Pitch control of a wind turbine blade in standby mode

Abstract

A method of operating a wind turbine generator (10) comprising a plurality of blades (12), the method comprising iterating the steps of comparing an indicated rotor speed with a rotor speed target to determine a rotor speed error; generating a modified rotor speed error by applying a control factor to the rotor speed error; controlling a pitch angle of the blades (12) in dependence on the modified speed error via a pitch control system; and altering the control factor in dependence on a magnitude of the indicated rotor speed.

Description

Technical Field

[0001] This disclosure relates to a method of operating a wind turbine generator (WTG), and more particularly to a method of controlling the blade pitch angle of the WTG when the WTG is operating in standby mode during grid loss. Background Technology

[0002] A wind power plant (WPP) (also known as a wind farm or wind farm) typically consists of multiple wind turbines or wind turbine generators (WTGs) configured to generate electricity to supply the external power grid. During normal operating conditions, the WPP's WTGs continuously generate electricity to supply the external grid. However, under certain operating conditions, the WPP may experience a "grid loss," where the WTGs are prevented from supplying electricity to the external grid, for example, because the external grid is unavailable or determined to have become unstable. In this case, the WTGs can switch to standby mode, where the WTGs continue to operate (i.e., the rotors continue to rotate) but do not operate to generate electricity to supply the external grid.

[0003] During a grid loss, WTGs may disconnect from their usual power source. In this situation, critical subsystems of the WTG, including, for example, the WTG's pitch control system, can be continuously powered by backup power systems, including battery storage systems and / or diesel generators. However, over time, these backup power systems may become depleted, in which case a complete shutdown of the WTG may be necessary.

[0004] In some situations, WTG can generate backup power while operating in standby mode, which can then be used to charge a backup power system. However, the backup power system can still be depleted, for example, if the power consumption of a critical subsystem exceeds the backup power generation over an extended period.

[0005] The purpose of this invention is to overcome the shortcomings of the known WPP of the above type. Summary of the Invention

[0006] According to one aspect of the invention, a method for operating a wind turbine generator is provided, the wind turbine generator comprising a rotor having a plurality of blades. The method includes iterative steps of: comparing an indicated rotor speed with a rotor speed target to determine a rotor speed error; generating a corrected rotor speed error by applying a control factor to the rotor speed error; controlling the pitch angle of each blade according to the corrected speed error via a pitch control system; and changing the control factor according to the magnitude of the indicated rotor speed.

[0007] Controlling the blade pitch angle based on a modified rotor speed error allows for greater flexibility, enabling customized control of the wind turbine generator through appropriate modifications to the control factor without altering the pitch control system itself. For example, if a reduction in the wind turbine's power consumption is desired, the control factor can correct for rotor speed errors in a way that effectively reduces the sensitivity of the pitch control system to deviations from the rotor speed target. This can be useful if the wind turbine is in idling mode. Conversely, in certain situations, the pitch control system can also be forced to respond more sensitively than usual.

[0008] Applying a control factor to the rotor speed error may require multiplying the two to generate a corrected speed error, which is then defined to the input of the pitch control system. In this case, the control factor can be considered as a gain factor.

[0009] Changing the control factor based on the magnitude of the indicated rotor speed may include: decreasing the control factor if the indicated rotor speed is within a first rotor speed range that includes the rotor speed target; decreasing the control factor when the indicated rotor speed is within the first rotor speed range; similarly, decreasing the control factor by a first reduction when the indicated rotor speed is within the first rotor speed range and greater than the rotor speed target; and decreasing the control factor by a second reduction when the indicated rotor speed is within the first rotor speed range and less than the rotor speed target.

[0010] Changing the control factor based on the magnitude of the indicated rotor speed may include: increasing the value of the control factor if the indicated rotor speed is outside a first rotor speed range. When the indicated rotor speed is outside the first rotor speed range, the control factor may be incremented. In one such method, when the indicated rotor speed is outside the first rotor speed range and greater than a rotor speed target, the control factor may be increased by a first increment; and when the indicated rotor speed is outside the first rotor speed range and less than a rotor speed target, the control factor may be increased by a second increment.

[0011] The method may include: increasing the value of the control factor when the indicated rotor speed is outside a second rotor speed range that is wider than a first rotor speed range and includes the first rotor speed range. The method may also include: keeping the control factor unchanged when the indicated rotor speed is outside the first rotor speed range but within the second rotor speed range.

[0012] The method may include setting a control factor to a predetermined value when the indicated rotor speed is outside a third rotor speed range that is wider than a second rotor speed range and includes the second rotor speed range. The predetermined value may be greater than 1.

[0013] This method can correspond to the standby pitch control mode of a wind turbine generator used when it is idling (e.g., during a grid loss). The responsiveness of the standby pitch control mode can be lower than that of the normal pitch control mode of the wind turbine generator, thereby reducing the power consumption of the wind turbine generator when operating in standby pitch control mode.

[0014] According to another aspect of the present invention, a controller configured to perform the above-described method is provided.

[0015] Another aspect of the present invention provides a control system for a wind turbine generator, the wind turbine generator including a rotor having a plurality of blades, the control system being configured to: compare an indicated rotor speed with a rotor speed target to determine a rotor speed error; generate a corrected rotor speed error by applying a control factor to the rotor speed error; control the pitch angle of each blade according to the corrected speed error; and change the control factor according to the magnitude of the indicated rotor speed.

[0016] It will be understood that preferred and / or optional features of each aspect of the invention may also be incorporated, individually or in appropriate combinations, into other aspects of the invention. Attached Figure Description

[0017] One or more embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, wherein:

[0018] Figure 1 This is a schematic diagram of a wind power plant applicable to embodiments of the present invention;

[0019] Figure 2 yes Figure 1 A schematic front view of the wind turbine generator of the wind power plant shown;

[0020] Figure 3 yes Figure 2 The schematic diagram of the wind turbine generator shown; and

[0021] Figure 4 This illustrates an embodiment of the invention for controlling [something] during normal operation and during power grid loss. Figure 2 A flowchart of a method for generating a wind turbine generator. Detailed Implementation

[0022] Figure 1A portion of a transmission network comprising a wind power plant (WPP) 1 according to a possible, non-limiting embodiment of the invention is illustrated schematically. The wind power plant may also be referred to as a wind farm or wind power plant. The WPP 1 includes a plurality of wind turbines or wind turbine generators (WTGs) 10 configured to generate electricity during normal operation of the WPP 1 to supply power to an external power grid 2. Figure 1 As shown, each of the WTG 10 is connected to the local power grid 3 of WPP 1. The local power grid 3 is in turn connected to the Point of Interconnection (POI) bus 6, through which power is fed to the external power grid 2 via the transmission line 4, which includes the main step-up transformer 5. The external power grid 2 can be a regional, national, or international power transmission network (e.g., the British National Grid).

[0023] WPP 1 is equipped with a Power Plant Controller (PPC) 7. PPC 7 is connected to the transmission network at a Point of Measurement (POM) 8 (from which PPC 7 can monitor the status of the transmission network) and to the Grid Operator or Transmission System Operator (TSO) 9 (from which PPC 7 can receive instructions regarding the operation of WPP 1). PPC 7 is also connected to WPP 1 and is configured to control the operation of WPP 1 based on a stored set of operating instructions and instructions received from TSO 9. Specifically, PPC 7 is configured to transmit information and instructions to the controller 20 of the corresponding WTG 10, which in turn controls the operation of WTG 10 and their various subsystems.

[0024] Will understand, Figure 1 The transmission network shown is schematic and in a highly simplified form for illustrative purposes only, and WPP 1 may include any desired number of WTG 10, which may be interconnected and connected to the external power grid 2 in any suitable manner.

[0025] Figure 2 The diagram schematically illustrates a typical WTG 10 from WPP 1. (As shown...) Figure 2 As shown, WTG 10 includes a rotor 11, which includes a plurality of blades 12 extending outward from a hub 13. The rotor 11 is connected to a nacelle 14, which in turn is connected to a tower 15. The nacelle 14 houses a generator configured to be driven by the rotor 11. WTG 10 is an onshore horizontal axis wind turbine (HAWT) with three blades 12. However, it will be understood that the invention is equally applicable to other types of WTGs (including offshore WTGs).

[0026] Figure 3 schematically illustrated Figure 2 The WTG 10 power generation and conversion equipment and various subsystems are shown. For example... Figure 3 As shown, the power generation and conversion equipment includes a generator 16, which is driven by a rotor 11 via a transmission 17. The transmission 17 may optionally include a gearbox, or alternatively, a direct-drive transmission. The generator 16 is configured to generate electricity in a normally compliant manner to supply electricity to an external power grid 2. The power generation and conversion equipment also includes a power converter 19, which is configured to convert the output of the generator 16 to a frequency compatible with the external power grid 2. The power converter 19 may optionally include, for example, […]. Figure 3 The machine-side AC-DC converter 19a, DC link 19b, and grid-side DC-AC converter 19c shown are full-size converters, although other types of converters may be used in other embodiments. The power generation and conversion equipment of WTG 10 is typically controlled by WTG controller 20.

[0027] Or as Figure 3 As shown, WTG 10 also includes a power system 23, which includes a battery storage system (BSS) 24 configured to be charged by generator 16 in any suitable manner according to the design of WTG 10. In some cases, power system 23 may be connected to power converter 19 and configured to be charged by power converter 19 (as shown in connection 25). In this case, the power system may optionally be connected to DC link 19b of power converter 19, or alternatively connected to any other suitable part of power converter 19. However, in other cases, power system 23 may be configured to be charged by generator 16 via a separate connection including other AC-DC converters (see connection 26). Power system 23 is also connected to various power consumption subsystems of WTG 10 and configured to supply DC current to the various power consumption subsystems, at least during grid loss conditions, as described in more detail below.

[0028] The WTG 10 also includes various power consumption subsystems 21, such as: a pitch control actuation system 22 including one or more actuators for controlling the pitch angle of the blades 12; a yaw control actuation system including one or more actuators for controlling the yaw angle of the nacelle 14; a lubrication system; a lifting system; a lighting system; a heating system; a cooling system; a ventilation system; a hydraulic pump system; and various sensor systems, which are typically also controlled by the WTG controller 20.

[0029] During normal operation of WTG 10 (where WPP 1 remains connected to external power grid 2), power consumption subsystem 21 can be powered in any suitable manner depending on the design of WTG 10 and WPP 1. For example, power consumption subsystem 21 can be powered by local power grid 3 and / or external power grid 2 (optionally via a dedicated power grid). In this case, WTG power system 23 can operate solely as a backup power system. Alternatively, power consumption subsystem 21 can be powered by WTG power system 23 during normal operation of WTG 10. However, in either case, WTG power system 23 is configured to supply DC current to at least some of the power consumption subsystems 21 during a power grid loss, enabling WTG 10 to continue operating during a power grid loss, even if the power consumption subsystems 21 have been disconnected from their primary power source.

[0030] WTG controller 20 is a control system comprising various control modules configured to control the operation of WTG 10 and its various subsystems 21. Specifically, WTG controller 20 includes a speed control module 30 and a pitch angle control module 40 (among other modules). Speed ​​control module 30 is configured to monitor the speed of wind turbine 10 (i.e., rotor speed and / or generator speed) at least substantially in real time, for example, using a speed sensor or alternatively by calculating or estimating the turbine speed. Pitch angle control module 40 is configured to set the pitch angle of blade 12, for example, by calculating a pitch angle setpoint and controlling the operation of pitch control actuation system 22 to achieve a desired blade pitch angle. For example, the pitch angle setpoint can be calculated based on the current wind turbine speed and the power demand received from PPC 7, although other operating parameters can be additionally considered when setting the pitch angle setpoint. Pitch angle control module 40 is thus able to adjust the rotor speed when controlling the pitch angle of blade 12.

[0031] Will understand, Figure 3 The basic WTG architecture shown is schematic and highly simplified for illustrative purposes only, and WTG 10 may also include other subsystems and control modules arranged in any suitable manner.

[0032] Now refer to Figure 4 The following flowchart describes the operation of the WTG 10 during normal operating conditions and during power outages. The following description refers to the control of a single WTG 10, although it will be understood that each of the WTG 10s can be controlled in an equivalent manner.

[0033] Under normal operating conditions, when WPP 1 is connected to external power grid 2 and external power grid 2 is available to receive power from WPP 1, WTG 10 operates in normal mode to generate power to supply external power grid 2 via local power grid 3 and transmission line 4, as indicated in step 1.

[0034] When WTG 10 is running in normal mode, the pitch angle control module 40 continues to run to calculate the pitch angle setpoint of the blade 12 according to the normal pitch control mode, and controls the operation of the pitch control actuation system 22 to achieve the desired blade pitch angle, as indicated in step 2.

[0035] As part of a closed-loop control process based on a speed error signal (which indicates the difference between the target rotor speed and the indicated rotor speed), the pitch angle control module 40 determines the desired pitch angle setpoint. Therefore, the pitch angle control module 40 operates by determining an appropriate pitch angle setpoint to minimize the rotor speed error.

[0036] The normal pitch control mode has a low threshold for performing pitch angle adjustment operations, thus providing basic, continuous adjustment of the pitch angle of blade 12, including responding to small and / or transient changes in operating conditions, to provide precise control of rotor and generator speeds, and to optimize and / or maximize the power output of WTG 10. In normal pitch control mode, the speed error signal input to the pitch angle control module 40 is the rotor speed error (which represents the difference between the indicated rotor speed and the target rotor speed), and is described in more detail below.

[0037] Under certain operating conditions, WTG 10 may become disconnected from the external power grid 2 and be prevented from supplying power to it, for example, if the external power grid 2 becomes completely unavailable or is determined to have become unstable. In this case, for example, a "power grid loss" condition is detected by PPC 7 and / or WTG controller 20, and WTG 10 automatically switches from normal mode to a separate standby mode, in which WTG 10 continues to operate (i.e., rotor 11 continues to rotate) but WTG 10 does not operate to supply power to the external power grid 2, as indicated in step 3. In some cases, rotor 11 may continue to rotate without interruption, but in other cases, rotor 11 may be initially stopped upon detecting a power grid loss and then restarted, for example, by feathering blades 12.

[0038] Although WTG 10 operates in standby mode during a power outage, power consumption subsystems 21 can be disconnected from their primary power source. To enable WTG 10 to continue operating during a power outage, WTG controller 20 and various other critical subsystems are powered at least intermittently by WTG power system 23. However, to minimize backup power consumption and maintain the lifespan of BSS 24, non-critical subsystems (including, for example, lifting systems, lighting systems, heating systems, cooling systems, ventilation systems, hydraulic pump systems, and non-critical sensor systems) may be automatically powered down in standby mode.

[0039] While rotor 11 continues to rotate in standby mode of WTG 10, WTG 10 can operate to generate backup power to supply BSS 24. In standby mode, WTG 10 uses the same generator 16 as the generator used during normal operation of WTG 10, but the output from generator 16 is delivered to a different power converter that is smaller than the power converter 19 used during normal operation. For example, the power converter 19 used during normal operation can generate approximately 9.5 MW, while the power converter used in standby mode can generate only approximately 15 kW. In some cases, BSS 24 can be charged at least substantially continuously (e.g., by trickle charging), but in other cases, BSS 24 can be charged only when charging conditions are met (e.g., when BSS 24 is below an energy threshold).

[0040] When WTG 10 enters standby mode, the normal pitch control mode is no longer used to control the pitch angle of blade 12. Instead, the pitch angle of blade 12 is controlled according to a separate standby pitch control mode, as detailed in steps 5 to 8, where the pitch angle control module 40 controls the blade pitch based on a corrected rotor speed error rather than the actual rotor speed error, as explained in more detail below. In other words, the speed error signal input to the pitch control module is the corrected speed error. The standby pitch control mode consumes less power than the normal pitch control mode by reducing the degree of blade pitch variation, thus reducing the power consumption of the WTG when it is running in standby mode.

[0041] In standby pitch control mode, the speed control module 30 continuously monitors the rotor speed (optionally indirectly by monitoring the generator speed) and generates an indication of the current speed of the rotor 11. In some embodiments, when the WTG is running in standby mode, the rotor speed can be monitored at a lower frequency (e.g., a monitoring period of 10 seconds) compared to when the WTG is running in normal mode.

[0042] The indicated rotor speed is then compared to a target rotor speed, as indicated in step 5. The target rotor speed can be a preset speed, which may be set, for example, during the design, installation, or calibration of the WTG 10. Alternatively, the target rotor speed can be variable and can be calculated during the operation of the WTG 10. The target rotor speed is preferably a speed highly suitable for generating backup power during grid loss. In this embodiment, the target rotor speed is preset to 60 rpm, but other values ​​(e.g., 80 rpm or 100 rpm) can also be selected equivalently.

[0043] In normal and standby pitch control modes, the speed control module 30 determines the rotor speed error by subtracting the rotor speed target from the indicated rotor speed. Therefore, if the indicated rotor speed exceeds the rotor speed target, the rotor speed error is positive; and if the indicated rotor speed is less than the rotor speed target, the rotor speed error is negative.

[0044] As described above, in the normal pitch control mode, the pitch angle control module 40 controls the blade pitch based on the rotor speed error.

[0045] However, in an embodiment of the invention, when the standby pitch control mode is activated, the pitch angle control module 40 controls the blade pitch based on a corrected rotor speed error rather than the actual rotor speed error. As indicated in step 6, the corrected rotor speed error is generated by applying a control factor in the form of a gain factor to the actual rotor speed error. In an embodiment of the invention, the rotor speed error is multiplied by a gain factor to generate the corrected speed error.

[0046] The value of the gain factor is variable, which allows the gain factor to be used to mitigate the effect of the pitch angle control module 40 on the actual rotor speed error. For example, when in standby pitch control mode, the response of the pitch angle control module 40 can be suppressed by setting the corrected rotor speed error to a value lower than the actual rotor speed error, thereby reducing power consumption.

[0047] In this embodiment, the value of the gain factor is changed according to the degree of deviation between the indicated rotor speed and the target rotor speed, as described below and as indicated in step 7. In most cases, the value of the gain factor will be between 0 and 1, although the gain factor may have a value greater than 1 in some cases. However, the gain factor is generally not negative.

[0048] When the value of the gain factor is changed, the speed control module 30 runs a closed loop in which the indicated rotor speed is monitored. The value of the gain factor is changed at the end of each iteration of the loop based on the magnitude of the indicated rotor speed.

[0049] When the indicated rotor speed is within the first rotor speed range, the value of the gain factor decreases at the end of each cycle of the control loop, thus shifting the value of the gain factor toward 0.

[0050] It is worth noting that once the gain factor reaches 0, the corrected rotor speed error also becomes 0. In this case, the pitch angle control module 40 functions as if the measured rotor speed matches the target rotor speed, and therefore takes no action.

[0051] The first rotor speed range is a relatively narrow range of rotor speeds that includes the target rotor speed. Therefore, the indicated rotor speed within the first rotor speed range indicates that the indicated rotor speed deviates from the target rotor speed by only a relatively small amount. In some embodiments of the invention, for example, for a target rotor speed of 60 rpm, the first rotor speed range can extend from 55 rpm to 70 rpm. Therefore, in this case, as long as the indicated rotor speed is between 55 rpm and 70 rpm, the value of the gain factor decreases in each control loop cycle until it reaches 0.

[0052] The decrease in the value of the gain factor can be uniform across the entire first rotor speed range; that is, if the indicated rotor speed is anywhere within the first rotor speed range, the gain factor can decrease by the first reduction at the end of the control loop cycle. In this case, when the indicated rotor speed is anywhere between 55 rpm and 70 rpm, the gain factor will therefore decrease by the same amount in each control loop cycle.

[0053] Alternatively, when the indicated rotor speed is within a first rotor speed range and greater than the rotor speed target, the gain factor can be reduced by a first decrement at the end of the control loop cycle, and when the indicated rotor speed is within the first rotor speed range and less than the rotor speed target, the gain factor can be reduced by a second decrement at the end of the control loop cycle. In this case, when the indicated rotor speed is between 60 rpm and 70 rpm, the gain factor will decrease by a certain amount, and when the indicated rotor speed is between 55 rpm and 60 rpm, the gain factor will decrease by a different amount. Under these parameters, the rotor speed range below the rotor speed target is narrower, so the second decrement can be greater than the first decrement to compensate for this.

[0054] In other embodiments, the reduction in the gain factor may be related to the degree of rotor speed error. For example, in these embodiments, the reduction in the gain factor may be inversely proportional to the rotor speed error, such that the amount of reduction in the gain factor can be larger when the rotor speed error is small. In other words, the gain factor decreases by a larger amount when the indicated rotor speed is closer to the rotor speed target compared to when the indicated rotor speed is further away from the rotor speed target (but still within the first rotor speed range).

[0055] Conversely, when the indicated rotor speed is outside the first rotor speed range, the value of the gain factor increases at the end of each control loop cycle, thus moving the value of the gain factor closer to 1. In some embodiments, the amount by which the gain factor increases can be the same regardless of how far the indicated rotor speed is outside the first rotor speed range. In other words, when the indicated rotor speed is outside the first rotor speed range, the value of the gain factor can increase by a first increment at each control loop iteration.

[0056] In an alternative embodiment, when the indicated rotor speed is outside the first rotor speed range but above the rotor speed target, the gain factor increases by a first increment at the end of the control loop cycle, and when the indicated rotor speed is outside the first rotor speed range but below the rotor speed target, the gain factor increases by a second increment at the end of the control loop cycle.

[0057] In other embodiments, when the rotor speed is outside the first rotor speed range, the increase in the gain factor in each control cycle can be proportional to the rotor speed error. In these embodiments, the increase in the gain factor can be larger when the speed error is large. In other words, the increase in the gain factor is larger when the indicated rotor speed is far from the rotor speed target compared to when the indicated rotor speed is closer to the rotor speed target but still outside the first rotor speed range.

[0058] In other embodiments of the invention, a second rotor speed range may exist that is wider than the first rotor speed range and includes the first rotor speed range. In some embodiments, the second rotor speed range extends, for example, from 35 rpm to 120 rpm.

[0059] In embodiments of the invention where a second rotor speed range exists, the gain factor increases in the manner described above only when the indicated rotor speed is outside the second rotor speed range. In other words, for embodiments where the second rotor speed range extends between 35 rpm and 120 rpm, the value of the gain factor increases only when the indicated rotor speed is greater than 120 rpm or less than 35 rpm. As stated above, regardless of the value of the indicated rotor speed, as long as it falls outside the second rotor speed range, the increase can be an increase of a first decrement; or, when the indicated rotor speed is greater than 120 rpm, a first increment can be added, and when the indicated rotor speed is less than 35 rpm, a second increment can be added. An indicated rotor speed outside the second rotor speed range therefore indicates a significant deviation of the indicated rotor speed from the rotor speed target.

[0060] In some embodiments, the value of the gain factor may remain unchanged when the indicated rotor speed is outside a first rotor speed range and within a second rotor speed range. In the above embodiment, this would correspond to the indicated rotor speed being between 35 rpm and 55 rpm or between 70 rpm and 120 rpm.

[0061] It will be apparent to those skilled in the art that although the change in the gain factor is presented here as a value based on the indicated rotor speed, an equivalent result can be achieved by instead considering the rotor speed error. For example, by converting the aforementioned rotor speed range to an equivalent rotor speed error, the value of the control factor can be decreased when the rotor speed error is between -5 rpm and +10 rpm, and the value of the control factor can be increased when the rotor speed error exceeds +60 rpm or is less than -25 rpm.

[0062] It is evident from the above description of how the value of the gain factor changes that the value of the gain factor is related to the indicated rotor speed and the history of rotor speed error. For embodiments where the gain factor value remains between 0 and 1, a gain factor value closer to 1 than 0 indicates that the indicated rotor speed spends more time outside the second rotor speed range than within the first rotor speed range, thus exhibiting a relatively high average deviation from the rotor speed target. On the other hand, a gain factor value closer to 0 than 1 indicates that the indicated rotor speed spends more time within the first rotor speed range than outside the first rotor speed range, thus exhibiting a relatively low average deviation from the rotor speed target.

[0063] Since the value of the corrected speed error depends on the value of the gain factor, changes in the gain factor as the rotor speed changes will affect the value of the corrected speed error. If the indicated rotor speed is outside the second rotor speed range for an extended period, the gain factor and the corrected speed error will continue to increase, thereby increasing the responsiveness of the pitch angle control module 40 to rotor speed errors. Conversely, if the indicated rotor speed is within the first rotor speed range, the corrected speed error will decrease as the gain factor decreases, effectively suppressing the response of the pitch angle control module 40 to save energy.

[0064] Since the gain factor is between 0 and 1, the corrected speed error is less than or equal to the rotor speed error. Therefore, unlike the normal pitch control mode, the degree of pitch angle change of blade 12 is generally reduced when operating in standby pitch control mode.

[0065] In some embodiments of the invention, a third rotor speed range may exist, which is wider than and encompasses the first and second rotor speed ranges. In some embodiments of the invention, the third rotor speed range may extend from 32 rpm to 123 rpm.

[0066] In embodiments of the invention where a third rotor speed range exists, when the indicated rotor speed is outside the third rotor speed range, the rotor speed is considered to be in a critical region and the gain factor can be immediately increased to a critical value. In other words, for embodiments where the third rotor speed range extends from 32 rpm to 123 rpm, the gain factor increases to a critical value when the indicated rotor speed is greater than 123 rpm or when the indicated rotor speed is less than 32 rpm. The critical value can be greater than 1, such that the corrected speed error is greater than the actual rotor speed error. This has the effect of making the pitch angle control module 40 more responsive to errors than it is in normal mode, thereby allowing the rotor speed error to be eliminated at a faster rate when the indicated rotor speed is detected to be in the critical region.

[0067] When WTG 10 is running in standby mode, PPC 7 and WTG controller 20 continuously monitor the availability of external power grid 2. Once the fault is cleared and external power grid 2 is once again available to receive power, WTG 10 reconnects to external power grid 2 and returns to normal mode to supply power to external power grid 2, as indicated in step 9. When WTG 10 re-enters normal mode, pitch angle control module 40 operates to continuously control the pitch angle of blade 12 according to the normal pitch control mode.

[0068] Other variations and modifications will be apparent to those skilled in the art. It is particularly important to understand that the values ​​cited above (e.g., the limitations on the first, second, and third rotor speed ranges) are merely indicative and will vary considerably in practice depending on the characteristics of the specific system in which they are implemented. Even for slightly different systems, the values ​​of these parameters can differ significantly.

Claims

1. A method of operating a wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12), the method comprising iterating the following steps: comparing an indicated rotor speed with a rotor speed target to determine a rotor speed error; generating a modified rotor speed error by applying a control factor to the rotor speed error; controlling a pitch angle of each blade (12) in dependence on the modified speed error via a pitch control system (20, 40); and varying the control factor in dependence on a magnitude of the indicated rotor speed by: decreasing the control factor if the indicated rotor speed is in a first rotor speed range comprising the rotor speed target; and increasing the value of the control factor if the indicated rotor speed is outside the first rotor speed range.

2. The method of claim 1, comprising: decrementing the control factor when the indicated rotor speed is in the first rotor speed range.

3. The method of claim 2, comprising: decreasing the control factor by a first decrement when the indicated rotor speed is within the first rotor speed range and greater than the rotor speed target; and decreasing the control factor by a second decrement when the indicated rotor speed is within the first rotor speed range and less than the rotor speed target. incrementing the control factor when the indicated rotor speed is outside the first rotor speed range.

4. The method of claim 1, comprising: increasing the control factor by a first increment when the indicated rotor speed is outside the first rotor speed range and greater than the rotor speed target; and increasing the control factor by a second increment when the indicated rotor speed is outside the first rotor speed range and less than the rotor speed target.

5. The method of claim 4, comprising: increasing the value of the control factor when the indicated rotor speed is outside a second rotor speed range wider than and containing the first rotor speed range; and leaving the control factor unchanged when the indicated rotor speed is outside the first rotor speed range but within the second rotor speed range. setting the control factor to a predetermined value when the indicated rotor speed is outside a third rotor speed range wider than and containing the second rotor speed range.

6. The method of any preceding claim, comprising: the predetermined value is greater than 1. the method corresponds to a standby pitch control mode of the wind turbine generator (10) for use when the wind turbine generator (10) is idling, the standby pitch control mode being less responsive than a normal pitch control mode of the wind turbine generator (10) such that power consumption of the wind turbine generator (10) is reduced when the wind turbine generator (10) is operated in the standby pitch control mode.

7. The method of claim 6, comprising: the standby pitch control mode is used when the wind turbine generator (10) is idling during a grid loss condition.

8. The method of claim 7, wherein, 11. A controller configured to perform the method of any one of claims 1 to 10.

9. The method of any one of claims 1-5, wherein, ​ 10. The method of claim 9, wherein, ​ ​ 12. A control system for a wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12), the control system being configured to: compare an indicated rotor speed with a rotor speed target to determine a rotor speed error; generate a modified rotor speed error by applying a control factor to the rotor speed error; control a pitch angle of each blade (12) in dependence on the modified speed error; and vary the control factor in dependence on a magnitude of the indicated rotor speed by: decreasing the control factor if the indicated rotor speed is in a first rotor speed range comprising the rotor speed target; and increasing the value of the control factor if the indicated rotor speed is outside the first rotor speed range.

13. A wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12) and a control system according to claim 12.

14. A wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12) and a control system according to claim 12, wherein the control factor is varied in dependence on a magnitude of the indicated rotor speed by: decreasing the control factor if the indicated rotor speed is in a first rotor speed range comprising the rotor speed target; and increasing the value of the control factor if the indicated rotor speed is outside the first rotor speed range.

15. A wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12) and a control system according to claim 12, wherein the control factor is varied in dependence on a magnitude of the indicated rotor speed by: decreasing the control factor if the indicated rotor speed is in a first rotor speed range comprising the rotor speed target; and increasing the value of the control factor if the indicated rotor speed is outside the first rotor speed range.

16. A wind turbine generator (10) comprising a rotor (11) having a plurality of blades (12) and a control system according to claim 12, wherein the control factor is varied in dependence on a magnitude of the indicated rotor speed by: decreasing

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