A method for controlling a grid forming wind turbine

Abstract

A method for controlling an active power output of a grid forming wind turbine (2) is disclosed A power plant controller (6) generates a frequency control power setpoint, Pref_freq, and the grid forming wind (2) turbine generates an active inertia response power setpoint, PI. An active power setpoint, Pref, is generated based on the frequency control power setpoint, Pref_freq, and the active inertia response power setpoint, PI. In the case that the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine (2), a modified active power setpoint, Pref_mod, is obtained which minimises and / or removes oscillations of the active power setpoint, Pref, with coinciding frequencies. The active power output of the grid forming wind turbine (2) is controlled based on the modified active power setpoint, Pref_mod.

Description

[0001] A METHOD FOR. CONTROLLING A GRID FORMING WIND TURBINE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for controlling an active power output of a grid forming wind turbine. The method according to the invention minimises the mechanical loads on the wind turbine during grid forming operation.

[0004] BACKGROUND OF THE INVENTION

[0005] In order to allow for a higher penetration of renewable energy sources, such as wind turbines, in power grids, it has been proposed that wind turbines, in particular power converters of wind turbines, are equipped with grid forming properties similar to conventional synchronous generators. This will allow the wind turbines to mimic the behaviour of a synchronous generator, thus contributing to maintaining the stability of the power grid, in particular with regard to power oscillations in the power grid. Such wind turbines are sometimes referred to as 'grid forming wind turbines'.

[0006] The grid forming properties can, e.g., be provided by configuring the wind turbines as virtual synchronous machines, and the mimicking behaviour may be modelled by a second order swing equation, including inertia response and damping power. The damping power part of the swing equation plays an important part in dampening frequency oscillations in the power grid. However, operating a wind turbine in grid forming mode causes additional mechanical loads on the wind turbine. Moreover, in the case that the grid forming wind turbine receives power setpoints from a power plant controller, while independently carrying out grid forming operation, this may significantly increase the mechanical loads on the wind turbine. DESCRIPTION OF THE INVENTION

[0007] It is an object of embodiments of the invention to provide a method for controlling an active power output of a grid forming wind turbine, in which mechanical loads on the wind turbine are minimised.

[0008] The invention provides a method for controlling an active power output of a grid forming wind turbine, the grid forming wind turbine forming part of a wind farm being connected to a power grid, the wind farm comprising a plurality of wind turbines and a power plant controller, the grid forming wind turbine being communicatively connected to the power plant controller, the method comprising the steps of:

[0009] - the power plant controller generating a frequency control power setpoint, Pref freq, based on a first measurement of a frequency of the power grid, and dispatching the frequency control power setpoint, Pref freq, to the grid forming wind turbine,

[0010] - the grid forming wind turbine generating an active inertia response power setpoint, Pi, based on a second measurement of the frequency of the power grid,

[0011] - generating an active power setpoint, Pref, based on the frequency control power setpoint, Pref_freq, and the active inertia response power setpoint, Pi,

[0012] - determining whether or not oscillations in the frequency control power setpoint, Pref _freq, and oscillations in the active inertia response power setpoint, Pi, result in an active power setpoint, Pref, having oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine,

[0013] - in the case that it is determined that the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine, modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, to minimise and / or remove oscillations of the active power setpoint, Pref, with coinciding frequencies, thus obtaining a modified active power setpoint, Pre mod, and

[0014] - subsequently controlling the active power output of the grid forming wind turbine based on the modified active power setpoint, Pref mod .

[0015] Thus, the method according to the invention is a method for controlling an active power output of a grid forming wind turbine. In the present context the term 'grid forming wind turbine' should be interpreted to mean a wind turbine that is configured to mimic synchronous generator behaviour. Thus, the wind turbine being controlled is of a kind which is capable of contributing to maintaining the stability of the power grid, thus allowing for higher penetration of renewable power sources in the power grid.

[0016] The grid forming wind turbine forms part of a wind farm being connected to a power grid. In the present context the term 'wind farm' should be interpreted to mean a plurality of wind turbines arranged within a specified geographical area, and which share some infrastructure, such as internal power grid, connection to an external power grid, substations, access roads, etc. Thus, the wind farm comprises a plurality of wind turbines, and further comprises a power plant controller being responsible for the overall control of the wind turbines within the wind farm. For instance, the power plant controller may be responsible for ensuring that the combined or total power output from the wind turbines to the power grid fulfils certain requirements, and for dispatching power setpoints to the individual wind turbines in order to achieve this.

[0017] The grid forming wind turbine is communicatively connected to the power plant controller. This enables the grid forming wind turbine to receive power setpoints from the power plant controller. Furthermore, other kinds of information may be communicated from the power plant controller to the grid forming wind turbine and / or from the grid forming wind turbine to the power plant controller.

[0018] In the method according to the invention, the power plant controller initially generates a frequency control power setpoint, Pref freq, based on a first measurement of a frequency of the power grid, and dispatches the frequency control power setpoint, Pref freq, to the grid forming wind turbine. The frequency control power setpoint, Pref freq, represents a power output required by the grid forming wind turbine in order to contribute to ensuring that the wind farm provides required frequency control to the power grid.

[0019] The first measurement of a frequency of the power grid may be performed by the power plant controller, e.g. at the point of common coupling between the wind farm and the power grid. The measured frequency may, e.g., be or represent a frequency of an oscillation occurring in the power grid.

[0020] Furthermore, the grid forming wind turbine generates an active inertia response power setpoint, Pi, based on a second measurement of the frequency of the power grid. The second measurement may be identical to the first measurement, or it may be a different measurement. This will be described in further detail below.

[0021] In an example, the second measurement of the frequency of the power grid is the rate of change of the frequency. Thus, the active inertia response power setpoint, PI, can be based on the rate of change of the frequency. With no other modification the power setpoint the grid forming wind turbine will then provide power based on the rate of change of frequency. In a specific embodiment the active inertia response power setpoint, PI, is proportional to the rate of change of frequency.

[0022] In an example, the first measurement and the second measurement is made substantially at the same time.

[0023] The active inertia response power setpoint, Pi, represents a power output required by the grid forming wind turbine in order to provide grid forming services towards the power grid.

[0024] Accordingly, the frequency control power setpoint, Pref freq, is generated centrally by the power plant controller and relates to the overall frequency control of the wind farm, while the active inertia response power setpoint, Pi, is generated locally by the grid forming wind turbine and relates to the grid forming operation of the wind turbine. Both power setpoints are provided to the grid forming wind turbine, and both need to be taken into account when controlling the grid forming wind turbine. Accordingly, an active power setpoint, Pref, is generated, based on the frequency control power setpoint, Pref _freq, and the active inertia response power setpoint, Pi.

[0025] Furthermore, it is determined whether or not oscillations in the frequency control power setpoint, Pref_freq, and oscillations in the active inertia response power setpoint, Pi, result in an active power setpoint, Pref, having oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine.

[0026] For instance, it may be determined whether or not one or more oscillations expected to occur in the active power setpoint, Pref, due to the combination of the frequency control power setpoint, Pref _freq, and the active inertia response power setpoint, Pi, include frequencies that coincide with critical frequencies related to the structure of the grid forming wind turbine. The critical frequencies could, e.g., be frequencies which are likely to increase mechanical loads on the wind turbine, such as eigenfrequencies and / or natural modes of various parts and components of the wind turbine.

[0027] The determination described above could, e.g., include analysing the frequency control power setpoint, Pref freq, as well as the active inertia response power setpoint, Pi, and possibly identify frequencies that may constructively interfere. As an alternative, the determination may be performed solely based on the first measurement of a frequency of the power grid, and a prediction of the impact thereon on the frequency control power setpoint, Pref_freq, and the active inertia response power setpoint, Pi, and consequently on the active power setpoint, Pref.

[0028] In the case that the active power setpoint, Pref, has oscillations which coincide with one or more critical frequencies related to the structure of the grid forming wind turbine, and the grid forming wind turbine is controlled based on the active power setpoint, Pref, then there is a risk that such critical frequencies of the structure of the grid forming wind turbine are excited, resulting in significant mechanical loads on the wind turbine.

[0029] Accordingly, when this is the case, the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, is / are modified, so as to minimise and / or remove oscillations of the active power setpoint, Pref, with coinciding frequencies. Modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, results in a modified active power setpoint, Pref mod . Due to the performed modifications, it is ensured that the modified active power setpoint, Pref mod, is essentially free of oscillations with critical or problematic frequencies. Therefore the grid forming wind turbine can be safely controlled in accordance with the modified active power setpoint, Pref mod, without risking that critical frequencies related to the structure of the grid forming wind turbine are excited, and thus minimising the risk of excessive mechanical loads on the grid forming wind turbine.

[0030] Accordingly, the active power output of the grid forming wind turbine is subsequently controlled based on the modified active power setpoint, Pref mod . This allows the wind turbine to operate in grid forming mode, i.e. providing grid forming services, while ensuring that the mechanical loads on the wind turbine are maintained within an acceptable level.

[0031] The method may further comprise the step of, in the case it is determined that the active power setpoint, Pref, does not have oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine, controlling the active power output of the grid forming wind turbine based on the active power setpoint, Pref.

[0032] In the case that the determining step reveals that there are no critical frequencies in the active power setpoint, Pref, then it can be assumed that it is safe to control the active power output of the grid forming wind turbine based on the active power setpoint, Pref. Therefore, in this case no modifications are performed, and the active power output of the grid forming wind turbine is simply controlled based on the original, unmodified active power setpoint, Pref. The step of modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, may comprise applying a lowpass filter or a bandpass filter to the frequency control power setpoint, Pref freq, and / or to the active power setpoint, Pref.

[0033] According to this embodiment, high frequencies or frequencies outside an applied bandpass filter are filtered out from the frequency control power setpoint, Pref_freq, and / or from the active power setpoint, Pref. Thereby potentially critical frequencies are removed, while ensuring that the active power output of the grid forming wind turbine is controlled in a manner which handles low frequency oscillations in the power grid.

[0034] Alternatively or additionally, the step of modifying the frequency control power setpoint, Pref_freq, and / or the active inertia response power setpoint, Pi, may comprise modifying a deadband and / or a droop slope of a frequency control loop.

[0035] Modifying a deadband of a frequency control loop changes the upper limit for acceptable deviations of the frequency of the power grid from a nominal frequency, e.g. 50 Hz. For instance, increasing the deadband will allow the frequency of the power grid to deviate more from the nominal frequency before active frequency control is initiated. This has the consequence that minor deviations from the nominal frequency are not handled, and this reduces the mechanical loads on the grid forming wind turbine.

[0036] Modifying a droop slope of a frequency control loop changes the rate at which the power setpoint is changed when the frequency of the power grid increases or decreases outside the deadband. For instance, decreasing the droop slope results in a less aggressive active frequency control, leading to a reduction in the mechanical loads on the grid forming wind turbine.

[0037] Alternatively or additionally, the step of modifying the frequency control power setpoint, Pref_freq, and / or the active inertia response power setpoint, Pi, may comprise limiting a ramp rate of a change in the active power setpoint, Pref. According to this embodiment, a limit is imposed on the ramp rate, or rate of change, of the active power setpoint, Pref, even if the generated frequency power setpoint, Pref_freq, and active inertia response power setpoint, Pi, indicate that a higher ramp rate is required in order to reach a required active power output of the grid forming wind turbine. This prevents fast and frequent changes in the active power setpoint, Pref, thus reducing the mechanical loads on the grid forming wind turbine.

[0038] The second measurement of the frequency of the power grid may be identical to the first measurement of the frequency of the power grid. According to this embodiment, the frequency control power setpoint, Pref freq, and the active inertia response power setpoint, Pi, are generated based on the same measurement of the frequency of the power grid.

[0039] For instance, the second measurement of the frequency of the power grid may be provided to the grid forming wind turbine by the power plant controller. In this case the first measurement may be performed by the power plant controller and subsequently provided to the grid forming wind turbine as the second measurement. The first measurement may, e.g., be performed at or near the point of common coupling between the wind farm and the external power. In this case at least the generated frequency control power setpoint, Pref _freq, reflects the conditions of the power grid at or near the point of common coupling.

[0040] As an alternative, the second measurement of the frequency of the power grid may differ from the first measurement of the frequency of the power grid. According to this embodiment, the first measurement and the second measurement may be performed at separate locations within the power grid and / or at separate points in time.

[0041] For instance, the grid forming wind turbine may perform the second measurement of the frequency of the power grid at a terminal of the grid forming wind turbine. In this case the second measurement, forming the basis of the active inertia response power setpoint, Pi, is performed locally at the location of the grid forming wind turbine. Accordingly, the generated active inertia response power setpoint, Pi, reflects the condition of the power grid at the location of the grid forming wind turbine.

[0042] The method may further comprise the step of determining oscillations in the grid frequency and / or in the frequency control power setpoint, Pref freq, and / or in the active inertia response power setpoint, Pi, and the step of determining whether or not the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine may be performed based on the determined oscillations.

[0043] According to this embodiment, it is investigated whether or not and to which extent oscillations are occurring in the grid frequency, the frequency control power setpoint, Pref_freq, and / or the active inertia response power setpoint, Pi. This may, e.g., include a frequency analysis of the relevant signals, and based thereon it may be determined whether or not the resulting active power setpoint, Pref, has oscillations which coincide with one or more of the critical frequencies related to the structure of the grid forming wind turbine. As part of such an analysis, it may, e.g., be determined whether or not oscillations occurring in two or more signals interfere constructively in a manner which creates oscillations in the active power setpoint, Pref, at or near one or more critical frequencies, with a significant amplitude.

[0044] The step of modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, may comprise switching operation of the grid forming wind turbine from a grid forming mode to a grid following mode.

[0045] According to this embodiment, the wind turbine is temporarily prevented from operating in grid forming mode, i.e. it is prevented from providing grid forming services towards the power grid. This ensures that there are no conflicts between the frequency control power setpoint, Pref _freq, provided by the power plant controller and the active inertia response power setpoint, Pi, generated locally by the wind turbine, in the sense that they create oscillations at critical frequencies in the resulting active power setpoint, Pref. However, since this has the consequence that the grid forming wind turbine is unable to provide grid forming services, it would normally be desirable to only switch to grid following mode if other measures have not resulted in a sufficient reduction in the mechanical loads on the wind turbine. Moreover, the wind turbine should preferably be switched back to operating in grid forming mode as soon as possible.

[0046] The one or more identified critical frequencies related to the structure of the grid forming wind turbine may include one or more eigenfrequencies of the grid forming wind turbine. For instance, the critical frequencies may include eigenfrequencies of the tower, the blades, the nacelle, etc. Alternatively or additionally, the one or more critical frequencies may include natural modes of various parts and components of the wind turbine.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention will now be described in further detail with reference to the accompanying drawings in which

[0049] Fig. 1 is a schematic view of a wind farm with a plurality of grid forming wind turbines being controlled in accordance with an embodiment of the method according to the invention,

[0050] Fig. 2 illustrates various frequency and power signals related to a grid forming wind turbine which may be controlled in accordance with a method according to an embodiment of the invention,

[0051] Fig. 3 is a block diagram illustrating an embodiment of the method according to the invention, and

[0052] Figs. 4-8 are block diagrams illustrating methods according to various embodiments of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

[0053] Fig. 1 is a schematic view of a wind farm 1 comprising a plurality of grid forming wind turbines 2, three of which are shown. The wind farm 1 is connected to an external power grid 3 via a point of common coupling 4. The grid forming wind turbines 2 are connected to the point of common coupling 4 via an internal or local power grid 5.

[0054] The wind farm 1 further comprises a power plant control (PPC) 6 being communicatively connected to the grid forming wind turbines 2 via communication connection 7. Accordingly, the power plant controller 6 is able to generate and dispatch power setpoints to the grid forming wind turbines 2, in order to ensure that the total power supplied from the wind farm 1 to the external power grid 3 is in accordance with the requirements of the external power grid 3.

[0055] Each of the grid forming wind turbines 2 may be controlled in the following manner. The power plant controller 6 generates a frequency control power setpoint, Pref_freq, based on a first measurement of a frequency of the power grid 3, 5, and dispatches the frequency control power setpoint, Pref j-eq, to the grid forming wind turbine 2. The measured frequency could be a frequency related to the external power grid 3 and / or to the internal, local power grid 5. For instance, the measured frequency may represent a frequency of oscillations detected at or near the point of common coupling 4. In any event, the frequency control power setpoint, Pref_freq, represents an active power output required by the grid forming wind turbine 2 in order to ensure that the total power output of the wind farm 1 to the external power grid 3, via the point of common coupling 4, fulfils requirements of the external power grid 3, at least in terms of frequency support.

[0056] Moreover, the grid forming wind turbine 2 generates an active inertia response power setpoint, Pi, based on a second measurement of the frequency of the power grid 3, 5. The active inertia response power setpoint, Pi, represents an active power setpoint for the grid forming wind turbine 2, related to the grid forming services of the grid forming wind turbine 2. An active power setpoint, Pref, for the grid forming wind turbine 2 is then generated, based on the frequency control power setpoint, Pref freq, and the active inertia response power setpoint, Pi, i.e. taking the power setpoint received from the power plant controller 6 as well as the power setpoint related to the grid forming services into account.

[0057] It is then investigated whether or not oscillations in the frequency control power setpoint, Pref_freq, and oscillations in the active inertia response power setpoint, Pi, result in an active power setpoint, Pref, having oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine 2. The critical frequencies could, e.g., include eigenfrequencies and / or natural modes of various parts and components of the grid forming wind turbine 2. If such coinciding behaviour occurs, there is a risk of exciting the critical frequencies if the active power output of the grid forming wind turbine 2 is controlled in accordance with the active power setpoint, Pref. Since this may result in high mechanical loads on the grid forming wind turbine 2, such a situation should be avoided.

[0058] Accordingly, if it is determined that the active power setpoint, Pref, has oscillations which coincide with one or more of the critical frequencies, the active power setpoint, Pref, is modified, thus obtaining a modified active power setpoint, Pref mod, in which the oscillations with coinciding frequencies have been minimised and / or removed. This may be obtained in various ways, e.g. as described below with reference to Figs. 4-8.

[0059] Finally, the active power output of the grid forming wind turbine 2 is controlled based on the modified active power setpoint, Pref_mod.

[0060] Fig. 2 shows four graphs, illustrating various frequency and power signals related to a grid forming wind turbine which may be controlled in accordance with a method according to an embodiment of the invention, as a function of time.

[0061] The first graph 8 illustrates the frequency of the power grid as a function of time. Accordingly, in graph 8 the second axis represents the frequency of the power grid and the first axis represents time, i.e. graph 8 illustrates change in frequency over time. It can be seen that the frequency of the power grid oscillates about a nominal frequency, e.g. 50 Hz, as a function of time. This oscillatory behaviour defines an oscillation frequency, which can be measured, e.g. by the power plant controller.

[0062] The second graph 9 illustrates a frequency control power setpoint, Pref _freq, generated by the power plant controller, in response to the oscillating frequency of the power grid illustrated in graph 8. The solid line 10 represents a frequency control power setpoint, Pref freq, generated purely based on the oscillating behaviour of the frequency of the power grid 8, and in order to cause the grid forming wind turbine to contribute to stabilising the frequency of the power grid. It can be seen that the solid line 10 oscillates essentially with the same frequency as graph 8, but in counterphase therewith.

[0063] The dashed line 11 corresponds to the solid line 10, except that a limitation has been imposed on the rate of change, i.e. on the ramp rate, of the frequency control power setpoint, Pref freq, in order to limit mechanical loads on the grid forming wind turbine caused by changes in power setpoint.

[0064] The third graph 12 represents an active inertia response power setpoint, Pi, generated by the grid forming wind turbine, in order to provide grid forming services. It can be seen that this setpoint is also oscillating, but not in a manner that coincides with the oscillations in the frequency control power setpoint, P ref_freq ■

[0065] The fourth graph represents an active power setpoint, Pref, for the grid forming wind turbine, which has been generated on the basis of the limited frequency control power setpoint, Pref freq, represented by line 11, and the active inertia response power setpoint, Pi, of graph 12. This setpoint is represented by the dashed line 14. The solid line 15 represents the machine side active power to be extracted from the generator.

[0066] In the case that the active power setpoint, Pref, represented by line 14, oscillates in a manner which coincides with one or more critical frequencies of the structure of the grid forming wind turbine, this may lead to high mechanical loads on the grid forming wind turbine. Therefore, such a situation needs to be handled, and this could, e.g., be done in the manner described below with reference to Figs. 4-8.

[0067] Fig. 3 is a block diagram illustrating an embodiment of the method according to the invention. A frequency of the power grid, e.g. as illustrated in graph 8 of Fig. 2, is measured and supplied to an oscillation detection block 16, where it is detected whether or not the grid frequency oscillates, i.e. whether or not the frequency of the power grid changes as a function of time, in an oscillating manner. Furthermore, an active power setpoint, Pref, for a grid forming wind turbine, which has been generated based on a frequency control power setpoint, Pref freq, received from a power plant controller and an active inertia response power setpoint, Pi, generated by the grid forming wind turbine, is supplied to a decision block 17, where it is investigated whether or not an active power setpoint, Pref, with oscillations will be generated. Moreover, it may be investigated whether or not the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine.

[0068] In the case that the grid frequency oscillates and the active power setpoint, Pref, has or can be expected to have oscillations which coincide the critical frequencies, cf. block 18, actions are taken in order to minimise and / or remove oscillations of the active power setpoint, Pref, with coinciding frequencies, cf. block 19. The actions could, e.g., be one or more of the actions described below with reference to Figs. 4-8.

[0069] Figs. 4-8 are block diagrams illustrating methods according to various embodiments of the invention. More particularly, the block diagrams of Figs. 4 illustrate various options for modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, in order to minimise and / or remove oscillations of the active power setpoint, Pref, with frequencies that coincide with one or more critical frequencies of the structure of the grid forming wind turbine. Fig. 4 illustrates a method in which a filter 20, such as a lowpass filter or a bandpass filter, is applied to the measured frequency of the power grid, in the case that oscillations are detected, e.g. as described above with reference to Figs. 2 and / or 3. Thus, when this is the case, the frequency control power setpoint, Pref_freq, generated based on the measured grid frequency only contains oscillations with frequencies that pass through the filter 20. This minimises and / or removes oscillations with unwanted frequencies from the frequency control power setpoint, Pref freq, and therefore also from the active power setpoint, Pref.

[0070] On the other hand, in the case that no oscillations are detected, the unfiltered signal is passed through switch 25, i.e. no frequencies are filtered out before generating the frequency control power setpoint, Pref freq .

[0071] Fig. 5 illustrates a method in which a filter 21, such as a lowpass filter or a bandpass filter, is applied to the frequency control power setpoint, Pref_freq, in the case that oscillations are detected. This has essentially the same effect as the method illustrated in Fig. 4, and the remarks set forth in this regard are equally applicable here.

[0072] Fig. 6 illustrates a method in which the deadband 22 of a frequency control loop is modified, in the case that oscillations are detected, e.g. as described above with reference to Figs. 2 and / or 3. This changes the upper limit for acceptable deviations of the frequency of the power grid from a nominal frequency, e.g. 50 Hz. For instance, increasing the deadband will allow the frequency of the power grid to deviate more from the nominal frequency before active frequency control is initiated. This has the consequence that minor deviations from the nominal frequency will not give rise to changes in the frequency control power setpoint, Pref freq . This minimises oscillations in the frequency control power setpoint, Prefjreq, and thus in the active power setpoint, Pref.

[0073] As described above with reference to Fig. 4, in the case that no oscillations are detected, the deadband 22 of the frequency loop is not modified. Fig. 7 illustrates a method in which the droop slope 23 of a frequency control loop is modified, in the case that oscillations are detected, e.g. as described above with reference to Figs. 2 and / or 3. This changes the rate at which the frequency control power setpoint, Pref _freq, is changed when the frequency of the power grid increases or decreases outside the deadband. For instance, decreasing the droop slope results in a less aggressive active frequency control, leading to a smoother change in the frequency control power setpoint, Pref _freq, and thus in the active power setpoint, Pref. Again, in the case that no oscillations are detected, the droop slope 23 is not modified.

[0074] Fig. 8 illustrates a method in which a ramp rate of change 24 of the frequency control power setpoint, Pref freq, is limited, in the case that oscillations are detected, e.g. as described above with reference to Figs. 2 and / or 3. Thereby the frequency control power setpoint, Pref_freq, is not allowed to change faster than the upper limit defined by the ramp limit. This results in a smooth change of the frequency control power setpoint, Pref freq, as well as prevents high frequency oscillations in the frequency control power setpoint, Pref_freq . Consequently, fast and frequent changes in the active power setpoint, Pref, are also avoided. Also in this case, if no oscillations are detected, ramp rate of change 24 remains unlimited.

Claims

1. CLAIMS1. A method for controlling an active power output of a grid forming wind turbine (2), the grid forming wind turbine (2) forming part of a wind farm (1) being connected to a power grid (3, 5), the wind farm (1) comprising a plurality of wind turbines (2) and a power plant controller (6), the grid forming wind turbine (2) being communicatively connected to the power plant controller (6), the method comprising the steps of:- the power plant controller (6) generating a frequency control power setpoint, Pref_freq, based on a first measurement of a frequency of the power grid (3, 5), and dispatching the frequency control power setpoint, Pref freq, to the grid forming wind turbine (2),- the grid forming wind (2) turbine generating an active inertia response power setpoint, Pi, based on a second measurement of the frequency of the power grid (3, 5),- generating an active power setpoint, Pref, based on the frequency control power setpoint, Pref_freq, and the active inertia response power setpoint, Pi,- determining whether or not oscillations in the frequency control power setpoint, Pref _freq, and oscillations in the active inertia response power setpoint, Pi, result in an active power setpoint, Pref, having oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine (2),- in the case that it is determined that the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine (2), modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, to minimise and / or remove oscillations of the active power setpoint, Pref, with coinciding frequencies, thus obtaining a modified active power setpoint, Pref_mod, and- subsequently controlling the active power output of the grid forming wind turbine (2) based on the modified active power setpoint, Pref mod .

2. A method according to claim 1, further comprising the step of, in the case it is determined that the active power setpoint, Pref, does not have oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine (2), controlling the active power output of the grid forming wind turbine (2) based on the active power setpoint, Pref.

3. A method according to claim 1 or 2, wherein the step of modifying the frequency control power setpoint, Pref _freq, and / or the active inertia response power setpoint, Pi, comprises applying a lowpass filter or a bandpass filter to the frequency control power setpoint, Pref _freq, and / or to the active power setpoint, Pref-4. A method according to any of the preceding claims, wherein the step of modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, comprises modifying a deadband and / or a droop slope of a frequency control loop.

5. A method according to any of the preceding claims, wherein the step of modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, comprises limiting a ramp rate of a change in the active power setpoint, Pref.

6. A method according to any of the preceding claims, wherein the second measurement of the frequency of the power grid (3, 5) is identical to the first measurement of the frequency of the power grid (3, 5).

7. A method according to claim 6, wherein the second measurement of the frequency of the power grid (3, 5) is provided to the grid forming wind turbine (2) by the power plant controller (6).

8. A method according to any of claims 1-5, wherein the second measurement of the frequency of the power grid (3, 5) differs from the first measurement of the frequency of the power grid (3, 5).

9. A method according to claim 8, wherein the grid forming wind turbine (2) performs the second measurement of the frequency of the power grid (3, 5) at a terminal of the grid forming wind turbine (2).

10. A method according to any of the preceding claims, further comprising the step of determining oscillations in the grid frequency and / or in the frequency control power setpoint, Pref freq, and / or in the active inertia response power setpoint, Pi, and wherein the step of determining whether or not the active power setpoint, Pref, has oscillations which coincide with one or more identified critical frequencies related to the structure of the grid forming wind turbine (2) is performed based on the determined oscillations.

11. A method according to any of the preceding claims, wherein the step of modifying the frequency control power setpoint, Pref freq, and / or the active inertia response power setpoint, Pi, comprises switching operation of the grid forming wind turbine (2) from a grid forming mode to a grid following mode.

12. A method according to any of the preceding claims, wherein the one or more identified critical frequencies related to the structure of the grid forming wind turbine (2) include one or more eigenfrequencies of the grid forming wind turbine (2).

Images