Wind turbine rotor positioning

Abstract

A method for operating a wind turbine to position a wind turbine rotor is provided. The wind turbine (100) comprises an electric generator (10) and a power converter system (20) mechanically coupled to a wind turbine rotor (101), and the power converter system (20) comprises at least two parallel power converters (21, 22) providing two parallel paths for exchanging electric power with the electric generator (10). The method comprises the steps of: controlling the power converter system (20) to supply electric power to the electric generator (10) to operate the electric generator as a motor; and, when a failure of the power converter (21) of the power converter system (20) is detected, controlling the power converter system (20) to maintain the operation of at least one other power converter (22) of the power converter system (20) to continue supplying electric power to the electric generator (10) to continue generating torque applied to the wind turbine rotor (101).

Description

Technology Field

[0001] The present invention relates to a method for operating a wind turbine to position the wind turbine rotor of the wind turbine. Additionally, the present invention relates to an individual wind turbine control system and an individual computer program. Background Technology

[0002] Rotor blades are often mounted to the hub of a wind turbine rotor by orienting the blades horizontally. Accordingly, the blade mount on the hub is positioned at a 90° angle (0° refers to a mount oriented vertically upward), and bolts extending from the blade's mounting flange are aligned with individual mounting holes in the blade mount on the hub. This assembly consequently requires the angular positioning of the hub. Conventionally, a dedicated rotor turning tool is transported into the nacelle, assembled, and mounted to rotate the hub. However, this procedure is time-consuming and costly, and requires additional purchase and maintenance of the rotor turning tool. The additional time required for the installation and operation of the tool can delay the installation of the wind turbine.

[0003] To obtain a desired angular position of the hub, it is also possible to use the wind turbine's generator as a motor to position the hub at the desired angle for mounting the rotor blades. To mount the first blade, the hub can be positioned at a 90° angle and locked by rotor locking pins. To mount the second rotor blade, the rotor locking pins can be removed again, and the hub with one blade is rotated to the next position to accommodate the second rotor blade. Considering the imbalance when one or two blades are installed on a 3-blade rotor hub, relatively large torques must be generated by the wind turbine's electric generator to rotate the hub. In the event of a failure in the wind turbine's electrical system, the torque applied by the generator is typically lost within milliseconds. If large torque is applied to rotate such an unbalanced hub, the drop in torque can result in a large step response that may lead to blade damage. If only one blade is mounted on the hub, the rotor's inertia is relatively low, whereas the total torque drop may need to be absorbed by this single blade.

[0004] Such blade damage is costly and will further delay the installation process because the rotor blades must be removed for repair. Two solutions have emerged to address this problem. In one solution, the aforementioned rotor turning tool continues to be used to avoid such damage, along with associated additional costs and installation time. As a second solution, the rotor blades have been reinforced to withstand the loads acting in these situations. While these reinforcements avoid damage to the blades, they are more expensive to produce and result in heavier blades that increase the load applied to all supporting components. This can reduce the lifespan or increase the risk of failure due to fatigue of individual components.

[0005] Accordingly, it is necessary to mitigate at least some of the aforementioned disadvantages and, in particular, reduce the risk of blade damage without the aforementioned associated disadvantages. In particular, it is desirable to reduce the time and cost associated with hub positioning.

[0006] This need is satisfied by the features of the independent claims. The dependent claims describe examples of the invention.

[0007] According to an aspect of the invention, a method for operating a wind turbine to position a wind turbine rotor is provided. The wind turbine comprises an electric generator mechanically coupled to the wind turbine rotor and a power converter system. The power converter system comprises at least two parallel power converters providing two parallel paths for exchanging electric power with the electric generator. The method includes the step of controlling the power converter system (in particular, at least two parallel power converters) to supply electric power to the electric generator to operate the electric generator as a motor. The supply of electric power is controlled so that the electric generator generates torque applied to the wind turbine rotor to position the wind turbine rotor, in particular to turn the wind turbine rotor and / or maintain the position of the wind turbine rotor. The method further includes the step of controlling the power converter system to maintain the operation of at least one other power converter of the power converter system so as to continue supplying electric power to the electric generator to continue generating torque applied to the wind turbine rotor when a failure of the power converter of the power converter system is detected.

[0008] The method may include ramping down unaligned dq current references to achieve a ramp down of torque.

[0009] The observer / feedback angle used for dq reference frame alignment can be frozen for low generator speeds of rotation, and / or integrated from the current (prevailing) rotational speed of the generator rotor for high rotational speeds.

[0010] By this method, the positioning of the rotor can be improved, and the risk of blade damage can be significantly reduced. By providing a parallel power converter system and maintaining the operation of at least one power converter in the event that another power converter fails, the drop in torque applied to the wind turbine rotor can be significantly reduced. Consequently, the instantaneous shock to one or more blades of the wind turbine rotor can be reduced proportionally. Additionally, this continued operation of at least one other power converter enables a gradual reduction in torque. Thus, the step response experienced in conventional systems can be smoothed out, which significantly reduces the high-frequency content of the mechanical shock experienced by the rotor blade(s) of the wind turbine rotor due to the drop in torque. Therefore, the method synergistically provides two ways in which blade load is reduced. This combined effect can ensure that the rotor blades do not require additional reinforcement to cope with the loads experienced during such failures of the wind turbine's power system; in particular, blade reinforcement may not be necessary. The method may be performed, for example, during the installation of rotor blades, the removal of rotor blades, and / or any service operation in which the rotor position (i.e., the angular position of the wind turbine rotor, in particular the angular position of the rotor hub) changes. For example, for a balanced wind turbine rotor, the method may also yield the benefit of allowing the continued positioning of the wind turbine rotor even if one or more of the parallel power converters fail.

[0011] For example, rotor blades in a 90° position (horizontal) or close to this horizontal position may experience specific hazardous loads. The loss of torque applied by the generator can result in a so-called whiplash effect, which can be caused by a steep and step-shaped drop in the applied torque. By a method, and particularly by reducing the step size and high-frequency components of this step response, this whiplash effect can be significantly reduced and the resulting critical load on the blade can be avoided. Generally, where 0° is defined as the angle at which the rotor blade points upward and 180° as the angle at which it points downward, the zone of blade angles where there is a risk of overloading the blade in a conventional system generally extends from 15° to 165°, and on the opposite side from 195° to 345°. In these regions, there may be a risk of blade damage in the event of a transducer fault. The most critical load typically occurs near the blade tip, for example, at about 70% to 100% of the blade span (0% is at the blade root). By maintaining the operation of at least one other power converter, the risk of such blade damage can be significantly reduced.

[0012] A wind turbine rotor may be an unbalanced wind turbine rotor. The wind turbine rotor may be an unbalanced rotor that includes fewer rotor blades than the default number but includes at least one rotor blade. For example, a method may be performed when installing or removing a rotor blade, which may result in operation using an individual unbalanced wind turbine rotor.

[0013] An unbalanced wind turbine rotor may include, for example, a 3-blade hub and one or two mounted rotor blades. A drop in torque in such a situation can result in significant blade loads due to the imbalance, which can be avoided by a method. The method can, of course, be operated during the entire installation / removal procedure and can provide benefits in the event of power converter loss while the generator holds the unbalanced rotor. Similarly, this method can be applied during the removal of rotor blades or during any service operation to achieve individual benefits.

[0014] At least two parallel power converters may be connected in parallel by providing electrical connections to their respective grid sides and their respective generator sides. Additionally, on their generator sides, it is possible for the parallel power converters to be connected to different generator windings. Additionally, on their grid sides, it is possible for the parallel power converters to be connected to different transformer windings. Combinations of these configurations are possible. In any case, the parallel power converters provide a parallel path for supplying electric power from the power grid (or other source) to the generator, or for supplying electric power received from the generator to the power grid.

[0015] The power converter system may receive electric power from a power grid combined with a wind turbine and / or from another power source, such as a power storage device, a diesel generator or another type of generator, or any other conceivable power source, and the power converter system converts this electric power and provides it to the generator.

[0016] In one example, maintaining the operation of at least one other power converter may include controlling the supply of electric power to the generator by at least one other power converter so as to gradually reduce the generated torque applied to the wind turbine rotor by the generator. Such a gradual reduction can avoid a steep step response and reduce the high-frequency component of the mechanical response. The generated torque can be reduced, for example, to below a threshold or to zero. Then, after such reduction, the wind turbine rotor may be allowed to find its equilibrium position in consideration of the applied gravitational torque, which will then be sufficiently damped by the inertia of the wind turbine rotor.

[0017] The method may further include the step of controlling a power converter system to stop supplying electric power to the generator after a cessation criterion is met. The cessation criterion may be, for example: the torque generated by the generator dropping below a threshold or reaching a value of zero; the elapsed of a predetermined time period; or at least one of an operating parameter, in particular the current supplied to the generator dropping below a threshold or reaching a value of zero. After such cessation of power supply, only gravitational torque and rotational friction may act on the wind turbine rotor, causing the rotor to swing out. Large step responses and resulting high frequencies can be avoided.

[0018] In one example, maintaining the operation of at least one other power converter may involve controlling at least one other power converter to provide a constant voltage to the generator. In the event of a power converter failure, the control of the power converter system may be disrupted and, over the long term, may result in control actions that deviate from the nominal operating conditions of the individual power converters or cause undesirable effects. By setting the output voltage of the power converter to a constant value, robust control can be achieved that enables the converter system to continue applying (reduced) torque to the wind turbine rotor through the generator. The constant voltage may, for example, correspond to the voltage supplied to the generator by at least one other power converter prior to the failure of the (failed) power converter. In other words, the output voltage of at least one other power converter may be frozen at the current value of this output voltage upon failure. Thus, at least one other power converter can continue to supply electric power to generate the same torque (at least temporarily). In this situation, although the dynamic control of the converter system may no longer be operational, the generator will therefore continue to apply torque to the wind turbine rotor in the correct direction, and thus essentially continue what the generator was doing prior to the failure. This enables a robust and efficient reduction of the step response.

[0019] A constant voltage may be provided for a predetermined amount of time, or until the torque generated by the generator drops below a predetermined threshold, specifically until it reaches a value of zero. This may constitute individual interruption criteria. Generally, the torque generated by the generator will depend on the electrical angle of the generator rotor. If the voltage is kept constant and the rotor moves due to gravity, taking into account the torque imbalance, the generated torque may begin to change (especially sinusoidally). Therefore, for example, interrupting the supply of the constant voltage at individual zero crossings of the torque may have the benefit of avoiding the reversal of the direction of the torque generated by the generator to actively drive the wind turbine rotor; that is, it may be avoided that the applied torque accelerates the rotor rather than brakes it.

[0020] For example, the method may further include the step of detecting or estimating the point in time when the torque generated by the generator reaches a value of zero, and the step of stopping the supply of electric power to the generator by the power converter when such zero value of the generated torque is detected. Such detection or estimation may be based on parameters measured by conventional converter control.

[0021] According to additional examples, maintaining the operation of at least one other power converter may involve controlling at least one other power converter according to the mode of operation in which the power converter was operated prior to the detection of a fault. This mode of operation may be, for example, a dynamic voltage control mode in which the voltage supplied to the generator is dynamically controlled. This dynamic control may be based, for example, on angular speed and / or angular position reference values ​​for the rotation of the wind turbine rotor. This may be, in particular, dynamic feedback and / or feedforward control. This operation may allow a relatively high torque to be continuously supplied to the wind turbine rotor by the generator. The control may, in particular, attempt to maintain the rotor position and / or rotor speed, and thus control at least one other power converter to supply electric power to the generator to generate a relatively high torque, particularly maximum torque. Consequently, the magnitude of the step response due to losses in the faulty power converter may be further reduced.

[0022] Control of at least one other power converter according to the operating mode may be performed for a predetermined time period. This time period may be, for example, 1 second, 500 ms, 300 ms, or less than 200 ms. This may be 50 ms to 300 ms, for example, 100 ms. If control in the previous operating mode continues, rotation of the wind turbine rotor due to torque imbalance (loss of the failed power converter) may disturb the control; in particular, the rotor position may drift away from the expected rotor position on which such control can be based. Therefore, operating in the operating mode only for a predetermined time period may have the advantage that the drift within this short time period is relatively low, so conventional control in the operating mode can still enable the generator to generate the desired amount of torque.

[0023] In an additional example, maintaining the operation of at least one other power converter may involve controlling at least one other power converter to ramp down the current supplied to the generator by at least one other power converter. By reducing the current supplied to the generator, the torque that the generator can produce may be reduced accordingly. Thus, by ramping down the current, the torque applied by the generator after a failure may be smoothly reduced, thereby avoiding additional step responses and high-frequency components of the mechanical response. The risk of blade load and damage may be reduced by this. For example, the current reference value of the current control loop of at least one other power converter may be reduced according to, for instance, predetermined characteristics. The ramp may be, in particular, a linear ramp or a non-linear ramp.

[0024] The current can be ramped down, for example, to a value below a threshold or to zero. The ramp-down may occur, for example, within a predetermined time period. This time period may preferably be less than 1 second, 500 ms, 300 ms, or 200 ms. For example, the predetermined time period may be 50 ms to 300 ms, such as 100 ms or 200 ms. Ramping down the current, and the consequently applied torque, within this time frame can reduce the risk of additional transducer damage due to overload while avoiding loads and high-frequency components associated with the step response. For example, the slope of the current ramp, particularly a linear slope, can be adapted to reach a current value of zero within the predetermined time period.

[0025] It should be clear that the aforementioned methods for controlling at least one other power converter can be combined to achieve beneficial control.

[0026] For example, maintaining the operation of at least one other power converter may include controlling at least one other power converter according to an operation mode during a first predetermined time period, and ramping down the current supplied to the generator by at least one other power converter within a second predetermined time period following the first predetermined time period. For example, in a two-converter system where the torque generated by the generator drops by 50% if one power converter is lost, the continued operation of the remaining power converter in the operation mode may maintain 50% of the torque during the first time period regardless of the azimuth position of the wind turbine rotor. Then, in the second time period, the current may be lowered from the actual value of the current to zero, so that the resulting smooth torque reduction does not generate any additional blade load.

[0027] In an additional example, maintaining the operation of at least one other power converter may include controlling at least one other power converter to provide a constant voltage to the generator during a first time period, and ramping down the current provided to the generator by at least one other power converter after the first time period. The first time period may preferably be less than 1 second, 500 ms, 300 ms, or 200 ms. This may be, for example, between 50 ms and 200 ms. By holding the voltage constant only during a predetermined time period and then ramping down the current, the need to detect zero crossing of the torque can be avoided. Since the current can simply ramp down until the current reaches zero, the timing of the control can be facilitated. The current may ramp down within a second time period that can take any of the aforementioned values. In particular, in any of the examples, the second time period may be determined, for example, by the slope of the current ramp. Therefore, the control can simply provide a specific slope of the current ramp and ramp down the current from the current value to a value of 0.

[0028] The second time period during which the current ramps down to zero may have a duration of 0.5 to 2 times the duration of the first time period during which the voltage is held constant. By doing so, a relatively fast and efficient torque reduction can be obtained, which avoids large steps and reduces the high-frequency components of the mechanical response. The durations of both time periods may be the same.

[0029] The wind turbine is preferably a direct-drive wind turbine in which the generator is mechanically connected to the wind turbine rotor without an intermediate gearbox. Since there is no intermediate gearbox capable of providing damping in the event of torque loss from the generator, the method can develop specific benefits in this wind turbine configuration.

[0030] According to a further aspect, a control system for controlling the operation of a wind turbine is provided. The wind turbine comprises a wind turbine rotor comprising at least one or two rotor blades, an electric generator mechanically coupled to the wind turbine rotor, and a power converter system. The power converter system comprises at least two parallel power converters providing two parallel power paths for exchanging electric power with the electric generator. The control system is configured to perform any of the methods disclosed herein.

[0031] According to a further aspect of the invention, individual wind turbines comprising such a control system are provided.

[0032] According to a further aspect of the invention, a computer program for controlling the operation of a wind turbine is provided, wherein the wind turbine comprises a power converter system having at least two parallel power converters that provide two parallel paths for exchanging electric power with an electric generator of the wind turbine. The computer program comprises control instructions that, when executed by a processing unit of a control system for controlling the operation of the wind turbine, cause the processing unit to perform any of the methods disclosed herein. Such computer program, in particular individual control instructions, may be provided on a volatile or non-volatile storage medium or data carrier and / or may be provided via a communication connection such as a wired or wireless network connection.

[0033] By such a control system, wind turbine, or computer program, advantages corresponding to the additional advantages described above in relation to the method can be achieved.

[0034] It should be understood that the features mentioned above and the features still described below may be used not only in the individual combinations indicated without departing from the scope of the invention, but also in other combinations or alone. In particular, features of different aspects and examples of the invention may be combined with one another unless otherwise specified. Brief explanation of the drawing

[0035] The foregoing and other features and advantages of the invention will become more apparent from the following detailed description, read together with the accompanying drawings. In the drawings, like reference numerals refer to like elements. FIG. 1 is a schematic diagram illustrating a wind turbine according to an example of the invention. FIG. 2 is a schematic diagram illustrating a control system of a wind turbine having parallel power converters according to an example of the invention. Figure 3 is a flowchart for operating a wind turbine according to an example of the invention. FIG. 4 is a diagram schematically illustrating the change in the angular position of a wind turbine rotor in the event of a failure of a power converter according to an example of the invention. FIG. 5 is a diagram schematically illustrating the torque generated by a generator of a wind turbine when operating according to different exemplary methods of the invention. FIG. 6 is a flowchart of an exemplary implementation of the step of maintaining the operation of a parallel power converter of the method of FIG. 3. Figure 7 is a flowchart of another exemplary implementation of the step of maintaining the operation of the parallel power converter of the method of Figure 3. Figure 8 is a flowchart of another exemplary implementation of the step of maintaining the operation of the parallel power converter of the method of Figure 3. Specific details for implementing the invention

[0036] In the following, embodiments and / or examples of the invention will be described in detail with reference to the accompanying drawings. It should be understood that the description of the embodiments below is provided for illustrative purposes only and should not be interpreted in a limiting sense. The drawings should be regarded merely as schematic representations, and it should be noted that the elements of the drawings are not necessarily scaled to one another. Rather, the representation of the various elements is selected so that their function and general purpose may be clear to those skilled in the art. As used herein, the singular forms “one,” “one,” and “the above” are intended to include plural forms unless the context clearly indicates otherwise. The terms “comprising,” “having,” “including,” and “containing” should be interpreted as open-ended terms (i.e., meaning “including but not limited thereto”) unless otherwise specified.

[0037] FIG. 1 schematically illustrates a wind turbine (100) having a tower (105) and a nacelle (104). The wind turbine rotor (101) of the wind turbine (100) has two rotor blades (102) mounted on the hub (110) of the wind turbine (100). In FIG. 1, the wind turbine (100) is illustrated during the installation or removal of a third rotor blade (103). The hub (110) is rotated to a position in which the blade mount (111) is oriented horizontally so that the rotor blade (103), which is suspended horizontally by a crane and yoke assembly (not illustrated), can be mounted on the blade mount (111) (or so that the rotor blade (103) can be correspondingly removed). If the blade mount (111) points vertically upward, the angular position of the wind turbine rotor (101) is considered to be 0°, and the position illustrated in FIG. 1 corresponds to a 90° angular position.

[0038] It will be easily apparent that the wind turbine rotor (101) is in an unbalanced state, as the weight of the two rotor blades (102) acts on one side of the rotor (101)'s axis of rotation, causing a torque due to gravity to be applied through the rotor (101). The wind turbine (100) may be a direct-drive wind turbine in which the wind turbine rotor (101) is mechanically connected to a wind turbine generator without an intermediate gearbox. The generator may be operated as a motor to generate torque that causes the wind turbine rotor (101) to move to the position illustrated in FIG. 1. Thus, a significant amount of torque must be applied to the wind turbine rotor (101) by the generator to overcome the imbalance. During the mounting of the rotor blades (103), the rotor (101) may be held in place by individual locking pins. However, when the locking pins are released and the orientation of the rotor (101) is changed, the gravitational torque applied by the rotor (101) is borne by the generator. Accordingly, when the generator loses power supply and consequently stops applying torque, the wind turbine rotor (101) will experience an immediate and significant drop in torque and consequently experience a step response. As described above, such a sudden drop in torque is associated with a significant risk of blade damage. The high-frequency component of this step response, and in particular the whipping effect caused by such a sudden drop in torque, can lead to a significant structural load on the blades (102) and consequent damage.

[0039] The solution disclosed in this specification can avoid such a sudden and large torque drop by providing parallel power converters and, in the event of a failure of one of the parallel power converters, continuing to supply electric power to the generator and thereby continuing to supply torque to the rotor (101) by the generator.

[0040] FIG. 2 schematically illustrates a specific embodiment of a wind turbine (100) having an electric power system (40) comprising a generator (10), a converter system (20), and a wind turbine transformer (30). The generator (10) may be a permanent magnet generator, in particular a permanent magnet synchronous generator (PMSG). The transformer (30) is coupled to the power grid (200) to exchange electric power with the power grid (200), and in particular to draw power from the power grid (200) during the operation of the generator (10) as a motor. The converter system (20) includes two or more parallel power converters (21, 22). These may be implemented as back-to-back converters, which may include, for example, a generator-side converter, a grid-side converter, and an intermediate DC-link. Although two parallel power converters are illustrated, a larger number of parallel power converters may exist. Parallel power converters (21, 22) provide a parallel power path from an electric power source (e.g., power grid (200)) to a generator (10). In the example of FIG. 2, the terminals of the generator-side converters are connected together, and the terminals of the grid-side converters are connected together to provide this parallel power path. In other examples, the terminals of the generator-side converters may be connected to different generator windings of the generator (10), and / or the terminals of the grid-side power converters may be connected to different windings of the transformer (30). Also in this way, a parallel power path between the power source and the generator (10) is established. In addition to using the power grid (200) as a power source, other power sources such as diesel generators, power storage devices, fuel cells, etc., may be used.

[0041] The converter system (20) is controlled by a control system (50). The control system (50) may be implemented as a single controller or distributed across multiple controllers. This may include, for example, a converter controller for parallel power converters (21, 22) and may further include a wind turbine controller for controlling additional functions of the wind turbine (100). During the installation (or removal) of the rotor blade (103), the control system (50) may control the power converter system (20) according to, for example, a position reference value for the angular position of the wind turbine rotor (101) and / or a speed reference value for the angular velocity. Accordingly, the converter system (20) may provide electric power to the generator (10) so that the generator (10) applies a torque that drives the rotor (101) to the reference angular position. Individual feedback control may be used.

[0042] The control system (50) may include a processing unit (51) and a memory (52). The processing unit (51) may include any type of processor, such as a microprocessor, an application-specific integrated circuit, a field-programmable gate array, a digital signal processor, etc. The memory (52) may include any type of memory, such as RAM, ROM, EEPROM, flash memory, a hard disk drive, etc. The control system (50) may include additional components common to computer systems, such as individual input / output interfaces, a data communication bus for interconnecting different components, and a user interface. The memory (52) may store control commands to be executed by the processing unit (51). These control commands may be configured to cause the control system (50) to perform any of the methods described herein.

[0043] FIG. 3 illustrates a method according to an example that can be performed by a control system (50). In step S1, the converter system (20) is operated to supply electric power to a generator (10) to generate torque for positioning a wind turbine rotor during blade installation, for example, to hold an unbalanced rotor at a specific angle position or to rotate an unbalanced rotor to a desired angle position. The control system (50) may issue individual control commands to the converter system (20) to provide individual operations.

[0044] The control system (50) can operate as a current dq controller, or another controller operating as a current dq controller may be included in the system and method.

[0045] Individual power converters are generally implemented with high-power semiconductor switches, which often suffer from excessive heat generation when switching high currents. Consequently, the risk of failure of the power converters in the power converter system (20) is particularly high during such operation.

[0046] In step S2, a failure of the parallel power converter is detected. For example, it may be detected that the converter (21) has failed. In a conventional converter system using parallel converters, a failure of one power converter will generally lead to a shutdown of the entire converter system, particularly an emergency shutdown of the converter system.

[0047] In one embodiment, the failure relates to a control system (50) operating as current dq controllers that have lost their observer / feedback angles used for dq reference frame alignment.

[0048] In another embodiment, the failure relates to current dq controllers that have lost their observer / feedback angles used for dq reference frame alignment.

[0049] In any embodiment, the unaligned dq current reference values ​​may be ramped down (e.g., by the control system (50) and / or the dq controller) to achieve a ramp down of torque. This ramp down may be entirely controlled by the ramp down of the dq current reference values, or this ramp down may be one parameter of the ramp down of the dq current reference values. At the same time, the observer / feedback angle used for aligning the dq reference frames may be frozen (e.g., in the case of low speed). Alternatively, this may be integrated from the current speed of the generator's rotor (e.g., in the case of high speed). In this context, low speed means lower than high speed, and more specifically means relatively close to zero in relation to the rated rotational speed. In the same way, in this context, high speed means higher than low speed, and more specifically means relatively close to the rated rotational speed and / or at least exceeding a threshold of rotational speed.

[0050] In contrast, in exemplary implementations of the present method, at step S3, the operation of one or more other power converters (other than the failed power converter) of the converter system (20) is maintained. For example, the operation of the power converter (22) and optionally the operation of one or more other parallel power converters (not shown) continue after the failure. As a result, torque is continuously applied to the wind turbine rotor (101) by the generator (10). The drop in torque experienced by the rotor (101) can be significantly reduced accordingly.

[0051] In step S4, it may be further verified whether a shutdown criterion for stopping the operation of the power converter system is satisfied (step S4). If it is not satisfied, the application of torque by the generator by the continued operation of one or more of the remaining power converters continues in step S3. If the shutdown criterion is reached in step S4, the supply of electric power to the generator by the power converter system (20) is stopped in step S5. Preferably, the shutdown criterion is invoked when the torque generated by the generator (10) reaches a value of 0, which may be determined after the expiration of a predetermined time period, or after a monitored operation parameter reaches a threshold or drops below a threshold, such as after the current supplied to the generator drops to a value of 0, or by actually detecting that the torque generated by the generator (10) has dropped to a value of 0. However, these are merely examples, and other shutdown criteria may be used. Although FIG. 3 is described in relation to the installation of rotor blades, this can be performed in response to any other service operation requiring the removal of rotor blades or the positioning of the wind turbine rotor, particularly when the rotor is in an unbalanced state.

[0052] FIG. 4 schematically illustrates the angular position α of the rotor (101) in degrees. The horizontal axis represents time in seconds. The angular position curve (401) indicates that the angular position of the rotor (101) is maintained at 90° by balancing the gravitational torque of the rotor (101) with the torque generated by the generator (10) up to 10 seconds. At 10 seconds, one of the parallel power converters is lost, causing an individual torque imbalance. As a result, the rotor (101) can no longer be held in the individual angular position and begins to rotate. However, since torque is still applied to the wind turbine rotor by the generator by the disclosed solution, the change in position occurs somewhat slowly and smoothly.

[0053] The step of maintaining the operation of one or more other parallel power converters in step S3 of FIG. 3 can be performed in different ways. FIGS. 6, 7, and 8 illustrate different implementations of step S3 of FIG. 3, and FIG. 5 illustrates the resulting torque generated by the electric generator (10) in response to individual control of the converter system (20). FIG. 5 illustrates time in seconds corresponding to the time of FIG. 4 on the horizontal axis. The vertical axis represents the torque τ generated by the generator (10), normalized to the torque generated prior to the failure. At the point of 10 seconds, the power converter (21) may fail, for example. As a result, since the generator (10) no longer receives electric power from the power converter (21), a drop (501) in the torque generated by the generator (10) is observed.

[0054] In the example of FIG. 6, step S3 of FIG. 3 includes a step (step S31) of maintaining a constant voltage supplied to the generator (10) by at least one other power converter (e.g., power converter (22)). In conventional operation, the voltage is dynamically controlled, for example, using position feedback from the rotor (101). As shown in FIG. 4, since the rotor position begins to drift after a failure, this control is no longer accurate, so maintaining a constant voltage provides a robust way to continue operation. However, since the generator torque depends on the electrical angle of the rotor, the generated torque begins to change and follows the sinusoidal curve (510) of FIG. 5. At any position of the rotor (101), the torque will become zero and then change its sign (curve section (511)), which will result in unwanted acceleration of the rotor. Accordingly, the interruption criterion of step S4 can be set so that the supply of electric power to the generator (10) by the converter system (20) is interrupted when the generated torque reaches zero, that is, when the curve (510) crosses zero. As a result, the applied torque is gradually reduced to zero, thereby avoiding the generation of high-frequency load components. The curve section (511) is shown for illustrative purposes only, and such torque will not actually be provided by the generator (10).

[0055] FIG. 7 illustrates an alternative implementation of step S3 of FIG. 3. In step S32, at least one other power converter, e.g., power converter (22), continues to operate in the same operating mode as it was operated prior to the failure. In other words, it can continue to provide dynamic voltage control, for example, based on position feedback. As a result, the torque generated by the generator (10) can be maintained relatively constant over a short period of time, at least during which the drift of the rotor position is not too large. This is illustrated by the curve section (520) of FIG. 5. After a first predetermined time period (100 ms in FIG. 5), the current supplied to the generator by at least one other power converter ramps down within a second predetermined time period (step S33). By reducing the current supplied to the generator (10), the torque generated by the generator (10) is likewise gradually reduced. This is illustrated by the curve section (521) of FIG. 5. In this example, the slope of the current reduction is set so that a second predetermined time period lasts for 100 ms; other slopes or times may be used. In this implementation, the cessation criterion of step S4 may be, for example, that the second predetermined time period has elapsed, or that the current has reached a value of 0, or that the torque has reached a value of 0. Again, by implementing this method, the initial drop in torque can be significantly reduced, and the torque transitions smoothly to a value of 0.

[0056] FIG. 8 illustrates an additional exemplary implementation of step S3 of FIG. 3. The example in FIG. 8 is a combination of the exemplary methods of FIG. 6 and FIG. 7. In step S35, the voltage supplied to the generator by at least one other power converter, e.g., power converter (21), is kept constant for a first predetermined time period. Thus, step S35 essentially corresponds to step S31. Thus, the torque generated by the generator (10) follows the curve (510) as illustrated in FIG. 5, but only for the interval (530) of the curve (510). The first predetermined time period is 100 ms in the example of FIG. 5. Subsequently, in step S36, the current supplied to the generator by at least one other power converter is ramped down within a second predetermined time period. This ramping down of the current corresponds to step S33 of FIG. 7. Again, this ramp-down of current causes a corresponding gradual decrease in torque generated by the generator (10), which is exemplified by the curve (531) in FIG. 5. A second predetermined time period is selected to be 100 ms in the example of FIG. 5. FIG. 5 again illustrates a linear ramp-down of torque, but has a different slope than the curve (521) in the example of FIG. 7. In an implementation of this method, the interruption criterion of step S4 may again be selected as the expiration of the second predetermined time period, the current reaching a value of 0, the torque reaching a value of 0, or any other suitable parameter reaching or exceeding a threshold. The method of FIG. 8 provides the advantage of not requiring zero-crossing detection of torque while having robust control immediately after a failure of one power converter. In particular, both the methods of FIG. 7 and FIG. 8 are relatively insensitive to the timing of interruption of the supply of electric power to the generator (step S5), particularly to interrupting the operation of the power converter (20). The current ramp can be implemented, for example, by ramping down the current reference value to a value of 0.Once the current reference value reaches a value of 0, no more torque will be generated by the generator (10), so the shutdown timing of the power converter system (20) is not deterministic.

[0057] It should be clear that the timing and slope of the curves exemplified in FIG. 5 are merely illustrative and that different timings may be used. Additionally, the control method may include different combinations of steps of maintaining a constant voltage, continuing operation in a specific operating mode, and ramping down the current. Other implementations, such as ramping down different operating parameters, may also be considered. For the purpose of illustration, the curve section (520) is depicted as a straight line in FIG. 5. However, it should be clear that the generated torque may be affected by disturbances in the control method due to position drift.

[0058] In a conventional system, the torque would drop to zero upon the occurrence of a fault. As can be seen in FIG. 5, the drop in torque is significantly reduced by each of the exemplary methods, and the torque is brought to a value of zero in a relatively smooth manner, thereby avoiding the critical load on the blade(s). Thus, by using the examples of methods, there may be no need to provide additional tools to rotate the rotor, nor may there be no need to reinforce the rotor blades to withstand such critical loads. When the torque generated by the generator (10) reaches a value of zero, the rotor will simply swing out under gravitational torque and the rotational friction and aerodynamic drag experienced.

[0059] Therefore, by this method, if a fault occurs in one of the converter channels, the torque of the remaining channels does not drop to zero, as in conventional systems where the default action of the electrical system is to stop switching. However, the present solution prevents this default action and ensures that the remaining power converters continue to operate and then shut down in a manner that causes a significantly reduced load on the blades. This is achieved by a gradual reduction of the generated torque before shutting down the converter system, for example, within a relatively short time period of less than 1 second.

[0060] While specific embodiments are disclosed herein, various changes and modifications may be made without departing from the scope of the invention. These embodiments are to be regarded as exemplary and non-limiting in all respects, and all variations falling within the meaning and equivalent scope of the appended claims are intended to be included in the claims.

Claims

Claim 1 A method for operating a wind turbine to position a wind turbine rotor of a wind turbine, wherein the wind turbine (100) comprises an electric generator (10) mechanically coupled to the wind turbine rotor (101) and a power converter system (20), and the power converter system (20) comprises at least two parallel power converters (21, 22) providing two parallel paths for exchanging electric power with the electric generator (10), and the method comprises:- A step of controlling the power converter system (20) to supply electric power to the electric generator (10) to operate the electric generator as a motor — the supply of electric power is controlled to generate torque applied to the wind turbine rotor (101) so that the electric generator (10) positions the wind turbine rotor —; and- A method comprising the step of controlling the power converter system (20) to maintain the operation of at least one other power converter (22) of the power converter system (20) so as to continue supplying electric power to the electric generator (10) to continue generating torque applied to the wind turbine rotor (101) when a failure of the power converter (21) of the power converter system (20) is detected. Claim 2 In claim 1, the method comprises ramping down unaligned dq current references to achieve a ramp down of torque. Claim 3 In paragraph 2, the observer / feedback angle used for aligning the dq reference frame is frozen for low generator speeds of rotation and / or integrated from the current prevailing rotational speed of the generator's rotor for high rotational speeds. Claim 4 A method according to any one of claims 1 to 3, wherein maintaining the operation of the at least one other power converter (22) comprises controlling the provision of electric power to the generator (10) by the at least one other power converter (22) so as to gradually reduce the generated torque applied to the wind turbine rotor (101) by the generator (10). Claim 5 In any one of claims 1 to 4, the method further comprises the step of controlling the power converter system (20) to stop supplying electric power to the generator (10) when a cessation criterion is satisfied, wherein the cessation criterion is preferably:- The torque generated by the generator (10) drops below a threshold value or reaches a value of 0; The elapsed of a predetermined period of time; or- A method in which an operating parameter, in particular, the current supplied to the generator (10) drops below a threshold value or reaches a value of 0. Claim 6 A method according to any one of claims 1 to 5, wherein maintaining the operation of the at least one other power converter (22) comprises controlling the at least one other power converter (22) to provide a constant voltage to the generator (10), wherein the constant voltage corresponds to the voltage supplied by the at least one other power converter (22) to the generator (10) prior to the failure of the power converter (21). Claim 7 In claim 6, the constant voltage is provided for a predetermined amount of time, or until the torque generated by the generator (10) drops below a predetermined threshold or reaches zero. Claim 8 A method according to claim 6 or 7, further comprising the step of detecting or estimating the point in time when the torque generated by the generator (10) reaches a value of 0, and the step of stopping the provision of electric power to the generator (10) by the at least one other power converter (22) when the value of 0 of the generated torque is detected. Claim 9 A method according to any one of claims 1 to 8, wherein maintaining the operation of the at least one other power converter (22) comprises controlling the at least one other power converter (22) according to the operation mode in which the power converter was operated prior to detecting the fault. Claim 10 A method according to any one of claims 1 to 9, wherein maintaining the operation of the at least one other power converter (22) comprises controlling the at least one other power converter (22) to ramp down the current supplied to the generator (10) by the at least one other power converter (22). Claim 11 In claim 10, the current is ramped down to a value below a threshold or to zero, and the ramping down preferably occurs within a predetermined time period, and the predetermined time period is preferably less than 1 s, 500 ms, 300 ms, or 200 ms. Claim 12 A method according to claim 10 or 11, where, when directly or indirectly citing claim 8 or 9, maintaining the operation of the at least one other power converter (22) comprises controlling the at least one other power converter (22) according to an operation mode during a first predetermined time period, and ramping down the current supplied to the generator (10) by the at least one other power converter (22) within a second predetermined time period following the first predetermined time period. Claim 13 A method according to claim 10 or 11, wherein, when directly or indirectly citing any one of claims 4 through 7, maintaining the operation of the at least one other power converter (22) comprises controlling the at least one other power converter (22) to provide a constant voltage to the generator (10) during a first time period, and ramping down the current provided to the generator (10) by the at least one other power converter (22) after the first time period. Claim 14 A control system configured to control the operation of a wind turbine, wherein the wind turbine (100) comprises a wind turbine rotor (101) comprising at least one or two rotor blades (102), an electric generator (10) mechanically coupled to the wind turbine rotor (101), and a power converter system (20), wherein the power converter system (20) comprises at least two parallel power converters (21, 22) providing two parallel paths for exchanging electric power with the electric generator (10), and the control system (50) is configured to perform a method according to any one of claims 1 to 13. Claim 15 A computer program for controlling the operation of a wind turbine (100), wherein the wind turbine (100) comprises a power converter system (20) having at least two parallel power converters (21, 22) that provide two parallel paths for exchanging electric power with an electric generator (10) of the wind turbine (100), and wherein the computer program comprises control commands that cause the processing unit (51) to perform a method of any one of claims 1 to 13 when executed by a processing unit (51) of a control system (50) that controls the operation of the wind turbine (100).

Images