BACKGROUND

Field of the Invention

Embodiments presented in this disclosure generally relate to varying wind turbine characteristics, e.g., rotor speed and blade pitch of a wind turbine when operating in low power and partial load modes. More specifically, embodiments disclosed in this disclosure relate to varying blade pitch of a wind turbine at a given rotor speed of the wind turbine when operating in a low power mode to utilize available power for a specified fast lift.

Description of the Related Art

Grid compliance in some countries requires wind turbines to quickly ramp from a low (de-rated) output power to full production (i.e., a rated output power). For example, a grid code may require the turbines in a wind park to operate in a low power mode upon request so that the turbines are de-rated to output a fraction of the optimal output power for a given wind speed. But when output power decreases, the torque on the rotor also decreases assuming the rotational speed is maintained. Operating at low power is challenging for a wind turbine. Under low power, the power is kept relatively constant by the converter which means that oscillations present in the wind can result in torque variations. When the torque is very low there is a risk that these torque variations generate torque in the opposite direction (i.e., negative torque) than the driving torque causing gear-torque-reversals. As the torque on the rotor decreases, the risk that smearing in the drive train will occur (when ball bearings begin to slide rather than rotate) also increases which can damage the turbine. These incidents can harm the gear box and reduce its lifetime significantly. SUMMARY

One embodiment of the present disclosure is a wind turbine that includes a rotor comprising a blade and a controller. The controller is configured to receive a command to operate the wind turbine at a de-rated output power. The controller is configured to determine a first rotor speed and a first blade pitch angle for the blade that outputs the de-rated output power while maintaining a predefined reserve power when outputting the de-rated output power. The predefined reserve power is power that is added to the de-rated output power by pitching the blade from the first blade pitch angle to an optimal blade pitch angle while maintaining the first rotor speed. The controller is configured to adjust the rotor to the first rotor speed and the blade to the first blade pitch angle.

Another embodiment of the present disclosure is a method for operating a wind turbine. The method includes receiving a command to operate the wind turbine at a de-rated output power. The method also includes determining a first rotor speed and a first blade pitch angle for a blade that outputs the de-rated output power while maintaining a predefined reserve power when outputting the de-rated output power. The predefined reserve power is power that is added to the de-rated output power by pitching the blade from the first blade pitch angle to an optimal blade pitch angle while maintaining the first rotor speed. The method includes adjusting the rotor to the first rotor speed and the blade to the first blade pitch angle.

Another embodiment of the present disclosure is a controller that includes a processor and a memory. The memory is configured to store a program, which when executed by the processor performs an operation. The operation includes receiving a command to operate a wind turbine at a de-rated output power. The operation includes determining a first rotor speed and a first blade pitch angle for a blade that outputs the de-rated output power while maintaining a predefined reserve power when outputting the de-rated output power. The predefined reserve power is power that is added to the de-rated output power by pitching the blade from the first blade pitch angle to an optimal blade pitch angle while maintaining the first rotor speed. The operation includes adjusting the rotor to the first rotor speed and the blade to the first blade pitch angle.

BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

Fig. 1 illustrates a diagrammatic view of a wind turbine, according to an embodiment described in this present disclosure.

Fig. 2 illustrates a diagrammatic view of the components internal to the nacelle and tower of a wind turbine, according to an embodiment described in this present disclosure.

Fig. 3 illustrates a controller for operating a wind turbine according to an embodiment described in this present disclosure.

Fig. 4 depicts a flow chart for a method for operating a wind turbine according to embodiments described in this present disclosure.

Fig. 5 is a graph illustrating different rotor speeds, blade pitches, and output powers of a wind turbine based on wind speed, according to an embodiment described in this present disclosure.

Fig. 6 depicts a flow chart for a method for operating a wind turbine according to embodiments described in this present disclosure. Fig. 7 is a graph illustrating different rotor speeds, blade pitches, and output powers of a wind turbine based on wind speed, according to an embodiment described in this present disclosure.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS

To reduce the risk of gear torque reversal in a wind turbine, the average torque level can be raised during low-power-operation (LPO) by reducing the rotational speed of the rotor. However, when LPO is no longer needed, the grid may request that the turbine ramp quickly to output rated power. Unfortunately, the speed at which the turbine can ramp its power output from a low power level (e.g. , a de-rated power) is limited by the rotor speed. Thus, reducing the speed of the rotor to reduce the risk of gear torque reversal limits how fast the turbine can ramp.

When ramping (i.e., increasing) output power from a de-rated power value, a wind turbine may issue power requests to a power converter to increase the output power of the turbine. However, these requests generate a torque which will slow down the rotor and may cause a low-speed shutdown. To compensate for the increased torque imposed by ramping the power, the blades may be pitched in thereby extracting additional energy from the wind. However, if when operating in a low power mode the rotor is slowed down below what is the aerodynamically optimal rotor speed (in order to prevent smearing and torque reversals as described above), even if the blades are pitched to the optimal pitch angle, the turbine may be unable ramp to the requested power - e.g., the rated output power of the turbine. To finish ramping to the requested power, the turbine may enter a partial-load operation where the power ramping stops (or slows down) so that the rotor speed can be increased to improve aerodynamic efficiency. As the rotor is accelerated, the aerodynamically efficiency is improved and the power can be ramped up to the target value. Operating in partial load mode, however, results in the output power ramping much slower than ramping the output power by pitching in the blades - e.g., 30 to 40 seconds versus 3-4 seconds. As such, ramping from a low power mode to the rated power when operating in a low power mode with slow rotor speeds may not satisfy a stringent grid code that requires fast ramping.

To ramp quickly from a low power mode to full production, the rotor speed may be kept at the optimal aerodynamic speed, which, for wind speeds greater than the rated wind speed of the turbine is the maximum speed of the rotor. Thus, whenever a request is received from the grid to output full production - i.e., the rated output power - the turbine need only pitch in the blades to their optimal angle to output the optimal output power. This power ramp can take only a few seconds. However, as above, maintaining the rotor speed at the optimal rotor speed for the current wind conditions may cause smearing or torque reversals that can reduce the lifetime of the drive train or gear box in the turbine. Instead of maintaining the rotor speed at the optimal speed when in the low power mode, the embodiments herein reduce the speed of the rotor so that the short-term available power achieved by pitching the blades is sufficient to guarantee a specified fast lift. As used herein, short term available power is the power the wind turbine is capable of producing if the rotor speed is maintained at a given value and the blades are pitched to a position which gives maximum aerodynamically efficiency e.g., the optimal pitch of the blades.

A grid may require a wind turbine to operate in a low power mode e.g., where the turbine outputs a de-rated power specified by the grid. The grid may also require the wind turbine to ramp, within a certain amount of time, from the de-rated output power to a particular output power {e.g., some percentage of rated power). The ramp time specified by the grid may require the wind turbine to ramp very quickly - e.g., within a few seconds.

In some embodiments, as the rotor speed decreases, the turbine maintains the ability to ramp the output power to the rated power of the turbine only by pitching in the blades to an optimal blade pitch angle. Thus, upon receiving a request to cease operating in the low power mode, the turbine can increase the output power to the optimal output power without first increasing the rotor speed. For example, the time required to ramp the output power from a de-rated power outputted during the low- power mode to the optimal output power is the time needed to pitch the blades to the optimal pitch angle {e.g., a few seconds).

In one embodiment, a controller in the turbine ensures that the turbine can ramp up to a predefined percentage of the rated output power by only pitching in the blades {e.g., 90% of the rated power) when operating in the low-power mode. For example, the controller may determine a given speed of a rotor, while operating in a low power mode, to output a de-rated power that maintains power that is available by only pitching in the blades to the optimal blade angle. As used in this present disclosure, the amount of "available power" is the power increase gained {i.e., the amount of additional power the wind turbine may output) by only pitching the blades to an optimal blade angle. In some embodiments, the controller may determine or receive a reserve power which is the desired value of the available power. Put differently, the reserve power is an amount of additional power or a predetermined amount of additional power that a wind turbine is required to output by only pitching its blades to the optimal blade pitch angle - i.e., without increasing the rotor speed. Thus, in one embodiment, while outputting the de-rated power, the controller changes the blade pitch angle and/or the rotor speed such that the available power is equal to the desired reserve power. For example, once the reserve power is identified, then the controller can use the reserve power as a control parameter or signal to modify the rotor speed and blade pitch angle so that the available power matches, or is equal to, the requested reserve power. In some embodiments, when the wind speed increases beyond the rated wind speed when in low power operation, the controller can decrease the rotor speed and still guarantee that the output power can be ramped to 90% of the rated output power when the blades are pitched in. When receiving a request to ramp to full production {i.e., the rated power), the output power of the turbine ramps from the de-rated power {e.g., 10% of the rated output power) to the optimal output power which may be 90% of the rated output power in a matter of seconds by pitching in the blades. After achieving 90% of the rated output power, the turbine can switch to partial load operation to (where the rotor speed is increased) to increase the output power to the rated output power. In this manner, the blades can be pitched in to quickly ramp the output power to the defined percentage of the rated output power. The turbine can then enter partial-load operation (which is typically a slower power ramping technique) to continue ramping up to the rated output power.

EXAMPLE EMBODIMENTS

Fig. 1 illustrates a diagrammatic view of a horizontal-axis wind turbine generator 100. The wind turbine generator 100 typically comprises a tower 102 and a wind turbine nacelle 104 located at the top of the tower 102. A wind turbine rotor 106 may be connected with the nacelle 104 through a low speed shaft extending out of the nacelle 104. The wind turbine rotor 106 comprises three rotor blades 108 mounted on a common hub 1 10 which rotate in a rotor plane, but may comprise any suitable number of blades, such as one, two, four, five, or more blades. The blades 108 (or airfoil) typically has an aerodynamic shape with a leading edge 1 12 for facing into the wind, a trailing edge 1 14 at the opposite end of a chord for the blades 108, a tip 1 16, and a root 1 18 for attaching to the hub 1 10 in any suitable manner.

For some embodiments, the blades 108 may be connected to the hub 1 10 using pitch bearings 120 such that each blade 108 may be rotated around its longitudinal axis to adjust the blade's pitch. The pitch angle of a blade 108 relative to the rotor plane may be controlled by linear actuators, hydraulic actuators, or stepper motors, for example, connected between the hub 1 10 and the blades 108.

Fig. 2 illustrates a diagrammatic view of typical components internal to the nacelle 104 and tower 102 of a wind turbine generator 100. When the wind 200 pushes on the blades 108, the rotor 106 spins and rotates a low-speed shaft 202. Gears in a gearbox 204 mechanically convert the low rotational speed of the low-speed shaft 202 into a relatively high rotational speed of a high-speed shaft 208 suitable for generating electricity using a generator 206. A controller 210 may sense the rotational speed of one or both of the shafts 202, 208. If the controller decides that the shaft(s) are rotating too fast, the controller may signal a braking system 212 to slow the rotation of the shafts, which slows the rotation of the rotor 106 - i.e., reduces the revolutions per minute (RPM). The braking system 212 may prevent damage to the components of the wind turbine generator 100. The controller 210 may also receive inputs from an anemometer 214 (providing wind speed) and/or a wind vane 216 (providing wind direction). Based on information received, the controller 210 may send a control signal to one or more of the blades 108 in an effort to adjust the pitch 218 of the blades. By adjusting the pitch 218 of the blades with respect to the wind direction, the rotational speed of the rotor (and therefore, the shafts 202, 208) may be increased or decreased. Based on the wind direction, for example, the controller 210 may send a control signal to an assembly comprising a yaw motor 220 and a yaw drive 222 to rotate the nacelle 104 with respect to the tower 102, such that the rotor 106 may be positioned to face more (or, in certain circumstances, less) upwind.

Fig. 3 illustrates a controller 210 for operating a wind turbine, according to an embodiment described herein. Controller 210 includes a processor 305 and memory 310. Processor 305 represents one or more processing elements that each may include one or more processing cores. Memory 310 may include volatile memory, non-volatile memory, or a combination of both. Furthermore, controller 210 may be located on the turbine 100 as shown in Fig. 2 or may located remotely of the turbine {e.g., as part of a supervisory control and data acquisition (SCADA) system).

Memory 310 includes a power control module (PCM) 315 which controls the wind turbine when operating in various modes. For example, the PCM 315 may operate a wind turbine in a low power mode where the turbine's output power is de-rated. That is, even though the turbine could efficiently output more power, the PCM 315 purposively de-rates or decreases the output power of the turbine. For example, the turbine may be designed to output 3MW (i.e., the rated output power) when the rated wind speed is achieved. However, in response from a request from a grid controller, the PCM 315 may operate the turbine in the low power mode where the power is de- rated even if the current wind speed is at or above the rated wind speed. For example, the grid controller may request that the turbine output only 10% of the rated output power when in the low power mode. To de-rate the output power, the PCM 315 may send instructions to a power converter or the generator to output only a fraction of its rated output power. However, reducing the output power also reduces the torque on the rotor. If the rotor speed is maintained at the same speed used to generate the rated output power, the turbine may experience smearing or torque reversals as described above. Thus, in some embodiments, the rotor speed is reduced when operating in the low power mode to mitigate the likelihood of structural damage to the drive train or gear box. But reducing the rotor speed may also prevent the output power from ramping quickly when the grid controller instructs the controller 210 to cease operating in the low power mode and increase the turbine's output power (i.e., ramp to full production). For example, the grid controller may identify a spike in customer demand in the grid, and in response, request that the turbine ramp quickly to satisfy this demand.

To determine the rotor speed in the low power mode which permits fast ramping, the PCM 315 includes turbine parameters 320 and wind speed (not shown in Fig. 3). The turbine parameters 320 may be output power profiles and optimal pitch angles for the wind turbine at various wind speeds and/or set rotor speeds. However, the parameters 320 may vary depending on the type and configuration of the turbine. In one embodiment, the turbine parameters 320 may have been calculated or simulated beforehand so that the PCM 315 can determine the power outputted by the turbine for different rotor speeds, blade pitch angles, wind speeds, and the like. Example charts illustrating turbine parameters 320 such as rotor speed, blade pitch angle, and output power at a particular wind speed (i.e., 10 m/s) are shown in Figs. 5 and 7 which will be described in more detail below.

The PCM 315 also stores a reserve power 325 which is a desired amount of power to be output by the wind turbine by ramping the power output of the wind turbine by only pitching blades of the wind turbine to an optimal blade pitch angle. In one embodiment, the reserve power 325 is predetermined. For example, a controller 210 may receive a request from an operator, the grid, a control center, etc. , for the wind turbine to operate in a low power mode and chose an operating point for the wind turbine in order to maintain a certain amount of reserve power 325 so that the turbine can increase its power output by the reserve power 325 quickly {e.g., 2-5 seconds) by pitching its blades to the optimal blade pitch angle. As another example, the controller 210 may receive a predetermined reserve power request of 1000W. In this example, the controller 210 may operate a wind turbine at a given rotor speed during the low power mode so that when the blades are pitched to an optimal blade pitch angle, the output power of the wind turbine increases by 1000W— i.e., the reserve power 325.

In other embodiments, the reserve power 325 may be calculated by the controller 210. The controller 210 may receive a request to operate a wind turbine in low power mode to output a de-rated power. The controller 210 may also identify the reserve power 325 which can ramp the power output quickly (e.g., a few seconds) by pitching the blades to the optimal blade pitch angle. Once calculated, the controller 210 maintains the reserve power 325 during low power operation as described above.

The available power 330 is the amount of additional power a wind turbine can increase the de-rated output power, by only pitching the blades to the optimal blade pitch angle, to output a new output power. In other words, the available power 330 is the output power gained by the wind turbine (i.e., the amount of additional power the wind turbine may output) from only pitching the blades to an optimal blade pitch angle. In some aspects, the available power 330 is calculated by the controller 210. For example, the controller 210 may operate the wind turbine in low power mode by operating the turbine at a given rotor speed and/or blade pitch angle. With the turbine operating at the given rotor speed and/or blade pitch angle, the controller 210 calculates the available power 330 that can be added to the output of the wind turbine by only pitching the blades to an optimal blade pitch angle. Thus, the difference between the optimal output power (i.e., the power output the turbine will output once the blades are pitched to the optimal blade pitch angle) and the de-rated output power (i.e., power output by the turbine while operating in low power mode) is the available power 330.

In one example, the controller 210 may instruct the wind turbine to output a de-rated output power of 10% of the maximum rated output power. To do so, the controller 210 operates the wind turbine at a particular rotor speed and/or at a particular blade pitch angle for the turbine to output the de-rated output power. The controller 210 may then determine that if the blades are pitched to the optimal blade pitch angle the wind turbine will output an optimal output power that is 80% of the maximum rated output power. Thus, the available power 330 is 70% of the maximum rated output power (80%-10%). In one example, the controller 210 may adjust the operating point of the wind turbine such that the available power 330 equals the desired reserve power 325. For example, if the desired reserve power 325 is 50% the maximum rated output power, the controller 210 adjusts the rotor speed and pitch of the blades so that the turbine continues to output 10% of the maximum rated output power but has an available power 330 that is 50% of the maximum rated output power. That is, the turbine can increases its power output to 50% of the maximum rated output power by only pitching in the blades to the optimal blade pitch angle for the current rotor speed.

In this example, the controller 210 sets various parameters (e.g., blade pitch and rotor speed) during low power operation such that the reserve power 325 and available power 330 are substantially equal (e.g., within a set threshold). In some examples, the PCM 315 uses the reserve power 325 to set the available power 330. In some embodiments, reserve power 325 and/or available power 330 may be characterized as data, a parameter, information, a figure, a number, a numerical value, a threshold, and the like. In some aspects, reserve power 325 and/or available power 330 may be predetermined, calculated, or selected.

In one embodiment, the PCM 315 operates the wind turbine in a partial load power mode (i.e., partial load mode) as previously described. Partial load mode may occur when controller 210 receives instructions to accelerate the rotor speed of the wind turbine. For example, the controller 210 may cause the wind turbine to pitch its blades to an optimal blade pitch angle. Thus, the output power of the turbine ramps from the de-rated power (e.g., 10% of the maximum rated output power) to 90% of the maximum rated output power in a matter of seconds by pitching in the blades. After achieving 90% of the maximum rated output power, to continue ramping to 100% of the maximum rated output power, the controller 210 may enter partial-load operation (i.e., partial load mode).

Using the turbine parameters 320 and the reserve power 325 and/or the available power 330, the PCM 315 controls the rotor speed in order to enable the turbine to ramp quickly from the de-rated power to the rated output power or some predefined percentage thereof.

Fig. 4 is a method 400 for determining rotor speed when operating in a low power mode, according to an embodiment described in this present disclosure. For clarity, the blocks of method 400 are discussed in tandem with Fig. 5 which illustrates a graph 500 of parameters for controlling the power output of a wind turbine at a given wind speed.

Fig. 5 is a graph 500 illustrating, for a given wind speed, wind turbine parameters such as blade pitch and rotor speed corresponding to various output powers. In one embodiment, Fig. 5 is a graphical representation of information stored in a database that contains calculations and/or parameters for various wind speeds (e.g., wind speed parameters). These calculations and/or parameters may identify for a given wind speed a first blade pitch angle and a rotor speed to generate a particular derated power for operating a turbine in low power mode. The calculations and/or parameters may further identify the amount of reserve power, available power, and optimal blade pitch angle for the given wind speed based on the first blade pitch angle and the rotor speed.

The variety of information such as the calculations and/or parameters may be stored in a table - e.g., one or more Cp tables. The Cp table may be stored in the controller or remotely at a controller center or server. In one embodiment, the controller may include a respective Cp table for each different wind speed. That is, the controller may select which Cp table to use depending on the current wind speed at the wind turbine in order to determine the control parameters (e.g. , rotor speed and blade pitch angle) for operating the wind turbine at a desired output power. Other parameters that have not been described, which are relevant to operating a turbine in different modes to generate de-rated power and rated power may be included in the table as well.

As shown, the graph 500 illustrates the graphical representation of the Cp table when wind speed is 10 m/s. The graph 500 depicts that the Cp table may be based on the assumption that a full-load controller may control the pitch to obtain a requested generator speed (rotor speed) and an OptiPitch curve (optimal pitch curve 505) which gives an optimal pitch positon for a given rotor speed.

As shown, graph 500 illustrates the power outputted by a turbine for various pitch angles (the y-axis) and rotor speeds (the x-axis). To represent generated power, graph 500 is divided by a plurality of different grayscale power contours where the lighter contours represent lower output power and the darker contours represent higher output power. Generally, as the blades are pitched in (i.e., the blades become more aligned with the rotor plane) and the rotor speed increases, the greater power outputted by the turbine. The optimal pitch curve 505 is a line relating the optimal pitch to the generator (rotor) speed. Optimal pitch is the angle at which the blades are set at to produce an optimal output power (e.g. , usually a percentage of the maximum rated output power) obtained by maintaining rotor speed and only adjusting the pitch angle of the blades. The power levels on the optimal pitch curve 505 represent the maximum power which the turbine can produce in a steady state - i.e., with deaccelerating the rotor - for a current rotor speed.

Graph 500 also illustrates a de-rated power curve 510 (i.e., Pd). The de-rated power curve 510 represents the pitch angle and rotor speed combinations which result in the turbine outputting the de-rated power. Moreover, graph 500 illustrates a desired power curve 515 (i.e. Pd+D where D is the desired reserve power). The desired power curve 515 represents the various combinations of rotor speed and blade pitch that result in the turbine producing the summation of the de-rated power and the reserve power. Returning to method 400, at block 405, the PCM receives the command to operate the wind turbine at a de-rated output power. Previous to receiving this command, the PCM operates the wind turbine at operation point 520 as shown in Fig. 5. Put differently, operation point 520 marks the operation of a wind turbine at a first point in time at the wind speed of 10 m/s before the PCM has begun to operate the wind turbine in the low power mode. As shown, operation point 520 is located on the optimal pitch curve 505 where the wind turbine outputs 3000W at 87% of the maximum rotor speed and a blade pitch angle of 5°.

The command to de-rate the output power may be sent from a control center, a grid, or a user/operator of the wind turbine, etc. The command may be sent based on the time of the day, weather conditions, grid conditions, wind conditions, wind farm utilization parameters, wind farm conditions, and the like. In some instances, the command may be received based on certain wind turbine characteristics occurring or particular thresholds being reached. For example, the wind turbine (or a group of turbines in a wind park) may be used as a reserve power source. When demand for power on the grid is low, the grid PCM instructs the turbine to operate in the low power mode. As demand increases, the grid PCM can instruct the turbine PCM to increase its output power. In the example shown in Fig. 5, the PCM receives a command to output a de-rated power of 1000W. That is, the wind turbine should decrease its power output from 3000W (e.g., a rated output power) to 1000W. At block 410, the PCM identifies a predetermined reserve power. In some embodiments, the PCM may receive a predefined reserve power from a user/operator, a grid, a control center, etc. In other examples, the PCM calculates the reserve power using wind turbine characteristics, power grid characteristics, time of day, wind conditions, location of a wind turbine, history of previous low power operation, history of previous ramp rate request, history of previous requests to ramp to a particular value, etc. For example, a turbine may be operating during early morning, e.g., 5-7 a.m. , on a weekday, e.g., Monday - Friday. If operating in the low power mode during the morning hours when power demand is low, the PCM may use a lower reserve power than when operating in the low power mode during the heat of the day when power demand is greater. In another example, the PCM selects the current value of the reserve power using a history of low power operation. The turbine may have a history of operating in low power mode at night, e.g., 10 p.m. - 4 a.m. , with a ramp rate of 1000W. Thus, every night the turbine may adjust to operate with a reserve power of 1000W, unless otherwise instructed by an operator or the grid. In another example, the PCM varies the reserve power based on wind conditions. For example, if the wind is turbulent (i.e. , varies significantly or rapidly), the PCM may reduce the reserve power so that the rotor speed can be reduced when operating in the low power mode thereby reducing the risk of negative torque reversal. In another example, the reserve power varies depending on the location of the wind turbine. For example turbines located on one side of a wind farm may operate with a reserve power of 1500W, while turbines located in the opposite side operate with a reserve power of 1000W. For Figs. 4 and 5, it is assumed the reserve power is 1000W. At block 415, the PCM determines a rotor speed and a first blade pitch angle that outputs the de-rated output power while maintaining the predefined reserve power. The PCM determines a first blade pitch angle, an optimal blade pitch angle, and a given rotor speed in order to obtain the reserve power. For example, the PCM determines the rotor speed that that outputs the de-rated output power requested by the grid but also maintains the reserve power. The PCM then determines the corresponding blade pitch angle that maintains the rotor speed in order to output the de-rated power. In one embodiment, to determine the rotor speed and optimal blade pitch angle, the PCM uses the Cp chart to identify the operating point where the optimal pitch curve 505 intersects the desired power curve 510 which corresponds to specific rotor speed. As described above, the Cp table will have values for all desired parameters in relation to each other, e.g., optimal blade pitch angle, first blade pitch angle, rotor speed, etc. In other embodiments, the PCM determines the blade pitch angle necessary to operate the wind turbine to output the optimal power output equivalent to the desired power output (i.e. , using the predefined reserve power). As seen in Fig. 5, the PCM determines the rotor speed corresponding to point 540 i.e. , 75% of the maximum rotor speed) and a first blade pitch angle, i.e., 15°, and an optimal blade pitch angle, i.e., 7 °, for the blades at that rotor speed which outputs 1000W of de-rated power and maintains the predefined reserve power, i.e., 1000W.

At block 420, the PCM adjusts the rotor to the rotor speed and the blades of the wind turbine to the first blade pitch angle. In some embodiments, the PCM may first adjust the speed of the rotor (i.e., rotor speed) and then adjust the blades to the first blade pitch angle. In some instances, the PCM may simultaneously adjust the rotor speed and the blades to the first blade pitch angle. As shown in Fig. 5, this is illustrated by arrow 525 illustrating the operation of the wind turbine moving from operation point 520 to operation point 530 on the de-rated power curve 510. As shown by the example in Fig. 5, the PCM adjusts the blades of the wind turbine to the first pitch angle, i.e., 15° and the rotor speed to 75% of the maximum rotor speed. The reserve power is shown graphically in Fig. 5 by the vertical distance between operation point 530 on the de-rated power curve 510 and the operation point 540 on the optimal blade pitch curve 505 and desired power curve 515.

At block 425, the PCM operates the wind turbine in the low power mode while outputting the de-rated power. As shown in Fig. 5, when at operating point 530, the wind turbine outputs the de-rated output power. More specifically, at operation point 530, the wind turbine outputs 1000W and has a reserve power of 1000W. That is, by only pitching in the blades to the optimal pitch curve 505 (for the given rotor speed), the PCM can ramp increase the output power by 1000W - i.e. , the reserve power. Thus, by identifying operation point 530 on the de-rated power curve 510, the wind turbine maintains the desired reserved power which can be obtained quickly by pitching in the blades to the corresponding blade pitch angle on the curve 505. At block 430, the PCM receives an instruction to cease operating at the de-rated output power. In some instances, the instruction requests that the wind turbine output a specified power greater than the de-rated power - e.g., the rated power or the optimal output power for the wind turbine. At block 435, the PCM adjusts the blades from the first blade pitch angle to the optimal blade pitch angle. By pitching to the optimal blade angle, the turbine increases its power output by the reserve power. In some embodiments, adjusting the blade from the first blade pitch angle to an optimal blade pitch angle causes the wind turbine to exit operating in the low power mode. As illustrated in Fig. 5, in response to cease low power operation, the PCM moves as designated by arrow 535 from operation point 530 to operation point 540 on to the curve 505 by only pitching the blades to the optimal blade pitch angle, i.e., 7 degrees, while maintaining the same rotor speed, i.e., 75% of the maximum rotor speed.

At block 440, the PCM operates the wind turbine to output optimal output power. As illustrated in Fig. 5, once at operation point 540, the wind turbine utilizes the predefined reserve power, i.e., 1000W, and outputs a rated power of 2000W at 75% of the maximum rotor speed which is a combination of the predefined reserve power and the de-rated output power. As illustrated in graph 500, operation point 540 is located at the intersection of the optimal pitch curve 505 and the desired power curve 515. In one embodiment, blocks 435 and 440 are performed in parallel to ensure balance between power intake and power production which results in a stable rotor speed.

At block 445, the PCM receives instruction to increase the output power to the maximum rated power. In some embodiments, maximum rated power may be reached by accelerating/increasing the rotor speed. At block 450, the PCM operates the wind turbine in partial load mode to increase the rotor speed and increase the output power to the requested maximum rated output power. Although not illustrated in Fig. 5, to increase to the rated output power, the PCM can accelerate the rotor until the PCM can pitch the blades to the portion of the optimal blade curve 505 that lies within the power gradient that corresponds to the rated output power.

The embodiment described above using method 400 and Fig. 5 depicts an instance where wind speed is constant. In some embodiments, wind speed may not remain constant. In these embodiments, the method 400 remains the same however a Cp table different than the one depicted in Fig. 5 may be used. For example, at a first point in time a wind turbine may operate at operation point 520. However, at a later point in time, the wind speed that the turbine is operating at may change from 10 m/s to 8 m/s. The PCM can still implement method 400; however, it will use a different Cp table to adjust the pitch and rotor speed so that the wind turbine outputs the derated output power and maintains the desired reserve power.

Fig. 6 is a method 600 for determining rotor speed when operating in a low power mode, according to an embodiment described in this present disclosure. For clarity, the blocks of method 600 are discussed in tandem with Fig. 7 which illustrates a graph 700 of parameters for controlling the power output of a wind turbine at a given wind speed. Graph 700 is similar to graph 500 of Fig. 5. The difference between graph 500 and graph 700 is the operation points of the wind turbine.

At block 605 of method 600, a PCM receives a command to operate a wind turbine at a de-rated output power (e.g., operate in low power mode). Graph 700 of Fig. 7 illustrates operation point 720 marking the operation of a wind turbine before receiving the command to operating in the low power mode. Graph 700 illustrates operation point 720 located on the optimal pitch curve 505 where the wind turbine outputs a power of 1700W at 71 % of maximum rotor speed and a blade pitch angle of 9°. In the example shown in Fig. 7, the command instructs the wind turbine to operate at a de-rated power of 1000W.

At block 610, the PCM operates the wind turbine to output the de-rated output power. In some embodiments, the PCM looks up the new operation point via the Cp table using the desired de-rated power. As described above, the Cp table, will have values for all desired parameters, e.g., optimal blade pitch angle, first blade pitch angle, rotor speed, etc. At block 610, the PCM may ensure the wind turbine moves quickly to output the de-rated power and ignore other parameters such as reserve power or available power.

As seen in Fig. 7, to output the de-rated output power the PCM determines and operates the wind turbine at a first blade pitch angle, i.e., 16 degrees, for the blades at given rotor speed, i.e., 71 % of the maximum rotor speed, based on outputting at 1000W of de-rated power. This causes the operation of the wind turbine to change, as illustrated in graph 700 by arrow 725, from operation point 720 located on the optimal pitch curve 510 to operation point 730 located on the de-rated power curve 510 where the wind turbine outputs 1000W.

At block 615, the PCM determines an available output power. In one embodiment, the PCM calculates the available output power using the current rotor speed and the Cp table to determine the amount of power that can be added by pitching the blades to the corresponding optimal blade pitch angle on curve 505. The available power is shown graphically in Fig. 7 by the vertical distance between the current operation point on the de-rated curve 510 (e.g. , operation point 730) and the optimal blade pitch curved 505. In this example, the available power is 700W, i.e. , 1700W - 1000W.

At block 620, the PCM compares the available power determined at block 615 to the reserve power. As described earlier, the reserve power may be provided to the PCM by an external source or may be calculated by the PCM based on any number of factors. In this embodiment, the PCM uses the reserve power as a PCM parameter to adjust the blade pitch and/or rotor speed until the available power - i.e. , the vertical distance between the operation point on de-rated power curve 510 and the optimal blade pitch curve 505 - is substantially equal to the reserve power.

If at block 625 the available power equals the reserve power, method 600 returns to block 615 to again monitor the available output power and ensure the available power equals the reserve power (which changes as wind speed changes). If, however, the PCM determines at block 630 that the available power does not equal the reserve power, method 600 proceeds to block 630 where the PCM adjusts the rotor speed while maintaining the de-rated output power to achieve the predefined reserve power. In Fig. 7, the available power does not equal the reserve power when the turbine is at operation point 730. Specifically, in this example, the available power, i.e., 700W is greater than the reserve power, i.e., 500W.

At block 630, the PCM adjusts the rotor speed while maintaining the de-rated output power to achieve the predefined reserve power. The PCM may select from the Cp table or determine a new rotor speed, thus adjusting the available power until the available power equals the reserve power. By decreasing the rotor speed, the turbine now operates at a different location on the de-rated curve 510 - i.e. , operation point 740. This is shown, in Fig. 7, by arrow 735, as the operation of the turbine moves from operation point 730 to operation point 740. In some embodiments, the PCM may also determine a new blade pitch angle along with the new rotor speed to adjust the available power to equal the reserve power. In other embodiments, the wind turbine simultaneously adjusts the rotor speed and the blade pitch angle. As shown, operation point 740 on the de-rated power curve has a smaller vertical distance to the optimal blade pitch curve 505 than operation point 730 which means the available power is less. Specifically, operation at point 740 means the available power equals the reserve power - i.e. , 500W. In some embodiments, after block 630 is completed, the method may proceed back to block 625 (not shown in method 600). Under this embodiment, this will allow the PCM to continue to adjust the available power such that it remains equal to the predefined reserve power as conditions change - e.g. , wind speed increases or decreases.

As seen in graph 700, to reach operation point 740, the PCM adjusts the rotor speed to 65% of the maximum rotor speed and to the first blade pitch angle of the blades to 17°. This causes the operation of the wind turbine to move along the de-rated power curve 510 from operation point 730 to operation point 740 as indicated by arrow 735. At operation point 740, the wind turbine may continue to output 1000W. Also, at operation point 740, the available power is reduced to 500W and is equal to the predefined reserve power of 500W. By reducing the rotor speed, the wind turbine increases the torque to the greatest value possible and still maintains the desired reserve power.

At block 635, the PCM receives an instruction to cease operating at the de-rated output power. In some instances, the instruction may be to operate the wind turbine to output a specified power. In some embodiments, the instruction may be to operate the wind turbine to output an optimal output power or rated power.

At block 640, the PCM adjusts the blades from the first blade pitch angle to an optimal blade pitch angle along curve 505 shown by the Cp chart illustrated in Fig. 7.

At block 645, the PCM operates the wind turbine to output optimal output power. Once the PCM adjusts the blade to the optimal blade pitch angle, i.e., 10 degress, operation of the wind turbine moves from the operation point 740 located on the derated power curve to the operation point 750 located at on the optimal power curve 505 and desired power curve 515, as indicated by arrow 745. As shown in Fig. 7, as indicated by operation point 750, the wind turbine increases its power output by the available/reserve power of 500W, and outputs a total optimal output power of 1500W at 65% of the maximum rotor speed and the blade pitch angle of 10° which is a combination of the reserve power and the de-rated output power.

In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the features and elements provided above, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the aspects, features, embodiments and advantages described herein are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). As will be appreciated by one skilled in the art, the embodiments disclosed herein may be embodied as a system, method or computer program product. Accordingly, aspects may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit," "module" or "system." Furthermore, aspects may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer- readable storage medium (or media) (e.g., a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

Aspects of the present disclosure are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments presented in this disclosure. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. The flowchart and block diagrams in the Figs, illustrate the architecture, functionality and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.