Wind turbines and methods

Abstract

The present disclosure relates to wind turbines (10) and methods (100) for operating wind turbines (10). A wind turbine (10) comprises a power converter (104), a main transformer (105), an auxiliary transformer (107), one or more auxiliary systems (108) and a plurality of power cables (109). The power cables (109) are connected to an output side of the main transformer (105). The power converter (104) is configured to operate with a pulse pattern for switching the switches of a line-side converter of the power converter (104) during island mode different than a pulse pattern to be used during normal operation of the wind turbine (10), the pulse pattern to be used during island mode being configured to avoid electrical resonance in the wind turbine (10).

Description

WIND TURBINES AND METHODS

[0001] The present disclosure relates to wind turbines, in particular to wind turbines configured to operate in island mode, and methods for operating wind turbines.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 to 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] While a wind turbine is disconnected from the electrical grid (utility grid), some relevant auxiliary systems, e.g. communication systems, ventilation and temperature regulation systems, pitch and yaw control systems and other, may keep operating until electric power provided by auxiliary power sources such as battery-based systems, supercapacitors such as uninterruptible power supplies (UPSs), or diesel generators run out of power as well. Solar panels may also be used to provide auxiliary power.

[0005] Another way to obtain auxiliary power may include operating the wind turbine in order to generate a small amount of electric power and supplying the generated electric power to one or more auxiliary systems. Such a self-sustaining mode may be referred to as “island mode”. In island mode, the wind turbine is disconnected from the electrical grid, and it may be operated to supply power to auxiliary systems. These auxiliary systems may be systems thatbelong to the wind turbine generating power or may even be systems that belong to other wind turbines or are located elsewhere in the wind farm.

[0006] A wind turbine may be in such an island mode until grid connection is regained, provided that an amount of auxiliary power available is actually sufficient for restarting normal operation.

[0007] The wind turbines of the wind farm may be electrically connected together to form a collector or wind farm grid. Each wind turbine in the wind farm grid may comprise its own transformer, usually referred to as “main transformer”. The main transformer steps up the voltage output from the wind turbine generator, e.g. from a few hundred volts, to a higher level suitable for supplying the wind farm grid.

[0008] Each wind turbine may also include a switchgear. Usually, a plurality of tower cables connects the main transformer and the switchgear. The tower cables may be connected to a surge arrester for protecting the electrical equipment inside the wind turbine by discharging surge currents, e.g. due to lightning. The switchgear may comprise a circuit breaker for protecting the electrical equipment inside the wind turbine in the event of a fault condition. The switchgear may also allow the wind turbine to be disconnected from the wind farm grid for maintenance purposes or for example during island mode.

[0009] However, it has been found that occasionally surges occur in the power cables. High voltage (HV) equipment, i.e. equipment at a high voltage side or region of the wind turbine, e.g. at and beyond the secondary side of the main transformer, may therefore be damaged. For example, one or more of the power cables, the HV windings of the main transformer or the surge arrester may be damaged during island mode. As electrical equipment of the wind turbine, for example its auxiliary systems, are connected to the main transformer, this equipment may also be damaged, or have a risk of being damaged, during island mode.

[0010] The present disclosure aims to avoid, or reducing the risk of, damage to the electrical equipment, in particular the HV equipment, of a wind turbine during island mode.SUMMARY

[0011] In an aspect of the present disclosure, a wind turbine is provided. The wind turbine comprises an electric generator configured to generate AC power and a power converter connected to the electric generator. The power converter comprises a machine-side converter configured to convert AC power to DC power, a DC-link, and a line-side converter configured to convert DC power to AC power. The line-side converter comprises a plurality of switches forconverting DC power to AC power. The wind turbine further comprises a main transformer and an auxiliary transformer connected to the power converter, one or more auxiliary systems connected to the auxiliary transformer, and a plurality of power cables connecting an output side of the main transformer with an electrical grid. The power converter is configured to operate with a first pulse pattern for switching the switches during normal operation of the wind turbine and the power converter is also configured to operate with a second pulse pattern different from the first pulse pattern for switching the switches during operation of the wind turbine in island mode. The second pulse pattern is configured to avoid electrical resonance in the wind turbine, optionally in the plurality of power cables.

[0012] It was surprisingly found that damage that occurred in the wind turbines (power cables and / or auxiliary components of the wind turbine) was mainly caused by resonance in the plurality of power cables. The resonance in the power cables was caused by harmonics of the pulse pattern employed during island mode. Surprisingly, the main transformer did not damp or avoid these resonances. In some cases, damage might additionally or alternatively occur due to resonance in e.g. the medium voltage (MV) power cables connecting the power converter, the main transformer and the auxiliary transformer, or the low voltage (LV) power cables connecting the auxiliary transformer and the one or more auxiliary systems.

[0013] According to this aspect, the wind turbine is now configured to operate with at least two different pulse patterns for switching the switches of the line-side converter of the power converter: one for normal operation and one for island mode of operation. Using a specific pulse pattern for island mode may allow to avoid or reduce electric resonance in e.g. the power cables connected to the output side of the main transformer, and therefore avoid or reduce damage to the high voltage equipment of the wind turbine, e.g. to the HV windings of the main transformer, the surge arrester and the power cables, during operation in island mode. Damage to the auxiliary systems may also be avoided or reduced during operation in island mode.

[0014] Throughout this disclosure, auxiliary systems are systems that support the wind turbine in operation, and in particular may refer to wind turbine systems or devices which should still be powered or are preferably also powered if the electrical grid is not available. For example, it may be necessary to maintain the communications systems and the temperature regulation and ventilation systems of the wind turbine in operation when the grid is unavailable.

[0015] Throughout this disclosure, it may be understood that a wind turbine is in operation (“normal operation”) when its rotor is driven by the available wind and rotating at a speed that is high enough to produce power, the electrical grid is available, and the generator of the wind turbine is producing electric power which is transferred into the electrical grid. The term “normaloperation” may be used herein to explicitly refer to such a situation and explicitly differentiate it from operation of a wind turbine in for example an island mode.

[0016] Throughout this disclosure, the term "island mode of operation" may refer to a mode of operation of a wind turbine in which the wind turbine is not supplying or obtaining electric power from the grid and the wind turbine is configured to operate independently from the electrical grid. In this mode, electric power may be obtained from rotation of the wind turbine rotor, and supplying power to auxiliary components of the wind turbine, particularly critical electric systems, e.g. one or more of communication systems, temperature and ventilation regulation systems, bearing lubrication systems, controller systems and navigation lights, operative.

[0017] When reference is made throughout this disclosure to the fact that a wind turbine is (electrically) disconnected from the grid, it may be understood that a wind turbine is not supplying or obtaining electric power to / from the grid. It may refer to a condition in which the wind turbine is electrically disconnected from the grid, either because a circuit breaker is opened or otherwise. It may refer to a condition in which the wind turbine has not operated yet, e.g. during erection or commissioning of a wind farm. It may also refer to a condition in which no grid is available, for example during installation and commissioning of a wind farm no grid may even exist yet.

[0018] In a further aspect of the present disclosure, a method for operating a wind turbine as described herein is provided. The method comprises determining that the electrical grid is not available, starting to operate the wind turbine in island mode, and switching the switches with the second pulse pattern during operation of the wind turbine in island mode such that electrical resonance in the wind turbine, e.g. in the plurality of power cables of the tower, is avoided. For example, if electrical resonance is present, it is eliminated or at least reduced.

[0019] In yet a further aspect of the present disclosure, an offshore wind turbine is provided. The wind turbine comprises an electric generator configured to generate an AC power signal and a power converter connected to the electric generator. The power converter comprises a machine-side converter configured to convert an AC power signal to a DC power signal, a Delink, and a line-side converter configured to convert a DC power signal to an AC power signal. The line-side converter comprises an electric circuit including a plurality of switches for converting a DC power signal to an AC power signal. The wind turbine further comprises an auxiliary transformer connected to the power converter, one or more auxiliary systems connected to the auxiliary transformer, an input side of a main transformer connected to the power converter, and a plurality of tower cables configured to carry an AC power signal connected to an output side of the main transformer. The power converter is configured tooperate with a first predefined pulse pattern for switching the switches during normal operation of the wind turbine and is configured to operate with a second predefined pulse pattern different from the first pulse pattern for switching the switches during operation of the wind turbine in island mode for avoiding, e.g. eliminating, reducing or preventing the appearance of, resonance in the tower cables in island mode.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 illustrates a perspective view of one example of a wind turbine;

[0021] Figure 2 illustrates a simplified, internal view of one example of the nacelle of the wind turbine of the figure 1 ;

[0022] Figure 3 schematically illustrates a wind turbine connected to an electrical grid and to one or more auxiliary power sources;

[0023] Figure 4 schematically illustrates an example of harmonics of an AC signal generated by a power converter of a wind turbine;

[0024] Figure 5 schematically illustrates an example of impedance for a power cable; and

[0025] Figure 6 shows a flow chart of an example of a method for operating a wind turbine.DETAILED DESCRIPTION OF EXAMPLES

[0026] 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 way of 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.

[0027] 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 rotatable hub 20 and at least one rotor blade 22 coupled to and extending outward from the hub 20. In the example, the rotor 18has 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.

[0028] 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.

[0029] 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.

[0030] 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 pitch system 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.

[0031] 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. Alternatively, the blade pitch for all rotor blades 22 may be controlled simultaneously by said control systems.

[0032] 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.

[0033] 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 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.

[0034] 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 an electric 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 between e.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, and transformer voltages may for instance be between 30 kV and 70 kV. Said electrical energy is conducted via power cables from the nacelle 16 into the tower 15.

[0035] In some examples, the wind turbine 10 may include one or more shaft sensors 51. The shaft sensors may be configured to monitor at least one of torque loads acting on the main shaft 44 and / or the high-speed shaft 48, and a rotational speed of the shaft 44, 48. In some examples, the wind turbine 10 may include one or more generator sensors 53. The generator sensors may be configured to monitor at least one of a rotational speed of the generator 42and a generator torque. Shaft sensors 51 and / or generator sensors 53 may include, for instance, one or more torque sensors (e.g., strain gauges or pressure sensors), optical sensors, accelerometers, magnetic sensors, speed sensors and Micro-Inertial Measurement Units (MIMUs).

[0036] 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 103. In the example, the nacelle 16 also includes a main forward support bearing 60 and a main 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.

[0037] 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 rotor 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, support 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] In the example, the pitch system 32 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.

[0042] 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.

[0043] 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 electric 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.

[0044] 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.

[0045] 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 power to 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 source 84 provides power to the pitch assembly 66 only during an electricpower loss event of the wind turbine 10. The electric 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 electric power loss event, in some examples the power generator 84 operates to provide electric power to the pitch assembly 66 such that pitch assembly 66 can operate during the electric power loss event.

[0046] In the example, the pitch drive system 68, the sensor 70, the pitch control system 80, cables, and the power source 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.

[0047] 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.

[0048] A wind turbine is provided according to an aspect of the present disclosure. Figure 3 schematically illustrates an example of a wind turbine 10 of a wind farm, e.g. an offshore wind turbine. Method 100 referred to below may for example be implemented in the wind turbine of this example.

[0049] The wind turbine 10 comprises an electric generator 42 configured to generate AC (alternating current) power and a power converter 104 connected to the electric generator 42. The power converter 104 comprises a machine-side converter configured to convert the AC power coming from the generator 42 to DC (direct current) power, a DC-link, and a line-side converter configured to convert DC power to AC power. The line-side converter comprises a plurality of switches for converting DC power to AC power.

[0050] The wind turbine further comprises a main transformer 105 and an auxiliary transformer 107 connected to the power converter 104. The wind turbine further comprises one or more auxiliary systems or components 108 connected to the auxiliary transformer and a plurality of power cables 109 connecting an output side of the main transformer 105 with an electrical grid. If the main transformer 105 is arranged in the nacelle 16, the power cables 109 may be tower cables and extend from the nacelle and down the wind turbine tower.

[0051] The electrical grid to which the cables are connected may particularly be a wind farm internal grid e.g. wind farm busbar 101. In other examples, the electrical grid may be the utility grid.

[0052] The power converter 104 is configured to operate with a first pulse pattern forswitching the switches during normal operation of the wind turbine. The power converter 104 is configured to operate with a second pulse pattern different from the first pulse pattern for switching the switches during operation of the wind turbine in island mode.

[0053] The second pulse pattern may be configured to avoid electrical resonance in the wind turbine, and specifically to avoid resonance in the power cables leading to the electrical grid. In this respect, the second pulse pattern may generate more electrical losses than other optimized pulse patterns, but the second pulse pattern may e.g. avoid resonance in the power cables, and thereby reduces wear and can avoid damage to different components, in particular HV components of the wind turbine.

[0054] In general, the wind turbine 10 is connected to a wind farm busbar 101 , and then connected to the public electrical grid 102 by a first wind farm switch 103. All the wind turbines of the wind park (not shown) are connected to the busbar 101 of the wind farm. The connection between the electrical grid 102 and the wind farm busbar 101 is regulated by the first wind farm switch 103.

[0055] The generator 42 of the wind turbine produces AC power of variable frequency due to varying wind conditions. At least one power converter 104 is provided for adjusting the power output from the generator 42 to one suitable for the grid 102, e.g. to an AC power having fixed frequency. The power converter 104 may further include a DC link connecting the machineside and the line-side converter. In some examples, one or more additional auxiliary power converters may be provided. Instead of or in addition to the main power converter, the auxiliary power converter(s) may be in charge of regulating generator torque during island mode.

[0056] The generator 42 of the wind turbine may be a permanent magnet generator comprising a generator rotor carrying a plurality of permanent magnets and a stator in some examples. The permanent magnet generator may be directly driven by the wind turbine rotor 18. The stator of the generator may be connected to the machine-side converter, which may be configured to convert the received AC voltage to DC voltage, the DC voltage then being delivered to the DC-link. The line-side converter may be configured to convert the DC voltage from the DC-link into a fixed frequency AC voltage.

[0057] The machine-side converter and the line-side converter may each comprise an electric circuit including a plurality of switches. The switches may in some examples be IGCTs (Integrated Gate Commutated Thyristors) or other suitable types of switches. Causing the voltage to go through the electric circuit and triggering the switches may convert an AC voltage to a DC voltage or a DC voltage to an AC voltage. The switches may have an ON state and an OFF state. If a switch is in the ON state, electric current may flow through it. If the switch isin the OFF state, electric current may be prevented from flowing through it.

[0058] Triggering the switches may be performed in different manners. For example, a pulse width modulated signal may be used to trigger the switches in a specific manner. Or a frequency modulated signal may be used to trigger the switches in a specific manner. It may also be possible to use signals which combine features of pulse width modulation (PWM) and pulse frequency modulation (PFM) to trigger the switches. The rhythm or pattern at which the switches are pulsed, i.e. when they are triggered and for how long they are kept in the corresponding state, affects the resulting AC signal.

[0059] The line-side converter may be connected to the wind farm busbar 101 through the main transformer 105. The main transformer 105 may be configured to step-up the voltage delivered by the power converter 104, e.g. to 66 kV. The main transformer 105 may be installed within the nacelle 16 or the tower 15 of the wind turbine in some examples. As the main transformer 105 is a set-up transformer, a side or winding of the transformer connected to the power converter 104 may be referred to as input side, primary side or low voltage side, whereas a side or winding of the transformer connected to the power cables 109 may be referred to as output side, secondary side or high voltage side. The auxiliary transformer 107 may be arranged between the power converter 104 and an input or low voltage side of the main transformer 105.

[0060] The auxiliary transformer 107 may be configured to provide a source of low-voltage power, e.g. about 200 - 400 V, to some electric elements of the wind turbine. The auxiliary transformer 107 may for example supply power to the auxiliary electric systems 108 of the wind turbine such as ventilation and temperature regulation systems. The auxiliary transformer 107 may be housed within the nacelle 16 of the wind turbine and may be connected to the main transformer 105.

[0061] The wind farm may further comprise a substation including e.g. wind farm transformers configured to convert power from a wind farm or collector voltage to a grid voltage.

[0062] One or more auxiliary power sources 84 may be provided in the wind turbine 10 for supplying auxiliary power to the auxiliary electric systems 108 during island mode if needed. For example, an auxiliary power source 84 may be used if e.g. the power obtained from rotation of the rotor 18 is insufficient for keeping the auxiliary electric systems 108 operative. The auxiliary power source 84 may e.g. be placed within the nacelle 16. In figure 2, an auxiliary power source 84 is shown for the pitch system 32 of a blade 22. It should be understood that such power source may be placed in other locations in other examples, e.g. in the tower, andother.

[0063] The main transformer 105 may be configured to receive electric power from the electrical grid 102 at a first voltage and electric power from one or more auxiliary power sources 84 at a second voltage different from the first voltage, e.g. lower than the first voltage. The electrical grid 102 may be configured to provide electric power to the wind farm busbar 101 , and therefore to the electric systems of the wind turbine in normal operation, and the generator 42 and the auxiliary power source(s) 84 may be configured to provide electric power to the auxiliary electric systems 108 e.g. in case of grid loss.

[0064] If the wind turbine can no longer receive power from the electrical grid 102, the auxiliary systems 108 may start to obtain it from the generator 42 and / or one or more auxiliary power sources 84. Critical electric components 108 of a wind turbine 10 may therefore be powered through the auxiliary transformer 107 of the wind turbine. The auxiliary wind turbine transformer 107 may transform the power received from the main transformer 105 of the wind turbine to a voltage level required by the electric components of the wind turbine it feeds, e.g. between 60 and 80 kV. If the power provided by the generator 42 exceeds the power required by the auxiliary systems 108, the auxiliary power sources 84 may also be charged.

[0065] The AC voltage generated in the converter which then travels to and through the main transformer 105 generally comprises a plurality of harmonics. The main transformer 105 may suppress at least some of the harmonics of the generated AC voltage signal, specifically the harmonics of the lowest frequencies. For example, the main transformer may be provided with rectifiers to this end. Electrostatic shields may be alternatively or additionally located between the primary and secondary windings for acting as a filter to prevent the transfer of potentially dangerous harmonics to the power cables 109.

[0066] The wind turbine 10 may further include a switchgear 106. The tower cables 109 connect the main transformer 105 and the switchgear 106. The switchgear may comprise a circuit breaker for disconnecting the wind turbine from the wind farm grid and the electrical grid 102 during island mode.

[0067] However, it has surprisingly been found that after the circuit breaker of the switchgear 106 is triggered for electrically disconnecting the wind turbine 10 from the wind farm grid and the wind turbine enters in island mode, one or more HV components of the wind turbine may be damaged. Other components connected to the main transformer, for example one or more auxiliary electric systems 108, may also be damaged in some cases.

[0068] It has been discovered that the main transformer 105 does not filter all the harmonics of the AC signal provided by the power converter 104, and that some of theunfiltered harmonics may cause electrical resonance for example in the power cables 109 of the tower. Figure 4 schematically illustrates an example of harmonics of an AC signal generated by the power converter 104. The AC signal may have been generated by using the first pulse pattern used in normal operation of the wind turbine. The x axis of the figure represents frequency, and the y axis of the figure represents amplitude of the harmonics.

[0069] A frequency range 111 which may cause resonance in the power cables 109 has been indicated in the example of figure 4. The power cables 109 may have a resonance frequency falling in that frequency range 111. Figure 5 schematically illustrates an example of impedance for a power cable 109. The frequency range 111 of figure 4 and a frequency range 112 in which the power cables have its resonance frequency overlap. Therefore, as the generated AC signal includes harmonics in the frequency range 111 in which the power cables 109 have their resonance frequency, resonance may appear in the power cables when the AC signal reaches the power cables.

[0070] If the second pulse pattern is used, the AC signal generated in the power converter may no longer include harmonics in the problematic frequency range 111. And therefore, electrical resonance in the wind turbine, specifically in the power cables 109, may be avoided. This may allow to avoid damage to the components of the HV side an the auxiliary electric systems 108 as well as to avoid other possible solutions which may be more expensive or complicated to implement. This may likewise allow to keep the auxiliary electric systems 108 connected to the main transformer 105 and the power cables 109 during island mode.

[0071] The first pulse pattern and the second pulse pattern may be predefined pulse patterns. As the theoretical resonance frequency of the power cables may be known in advance, it can be determined in which frequency range there should not be harmonics of the AC signal generated by the power converter. Therefore, a second pulse pattern may be predefined, i.e. known in advance., e.g. before the wind turbine is installed. The second pulse pattern (and in general any suitable pulse pattern) may for example be included in a memory of a controller of the power converter 104. The power converter may therefore have access to the first and second pulse patterns even before the wind turbine starts normal operation for the first time. For example, the second pulse pattern may be accessible and usable during commissioning of the wind turbine.

[0072] The second pulse pattern (and e.g. the third pulse pattern, see further below) may differ from the first pulse pattern in that the switches are caused to pulse in a different manner, e.g. with a different rhythm. For example, the second pulse pattern may cause the switches to pulse at different times and / or during different time periods and particularly with a different frequency or with different frequencies than the first pulse pattern. In some examples, a pulsewidth modulated signal used to trigger the switches with the second (or e.g. third) pulse pattern may be different, e.g. may have different pulse widths, than the pulse widths of the first pulse pattern.

[0073] The wind turbine 10 may further comprise a sensor 113 for measuring data indicative of electric resonance of harmonics in the wind turbine, specifically in the plurality of power cables, e.g. of harmonics in a resonance frequency range of the power cables or close to it. The sensor 113 may be a voltage sensor in some examples. Other types of sensors, for example a power sensor such as a power analyzer, may be used in other examples.

[0074] The sensor may be configured to directly provide the harmonics of a signal. For example, the sensor may be configured to perform a Fourier transform of the voltage signal. In other examples, the signal may be processed in a different device, e.g. in the wind turbine controller 36 or in the controller of the power converter 104.

[0075] The sensor 113 may be arranged in the wind turbine tower in some examples. In other examples, the sensor may be arranged in the switchgear 106. The sensor 113 may be arranged in any location which enables measuring data indicative of the presence of electrical resonance or of a risk of electrical resonance in the wind turbine, e.g. in the plurality of power cables. For example, as the power cables are electrically connected with other electrical components, e.g. the main transformer, the power converter, the auxiliary transformer and one or more auxiliary loads, the presence of electrical resonance in the power cables may be detected in different locations.

[0076] The sensor 113 may for example be arranged at a HV side, e.g. with the power cables 109 of the tower, or with a surge arrester, or with a HV winding of the main transformer 105, and may be configured to measure data indicative of harmonics in the power cables 109. Depending on which harmonics are detected, the presence of resonance or a high risk of electric resonance may be detected. Detecting certain harmonics, e.g. harmonics close to a resonance frequency of the power cables 109, may be indicative of a risk of resonance.

[0077] In other examples, the sensor 113 may be arranged at a medium voltage (MV) side of the wind turbine, e.g. at or between the power converter 104, the (primary side of the) main transformer 105 and the (primary side of the) auxiliary transformer 107. For example, the sensor, optionally a voltage sensor, may be arranged with the power converter 104 in some of these examples.

[0078] Still in other examples, the sensor 113 may be arranged at a low voltage (LV) side of the wind turbine, e.g. at or between the (primary side of the) auxiliary transformer 107 andthe one or more auxiliary systems 108. For example, a power analyzer may be provided at the LV side in some of these examples.

[0079] The wind turbine 10 may be configured to detect the presence or the risk of resonance, e.g. to detect certain harmonics with the sensor, and in response to the detection, the power converter 104 may be configured to change a pulse pattern. In some examples, the pulse pattern may be changed from the first pulse pattern to the second pulse pattern. In some examples, the pulse pattern may be changed from the first pulse pattern or the second pulse pattern to a third pulse pattern different from the first and second pulse patterns. The wind turbine may therefore be further configured to operate with the third pulse pattern for switching the switches during operation of the wind turbine in island mode.

[0080] By providing at least a sensor 113, it may for example be checked whether the signal includes harmonics which may overlap with a frequency range 112 in which the power cables 109 have their resonance frequency. If resonance or a risk of resonance is detected, then the pulse pattern of the switches of the line-side converter may be changed to avoid or decrease the risk of resonance.

[0081] If more than one sensor 113 is provided, a sensor may be provided in one of the HV, MV or LV side, and another sensor 113 may be provided in a same side or in another side. For example, a first voltage sensor and a second voltage sensor may be provided in the HV side. Or a voltage sensor may be provided in the HV side and a power sensor may be provided in the LV side in other examples. In some examples, one or more sensors may be provided in the HV side, one or more sensors may be provided in the MV side, and one or more sensors may be provided in the LV side.

[0082] As the resonance frequency of the power cables may be known in advance, one or more additional pulse patterns may not be required or provided in some examples. In some of these examples, one or more sensors may be provided, or one or more sensors may be dispensed with. When starting to operate in island mode, the first pulse pattern may be changed to the second pulse pattern. But as the resonance frequency of the power cables 109 may vary as the power cables 109 or related electronic accessories age, or e.g. when maintenance is carried out on the transformer, or a new component (transformer or cable) is installed, the sensor 113 may help to detect whether a different pulse pattern should be used instead of a current pulse pattern being used in some of these examples. Therefore, one or more additional pulse patterns may be provided, e.g. stored in a memory of the power converter 104. For example, the second pulse pattern may be changed to the third pulse pattern if a sensor detects electrical resonance or risk of electrical resonance in one or more of the HV, MV or LV sides.

[0083] In other examples, one or more sensors 113 may be provided, and the first pulse pattern may be changed to the second pulse pattern once resonance or risk thereof is detected based on at least one of the sensors 113. In these examples, the wind turbine 10 may start to operate in island mode, and the pulse pattern may not be changed until risk or resonance is determined. In these examples, it may also be possible to change to additional pulse patterns, e.g. to the third pulse pattern, based on measurements of one or more of the sensors 113 later on.

[0084] Depending on where electrical resonance or risk of electrical resonance is detected, e.g. measured, the current pulse pattern may be changed to a different pulse pattern. For example, if electrical resonance is detected, e.g. measured, in the LV side, the current pulse pattern may be changed to a pulse pattern specifically configured to eliminate or reduce the resonance in the LV side. Although the HV, MV and LV sides are connected, each side may be seen as a “micro grid” of the wind turbine. A frequency spectrum may be different in each of the HV, MV and LV sides. Pulse patterns configured to change the frequency spectrum specifically in one or more of these sides or internal micro grids of the wind turbine may be provided.

[0085] In a further aspect of the disclosure, a method 100 for operating a wind turbine as described herein is provided. The flowchart of the method is illustrated in figure 6. The method comprises, at block 110, determining that the electrical grid 102 is not available. The method further comprises, at block 120, starting to operate the wind turbine 10 in island mode. The method further comprises, at block 130, switching the switches with the second pulse pattern during operation of the wind turbine in island mode such that electrical resonance in the wind turbine, specifically in the power cables of the tower, is avoided. For example, if electrical resonance is present, it may be eliminated or at least reduced.

[0086] Therefore, when the electrical grid 102 is not available and the wind turbine operates, e.g. starts to operate, in island mode, i.e. starts to generate power for its own consumption by rotation of the wind turbine rotor 18, the second pulse pattern may be used for avoiding or at least reducing the risk of electric resonance.

[0087] In island mode, the wind turbine rotor 18 may be rotating at a rotational speed such that the wind turbine generator 42 is generating power for feeding auxiliary systems 108 of the wind turbine such as temperature regulation systems and communication systems. In some examples, the rotation speed of the rotor 18 may be between 4 and 8 RPM, for example between 5 and 7 RPM. The rotational speed and the generated electrical power may be kept at a desired value by actively controlling the pitch angle of the blades 22. The generator 42 torque can also be controlled, e.g. with a main converter or an additional converter. Thegenerator 42 may provide between 100 and 300 kW in the island mode of operation in some examples.

[0088] The method may further comprise keeping the auxiliary systems 108 connected to the main transformer 105 while operating the wind turbine in island mode. As the risk of resonance is low or very low, disconnecting the auxiliary systems 108 from the main transformer 105 may not be required. Expensive elements and difficult methods for installing them in the wind turbine for disconnecting the auxiliary systems 108 from the main transformer 105 may be avoided.

[0089] Before the wind turbine 10 starts to operate in island mode, the method may further comprise electrically disconnecting the wind turbine 10 from the electrical grid 102. For example, disconnecting may comprise triggering a circuit breaker. The circuit breaker may be arranged in a switchgear 106 of the wind turbine 10.

[0090] If before the wind turbine 10 is operated in the island mode, it was operated in normal mode, the method may further comprise switching the switches with the first pulse pattern during normal operation of the wind turbine. The first pulse pattern may have features adapted for normal operation and the second pulse pattern may have features adapted for operation in the island mode. For example, if PWM is used for generating the first and second pulse patterns, the frequency of the carrier wave for generating the first pulse pattern may be different, e.g. lower, than the frequency of the carrier wave for generating the second pulse pattern. In general, the pulse patterns may be generated in any suitable manner known in the art.

[0091] The method may further comprise measuring information indicative of harmonics in the wind turbine, specifically in the plurality of the power cables of the tower. The harmonics of the signal measured may provide information about the risk of resonance and whether the pulse patterns being used are working appropriately. The information indicative of harmonics may be measured with a voltage sensor. The voltage sensor 113 may be configured to be arranged with the power cables 109 of the wind turbine tower in some examples. Measuring the information indicative of harmonics in the power cables of the tower may be particularly suitable as resonance may occur in the power cables. However, harmonics, and in general the presence or the risk of resonance, may also be measured in a different place in other examples, e.g. near the auxiliary systems 108, optionally with a power analyzer, or in the power converter 104. In some examples, in addition to measure the information indicative of harmonics in the plurality of power cables of the tower, it may be measured between the power converter 104 and the main transformer 105.

[0092] The method may further comprise changing the operation of the power converter 104 to a third pulse pattern in response to detecting a presence or a risk of resonance in the wind turbine 10, for example in the power cables of the tower, e.g. in response to detecting harmonics in the resonance frequency range of the power cables in the power cables of the tower.

[0093] As mentioned before, in some examples the wind turbine 10 may start to operate in island mode and immediately change from the first to the second pulse pattern. In some of these examples, one or more sensors 113 may in addition be used to detect the presence or the risk of electrical resonance in one or more of the LV, MV or HV sides and to change to another pulse pattern, e.g. the third pulse pattern. Still in other examples, the wind turbine 10 may start to operate in island mode and use one or more sensors 113 to detect the presence or the risk of electrical resonance. After the detection, the first pulse pattern may be changed to another pulse pattern, e.g. to the second pulse pattern. If later on, an electrical resonance or a risk of electrical resonance is detected, the pulse pattern may be changed again e.g. to the third pulse pattern.

[0094] As a frequency spectrum may be different in the HV side, the MV side and the LV side, depending on where the resonance is detected, e.g. on where it is measured, one or another pulse pattern may be chosen as the new pulse pattern. For example, if resonance or risk of resonance is detected in the LV side, the current pulse pattern may be changed to a certain pulse pattern configured to avoid, eliminate or reduce resonance in the LV side. But if resonance or risk thereof is detected in the HV side, the current pulse pattern may be changed to another pulse pattern configured to avoid, eliminate or reduce resonance in the HV side.

[0095] Details and explanations provided for this method can be applied and combined with the wind turbine of the previous aspect, and vice versa.

[0096] In a further aspect of the disclosure, an offshore wind turbine is provided. Details and explanations for this wind turbine can be applied and combined with the wind turbine and method of the previous aspects, and vice versa.

[0097] The wind turbine 10 comprises an electric generator 42 configured to generate an AC power signal. The wind turbine 10 further comprises a power converter 104 connected to the electric generator 42 which comprises a machine-side converter configured to convert an AC power signal to a DC power signal, a DC-link, and a line-side converter configured to convert a DC power signal to an AC power signal. The line-side converter comprises an electric circuit including a plurality of switches for converting a DC power signal to an AC power signal.

[0098] The wind turbine 10 further comprises an auxiliary transformer 107 connected tothe power converter 104 and one or more auxiliary systems 108 connected to the auxiliary transformer 107. The wind turbine 10 further comprises a main transformer 105. An input side of the main transformer 105 is connected to the power converter 104 and an output side of the main transformer 105 is connected to a plurality of tower cables 109 configured to carry an AC power signal.

[0099] The power converter 104 is configured to operate with a first predefined pulse pattern for switching the switches during normal operation of the wind turbine and is configured to operate with a second predefined pulse pattern different from the first pulse pattern for switching the switches during operation of the wind turbine in island mode for avoiding or reducing resonance in the tower cables 109 in island mode.

[0100] The wind turbine 10 may further comprise a sensor 113 for measuring voltage in one or more cables connected to the main transformer 105. For example, the sensor 113 may be arranged between the main transformer 105 and the auxiliary transformer 107, or between the auxiliary transformer 107 and the auxiliary systems 108, or between the power converter 104 and the main transformer 105. The sensor 113 for measuring voltage may specifically be arranged with the tower cables 109 for measuring voltage in the tower cables 109.

[0101] The voltage may be indicative of a presence of harmonics within a frequency range including a resonance frequency of the tower cables 109. The wind turbine may be configured to detect harmonics within a frequency range including a resonance frequency of the tower cables, and when detected, the power converter 104 may be configured to operate the wind turbine in island mode with a third pulse pattern for switching the switches different from the first and second pulse patterns.

[0102] 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 wind turbine (10) comprising: an electric generator (42) configured to generate AC power; a power converter (104) connected to the electric generator (42) and comprising a machine-side converter configured to convert AC power to DC power, a DC-link, and a lineside converter configured to convert DC power to AC power, the line-side converter comprising a plurality of switches for converting DC power to AC power; a main transformer (105) and an auxiliary transformer (107) connected to the power converter (104); one or more auxiliary systems (108) connected to the auxiliary transformer (107); a plurality of power cables (109) connecting an output side of the main transformer with an electrical grid; wherein the power converter (104) is configured to operate with a first pulse pattern for switching the switches during normal operation of the wind turbine (10) and wherein the power converter (104) is configured to operate with a second pulse pattern different from the first pulse pattern for switching the switches during operation of the wind turbine (104) in island mode, wherein the second pulse pattern is configured to avoid electrical resonance in the wind turbine (10).

2. The wind turbine of claim 1 , wherein the second pulse pattern is configured to avoid electrical resonance in the plurality of power cables (109).

3. The wind turbine of claim 1 or claim 2, wherein the first pulse pattern and the second pulse pattern are predefined pulse patterns.

4. The wind turbine of any of claims 1 - 3, further comprising a sensor (113) for measuring data indicative of electric resonance of harmonics in the wind turbine (10), specifically in the plurality of power cables (109).

5. The wind turbine of claim 4, wherein the sensor (113) is a voltage sensor.

6. The wind turbine of claim 4 or claim 5, wherein the sensor (113) is arranged with the plurality of power cables (109) and is configured to measure data indicative of harmonics in the power cables (109).

7. The wind turbine of any of claims 4 - 6, wherein the wind turbine (10) is configured to detect a presence or a risk of electrical resonance with the sensor (113) and, wherein the power converter (104) is configured to change a pulse pattern in response to the detection.

8. The wind turbine of claim 7, further configured to operate with a third pulse pattern for switching the switches during operation of the wind turbine (10) in island mode, the third pulse pattern being different from the first and second pulse patterns.

9. A method (100) for operating the wind turbine of claims 1 - 8, the method comprising: determining (110) that the electrical grid (102) is not available; starting (120) to operate the wind turbine (10) in island mode; and switching (130) the switches with the second pulse pattern during operation of the wind turbine (10) in island mode such that electrical resonance in the wind turbine (10) is avoided.

10. The method of claim 9, further comprising keeping the auxiliary systems (108) connected to the main transformer (105) while operating the wind turbine (10) in island mode.

11. The method of claim 9 or claim 10, further comprising electrically disconnecting the wind turbine (10) from the electrical grid (102) before starting to operate in the island mode.

12. The method of any of claims 9 - 11 , further comprising switching the switches with the first pulse pattern during normal operation of the wind turbine (10).

13. The method of any of claims 9 - 12, further comprising measuring information indicative of harmonics in the wind turbine (10), specifically in the plurality of power cables (109).

14. The method of claim 12 or claim 13, wherein the information indicative of harmonics is measured in the plurality of power cables (109) and is optionally additionally measured between the power converter and the main transformer.

15. The method of claim 13 or claim 14, further comprising changing operation of the power converter to a third pulse pattern in response to detecting a presence or a risk of resonance in the wind turbine (10).

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