Methods for operating wind turbines and wind turbines

Abstract

The present disclosure relates to wind turbines (10) and methods (100) for operating wind turbines (10). A method (100) for operating a wind turbine (10) comprises determining (140) a voltage below a predetermined level between the power converter (104) and the main transformer (105); and in response to determining (140) the voltage below the predetermined level, an auxiliary energy source (84) may supply power to one or more of the auxiliary components (108, 109). The auxiliary transformer (106) may be de-energized (160) and the auxiliary transformer (107) may be changed (170) to a different transformer ratio and then power may be delivered (180) to the auxiliary components (108, 109) using the auxiliary transformer (107) with the changed transformer ratio. Suitable control systems (36) and wind turbines (10) are disclosed as well.

Description

METHODS FOR OPERATING WIND TURBINES AND WIND TURBINES

[0001] The present disclosure relates to methods for operating wind turbines, in particular to methods for operating wind turbines while supplying power to auxiliary components. The present disclosure further relates to methods for operating wind turbines which allow increasing a range of reactive power supplied by a main transformer of a wind turbine to the electrical grid without damaging auxiliary loads of the wind turbine. The present disclosure further relates to wind turbines configured for performing such methods.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] A grid code defines the parameters that a wind farm has to meet to ensure safe, secure and proper functioning of the public electrical grid. More and more, wind farms are expected to support the grid like conventional power plants, and a grid code usually specifies the required behavior of the wind farm during disturbances in the electrical grid. For example, a grid code may include requirements regarding voltage regulation, frequency regulation and active power and reactive power supply. A grid code may also include requirements regarding a response to a fault in the electrical grid, e.g. a short-circuit, and requirement to ride throughshort interruptions of the connection to the electrical grid. The requirements vary between countries and their severity usually depends on the wind power penetration level as well as on the robustness of the national or regional electrical grid.

[0005] Each wind turbine in the wind farm grid usually comprises its own transformer, usually referred to as “main transformer”. The main transformer steps up the voltage output from the wind turbine generator and converter, e.g. from a few hundred volts (“medium voltage level”), to a higher level suitable for an internal wind farm grid. The wind farm may further comprise a substation with a transformer and a point of common coupling (PCC) which steps the voltage of the wind farm grid to the public utility grid.

[0006] The wind turbines also include auxiliary systems, for example communication systems, ventilation and temperature regulation systems, pitch and yaw control systems and others. In order to supply power to the auxiliary systems, the wind turbines comprise an auxiliary transformer for providing a source of low-voltage power, e.g. about 200 - 400 V, to the auxiliary systems.

[0007] The power converter, the main transformer and the auxiliary transformer are generally connected to each other through electrical cables. Therefore, power from the power converter may travel to and through the main transformer and the auxiliary transformer, and be supplied to the electrical grid and the auxiliary systems. Power from the electrical grid may also flow to and through the main transformer and the auxiliary transformer, and be supplied to the auxiliary systems.

[0008] The auxiliary systems can operate within a specific voltage range, and the power converter, the main transformer and the auxiliary transformer are electrically connected to each other. In practice, this can mean that the reactive power capability of the wind turbine is limited by the need of a specific voltage range for the auxiliary components. This may be more so in the case of main transformers with a high impedance. Therefore, reactive power capability (or rather, the lack thereof) may need to be compensated, in particular increased, in order to be able to meet the requirements of the corresponding grid code. Reactive power compensators, for example static synchronous compensators (STATCOM), may be provided in a wind farm for increasing the reactive power provided by the wind turbines and fulfil the grid code requirements.

[0009] The present disclosure aims to solve at least in part one or more of the abovementioned problems.SUMMARY

[0010] In an aspect of the present disclosure, a method for operating a wind turbine is provided. The method comprises operating the wind turbine to generate power with a generator, and a power converter converting the generated power to a medium voltage power. The method further comprises a main transformer stepping up the converted power to a high voltage power and delivering the high voltage power to an electrical grid, and an auxiliary transformer delivering power to auxiliary components using a first transformer ratio. The method further comprises determining a voltage below a predetermined level between the power converter and the main transformer. The method then comprises, in response to determining the voltage below the predetermined level, an auxiliary energy source supplying power to one or more of the auxiliary components and de-energizing the auxiliary transformer. The method further comprises changing the auxiliary transformer to a second transformer ratio and delivering power to the auxiliary components using the auxiliary transformer with the second transformer ratio.

[0011] In accordance with this method, a no-load tap changer is provided which enables a cost-effective method for increasing an operational voltage range of the wind turbine, and a reactive power capability. The operational voltage range and reactive power capability is increased by effectively decoupling the main converter from the auxiliary components. A no- load tap changer is not only cost-effective but also less prone to wear than on-load tap changers.

[0012] Throughout this disclosure, auxiliary components (or auxiliary systems) are to be regarded as components 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 when 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.

[0013] 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 “normal operation” may be used herein to explicitly refer to such a situation.

[0014] In a further aspect of the present disclosure, a control system for a wind turbine which is configured to carry out a method according to the first aspect is provided. I.e. the control system is configured to operate the wind turbine to generate power with a generator, and operate a power converter converting the generated power to a medium voltage power. The control system is further configured to operate a main transformer to step up the convertedpower to a high voltage power and deliver the high voltage power to an electrical grid, operate an auxiliary transformer to deliver power to auxiliary components using a first transformer ratio and to determine a voltage below a predetermined level between the power converter and the main transformer. The control system is further configured to perform the following steps in response to detecting the voltage below the predetermined level: an auxiliary energy source supplying power to one or more of the auxiliary components, de-energizing the auxiliary transformer, changing the auxiliary transformer to a second transformer ratio and delivering power to the auxiliary components using the auxiliary transformer with the second transformer ratio.

[0015] In yet a further aspect a wind turbine is provided, which comprises a generator to generate power, a main power converter configured to convert the generated power, a main transformer connected to the power converter and an auxiliary transformer connected to the power converter and the main transformer, wherein the auxiliary transformer comprises a no- load tap changer, NLTC, with a plurality of taps. The wind turbine further comprises one or more auxiliary components connected to the auxiliary transformer and an auxiliary energy source connected to the auxiliary components for supplying power to the first auxiliary system during a change of tapping position of the NLTC.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 illustrates a perspective view of one example of a wind turbine;

[0017] Figure 2 illustrates a simplified, internal view of one example of the nacelle of the wind turbine of the figure 1 ;

[0018] Figure 3 schematically illustrates a wind turbine connected to an electrical grid and comprising auxiliary components;

[0019] Figure 4 schematically illustrates an example of three voltage-power curves at a primary side of a wind turbine main transformer; and

[0020] Figure 5 shows a flow chart of an example of a method for operating a wind turbine.DETAILED DESCRIPTION OF EXAMPLES

[0021] 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.

[0022] 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 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.

[0023] 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.

[0024] 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.

[0025] 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 rotorblades 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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, andtransformer 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.

[0030] 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 42 and 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).

[0031] 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.

[0032] 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.

[0033] 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 (active) power than a wind turbine with a gearbox.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 windturbine 10, the wind turbine controller 36 may control the pitch drive system 68 to adjust a pitch angle of rotor blades 22.

[0040] 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 electric power 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.

[0041] 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.

[0042] 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.

[0043] Figure 3 schematically illustrates an example of a wind turbine 10 of a wind farm, e.g. an offshore wind turbine. The wind turbine 10 of this example comprises an electric generator 42 configured to generate AC (alternating current) power and a power converter 104 connected to the electric generator 42. 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 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.

[0044] 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 components 108, 109 connected to the auxiliary transformer and aplurality of power cables 112 connected to an output side of the main transformer 105 which leads to an electrical grid, typically an internal grid of the wind farm. If the main transformer 105 is arranged in the nacelle 16, the power cables 112 may be tower cables and extend from the nacelle and down the wind turbine tower.

[0045] The wind turbine 10 may be connected to a wind farm busbar 101 , and then connected to the public utility grid 102 at a Point of Common Coupling (PCC), in this case represented 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 at the PCC may be regulated by a wind farm substation.

[0046] 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 grid frequency may slightly vary over time. For example, the grid frequency may be about 50 Hz. In some examples, one or more additional power converters may be provided.

[0047] 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. A side or winding of the transformer connected to the power converter 104 may be referred to as input side or secondary side of the transformer, whereas a side or winding of the transformer connected to the power cables 112 may be referred to as output side or primary side.

[0048] The auxiliary transformer 107 may be arranged between the power converter 104 and an input or secondary side of the main transformer 105. The auxiliary transformer 107 may likewise comprise a primary side and a secondary side. The one or more auxiliary systems 108, 109 are connected to the secondary side of the auxiliary transformer 107.

[0049] The auxiliary transformer 107 may be configured to provide a source of low-voltage power, e.g. about 200 - 400 V, to some auxiliary components of the wind turbine. The auxiliary transformer 107 may for example supply active power to the auxiliary electric systems 108, 109 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 electrically connected to the main transformer 105 and the power converter 104.

[0050] 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 voltageand one or more reactive power compensators, e.g. one or more STATCOMs.

[0051] A grid code may require that a wind farm supplies a certain reactive power Q and / or a certain voltage II to the electrical grid 102, in particular depending on the situation of the utility grid. This reactive power and voltage may be referred to as nominal reactive power and nominal voltage, respectively. The grid code may also define a reactive power range AQ and a voltage range AU around the nominal reactive power and voltage within which the reactive power Q and voltage II may vary. For example, a voltage range may be from -10% to +10% of the nominal voltage. If the nominal voltage is expressed in a per unit system, the nominal voltage is U = 1 pu. And the -10% to 10% example of voltage range would be expressed as 0.9 pu - 1.1 pu. The same is applicable to the reactive power Q and to other magnitudes, i.e. a range of reactive power may be expressed as a percentage range around the nominal reactive power or as a range in pu units where the nominal reactive power is 1 pu.

[0052] At rated conditions of reactive power Q, the voltage range AU at the primary side of the main transformer may be limited. For example, a high impedance of the main transformer 105 means that a significant voltage drop occurs over the main transformer. Therefore, if a certain voltage and voltage range at the primary side is desired or required, e.g. between 0.9 and 1.1 pu, a higher voltage and voltage range should be available at the secondary side of the main transformer.

[0053] The voltage at the secondary side of the main transformer may however be limited by the auxiliary loads, and their operational range. In this regard, as the power converter, the secondary side of the main transformer and the primary side of the auxiliary transformer are connected, the voltage at the secondary side of the main transformer (M), UM P, and at the primary side of the auxiliary transformer (A), UA P, is the same.

[0054] The auxiliary transformer 107 may be configured to operate with a specific turn ratio or “transformer ratio” RA= NP / NS. Since RA= UA P / - UA S, the voltage at the secondary side of the auxiliary transformer NP / NSwill be UA S= UA P / RA, where UA Pis the voltage at the secondary side of the auxiliary transformer. Therefore, if RAis fixed, and a value of UA Smay vary in a certain range for suitably supplying (active) power to the auxiliary loads, the values of UA P, i.e. the range of voltage within which the power converter may operate, will be limited. And therefore, the values of the voltage at the primary side of the main transformer UM Swill also be limited.

[0055] In accordance with a first aspect, a method 100 for operating a wind turbine is provided. The method is illustrated in the flow chart of figure 5. The method comprises, at block 110, operating the wind turbine to generate power with the generator 42, and the powerconverter 104 converting the generated power to a medium voltage power. The main transformer 105 steps up, at block 120, the converted power to a high voltage power and the high voltage power is delivered to the electrical grid. The auxiliary transformer 107 delivers, at block 130, power to auxiliary components 108, 109 using a first transformer ratio. The first transformer ratio may be used in one operational scenario.

[0056] The method further comprises determining, at block 140, a voltage below a predetermined level between the power converter 104 and the main transformer 105. The determination might be made based on a local voltage measurement. A voltage may be detected at the wind farm grid or between the main transformer and the converter.

[0057] And in response to determining the voltage below the predetermined level, the method comprises, at block 150, an auxiliary energy source 84 supplying power to one or more of the auxiliary components 108, 109. The auxiliary transformer 107 may be de-energized, and the auxiliary transformer may be changed to a second transformer ratio at blocks 160 and 170. The second transformer ratio may be such that even in the case of a voltage reduction at the output of the power converter (or the secondary side of the main transformer), a suitable voltage is supplied to the auxiliary components 108, 109.

[0058] A suitable switching system 95, e.g. one or more contactors 95, see the example of figure 3, may be provided between the secondary side of the auxiliary transformer 107 and the auxiliary components 108, 109 for changing a connection of the auxiliary components between taps, i.e. from one tap to another tap. For example, when the auxiliary components are connected to a first tap, a contact of a contactor linking the first tap and the auxiliary components is closed. The remaining contacts of the contactor are open. When the auxiliary components are connected to a second tap, the contact of the contactor linking the second tap and the auxiliary components is closed, and the remaining contacts of the contactor are open.

[0059] The method may thus further comprise delivering power to the auxiliary components 108, 109 using the auxiliary transformer with the second transformer ratio at block 180.

[0060] The auxiliary energy source may be a yaw backup system, i.e. a yaw backup power supply. In some examples, the auxiliary energy source may comprise one or more batteries, ultracaps or UPS (Uninterruptible Power Supply). Such an auxiliary energy source may be charged in normal operation through the auxiliary transformer.

[0061] In yet further examples, the auxiliary energy source may be a DC-link of a power converter.

[0062] In some examples, one or more of the auxiliary components may be switched off when power is supplied from the auxiliary energy source. Not all auxiliary components requireuninterrupted power supply. In examples, the auxiliary components that are switched off may include a fan, or a heater, or a yaw system.

[0063] After a tap change has been made and the auxiliary transformer is energized again, the same auxiliary components may again be reconnected.

[0064] In some examples, in order for the tap change to be carried out, (active) power may be delivered to the auxiliary components by the auxiliary energy source between 2 and 30 seconds, specifically between 5 and 10 seconds.

[0065] De-energizing the auxiliary transformer in order to carry out the tap change may comprise the auxiliary energy source supplying power at a voltage level that is substantially equal to the voltage level at a secondary side of the auxiliary transformer. The auxiliary energy source may include a power converter to adapt the voltage level supplied.

[0066] After the tap change has been made, the method may further comprise synchronizing the voltage supplied by the auxiliary energy source to a voltage at a primary side of the auxiliary transformer, and subsequently reconnecting.

[0067] Once a suitable voltage level at the wind farm grip and between the transformer and power converter has been re-established, the auxiliary transformer may again be re-energized, and a tap change may be carried out to revert to the original setting. Also during this procedure, the auxiliary components may be temporarily supplied by the auxiliary energy source, and once the tap change has been made, power may again be supplied to the auxiliary components through the auxiliary transformer.

[0068] With examples of the present disclosure, as may be seen in the example of figure 4, the reactive power that may be injected into the electrical grid by the wind turbine may be increased by the tap change. In particular, for voltages of U > 1 pu, the capacity for injecting reactive power may be increased.

[0069] For the sake of simplicity, let us assume that ideally a wind turbine 10 should provide a certain nominal reactive power Q, and that a certain range of variation around the nominal value should be allowed. Power, both active P and reactive Q, may be related to voltage II through power curves. A power curve describes a relationship between a power delivered to an electrical load and a voltage at the terminals of the load in an electric power system. Therefore, a voltage range which the wind turbine should ideally provide may also be known. Such a desired voltage range may e.g. be between 90% - 110% of the nominal voltage (or equivalently between 0.9 and 1.1 pu).

[0070] Figure 4 schematically illustrates an example of three voltage-power curves at aprimary side of the main transformer. In the context of a wind turbine, the SS side may also be referred to as high voltage side. In this example, the power converter, the secondary side of the main transformer and the primary side of the auxiliary transformer are connected, as schematically illustrated for example in figure 3. The X axis represents a ratio of reactive power Q and active power P. The active power is rated active power, i.e. the maximum theoretical power that can be provided by the wind turbine. The Y axis represents voltage II (in pu units). The tapping position of the NLTC is at a default position for the first power curve (dash-dotted line).

[0071] For the second power curve (dashed line), the tapping position of the NLTC has been increased by one from the default tapping position. And for the third power curve (solid line), the tapping position of the NLTC has been increased by two from the default tapping position. When the tapping position has been increased, the capability to provide reactive power to the electrical grid has also been increased, specifically for voltages UM s> 1 pu. Therefore, when the grid code requires a specific injection of reactive power, the power converter 105 may provide a suitable level of voltage and reactive power for satisfying the requirement. This may be performed without damaging the auxiliary systems of the wind turbine 10.

[0072] Steps of increasing the reactive power may be performed after the wind turbine detects a disturbance in the electrical grid 102, optionally after the voltage decreases in an electrical grid to which the wind turbine is connected, in some examples.

[0073] The wind turbine controller 36, or any other suitable controller, may be configured to control the production of reactive power and / or the change of tapping position of the NLTC. For example, if a certain reactive power should be injected into the electrical grid, the wind turbine controller 36 may instruct the power converter 104 to increase the production of reactive power, may instruct the NLTC to change a tapping position and may instruct the storage energy system to supply power to the first auxiliary system.

[0074] As the tap changer is specifically a no-load tap changer, the NLTC is to be deenergized before switching the tapping position of the NLTC. Therefore, the method may further comprise supplying active power to the first auxiliary system 108 with the auxiliary energy source 84 before switching the tapping position, as well as de-energizing the auxiliary transformer 107. After switching to the new tapping position, the method may further comprise reenergizing the auxiliary transformer 107.

[0075] According to a further aspect of the present disclosure, a control system 36 is provided which is configured to carry out any of the examples of the methods herein disclosed.In a further aspect, a wind turbine comprising such a control system is provided.

[0076] The control system for a wind turbine may be configured to: operate the wind turbine to generate power with a generator, operate a power converter converting the generated power to a medium voltage; operate a main transformer to step up the converted power to a high voltage power and deliver the high voltage power to an electrical grid; operate an auxiliary transformer to deliver power to auxiliary components using a first transformer ratio; determine a voltage below a predetermined level between the power converter and the main transformer; and to perform the following steps in response to detecting the voltage below the predetermined level: an auxiliary energy source supplying power to one or more of the auxiliary components; de-energizing the auxiliary transformer; changing the auxiliary transformer to a second transformer ratio; and delivering power to the auxiliary components using the auxiliary transformer with the second transformer ratio.

[0077] The control system may be further configured to de-energize the auxiliary transformer, to change the auxiliary transformer back to the first transformer ratio and delivering power to the auxiliary components using the auxiliary transformer with the first transformer ratio.

[0078] According to a further aspect of the disclosure, a wind turbine is provided. The wind turbine may be a direct drive wind turbine in some examples. The wind turbine 10 comprises a power converter 104, a main transformer 105 connected to the power converter 104, an auxiliary transformer 107 connected to the power converter and the main transformer 105. The auxiliary transformer 107 comprises a no-load tap changer, NLTC, with a plurality of taps. The wind turbine further comprises an auxiliary components 108 connected to the auxiliary transformer 107 and an auxiliary energy source 84 connected to the auxiliary component 108 for supplying power to the auxiliary component 108 during a change of tapping position of the NLTC. Details and explanations with respect to the wind turbine of this aspect may be combined and applied to method 100 and vice versa.

[0079] Although reference has herein generally been made to a single auxiliary energy source herein, it may be possible to use more than one energy source and / or more than one energy storage system.

[0080] 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 withinthe 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) comprising: operating (110) the wind turbine (10) to generate power with a generator (42), and a power converter (104) converting the generated power to a medium voltage; a main transformer (10) stepping up (120) the converted power to a high voltage power and delivering the high voltage power to an electrical grid (102); an auxiliary transformer (107) delivering power to auxiliary components (108, 109) using a first transformer ratio; determining (140) a voltage below a predetermined level between the power converter (104) and the main transformer (105); and in response to determining (140) the voltage below the predetermined level, the method comprising: an auxiliary energy source (84) supplying (150) power to one or more of the auxiliary components (108, 109); de-energizing (160) the auxiliary transformer (107); changing (170) the auxiliary transformer (107) to a second transformer ratio; and delivering (180) power to the auxiliary components (108, 109) using the auxiliary transformer (107) with the second transformer ratio.

2. The method of claim 1 , wherein the auxiliary energy source (84) is a yaw backup system.

3. The method of claim 1 , wherein the auxiliary energy source (84) comprises one or more batteries, ultracaps or UPS.

4. The method of claim 2 or claim 3, the method further comprising charging the auxiliary energy source (84) through the auxiliary transformer (107).

5. The method of claim 1 , wherein the auxiliary energy source (84) is a DC-link of a power converter (104).

6. The method of any of claims 1 - 5, wherein the auxiliary components (108, 109) include a pitch system, a yaw system or a wind turbine controller.

7. The method of any of claims 1 - 6, further comprising switching off one or more of the auxiliary components (108, 109).

8. The method of claim 7, wherein the auxiliary components (108, 109) that are switched off include a fan or a heater, or a yaw system.

9. The method of claim 7 or claim 8, further comprising switching the auxiliary components (108, 109) on again after reenergizing the auxiliary transformer (107).

10. The method of any of claims 1 - 9, wherein active power is delivered to the auxiliary components (108, 109) by the auxiliary energy source between 2 and 30 seconds, specifically between 5 and 10 seconds.

11. The method of any of claims 1 - 10, wherein de-energizing (160) the auxiliary transformer (107) comprises the auxiliary energy source (84) supplying power at a voltage level that is substantially equal to the voltage level at a secondary side of the auxiliary transformer (107).

12. The method of any of claims 1 - 11 , further comprising synchronizing the voltage supplied by the auxiliary energy source (84) to a voltage at a primary side of the auxiliary transformer (107), and subsequently reconnecting.

13. A control system (36) for a wind turbine (10) configured to: operate (110) the wind turbine (10) to generate power with a generator (42), and operate a power converter (104) converting the generated power to a medium voltage; operate (120) a main transformer (105) to step up the converted power to a high voltage power and deliver the high voltage power to an electrical grid (102); operate (130) an auxiliary transformer (107) to deliver power to auxiliary components (108, 109) using a first transformer ratio; determine (140) a voltage below a predetermined level between the power converter (104) and the main transformer (105); the control system (36) further configured to perform the following steps in response to detecting the voltage below the predetermined level:an auxiliary energy source (107) supplying (150) power to one or more of the auxiliary components (108, 109); de-energizing (160) the auxiliary transformer (107); changing (170) the auxiliary transformer (107) to a second transformer ratio; and delivering (180) power to the auxiliary components (108,109) using the auxiliary transformer (107) with the second transformer ratio.

14. A wind turbine (10) comprising: a generator (42) to generate power; a main power converter (104) configured to convert the generated power; a main transformer (105) connected to the power converter (104); an auxiliary transformer (107) connected to the power converter (105) and the main transformer (105), wherein the auxiliary transformer (107) comprises a no-load tap changer, NLTC, with a plurality of taps; and an auxiliary component (108, 109) connected to the auxiliary transformer (107); and an auxiliary energy source (84) connected to the auxiliary component (108, 109) for supplying power to the auxiliary component (108, 109) during a change of tapping position of the NLTC.

15. The wind turbine of claim 14, wherein the auxiliary energy source (84) comprises batteries and / or ultracaps and / or UPS.

Images