Wind turbine with virtual synchronous generator and DC link control

Abstract

This invention relates to a method for controlling a wind turbine, which acts as a virtual synchronizer, by determining the synchronizer rotational speed ωVSM and the synchronizer angle θVSM. The virtual synchronizer rotational speed ωVSM is determined based on a combination of feedback of damping power Pd, a power reference Pref for the desired power output of the wind turbine, grid power Pgrid supplied by the wind turbine to the grid, and chopper power Pchop dissipated by the chopper, as well as an inertial integral model. The synchronizer angle θVSM is determined based on the integral of the synchronizer rotational speed ωVSM, and the damping power Pd is determined based on the virtual synchronizer rotational speed ωVSM.

Description

Technical Field

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

[0002] To enable greater integration of renewable energy sources such as wind turbines into the power grid, some countries have proposed requirements for equipping power converters with grid-forming properties similar to those of traditional synchronous generators. These requirements can be addressed by configuring renewable power generation units as virtual synchronous machines (VSMs).

[0003] In a normal implementation of a virtual synchronous machine, the power delivered to the grid may fluctuate dramatically after low-voltage conditions, and the power of the wind turbine generator is unloaded during a fault and fluctuates after the fault.

[0004] Therefore, the implementation of virtual synchronizers presents a problem in generating power oscillations and increasing mechanical loads on the drivetrain and other mechanical components after a grid failure. Thus, there is a strong need for implementations of virtual synchronizers that address these issues. Summary of the Invention

[0005] The object of the present invention is to improve the control of wind turbines to mitigate one or more of the aforementioned problems, and therefore to provide a method that provides an improved control method for virtual synchronizers.

[0006] In a first aspect of the invention, a method for controlling a wind turbine is provided, wherein the wind turbine includes a generator, a machine-side converter, a line-side converter, a direct current (DC) link, and a chopper electrically connected to the output of the machine-side converter and the input of the line-side converter, the method comprising:

[0007] - Determine the rotational speed and synchro angle, wherein the virtual synchro rotational speed is determined based on the feedback of damping power, the power reference of the wind turbine's expected power output, the combination of the grid power supplied by the wind turbine to the grid and the chopper power dissipated by the chopper, and the inertial integral model, wherein the synchro angle is determined based on the integral of the synchro rotational speed, and wherein the damping power is determined based on the virtual synchro rotational speed.

[0008] The chopper power dissipated by the chopper can be added to the grid power, and the difference between the power reference and the sum of the chopper power and the grid power is fed into an inertial integral model, which determines the integral of the power difference.

[0009] Therefore, given the inertial integral, the derivative of the synchronous machine's rotational speed indicates the deviation (e.g., the difference) between the power reference of the wind turbine's desired power output and the sum of the grid power, chopper power, and damping power supplied by the wind turbine to the grid.

[0010] The first aspect of the invention is to dissipate power in a DC-link chopper and use this power dissipation in the oscillation equations of a virtual synchronous machine model. This results in smoother performance during low-voltage periods and voltage ride-through events, with more stable and controllable current injection during faults and reduced power oscillations and mechanical loads during the voltage recovery phase following a grid fault.

[0011] Advantageously, by including the chopper power in the power deviation between the determined power reference and the generated grid power, the power generated by the generator can be gradually reduced, thereby reducing drivetrain oscillations.

[0012] According to one embodiment, the power output from the wind turbine is controlled based on the synchro angle.

[0013] According to one embodiment, the chopper power reference of the chopper is determined based on a comparison between the DC link voltage and a DC link voltage reference and / or a comparison between the grid power and a power reference.

[0014] According to one embodiment, determining the damping power based on the rotational speed of the virtual synchronizer includes high-pass filtering the rotational speed of the synchronizer and determining the damping power based on the high-pass filtered signal.

[0015] According to one embodiment, the method includes:

[0016] - Obtain the network voltage at the connection point of the wind turbine output.

[0017] - Determining the rotational speed of the power grid based on network voltage, and

[0018] - The damping power is determined based on the rotational speed of the virtual synchronous machine and the rotational speed of the power grid.

[0019] According to one embodiment, the comparison of the DC link voltage with the DC link voltage reference includes determining the contribution to the chopper power reference based on the voltage difference between the DC link voltage and the DC link voltage reference, and a comparison of the voltage difference with a voltage threshold.

[0020] According to one embodiment, the comparison between grid power and a power reference includes determining the contribution to the chopper power reference based on the power difference between the grid power and the power reference and a comparison of the power difference with a power threshold.

[0021] According to one embodiment, the chopper power is determined based on a voltage measurement of the DC link voltage, or alternatively based on both the DC link voltage and a measured current flowing into / out of the DC link capacitor.

[0022] According to one embodiment, the method includes:

[0023] - The machine-side power reference of the machine-side converter is determined based on a power reference and a power adjustment value, wherein the power adjustment value is determined based on the difference between the DC link voltage reference and the DC link voltage.

[0024] Advantageously, the DC link voltage is maintained at its reference value by feeding forward the grid power reference and controlling the DC link voltage using a DC link controller that adjusts the generator power reference.

[0025] A second aspect of the invention relates to a control system for controlling a wind turbine, the wind turbine including a generator, a machine-side converter, a line-side converter, a DC link, and a chopper electrically connected to the output of the machine-side converter and the input of the line-side converter, the control system being arranged as follows:

[0026] - Determine the virtual synchro rotation speed and synchro angle, wherein the virtual synchro rotation speed is determined based on the feedback of damping power, the power reference of the expected power output of the wind turbine, the combination of the grid power supplied by the wind turbine to the grid and the chopper power dissipated by the chopper, and the inertial integral model, wherein the synchro angle is determined based on the integral of the synchro rotation speed, and wherein the damping power is determined based on the virtual synchro rotation speed.

[0027] A third aspect of the invention relates to a computer program product comprising software code, which, when executed on a data processing system, is adapted to control a wind turbine, and the computer program product is adapted to perform the method of the first aspect.

[0028] A fourth aspect of the invention relates to a wind turbine comprising a control system according to the first aspect.

[0029] 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 be apparent and elucidated with reference to the embodiments described below. Attached Figure Description

[0030] Embodiments of the invention will be described by way of example only, with reference to the accompanying drawings, in which:

[0031] Figure 1 A wind turbine was shown.

[0032] Figure 2A The electrical system of the wind turbine is shown.

[0033] Figure 2B The diagram shows control components arranged to control the generation of active current and active power, as well as reactive current and reactive power.

[0034] Figure 3A -B shows an example of a control system used to determine the angle of a synchronizing machine.

[0035] Figure 4A The circuit for determining the chopper power reference is shown.

[0036] Figure 4B The circuitry for determining the power reference of the machine-side converter is shown.

[0037] Figure 5 The response curves of a wind turbine equipped with a virtual synchronizer are shown in the case of a low-pressure ride-through event.

[0038] Figure 6 The response curves of a wind turbine equipped with a virtual synchronizing machine according to an embodiment of the present invention are shown in the case of a low-pressure ride-through event. Detailed Implementation

[0039] Figure 1 A 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 rotor 102 can rotate under the action of wind. The rotational energy of the rotor blades 103 caused by the wind is transferred to the generator via a shaft. Thus, the wind turbine 100 is able to convert the kinetic energy of the wind into mechanical energy by means of the rotor blades, and subsequently into electrical energy by means of the generator. The generator is connected to a power converter, which includes a machine-side converter and a line-side converter. The machine-side converter converts the alternating current (AC) generated by the generator into direct current (DC), and the line-side converter converts the DC into AC for injection into the public power grid.

[0040] Figure 2AAn example of an electrical system 200 for a wind turbine 100 according to one embodiment is shown. The electrical system includes a generator 201 and a power converter 202. The power converter 202 includes a machine-side converter 203, a line-side converter 204, a DC link 205, and a resistor 207 connected to a controllable switch 206. The resistor and switch form a power dissipation device (also called a chopper) 209 for dissipating active power. The DC link 205 includes one or more DC link capacitors charged by the DC output current from the machine-side converter 203 or the current from the line-side converter 204. The output AC current from the line-side converter 204 is supplied to the power line 220 via an output inductor 206 and possibly via a wind turbine transformer 208.

[0041] 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 park, which is arranged to supply power to the public grid for power distribution.

[0042] The power converter 202 can be a full-scale converter (including forced commutation and line commutation converters) configured according to different principles.

[0043] The power system 200 is shown in principle and therefore does not explicitly reveal that the system can be a three-phase system. However, the principles of the described embodiments are applicable to both single-phase and multi-phase systems.

[0044] The line-side converter 204 uses some variations of pulse width modulation (PWM) to convert direct current into alternating current. The control system 250 is used to control the modulation of the line-side converter 204 and to control the reactive and active current generated by the line-side converter 204.

[0045] Figure 2AThe diagram shows that the grid voltage Ugrid (here referring to the voltage on the low-voltage LV side of transformer 208) can be measured. The grid voltage Ugrid can be used to determine the virtual synchro angle θVSM (as described elsewhere) and to control the converter's power output based on Pgrid, determined from the grid voltage Ugrid and grid current 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 reference Uqref can be used instead of the measured voltage Ugrid. Thus, in an alternative, an internal voltage reference such as Uqref can be used to determine Pgrid and therefore the synchro angle θVSM. The grid current Igrid supplied to the grid can also be measured. The grid voltage angle θgrid can be determined, for example, from the grid voltage Ugrid.

[0046] Figure 2B An example of a control unit 260 is shown, which is arranged to control the generation of active current Iq and active power Pgrid, as well as reactive current Id and reactive power Q, supplied by the wind turbine to the grid at power output 270. Control unit 260 may form part of control system 250. Alternatively, control unit 260 receives control signals from control system 250.

[0047] The references for active and reactive currents can be received from the power plant controller (PPC) or the transmission system operator (TSO), or determined based on, for example, active and reactive power references from the grid operator.

[0048] The active power Pgrid is controlled via a virtual synchronizer angle θVSM. An example for determining the synchronizer angle θVSM is given elsewhere.

[0049] The synchronizing angle θVSM can be used to transform signals from a rotating DQ coordinate system to a non-rotating coordinate system (such as αβ or abc coordinate systems) and vice versa. Based on the synchronizing angle θVSM and the voltage amplitude reference Uqref, control signals for the desired active and reactive power are determined.

[0050] Therefore, the synchronizing angle θVSM can be defined in a rotating DQ coordinate system defined by the angular position θVSM. Based on the synchronizing angle θVSM, the control signal (i.e., the angle of the modulated voltage signal used for the pulse width modulator PWM 265) is determined and transformed into a non-rotating coordinate system (such as the αβ or abc coordinate system). The modulated Uqref voltage signal controls the reactive current Id and the active current Iq.

[0051] The coordinate system transformation and control unit 266 determines the voltage reference signal and transforms the voltage control signal from the DQ coordinate system to the αβ or abc coordinate system. The coordinate system transformation output signal from the control unit 266 is converted by the pulse width modulator 265 into a modulation signal for the grid-side converter 204 to generate a voltage based on the θVSM angle, which will give the grid power according to the grid power reference.

[0052] The reactive power Q is controlled by the amplitude of the grid voltage reference Uqref, which is determined based on the reactive power reference. The voltage reference Uqref is transformed from the DQ coordinate system to the αβ or abc coordinate system and output as a control signal from the control unit 266 to the pulse width modulator PWM 265, which determines the modulation signal for the grid-side converter 204.

[0053] Figure 3A and Figure 3B An example of a control system 391 for determining the synchronizing angle θVSM is shown. The synchronizing control system 391 may be included by the control system 250.

[0054] The synchronous machine angle θVSM is determined based on the concept of virtual synchronous machine control, which aims to generate a power response that corresponds to the power response from a real synchronous generator (which includes the inertia of the synchronous generator).

[0055] In response to grid voltage fluctuations (e.g., reflected in the measured Ugrid and Pgrid), the virtual machine may accelerate or decelerate to reach a new equilibrium condition. A new equilibrium is reached when the measured grid power Pgrid again follows the Pref.

[0056] The concept of virtual synchronous machine control is utilized in the line-side converter 204, which uses the swing equation to calculate θVSM.

[0057] During an undervoltage ride-through (UVRT) event, the angular velocity ωVSM will increase faster than the grid angular velocity ωL, and the turbine is at risk of becoming unstable and tripping or shutting down during low grid voltage and prolonged faults. While the inertia constant H of the swing equation can be changed to a high value during a fault, such control will not be able to adapt to phase changes or actual frequency variations during UVRT. Embodiments and examples of the present invention dissipate energy in the DC-link chopper 209 and use the dissipated energy in the swing equation to achieve smoother performance during UVRT / OVRT, where current injection is more stable and controllable during a fault, and power swing is reduced during the voltage recovery phase. The advantages are a wider voltage tolerance curve without loss of synchronicity and less mechanical load during the voltage recovery phase.

[0058] Figure 3AAn example implementation of the virtual synchronization model 301 is shown. The virtual synchronization model 301 includes a closed loop where the virtual synchronizer rotational speed ωVSM is determined based on a combination of feedback of the damping power Pd, a power reference Pref of the wind turbine's desired active power output, the active grid power Pgrid supplied by the wind turbine to the grid via power line 220, and the chopper power Pchop dissipated by chopper 209, and an 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 combination Pref-Pd-Pgrid-Pchop is used as the input to the inertial integral model 311.

[0059] Since the derivative of the synchronous machine rotational speed ωVSM corresponds to the deviation between the power reference Pref and the grid power Pgrid, the integral of the difference Pref-Pd-Pgrid-Pchop gives the synchronous machine rotational speed ωVSM.

[0060] The grid power Pgrid can be determined based on the measured grid voltage Ugrid and the measured current Igrid (e.g., measured on the LV or HV side of the transformer).

[0061] The change in power reference Pref (i.e., the change per unit time) can be slope-limited according to slope limiter 312.

[0062] The damping power Pd is determined by multiplying the difference between the grid's rotational speed ωL and the synchronous machine's rotational speed ωVSM by the damping coefficient Dp. The grid's rotational speed ωL (i.e., the grid frequency) is determined by the measured grid voltage Ugrid.

[0063] The synchronous generator angle θVSM is determined by integrating ωr / s based on the synchronous generator rotation speed ωVSM, where ωr is the rated synchronous generator speed.

[0064] The chopper power Pchop can be determined based on the equation Pchop = chop_on * UDC * UDC / Rchop, using a voltage detector arranged to measure the voltage across the DC link capacitor 205, for example, by means of a voltage detector. Rchop is the resistance of the chopper resistor 207, and chop_on is a value between 0 and 1 that indicates the duty cycle of switch 206 (i.e., the fraction of time switch 206 is closed), where chop_on = 0.5 indicates that the switch is closed for 50% of the switching cycle.

[0065] Figure 3BAn alternative virtual synchronization model 301 is shown, which is not based on the measured grid voltage Ugrid, but the damping component (e.g., damping power Pd) is determined based on a high-pass filter 313 of the synchro speed ωVSM.

[0066] Generally, the virtual synchronization model 301 determines the virtual machine angle θVSM based on the power combination Pref-Pd-Pgrid-Pchop, the inertial integral model 311 (e.g., implemented as 1 / (2Hs)) and the feedback of the damping power Pd determined based on the integral of ωVSM and ωVSM.

[0067] The control system 391 can be implemented based on power values ​​Pref, Pd, Pgrid, Pchop, or it can be implemented equivalently based on the relationship that power equals torque multiplied by rotational frequency (e.g., the rotational speed ωL of the power grid) and the corresponding torque values ​​Tref, Td, Tgrid, Tchop.

[0068] Figure 4A Circuit 410 is shown for determining the chopper power reference Pchop_ref for chopper 209. As shown, Pchop_ref can be determined based on the difference between the DC link voltage UDC and the DC link voltage reference UDC_ref. The DC link voltage is the voltage across the DC link capacitor 205. An increase in the DC link voltage above the reference can be compensated for by dissipating energy in the DC link capacitor in the chopper according to the chopper reference Pchop_ref. Alternatively, Pchop_ref can be determined based on the difference between the measured grid power Pgrid and the power reference Pref. If the power reference Pref is greater than the grid power Pgrid (e.g., due to a low-voltage grid event), the excess energy will cause the DC link voltage UDC to increase. The excess energy can be compensated for by activating the chopper according to the chopper reference Pchop_ref.

[0069] The contribution of the chopper power reference Pchop_ref based on the DC link voltage UDC and / or grid power Pgrid can depend on a comparison of the voltage and / or power difference with corresponding voltage and power thresholds (defined by voltage and power limiting functions 401, 402). Therefore, if the difference is below the threshold, the contribution to the chopper reference from any comparison or difference calculation can be zero, and if the difference is above the threshold, limiting functions 401, 402 provide a monotonically increasing output as a function of the difference (i.e., the voltage or power difference).

[0070] As shown in the figure, the chopper reference Pchop_ref can also be determined based on a combination (e.g., the sum) of contributions from the DC link voltage deviation (UDC-ref-UDC) and the active power deviation (Pref-Pgrid).

[0071] Figure 4B Circuitry 420 is shown for determining a power reference P_MSC_ref for the machine-side converter 203 based on the difference between the DC link voltage reference UDC_ref and the measured DC link voltage UDC, and a power reference Pref. A DC link controller 421 determines a power adjustment P_corr to be combined with the power reference Pref. Therefore, if the DC link voltage UDC is too high (e.g., above a threshold), P_corr becomes negative relative to the DC link reference UDC_ref, causing the machine-side power reference P_MSC_ref to decrease relative to the power reference Pref. In this way, the voltage across the DC link capacitor 205 is controlled.

[0072] Figure 5 The response curves of a wind turbine equipped with a virtual synchro for a low-pressure ride-through event are shown. This virtual synchro is not suitable for including chopper power Pchop (i.e., the power deviation ΔP is equal to Pref - Pgrid - Pd).

[0073] Curve 501 shows the measured voltage drop at Ugrid. Due to this voltage drop, the electrical power Pgrid to the grid (curve 502) immediately decreases. During the fault, the grid power increases because the virtual synchronous machine (VSM) control increases the angle between the grid and the synchronous machine angle θVSM. After the grid fault, the grid power Pgrid oscillates because the virtual synchronous machine has accelerated during the fault and will oscillate back to its pre-fault power level.

[0074] The power dissipated in the DC link chopper, Pchop (e.g., according to...) Figure 4A The chart (as defined in the diagram) is shown in curve 503. The chopper is activated due to a mismatch between the high DC link voltage UDC or the generator power (i.e., the machine-side power P_MSC) and the grid power Pgrid. As shown, Pchop can be reduced during a fault to decrease the energy capacity required in the chopper (e.g., by adjusting the chop_on duty cycle).

[0075] Curve 504 shows that the machine-side power P_MSC remains stable in this example because the chopper is dissipating power not being delivered to the grid. After the fault, the P_MSC power oscillates until the grid-side VSM control returns to its steady-state condition.

[0076] Due to the VSM response, the active current Iq (curve 505) increases.

[0077] Due to the VSM response with reactive current to support grid voltage, the reactive current Id (curve 506) towards the grid increases during a fault.

[0078] Due to the deviation between the power reference Pref and the grid power Pgrid, the synchronizing angle difference (θgrid-θVSM) between the grid angle and the synchronizing angle (curve 507) increases during the fault.

[0079] Figure 6 The response curves of a wind turbine equipped with a virtual synchro under a low-pressure ride-through event are shown. The virtual synchro is adapted to include chopper power Pchop, i.e., such that the power deviation ΔP is equal to Pref - Pgrid - Pd - Pchop.

[0080] Curve 601 shows the measured voltage drop at Ugrid. Due to this voltage drop, the electrical power Pgrid to the grid (curve 602) decreases immediately. After a grid fault, the grid power Pgrid recovers slowly without oscillation. This is because the control method includes chopper power Pchop, which has the effect of preventing the virtual synchronizer from accelerating during a fault. That is, the angle difference between the grid θgrid and the synchronizer angle θVSM does not change significantly.

[0081] The power dissipated in the DC link chopper, Pchop (e.g., according to...) Figure 4A The chart (as defined in the diagram) is shown in curve 603. The chopper is activated due to a mismatch between the high DC link voltage UDC or the generator power (i.e., the machine-side power P_MSC) and the grid power Pgrid. As shown, Pchop can be reduced during a fault to decrease the energy capacity required in the chopper (e.g., by adjusting the chop_on duty cycle).

[0082] Curve 604 illustrates a slight unloading of the generator-side power P_MSC during a fault and its recovery after the grid fault. During normal, fault-free periods, the generator-side power P_MSC is typically equal to the grid power, but during fault periods, reducing generator power more slowly than the grid power can be useful to avoid drivetrain load and tower oscillations.

[0083] The active current Iq (curve 605) is constant, but it can be increased or decreased during faults to match grid requirements.

[0084] The reactive current Id toward the grid (curve 606) increases during faults to match the VSM response or a more converter-controllable value and provide grid voltage support.

[0085] Because of the synchronous machine control of the chopper power Pchop, the synchronous machine angle difference (θgrid-θVSM) (curve 607) between the grid angle and the synchronous machine angle remains almost unchanged during a fault. This increases stability during a fault and minimizes power recovery oscillations after a fault.

[0086] Although 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 will be interpreted in accordance with the appended claims. In the context of the claims, the terms "comprising" or "including" do not exclude other possible elements or steps. Furthermore, references such as "a" or "an" should not be construed as excluding multiples. 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 may 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 controlling a wind turbine (100), the wind turbine including a generator (201), a machine-side converter (203), a line-side converter (204), a DC link (205), and a chopper (209) electrically connected to the output of the machine-side converter and the input of the line-side converter, the method comprising: - Determine the virtual synchro rotational speed (ωVSM) and / or synchro angle (θVSM), wherein the virtual synchro rotational speed (ωVSM) is determined based on a combination of feedback of damping power (Pd), power reference (Pref) of the desired power output of the wind turbine, grid power (Pgrid) supplied by the wind turbine to the grid and chopper power (Pchop) dissipated by the chopper (209), and an inertial integral model (311), wherein the synchro angle (θVSM) is determined based on the integral of the synchro rotational speed (ωVSM), and wherein the damping power (Pd) is determined based on the virtual synchro rotational speed (ωVSM).

2. The method of claim 1, comprising controlling the power output from the wind turbine based on the synchro angle (θVSM).

3. The method according to any one of claims 1-2, comprising determining the chopper power reference (Pchop_ref) of the chopper based on a comparison of DC link voltage (UDC) with DC link voltage reference (UDC_ref) and / or a comparison of grid power (Pgrid) with power reference (Pref).

4. The method according to any one of claims 1-2, wherein, Determining the damping power (Pd) based on the virtual synchronous machine rotational speed (ωVSM) involves high-pass filtering the synchronous machine rotational speed (ωVSM) and determining the damping power (Pd) based on the high-pass filtered signal.

5. The method according to any one of claims 1-2, comprising: - Obtain the network voltage (Ugrid) at the connection point of the wind turbine output. - Determine the rotational speed (ωL) of the power grid based on the network voltage (Ugrid), and - Determine the damping power (Pd) based on the rotational speed of the virtual synchronous machine (ωVSM) and the rotational speed of the power grid (ωL).

6. The method according to claim 3, wherein, The comparison between the DC link voltage (UDC) and the DC link voltage reference (UDC_ref) includes determining the contribution to the chopper power reference (Pchop_ref) based on the voltage difference between the DC link voltage (UDC) and the DC link voltage reference (UDC_ref) and the comparison of the voltage difference with a voltage threshold.

7. The method according to claim 3, wherein, The comparison between grid power (Pgrid) and power reference (Pref) includes determining the contribution to chopper power reference (Pchop_ref) based on the power difference between grid power (Pgrid) and power reference (Pref) and the comparison of said power difference with a power threshold.

8. The method according to any one of claims 1-2, wherein, The chopper power (Pchop) is determined based on the voltage measurement results of the DC link voltage UDC.

9. The method according to any one of claims 1-2, comprising: - The machine-side power reference (P_MSC_ref) of the machine-side converter (203) is determined based on the power reference (Pref) and the power adjustment value (P_corr), wherein the power adjustment value (P_corr) is determined based on the difference between the DC link voltage reference (UDC_ref) and the DC link voltage (UDC).

10. A control system (250) for controlling a wind turbine, the wind turbine including a generator (201), a machine-side converter (203), a line-side converter (204), a DC link (205), and a chopper (209) electrically connected to the output of the machine-side converter and the input of the line-side converter, the control system being arranged as follows: - Determine the virtual synchronizer rotational speed (ωVSM) and / or synchronizer angle (θVSM), where, The virtual synchro rotational speed (ωVSM) is determined based on a combination of feedback of damping power (Pd), power reference (Pref) of the wind turbine’s desired power output, grid power (Pgrid) supplied by the wind turbine to the grid and chopper power (Pchop) dissipated by the chopper (209), and an inertial integral model (311), wherein the synchro angle (θVSM) is determined based on the integral of the synchro rotational speed (ωVSM), and wherein the damping power (Pd) is determined based on the virtual synchro rotational speed (ωVSM).

11. A computer program product comprising software code, which, when executed on a data processing system, is adapted to control a wind turbine (100), the computer program product being adapted to perform the method according to any one of claims 1-9.

12. A wind turbine (100) comprising the control system according to claim 10.

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