Wind turbine with virtual synchronous generator with damping control

Abstract

The invention relates to a method for damping drivetrain oscillations of a wind turbine in a VSM configuration. The method comprises determining a drivetrain damping power signal based on a speed signal representing a generator speed, determining a power deviation based on a combination of a power reference for a desired power production, the drivetrain damping power signal, a grid power supplied to a grid by a line side converter, and a damping power, determining a virtual synchronous machine angle based on the power deviation such that a derivative of a virtual synchronous machine rotational speed is indicative of the power deviation, determining a converter reference for controlling the line side converter to generate a desired active power based on the virtual synchronous machine angle and a voltage reference for a voltage magnitude to be generated by the line side converter, and applying the converter reference to the line side converter.

Description

Technical Field

[0001] This invention relates to the control of wind turbines, and particularly to the control of wind turbines configured to exhibit a virtual synchronous generator response. Background Technology

[0002] To increase the penetration of renewable energy sources (such as wind turbines) into the power grid, some countries have mandated that power converters be equipped with grid-forming characteristics similar to those of traditional synchronous generators. These requirements can be addressed by configuring renewable power generation units as virtual synchronous machines.

[0003] Traditional wind turbines can be configured to dampen mechanical oscillations in the drivetrain or other parts of the wind turbine by modulating the generated active power. This can be achieved by modifying the power reference according to a signal representing the oscillation, and by controlling the generated active power based on the modified power reference through a machine-side converter.

[0004] This method is considered unsuitable for virtual synchronizers (VSMs) configured for wind turbines. Therefore, the problem is that wind turbines with VSM configurations may not be able to utilize existing drivetrain damping methods for wind turbines. Thus, a drivetrain damping method, or a method commonly used to dampen mechanical oscillations, is needed that is suitable for controlling wind turbines with VSM configurations. Summary of the Invention

[0005] The object of the present invention is to improve the control of power generation units such as wind turbines to mitigate one or more of the aforementioned problems, and thus to provide a method for providing an improved virtual synchronous machine control method, particularly a power generation-based method for damping mechanical oscillations.

[0006] In a first aspect of the invention, a method for damping mechanical oscillations in a drivetrain such as that of a wind turbine, the wind turbine comprising a drivetrain driven by a rotor, a generator driven by the drivetrain, a machine-side converter, and a line-side converter arranged to supply power to a power grid.

[0007] The method includes:

[0008] - Obtain a speed signal representing the rotational speed of the generator.

[0009] - Determine the transmission system damping power signal based on the speed signal.

[0010] - The power deviation is determined based on the difference between the power reference and damped power implemented by the virtual synchronous generator for the wind turbine, and the grid power supplied to the grid by the line-side converter.

[0011] - Determine the virtual synchronizer speed and / or virtual synchronizer angle based on the power deviation, such that the derivative of the virtual synchronizer speed indicates the power deviation.

[0012] - Provides a voltage reference for the voltage amplitude or reactive power to be generated by the line-side converter.

[0013] - Based on the virtual synchronizer angle and the voltage reference, determine the converter reference used to control the line-side converter to generate the desired active power, and

[0014] - The line-side converter is controlled based on the converter reference.

[0015] Advantageously, the line-side converter is used to generate the change in active power based on the virtual synchro angle by determining the virtual synchro angle based on the transmission damping power signal that reflects mechanical oscillations via the speed signal.

[0016] According to one embodiment, the drivetrain damping power signal is determined such that it includes a signal component dependent on the mechanical oscillation. According to another embodiment, determining the drivetrain damping power signal includes applying a filter, such as a bandpass filter, to the velocity signal. Advantageously, the filter is designed to allow frequencies within a frequency range that includes the oscillation frequency to pass through. Therefore, the filter can be designed based on the known eigenfrequency of the wind turbine (e.g., the eigenfrequency of the drivetrain or rotor blades).

[0017] According to an embodiment, determining the drivetrain damping power signal includes applying a bandwidth modification filter to increase the bandwidth and adjust the phase of the transfer function, the transfer function being defined as the relationship between the drivetrain damping power signal and the grid power.

[0018] Advantageously, by increasing the bandwidth, the frequency component of the velocity signal corresponding to the frequency of the oscillation can be prevented from being damped or phase-shifted due to the limited bandwidth of the transfer function in other cases.

[0019] According to an embodiment, the bandwidth modification filter is determined based on the short-circuit ratio of the power grid, wherein the bandwidth of the transfer function depends on the short-circuit ratio.

[0020] According to an embodiment, the method includes: modifying the amplitude of the drivetrain damping power signal in a manner such as reduction, depending on the power reference.

[0021] Specifically, for low-power generation conditions with a low power reference, the amplitude of the drivetrain damping power signal can be reduced. Otherwise, a high amplitude of the drivetrain damping power signal would be added to the low power reference, leading to a significant change in power output.

[0022] In another embodiment, the modification to the amplitude is determined such that the amplitude of the drivetrain damping power signal is reduced when the power reference is below a given power threshold.

[0023] According to an embodiment, the method further includes: modifying the damping power, for example by increasing, depending on the power reference.

[0024] Advantageously, the damping power can be increased, for example, for a low value of the power reference, to improve the stability of the closed-loop control of the active power.

[0025] In another embodiment, the modification to the damping power is determined such that the damping power is increased when the power reference is below a given power threshold. For a low power reference, the increase in damping power may be performed with a fixed increase factor multiplied by the determined damping power. For power reference values ​​above the given power threshold, or above another higher power threshold, the damping power may be unaffected.

[0026] In another embodiment, the rate of change of the damping power can be limited by using a rate limiter to avoid sudden steps in the damping power and thus improve the stability of the closed-loop control of active power generation.

[0027] According to an embodiment, the virtual synchronous machine speed is determined based on a combination of the following: the damping power feedback, the power reference, the transmission system damping power signal, the grid power, and the inertial integral model, wherein the virtual synchronous machine angle is determined based on the integral of the virtual synchronous machine speed, and wherein the damping power is determined based on the virtual synchronous motor speed.

[0028] According to an embodiment, the method includes:

[0029] - Obtain the network voltage at the connection point of the output of the line-side converter.

[0030] - Determine the rotational speed of the power grid based on the network voltage, and

[0031] The damping power is determined based on the rotational speed of the virtual synchronous machine and the rotational speed of the power grid.

[0032] A second aspect of the invention relates to a control system configured to dampen mechanical oscillations in a drivetrain such as a wind turbine, the wind turbine including a rotor-driven drivetrain, a generator driven by the drivetrain, a machine-side converter, and a line-side converter arranged to supply power to the grid, wherein the control system is configured to perform the method according to the first aspect.

[0033] A third aspect of the invention relates to a wind turbine comprising a control system according to the second aspect.

[0034] In general, various aspects and embodiments of the present invention can be combined and coupled in any possible manner within the scope of the invention. These and other aspects, features, and / or advantages of the invention will become apparent from the embodiments described below and will be illustrated by the embodiments. Attached Figure Description

[0035] Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings, wherein:

[0036] Figure 1 A wind turbine was shown.

[0037] Figure 2A An example of an electrical system for a wind turbine or power generation unit is shown.

[0038] Figure 2B The diagram shows control components arranged to control the generation of active and reactive power supplied to the grid at the power output of a wind turbine or power generation unit.

[0039] Figure 3 An embodiment of a damping method for a wind turbine configured as a virtual synchronous generator is shown.

[0040] Figure 4A An overview of the power control system is shown.

[0041] Figure 4B The Bode plots of the magnitude and phase of the power transfer function Gvsm are shown, as well as

[0042] Figure 5 The improved damping effect achieved by the damping control system is shown. Detailed Implementation

[0043] Figure 1A wind turbine 100 (WTG) is shown, comprising a tower 101 and a rotor 102 having at least one rotor blade 103, such as three blades. The rotor is connected to a nacelle 104, which is mounted on top of the tower 101 and adapted to drive a generator located within the nacelle via a drivetrain. The drivetrain includes a shaft connecting the rotor 102 to a gearbox or generator. The rotor 102 can be rotated by the action of wind. The rotational energy of the rotor blades 103 caused by the wind is transferred to the generator via the shaft. Thus, the wind turbine 100 is able to convert the kinetic energy of the wind into mechanical energy through the rotor blades, and subsequently into electrical energy through the generator. The generator is connected to a power converter, which includes a generator-side converter and a line-side converter. The generator-side converter converts AC power generated by the generator into DC power, and the line-side converter converts DC power into AC power for injection into the public power grid.

[0044] Figure 2A An example of an electrical system 200 for a wind turbine 100 according to an embodiment is shown. The electrical system includes a generator or power source 201 and a power converter 202.

[0045] Power converter 202 includes a machine-side converter 203 and a line-side converter 204. Power converter 202 may also include a DC link 205 and a resistor 207 connected to a controllable switch 206. The resistor and switch form a power dissipation device, also known as a chopper 209, for dissipating active power. The DC link 205 includes one or more DC link capacitors, which are charged by the DC output current from the generator-side converter 203 and supply DC power to the line-side converter 204. Alternatively, power converter 202 can be configured as a multilevel converter, where the DC capacitors are distributed across the arms of the converter. In this case, Figure 2A The DC link capacitor and possibly switch 206 may not be present. However, power converter 202 typically includes machine-side converter 203 and line-side converter 204.

[0046] Power converter 202 can be a full-scale converter configured according to different principles, including forced commutation and line-commutation converters. The output AC current from line-side converter 204 can be supplied to power line 220 via output inductor 215 and possibly via wind turbine transformer 208. In this example, the output AC current is a three-phase current output. Harmonic filter capacitor 216 can be arranged between the output conductors, forming a harmonic filter together with inductor 215 to convert the square wave voltage signal from line-side converter 204 into a sinusoidal voltage signal.

[0047] Power line 220 may be a medium-voltage power bus that receives power from other wind turbines 100. Power line 220 may be connected to a high-voltage network, for example, via other transformers. Thus, power line 220 and one or more power systems 200 corresponding to the wind turbines constitute a wind power plant or wind farm, which is arranged to supply power to the public grid for power distribution.

[0048] The power system 200 is primarily shown and therefore not explicitly disclosed as a three-phase system. However, the principles of the described embodiments apply to both single-phase and multi-phase systems. The line-side converter 204 uses some variant of pulse width modulation (PWM) to convert DC power to AC power. The control system 250 is used to control the modulation of the line-side converter 204 and to control the active power P and reactive power Q generated by the line-side converter 204.

[0049] Figure 2A The diagram shows the measurable grid voltage Ugrid, here the voltage on the low-voltage LV side of transformer 208. Based on the active power Pgrid determined from the grid voltage Ugrid and grid current Igrid, the grid voltage Ugrid can be used to determine the virtual synchro angle θVSM (as described elsewhere) and to control the power output of the converter. The reactive power Qgrid can be determined similarly from Ugrid and Igrid. Alternatively, the grid voltage Ugrid can be measured on the high-voltage HV side of the transformer and corrected based on the transformer's turns ratio, or an internal voltage amplitude reference Vqref can be used instead of the measured voltage Ugrid. Therefore, in an alternative, an internal voltage amplitude reference such as Vqref, Vdqref, or Vαβref can be used to determine Pgrid and thus the synchro angle θVSM. The grid current Igrid supplied to the grid can also be measured.

[0050] Figure 2BAn example of control component 260 is shown, which is arranged to control the generation of active power Pgrid and reactive power Qgrid supplied to the grid at the power output 270 from the wind turbine 100 or the power generation unit 199. That is, control component 260 can be arranged to control the output active power Pgrid and output voltage amplitude of the low-voltage side LV, or alternatively, to control the output active power Pgrid and output reactive power Qgrid of the low-voltage side LV. Control system 250 may include one or more control components 260, such as frame conversion unit 266. For example, control system 250 may include frame conversion unit 266 that provides a determined converter reference Vabc for line-side converter 204 or for a PWM modulator 265 that line-side converter 204 may include. In this way, line-side converter 204 is controlled based on converter reference Vabc.

[0051] References for active and reactive power can be received from the power plant controller (PPC) or the grid operator (TSO), or determined based on, for example, active and reactive power references from the grid operator.

[0052] Active power Pgrid is controlled via a virtual synchro angle θVSM. In short, the synchro angular acceleration (twice the time derivative of θVSM) indicates the difference between the power reference Pref used for the desired power output of the wind turbine and the grid power Pgrid supplied to the grid by the wind turbine. An example for determining the synchro angle θVSM is given elsewhere.

[0053] The synchro angle θVSM can be used to convert a signal from a rotated DQ frame to a non-rotated frame, such as an αβ or abc frame, and vice versa. Based on the synchro angle θVSM and the voltage amplitude reference Vqref, control signals for the desired active and reactive power are determined.

[0054] Therefore, the synchronizer angle θVSM can be defined within a rotating DQ frame defined by the angular position θVSM. Based on the synchronizer angle θVSM, the control signal (i.e., the angle of the modulated voltage signal for the pulse width modulator PWM 265) is determined, and this control signal is converted into a non-rotating frame, such as an αβ frame or an abc frame. The resulting converter reference signal Vabc controls the active power Pgrid and reactive power Qgrid.

[0055] Frame conversion unit 266 converts the control signal from the DQ frame into an αβ or abc frame and determines the sine converter reference signal Vabc for PWM 265. The frame conversion output signal from frame conversion unit 266 is converted by pulse width modulator PWM 265 into a modulation signal for grid-side converter 204 to generate the desired active power Pgrid, reactive power Qgrid, and / or voltage amplitude Ugrid.

[0056] The reactive power Qgrid generated by the line-side converter 204 can be controlled based on the voltage amplitude reference Vqref.

[0057] Generally, the voltage reference Vqref can be a reference for either the voltage amplitude Ugrid or the reactive power Qgrid to be generated by the line-side converter 204. Thus, based on the voltage reference Vqref and the synchronizing machine angle θVSM, the converter reference Vabc is determined to control the line-side converter 204 to generate the desired active power Pgrid, and therefore the reactive power Qgrid.

[0058] The voltage amplitude reference Vqref can be defined in the DQ frame.

[0059] The voltage amplitude reference Vqref rotates at the speed ωVSM of a virtual synchronous machine, i.e., under steady-state conditions at the base frequency, such as 50Hz of the AC grid voltage. The voltage amplitude reference Vqref can be converted from a DQ frame to an αβ or abc frame and output from the frame conversion unit 266 as a converter reference Vabc to a pulse width modulator PWM 265, which determines the modulation signal for the grid-side converter 204.

[0060] Figure 3 An example of a damping control system 300 for a virtual synchronous generator implemented using a transmission damping method for damping mechanical oscillations is shown. The damping control system 300 may be included, for example, in a control system 250, such that the damping control system provides the synchronizing angle θVSM to the frame conversion unit 266.

[0061] The idea behind the damping method is to use modulated active power to dampen mechanical oscillations in the transmission system, rotor blades 103, tower 101, or other components. Damping is based on a speed signal Wgen representing the generator's rotational speed. When mechanical oscillations, such as blade or tower oscillations, affect the generator speed Wgen, the corresponding change in generator speed represents the mechanical oscillation. Damping of mechanical oscillations can be achieved by determining and generating changes in active power based on changes in generator speed.

[0062] Traditionally, this damping of mechanical oscillations is performed by modulating the power conversion of the machine-side converter 203 based on the generator speed signal Wgen via a modulation signal determined by a filter assembly. This filter assembly may correspond to or be equivalent to... Figure 3 The filter component DTD in the process.

[0063] The speed signal Wgen can be the measured or estimated rotational speed of the shaft in the transmission system.

[0064] A filter element DTD is typically configured to pass through variations in the generator speed signal Wgen, generated in response to mechanical oscillations. For example, a filter element DTD can be a bandpass filter, where the bandpass frequency range is configured to allow the frequencies at which the mechanical oscillations are desired to be damped to pass through. Therefore, the filter element DTD can be designed based on the eigenfrequency of the mechanical component.

[0065] Therefore, the transmission damping power signal Pdtd provided by the filter component DTD is determined to include a signal component of mechanical oscillations that depends on the desired damping.

[0066] The drivetrain damping power signal Pdtd is provided to the transfer function Gff, which is configured as a bandwidth modification filter Gff, arranged to increase the bandwidth and adjust the phase of the power transfer function Gvsm of the active power control loop of the virtual synchronous generator. The power transfer function Gvsm defines the relationship between the drivetrain damping power signal Pdtd and the active grid power Pgrid supplied to the grid or the active power generated by the line-side converter 204.

[0067] Figure 4B Bode plots of magnitude 451 and phase 452 of the power transfer function Gvsm without the bandwidth modification filter Gff are shown, as well as Bode plots of magnitude 453 and phase 454 of the power transfer function Gvsm with the bandwidth modification filter Gff.

[0068] like Figure 4B As shown, due to the increased bandwidth, the frequency component of the transmission damping power signal Pdtd generated in response to mechanical oscillations in the indicated frequency range of 460 will be less affected by the phase changes and attenuation of the power transfer function Gvsm when the bandwidth modification filter Gff is applied.

[0069] The bandwidth modification filter Gff is optional because the damping effect of the damping control system 300 can be achieved without the bandwidth modification filter Gff, but the damping effect may be less efficient.

[0070] Since the bandwidth of the power transfer function Gvsm depends on the short-circuit ratio (SCR) of the power grid, the bandwidth modification filter Gff can be determined based on the short-circuit ratio (SCR) of the power grid.

[0071] The short-circuit ratio can be measured at the turbine, for example, at the output of the line-side converter, or at the common coupling point of the electrical connections of multiple wind turbines. Alternatively, the short-circuit ratio can be estimated based on other estimated or measured electrical parameters, or received, for example, from a service provider.

[0072] The power transfer function between the transmission system damping power signal Pdtd and the active grid power Pgrid, including the bandwidth modification filter Gff, can be expressed as:

[0073]

[0074] Where Gvsm is the transfer function of the filter Gff without bandwidth modification, and Xr is the reactance of the output inductor 215.

[0075] A bandwidth-modifying filter Gff can be designed to increase the bandwidth of Pgrid / Pdtd by a desired spectral range so that no phase shift or amplitude change occurs in the low-frequency range of the expected oscillation frequency of the Pdtd signal. For example, the bandwidth-modifying filter Gff can be designed as a pre-filter according to Gff≈Gp / Gvsm, where Gp is ​​the desired low-pass filter response, for example, as described in amplitude Bode plot 453.

[0076] The output Pdtd-ff from the bandwidth modification filter Gff is combined with the power reference Pref to generate the power reference Pref_vsm for the virtual synchronous generator implementation. The product of the output Pdtd-ff and the gain factor Am is optional and is described elsewhere.

[0077] Figure 3 The implementation of the swing equation based on the virtual synchronous generator is shown to determine the principle of the synchronizing angle θVSM of the virtual synchronous generator.

[0078] The synchronous motor angle θVSM is determined based on the concept of virtual synchronous machine control, which aims to generate a power response corresponding to the power response from a real synchronous generator, including the inertia of the synchronous generator.

[0079] The power deviation ΔP is defined as Pref_vsm - Pd - Pgrid, where Pd is the damped power determined according to the virtual synchronization model 301, and Pgrid is the generated active power. Under steady-state conditions, the power error ΔP is zero.

[0080] In response to the change in power reference Pref_vsm caused by the change in the drivetrain damping power signal Pdtd, the power deviation ΔP becomes non-zero. This causes the virtual synchronization angle θVSM to increase or decrease, thereby reducing the power error ΔP. For example, if the change in the drivetrain damping power signal Pdtd results in a positive power deviation ΔP, the synchronizer speed ωVSM will increase.

[0081] Therefore, in response to fluctuations in the drivetrain damping power signal Pdtd, the synthesized inertial response becomes non-zero, causing the virtual machine to accelerate or decelerate to reach a new equilibrium condition. Changes in acceleration induce changes in power generation via the virtual synchronization angle θVSM, which reduces the variation in the drivetrain damping power signal Pdtd and thus dampens mechanical oscillations.

[0082] Using the oscillation equation, the concept of virtual synchronous machine control is used on the online-side converter 204 to calculate θVSM.

[0083] The virtual synchronization model 301 includes a closed loop that determines the virtual synchronizer speed ωVSM based on the power deviation ΔP and the inertial integral model 311. The inertial integral model 311 is implemented as 1 / (2Hs), where H is the inertial time constant and 1 / s is the integral in the s-domain. Therefore, the power deviation ΔP is used as the input to the inertial integral model 311.

[0084] Since the derivative of the synchronous machine speed ωVSM is proportional to the deviation between the power reference Pref_vsm and the grid power Pgrid, the integral of the difference ΔP gives the synchronous machine speed ωVSM.

[0085] The grid power Pgrid can be determined based on the measured grid voltage Ugrid or an internal voltage reference (e.g., amplitude reference Vqref) or its transformation, and the measured grid current Igrid.

[0086] The damping power Pd is determined by multiplying the difference between the grid rotational speed ωL and the synchronous machine speed ωVSM by the damping factor Dp. The grid rotational speed ωL (i.e., the grid frequency) is determined from the measured grid voltage Ugrid.

[0087] The synchronous generator angle θVSM is determined based on the synchronous generator speed ωVSM as an integral of ωr / s, where ωr is the rated synchronous generator speed.

[0088] Alternatively, the grid speed ωL can be determined based on a high-pass filter of the determined synchronous machine speed ωVSM. That is, the grid speed ωL is determined as the output of a high-pass filter configured to filter the input synchronous machine speed ωVSM. In this alternative, the damping power Pd is determined based on the high-pass filter of the synchronous machine speed ωVSM.

[0089] Generally, the virtual synchronous model 301 determines the angle θVSM of the virtual machine based on the power deviation ΔP, the inertia integration model 311 (e.g., implemented as 1 / (2Hs)), and the feedback of the damping power Pd determined based on the integration of ωVSM and ωVSM. In other words, the synchronous machine speed ωVSM and the synchronous machine angle θVSM are determined such that they indicate the integral deviation between the power reference Pref for the desired power output of the wind turbine and the virtual grid power Pvsm.

[0090] The control system 250 and the damping control system 300 can be implemented based on the power values Pref, Pdtd, Pd, Pgrid, but can equivalently be implemented based on the relationship that power equals torque multiplied by the rotational frequency (e.g., the synchronous machine speed ωVSM), based on the corresponding torque values Tref, Tdtd, Td, Tgrid.

[0091] In the case where the power reference Pref is low, it may be problematic if a large change in the drivetrain damping power signal Pdtd or the filtered drivetrain damping power signal Pdtd-ff is added to the power reference Pref and thus provided as the virtual synchronous power reference Pref_vsm for the virtual synchronous model 301. Additionally, in another example, when the turbine active power output approaches zero in response to a decrease in the power reference Pref, the control oscillation in the drivetrain may change frequency, e.g., because the oscillation of the first drivetrain mode decreases compared to the 3P oscillation generated due to the leeward side effect of the rotor blade 103.

[0092] Therefore, it may be beneficial to reduce or limit the magnitude of the change in the rotor speed signal Wgen or the output of the filter component DTD.

[0093]

[0094] Figure 3 An example is shown for reducing the magnitude of the rotor speed change by multiplying the drivetrain damping power signal Pdtd or the filtered drivetrain damping power signal Pdtd-ff by a gain factor Am. The gain factor Am can be determined by the gain factor function 321 based on the power reference Pref. For Pref > Plim, the gain factor Am can be set to be 1; and for Pref < Plim, the gain factor Am can be set to a value less than 1, or to a variable value that is less than 1 for Pref < Plim, where the variable value is a function of the power reference Pref. ​

[0095] For example, as Figure 3 shown, for Pref < Plim, the gain factor Am can be Pref-lpf / Plim or Pref / Plim, and for Pref > Plim, the gain factor Am can be set to 1. Pref-lpf is a low-pass filtered version of the power reference, where the low-pass filter LPF is used to ensure that the gain factor Am changes smoothly and robustly.

[0096] As an example, the power threshold Plim can be set to 0.05PU (corresponding to 5% of the rated power of the wind turbine), such that the gain factor Am linearly increases from 0 to 1 at the power threshold Plim. Obviously, other functions 321 such as non-linear functions, step functions, or look-up tables can be used instead of the linear function Am = Pref-lpf / Plim.

[0097] [[ID=⑨]]In another example, a hysteresis function combined with a rate limiter is used to determine the gain factor Am based on the power reference Pref. <00002②0>As a supplement or alternative to modifying the gain factor Am, the damping power Pd can be determined based on the power reference Pref, for example, such that the damping power Pd is increased according to the power reference, or such that the damping power Pd is increased when the power reference Pref is below a given power threshold Plow. For low values of the power reference Pref, the increase in the damping power provides an increase in the stability of the damping control system 300.

[0099] As Figure 3 shown, the adjustment of the damping power Pd can be achieved by applying a hysteresis function 322 that determines a damping factor Dp depending on the power reference Pref, such that for Pref < Plow, the damping factor Dp is set to DpH, and for Pref > Plow, the damping factor Dp is set to DpL, where DpH is greater than DpL. To ensure that there is no step change in the value of the damping factor Dp, the damping factor is passed through a rate limiter 323 to generate a smooth change in the damping factor Dp. The determined damping factor is input into the virtual synchronous model 301, such that the damping power Pd is adjusted according to the damping factor Dp by multiplying the damping power Pd by the damping factor Dp.

[0100] ... In another example, a function equivalent to the function 321 and a method for determining the gain factor Am are used to determine the damping factor Dp. <②000227>In the example, the power thresholds for the power references Plow and Plim are equal. <00②0229> Figure 4A It is shown that Figure 2B and Figure 3 An overview of the intent, in which a potentially constrained transmission damping signal Pdtd-ff-lim is combined with a power reference Pref to provide input to a virtual synchronous generator model 301 to provide damping for mechanical oscillations.

[0103] Figure 5 The diagram illustrates the change in generator torque 501 when the damping control system 300 is used to dampen mechanical oscillations, and the change in generator torque 502 when a bandwidth modification filter Gff is applied to the damping control system 300. Therefore, applying the bandwidth modification filter Gff to the damping control system 300 provides improved damping of generator torque oscillations caused by mechanical oscillations.

[0104] While the invention has been described in conjunction with specific embodiments, it should not be construed as limiting it in any way to the presented examples. The scope of the invention should be interpreted in accordance with the appended claims. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Furthermore, references such as "a" or "an" should not be construed as excluding the plural. The use of reference numerals in the claims relating to elements indicated in the drawings should also not be construed as limiting the scope of the invention. Moreover, the various features mentioned in different claims can be advantageously combined, and the mention of these features in different claims does not preclude the possibility and advantage of combining features.

Claims

1. A method for damping mechanical oscillations in a drivetrain such as a wind turbine (100), the wind turbine (100) comprising a drivetrain driven by a rotor, a generator (201) driven by the drivetrain, a machine-side converter (203), and a line-side converter (204) arranged to supply power to a power grid. The method includes: - Obtain a speed signal (Wgen) representing the rotational speed of the generator. - Determine the transmission system damping power signal (Pdtd) based on the speed signal. - The power deviation (ΔP) is determined based on the difference between the power reference Pref_vsm implemented for the virtual synchronous generator of the wind turbine, the damping power (Pd), and the grid power (Pgrid) supplied to the grid by the line-side converter (204). - Determine the virtual synchronizer speed (ωVSM) and / or virtual synchronizer angle (θVSM) based on the power deviation (ΔP), such that the derivative of the virtual synchronizer speed (ωVSM) indicates the power deviation (ΔP). - Provides a voltage reference (Vqref) for the voltage amplitude (Ugrid) or reactive power (Qgrid) to be generated by the line-side converter (204). - Based on the virtual synchro angle (θVSM) and the voltage reference (Vqref), determine the converter reference (Vabc) for controlling the line-side converter (204) to generate the desired active power (Pgrid), and - The line-side converter (204) is controlled based on the converter reference (Vabc).

2. The method according to claim 1, wherein, The transmission system damping power signal (Pdtd) is determined such that the transmission system damping power signal (Pdtd) includes a signal component that depends on the mechanical oscillation.

3. The method according to claim 1 or 2, wherein, Determining the drivetrain damping power signal (Pdtd) involves applying a bandpass filter (DTD) to the speed signal (Wgen).

4. The method according to claim 1 or 2, wherein, Determining the drivetrain damping power signal (Pdtd) involves applying a bandwidth modification filter (Gff) to increase the bandwidth and adjust the phase of the transfer function (Gvsm), which is defined as the relationship between the drivetrain damping power signal (Pdtd) and the grid power (Pgrid).

5. The method according to claim 4, comprising: The bandwidth modification filter (Gff) is determined based on the short-circuit ratio (SCR) of the power grid, wherein the bandwidth of the transfer function (Gvsm) depends on the short-circuit ratio (SCR).

6. The method according to claim 1 or 2, further comprising: The amplitude of the drivetrain damping power signal (Pdtd) is modified in a manner such as reduction, depending on the power reference (Pref).

7. The method according to claim 6, wherein, The modification to the amplitude is determined such that the amplitude is reduced when the power reference (Pref) is below a given power threshold (Plim).

8. The method according to claim 1 or 2, further comprising: The damping power (Pd) is modified, for example, by increasing, depending on the power reference (Pref).

9. The method according to claim 8, wherein, The modification to the damping power (Pd) is determined such that the damping power is increased when the power reference (Pref) is below a given power threshold (Plow).

10. The method of claim 9, comprising: Limit the rate of change of the damping power.

11. The method according to claim 1 or 2, wherein, The virtual synchronizer speed (ωVSM) is determined based on a combination of the following: feedback of the damping power (Pd), the power reference (Pref), the transmission damping power signal (Pdtd), the grid power (Pgrid), and the inertial integral model (311), wherein the virtual synchronizer angle (θVSM) is determined based on the integral of the virtual synchronizer speed (ωVSM), and wherein the damping power (Pd) is determined based on the virtual synchronizer speed (ωVSM).

12. The method according to claim 1 or 2, comprising: - Obtain the network voltage (Ugrid) at the connection point of the output of the line-side converter (204). - The rotational speed (ωL) of the power grid is determined based on the network voltage (Ugrid), and - The damping power (Pd) is determined based on the virtual synchronous machine speed (ωVSM) and the power grid speed (ωL).

13. A control system (250) configured to dampen mechanical oscillations in a drivetrain such as a wind turbine (100), the wind turbine (100) comprising a rotor-driven drivetrain, a generator (201) driven by the drivetrain, a machine-side converter (203), and a line-side converter (204) arranged to supply power to the power grid, wherein, The control system is configured to perform the method according to any one of claims 1-12.

14. A wind turbine (100) comprising a control system (250) according to claim 13.

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