Control of power amplitude of stabilizing power provided by a grid forming converter

Abstract

Disclosed is methods, electronic devices, and wind turbines that enable control of the wind turbine to reduce or avoid excitation of structural eigenmodes of the wind turbine due to an oscillating stabilizing power injected into the power grid in response to a grid frequency oscillation event. Information indicative of a stabilizing power with a stabilizing frequency injected in response to a grid disturbance is received. The stabilizing frequency is compared with an eigenfrequency of a structural eigenmode of the wind turbine, and upon determining that the stabilizing frequency overlaps with the eigenfrequency, determining an indicator of oscillations in the structural eigenmode. It is then determined whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator, andupon determining to reduce the injected stabilizing power, controlling the converter to reduce the injection of the stabilizing power.

Description

[0001] CONTROL OF POWER AMPLITUDE OF STABILISING POWER PROVIDED BY A GRID FORMING CONVERTER

[0002] The present disclosure pertains to the field of wind turbine control. The present disclosure relates to a method, a wind turbine, and an electronic device for providing grid-forming control of a wind turbine.

[0003] BACKGROUND

[0004] The frequency of the electric power grid is traditionally controlled by large power plants with synchronous generators, where a power system stabilizer is typically used to dampen low-frequency oscillations of the grid to maintain a stable nominal grid frequency (50 or 60 Hz). Renewable energy generation units such as wind turbines or photovoltaic systems has only little or no inertia and their power output varies depending on the weather and the units' components limitations. As these renewable energy generation units deliver an increasing part of the power, their AC power converters are being operated in a grid forming mode where they can be used for stabilizing the grid.

[0005] A transmission system operator provides a power grid code that specify minimum, capabilities of generation units connected to the grid, such as nominal values and ranges for operating parameters, e.g.: AC voltage, frequency, active power, AC current, and reactive power. For grid forming generation units, the grid code also specifies how to react in case of grid disturbances, typically by adjusting the injected active power to stabilise the grid disturbance. The reaction is performed by a converter of the wind turbine, and the injected stabilising active power is coupled to the load on the wind turbine.

[0006] In the case of a grid frequency oscillation event (such as inter-area or intra-area oscillations), the specified reaction is to inject an oscillating stabilising power with a frequency and phase that counteracts the disturbance. There is a risk that such oscillations can couple to and excite structural eigenmodes of the wind turbine.

[0007] SUMMARY

[0008] It is an object of the present invention to provide grid-forming control of a wind turbine that is capable of a controlled reaction to potential excitation of structural eigenmodes of the wind turbine when reacting to grid frequency oscillation events. Accordingly, it would be a benefit to provide a method, a wind turbine, and an electronic device that is prepared for potential excitation of structural eigenmodes of the wind turbine when reacting to grid frequency oscillation events and take appropriate action.

[0009] Disclosed is a method for providing grid-forming control of a wind turbine electrically coupled to a power grid. The wind turbine comprises a generator driven by a rotor to produce electrical power; a wind turbine controller, WTC; a converter coupled to the generator and the power grid for converting the produced electrical power to generate AC power injected into the power grid at a nominal AC frequency; and a converter controller for controlling operation of the converter according to a grid forming converter, GFC, control scheme. The method comprising, by the converter controller, injecting a stabilizing power in response to a grid frequency oscillation, wherein the injected stabilizing power oscillates around the nominal AC frequency at a stabilizing frequency corresponding to a frequency of the grid frequency oscillation. The method comprising, by the WTC, receiving information indicative of the frequency of the grid frequency oscillation and receiving, from the converter controller, information indicating that a stabilizing power is being injected. The method comprises, by the WTC, comparing the frequency of the grid frequency oscillation with an eigenfrequency of a structural eigenmode of the wind turbine. The method comprises, by the WTC, upon determining that the frequency of the grid frequency oscillation with the eigenfrequency, determining, based on values from one or more sensors of the wind turbine, an indicator of oscillations in the structural eigenmode. The method comprises, by the WTC, determining to whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator. The method comprises, upon determining to reduce the injected stabilizing power, controlling the converter to reduce the injection of the stabilizing power.

[0010] Disclosed is a wind turbine comprising a generator driven by a rotor to produce electrical power; a wind turbine controller, WTC; a converter coupled to the generator and the power grid for converting the produced electrical power to generate AC power injected into the power grid at a nominal AC frequency; and a converter controller for controlling operation of the converter according to a grid forming converter, GFC, control scheme. The WTC and the converter controller are configured to perform any of the methods according to the disclosed methods. Disclosed is an electronic device comprising a memory circuitry, a processor circuitry, and an interface, wherein the electronic device is configured to perform any of the methods according to the disclosed methods.

[0011] Disclosed is a computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by an electronic device cause the electronic device to perform any of the methods according to the disclosed methods.

[0012] It is an advantage of the present disclosure that the disclosed method ensures that the wind turbine can, in certain situations, in view of the indicator of oscillations in the structural eigenmode inject a stabilizing power in reaction to a grid frequency oscillation event without risk of damaging of the wind turbine.

[0013] It is an advantage of the present disclosure that the disclosed method ensures that the wind turbine can inject a stabilizing power in reaction to a grid frequency oscillation event even when the stabilizing frequency overlaps with an eigenfrequency of a structural eigenmode of the wind turbine, in view of the indicator of oscillations in the structural eigenmode.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and other features and advantages of the present disclosure will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:

[0016] Fig. 1 is a diagram illustrating a wind turbine,

[0017] Fig. 2 is a schematic drawing showing a wind turbine and signal flows illustrating an exemplary operation of the wind turbine controller according to the disclosure.

[0018] Figs. 3-5 and 7 are flow-charts illustrating an exemplary method for providing grid-forming control of a wind turbine according to this disclosure, and

[0019] Figs. 6A-E are graphs illustrating grid frequency oscillations, stabilising power, indicator development, modification factor development, and modified stabilising power according to exemplary embodiment of this disclosure. DETAILED DESCRIPTION

[0020] Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the disclosure or as a limitation on the scope of the disclosure. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described.

[0021] As mentioned previously, a transmission system operator (TSO) provides a power grid code that specifies capabilities for generation units of the grid in terms of the operating parameters that they inject into the grid. These capabilities are values and value ranges - so-called rated or nominal values and rated or nominal ranges - which must be observed when operating a generation unit connected to the grid. As an example, a power grid code may specify that a generation unit connected to the grid shall, as a minimum, have the following capabilities in terms of the operating parameter frequency (from EirGrid Grid Code):

[0022] 1. operate continuously at normal rated output at grid frequencies in the range 49.5Hz to 50.5Hz;

[0023] 2. remain synchronised to the grid at grid frequencies within the range 47.5Hz to 52.0Hz for a duration of 60 minutes;

[0024] 3. remain synchronised to the grid at grid frequencies within the range 47.0Hz to 47.5Hz for a duration of 20 seconds required each time the frequency is below 47.5Hz; etc.

[0025] A power grid code will typically provide similar required capabilities in terms of other operating parameters such as AC voltage, AC current; active power; and reactive power.

[0026] Classical synchronous power generators driven by gas, coal, nuclear power etc. will have grid forming capabilities as their steam turbines are operated a fixed frequencies and thereby define the frequency of the AC voltage and AC current in the grid. Power generation units that are not operated at fixed frequencies, such as wind power, solar power etc., are often referred to as asynchronous generation units. As such units constitute larger and larger parts of the grid power supply, they are also required to contribute to the formation of stable grid voltage and frequency. Asynchronous generation units with grid forming capabilities can be operated to react to disturbances in the grid operating parameters by adjusting the injected power.

[0027] In the present disclosure, the grid forming capability of the wind turbine is provided by the converter controller that controls the converter to inject AC power according to a grid forming converter control scheme.

[0028] A TSO may specify how a grid forming generation unit shall react in case of grid disturbances in the form of deviations in one or more operating parameters of the power grid. A disturbance of the power grid may for example be a grid frequency oscillation event where the AC frequency of the grid oscillates around the nominal frequency of the grid, fnOm, typically 50 or 60 Hz. The oscillations typically have frequencies in the range of tenths of Hz to a few Hz and amplitudes of the order of tenths of Hz. In an exemplary grid frequency oscillation event, the AC frequency of the grid may oscillate between 49.85Hz and 50.15Hz with an oscillation frequency of 0.8Hz. Such grid frequency oscillation events may be caused by a combination of different factors such as very low consumption whereby the load contribution to damping is decreased, high voltage angle differences between different parts of the grid, unavailability of some generators leading to a nonstandard power flow in the system, and large imports from other grids. Depending on which parts of the power grid is affected by a grid frequency oscillation event, it is common to refer to the mode of oscillation being inter-area, intra-area, local or plant.

[0029] In the case of a grid frequency oscillation event, the specified reaction is to oscillate the injected active power at a frequency and phase that counteracts the disturbance. This reaction involves injecting active power (referred to as the stabilising power hereinafter) with a frequency oscillation around the nominal frequency fnOm in the range of tenths of Hz to a few Hz - hereafter referred to as the stabilising frequency fstab to distinguish it from the actual and nominal AC frequency of the grid. In such frequency ranges, there is a risk that this oscillation can couple to or resonate with structural eigenmodes of the wind turbine, resulting in excitation of structural eigenmodes that may damage the wind turbine. It is such coupling of an oscillating stabilising power injected in response to a grid frequency oscillation event to structural eigenmodes of the wind turbine which is to be handled to protect the wind turbine.

[0030] The present disclosure provides methods, electronic devices and wind turbines that enable a controlled reaction to potential excitation of structural eigenmodes of the wind turbine due to an oscillating stabilising power injected into the power grid in response to a grid frequency oscillation event.

[0031] Fig. 1 is a diagram illustrating a wind turbine 10 connected to a power grid 11. The wind turbine comprises a tower 12, and nacelle 14 and a rotor 18.

[0032] It is well known that wind turbines are subject to mechanical vibrations, manifested in structural eigenmodes of vibration in the different parts. A component such as the tower or a rotor blade may have several structural eigenmodes related to different vibrations therein. Typical structural eigenmodes of a wind turbine are related to vibrations in the tower, rotor, and drive-train. Each structural eigenmode has a corresponding eigenfrequency which may vary depending on for example environmental parameters such as wind speed and temperature, operational mode of the wind turbine, and operational parameters of the wind turbine. Many of the typical eigenmodes of a wind turbine have eigenfrequencies in the range of 0.1 Hz - 5Hz. Different eigenmodes may be coupled, either to form a coupled eigenmode to provide coupled motion of the structural elements or may be couple to influence each other so that even a weak vibration in one eigenmode may excite another eigenmode. As an example, oscillations on the blade edge moments indicative of mechanical vibration on the blades can also manifest itself on the generator speed measurements due to the mechanical coupling of the rotor / d rive-train.

[0033] Fig. 2 is a diagram schematically illustrating elements of a wind turbine, a wind turbine control system 19 comprising a wind turbine controller (WTC) 20, and a converter controller (CC) 40. In one or more example methods, the WTC 20 controls the power production of the wind turbine. In one or more example methods, the WTC 20 is implemented via an electronic device 22 comprising a memory circuitry 23, a processor circuitry 24, and an interface 25, and may be located in the wind turbine, external to the wind turbine, or may be a distributed in units located both in and external to the wind turbine. The memory circuitry 23 may store one or more programs comprising instructions, which when executed by the processor circuitry 24 cause the electronic device to perform any of the method steps according to the disclosed methods. The wind turbine elements comprise a generator 32 driven by the rotor typically via a shaft (not shown) and a gear box (not shown) to produce electrical power. In one or more example methods, the generator 32 may be controlled by the WTC 20 via a power reference.

[0034] The wind turbine elements comprise a converter 34 coupled to the generator 32 via a coupling 33 for converting electrical power produced by generator 32 to power PAC at the grid frequency that is bidirectionally exchanged between the generator 32 and the power grid 11.

[0035] The converter controller 40 is configured to control the converter 34 to inject AC power according to a grid forming converter, GFC, control scheme. In one or more example methods, the CC 40 may be implemented via an electronic device 42 comprising a memory circuitry 43, a processor circuitry 44, and an interface 45, and may be located in the wind turbine, external to the wind turbine, or may be a distributed in units located both in and external to the wind turbine. The memory circuitry 43 may store one or more programs comprising instructions, which when executed by the processor circuitry 44 cause the electronic device to perform any of the method steps according to the disclosed methods.

[0036] The converter 34 is illustrated as a single unit, however in general it will comprise a machine side converter and a grid side converter, separated by a DC link. The machine side converter being connected to the generator and is arranged for receiving the variable frequency power of the generator and converting it into the DC power of the DC link. The grid side converter being connected to the grid and is arranged for receiving the DC power of the DC link and it into the AC power of the grid. The converter controller is arranged to control both the machine side converter and the grid side converter.

[0037] The wind turbine control system 19, such as the WTC 20 and the CC 40, is configured to perform any of the methods or method steps according to the disclosed methods. The WTC 20 and CC 40 may exchange information and instructions via interfaces 25 and 45.

[0038] The power produced by the generator (also commonly referred to as the machine-side or rotor-side power) is coupled to the power injected into the grid by the converter (also commonly referred to as the grid-side power) via coupling 33 in different commonly known ways. The mechanical and electrical couplings between the machine-side power and the grid side power and how they influence each other depends on a lot of factors well known to the skilled person. Depending on how the machine-side and grid-side converters are connected, frequency oscillations on the injected power may couple to the generator 32 and from there to the mechanical structure of the wind turbine and potentially excite structural eigenmodes of turbine elements. If, for example, the converter 34 is controlled to inject a stabilising power with a stabilising frequency in the range of tenths of Hz to a few Hz, vibrations at this stabilising frequency may couple through the generator 32.

[0039] The wind turbine elements comprise sensors 35, 36, 37, 38 positioned on or in different parts of the wind turbine and connected for the WTC 20 to obtain sensed values from the sensors. Sensors 35-38 are selected and positioned to sense vibrations in structural eigenmodes of the wind turbine and may include sensors for sensing acceleration, velocity, position, force, stress, and others. While not illustrated, sensors may also be connected to the converter controller.

[0040] Fig. 3 shows a flow diagram of an example methods 90 and 100 performed by the WTC 20 and the CC 40. In one or more example methods, the WTC 20 and the CC 40 are implemented by electronic devices and the methods may thus be performed by one or more electronic device disclosed herein, such as electronic devices 22 and 42 of Fig. 2.

[0041] If a grid frequency oscillation event is occurring, it may be detected by the TPO, by the individual parts of a wind turbine such as the WTC 20 and the CC 40, as well as at many different levels therebetween. In principle, any entity that can detect the current grid power and knows the grid code may be configured to detect a grid frequency oscillation event. Similarly, the required response, in terms of a stabilizing power with corresponding stabilizing frequency and phase, can be determined by any entity that knows the current grid power and the grid code. As is illustrated in Figs. 6A and 6B, the frequency of the grid frequency oscillation (6A) and the stabilizing frequency (6B) are the same, with the stabilizing frequency being phase shifted to counteract the grid frequency oscillation.

[0042] With reference to Fig. 3, the method 90 comprises, by the CC 40, injecting S101 a stabilizing power in response to a grid frequency oscillation, wherein the injected stabilizing power oscillates around the nominal AC frequency fnOm at a stabilizing frequency fstab corresponding to a frequency of the grid frequency oscillation. The CC 40 may itself have detected the grid frequency oscillation event from the grid power and determined the stabilizing power from knowledge of the grid code, or it may have received this information from another entity. With reference to Fig. 3, the method 100 performed by the WTC comprises receiving S102 information indicative of the frequency of the grid frequency oscillation As described above, the current grid power entails information of the frequency of the grid frequency oscillation. By receiving indicative information such as the current grid power, the stabilizing power, or a power signal corresponding to the power generated by generator 32, the WTC 20 may, in one or more embodiments, itself determine S103 the frequency of the grid frequency oscillation and thus the stabilizing frequency. Alternatively or additionally, WTC 20 may receive other information allowing it to determine S103 the frequency of the grid frequency oscillation, or it may receive the frequency of the grid frequency oscillation or the stabilizing frequency directly. The method 100 performed by the WTC comprises receiving, from the converter controller, information indicating that a stabilizing power is being injected. The WTC does not necessarily need to know how large the stabilizing power is, as it merely looks at its effect and whether it need regulation.

[0043] As the CC 40 informs the WTC 20 that it injects a stabilising power, it would be straightforward that the converter controller also informs the WTC 20 of the stabilising frequency. Thus, in one or more embodiments, the step of receiving S102 information comprises receiving, from the converter controller, information indicative of stabilizing power injected in response to a grid disturbance, where the information comprises a stabilizing frequency of an oscillation of the injected stabilizing power around the nominal AC frequency.

[0044] The method 100 comprises determining S106 an indicator of oscillations in a structural eigenmode Ci, and determining S108 whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator . Upon determining to reduce the injected stabilizing power, the method 100 comprises reducing S110 the injected stabilizing power.

[0045] In one or more embodiments the method 100 comprises obtaining S104 values from one or more sensors 35-38 of the wind turbine, and determining indicator based on the obtained values. Values from different sensors may be combined to determine an indicator for a given eigenmode, and values from a given sensor may be used in the determination of indicators for different eigenmodes. In one or more embodiments, determining an indicator of oscillations in the structural eigenmode comprises determining an indicator that is proportional to an amplitude of the oscillations in the structural eigenmode.

[0046] The indicator is a variable indicative of oscillations in eigenmode Ci, with different eigenmodes of the wind turbine being referred to by index i = 1 , 2, N. An indicator may be any quantification of oscillations in or excitation of the corresponding eigenmode. Hence, an exemplary indicator for a tower side-side eigenmode may be an acceleration of the top or mid-height of the tower obtained from a sensor positioned in the top and elsewhere on the tower. An exemplary indicator for drive-train eigenmode excitation may be oscillations on the generator speed. An exemplary indicator for rotor blade eigenmode excitation may be oscillations on the blade edge moments on each blade.

[0047] In one or more embodiments, the method 100 comprises, upon determining S108 not to reduce the injected stabilizing power based on indicator , returning to the step of determining S106 indicator , such as determining indicator after a set period of time, such as periodically determining indicator , such as monitoring indicator . In one or more embodiments, the indicator is determined periodically based on updated values from the one or more sensors and the determining S108 is repeated for each new determination of .

[0048] An indicated in Fig. 3, steps that are specific to eigenmode Ci, such as S106 and S108 and other eigenmode-specific steps described later, may be performed for all eigenmodes Cl. .N.

[0049] In one or more embodiments, the step of reducing S110 the injected stabilizing power comprises controlling the converter 34 to reduce the injection of the stabilizing power, such as sending an instruction or a signal to the CC 40 for this to control the converter 34 to reduce the injection. To reduce the injection of the stabilizing power effectively means to reduce at least an amplitude of the oscillation of the active power around the nominal AC frequency (i.e. amplitude of sine-curve in Fig. 6B). In one or more embodiments, reducing the injection of the stabilizing power comprises reducing the injected stabilizing power by an amount that is proportional to an amplitude of the oscillations in the structural eigenmode, such as proportional to the indicator.

[0050] In one or more embodiments, method 100 is performed at regular intervals, such as continuously in accordance with a sampling frequency of the sensors. After a reduction in the injected stabilizing power, later sizes and / or a temporal development of indicators may show that there is currently no or little excitation of eigenmodes. For example, if the operational envelope of the wind turbine has changed, some eigenmodes may no more be of risk of excitation. In such cases, it may be of interest to try to increase the injected stabilizing power again. Therefore, in one or more embodiments, determining whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator comprises determining to increase the injected stabilizing power.

[0051] Fig. 4 illustrates different embodiments of method 100 of Fig. 3, related to the determination S106 of the indicator.

[0052] Since eigenmodes have different eigenfrequencies, not all eigenmodes may be at risk of being excited by a given stabilising frequency, and it may be advantageous to identify the eigenmodes that are at risk and sort out those that are not at risk of being excited. With reference to Fig. 4, one or more embodiments the method 100 comprises, after receiving S102 information indicative of the injected stabilizing power and stabilizing frequency, comparing S130 the stabilizing frequency with an eigenfrequency of structural eigenmode Ci of the wind turbine. Upon determining that the stabilizing frequency overlaps with the eigenfrequency of Ci, the determining S106 of indicator is carried out. Upon determining that the stabilizing frequency does not overlap with the eigenfrequency of Ci, no indicator needs be determined, and the process may stop for eigenmode Ci. An indicated in Fig. 3, the comparing S130 may be performed for all eigenmodes CI...N. For some eigenmodes of the rotor blades, the eigenfrequencies can depend on the operational state of the turbine, such as the rotor speed and the blade pitch. For example, such eigenmodes and eigenfrequencies are commonly referred to as whirling modes and whirling frequencies. Hence, the determination of frequency overlap may comprise first determining the eigenfrequencies for the current operational state of the turbine. Coupling to an eigenmode is also possible when the stabilising frequency and the eigenfrequency are near but not identical. Therefore, in one or more embodiments, determining S130 whether the stabilizing frequency overlaps with the eigenfrequency comprises determining that the stabilizing frequency lies within a predetermined range of the eigenfrequency, such as within + / - 5% of the eigenfrequency. The size of the predetermined range may be specific to each eigenmode and may be a tuneable parameter. It is worth mentioning that an overlap between the stabilizing frequency and the eigenfrequency does not mean that the eigenmode will definitely be excited by the stabilising power injection, only that there is a risk. Wind turbines are complex structures, and whether there is a mechanical coupling between the eigenmode and the injection of stabilising power - i.e. whether the eigenmode is actually excited thereby - is very difficult to predict.

[0053] Some oscillations with small amplitude in an eigenmode may not pose a serious problem, whereas a large or an increasing amplitude may. Therefore, in one or more embodiments of method 100, the indicator is indicative of an amplitude of oscillations in the structural eigenmode, the amplitude being determined based on the values from the one or more sensors. With reference to Fig. 4, one or more embodiments the method 100 comprises obtaining S104A-C values from one or more sensors A-C and calculating an amplitude Aj of oscillations in the eigenmode Ci using the obtained sensor values. The determined amplitude Aj is then used in the determination S106 of indicator .

[0054] Wind turbines will generally have active damping systems for reducing vibration in selected eigenmodes, such as active tower damping using pitch actuation of the rotor blades. If a wind turbine has an active damping feature connected to a given eigenmode, the eigenmode vibrations may be controlled even though the eigenmode is in effect excited by the stabilising power injection. Some active damping systems uses the generator as an actuator to counteract vibrations, and such active damping systems are herein referred to as active damping power features. If an active damping power feature is used to damp oscillations in structural eigenmode Ci having an eigenfrequency that overlaps with the stabilising frequency, there is a risk that the active damping power feature and the injected stabilizing power are counteracting one another since both relies on the power generation in the generator 32. Therefore, in one or more embodiments, the indicator is indicative of an actuation level of an active damping power feature used to damp oscillations in the structural eigenmode. With reference to Fig. 4, actuation S105 of an active power damping feature for eigenmode Ci is typically set based on values from the one or more sensors A-C that may also be used to determine oscillations in the eigenmode. Hence, an actuation level of the active damping power feature may be determined from the obtained sensor values. In one or more embodiments, determining S108 whether to reduce the injected stabilizing power comprises determining, from the actuation level of the active damping power feature, that the active damping power feature and the injected stabilizing power are counteracting one another. With reference to Fig. 4, this may comprise comparing S134 the injected stabilizing power and actuation of the active damping power feature, such as comparing temporal variations in the injected stabilizing power and actuation of the active damping power feature. Fig. 5 illustrates different embodiments of method 100 of Fig. 3, related to the determination S108 of whether to reduce the injected stabilising power and the reduction S110 of the injected stabilising power.

[0055] Having determined the indicator in step S106, the method 100 comprises determining S108 whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator. In one or more embodiments, there is provided a predetermined maximum threshold, II MAX for the indicator for the eigenmode Ci, indicating a maximum allowed excitation of the eigenmode. The maximum allowed excitation of an eigenmode, and thus the corresponding predetermined maximum threshold, may for example be determined during the design of the corresponding wind turbine part, but could also be a dynamical threshold depending on operational parameters of the wind turbine.

[0056] With reference to Fig. 5, in one or more embodiments, determining S108 whether to reduce the injected stabilizing power comprises determining S112 whether the current indicator value determined in S106 fulfils a condition linked to the predetermined maximum threshold. Preferred conditions may be whether the indicator value determined in S106 is equal to the predetermined maximum threshold II MAX, whether it exceeds II AX, whether it lies within a predetermined range of II MAX, or whether a temporal development of the indicator exceeds a predetermined threshold for temporal development of the indicator. If the chosen condition is fulfilled, it is determined to reduce S110 the injected stabilising power.

[0057] For some structural eigenmodes of the wind turbine, it may be desired that the injection of stabilising power with the stabilising frequency be stopped immediately when the chosen indicator condition is fulfilled. Therefore, in one or more embodiments, the method 100 comprises determining that the indicator fulfils a condition linked to the predetermined maximum threshold; and wherein controlling the converter to reduce the injection of the stabilizing power comprises stopping S110A injection of the stabilizing power.

[0058] For some structural eigenmodes of the wind turbine, it may be desired that the injection of stabilising power with the stabilising frequency be reduced when the chosen indicator condition is fulfilled or when the indicator approaches the predetermined maximum threshold, such as when temporal development of the indicator (t) is growing faster than a predetermined rate. The reduction in injection of stabilising power may be by a fixed amount or may depend on the strength of the oscillations in the eigenmode so that the larger the oscillations, the more the injected stabilizing power will be reduced in order to protect the structural integrity of the wind turbine. With reference to Fig. 5, in one or more embodiments, determining S108 whether to reduce the injected stabilizing power comprises determining S114 a difference between the indicator value determined in S106 and the predetermined maximum threshold. Further, controlling S110 the converter to reduce the injection of the stabilizing power may comprise determining S122 a reduction factor R based on the difference, wherein the reduction factor is used to reduce the injected stabilizing power or at least an amplitude of the oscillation of the injected stabilizing power around the nominal AC frequency. The step of reducing S110 will then comprise reducing S110B the stabilizing power by reduction factor R.

[0059] The graphs shown in Figs. 6A-E illustrate an embodiment of reducing S110B the injected stabilizing power by a reduction factor Rj determined based on the indicator . The graph of Fig. 6A shows the actual grid frequency oscillating around the nominal grid frequency for a grid frequency oscillation event. The graph of Fig. 6B shows the stabilizing active power to be injected by the converter of the wind turbine. The stabilizing power is intended to counteract the grid frequency oscillations and here oscillates with the same frequency as the grid oscillations and with a shifted phase relative to the grid oscillations. Fig. 6B indicates the amplitude of the oscillation of the stabilizing power around an average value. The nominal AC frequency fnOm is not indicated and is typically much larger than fstab. The graph of Fig. 6C shows an exemplary determined indicator value for an eigenmode Ci of the wind turbine. Here, the injected stabilizing power couples to the eigenmode which starts oscillating with increasing amplitude, resulting in the increase in indicator value over time. The graph of Fig. 6C also shows the predetermined maximum threshold for the indicator. The graph of Fig. 6D shows an exemplary normalized reduction factor R, where R = 1 equals no reduction of the injected stabilizing power, and R = 0 equals complete reduction, i.e., stopping injected stabilizing power. For example, R may be determined as inversely proportional to the indicator I j or as proportional to a normalized difference between and II MAX. Lastly, the graph of Fig. 6E shows the injected stabilizing power being reduced by the reduction factor R in response to the stabilizing power injection exciting eigenmode Cj.

[0060] With reference to Fig. 3, the wind turbine may have N different eigenmodes Cj with i = 1 ... N, and performing the steps of determining S106 indicator and possibly determining S108 whether to reduce the injected stabilising power based on may be carried out for each eigenmode individually, or at least for each eigenmode whose eigenfrequency overlaps with the stabilising frequency. These repetitions may comprise any of the embodiments described in relation to Figs. 4 and 5.

[0061] In embodiments predetermined maximum thresholds are set, either as a value or range or as predetermined threshold for temporal development. Typically such thresholds values, ranges and temporal developments are set for normal production to ensure the wind turbine stays within safe operation. In embodiments, when operated in a grid forming mode, and the grid forming converter is controlled to inject stabilizing power, these thresholds are changed from the normal production supervision thresholds to specific changed predetermined thresholds. This is done to ensure that the wind turbine is not shutting down for triggering a normal production threshold while operating in the special operational state of injecting stabilizing power. In embodiments, thresholds for monitoring drive train oscillations may be set higher as increased drive train oscillations may be seen. In embodiments, thresholds for monitoring sidewards tower oscillations may be set higher as increased sidewards tower oscillation may be tolerated. In another embodiment, a threshold for monitoring the output power deviating from the power reference may also be set higher to accommodate for the potentially increased power variation during injection of stabilizing power. Injection of stabilizing power during grid forming control is a special operational mode, and therefore trigger limits can be changed without jeopardizing the mechanical safety of the wind turbine.

[0062] Fig. 7 illustrates the reduction of the injected stabilising power based on the determined indicator for N eigenmodes. With reference to Fig. 7, in one or more embodiments, the method 100 comprises performing the steps of determining S106 the indicator and determining S114 the difference between the indicator and the predetermined maximum threshold for two or more different structural eigenmodes of the wind turbine, and normalizing S116 and comparing S118 the normalised differences to determine the smallest normalised difference, i.e. difference corresponding to the eigenmode with the highest excitation relative to its maximum allowed excitation. With reference to Fig. 7, determining S108 whether to reduce the injected stabilizing power may comprise determining whether to reduce the injected stabilizing power based on the determined smallest normalized difference. In one or more embodiments, the method 100 comprises determining 122 the reduction factor R based on the smallest normalised difference corresponding to the most excited eigenmode. In case of a grid frequency oscillation event, the WTC and / or the converter controller has either determined or received a requested stabilising power specified by the TSO such as in the grid code. With reference to Fig. 7, the TSO may send information 50 regarding a grid frequency oscillation event. Having determined to reduce the injected stabilising power and the reduction factor R, the WTC or the converter controller uses the requested or the currently injected stabilising power to reduce the stabilising power in accordance with the reduction factor R.

[0063] Fig. 2 includes block diagrams of exemplary electronic devices 22 and 42 according to the disclosure. The electronic devices 22 and 42 are configured to perform any of the methods or method steps disclosed in Figs. 3, 4, 5, and 7.

[0064] The electronic device 22, 42 is configured to obtain (e.g., via processor circuitry 24, 44 and / or interface 25, 45). The processor circuitry 24, 44 is optionally configured to perform any of the steps and operations disclosed in Figs. 3, 4, 5, and 7. The operations of the electronic device 22, 42 may be embodied in the form of executable logic routines (e.g., lines of code, software programs, etc.) that are stored on a non-transitory computer readable medium (e.g., the memory circuitry 23, 43) and are executed by the processor circuitry 24, 44).

[0065] Furthermore, the operations of the electronic device 22, 42 may be considered a method that the electronic device 22, 42 is configured to carry out. Also, while the described functions and operations may be implemented in software, such functionality may as well be carried out via dedicated hardware or firmware, or some combination of hardware, firmware and / or software.

[0066] The memory circuitry 23, 43 may be one or more of a buffer, a flash memory, a hard drive, a removable media, a volatile memory, a non-volatile memory, a random-access memory (RAM), or other suitable device. In a typical arrangement, the memory circuitry 23, 43 may include a non-volatile memory for long term data storage and a volatile memory that functions as system memory for the processor circuitry 24, 44. The memory circuitry 23, 43 may exchange data with the processor circuitry 24, 44 over a data bus. Control lines and an address bus between the memory circuitry 23, 43 and the processor circuitry 24, 44 also may be present (not shown in Fig. 2). The memory circuitry 23, 43 is considered a non-transitory computer readable medium. It is noted that any reference signs do not limit the scope of the claims, that the exemplary embodiments may be implemented at least in part by means of both hardware and software, and that several "means", "units" or "devices" may be represented by the same item of hardware.

[0067] The various exemplary methods, devices, nodes, and systems described herein are described in the general context of method steps or processes, which may be implemented in one aspect by a computer program product, embodied in a computer- readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Program circuitries may include routines, programs, objects, components, data structures, etc. that perform specified tasks or implement specific abstract data types. Computer-executable instructions, associated data structures, and program circuitries represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

[0068] Although features have been shown and described, it will be understood that they are not intended to limit the claimed disclosure, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the claimed disclosure. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed disclosure is intended to cover all alternatives, modifications, and equivalents.

Claims

CLAIMS1. A method for providing grid-forming control of a wind turbine electrically coupled to a power grid, the wind turbine comprising:- a generator driven by a rotor to produce electrical power;- a wind turbine controller, WTC;- a converter coupled to the generator and the power grid for converting the produced electrical power to generate AC power injected into the power grid at a nominal AC frequency; and- a converter controller for controlling operation of the converter according to a grid forming converter, GFC, control scheme; the method comprising, by the converter controller:• injecting a stabilizing power in response to a grid frequency oscillation, wherein the injected stabilizing power oscillates around the nominal AC frequency at a stabilizing frequency corresponding to a frequency of the grid frequency oscillation; the method comprising, by the WTC:• receiving information indicative of the frequency of the grid frequency oscillation and receiving, from the converter controller, information indicating that a stabilizing power is being injected;• comparing the frequency of the grid frequency oscillation with an eigenfrequency of a structural eigenmode of the wind turbine;• upon determining that the frequency of the grid frequency oscillation overlaps with the eigenfrequency, determining, based on values from one or more sensors of the wind turbine, an indicator of oscillations in the structural eigenmode;• determining whether to reduce the injected stabilizing power based on the size and / or a temporal development of the indicator; and the method comprising, upon determining to reduce the injected stabilizing power, controlling the converter to reduce the injection of the stabilizing power.

2. The method according to claim 1 , wherein the indicator is indicative of an amplitude of oscillations in the structural eigenmode, the amplitude being determined based on the values from the one or more sensors.

3. The method according to any of the preceding claims, wherein the indicator is indicative of an actuation level of an active damping power feature used to damp oscillations in the structural eigenmode.

4. The method according to claim 3, wherein determining whether to reduce the injected stabilizing power comprises determining, from the actuation level of the active damping power feature, that the active damping power feature and the injected stabilizing power are counteracting one another.

5. The method according to any of the preceding claims, wherein determining that the frequency of the grid frequency oscillation overlaps with the eigenfrequency comprises determining that the frequency of the grid frequency oscillation lies within a predetermined range of the eigenfrequency.

6. The method according to any of the preceding claims, wherein the indicator is determined periodically based on updated values from the one or more sensors.

7. The method according to any of the preceding claims, wherein the indicator for the eigenmode has a predetermined maximum threshold indicating maximum allowed excitation of the eigenmode and wherein determining whether to reduce the injected stabilizing power comprises determining whether the indicator fulfils a condition related to the predetermined maximum threshold.

8. The method according to claim 7, wherein determining whether to reduce the injected stabilizing power comprises determining whether the indicator lies within a predetermined range of the predetermined maximum threshold.

9. The method according to claim 7, wherein determining whether to reduce the injected stabilizing power comprises determining that the indicator fulfils a condition linked to the predetermined maximum threshold; and wherein controlling the converter to reduce the injection of the stabilizing power comprises stopping injection of the stabilizing power.

10. The method according to claim 7, wherein determining whether to reduce the injected stabilizing power comprises determining a difference between the indicator and the predetermined maximum threshold.

11. The method according to claim 10, wherein controlling the converter to reduce the injection of the stabilizing power comprises determining a reduction factor based on the difference, wherein the reduction factor is used to reduce an amplitude of the oscillation of the injected stabilizing power around the nominal AC frequency.

12. The method according to claim 10, comprising, in the WTC, performing the steps of determining the indicator and determining the difference between the indicator and the predetermined maximum threshold for two or more different structural eigenmodes of the wind turbine; and normalizing and comparing the determined differences to identify the smallest normalized difference; wherein determining to reduce the injected stabilizing power comprises determining whether to reduce the injected stabilizing power based on the smallest normalized difference.

13. The method according to claim 12, wherein controlling the converter to reduce the injection of the stabilizing power comprises determining a reduction factor based on the smallest normalized difference, wherein the reduction factor is used to reduce an amplitude of the oscillation of the injected stabilizing power around the nominal AC frequency.

14. The method according to any of the preceding claims, wherein determining whether to reduce the injected stabilizing power comprises determining whether a temporal development of the indicator exceeds a predetermined threshold for temporal development of the indicator.

15. A wind turbine comprising:- a generator driven by a rotor to produce electrical power;- a wind turbine controller, WTC;- a converter coupled to the generator and the power grid for converting the produced electrical power to generate AC power injected into the power grid at a nominal AC frequency; and- a converter controller for controlling operation of the converter according to a grid forming converter, GFC, control scheme; wherein the WTC and the converter controller are configured to perform any of the methods according to any of claims 1-14.

16. An electronic device comprising a memory circuitry, a processor circuitry, and a wireless interface, wherein the electronic device is configured to perform any of the methods according to any of claims 1-14.

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