Method for operating a wind turbine and wind turbine

Abstract

The present disclosure relates to methods (100, 200) for operating a wind turbine (10). The wind turbine (10) has a generator (42) for generating power and a power converter (61) with switches (73, 75) for converting the generated power. The method (100, 200) comprises, in a situation with the wind turbine (10) disconnected from a utility grid (102), generating power with the generator (42) and operating the switches (73, 75) to convert the generated power. Furthermore, the method comprises determining an electrical characteristic indicative of an electrical resonance downstream of the power converter (61). The method also comprises defining, at least partially based on the determined electrical characteristic, a preferred switching frequency for operating the switches (73, 75). Then, the method comprises operating the switches (73, 75) with the preferred switching frequency. The present disclosure relates also to a wind turbine (10) with a controller unit (36, 611) configure to implement such methods.

Description

GENERAL ELECTRIC RE OVABLES ESPANA S.L. MARCH 14, 2025 701148-WO-1 P5591 PC00METHOD FOR OPERATING A WIND TURBINE AND WIND TURBINE

[0001] The present disclosure relates to methods for operating a wind turbine. More particularly, the present disclosure relates to wind turbines comprising power converters with switches, and to methods for operating the switches of the power converter while the wind turbine is disconnected from a utility grid.BACKGROUND

[0002] Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly (“directly driven” or “gearless”) or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.

[0003] The wind turbine hub may be rotatably coupled at a front of the nacelle. The wind turbine hub may be connected to a rotor shaft, and the rotor shaft may then be rotatably mounted in the nacelle using one or more rotor shaft bearings arranged in a frame inside the nacelle. The nacelle is a housing arranged on top of a wind turbine tower that may contain and protect the gearbox (if present) and the generator (if not placed outside the nacelle) and, depending on the wind turbine, further components such as a power converter, and auxiliary systems.

[0004] A wind turbine may become disconnected from a utility grid due to different reasons, such as faults or regular maintenance. Furthermore, the wind turbine may also be disconnected from the utility grid during the erection and commissioning phase, i.e. prior to starting commercial operation. While a wind turbine is disconnected from the utility grid, operation of some auxiliary systems of the wind turbine may be required. Some relevant auxiliary systems include, e.g. control systems, communication systems, air conditioning system, temperature regulation systems, or yaw operation systems. Operation of such auxiliary systems may be desired for safety or maintenance reasons. To this end, a wind turbine may operate in a self-sustaining mode of operation. Such a self-sustaining mode maybe referred to as island mode, Turbine Island Mode (TIM). Hence, in a situation in which the wind turbine is disconnected from a utility grid, the wind turbine may be operated to generate a relatively small amount of power to feed its own auxiliary systems.

[0005] Often, a plurality of wind turbines in a specific location forms a wind farm. The wind turbines may be electrically connected via one or more collector cables, which may form a collector grid or wind farm grid. In particular, each wind turbine may comprise a switchgear, which may include a wind turbine switch, e.g. a circuit braker, to connect the wind turbine to the collector grid. This collector grid may be connected to the utility grid at a point of interconnection.

[0006] As with individual wind turbines, a wind farm may also be or become disconnected from a utility grid. Accordingly, a wind farm may also be configured to operate in an island mode. That is, in a situation in which the wind farm is disconnected from a utility grid, one or more wind turbines may be operated to supply power to auxiliary systems. These auxiliary systems may belong to the wind turbines generating electrical power or to other wind turbines connected to the same collector grid.

[0007] In particular, it is known that wind farms, especially when comprising a large number of wind turbines, can be arranged in several strings of wind turbines. Each string comprises a plurality of wind turbines, which can be electrically connected in series. In known configurations, the different strings are connected to each other at a collection point, such that power generated by the wind turbines of the different strings is collected and, finally, injected into a utility grid. If disconnected from the utility grid, one or more wind turbines in a string can be operated in an island mode of operation to supply power to their own auxiliary systems and to the auxiliary systems of at least some of the other wind turbines in the string. In such a case, the string is operated in a so-called string island mode, SIM. In this case, the wind turbines receiving the electrical power act as a load.

[0008] Also during normal operation, i.e. not during island modes of operation, a wind turbine may be disconnected from a utility grid during a startup sequence of the wind turbine. Indeed, grid synchronization may be required before establishing electrical connection between the wind turbine and the utility grid. To this end, a wind turbine may be operated to synchronize with the utility grid before it is actually connected to the utility grid.

[0009] Operation of a wind turbine while being disconnected from the utility grid, either when installed as a single unit or as part of a wind farm, and either due to, e.g. an islanding mode of operation or to a pre-connection synchronization sequence, may comprise specific challenges. In particular, the connection of the wind turbine to a lightly loaded system, i.e. asystem with a low load, can lead to electrical faults in the system. Specifically, overvoltages or overcurrents may originate when the wind turbine is not connected to the large load provided by the utility grid. Such overvoltages or overcurrents can induce damage faults in the wind turbine or in the collector grid, i.e. in the collector cables connecting the different wind turbines among themselves or with a point of interconnection to the utility grid.

[0010] In order to prevent such potential damage, different mitigation strategies are used. It is known to provide sensors to monitor the operating conditions of the system. In case an abnormal operating condition is detected, operation of the wind turbine can be stopped such that generation and injection of electrical power is interrupted. However, overvoltages or overcurrents can still arise when employing such reactive methods. Furthermore, interruption of the operation of the wind turbine may result in delays and / or in the absence of power for critical loads. In some other cases, the technical specification of components may be defined to withstand such faults. However, this inevitably leads to increased costs and, consequently, to reduced competitiveness.

[0011] The present disclosure aims to provide improved methods for operating a wind turbine or, more specifically, a wind turbine disconnected from the utility grid so as to at least partially reduce the above-mentioned limitations.SUMMARY

[0012] In an aspect of the present disclosure, a method for operating a wind turbine is provided. The wind turbine has a generator for generating power and a power converter with switches for converting the generated power. The method comprises, in a situation with the wind turbine disconnected from a utility grid, generating power with the generator and operating the switches to convert the generated power. Furthermore, the method comprises determining an electrical characteristic indicative of electrical resonance at a point downstream of the power converter, and defining, at least partially based on the determined electrical characteristic, a preferred switching frequency for operating the switches. The method also comprises operating the switches with the preferred switching frequency.

[0013] According to this aspect of the disclosure, an improved method to operate a wind turbine during a period when it is disconnected from the utility grid is provided. In particular, the method provides increased flexibility and versality by adjusting the switching frequency of the switches of the power converter. Hence, instead of using a fixed, predefined, switching frequency, a variable frequency, which can be adjusted depending on prevailing conditions, is provided in the present method.

[0014] The versatility provided by the method is advantageous when dealing with situations in which the wind turbine is not connected to the utility grid. In such cases, the load is significantly lower than when connected to the utility grid. Consequently, the damping effect provided by the utility grid is not present. The connection of a wind turbine to such a low load poses significant risks on the system in certain scenarios and applications. As an example, high voltage cables used in offshore wind farms exhibit a high capacitance. It has been found that this increases the possibility of resonance between such cable capacitance and system reactance, i.e. reactance from cable itself, transformer, or filters, when power is injected into the system while the wind turbine is not connected to the utility grid. As indicated, such situations can occur under different relatively common scenarios, such as when operating in island mode, i.e. with a wind turbine generating a certain amount of power to feed auxiliary loads, or during the grid synchronization process, i.e. during the pre-connection process required for synchronizing the power generated by the wind turbine (or wind farm) with the utility grid before electrical connection and power injection into / from the grid.

[0015] Electrical resonance can lead to significant amplification of voltage and / or currents in the system, e.g. on the high voltage cables. In systems comprising converter-based power sources, such resonance can be excited by the switching frequency of the switches of the power converter. Furthermore, not only the fundamental switching frequency of the switches is to be considered, but also the corresponding harmonics can lead to resonance. In other words, the reactance and capacitance of the system form an electrical LC tank, which experiences resonance at certain specific resonance frequencies. According to the present disclosure, the switching pattern of the switches can be modified so that a preferred switching frequency is used such that neither the switching frequency nor the corresponding harmonics are close to the resonance frequency of the system. In this manner, resonance phenomena are avoided or, at least, restricted and kept under control.

[0016] Moreover, the value of the preferred switching frequency is based on a determined electrical characteristic. Hence, the method does not rely on a previously determined theoretical resonance frequency, e.g. by means of simulations, but it includes a determination of a characteristic to analyze the actual response of the system. Accordingly, changes in the electrical system are accounted for as the electrical characteristic can be determined in real time. Those changes may arise from different operating conditions, e.g. different loads being connected to the wind turbine. Furthermore, aging of different components, such as cables or transformers, can also lead to slow changes in the impedance parameters of the different components. Those changes in the impedance parameters translate into different resonance frequencies. Based on the method according to the present disclosure, such changes areeffectively captured by dynamically determining an electrical characteristic downstream of the power converter.

[0017] In another aspect of the present disclosure, another method for operating a wind turbine comprising a generator for generating power, and power converter connected to the generator is provided. The power converter comprises switches for converting the generated power. The method comprises generating electrical power with the generator and operating the switches according to a frequency sweep while converting the generated power. An electrical characteristic of an electrical power downstream of the power converter is measured while sweeping the switching frequency of the switches. Moreover, the method comprises defining a preferred switching frequency at least partially based on the measured electrical characteristic. The method also comprises continuing operating the switches of the power converter at the preferred switching frequency.

[0018] According to this aspect of the disclosure, a particularly efficient method is provided to find a preferred switching frequency. As already explained, it has been found that certain switching frequencies (or the corresponding harmonics) for the switches of power converter can be closer to resonance frequencies of the LC tank system created by the impedances of the electrical system. This is especially relevant for situations wherein the wind turbine is not connected to the utility grid because, when connected to the utility grid, the latter provides a significant load that dampens potential resonance oscillations.

[0019] According to this aspect of the disclosure, an efficient method to find an optimum or, at least a safe, switching frequency, is achieved. Indeed, by operating the switches with a frequency sweep, a scan over a continuous and desired range of frequencies is implemented. In this manner, a direct determination of the effect of the switching frequency on the response of the system is obtained in a precise manner. Such direct determination provides a safer and more realistic determination of the performance of the system than, e.g. simulations. In particular, time evolution of the system due to, e.g. different operating conditions or aging of the components, can be achieved. As shown in more detail below, different parameters can be provided to the frequency sweep used for the switching frequency to optimize the operation.

[0020] The simultaneous implementation of the frequency sweep and the determination of an electrical characteristic permits a real-time assessment of the system. Accordingly, a fast and efficient reaction is provided.

[0021] In still a further aspect of the present disclosure, a wind turbine is provided. The wind turbine comprises a generator for generating electrical power and a power converter connected to the generator for converting the electrical power generated by the generator. Thepower converter comprises switches. The wind turbine also comprises a control unit for controlling the power converter. The control unit is configured for receiving an electrical characteristic of an electrical power downstream of the power converter. The electrical characteristic is indicative of electrical resonance. The control unit is also configured for defining a preferred switching frequency for switching the switches of the power converter at least partially based on the determined electrical characteristic. Furthermore, the control unit is configured for controlling the switches of the power converter according to the preferred switching frequency.

[0022] According to this aspect of the disclosure, a wind turbine with improved capabilities and, more particularly, with improved safety when operating disconnected from the utility grid is provided. Specifically, increased safety is achieved when operating is island mode, e.g. turbine island mode or string island mode, or during the synchronization phase during starting up of the wind turbine that is implemented previous to the connection to the utility grid.

[0023] The control unit is configured to implement a method according to the previously described aspects. Accordingly, the associated improvements and / or benefits, specifically in terms of safety and flexibility, are provided to the wind turbine. It is worth noting that, although the control unit is for controlling the power converter, this is not limited to a single controller, i.e. to a single isolated piece of equipment. The control unit can be a distributed control unit. Furthermore, the control unit can, in some cases, also be configured for controlling other components of the wind turbine.

[0024] Throughout this disclosure, the expression downstream of the power converter is understood as referring to any point that is electrically arranged after the power converter when seen from the point of view of the wind turbine generator, i.e. any point at which the power generated by the generated is already converted and being fed into the system. In particular, this point can range from a point at the output terminals of the power converter itself, or to any point in the corresponding collector cables of a collector grid.

[0025] Furthermore, it is understood that different names may be given to the electrical characteristic indicative of electrical resonance. In some cases, such an electrical characteristic may be identified as, e.g. a harmonic overvoltage or as a rms overvoltage. However, regardless of the specific name or nature of the determined characteristic, it is understood that the present disclosure refers to any electrical characteristic that can be associated with the phenomenon of electrical resonance downstream of the power converter.

[0026] Also, throughout this disclosure, the term switching frequency is used to refer to the rate at which the switches, e.g. MOSFET or IGBTS, of a power converter, are turned on andoff in a repetitive manner, i.e. to the number of times the switch transitions between on and off per unit of time. As known by the skilled person, such switching frequency impacts the performance of the power converter. Typical values for the switching frequency are dependent on the application. Hence, low-frequency power converters typically employ switching frequencies in the range from 1 kHz to 20kHz. However, higher switching frequencies, up to several MHz, can also be selected for high-frequency converters.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Non-limiting examples of the present disclosure will be described in the following, with reference to the drawings, in which:Figure 1 illustrates a perspective view of one example of a wind turbine;Figure 2 illustrates a simplified, internal view of one example of the nacelle of the wind turbine of the figure 1 ;Figure 3 schematically illustrates the main electrical components, including a power converter, of a wind turbine according to an example;Figure 4 schematically illustrates a power converter comprising a plurality of switches according to an example;Figure 5 schematically illustrates a wind farm according to an example;Figure 6 shows an example of a frequency response of an electrical network;Figure 7 shows a spectral analysis of different electrical characteristics according to an example;Figure 8 shows a flowchart of an example of a method for operating a wind turbine;Figure 9 shows a flowchart of another example of a method for operating a wind turbine; Figure 10 illustrates an implementation of an example of a method for operating a wind turbine according to the present disclosure; andFigure 11 illustrates another implementation of an example of a method for operating a wind turbine according to the present disclosure.DETAILED DESCRIPTION OF EXAMPLES

[0028] Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by wayof explanation only, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0029] Figure 1 is a perspective view of an example of a wind turbine 10. In the example, the wind turbine 10 is a horizontal-axis wind turbine. Alternatively, the wind turbine 10 may be a vertical-axis wind turbine. In the example, the wind turbine 10 includes a tower 15 that extends from a support system 14 on a ground 12, a nacelle 16 mounted on tower 15, and a rotor 18 that is coupled to nacelle 16. The rotor 18 includes a hub 20 and at least one rotor blade 22 coupled to and extending outward from the hub 20. In the example, the rotor 18 has three rotor blades 22. In an alternative embodiment, the rotor 18 includes more or less than three rotor blades 22. The tower 15 may be fabricated from tubular steel to define a cavity (not shown in Figure 1) between a support system 14 and the nacelle 16. In an alternative embodiment, the tower 15 is any suitable type of a tower having any suitable height. According to an alternative, the tower can be a hybrid tower comprising a portion made of concrete and a tubular steel portion. Also, the tower can be a partial or full lattice tower.

[0030] The rotor blades 22 are spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. The rotor blades 22 are mated to the hub 20 by coupling a blade root region 24 to the hub 20 at a plurality of load transfer regions 26. The load transfer regions 26 may have a hub load transfer region and a blade load transfer region (both not shown in Figure 1). Loads induced to the rotor blades 22 are transferred to the hub 20 via the load transfer regions 26.

[0031] In examples, the rotor blades 22 may have a length ranging from about 15 meters (m) to about 90 m or more. Rotor blades 22 may have any suitable length that enables the wind turbine 10 to function as described herein. For example, non-limiting examples of blade lengths include 20 m or less, 37 m, 48.7 m, 50.2m, 52.2 m or a length that is greater than 91 m. As wind strikes the rotor blades 22 from a wind direction 28, the rotor 18 is rotated about a rotor axis 30. As the rotor blades 22 are rotated and subjected to centrifugal forces, the rotor blades 22 are also subjected to various forces and moments. As such, the rotor blades 22 may deflect and / or rotate from a neutral, or non-deflected, position to a deflected position.

[0032] Moreover, a pitch angle of the rotor blades 22, e.g. an angle that determines an orientation of the rotor blades 22 with respect to the wind direction, may be changed by a pitchsystem 32 to control the load and power generated by the wind turbine 10 by adjusting an angular position of at least one rotor blade 22 relative to wind vectors. Pitch axes 34 of rotor blades 22 are shown. During operation of the wind turbine 10, the pitch system 32 may particularly change a pitch angle of the rotor blades 22 such that the angle of attack of (portions of) the rotor blades are reduced, which facilitates reducing a rotational speed and / or facilitates a stall of the rotor 18.

[0033] In the example, a blade pitch of each rotor blade 22 is controlled individually by a wind turbine controller 36 or by a pitch control system 80 (see Figure 2). Alternatively, the blade pitch for all rotor blades 22 may be controlled simultaneously by said control systems.

[0034] Further, in the example, as the wind direction 28 changes, a yaw direction of the nacelle 16 may be rotated about a yaw axis 38 to position the rotor blades 22 with respect to wind direction 28.

[0035] In the example, the wind turbine controller 36 is shown as being centralized within the nacelle 16, however, the wind turbine controller 36 may be a distributed control system throughout the wind turbine 10, on the support system 14, within a wind farm, and / or at a remote-control center. The wind turbine controller 36 may include one or more processors 40 configured to perform one or more of the steps of the methods described herein. Further, many of the other components described herein include one or more processors. The wind turbine controller 36 may also include a memory, e.g. one or more memory devices. As used herein, a memory may comprise memory element(s) including, but not limited to, a computer readable medium (e.g., random access memory (RAM)), a computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magnetooptical disk (MOD), a digital versatile disc (DVD) and / or other suitable memory elements.

[0036] Figure 2 is an enlarged sectional view of a portion of the wind turbine 10. In the example, the wind turbine 10 includes the nacelle 16 and the rotor 18 that is rotatably coupled to the nacelle 16. More specifically, the hub 20 of the rotor 18 is rotatably coupled to a generator 42 positioned within the nacelle 16 by the main shaft 44, a gearbox 46, a high-speed shaft 48, and a coupling 50. In the example, the main shaft 44 is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle 16. A rotation of the main shaft 44 drives the gearbox 46 that subsequently drives the high-speed shaft 48 by translating the relatively slow rotational movement of the rotor 18 and of the main shaft 44 into a relatively fast rotational movement of the high-speed shaft 48. The latter is connected to the generator 42 for generating electrical energy with the help of a coupling 50. Furthermore, a transformer 90 and / or suitable electronics, switches, and / or inverters may be arranged in the nacelle 16 in order to transform electrical energy generated by the generator 42 having a voltage betweene.g. 400V to 1000 V into electrical energy having medium voltage (e.g. 10 - 35 kV). Offshore wind turbines may have, for example, generator voltages between 650 V and 3500 V. Transformer voltages may for instance be between 30 kV and 70 kV, or even higher, e.g. 132 kV. In particular, higher voltages result in increased challenges when dealing with faults so faster protection requirements are needed. Said electrical energy is conducted via power cables from the nacelle 16 into the tower 15.

[0037] The gearbox 46, generator 42 and transformer 90 may be supported by a main support structure frame of the nacelle 16, optionally embodied as a main frame 52. The gearbox 46 may include a gearbox housing that is connected to the main frame 52 by one or more torque arms. In the example, the nacelle 16 also includes a forward support bearing 60 and an aft support bearing 62. Furthermore, the generator 42 can be mounted to the main frame 52 by decoupling support means 54, in particular in order to prevent vibrations of the generator 42 to be introduced into the main frame 52 and thereby causing a noise emission source.

[0038] Optionally, the main frame 52 is configured to carry the entire load caused by the weight of the rotor 18 and components of the nacelle 16 and by the wind and rotational loads, and furthermore, to introduce these loads into the tower 15 of the wind turbine 10. The main shaft 44, generator 42, gearbox 46, high-speed shaft 48, coupling 50, and any associated fastening, support, and / or securing device including, but not limited to, main frame 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0039] In some examples, the wind turbine may be a direct drive wind turbine without gearbox 46. Generator 42 operates at the same rotational speed as the rotor 18 in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox 46 for providing a similar amount of power than a wind turbine with a gearbox.

[0040] The nacelle 16 may also include a yaw drive mechanism 56 that may be used to rotate the nacelle 16 and thereby also the rotor 18 about the yaw axis 38 to control the perspective of the rotor blades 22 with respect to the wind direction 28.

[0041] For positioning the nacelle 16 appropriately with respect to the wind direction 28, the nacelle 16 may also include at least one meteorological measurement system which may include a wind vane and an anemometer. The meteorological measurement system 58 can provide information to the wind turbine controller 36 that may include wind direction 28 and / or wind speed.

[0042] In the example, the pitch system 32 (see Figure 1) is at least partially arranged as a pitch assembly 66 in the hub 20. The pitch assembly 66 includes one or more pitch drive systems 68 and at least one sensor 70. Each pitch drive system 68 is coupled to a respective rotor blade 22 (shown in figure 1) for modulating the pitch angle of a rotor blade 22 along the pitch axis 34. Only one of three pitch drive systems 68 is shown in figure 2.

[0043] In the example, the pitch assembly 66 includes at least one pitch bearing 72 coupled to hub 20 and to a respective rotor blade 22 (shown in figure 1 ) for rotating the respective rotor blade 22 about the pitch axis 34. The pitch drive system 68 includes a pitch drive motor 74, a pitch drive gearbox 76, and a pitch drive pinion 78. The pitch drive motor 74 is coupled to the pitch drive gearbox 76 such that the pitch drive motor 74 imparts mechanical force to the pitch drive gearbox 76. The pitch drive gearbox 76 is coupled to the pitch drive pinion 78 such that the pitch drive pinion 78 is rotated by the pitch drive gearbox 76. The pitch bearing 72 is coupled to pitch drive pinion 78 such that the rotation of the pitch drive pinion 78 causes a rotation of the pitch bearing 72.

[0044] Pitch drive system 68 is coupled to the wind turbine controller 36 for adjusting the pitch angle of a rotor blade 22 upon receipt of one or more signals from the wind turbine controller 36. In the example, the pitch drive motor 74 is any suitable motor driven by electrical power and / or a hydraulic system that enables pitch assembly 66 to function as described herein. Alternatively, the pitch assembly 66 may include any suitable structure, configuration, arrangement, and / or components such as, but not limited to, hydraulic cylinders, springs, and / or servomechanisms. In certain embodiments, the pitch drive motor 74 is driven by energy extracted from a rotational inertia of hub 20 and / or a stored energy source (not shown) that supplies energy to components of the wind turbine 10.

[0045] The pitch assembly 66 may also include one or more pitch control systems 80 for controlling the pitch drive system 68 according to control signals from the wind turbine controller 36, in case of specific prioritized situations and / or during rotor 18 overspeed. In the example, the pitch assembly 66 includes at least one pitch control system 80 communicatively coupled to a respective pitch drive system 68 for controlling pitch drive system 68 independently from the wind turbine controller 36. In the example, the pitch control system 80 is coupled to the pitch drive system 68 and to a sensor 70. During normal operation of the wind turbine 10, the wind turbine controller 36 may control the pitch drive system 68 to adjust a pitch angle of rotor blades 22.

[0046] According to an embodiment, a power generator 84, for example comprising a battery and electric capacitors, is arranged at or within the hub 20 and is coupled to the sensor 70, the pitch control system 80, and to the pitch drive system 68 to provide a source of powerto these components. In the example, the power generator 84 provides a continuing source of power to the pitch assembly 66 during operation of the wind turbine 10. In an alternative embodiment, power generator 84 provides power to the pitch assembly 66 only during an electrical power loss event of the wind turbine 10. The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine 10, and / or failure of the wind turbine controller 36. During the electrical power loss event, the power generator 84 operates to provide electrical power to the pitch assembly 66 such that pitch assembly 66 can operate during the electrical power loss event.

[0047] In the example, the pitch drive system 68, the sensor 70, the pitch control system 80, cables, and the power generator 84 are each positioned in a cavity 86 defined by an inner surface 88 of hub 20. In an alternative embodiment, said components are positioned with respect to an outer surface of hub 20 and may be coupled, directly or indirectly, to the outer surface.

[0048] As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein.

[0049] Figure 3 schematically illustrates the main electrical components of an example of a wind turbine 10, including a power converter 61. The connection of the wind turbine 10 to a utility grid 102 via a collector cable 107 is also schematically illustrated. In this example, the wind turbine 10 is a direct-drive wind turbine 10, i.e. the generator 42 is directly driven by the rotor 18. The generator 42 of the wind turbine 10 may be a permanent magnet generator comprising a generator rotor carrying a plurality of permanent magnets and a stator in some examples. In other examples, the generator 42 may be a squirrel caged induction generator, which may be connected to a power converter 61 providing full power conversion. In still other examples, the wind turbine 10 may comprise a geared wind turbine and the generator 42 may comprise a doubly-fed induction generator (DFIG).

[0050] The generator 42 generates AC (alternating current) power of variable frequency due to varying wind conditions. The power converter 61 may be provided for adjusting the power output from the generator 42 to one suitable for the utility grid 102, e.g. to an AC power having fixed frequency. The power converter 61 may comprise a machine-side converter 63, a line-side converter 65, and a DC (direct current) link 67 connecting the machine-side converter 63 and the line-side converter 65.

[0051] In the case of a direct-drive wind turbine 10, the stator of the generator 42 may be connected to the machine-side converter 63, which may be configured to convert the received AC voltage to DC voltage, the DC voltage being delivered to the DC-link 67. The line-side converter 65 may be configured to convert the DC voltage from the DC-link 67 into a fixed frequency AC voltage.

[0052] As also shown in Figure 3, the line-side converter 65 may be connected to a wind turbine transformer 90. The wind turbine transformer 90 may be configured to step-up the voltage delivered by the power converter 61 , e.g. to 33 kV. In some cases, the transformer 90 may be arranged in the nacelle 16, so that high voltage tower cables 103 may be arranged along the tower 15 of the wind turbine 10. In other examples, the transformer 90 may be arranged down-tower. A filter 95 may also be provided in the wind turbine 10, e.g. between the power converter 61 and the transformer 90. The filter 95 may be provided to reduce harmonics, i.e. switching harmonics arising from the switching operation of the switches of the power converter 61 or, more generally, to improve power quality. The filter 95 may comprise different types of filters such as L-filters comprising only inductances, or LC-filters combining inductances and capacitors.

[0053] An output terminal of the wind turbine 10 may comprise a wind turbine switch 108. As also shown in Figure 3, the wind turbine 10 may be connected to a collector cable 107, i.e. to a collector grid, by means of the wind turbine switch 108. Accordingly, the collector cable 107 is understood as an external conductor to the wind turbine 10, which is configured to collect the electrical power generated by the wind turbine 10. As shown in more detail below (see Figure 5), the collector cable 107 may be typically connected to additional wind turbines in standard applications, thus forming a collector or wind farm grid. Specifically, the wind turbine 10 may comprise a switchgear, which may comprise the wind turbine switch 108 for connecting the wind turbine 10 to the collector cable 107. The switchgear may also comprise additional switches for connecting the wind turbine 10 to other wind turbines 10 in the system. The wind turbine switch 108 may comprise a circuit breaker. Furthermore, the collector cable 107 may be connected to the utility grid 102 at a point of interconnection 109. In particular, as shown in Figure 3, a main switch 104 or wind farm switch may be provided between the collector cable 107, or collector grid, and the point of interconnection 109 to the utility grid 102.

[0054] In the scenario depicted in the example of Figure 3, both the wind turbine switch 108 and the main switch 104 are open. Accordingly, the wind turbine 10 is electrically disconnected from the collector cable 107 and, accordingly, also disconnected from the utility grid 102. In some other scenarios, the wind turbine switch 108 may be closed, in which case the wind turbine 10 would be electrically connected to the collector cable 107 and the poweroutputted by the wind turbine 10 may be received by the collector cable 107. In these examples, the main switch 104 may be open, so that the wind turbine 10 may remain disconnected from the utility grid 102. Consequently, in this example, the collector cable 107 may be present in the electrical network seen by the wind turbine 10.

[0055] In order to control operation of the power converter 61 , a power converter controller 611 may be provided. The power converter controller 611 may be configured to control the machine-side converter 63 by generating control pulses 631 for a plurality of converter switches of the machine-side converter 63. Similarly, the power converter controller 611 may be configured for controlling the line-side converter 65 by generating control pulses 651 for a plurality of converter switches of the line-side converter 65. Overall, the power converter 61 may comprise a plurality of switches, and the switches may be controlled by control pulses 631, 651, which may be generated by a power converter controller 611. Specifically, the switches of the power converter 61 may be controlled according to a switching pattern exhibiting a certain switching frequency, i.e. they may be controlled with a certain rate. Although dependent on the specific application, switching frequencies in the range of 1 kHz to 20 kHz may be typically employed for power conversion in wind turbines.

[0056] Figure 4 schematically illustrates a more detailed view of an example of a power converter 61. In particular, a three-phase back-to-back power converter 61 is illustrated in Figure 4. The power converter 61 comprises a machine-side converter 63, a DC-link 67, and a line-side converter 65 as described above. A plurality of switches 73, 75 are arranged in the machine-side converter 63 and in the line-side converter 65. Different types of switches 73, 75 may be envisaged. E.g., insulate-gate bipolar transistors (IGBTs) or other types of switches, such as silicon-carbide MOSFETs, may be employed. Furthermore, as depicted in Figure 4, freewheeling or anti-parallel diodes may also be provided with the switches 73, 75, thus forming so-called switching modules. Regarding the DC-link 67, a capacitor 77, or capacitor bank, may be provided as an energy storage element for DC-voltage stabilization between the machine-side converter 63 and the line-side converter 65.

[0057] Figure 5 schematically illustrates a wind farm 101 according to an example. The wind farm 101 may comprise a plurality of wind turbines 10 e.g., as those described above with reference to Figures 1 to 3. As with the case of the single wind turbine 10 depicted in Figure 3, the wind farm 101 may also be connected to a utility grid 102 at a point of interconnection 109. The wind farm 101 may comprise a substation 106 including e.g. a wind farm transformer configured for converting power from a wind farm voltage to a utility grid voltage, and a wind farm switch (not shown) equivalent to the main switch 104 depicted in Figure 3.

[0058] The wind turbines 10 of the wind farm 101 may be arranged in clusters, specifically in strings 105. In the example depicted in Figure 5, a wind farm 101 comprising three strings 105 is shown. Besides, a detail of one of the strings 105, comprising three wind turbines 10, is depicted. It is understood that, in other examples, a different number of strings 105 and a different number of wind turbines 10 per string 105 may be provided. Each string 105 may be connected to, or disconnected from, the substation 106 via a string switch 115. In some examples, the string switches 115 or breakers may be arranged at the substation 106.

[0059] As also depicted in Figure 5, the electrical power generated by each wind turbine 10 may be collected in a collector cable 107 which, in this example, may also be referred to as a string conductor. Each wind turbine 10 may be connected to, or disconnected from, a corresponding collector cable 107 via the corresponding wind turbine switch 108.

[0060] As mentioned above, a wind turbine 10 may operate in a power generating mode without being connected to the utility grid 102. As an example, the wind turbine 10 may be operated in a self-sustaining mode, or turbine island mode. Indeed, referring to Figure 3, the generator 42 may be operated to generate electrical power to feed electrical auxiliaries of the wind turbine 10 itself. To this end, the power converter 61 may be operated to convert the generated power. The wind turbine switch 108 may be opened, i.e. the wind turbine 10 may be isolated from the collector cable 107, such that it may not inject any power into the collector cable 107. In other examples, the wind turbine switch 108 may be closed whereas a main switch 104 may be open. In this manner, the power generated by the wind turbine 10 may be fed into the collector cable 107 or collector grid. In an example, additional loads may be connected to the collector cable 107.

[0061] Specifically, in examples like the one depicted in Figure 5, the wind turbine switch 108 of one or more of the wind turbines 10 of a string 105 may be closed, and corresponding generated power may be fed into the collector cable 107 which, in this example, may be referred to as a string conductor. In such examples, the power injected into the collector cable 107 may be used to feed the auxiliary loads of one or more of the remaining wind turbines 10. In this manner, a string island mode configuration may be implemented.

[0062] Furthermore, in a variant, at least one of the strings 105 may be connected to the substation 106, i.e. the corresponding switch 115 may be closed, while the wind farm 101 is disconnected from the utility grid 102, i.e. no power is injected into the utility grid 102 through the point of interconnection 109. Specifically, a main wind farm switch (not shown) in the substation 106 may be open. In such a variant, a wind turbine 10 of the string 105 connected to the substation 106 may be operated to generate power to feed, not only its own auxiliariesand, probably, the auxiliaries of other wind turbines 10 in the same string 105, but also auxiliary systems of the substation 106.

[0063] In still another scenario, a wind turbine 10 may operate disconnected from the utility grid 102 during a startup maneuver, specifically, during a pre-connection synchronization phase. Accordingly, before connecting with the utility grid 102, power may be generated with the generator 42 and the switches 75 of the line-side converter 65 of the power converter 61 may be switched to ramp up the output voltage in a controlled manner. The generated electrical power may then be synchronized with the utility grid 102 prior to enabling direct connection between the wind turbine 10 and the utility grid 102, e.g. by closing the main switch 104.

[0064] In all such examples comprising operation of the wind turbine 10 disconnected from the utility grid 102, oscillations may arise on, e.g. the different high voltage cables downstream of the transformer 90. These may include the high voltage tower cables 103, or the collector cable 107 when the wind turbine 10 is connected to the same via the corresponding wind turbine switch 108. Such oscillations may increase significantly, thus leading to damaging resonance effects, due to the absence of the damping provided by the large load of the utility grid 102. In other words, such scenarios may result in lightly loaded systems, i.e. systems connected to very low loads. In particular, high voltage cables employed in offshore wind farms may exhibit particularly high capacitance values, which may increase the likelihood of significant resonance phenomena between cable capacitance and system reactance (including transformer 90, cables, filters). Such resonance phenomena may lead to significant amplification of voltage in the system, e.g. in the collector cable 107 or in the substation 106 of a wind farm 101 in cases comprising connection of the wind turbine 10 to the substation 106 as described above. Consequently, the voltage can reach dangerous levels, which can result in failure of equipment and / or in fire events.

[0065] The system reactance and capacitance may be characterized as an LC tank. A frequency response of the system may be obtained, e.g. by simulation or experimentally, to analyze the behavior of the system in front of excitations of different frequency. In some cases, such a frequency response characterization may be carried out prior to the operation of the wind turbine 10. As an example, Figure 6 illustrates a Bode plot 300 of the frequency response for a system like the one depicted in Figure 3. Specifically, the frequency response is obtained with a configuration in which the wind turbine 10 is not connected to the collector cable 107, i.e. the wind turbine switch 108 is open. Correspondingly, the wind turbine 10 is also disconnected from the utility grid 102. It is understood that this frequency response is provided for illustrative purposes only. In practice, the specific behavior will be dependent on the reactance and capacitance of the specific components and cables employed and also on thenetwork configuration, i.e. on the load experienced by the wind turbine 10 while being disconnected from the utility grid 102. Hence, a different configuration may comprise a configuration in which the wind turbine 10 is connected to the collector cable 107, i.e. with the wind turbine switch 108 closed. In still a further configuration, the wind turbine 10 may be connected to a substation 106 of a wind farm 101. In each case, different LC tanks or LC networks may arise.

[0066] As shown in Figure 6, both the Bode magnitude plot 301 (upper figure) and the Bode phase plot 302 (lower figure) show a resonance at an excitation frequency ”fr”. Accordingly, in this example, this “fr” would correspond to a so-called resonance frequency, and an excitation signal with a frequency exhibiting such a value, or a value close to it, may induce significant oscillations. These oscillations may amplify and, eventually, damage the system. The specific value of “fr” may depend on the conditions of the system and it may be in the range of a few kHz.

[0067] As previously indicated, switching frequencies used for the switches 73, 75 of the power converter 61 may happen to be in such a range, i.e. a range of a few kHz. And this switching frequency may also be found in normal operation when connected to the utility grid. However, when operating in a relatively lightly loaded mode, i.e. disconnected from the utility grid 102, a resonance of the LC tank may arise depending on the precise switching frequency.

[0068] Furthermore, not only the fundamental switching frequency may induce such phenomena. Also harmonics of the switching frequency can exist in the system, and these may need to be considered as well. As an example, Figure 7 schematically depicts the spectral analysis 400 of different electrical signals obtained at different points of an electrical system like the one depicted in Figure 3 after the power converter 61. In particular, a switching pattern with a switching frequency ”fi” is used for the power converter 61 in this example.

[0069] The x axis of Figure 7 represents frequency, and the y axis represents the amplitude. In particular, the spectrum of the following electrical variables are represented (from top to bottom): voltage 401 at the output terminals of the power converter 61 , current 402 at the output terminals of the power converter 61, voltage 403 at the output terminal of the transformer 90, and voltage 404 at a location of the wind turbine switch 108, which is open in this case such that the wind turbine 10 is disconnected from the collector cable 107. As shown in all graphs, different frequency responses occur around the switching frequency fi, but also around the harmonics of this switching frequency, i.e. around fa and fs. If such frequency ranges (either of the fundamental or of the harmonics) are relatively close or overlap with a resonance frequency of the system, significant amplification of voltage oscillations may arise, thus leading to resonance in the system.

[0070] It is understood that, in other examples, the wind turbine switch 108 may be closed, such the wind turbine 10 may be electrically connected to the collector cable 107. In such case, the collector cable 107 may be present in the network as seen by the wind turbine 10 and the location of the resonance frequencies of the resulting network may change.

[0071] In an aspect of the present disclosure, a method 100 for operating a wind turbine 10 to address the risk described with reference to Figures 6 and 7 is provided. This method 100 is schematically illustrated in the flowchart of Figure 8. The wind turbine 10 has a generator 42 for generating power and a power converter 61 with switches 73, 75 for converting the generated power. The method 100 comprises, in a situation with the wind turbine 10 disconnected from a utility grid 102, generating power with the generator 42 in block 110. Block 120 comprises operating the switches 73, 75 to convert the generated power. Moreover, block 130 comprises determining an electrical characteristic indicative of electrical resonance at a point downstream of the power converter 61. A preferred switching frequency for operating the switches 73, 75 is defined at least partially based on the determined electrical characteristic in block 140. Then, block 150 comprises operating the switches 73, 75 with the preferred switching frequency.

[0072] The method 100 shown in the flowchart of Figure 8 provides increased flexibility and versality for operating a wind turbine 10 during periods in which it is disconnected from the utility grid 102. Hence, instead of using a fixed, i.e. nominal, or one or more predefined, switching frequency for the switches 73, 75 of the power converter 71, a varying switching frequency, which is adjusted depending on prevailing conditions is employed for the switching pattern.

[0073] In this manner, the previously mentioned resonance oscillations, which may arise in lightly loaded systems, i.e. in systems that are not connected to the large load given by the utility grid 102, are accounted for. Indeed, by shifting or adapting the switching frequency of the switches 73, 75, the resonance peak shown in Figure 6 can be effectively avoided. It is understood that the location of such resonance peak may be shifted depending on operational conditions. To this end, the method 100 includes, in block 130, a determination of the electrical characteristics downstream of the power converter 61 , i.e. after the electrical power generated by the generator 42 has been converted. In this manner, adynamic actuation is achieved which results in improved adaptation to varying conditions.

[0074] In examples of the present disclosure, defining the preferred switching frequency may comprise defining a switching frequency such that the electrical resonance is curtailed. Specifically, in a variant of these examples, an output terminal of the wind turbine 10 may be connected to a collector cable 107 through a wind turbine switch 108 and the electricalcharacteristic may be indicative of electric resonance in the collector cable 107. Furthermore, defining the preferred switching frequency may comprise defining a switching frequency such that electrical resonance in the collector cable 107 is restricted.

[0075] The collector cable 107 may comprise a high voltage (HV) cable, which may exhibit a high capacitance. This may be particularly the case for offshore installations. The collector cable 107 may be connected to an output terminal of the wind turbine 10 and, as the name indicates, it may be used to collect the power generated by the wind turbine 10. To this end, a wind turbine switch 108 may be provided to enable connection between an output of the wind turbine 10 and the collector cable 107. An increased risk of a detrimental resonance between the collector cable 107 capacitance and the system reactance may arise in offshore applications due to the mentioned high capacitance of the typically employed collector cables 107. Potential risks may be mitigated by, in block 130, determining an electrical characteristic that is indicative of such an increased oscillation in the collector cable 107. In this manner, effective and fast response may be provided in a dynamic fashion, so that electrical resonance phenomena may be avoided or, at least, kept under control. In order to determine the electrical characteristic indicative of increased oscillation in the collector cable, a sensor may be arranged at different locations such as, e.g. at a switchgear including the wind turbine switch 108.

[0076] In other examples, the wind turbine 10 may be provided in a wind farm 101 comprising a plurality of wind turbines 10, e.g. a wind farm 101 like the one described with reference to Figure 5. The wind turbine 10 may be connected to the collector cable 107 of one of the strings 105 and it may also be connected to the substation 106. In such case, the electrical characteristic downstream of the power converter 61 may also be indicative of an electrical resonance in the substation 106. Overall, the electrical characteristic may be indicative of an electrical resonance at any point of the collection system, i.e. the system comprising the collector cables 107 of the strings, as well as the substation 106. In order to determine the electrical characteristic indicative of electrical resonance at the substation 106, a sensor may be arranged at different locations such as, e.g. at a switchgear including the wind turbine switch 108.

[0077] In still other examples, the electrical characteristic downstream of the power converter 61 may be indicative of an electrical resonance in the high voltage tower cables 103 arranged right after the transformer 90 of the wind turbine 10. As in previous cases, different locations may be provided for a sensor to determine the electrical characteristics downstream of the power converter 61. In particular, a sensor may be provided in a switchgear comprising the wind turbine switch 108.

[0078] In order to determine the most appropriate switching frequency, an example of the disclosure may provide, in order to define the preferred switching frequency in block 140, operating the switches 73, 75 at different switching frequencies, and determining the electrical characteristic at the different switching frequencies.

[0079] According to this example, a direct analysis of the influence of the switching frequency on the performance or response of the system may be obtained. In particular, the switches 73, 75 of the power converter 61 may be operated at different switching frequencies within a certain predefined range and the response of the system may be obtained by assessing the value of a certain electrical characteristic at such frequencies. Moreover, by manipulating the different switching frequencies, different trends can be observed.

[0080] In particular, in a variant of this example, operating the switches 73, 75 at different switching frequencies may comprise performing a switching frequency sweep from a first frequency value to a second frequency value over a period of time. Different types of frequency sweeps may be implemented, such as linear or logarithmic sweeps. Moreover, by controlling not only the frequency range, but also the period of time for the frequency sweep, a more detailed analysis of the response may be achieved. In particular, the dynamics of the system may be considered by adjusting the ramp, i.e. the rate at which the switching frequency is varied.

[0081] Figure 9 shows a flowchart with another example of a method 200 for operating a wind turbine 10 in which such a switching frequency sweep is implemented. In particular, Figure 9 shows a method 200 for starting up a wind turbine 10. The wind turbine 10 comprises a generator 42 for generating power, and a power converter 61 connected to the generator 42. The power converter 61 comprises switches 73, 75 for converting the generated power. The method 200 comprises, in block, 210, generating electrical power with the generator 42. The method 200 also comprises operating the switches 73, 75 according to a frequency sweep while converting the generated power. Furthermore, block 230 comprises measuring an electrical characteristic of an electrical power downstream of the power converter 61 while sweeping the frequency of the switches 73, 75. Then, in block 240, the method 200 comprises defining a preferred switching frequency at least partially based on the measured electrical characteristic. The method 200 then comprises operating the switches 73, 75 of the power converter 61 at the preferred switching frequency.

[0082] According to this method 200, a safer starting up of the wind turbine 10 may be implemented. Hence, by taking into account the frequency response of the system during the startup, resonance phenomena can be accounted for. As shown in more detail below, the startup of the wind turbine 10 may be carried out for different purposes and in differentscenarios, e.g. in an island mode scenario or in a pre-connection grid synchronization scenario.

[0083] As an example, a frequency sweep with a period of time of less than 20 seconds, specifically less than 10 seconds, or more specifically less than 1 second, may be selected.. The duration of the frequency sweep may also be dependent on the intended application and on the expected severity of potential resonance effects.

[0084] In an example of the method 100 (or of the method 200), the electrical characteristic determined in block 130 (or 230) may comprise a voltage and / or a current value. Then, defining the preferred switching frequency in block 140 (or 240) may comprise picking a switching frequency at which the voltage and / or current value is less than a threshold. Hence, the definition of the switching frequency may not be based on any pre-defined value or on previously conducted theoretical analysis.

[0085] The threshold may be adjusted to a maximum permissible value for the specific electrical characteristic. In this manner, different switching frequencies may be employed, provided they do not lead to voltages and / or currents in the system, e.g. in the collector cable 107, exceeding dangerous values. In other words, the switching frequencies may be selected within a continuous range of switching frequencies, the range comprising safe switching frequencies. The selection of one specific switching frequency or another may depend on other considerations, such as conversion efficiency.

[0086] The voltage and / or current values may specifically correspond to a value of an amplitude of a voltage and / or current oscillation. Other magnitudes, such as the rms value of the detected voltage and / or current may also be used.

[0087] In still another example of the disclosure where the electrical characteristic may comprise a voltage and / or a current value, defining the preferred switching frequency may comprise finding the switching frequency at which the voltage and / or current value is minimum

[0088] Hence, in this example, a more optimum performance, from the point of view of mitigating potential resonance phenomena occurring while the wind turbine 10 is disconnected from the utility grid 102 may be achieved. Indeed, according to this example, the electrical characteristics may not only be kept within acceptable limits, but they may be effectively minimized. Accordingly, the potential risk of a sudden increase of the amplitude of the oscillations may be mitigated. Furthermore, by finding the switching frequency giving place to the lowest oscillations, a gentler operation of the overall system may be achieved.

[0089] In some examples of the present disclosure, the power converter 61 may be operated at an operating voltage value that is less than a rated voltage of the power converter61 until the preferred switching frequency is defined. In particular, the power converter 61 may be operated at a value of about 50% of the rated voltage or less than 50% of the rated voltage. In another example, an even lower operating voltage, e.g. about 10% or lower of the rated voltage of the power converter 61 , may be selected.

[0090] By operating at a reduced operating voltage while the wind turbine 10 is disconnected from the utility grid 102, the risk of over-excitation, which can lead to abnormally high voltage oscillations can be mitigated. In particular, resonance phenomena can build up in a short period of time, so a potential risk may exist even in the limited time required for defining the preferred switching frequency. For this reason, in order to mitigate such a risk, excitation of the system may be contained by limiting the operating power of the power converter 61. In this manner, the power injected into the system may be high enough to identify characteristics of the frequency response of the system but, at the same time, low enough to avoid any significant excitation even when operating at the resonance frequency for a certain period of time.

[0091] In a variant of this example, the method 100 may comprise, after having defined the preferred switching frequency for the switches 73, 75, ramping up an operating voltage of the power converter 61 to the rated voltage of the power converter 61. Hence, according to this variant, the power converter 61 may be allowed to operate at its nominal voltage, but only after ensuring that the switching frequency used for the switching pattern of the switches 73, 75 is safe. Indeed, by defining a switching frequency that is sufficiently apart from the values that are likely to induce some resonance phenomena, a normal operation of the power converter 61 can be established, thus improving performance, e.g. efficiency, of the power conversion.

[0092] As indicated, different electrical characteristics may be used to implement the method 100 in different examples of the disclosure. In particular, in an example, determining the electrical characteristic in block 130 of Figure 8 may comprise measuring with a sensor at an output terminal of the power converter 61.

[0093] In this example, current or voltage sensors may be provided at the output of the power converter 61. Furthermore, some power converters 61 may be already equipped with such sensors, i.e. for other purposes. In particular, high accuracy sensors may be integrated in the power converter 61 to measure the electrical values right at the output terminal of the same. Such integration may facilitate operation and control of the signals while reducing associated costs. Other types of sensors, for example a power sensor, such as a power analyzer, may be used in other examples.

[0094] In cases in which the wind turbine 10 comprises a transformer 90 connected to an output of the power converter 61 (see Figure 3), an example of the disclosure may comprise determining the electrical characteristic in block 130 of the method 100 by measuring with a sensor at an output of the transformer 90. As in the previously mentioned example, current and / or voltage sensors may be provided on the high voltage side of the transformer 90. By arranging the sensors in the transformer, an improved representation of the effects induced by the reactance of the transformer may be obtained. A power analyzer may also be provided in other variant of this example.

[0095] In other examples, determining the electrical characteristic in block 130 of the method 100 may comprise measuring with a sensor arranged at a wind turbine switch 108 or, more specifically, measuring with a sensor arranged at a switchgear comprising the wind turbine switch 108.

[0096] In still other examples, in which the wind turbine 10, i.e. an output terminal of the wind turbine 10, is connected to a collector cable 107 (as shown in Figures 3 or 5) through a wind turbine switch 108, determining the electrical characteristic may comprise measuring with a sensor at a location along the collector cable 107. In this example, a direct observation of the formation of oscillations in the collector cable 107 may be achieved. This may be particularly relevant when using high voltage cables in, e.g. offshore applications. Furthermore, a plurality of locations, e.g. a first location in the proximity of the wind turbine 10, and a second location in the proximity of a main switch 104 may be envisaged. As in previous cases, the sensors may comprise current, voltage, or power sensors. Specifically, in a variant of these examples, the electrical characteristic may comprise measuring with a sensor arranged at the switchgear comprising the wind turbine switch 108.

[0097] Also in examples comprising connection of a wind turbine 10 to a collector cable 107, and connection to other wind turbines 10 connected to the same collector cable 107, e.g. when operating in string island mode, determining the electrical characteristic downstream of the power converter 61 may comprise measuring, e.g. with a sensor, at one of the other wind turbines 10, i.e. at a neighboring wind turbine 10.

[0098] It is also understood that previously mentioned examples may be combined. Hence, in order to achieve an improved characterization of the frequency response of the system, a plurality of sensors may be distributed at different locations and in different components.

[0099] The method 100 described with reference to Figure 8 may be implemented in different scenarios, i.e. under different operating conditions. Hence, in an example of the disclosure, the method 100 may comprise feeding at least an auxiliary system of the windturbine 10 with the generated power while the wind turbine 10 is disconnected from the utility grid 102.

[0100] This scenario may correspond to the so-called turbine island mode (TIM). Hence, during normal operation, i.e. with the wind turbine 10 connected to the utility grid 102, feeding power to the auxiliary systems, e.g. cooling system, control units, etc. may comprise absorbing power from the utility grid 102. However, in cases when the wind turbine 10 is disconnected from the utility grid 102, e.g. due to maintenance or disruption in the utility grid 102, no power may be obtained from the utility grid 102 and the wind turbine 10 may be disconnected by opening at least one of the wind turbine switch 108 or the main switch 104 depicted in Figure 4 (and / or the string switch 115 in case a configuration like the one depicted in Figure 5 is considered).

[0101] Some of the auxiliary systems may need to be kept operational even while the wind turbine 10 is not operating normally, e.g. cooling / heating systems to prevent corrosion, illumination system for safety purposes, or yaw system for load mitigation. In order to feed those auxiliaries, the wind turbine 10 may operate in a controlled manner with a reduced power level. However, resonance phenomena may arise from such a lightly loaded situation. Accordingly, the method 100 according to the present disclosure may be implemented to prevent undesired oscillation.

[0102] Furthermore, as illustrated in, e.g. Figure 5, the wind turbine 10 may be connected to a collector cable 107 in a wind farm 101 and the method 100 may further comprise feeding at least another load connected to the collector cable 107 with the generated power. The at least another load may also be disconnected from the utility grid 102. Specifically, in an example, the at least another load may comprise another wind turbine 10.

[0103] In this case, the wind turbine 10 may not only generate power to feed its own auxiliary system, but also to feed other systems and, in some cases, to feed one or more additional wind turbines 10. The scenarios leading to such a situation are basically the same as those described above in regards to the turbine island mode. However, in this case, the risk of a potential resonance occurring at the collector cable 107 is further exacerbated. Furthermore, such resonance can have increasingly damaging effects by affecting other wind turbines 10 as well. Accordingly, the implementation of a method 100 for operating a wind turbine 10 according to the present disclosure may become especially relevant.

[0104] In a variant of this example, the wind turbine 10 may be provided in a string 105 that may be connected to the substation 106, i.e. the corresponding switch 115 may be closed.Accordingly, the at least another load may comprise not only another wind turbine 10, but also auxiliary components of the substation 106.

[0105] In still other examples, a plurality of wind turbines 10 connected to the same collector cable 107 may cooperate to inject power into the system. In such a case, a method 100 like the one described with reference to Figure 8 (or a method 200 like the one described with reference to Figure 9) may be implemented for each of the wind turbines 10 in the plurality of wind turbines 10 so as to prevent the excitation of the system by each of the wind turbines 10.

[0106] Apart from island modes of operation, other situations may also pose a risk for the safety of the system due to the operation of a wind turbine 10 in a lightly loaded condition. Hence, in another example of the disclosure, the method 100 may comprise synchronizing the electrical power converted by the power converter 91 with the utility grid 102 while the wind turbine 10 is disconnected from the utility grid 102 and, subsequently, electrically connecting the wind turbine 10 with the utility grid 102. Indeed, during the startup sequence of a wind turbine 10, a pre-connection synchronization phase may be typically required. Specifically, in an example, an output terminal of the wind turbine 10 may be connected to a collector cable 107 through a wind turbine switch 108 and the collector cable 107 may be connectable to the utility grid 102 through at least a further switch 104, 115. The at least further switch 104, 115 may be open during the synchronization and it may be closed after synchronization for electrically connecting the wind turbine 10 with the utility grid 102.

[0107] The grid synchronization phase is needed so that the power injected by the wind turbine 10 has the same frequency and phase as the signal in the utility grid. Otherwise, serious consequences for both the utility grid 102 and the wind turbine 10 may arise such as, e.g. grid instability, current surges, or reverse power flows. However, during such grid synchronization, the wind turbine 10 is obviously not yet connected to the high load provided by the utility grid 102. Accordingly, the above-mentioned resonance phenomena can arise if the switching frequency of the switches 73, 75 of the power converter 61 lies proximate to a resonance frequency of the system. In order to address this potential risk, a method 100 (or method 200) according to the present disclosure may also be implemented during startup of the wind turbine 10.

[0108] It is understood that, in different variants of this example, and depending on, e.g. the configuration of the wind farm 101 , different switches may be present in the system. Hence, in an example, the wind turbine 10 may be connected to the collector cable 107 with the wind turbine switch 108 closed, whereas the main switch 104 may be closed only after grid synchronization. Similarly, in the example of Figure 5, the wind turbine switches 108 may beclosed, such that the wind turbines 10 may be electrically connected to the collector cable 107 which, in this case, would constitute a collector grid. Then, the string switch 115 and the wind farm switch (not shown) may be the switches that are only closed after grid synchronization. In still other examples, the wind turbine switch 108 may be open during the synchronization phase, such that connection to the collector cable 107 (and from here to the utility grid 102 via the corresponding switches), may only occur after synchronization.

[0109] In a variant of examples comprising such a pre-connection or synchronization phase, the method 100 may further comprise shifting the switching frequency of the switches 73, 75 from the preferred switching frequency to a nominal switching frequency after electrically connecting the wind turbine 10 to the utility grid 102, i.e. after closing the switches after synchronization. Indeed, even if a certain preferred switching frequency may be desired during disconnected operation, i.e. while the wind turbine 10 is not connected to the utility grid 102, this may no longer be necessary after synchronization and grid connection. In particular, the utility grid 102 comprises a very large load and, accordingly, potential resonance effects, which may have a significant relevance while isolated from the utility grid 102, may not pose any significant risk. On the contrary, by operating at a frequency different from the nominal switching frequency of the power converter 61, lower than expected performance may be achieved. As an example, efficiency of the power converter 61 may be reduced due to higher than expect switch losses. For this reason, in this example, the switching frequency may be finally adjusted to the nominal value.

[0110] In some examples of the disclosure, the method 100 may be carried out each time the wind turbine 10 is started. More particularly, the definition of the preferred switching frequency at least partially based on the determined electrical characteristic may be carried out each time the wind turbine 10 is started. In this manner, any differences that may arise from one moment to another may be taken into account. The frequency response of the system is dependent on real-time characteristics of the system which may change dynamically. Specifically, different aspects, such as the configuration of the system, e.g. the loads connected to the respective wind turbine 10, the prevailing conditions, or the aging of the different components (cables, transformer, etc.), may evolve over time. Accordingly, by implementing the method each time the wind turbine 10 is started, a proper characterization may be achieved and increased overall safety may be accomplished. To this end, the method 100 may be carried out in a relatively short period of time. Hence, a frequency sweep with a short duration of, e.g. of 0.1 seconds, may be implemented to scan the switching frequency and to characterize the system over the range of interest. Subsequently, the defined switchingfrequency may be employed to complete the startup sequence, e.g. including synchronization with the utility grid 102.

[0111] Similarly, the method 100 may be implemented every time an island mode of operation, either at a wind turbine level or at, e.g. a string level, is implemented in the wind turbine 10.

[0112] In still other examples, the method 100 may comprise that defining the preferred switching frequency at least partially based on the determined electrical characteristic is carried out on a periodic basis. In particular, in order to account for long-term aging effects, a periodic assessment may be carried out, e.g. on a monthly basis.

[0113] Alternatively, the assessment may be carried in a proactive manner any time an electrical network reconfiguration occurs. Such network reconfiguration may result in a modification of the load connected to the wind turbine 10. This may be the case if, for instance, the wind turbine 10 is arranged in a wind farm 101 comprising multiple strings 105 and one or more of these strings 105 are disconnected (or if a new string is added). In such cases, the resonant frequency may change and, accordingly, the preferred switching frequency may also be favorably adjusted.

[0114] Figure 10 illustrates the results 500 obtained while implementing a method according to the present disclosure. In particular, in the example depicted in Figure 10, a system like the one described with reference to Figure 3 is provided. Hence, a wind turbine 10 is disconnected from a collector cable 107, i.e. the wind turbine switch 108 is open. From top to bottom, the different graphs in Figure 5 show: the voltage signals 501 obtained at the output terminal of the power converter 61 , current signals 502 obtained at the output of the power converter 61, the voltage signals 503 obtained at the output of the transformer 90, and the voltage signals 504 obtained at a location of the wind turbine switch 108. Furthermore, the bottom graph shows the evolution of the switching frequency used for the switches 75 of the line-side converter 65. In this case, a frequency sweep 505 is implemented. Specifically, as shown in Figure 10, the switching frequency is changed from a first value, fstart, to a second value, fend- In this example, the frequency is ramped up for a period of 0.1 seconds. Later on, as shown in the curve, the frequency is ramped down. The process can be repeated as also illustrated in the example of Figure 10.

[0115] In this example, four different electrical characteristics 501, 502, 503, 504 are determined, e.g. in block 130, while varying the switching frequency. A clear oscillation is detected for the current signals 502 at the output terminal of the power converter, the voltage signals 503 at the output of the transformer, or the voltage signals 504 at the wind turbineswitch 108. Accordingly, based on this frequency response, the adequate switching frequency for the switches 73, 75 can be defined. Specifically, it is clear that maximum resonance behavior is observed at frequencies around fsi. Consequently, such switching frequency is to be avoided. On the contrary, very low amplitudes, indicative of very low or non-significant resonance, is found for frequencies in the range of fS2. Accordingly, while implementing block 140 of method 100 depicted in Figure 8, a preferred switching frequency in the range of fS2 may be defined under this scenario. Based on this, operation of the switches 73, 75 may be carried out at such preferred switching frequency fS2.

[0116] Figure 11 illustrates the results 600 obtained while simulating a method 100 according to the present disclosure in another scenario. In a similar manner to Figure 10, Figure 11 also shows, from top to bottom: the voltage signals 601 obtained at the output terminal of power converter 61 , current signals 602 obtained at the output terminal of the power converter 61, the voltage signals 603 obtained at the output of the transformer 90, and the voltage signals 604 obtained at the location of the wind turbine switch 108. Furthermore, the bottom graph shows also in this case the evolution of the switching frequency used for the switches 75 of the line-side converter 65. A frequency sweep 605, which has the same parameters (fstart to fend, in 0.1 seconds) as the one described with reference to Figure 10, is also implemented.

[0117] In the scenario considered in Figure 11, the wind turbine 10 is initially connected only to a first collector cable 107, i.e. with the wind turbine switch 108 closed, until a time of t=0.2 seconds. At t=0.2 seconds, a switch connecting the first collector cable 107 to a second collector cable (not shown in Figure 3) is closed. In any case, the wind turbine 10 remains disconnected from the utility grid 102. Hence, in this example, the utility grid 102 may be connectable to an end of the second collector cable by means of a switch which, in the situation depicted in Figure 11 , would be open (in a similar manner as the main switch 104 in Figure 3). In this simulation, the second collector cable is particularly long, e.g. with a length of about 2 kilometers, so a significant impedance is added to the wind turbine 10 at t = 0.2 seconds. Consequently, the frequency response of the system is expected to change due to the reconfiguration in the network.

[0118] Indeed, the evolution of the different signals, especially of the current signals 602 measured at the output terminal of the power converter 61 , clearly show a different frequency response after connecting the second cable, i.e. after increasing the impedance seen by the wind turbine 10. Accordingly, a different range of switching frequencies is now seen as giving place to relatively high amplitudes. However, in this example, the same range of switching frequencies that provides low resonance for the system with just the first collector cable 107,i.e. a range of frequencies of about fS3, is also favorable for the new configuration. Hence, minimal or no oscillations are present after connecting the 2 km long second collector cable when operating with a switching frequency of fS3- It is understood that the example shown in Figure 11 is provided only for illustrative purposes. In particular, in other examples, the alteration of the frequency response given by the modification of the impedance may indeed result in a need to adapt the switching frequency of the switches 73, 75 so as to keep resonance oscillations under control.

[0119] In some other examples of the disclosure, specific actions may be carried out to selectively reduce certain frequency harmonics. Accordingly, apart from defining a preferred switching frequency for the switching pattern used for the switches 73, 75, either hardware, e.g. arrangement of the windings of the transformer, or control features may be introduced to reduce harmonics in a selective manner. In these examples, a further improved system may be provided.

[0120] In another aspect of the present disclosure, a wind turbine 10 is provided. The wind turbine comprises a generator 42 for generating electrical power, and a power converter 61 connected to the generator 42 for converting the electrical power generated by the generator 42. The power converter 61 comprising switches 73, 75. Furthermore, the wind turbine 10 comprises a control unit 36, 611 for controlling the power converter 61. The control unit 36, 611 is configured for: receiving an electrical characteristic of an electrical power downstream of the power converter 61 , defining a preferred switching frequency for switching the switches 73, 75 of the power converter 61 at least partially based on the determined electrical characteristic, and controlling the switches 73, 75 of the power converter 61 according to the preferred switching frequency.

[0121] According to this aspect of the disclosure, a wind turbine 10 with improved capabilities is provided. In particular, a wind turbine 10 capable of implementing the abovedescribed methods is obtained. Consequently, the operation of the wind turbine 10 in conditions wherein connection to the utility grid 102 is not present can be carried out in a safer and more reliable manner.

[0122] Furthermore, it is worth noting that the control unit 36, 611 may be embodied in different forms. In some examples, the control unit may comprise a dedicated power converter controller 611 as schematically depicted in Figure 4. In other examples, the control unit according to this aspect of the disclosure may be implemented in the wind turbine controller 36. Moreover, the control unit may be a distributed system. Accordingly, different physical controllers may be provided to implement the mentioned functionality.

[0123] In an example, the wind turbine 10 may be configured for connection to a high voltage collector cable 107. In particular, an output terminal of the wind turbine 10 may be connected to the high voltage collector cable 107 by means of a wind turbine switch 108. Connection to high voltage cables may be particularly prone to resonance oscillations when the wind turbine 10 is not connected to the utility grid 102. Consequently, the provision of a control unit 36, 611 configured to operate as mentioned above may be particularly relevant in those cases comprising connection to collector cables with significantly high capacitance, such as high voltage cables typically used in offshore wind farms.

[0124] Furthermore, in examples of the disclosure, a wind farm 101 may be provided. The wind farm may comprise a high voltage collector cable 107 and a plurality of wind turbines 10 may be connected to the high voltage collector cable 107. At least one of the wind turbines 10 may comprise a control unit as described above.

[0125] This written description uses examples to disclose a teaching, including the preferred embodiments, and also to enable any person skilled in the art to put the teaching into practice, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspects, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments and techniques in accordance with principles of this application. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

CLAIMS1. A method (100) for operating a wind turbine (10), the wind turbine (10) having a generator (42) for generating power and a power converter (61) with switches (73, 75) for converting the generated power;wherein the method (100) comprises, in a situation with the wind turbine (10) disconnected from a utility grid (102):generating power with the generator (42);operating the switches (73, 75) to convert the generated power;determining an electrical characteristic indicative of an electrical resonance at a point downstream of the power converter (61);defining, at least partially based on the determined electrical characteristic, a preferred switching frequency for operating the switches (73, 75); andoperating the switches (73, 75) with the preferred switching frequency.

2. The method (100) of claim 1, wherein defining the preferred switching frequency comprises defining a switching frequency such that the electrical resonance is curtailed.

3. The method (100) of claim 2, wherein an output terminal of the wind turbine (10) is connected to a collector cable (107) through a wind turbine switch (108) and the electrical characteristic is indicative of electric resonance in the collector cable (107).

4. The method (100) of any of claims 1 to 3, wherein defining the preferred switching frequency comprises operating the switches (73, 75) at different switching frequencies, and determining the electrical characteristic at the different switching frequencies.

5. The method (100) of claim 4, wherein operating the switches (73, 75) at different switching frequencies comprises performing a switching frequency sweep from a first frequency value to a second frequency value over a period of time.

6. The method (100) of any of claims 4 or 5, wherein the electrical characteristic comprises a voltage and / or a current value, and wherein defining the preferred switching frequency comprises picking a switching frequency at which the voltage and / or current value is less than a threshold.

7. The method (100) of any of claims 4 to 6, wherein the electrical characteristic comprises a voltage and / or a current value, and wherein defining the preferred switching frequency comprises finding the switching frequency at which the voltage and / or current value is minimum.

8. The method (100) of any previous claim, wherein the power converter (61 ) is operated at an operating voltage value that is less than a rated voltage of the power converter (61 ) until the preferred switching frequency is defined, specifically wherein an operating voltage of the power converter (61) is ramped up to the rated voltage of the power converter (61) after the preferred switching frequency is defined.

9. The method (100) of any previous claim, wherein determining the electrical characteristic comprises measuring with a sensor at an output terminal of the power converter (61).

10. The method (100) of any previous claim, wherein the wind turbine (10) comprises a transformer (90) connected to an output of the power converter (61), and wherein determining the electrical characteristic comprises measuring with a sensor at an output of the transformer (90).

11. The method (100) of any previous claim, wherein an output terminal of the wind turbine (10) is connected to a collector cable (107) through a wind turbine switch (108), and determining the electrical characteristic comprises measuring with a sensor at a location of the wind turbine switch (108).

12. The method (100) of any previous claim, wherein an output terminal of the wind turbine (10) is connected to a collector cable (107) through a wind turbine switch (108), the method further comprising feeding at least another load connected to the collector cable (107) with thegenerated power, the at least another load being also disconnected from the utility grid (102), specifically wherein the at least another load comprises another wind turbine (10).

13. The method (10) of any previous claim, comprising synchronizing the electrical power converted by the power converter (61) with the utility grid (102), and subsequently electrically connecting the wind turbine (10) with the utility grid (102).

14. The method (100) of claim 13, comprising varying the switching frequency of the switches (73, 75) from the preferred switching frequency to a nominal switching frequency after electrically connecting the wind turbine (10) with the utility grid (102).

15. A wind turbine (10) comprising:a generator (42) for generating electrical power;a power converter (61) connected to the generator (42) for converting the electrical power generated by the generator (42), the power converter (61) comprising switches (73, 75), and a control unit (36, 611 ) for controlling the power converter (61 ), the control unit (36, 611 ) being configured for:receiving an electrical characteristic of an electrical power indicative of an electrical resonance at a point downstream of the power converter (61);defining a preferred switching frequency for switching the switches (73, 75) of the power converter (61) at least partially based on the determined electrical characteristic; and controlling the switches (73, 75) of the power converter (61) according to the preferred switching frequency.

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