Applying restriction to active power delivery from grid forming converter due to torque limitation

Abstract

Disclosed is a method for providing grid-forming control of a wind turbine electrically coupled to a power grid. The method comprises a wind turbine controller, (WTC), defining a first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging the wind turbine, and a second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without damaging the wind turbine. The method comprises the WTC, determining, in agreement with the second upper limit, a maximum power threshold for AC power injected into the power grid by the converter in case of a grid disturbance, and providing the maximum power threshold to a converter controller for controlling operation of the converter according to a grid forming converter, (GFC), control scheme. The method comprises the converter controller controlling the converter to inject AC power in accordance with the GFC control scheme while respecting the maximum power threshold on the injected AC power.

Description

[0001] APPLYING RESTRICTION TO ACTIVE POWER DELIVERY FROM GRID FORMING CONVERTER DUE TO TORQUE LIMITATION

[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, and the inertia of these grid forming generation units stabilizes the power grid to a nominal or rated 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. As these renewable energy generation units deliver an increasing part of the power, their AC power converters can be operated in a grid forming mode 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 to assist in stabilising grid operation. The reaction includes adjusting operating parameters, typically resulting in a brief (seconds to minutes) increase in injected active power, often to be delivered within hundreds of milliseconds of the disturbance. The reaction is performed by a converter of the wind turbine, and the increase in injected active power leads to an extra load on the wind turbine. This leads to a risk of damaging the structural integrity of the wind turbine.

[0006] SUMMARY

[0007] It is an object of the present invention to improve the ability of wind turbines with grid forming capabilities to react to grid disturbances without risk of damaging the structural integrity of the wind turbine.

[0008] Accordingly, it would be a benefit to provide a method, a wind turbine and an electronic device that may allow reaction to grid disturbances without putting a risk on the structural integrity of the wind turbine. Accordingly, it would be a benefit to provide a method, a wind turbine, and an electronic device for controlling operation of a wind turbine, which mitigates, alleviates or addresses the existing shortcomings.

[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; a converter coupled to the power grid for converting the produced electrical power and inject AC power into the power grid; and a converter controller for controlling operation of the converter according to a grid forming converter control scheme. The wind turbine controller defines a first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging a structural integrity of the wind turbine, and a second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without risk of damaging the structural integrity of the wind turbine. The method comprises, by the wind turbine controller, determining, in agreement with the second upper limit, a maximum power threshold for AC power injected into the power grid by the converter, and providing the maximum power threshold to the converter controller. The method comprises, by the converter controller: receiving input relating to the maximum power threshold; and, based on the received input relating to the maximum power threshold, controlling the converter to inject AC power in accordance with the grid forming converter control scheme while respecting the maximum power threshold on the injected AC power.

[0010] Disclosed is a wind turbine comprising a generator driven by a rotor to produce electrical power; a wind turbine controller; a converter coupled to the power grid for converting the produced electrical power to generate and inject AC power into the power grid; and a converter controller for controlling the converter to inject AC power according to a grid forming converter control scheme. The wind turbine controller and the converter controller are configured to perform any of the methods according to the disclosed methods.

[0011] Disclosed is 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 the disclosed methods. 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 grid forming converter can react to grid disturbances autonomously without risk of overloading and damaging the generator.

[0013] It is an advantage of the present disclosure that the grid forming converter can react to grid disturbances quickly, without first having to confer with the wind turbine controller.

[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 a wind turbine controller, and a signal flow illustrating an exemplary operation of the wind turbine controller according to the disclosure, and

[0018] Fig. 3 is a flow-chart illustrating an exemplary operation of the wind turbine controller according to the disclosure.

[0019] 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. 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 p. 91):

[0021] 1. operate continuously at normal rated output at grid frequencies in the range 49.5Hz to 50.5Hz;

[0022] 2. remain synchronised to the grid at grid frequencies within the range 47.5Hz to 52.0Hz for a duration of 60 minutes;

[0023] 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.

[0024] 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.

[0025] 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 consitute larger and larger parts of the grid power supply, they may beneficially 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.

[0026] 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. 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 system value deviating from a nominal value or a nominal range of an operating parameter, where a ‘system value’ of an operating parameter is the actual value currently measured in the power grid and the ‘nominal value’ and ‘nominal range’ of the operating parameter are the value and value range provided in the power grid code. Hence, the disturbance may be a deviation in a system AC voltage amplitude from a nominal amplitude or a nominal amplitude range and / or a deviation in a system frequency from a nominal frequency or a nominal frequency range specified in the power grid code.

[0027] For a voltage dip event, the required reaction typically includes injecting reactive current, commonly referred to as “ride-through” capability. For a frequency oscillation event, the required reaction typically include injecting active power with an oscillation frequency and amplitude that counteracts the grid oscillations.

[0028] 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.

[0029] Fig. 2 is a diagram schematically illustrating elements of a wind turbine and elements of a wind turbine controller (WTC) 20. Fig. 2 also shows a signal flow chart illustrating a method for operating the wind turbine 10 according to the disclosure. In one or more example methods, the WTC 20 controls operation of the wind turbine 10 and at least indirectly (e.g. via units 40, 34) controls power generation by generator 32. In one or more example methods, the WTC 20 comprises a main controller, also sometimes referred to as a load and production controller, being 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 10, external to the wind turbine 10, or may be a distributed in units located both in and external to the wind turbine 10. 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 methods or method steps according to the disclosed methods.

[0030] The wind turbine 10 comprises a generator 32 driven by the rotor typically via a shaft (not shown) and a gear box (not shown) to produce electrical power. In direct drive systems, and other systems, the gear box may not be present. In one or more example methods, the generator 32 may be controlled by the WTC 20. The WTC 20 defines or determines a first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging a structural integrity of the wind turbine. The WTC 20 also defines or determines a second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without risk of damaging the structural integrity of the wind turbine. In this respect, continuously without risk means that the generator can be operated with this maximum torque for optimal power production for an indefinite period of time. In case of extraordinary stress or load events such as sudden wind gusts, additional power demands etc., the WTC will allow the generator torque to increase the first upper limit. In this respect, the first upper limit may be referred to as a “soft” upper limit. Also, in this respect, non-continuous operation means operation for a limited time, such in case of overboost operation to meet an increased short-time power demand. Here, overboost operation is when the wind turbine generator produces a power output that exceeds the power output that is available from ambient wind conditions at the wind turbine generator or produces a power output that exceeds the rated power output at the current wind speed. In various embodiments, the additional power output provided through overboosting may be derived from the kinetic energy of the rotor during its power generation operation. The wind turbine generators may have limited amounts of overboost capacity before their respective rotors are slowed to a minimum allowable speed. During non-continuous operation up to the second upper limit, any change in other operating parameters may result in the WTC decreasing the generator torque so as not to exceed the second upper limit. In this respect, the second upper limit may be referred to as a “hard” upper limit.

[0031] In embodiments, the second upper limit may be based on the rated power and be set as a predetermined amount of the rated power being permitted for a predetermined time period. In below rated operation, such as in partial load operation, the rated power may be understood as the power defined by the power curve for the operating point. In an embodiment, the predetermined amount of the rated power may be set as a percentage of the rated power, such as between 2% and 20%, such as between 5% and 15% or such as 10%. The second upper limit may be allowed for non-continuous operation, defined as an allowed predetermined time period. The predetermined time period may be below 1 minute, such as between 1 second and 45 seconds, such as between 2 seconds and 30 seconds, such as between 5 seconds and 20 seconds, such as 10 seconds.

[0032] In an embodiment, the second upper limit may be allowed for non-continuous operation, defined by a period where a temperature of an electrical component is below a predefined threshold temperature. When operating at the second upper limit the temperature may increase in in the electrical components. Such temperature increase may be determined by a sensor reading or by use of an estimator, implemented to provide a temperature estimate of a given electrical component. Use of a temperature signal or estimate to limit the period of the non-continuous operation may be done in combination with, or as an alternative to, a pre-defined time period.

[0033] In an embodiment, upon expiry period or the period where non-continuous operation at the second upper limit is allowed, subsequent operation may be done at the first upper limit.

[0034] The wind turbine 10 comprises a converter 34 coupled to the generator 32 for converting the electrical power produced by generator 32 to power PAC at the grid frequency for exchange with the grid. The converter 34 (grid-side) may be connected to the generator 32 (machine-side) via a DC link. The converter 34 is coupled to the power grid 11 and may exchange electrical power P c with the power grid 11. The electrical power PACmay comprise active and / or reactive power.

[0035] The wind turbine 10 comprises a converter controller 40 for controlling the converter 34 to inject AC power according to a grid forming converter, GFC, control scheme. In one or more example methods, the converter controller 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 10, external to the wind turbine 10, or may be a distributed in units located both in and external to the wind turbine 10. 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 methods or method steps according to the disclosed methods.

[0036] The WTC 20 and the converter controller 40 are configured to perform any of the methods or method steps according to the disclosed methods. The WTC 20, hereunder the main controller 22, and converter controller 40 may exchange information and instructions via interfaces 25 and 45.

[0037] The converter 34 may convert electrical power that is exchanged between the generator 32 and the power grid 11. The converter 34 is operated in accordance with a GFC control scheme in which the converter system is controlled so as to behave as an ideal voltage source. Accordingly, converter 34 may be operated so as to maintain a voltage amplitude, a voltage frequency and / or a voltage phase angle to control the electrical (active and reactive) power being exchanged with the power grid 11.

[0038] Fig. 3 shows flow diagrams of an example method 100 comprising method 200 performed by the WTC 10 and method 400 performed by the converter controller 40, respectively, for providing grid forming control of a wind turbine according to the disclosure. In one or more example methods, the WTC 20 and the converter controller 40 are implemented by electronic devices and the method may thus be performed by one or more electronic device disclosed herein, such as electronic devices 22 and 42 of Fig. 2

[0039] With reference to Fig. 3, the method 200 comprises defining or determining S202A the first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging a structural integrity of the wind turbine. The WTC 20 also defines or determines S202B a second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without risk of damaging the structural integrity of the wind turbine.

[0040] With reference to Fig. 3, the method 200 performed by the WTC 20 comprises, determining S204B, in agreement with the second upper limit, a maximum power threshold, PMAX, S206 for AC power injected into the power grid 11 by the converter 34. The maximum power threshold is thus based on the second upper limit of generator torque and therefore also related to a maximum power produced by the generator for a given rotor speed. The power produced by the generator (also commonly referred to as the machine-side or rotor-side power) can be coupled to the power injected into the grid by the converter (also commonly referred to as the grid-side power) in different commonly known ways. For example, in a common setup, a machine-side converter is connected to the grid side converter via a DC link. In this as well as other cases, the relation between the machine-side power and the grid side power will be well known to the skilled person. Given well-known relations between generator torque and machine-side power as a function of rotor speed allows the WTC to determine a maximum power threshold for injected power based on the second upper limit on generator torque, where the dependency on rotor speed lies either at the second upper limit or the maximum power threshold.

[0041] In one or more example methods, the first and second upper limits are determined based on a rotor speed R, such obtained from by a rotation frequency of the generator, and wherein values of the first and second upper limits are periodically updated using a current rotor speed, such as updated continuously, such as at periodic intervals smaller than 1 / 10 seconds. The WTC 20 may also base the determination of the first and second upper limits based on a current operating point of the wind turbine in terms of at least rotor speed or active power output, but potentially also including pitch angle, available wind estimation or various measurements.

[0042] In one or more example methods, the first and second upper limits are calculated using one or more functions having at least the rotor speed or a related parameter such as rotation frequency of the generator. In one or more example methods, the first and second upper limits are stored in a look-up table. For example, the look-up table may store and return the first and second upper limits given at least the rotor speed or a related parameter.

[0043] The WTC 20 provides S206 the maximum power threshold to the converter controller 40. With reference to Fig. 2, the step of providing S206 PMAX to the converter controller 40 may be implemented by signalling from the electronic device 22 to the electronic device 42. That the maximum power threshold is determined in agreement with the second upper limit means that when the converter generates and injects AC power below the maximum power threshold, then the generator torque will stay below the second upper limit. In an embodiment the WTC determines and provides the maximum power threshold to the converter controller periodically and not in response to having been informed of a grid disturbance or of an energy or power either already delivered or to be delivered in reaction to a grid disturbance. The WTC may normally determine and provide the maximum power threshold without using input relating to an energy or power to be injected in reaction to a grid disturbance, i.e. a determined value of the maximum power threshold is independent of, such as not based on, an energy or power to be delivered in reaction to a grid disturbance. Therefore, in one or more example methods, the WTC determines and provides the maximum power threshold independently of the AC power injected into the power grid. In one or more example methods, the WTC determines and provides the maximum power threshold independently of an operating status of or any disturbances in the power grid.

[0044] In one or more example methods, determining S204B and providing S206 the maximum power threshold comprises periodically determining an updated value of the maximum power threshold PMAX for AC power injected into the power grid by the converter and periodically providing the updated value of the maximum power threshold to the converter controller. In an exemplary embodiment, the WTC calculates the maximum power threshold for injected power as a function of rotor speed and periodically determines and provides updated values of the maximum power threshold based on the current rotor speed and a static or an updated value of the second upper limit. In an exemplary embodiment, the WTC determines the maximum power threshold for injected power based on an updated value of the second upper limit, where the updated value of the second upper limit is determined as a function of rotor speed and / or other parameters of the wind turbine.

[0045] With reference to Fig. 3, the method 400 performed by the converter controller 40 comprises receiving S402 input relating to the maximum power threshold PMAX- The method 400 comprises controlling S404, based on the received input relating to the maximum power threshold, the converter 34 to inject AC power in accordance with the GFC control scheme while respecting the maximum power threshold on the injected AC power.

[0046] In one or more example methods, the converter controller periodically receives S402 input relating to an updated value of the maximum power threshold from the WTC 20. The converter controller controls S404 operation of the converter based on the most recently received input relating to the maximum power threshold. In one or more example methods, the received input relating to the maximum power threshold is a value of the maximum power threshold.

[0047] In one or more example methods, controlling S404 the converter to inject AC power while respecting the maximum power threshold on the injected AC power comprises: - monitoring the injected AC power, PAc;

[0048] - comparing the input, such as the most recently received input, relating to the maximum power threshold, PMAX, with the injected AC power; and

[0049] - before the injected AC power reaches the maximum power threshold, controlling the converter to reduce the injected AC power to keep the injected AC power under the maximum power threshold.

[0050] In one or more example methods, controlling S404, by the converter controller 40, the converter 34 to inject AC power in accordance with the GFC control scheme comprises:

[0051] - Operating the converter 40 to control S404A the exchange of electrical power between the generator 32 and the power grid 11 in accordance with operating parameters of the power grid 11 , such as prescribed by a power grid code. The operating parameters comprise at least one of: AC voltage, frequency, AC current; active power; and reactive power. This compares to normal operation without grid disturbances.

[0052] - In case of a power grid disturbance D, controlling S420 the converter 34 to inject AC power with one or more stabilising components of the injected AC power that seek to counteract the disturbance.

[0053] As controlling S404 operation of the converter respects the maximum power threshold, it follows that the injected AC power from the converter is not allowed to surpass the maximum power threshold, not even in case of grid disturbances. Thus, the injected AC power with stabilising component is also not allowed to surpass the maximum power threshold.

[0054] Typical operating parameters for the power grid 11 may comprise power (W), voltage amplitude (V), frequency (Hz or rad / s) of the grid voltage or the grid current, and phase angle between voltage and current. A grid disturbance may be any deviation from rated values or rated ranges of any of these grid operating parameters. When a disturbance occurs, the converter controller 40 may determine a disturbance power Pd and a disturbance energy Ed indicative of the electrical power and energy that is required to be exchanged with the power grid 11 in response to the disturbance D, such as in accordance with the grid code. The stabilizing component is the amount of electrical power Pd and energy Ed that is exchanged between the grid 11 and the wind turbine 10 for the support of the power grid in response to a grid disturbance. The total injected AC power, i.e. with or without the stabilising component, is referred to as PAc in the following.

[0055] Since the converter 40 controller has already received S402 the the maximum power threshold on the injected AC power from the WTC 20, it does not need to check the determined stabilizing componment with the WTC 20 ensure feasibility of the wind turbine 10, in particular the generator 32. Instead, the converter controller 40 can determine or decide how large a stabilizing component to inject independetly of the WTC.

[0056] As the reaction to the grid disturbance should be fast, it is an advantage that the converter 34 is operated to react without the WTC first having to evaluate for system feasibility in view of current operating conditions (e.g. wind speed and generator torque). Also, to maximise the stabilizing contribution, and thus to live up to the grid forming requirements, it is advantageous that the converter can react with a stabilizing component that is as large as possible without risk to the wind turbine. It is thus an advantage that the converter controller can react to grid disturbances autonomously, i.e. independently of the WTC. The WTC may be informed of grid disturbances and of planned or injected stabilising components, but the PMAX value set by the WTC does not depend thereon. Typically, the converter and / or converter controller utilise a much higher sampling rate than the WTC, so in effect the WTC will learn about grid disturbances and planned or injected stabilising components after the converter controller has initialised controlling the converter to inject AC power with a stabilising component. Therefore, in one or more example methods, upon receiving input relating to a disturbance of the power grid, the converter controller initiates controlling the converter to inject AC power with a stabilising component prior to informing the WTC of the disturbance or of a determined or injected stabilising component.

[0057] That the converter controller may operate autonomously means that it will not wait for approval or updated PMAX values before initialising controlling the converter to inject AC power with a stabilising component, but instead rely on already received values of P AX, such as the most recently received information relating to PMAX. In one or more example methods, the converter controller stores the received input relating to the maximum power threshold and, upon receiving input relating to a disturbance of the power grid, controls the converter to inject AC power with a stabilising component based on a stored value of the maximum power threshold. With reference to Fig. 3, controlling S420 the converter 34 to inject AC power with one or more stabilising components of the injected AC power that seek to counteract the disturbance may be implemented in different ways. In one or more example methods, the method 400 comprises, upon receiving S404B input relating to a disturbance D of the power grid, the converter controller determines S406 the stabilising component of the injected AC power that seeks to counteract the disturbance.

[0058] In one or more example methods, receiving S404B input relating to a disturbance D comprises monitoring S405 operating parameters of the power grid 11 and determining the disturbance D from deviations between the monitored operating parameters and the rated operating parameters.

[0059] In one or more example methods, the determination S406 of the stabilising component may comprise receiving input relating to the stabilising component from the grid operator, or the determination S406 of the stabilising component may comprise calculating the stabilizing component as a function of the disturbance D or looking up the disturbance in a look-up table using the disturbance D. As described previously, the required reaction in case of grid disturbances is typically specified in the grid code, which may thus provide the basis for how to determine the stabilizing component.

[0060] When the stabilizing component has been determined, the converter controller may proceed in different ways.

[0061] For example, the converter controller 40 may directly inject the determined stabilizing component. Thus, in one or more example methods, the method 400 comprises:

[0062] - upon receiving S404B input relating to a disturbance D of the power grid, determining S406 a stabilising component of the injected AC power that seeks to counteract the disturbance; and

[0063] - controlling S420 the converter to inject AC power with the stabilising component.

[0064] In another example, the converter controller 40 may calculate the effects of the determined stabilizing component before controlling the converter to inject it. Thus, in one or more example methods, the method 400 comprises: upon receiving S404B input relating to a disturbance D of the power grid, determining S406 a stabilising component of the injected AC power that seeks to counteract the disturbance;

[0065] - calculating S408 a value of the injected AC power with the stabilising component; and

[0066] - upon determining S410 that the calculated value is lower than the maximum power threshold, controlling S420 the converter to inject AC power with the stabilising component.

[0067] Both the stabilising component and the maximum power threshold are dynamical and may vary over time as other parameters in both the wind turbine and the power grid changes, and they may thus change during injection of AC power with the stabilising component. Therefore, in one or more example methods, the method 400 comprises.

[0068] - monitoring S422 the injected AC power with the stabilising component;

[0069] - comparing S424 the input relating to the maximum power threshold PMAX with the injected AC power with the stabilising component PAC; and

[0070] - before the injected AC power with the stabilising component reaches the maximum power threshold, controlling the converter to reduce the stabilising component to keep the injected AC power with the stabilising component under the maximum power threshold.

[0071] The comparison between the maximum power threshold and the injected AC power with the stabilising component may comprise determining whether P c falls within a predetermined range of PMAX, such as determining whether P c > PMAX - r, where r is a predetermined range. The comparison between the maximum power threshold and the injected AC power with the stabilising component may comprise determining a rate of change of PAC, and estimating whether PAC is likely to exceed P AX within a predetermined period. Such determinations of PAC relative to PMAX may be combined and varied to ensure that the stabilising component is reduced before PAC reaches PMAX.

[0072] In one or more example methods, the method 400 comprises, if the comparing S424 leads to that PAC should be reduced, then reducing S426 the stabilising component and controlling S420 the converter to inject AC power with the reduced stabilising component. If the comparing S424 leads to that PAc should be reduced, then the monitoring S422 of PAC is continued.

[0073] As described previously, the WTC 20 defines or determines both the first upper limit - the “soft” upper limit - and the second upper limit - the “hard” upper limit for the generator torque. The maximum power threshold PMAX is determined in agreement with the second upper limit, so that when the injected AC power stays below the maximum power threshold, then the generator torque will stay below the second upper limit.

[0074] Still with reference to Fig. 3, in one or more example methods, the method 200 comprises the WTC 20 determining S204A, in agreement with the first upper limit, a power limit PH for AC power continuously injected into the power grid by the converter and providing S207 the power limit to the converter controller. With reference to Fig. 2, the step of providing S207 PH to the converter controller 40 may be implemented by signalling from the electronic device 22 to the electronic device 42. In one or more example methods, the method 400 comprises, by the converter controller 40:

[0075] • receiving S402 input relating to the power limit;

[0076] • based on the received input relating to the power limit, controlling S404A the converter to inject AC power while respecting the power limit on the injected AC power;

[0077] • upon receiving S404B input relating to a disturbance of the power grid, determining S406 a stabilising component of the injected AC power that seeks to counteract the disturbance and controlling S420 the converter to inject AC power with the stabilising component while respecting the maximum power threshold, even when the AC power with the stabilising component exceeds the power limit.

[0078] Accordingly, in case of a grid disturbance the injected AC power is allowed to exceed PH, but not PMAX.

[0079] Although grid disturbances are typically transitory events, under some circumstances, the period where the injected AC power is allowed to exceed PH, may last so long as to effectively resemble continuous operation. Therefore, in one or more example methods, the input relating to the power limit and / or the maximum power threshold comprises a maximum period for which the AC power with the stabilising component is allowed to exceed the power limit.

[0080] The considerations relating to the relation between the second upper limit and the maximum power threshold PMAX are generally also valid to the relation between the first upper limit and the power limit PH. Hence, in an exemplary embodiment, the WTC calculates the power limit for injected power as a function of rotor speed and periodically determines and provides updated values of the power limit based on the current rotor speed and a static or an updated value of the first upper limit. In an exemplary embodiment, the WTC determines the power limit for injected power based on an updated value of the first upper limit, where the updated value of the first upper limit is determined as a function of rotor speed and / or other parameters of the wind turbine.

[0081] 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 Fig. 3.

[0082] 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 Fig. 3. 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).

[0083] 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.

[0084] 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.

[0085] It should further be 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.

[0086] 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. Generally, 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.

[0087] 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, defining a first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging a structural integrity of the wind turbine, and a second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without risk of damaging the structural integrity of the wind turbine;- a converter coupled to the power grid for converting the produced electrical power to inject AC power into the power grid; and- a converter controller for controlling operation of the converter according to a grid forming converter, GFC, control scheme; the method comprising, by the WTC, determining, in agreement with the second upper limit, a maximum power threshold for AC power injected into the power grid by the converter, and providing the maximum power threshold to the converter controller; the method comprising, by the converter controller:• receiving input relating to the maximum power threshold; and• based on the received input relating to the maximum power threshold, controlling the converter to inject AC power in accordance with the GFC control scheme while respecting the maximum power threshold on the injected AC power.

2. The method according to claim 1, wherein the first and second upper limits are determined based on a rotor speed and wherein values of the first and second upper limits are periodically updated using a current rotor speed.

3. The method according to claim 2, wherein determining and providing the maximum power threshold comprises periodically determining, in agreement with an updated valueof the second upper limit, an updated value of the maximum power threshold for AC power injected into the power grid by the converter and periodically providing the updated value of the maximum power threshold to the converter controller; and wherein the converter controller controls operation of the converter based on the most recently received input relating to the maximum power threshold.

4. The method according to any of the preceding claims, wherein controlling the converter to inject AC power while respecting the maximum power threshold on the injected AC power comprises:- monitoring the injected AC power;- comparing the input relating to the maximum power threshold with the injected AC power; and- before the injected AC power reaches the maximum power threshold, controlling the converter to reduce the injected AC power to keep the injected AC power under the maximum power threshold.

5. The method according to any of the preceding claims, wherein controlling the converter to inject AC power in accordance with the GFC control scheme comprises:- operating the converter to control the exchange of electrical power between the generator and the power grid in accordance with operating parameters of the power grid, the operating parameters comprising at least one of: AC voltage, frequency, AC current; active power; and reactive power; and- upon receiving input relating to a disturbance of one or more operating parameters; controlling the converter to inject AC power with one or more stabilising components of the injected AC power that seek to counteract the disturbance.

6. The method according to any of the preceding claims, wherein controlling the converter to inject AC power in accordance with the GFC control scheme while respecting the maximum power threshold on the injected AC power comprises, in the converter controller:upon receiving input relating to a disturbance of the power grid, determining a stabilising component of the injected AC power that seeks to counteract the disturbance; and- controlling the converter to inject AC power with the stabilising component.

7. The method according to any of the preceding claims, wherein controlling the converter to inject AC power in accordance with the GFC control scheme while respecting the maximum power threshold on the injected AC power comprises, in the converter controller:- upon receiving input relating to a disturbance of the power grid, determining a stabilising component of the injected AC power that seeks to counteract the disturbance;- calculating a value of the injected AC power with the stabilising component; and- upon determining that the calculated value is lower than the maximum power threshold, controlling the converter to inject AC power with the stabilising component.

8. The method according to any of the preceding claims, wherein, upon receiving input relating to a disturbance of the power grid, the converter controller initiates controlling the converter to inject AC power with a stabilising component prior to informing the WTC of the disturbance.

9. The method according to any of the preceding claims, wherein the converter controller stores the received input relating to the maximum power threshold and, upon receiving input relating to a disturbance of the power grid, controls the converter to inject AC power with a stabilising component based on a stored value of the maximum power threshold.

10. The method according to any of claims 5-9, wherein controlling the converter to inject AC power with the stabilising component comprises:- monitoring the injected AC power with the stabilising component;- comparing the input relating to the maximum power threshold with the injected AC power with the stabilising component; and- before the injected AC power with the stabilising component reaches the maximum power threshold, controlling the converter to reduce the stabilising component to keep the injected AC power with the stabilising component under the maximum power threshold.

11. The method according to any of the preceding claims, the method comprising: by the WTC, determining, in agreement with the first upper limit, a power limit for continuously AC power injected into the power grid by the converter, and providing the power limit to the converter controller; by the converter controller:• receiving input relating to the power limit;• based on the received input relating to the power limit, controlling the converter to inject AC power while respecting the power limit on the injected AC power;• upon receiving input relating to a disturbance of the power grid, determining a stabilising component of the injected AC power that seeks to counteract the disturbance and controlling the converter to inject AC power with the stabilising component while respecting the maximum power threshold, even when the AC power with the stabilising component exceeds the power limit.

12. The method according to claim 11 , wherein the input relating to the maximum power threshold comprises a maximum period for which the AC power with the stabilising component is allowed to exceed the power limit.

13. The method according to any of the preceding claims, wherein the WTC determines and provides the maximum power threshold independently of the AC power injected into the power grid.

14. The method according to any of the preceding claims, wherein the WTC determines and provides the maximum power threshold independently of an operating status of the power grid or any disturbances in the power grid.

15. A wind turbine comprising:- a generator driven by a rotor to produce electrical power;- a wind turbine controller, WTC, of the wind turbine, defining a first upper limit indicating a maximum generator torque with which the wind turbine may continuously operate without risk of damaging a structural integrity of the wind turbine, and second upper limit being larger than the first upper limit and indicating a maximum generator torque with which the wind turbine may operate non-continuously without damaging the structural integrity of the wind turbine;- a converter coupled to the power grid for converting the produced electrical power to generate AC power injected into the power grid; 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.

17. 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 of claims 1-14.

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