Virtual synchronous generator with active damping

Abstract

The present invention relates to a method for controlling a power generation unit, such as a wind turbine, configured as a virtual synchronous machine. A capacitor voltage signal obtained from a voltage measurement of an output capacitor is filtered to reduce the magnitude of and / or shift an impedance peak present in the impedance characteristic of the output of the power generation unit. The filter-compensated voltage signal obtained from the output capacitor is combined with a voltage amplitude reference to obtain a filtered capacitor voltage signal, which is used to control a line-side converter to influence the impedance peak in a desired manner.

Description

Technical Field

[0001] The present invention relates to the control of power generation units, particularly renewable power generation units such as wind turbines, and in particular to the control of power generation units configured to exhibit a virtual synchronous generator response. Background Art

[0002] To achieve higher penetration of renewable energy sources such as wind turbines into the power grid, some countries are requiring power converters to have 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 (VSMs).

[0003] Some grid operators prefer that the impedance spectrum of a virtual synchronous machine resemble that of a voltage source behind a pure inductor.Since most inverters include capacitive harmonic filters, there are resonance points where the resulting impedance becomes very high and the damping is low.

[0004] Therefore, it is a problem that the virtual synchronous machine exhibits an impedance characteristic that may deviate from the impedance characteristic required by the grid operator.Therefore, there is a need for implementations of virtual synchronous machines that address these issues. Summary of the Invention

[0005] It is an object of the present invention to improve the control of a power generating unit such as a wind turbine to alleviate one or more of the above problems and therefore to provide a method which provides an improved control method for a virtual synchronous machine.

[0006] In a first aspect of the present invention, a method for controlling a power generation unit is provided, wherein the power generation unit includes a power source, a machine-side converter, a line-side converter, a DC link electrically connected to an output of the machine-side converter and an input of the grid-side converter, and a filter inductor and a filter capacitor arranged at the output of the power generation unit, the method comprising:

[0007] - obtaining a capacitor voltage signal in a measurement coordinate system by measuring the capacitor voltage of one or more of the filter capacitors,

[0008] - determining a virtual synchronous machine rotational speed and / or a synchronous machine angle, wherein a derivative of the synchronous machine rotational speed is indicative of a deviation between a power reference of a desired power output of the power generation unit and a grid power combined damping power supplied to the output by the power generation unit,

[0009] - providing a voltage magnitude reference for the desired reactive power to be generated by the line-side converter,

[0010] - optionally, transforming the capacitor voltage signal from a measurement coordinate system to a target coordinate system to generate a transformed capacitor voltage signal,

[0011] filtering the capacitor voltage signal or the transformed capacitor voltage signal by means of a compensation filter into a filtered capacitor voltage signal, wherein the compensation filter is designed to reduce the magnitude of and / or shift the impedance peak, wherein the impedance peak is present in the impedance characteristic of the output of the power generation unit, and

[0012] - determining a filter-compensated voltage reference by combining the voltage magnitude reference or a transformation thereof with the filtered capacitor voltage signal.

[0013] Advantageously, by feeding forward the measured voltage of the filter capacitor of the capacitive harmonic filter and filtering this voltage with a compensation filter, and then combining the filtered voltage signal with a voltage amplitude reference of the desired reactive power, the impedance peak is influenced such that undesirable resonance effects of the filter capacitor are eliminated or reduced, at least in the frequency range of interest.

[0014] Transforming the capacitor voltage signal from the measurement coordinate system to the target coordinate system to generate a transformed capacitor voltage signal is an optional transformation, as are embodiments in which the signal obtained in the measurement coordinate system is maintained in the measurement coordinate system without further transformation.

[0015] According to one embodiment, the method comprises controlling power output from a power generation unit by:

[0016] - controlling the generation of reactive power generation from the line side converter based on the filter compensated voltage reference or a transformation thereof, and

[0017] - Controlling the generation of active power generation from the line side converter based on the synchronous machine angle or a transformation thereof.

[0018] According to one embodiment, the compensation filter or the series-connected filters are further designed to decouple the voltage amplitude reference from the filtered capacitor voltage signal at the fundamental frequency.

[0019] Advantageously, the capacitor voltage signal is filtered to remove signal content at a fundamental frequency (eg grid frequency or zero Hertz frequency, depending on the rotational transformation used in the processing of the capacitor voltage signal).

[0020] According to one embodiment, the virtual synchronous machine rotational speed is determined based on feedback of the damping power, a power reference, a combination of the grid power and an inertia integration model, wherein the synchronous machine angle is determined based on an integration of the synchronous machine rotational speed, and wherein the damping power is determined based on the virtual synchronous machine rotational speed.

[0021] According to one embodiment, determining the damping power based on the virtual synchronous machine rotation speed comprises high-pass filtering the synchronous machine rotation speed and determining the damping power based on the high-pass filtered signal.

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

[0023] - obtaining the network voltage at the connection point of the output of the power generation unit,

[0024] - determining the rotation speed of the grid based on the network voltage, and

[0025] - Determining the damping power based on the virtual synchronous machine rotation speed and the grid rotation speed.

[0026] According to one embodiment, the compensation filter comprises a lead-lag filter or a high-pass filter.

[0027] The compensation filter can be designed to limit the resonance caused by the harmonic filtering capacitors by an approximate pole cancelling method.

[0028] According to one embodiment, the voltage amplitude reference is defined in a DQ coordinate system (ie, a direct quadrature frame).

[0029] According to one embodiment, the method includes transforming a voltage magnitude reference in a DQ coordinate system to a non-rotating target coordinate system, and determining a filter-compensated voltage reference by combining the transformed voltage reference with a filtered capacitor voltage signal.

[0030] According to one embodiment, the filter-compensated voltage reference is determined by combining a voltage magnitude reference—or a derived voltage reference derived from the voltage magnitude reference—with the filtered capacitor voltage signal.

[0031] According to one embodiment, the capacitor voltage signal is transformed from a measurement coordinate system to a target coordinate system to generate a transformed capacitor voltage signal.

[0032] According to one embodiment, the method comprises transforming the filter-compensated voltage reference from a target coordinate system to a measurement coordinate system.

[0033] According to one embodiment, the power generating unit is a wind turbine.

[0034] A second aspect of the present invention relates to a control system for controlling a power generation unit, the power generation unit comprising a power source, a machine-side converter, a line-side converter, a DC link electrically connected to an output of the machine-side converter and an input of the grid-side converter, and a filter inductor and a filter capacitor arranged at the output of the power generation unit, the control system being arranged to:

[0035] - obtaining a capacitor voltage signal in a measurement coordinate system by measuring the capacitor voltage of one or more of the filter capacitors,

[0036] - determining a virtual synchronous machine rotational speed and / or a synchronous machine angle, wherein a derivative of the synchronous machine rotational speed is indicative of a deviation between a power reference of a desired power output of the power generation unit and a grid power combined damping power supplied to the output by the power generation unit,

[0037] - providing a voltage magnitude reference for the desired reactive power to be generated by the line-side converter,

[0038] - optionally, transforming the capacitor voltage signal from a measurement coordinate system to a target coordinate system to generate a transformed capacitor voltage signal,

[0039] filtering the capacitor voltage signal or the transformed capacitor voltage signal by means of a compensation filter into a filtered capacitor voltage signal, wherein the compensation filter is designed to reduce the magnitude of and / or shift the impedance peak, wherein the impedance peak is present in the impedance characteristic of the output of the power generation unit, and

[0040] - determining a filter-compensated voltage reference by combining the voltage magnitude reference or a transformation thereof with the filtered capacitor voltage signal.

[0041] A third aspect of the invention relates to a power generation unit comprising a control system according to the second aspect.

[0042] A fourth aspect of the invention relates to a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0043] In general, the various aspects and embodiments of the present invention may be combined and coupled in any possible manner within the scope of the present invention. These and other aspects, features and / or advantages of the present invention will be apparent and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0045] Figure 1A wind turbine is shown,

[0046] Figure 2A An example of an electrical power system showing a wind turbine or power generation unit,

[0047] Figure 2B shows control components arranged to control the generation of active and reactive power supplied to a grid at a power output of a wind turbine or power generation unit,

[0048] Figure 3A -B shows an example of a control system for determining the angle of a synchronous machine,

[0049] Figure 4 An equivalent diagram showing the output of a virtual synchronous machine,

[0050] Figure 5 shows a graph showing that the impedance peak has been shifted to an acceptable frequency range and / or peak amplitude,

[0051] Figure 6A -B shows an example of implementation of the compensation filter Gff. DETAILED DESCRIPTION

[0052] 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 is adapted to drive a generator located within the nacelle via a drive train. The rotor 102 can be rotated by the action of the 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 with the aid of the rotor blades, and subsequently convert it into electrical energy with the aid of 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 the generator alternating current (AC) power into direct current (DC) power, and the line-side converter converts the DC power into AC power for injection into the utility grid.

[0053] 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 or power source 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 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. The output AC current from the line-side converter 204 is supplied to a power line 220 via an output inductor 206 and, possibly, a wind turbine transformer 208. A harmonic filter capacitor 216, arranged between the output conductors, forms a harmonic filter together with the inductor 206, which converts the square wave voltage signal from the line-side converter 204 into a sinusoidal voltage signal.

[0054] Since the power system 200 is also applicable to other power generation units 199 configured with full-scale power converters 202, the examples and embodiments of the present invention are equally applicable to other power generation units (such as renewable power generation units, for example, solar power units or photovoltaic power generation units). That is, the generator or power source 201 can be embodied as a solar power source (such as a photovoltaic power source), a wind turbine generator, or other power sources or generators. Therefore, the power generation units 199 comprising the power system 200 can be wind turbines, solar power plants or units, or other power units such as renewable power generation units.

[0055] 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 another transformer. Thus, power line 220 and one or more power systems 200 corresponding to wind turbines constitute a wind farm or park, which is arranged to supply power to a utility grid for distribution.

[0056] The power converter 202 may be a full-scale converter configured according to various principles (including force-commutated and line-commutated converters).

[0057] The power system 200 is shown schematically and therefore does not explicitly disclose that the system may be a three-phase system. However, the principles of the described embodiments are applicable to single-phase and multi-phase systems.

[0058] The line side converter 204 uses some variation of pulse width modulation (PWM) to convert DC power to AC power. A control system 250 is used to control the modulation of the line side converter 204 and to control the reactive power P and active power Q generated by the line side converter 204.

[0059] Figure 2A It is shown that the grid voltage Ugrid (here, the voltage on the low-voltage (LV) side of the transformer 208) can be measured. The grid voltage Ugrid can be used to determine the virtual synchronous machine angle θVSM (as described elsewhere) and to control the power output of the converter based on determining the active power Pgrid from the grid voltage Ugrid and the grid current Igrid. The reactive power Qgrid can be similarly determined 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 measuring the voltage Ugrid. Therefore, in alternative solutions, an internal voltage amplitude reference such as Vqref, Vdqref, or Vαβref can be used to determine Pgrid and, therefore, the synchronous machine angle θVSM. Therefore, the grid current Igrid supplied to the grid can also be measured.

[0060] Figure 2B An example of a control component 260 is shown, which is arranged to control the generation of active power Pgrid and reactive power Qgrid supplied from the wind turbine to the grid at power output 270. That is, the control component 260 can be arranged to control the output active power Pgrid and the output voltage amplitude of the low-voltage side LV, instead of controlling the output active power P and the output reactive power Qgrid of the low-voltage side LV. The control component 260 can form part of the control system 250. Alternatively, the control component 260 receives control signals from the control system 250.

[0061] The references for active power and reactive power may be received from the power plant controller PPC or the grid operator TSO or determined from active power references and reactive power references, eg from the grid operator.

[0062] The active power Pgrid is controlled via the virtual synchronous machine angle θVSM. In short, the synchronous machine angle acceleration is the difference between the power reference Pref, which indicates the desired power output of the wind turbine, and the grid power Pgrid supplied by the wind turbine to the grid. Examples for determining the synchronous machine angle θVSM are given elsewhere.

[0063] The synchronous machine angle θVSM can be used to transform the signal from the rotating DQ coordinate system to a non-rotating coordinate system (such as αβ or abc coordinate system) and vice versa. Based on the synchronous machine angle θVSM and the voltage amplitude reference Vqref, the control signals of the desired active power and reactive power are determined.

[0064] Therefore, the synchronizer angle θ VSM can be defined in the rotating DQ coordinate system defined by the angular position θ VSM. Based on the synchronizer angle θ VSM, the control signal (i.e., the angle of the modulated voltage signal 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 voltage reference signal controls the active and reactive powers P grid and Q grid.

[0065] The coordinate system conversion unit 266 transforms the control signal from the DQ coordinate system to the αβ or abc coordinate system and determines the sinusoidal voltage reference of the PWM 265. The coordinate system conversion output signal from the control unit 266 is converted by the pulse width modulator PWM 265 into a modulation signal for the grid-side converter 204 to generate the desired active power and reactive power and / or voltage amplitude.

[0066] The reactive power Qgrid to be generated by the line side converter 204 may be controlled based on the voltage magnitude reference Vqref.

[0067] The voltage amplitude reference Vqref may be defined in a DQ coordinate system that rotates at the rotational speed ωVSM of the virtual synchronous machine (i.e., under steady-state conditions, at the fundamental frequency of the AC grid voltage, such as 50 Hz). The voltage amplitude reference Vqref or a modification thereof (as described below) may be converted from the DQ coordinate system to an αβ or abc coordinate system and output from the control unit 266 as a control signal to the pulse width modulator PWM 265, which determines a modulation signal for the grid-side converter 204.

[0068] Figure 3A and Figure 3B An example of a control system 391 for determining the synchronizer angle θ VSM is shown.

[0069] The synchronous machine angle θ VSM is determined based on a virtual synchronous machine control concept, which aims to generate a power response corresponding to the power response from a real synchronous generator (which includes the inertia of the synchronous generator).

[0070] Under steady-state conditions, the power error ΔP has a value of zero. In response to grid voltage fluctuations (e.g., as reflected in the measured Ugrid) or in response to a change in the power reference Pref, the power error ΔP becomes non-zero, causing the angle θVSM to increase or decrease to reduce the power error ΔP. For example, during a low voltage ride-through (LVRT) event or under voltage ride-through (UVRT), the power error ΔP becomes positive, and the synchronous machine speed ωVSM increases.

[0071] Therefore, in response to grid voltage fluctuations (e.g., reflected in the measured Ugrid and Pgrid), the synthetic inertial response value becomes non-zero, which causes the virtual machine to accelerate or decelerate to reach a new equilibrium condition. The new equilibrium is reached when the measured grid power Pgrid again follows Pref.

[0072] The virtual synchronous machine control concept is utilized on the line side converter 204 using the swing equation to calculate θ VSM .

[0073] Figure 3A An example of an implementation of a virtual synchronous model 301 is shown. The virtual synchronous model 301 comprises a closed loop in which the virtual synchronous machine rotational speed ω VSM is determined based on a combination of feedback of the damping power Pd, a power reference Pref of the desired active power output of the wind turbine, the active grid power Pgrid supplied by the wind turbine to the grid via the power line 220, and an inertia integration model 311. The inertia integration model 311 is implemented as 1 / (2Hs), where H is the inertia time constant and 1 / s is the integral in the S domain. Thus, the power combination Pref - Pd - Pgrid = ΔP is used as input to the inertia integration model 311.

[0074] Since the derivative of the synchronous machine rotation speed ω VSM is proportional to the deviation between the power reference Pref and the grid power Pgrid, the integration of the difference ΔP gives the synchronous machine rotation speed ω VSM.

[0075] The grid power Pgrid may be determined based on the measured grid voltage Ugrid or an internal voltage reference such as a magnitude reference Vqref or a transformation thereof such as Vαβref or Vdqref and the measured grid current Igrid.

[0076] The damping power Pd is determined as the difference between the grid rotation speed ωL and the synchronous machine rotation speed ωVSM multiplied by the damping coefficient Dp. The grid rotation speed ωL (ie the grid frequency) is determined from the measured grid voltage Ugrid.

[0077] The synchronous machine angle θ VSM is determined based on the integration of the synchronous machine rotation speed ω VSM according to ωr / s, where ωr is the rated synchronous generator speed.

[0078] Figure 3B An alternative virtual synchronous model 301 is shown, which is not based on the measured grid voltage Ugrid, but the grid rotation speed ωL is determined based on high-pass filtering of the determined synchronous machine rotation speed ωVSM, i.e. by determining the grid rotation speed ωL as the output of a high-pass filter 313, which is arranged to filter the input synchronous machine rotation speed ωVSM.

[0079] Therefore, the alternative virtual synchronous model 301 is not based on the measured grid voltage Ugrid, but the damping portion (eg the damping power Pd) is determined based on a high-pass filtering 313 of the synchronous machine rotation speed ω VSM.

[0080] In general, the virtual synchronous model 301 determines the virtual machine angle θVSM based on a combination of the powers Pref, Pd, and Pgrid, an inertia integral model 311 (e.g., implemented as 1 / (2Hs)), and feedback based on ωVSM and the damping power Pd determined by the integration of ωVSM. In other words, the synchronous machine rotational speed ωVSM and the synchronous machine angle θVSM are determined so that they indicate the integral deviation between the power reference Pref of the desired power output of the wind turbine and the grid power Pgrid supplied to the output by the wind turbine.

[0081] The control system 391 can be implemented based on the power values ​​Pref, Pd, and Pgrid, or equivalently implemented based on the corresponding torque values ​​Tref, Td, and Tgrid based on the relationship that power equals torque multiplied by rotation frequency (eg, synchronous machine rotation speed ωVSM).

[0082] Due to certain grid code requirements, or because some TSOs prefer the impedance spectrum of a virtual synchronous machine to resemble the impedance spectrum of a voltage source behind a pure inductor, embodiments of the present invention propose a solution to modify the impedance spectrum. Due to the capacitive harmonic filter formed by harmonic filter capacitor 216 and inductor 206, there will be a resonance point where the resulting impedance becomes very high and the damping is low.

[0083] Embodiments of the present invention address this issue by feeding forward the measured filter voltage (ie, the measured voltage across the harmonic filter capacitor 216) to eliminate or reduce the effect of the capacitor in the frequency range of interest.

[0084] Figure 4 An equivalent diagram 401 of the output of a virtual synchronous machine is shown, i.e., an equivalent diagram of the output of the line-side converter 204 viewed from the power output location 270 (i.e., between the filter capacitor 216 and the transformer 208) toward the converter 204. L is the equivalent inductance of the inductor 206, C is the equivalent capacitance of the filter capacitor 216, and Uout is the output voltage. Gd is a delay model of the pulse width modulator 265 and the digital controller. Gff is a compensation filter designed to reduce the magnitude of and / or shift the impedance peak 511, which is present in the impedance characteristic Zpp of the equivalent diagram 401. The equation for Zpp and the transfer function for Gd are shown as follows: Figure 4 As shown in A.

[0085] In the case where grid voltage feedforward is not used (i.e., when Gff=0), the pole of the transfer function Zpp results in Figure 5 The influence of the impedance peak 511 can be reduced by adding the grid voltage feedforward Gff to the voltage amplitude reference Vqref (or its transformation, i.e. Vdqref or Vαβref) of the virtual synchronous machine via a compensation filter Gff in series with the series-connected filter Gff2.

[0086] Figure 6A An example of implementation of the compensation filter Gff is shown.

[0087] One or more voltage signals Ucap are obtained from the measurement of the voltage across the corresponding filter capacitor C 216. The capacitor voltage Ucap can be combined with Figure 2A The grid voltage Ugrid obtained on the low-voltage side is equivalent or identical to that described. Therefore, Ucap can be replaced by Ugrid.

[0088] The voltage signal Ucap is obtained in a fixed measurement coordinate system abc. For example, three voltage signals Ucap can be obtained, one for each of the three phases.

[0089] In this example, the voltage Ucap is transformed from the measurement coordinate system to the DQ coordinate system, which rotates at the virtual synchronous angular frequency. The abc / DQ transformation is performed based on the virtual synchronous machine angle θVSM. The transformed capacitor voltage signal Ucap is referred to as the transformed capacitor voltage signal Udq.

[0090] The virtual synchronizer angle θVSM is used for the coordinate system conversion 266 from the DQ coordinate system to the abc coordinate system and is provided as a control signal for the pulse width modulator PWM 265 to generate active power P based on θVSM, as shown in combination with Figure 2A As described. The virtual synchronous machine angle θVSM is combined with Figure 3A -B is as described.

[0091] The transformed capacitor voltage signal Udq is filtered into a filtered capacitor voltage signal Udq_f by a compensation filter Gff to reduce the amplitude of the impedance peak 511 and / or move the impedance peak 511 (eg, move the impedance peak 511 to a lower frequency).

[0092] Since the transformed capacitor voltage signal Udq is represented as a DC signal, a high-pass filter Gff2 is arranged in series with the compensation filter Gff, or a high-pass filter is included in the compensation filter Gff, so as to decouple the voltage reference Vdqref from the filtered capacitor voltage signal Udq_f at the fundamental frequency (for example, 50 Hz grid frequency).

[0093] The voltage reference Vdqref is a voltage reference in the DQ coordinate system generated based on the voltage magnitude reference Vqref.

[0094] The filtered capacitor voltage signal Udq_f is combined (eg, added) to the voltage reference Vdqref to generate a filter-compensated voltage reference Vdq_f for the desired reactive and active power to be generated by the line side converter 204 .

[0095] Similar to the control signal derived from the synchronizer angle θ VSM , the filter-compensated voltage reference Vdq_f is transformed from the DQ coordinate system to the measurement coordinate system abc.

[0096] The generation of reactive power Q and active power P is controlled based on the filter-compensated voltage reference Vdq_f and the synchronizer angle θVSM.

[0097] Figure 6B An alternative example of implementation of the compensation filter Gff is shown. Figure 6B The solution in Figure 6A The solutions in are equivalent, but based on other coordinate system transformations.

[0098] In this example, the voltage Ucap is transformed from the measurement coordinate system to the stationary αβ coordinate system. The abc / αβ transformation transforms the three vector values ​​in the measurement coordinate system abc into two vector values ​​in the αβ coordinate system. The transformed capacitor voltage signal Ucap is called the transformed capacitor voltage signal Uαβ.

[0099] Vαβref is the DQ transformed version of Vdqref, where Vdqref is based on the voltage amplitude reference Vqref ( Figure 6A The voltage reference Vdqref in the DQ coordinate system is generated (not explicitly shown in FIG).

[0100] The virtual synchronizer angle θ VSM is used in the coordinate system conversion unit 266 from the DQ coordinate system to the αβ coordinate system, and is transformed into the measurement coordinate system abc by the transformation element 621 and provided as a control signal of the pulse width modulator PWM.

[0101] The transformed capacitor voltage signal Uαβ is filtered into a filtered capacitor voltage signal Uαβ_f by a compensation filter Gff, so as to reduce the amplitude of the impedance peak 511 and / or shift the impedance peak 511 .

[0102] Since the transformed capacitor voltage signal Uαβ is represented as a time-varying signal, a notch filter Gff3 is arranged in series with the compensation filter Gff or is included in the compensation filter Gff to decouple the voltage reference Vαβ from the filtered capacitor voltage signal Vαβ_f at the fundamental frequency (e.g., 50 Hz grid frequency). The notch filter is therefore arranged to select frequency components of the filtered capacitor voltage signal Uαβ_f near the fundamental frequency.

[0103] The filtered capacitor voltage signal Uαβ_f is combined with (eg, added to) the transformed voltage reference Vαβref to generate a filter-compensated voltage reference Vαβ_f.

[0104] Similar to the control signal derived from the synchronizer angle θ VSM , the filter-compensated voltage reference V α β_f is transformed from the α β coordinate system to the measurement coordinate system abc. Thus, the two signals are transformed by the α β / abc transformation element 621 .

[0105] Figure 6A -B filter circuit or other similar filter circuits can be implemented in the control system 250.

[0106] Figure 5 The graph shows the impedance characteristic Zpp as a function of frequency f. Figure 6A -The result of the solution in B. Therefore, Figure 5 The impedance peak 511 of the impedance graph obtained from the virtual synchronous machine implemented without the compensation filter Gff is shown to have been transformed into the impedance peak 512 of the graph obtained when the compensation filter is implemented. The latter graph shows that the impedance peak is reduced in magnitude and shifted to lower frequencies, i.e., reduced from a frequency or frequency range where the peak is unacceptable to a frequency or frequency range where the reduced peak is acceptable.

[0107] Although the present invention has been described in conjunction with specific embodiments, it should not be interpreted as being limited in any way to the examples presented. The scope of the invention is to be interpreted in the light of the appended claims. In the context of the claims, the terms "comprise" or "comprising" do not exclude other possible elements or steps. Furthermore, references such as "a" or "an" should not be interpreted as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the drawings should also not be interpreted as limiting the scope of the invention. Furthermore, individual features mentioned in different claims may possibly be advantageously combined, and mentioning these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method for controlling a power generation unit (199), the power generation unit comprising a power source (201), a machine-side converter (203), a line-side converter (204), a DC link (205) electrically connected to an output of the machine-side converter and an input of a grid-side converter, and a filter inductor (L) and a filter capacitor (C) arranged at an output of the power generation unit, the method comprising: - obtaining a capacitor voltage signal (Ucap) in a measurement coordinate system by measuring the capacitor voltage (Ucap) of one or more of the filter capacitors, - determining a virtual synchronous machine rotation speed (ωVSM) and / or a synchronous machine angle (θVSM), wherein a derivative of the synchronous machine rotation speed (ωVSM) indicates a deviation between a power reference (Pref) of a desired power output of the power generation unit and a grid power (Pgrid) supplied to the output by the power generation unit combined with a damping power (Pd), wherein the virtual synchronous machine rotation speed (ωVSM) is determined based on feedback of the damping power (Pd), a combination of the power reference (Pref), the grid power (Pgrid) and an inertia integral model (311), wherein the synchronous machine angle (θVSM) is determined based on an integration of the synchronous machine rotation speed (ωVSM), and wherein the damping power (Pd) is determined based on the virtual synchronous machine rotation speed (ωVSM), - providing a voltage magnitude reference (Vqref) of the desired reactive power (Qgrid) to be generated by the line side converter (204), - transforming the capacitor voltage signal (Ucap) from the measurement coordinate system to the target coordinate system (dq, αβ) to generate a transformed capacitor voltage signal (Udq, Uαβ), - filtering the transformed capacitor voltage signal (Udq, Uαβ) into a filtered capacitor voltage signal (Udq_f, Uαβ_f) by means of a compensation filter (Gff), wherein the compensation filter (Gff) is designed to reduce the magnitude of an impedance peak (511) and / or to shift the impedance peak (511), wherein the impedance peak is present in the impedance characteristic of the output of the power generation unit, and - determining a filter-compensated voltage reference (Vdq_f, Vαβ_f) by combining the voltage amplitude reference (Vqref) or its transformation (Vdqref, Vαβref) with the filtered capacitor voltage signal (Udq_f, Uαβ_f).

2. The method of claim 1 , comprising controlling the power output from the power generation unit by: - controlling the generation of reactive power generation from the line side converter (204) based on the filter compensated voltage reference (Vdq_f) or a transformation thereof, and - Controlling the generation of active power generation from the line side converter (204) based on the synchronous machine angle (θVSM) or a transformation thereof.

3. The method according to claim 1 or 2, wherein The compensation filter (Gff) or series-connected filters (Gff2, Gff3) are also designed to decouple the voltage amplitude reference (Vqref) from the filtered capacitor voltage signal (Udq_f, Uαβ_f) at the fundamental frequency.

4. The method according to claim 1 or 2, wherein: Determining the damping power (Pd) based on the virtual synchronous machine rotation speed (ω VSM) includes high-pass filtering the synchronous machine rotation speed (ω VSM) and determining the damping power (Pd) based on the high-pass filtered signal.

5. The method according to claim 1 or 2, comprising: - obtaining the network voltage (Ugrid) at the connection point of the output of the power generation unit, - determining the rotation speed of the grid (ωL) based on the grid voltage (Ugrid), and - Determining the damping power (Pd) based on the virtual synchronous machine rotation speed (ω VSM ) and the rotation speed of the grid (ω L ).

6. The method according to claim 1 or 2, wherein: The compensation filter (GW) includes a lead-lag filter or a high-pass filter.

7. The method according to claim 1 or 2, wherein: The voltage magnitude reference (Vqref) is defined in the DQ coordinate system.

8. The method according to claim 7, wherein: The method comprises transforming a voltage magnitude reference (Vqref) in a DQ coordinate system to a non-rotating target coordinate system (αβ), and wherein a filter-compensated voltage reference (Vαβ_f) is determined by combining the transformed voltage reference (Vαβref) with a filtered capacitor voltage signal (Udq_f, Uαβ_f).

9. The method according to claim 7, wherein: The filter-compensated voltage reference (Udq_f) is determined by combining the voltage amplitude reference (Vqref) or the derived voltage reference (Vdqref) with the filtered capacitor voltage signal (Udq_f, Uαβ_f).

10. The method according to claim 1 or 2, wherein: The capacitor voltage signal (Ucap) is transformed from the measurement coordinate system to the target coordinate system (DQ, αβ) to generate a transformed capacitor voltage signal (Udq, Uαβ).

11. The method according to claim 1 or 2, wherein: The method includes transforming a filter-compensated voltage reference (Vdq_f, Vαβ_f) from a target coordinate system (DQ, αβ) to a measurement coordinate system (abc).

12. The method according to claim 1 or 2, wherein: The power generation unit (199) is a wind turbine (100).

13. A control system (250) for controlling a power generation unit (199), the power generation unit comprising a power source (201), a machine-side converter (203), a line-side converter (204), a DC link (205) electrically connected to an output of the machine-side converter and an input of a grid-side converter, and a filter inductor (L) and a filter capacitor (C) arranged at an output of the power generation unit, the control system being arranged to: - obtaining a capacitor voltage signal (Ucap) in a measurement coordinate system by measuring the capacitor voltage (Ucap) of one or more of the filter capacitors, - Determine the virtual synchronous machine rotational speed (ω VSM ) and / or the synchronous machine angle (θ VSM ), wherein A derivative of the synchronous machine rotation speed (ωVSM) indicates a deviation between a power reference (Pref) of a desired power output of the power generation unit and a grid power (Pgrid) supplied to the output by the power generation unit combined with a damping power (Pd), wherein the virtual synchronous machine rotation speed (ωVSM) is determined based on feedback of the damping power (Pd), the power reference (Pref), a combination of the grid power (Pgrid) and an inertia integration model (311), wherein the synchronous machine angle (θVSM) is determined based on an integration of the synchronous machine rotation speed (ωVSM), and wherein the damping power (Pd) is determined based on the virtual synchronous machine rotation speed (ωVSM), - providing a voltage magnitude reference (Vqref) of the desired reactive power (Qgrid) to be generated by the line side converter (204), - transforming the capacitor voltage signal (Ucap) from the measurement coordinate system to the target coordinate system (dq, αβ) to generate a transformed capacitor voltage signal (Udq, Uαβ), - filtering the transformed capacitor voltage signal (Udq, Uαβ) into a filtered capacitor voltage signal (Udq_f, Uαβ_f) by means of a compensation filter (Gff), wherein the compensation filter (Gff) is designed to reduce the magnitude of an impedance peak (511) and / or to shift the impedance peak (511), wherein the impedance peak is present in the impedance characteristic of the output of the power generation unit, and - determining a filter-compensated voltage reference (Vdq_f, Vαβ_f) by combining the voltage amplitude reference (Vqref) or its transformation (Vdqref, Vαβref) with the filtered capacitor voltage signal (Udq_f, Uαβ_f).

14. A power generation unit (199) comprising a control system (250) according to claim 13.

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