbine <NUM>. However, once the temperature has risen high enough to be trigger an alarm, it may already be too late for the rolling element bearing <NUM> to be saved and may need replacing. This may lead to additional cost of replacing the bearing <NUM> accompanied with the loss due to downtime of the wind turbine.

Therefore, by using the pressure sensor <NUM> in fluid communication with the grease <NUM> in the space within the bearing housing <NUM> such an incident may be avoided and provide further maintenance benefits which may increase the operating lifespan of the rolling element bearing <NUM>. The pressure sensor <NUM> may also be used to indicate a low grease level and prompt additional grease <NUM> to be added to the bearing from the lubrication system. Such pre-emptive 'bridging' action between normal scheduled maintenance may significantly reduce the number of additional maintenance services required between the normal scheduled maintenance intervals.

<FIG> illustrates the method of monitoring a rolling element bearing <NUM> of a wind turbine <NUM> of at least <FIG>, <FIG>, <FIG>, or <FIG>. Firstly, at step S1, the main shaft is rotated, typically by operation of the wind turbine <NUM>. The rotation of the main shaft causes the inner race <NUM> to move with respect to the cage <NUM>. The rotation of the main shaft causes the cage <NUM> to move with respect to the outer race <NUM>. This causes pressure fluctuations of the grease <NUM> which corresponds to: (i) the amount of grease <NUM>; and, (ii) the rotation of the plurality of rolling element bearings <NUM>, as will be described below. These pressure fluctuations may be sensed by the pressure sensor <NUM>.

At step S2, the pressure fluctuations of the grease <NUM> may be measured by the pressure sensor <NUM> to generate a pressure signal. The pressure signal is shown with respect to time at <FIG> illustrates pressure along the y-axis and time across the x-axis.

At step S3, the amount of grease <NUM> in rolling element bearing <NUM> may be determined by isolating the pressure signal components, e.g. by decomposing the pressure signal into its constituent frequencies, and identifying the frequency which corresponds to the angular rotation speed of the plurality of rotating elements <NUM>. Referring back to <FIG>, the pressure signal component <NUM> is shown to correspond to the rotation speed of the plurality of rolling elements. The component <NUM> may be a relatively high frequency signal in comparison to other signal components from the pressure sensor <NUM>. The component <NUM> may also have a relatively stable amplitude <NUM> over many periods. The component <NUM> may be a consistent signal and may be present in relative isolation for a temporal majority, as shown in <FIG>.

The presence of large magnitude pressure signals distinct from the component <NUM> may be seen from <FIG>. These may be an artefact of the specific test rig used to gather the measurements and may not be present in operation of a wind turbine.

At step S4, the amplitude of the pressure signal components <NUM> is measured as shown in <FIG>, the result is a measured amplitude <NUM>. Alternatively, the measured amplitude <NUM> may be an average of amplitude over a period of time.

At step S5, the amount of grease <NUM> in the rolling element bearing <NUM> may be estimated based on the measured amplitude <NUM>. If the grease level is normal then the amplitude <NUM> will be larger. If the grease level is low then the amplitude <NUM> will be smaller. If no pressure signal components due to the angular rotation speed of the plurality of rotating elements <NUM> are observed in the pressure signal, then it can be determined that the grease level is very low or out as there is no grease between the pressure sensor <NUM> and the roller bearing elements to transfer the impulse. The measured pressure signal component <NUM> and the amplitude <NUM> of the pressure signal component <NUM> may therefore be a good proxy for the amount of grease <NUM> in the rolling element bearing <NUM>. If there is a sub-optimal amount of grease <NUM> in the rolling element bearing <NUM>, then the pressure fluctuations of the grease <NUM> will be attenuated more. This is because the pressure fluctuations of the grease <NUM> require grease <NUM> in order to propagate to the pressure sensor <NUM>.

In addition to, or as an alternative to, generating an estimate of the amount of grease <NUM> in the rolling element bearing <NUM>, the pressure signal may be used to estimate the cage slip ratio of the rolling element bearing <NUM>. Cage slip ratio may be defined as a ratio of the angular rotational speed of the cage with respect to the idealised speed of the cage, for a particular angular rotational speed of the inner race <NUM> (i.e. the shaft rotational speed). Idealised cage slip ratios for a range of angular rotational speeds of the inner race can be calculated from the bearing dimensions or may be provided by the bearing manufacturer. The cage slip ratio is a proxy (or tracer) for bearing wear. Specifically, the cage slip ratio may be used to determine the wear of one or more rolling elements <NUM> of the plurality of rolling elements <NUM>. The more the measured cage slip deviates from the idealised cage slip (either calculated from bearing dimensions or received from the manufacturers datasheet), the more wear will be present. This wear may be indicated by the pressure signal well in advance of any notable loss of performance or failure of the bearing. Maintenance to repair or replace the bearing may therefore be scheduled accordingly well in advance. The operation of the wind turbine may be adjusted, e.g. by limiting the rotor speed, to extend the operation of the turbine to bridge until the next scheduled maintenance. This may avoid unscheduled interim maintenance that may otherwise have become necessary.

This process begins at step S6, wherein a cage frequency may be determined, which corresponds to an angular rotation speed of the plurality of rolling elements <NUM>. Referring to <FIG>, which illustrates a Fast Fourier Transform (FFT) of the signal shown in <FIG> demonstrates there is a frequency component <NUM> at about <NUM> (although the value depends on a multitude of factors). The frequency component <NUM> corresponds to the FFT of the time domain component <NUM>. The frequency component <NUM> may be a relatively high frequency signal in comparison to other signal components from the pressure sensor <NUM>. The frequency component <NUM> may be characterised by a relatively large magnitude due to the consistency of the component <NUM> (i.e. the pressure signal). The frequency component <NUM> may be the highest frequency signal with one of the largest magnitudes. The frequency component <NUM> may have a peak magnitude greater than <NUM>% the peak magnitude of any other frequency component within a <NUM> or <NUM> range of the frequency component <NUM>. The frequency component <NUM> represents the cage <NUM> rotation frequency multiplied by the number of rolling elements of the plurality of rolling elements <NUM>. e.g. If there are ten rolling elements in the rolling element bearing <NUM>, then ten rolling elements will pass a fixed position of the outer race <NUM> during one cage <NUM> rotation. The fixed position may be the location of the pressure sensor <NUM>.

At step S7, the main shaft frequency may be measured from the main shaft angular rotation speed. The main shaft rotation frequency may be measured by a main shaft Tachometer (not shown) or any other sensor of the wind turbine <NUM>, built in or otherwise.

At step S8, the cage slip ratio of the rolling element bearing <NUM> is calculated based on: the determined cage frequency and an idealised cage frequency at the measured shaft angular rotation speed. The idealised cage frequency at the measured main shaft rotation frequency may be known based on the design of the rolling element bearing <NUM> and/or may be received from (or easily calculated from) the bearing manufacturer's data sheets. Referring to <FIG>, the idealised cage frequency is denoted ωcr where: <MAT>.

Where Rr is the radius of a rolling element of the plurality of rolling elements <NUM>, Rm is the bearing pitch radius, and ωt is the angular speed of inner race <NUM> = the angular speed of the main shaft.

The cage slip ratio may be defined as: <MAT>.

Where ωc is the angular speed of the cage <NUM>.

<FIG> illustrates a schematic view of the rolling element bearing <NUM> and may aid in defining the parameters used in the equations above (<NUM>), (<NUM>).

Based on the pre-determined recommended amount of grease <NUM>, or, alternatively, the limits of the amount of grease <NUM> for acceptable functionality of the rolling element bearing <NUM>, the wind turbine <NUM> may transmit a maintenance request. Additionally, or alternatively, the wind turbine <NUM> may transmit a maintenance request based on the cage slip ratio. A microcontroller (not shown) of the control system (not shown) of the wind turbine <NUM> may process incoming data and transmit a maintenance request based on the determined cage slip ratio and/or, the amount of grease <NUM>. Alternatively, the control system (not shown) of the wind turbine <NUM> may further process the results of step S8 and step S5 to determine when, if at all, a maintenance request may be transmitted.

At step S10, the control system (not shown) of the wind turbine <NUM> may switch the operational state of the wind turbine <NUM>. The calculated slip ratio and/or estimate of the amount of grease may be compared to predetermined or variable thresholds and, if exceeded, may control the wind turbine <NUM> to reduce the speed of the main shaft <NUM>. The speed of the main shaft <NUM> may be reduced via a braking system (not shown), pitching the blades <NUM>, or other mechanism.

Alternatively, the calculated slip ratio and/or estimate of the amount of grease may be compared to predetermined or variable thresholds and, if exceeded, may control the wind turbine <NUM> to halt operation of the wind turbine <NUM> completely, or switch the wind turbine <NUM> into an idle operating state.

A cage slip ratio of less than <NUM>% may be considered to be within normal operational limits. The predetermined or variable threshold associated with the cage slip ratio may be <NUM>%, <NUM>%, <NUM>%, <NUM>%, or any value which may be considered to require maintenance (this may be determined from manufacturer's datasheets). The threshold may further depend on: the type of rolling element bearing <NUM>; and/or, the conditions of operation. The predetermined or variable threshold associated with the amount of grease <NUM> may be a percentage of the maximum amount of grease <NUM> for a particular rolling element bearing <NUM>, such as <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% or any value which may be considered to require maintenance (this may be determined from manufacturer's datasheets). The threshold may further depend on, the type of rolling element bearing <NUM>, and/or the conditions of operation.

As an example, the step of S10 may be required if there is a dramatic loss of the amount of grease as estimated at step S5. This may indicate a sudden grease <NUM> pressure loss which could cause damage to the rolling element bearing <NUM>. Alternatively, a large change in the value of cage slip ratio may be indicative of a damaged rolling element bearing <NUM>.

Although not shown in the Figures, an air breather/pressure relief valve may be used with the grease cavity or space of the bearing housing <NUM> for accommodating pressure changes with temperature. This pressure change with temperature may be factored into any calculations for the amount of grease <NUM> and/or cage slip ratio. A temperature sensor may be utilised for this purpose.

At step S11, maintenance is performed on the rolling element bearing <NUM> based on the determined cage slip ratio and/or, the amount of grease <NUM>. Maintenance may comprise adding grease to the rolling element bearing <NUM>. Maintenance may comprise replacing the grease <NUM> in the rolling element bearing <NUM>. Maintenance may comprise dis-assembly of the rolling element bearing <NUM> and the replacement of some or all components or parts. For example, at least some of the plurality of rolling elements <NUM>, or any other components of the rolling element bearing <NUM> may be replaced.

Any of the steps S9, S10 and S11 may take place in any order. At least some of the steps S9, S10 and S11 may not take place for some operations, or even at all. For example the steps S1 - S8 may be performed during a maintenance test, therefore steps S9 and S10 are not required to be performed during this test. Further, the step of S11 only needs to be performed if there are results from step <NUM> and/or S5 which require maintenance on the rolling element bearing <NUM>. Moreover, it would be clear to the skilled person that there would be no requirement for steps S3 - S5 or steps S6 - S8 be performed at all, such that only one of the cage slip ratio and/or, the amount of grease <NUM> in the rolling element bearing <NUM> may be determined.

The main bearing housing <NUM> and/or the rolling element bearing <NUM> of <FIG> can be assembled or retro-fitted by the addition of the pressure sensor <NUM> to: a rolling element bearing <NUM>; main bearing housing <NUM>; or, any part of the wind turbine <NUM>. The addition of the pressure sensor <NUM> is such that the pressure sensor <NUM> is in fluid contact with the grease <NUM> of the rolling element bearing <NUM>. Since the only requirement is that the pressure sensor <NUM> is in fluid contact with the grease <NUM> of the rolling element bearing <NUM>, there may be few physical limitations to the location of the pressure sensor <NUM>. The pressure sensor <NUM> may have a small overall size, and/or be a wireless sensor (the pressure sensor may be wired in some applications). These features enable the pressure sensor <NUM> to have few physical location limitations when assembling or retro-fitting. For example, the pressure sensor <NUM> may be positioned in the grease outlet <NUM> during assembly.

Retro-fitting may place the pressure sensor <NUM> in a sub-optimal location for pressure sensing, but may result in minimal changes to the existing rolling element bearing <NUM> in order to accommodate the pressure sensor <NUM>. Therefore, this system may be optimal overall depending upon the application and purpose. Positioning the pressure sensor <NUM> in the grease outlet <NUM> may be especially useful for retro-fitting, because the grease outlet <NUM> is already an access point for the grease <NUM>. In another example, the pressure sensor <NUM> may be manufactured such that it is inside the rolling element bearing <NUM>, or outside the rolling element bearing <NUM> but at another location within the main bearing housing <NUM>. If the pressure sensor <NUM> was inside the rolling element bearing <NUM>, it may be positioned on the outside race <NUM> in fluid contact with the grease <NUM>, or on the inside race <NUM>, or at a side of the rolling element bearing <NUM> positioned such that the plurality of rolling elements <NUM> are not hindered in their path. In conclusion, the pressure sensor <NUM> may be manufactured to be positioned anywhere around the rolling element bearing <NUM> so long as it can measure the presence of the grease <NUM> within the bearing housing that is in contact with the rolling element bearing <NUM>.

As mentioned previously, the grease outlet <NUM> is particularly convenient for retro-fitting the pressure sensor an existing rolling element bearing housing <NUM>. The process of retro-fitting a pressure sensor <NUM> may require the removal of the plug from the grease outlet <NUM>, and the insertion of the pressure sensor <NUM> to the grease outlet <NUM>. Ideally this process occurs while the main shaft of the wind turbine <NUM> is stationary to avoid unnecessary grease <NUM> pressure loss and to allow maintenance personnel access. The end result of retro-fitting is that the pressure sensor <NUM> is secured in the grease outlet <NUM> so that it is suitable for containing the grease <NUM> inside the rolling element bearing <NUM> during operation of the wind turbine <NUM>. Thus, the pressure sensor <NUM> may be used as a fixed installed item for a condition monitoring system (CMS) or as a diagnostic tool for service technicians.

The method of manufacturing the wind turbine (specifically, the main bearing housing <NUM> and/or the rolling element bearing <NUM>) by assembling or retro-fitting the pressure sensor <NUM>, is also equally applicable if there are two pressure sensors 20a, 20b to be retrofitted or assembled to the same rolling element bearing <NUM> in the manner described above.

The benefits of the wind turbine and respective methods are:.

Claim 1:
A wind turbine (<NUM>) comprising: a bearing housing (<NUM>); a rolling element bearing (<NUM>) within the bearing housing and a space within the bearing housing for containing grease for lubricating the rolling element bearing; a shaft (<NUM>) rotatably supported by the rolling element bearing; wherein the rolling element bearing comprises a plurality of rolling elements (<NUM>) spaced by a cage (<NUM>), characterised in that the wind turbine further comprises a pressure sensor (<NUM>),
wherein the pressure sensor is in fluid communication with the space and configured to measure the pressure of grease in the space within the bearing housing.