Generator power peak limit in wind power plant

Abstract

The invention relates to a method for controlling wind turbine transients. The wind turbine generator includes a power converter including a machine-side converter and a line-side converter and a DC link. A grid voltage reference for controlling the line-side converter is determined, and the power (PLsc) supplied to the grid is controlled by the line-side converter using a grid formation controller. The machine-side converter and the generator are controlled according to the power reference (PMsc). An error between the measured DC link voltage value and a DC link reference value is determined, and a voltage correction component is derived in response to the error signal. The voltage correction component is added to an output voltage of the grid formation control, and the line-side converter is operated according to the combined output voltage.

Description

Field of the Invention

[0001] The present invention relates to the control of power generation in a wind turbine and, in particular, to the control of generator power peaks in response to transient grid events. Background Art

[0002] In order to allow for higher penetration of renewable energy sources, such as wind turbines, into the electrical grid, requirements have been proposed to equip the power converters of wind turbines with grid-forming characteristics similar to those of conventional synchronous generators. These requirements can be addressed, for example, by configuring the renewable power generation unit as a virtual synchronous machine (VSM).

[0003] When a wind turbine is configured to operate as a VSM or according to other grid-forming control schemes, grid disturbances, such as phase jumps, can cause peak values of generator power and / or generator torque, leading to undesirable transient mechanical loads.

[0004] Accordingly, a problem is that a wind turbine configured to operate according to a grid-forming control scheme may experience high mechanical loads in response to grid disturbances. Summary of the Invention

[0005] It is an object of the present invention to improve the control of a wind turbine including a power converter configured to be controlled according to a grid-forming control scheme in order to mitigate the problem of transient mechanical loads.

[0006] According to a first aspect of the present invention, there is provided a method for controlling a wind power device to limit motor transients, the wind power device including an aerodynamic rotor, an electric machine having a stator and a rotor driven by the aerodynamic rotor, a power converter including a machine-side converter connected to the stator and a line-side converter configured to supply power to the grid, and a DC link electrically connected to an output of the machine-side converter and an input of the line-side converter. The method includes:

[0007] determining a grid voltage reference for controlling the line-side converter;

[0008] controlling the power supplied by the line-side converter to the grid by controlling the line-side converter using a grid-forming controller configured to control the output voltage towards the grid voltage reference;

[0009] using a power reference for machine power control;

[0010] controlling the machine-side converter and the electric machine according to the power reference;

[0011] Using DC link voltage control, an error signal representing the error between the measured DC link voltage value and the DC link reference value is derived, the error being caused by a power imbalance between the line side converter power and the machine side converter power;

[0012] A DC link voltage correction component is derived in response to the error signal;

[0013] The DC link voltage correction component is added to the output voltage of the grid forming control; and

[0014] The line side converter is operated according to a combination of the output voltage from the grid forming controller and the DC link voltage correction component.

[0015] Regarding the power reference for machine power control, the power supplied to the grid can be fed forward to be used as the power reference, and / or the power reference can be received from, for example, a power plant controller PPC or a grid operator, or determined based on active and reactive power references.

[0016] The electric machine can be a generator.

[0017] As mentioned, it may be desirable or even necessary to equip a wind turbine power converter with requirements for grid forming characteristics that exhibit a similarity to the behavior of a conventional synchronous generator. This can be achieved by using a grid forming controller to operate the line side converter according to a grid forming control scheme.

[0018] In a grid forming control scheme, the line side converter is controlled to generate an output voltage according to a grid voltage reference, where the grid voltage reference can be, for example, a reference provided by an electrical grid operator or an operator of a wind farm, for example, and where multiple power sources can be connected to the electrical grid to supply power. The grid voltage reference can include the voltage amplitude and frequency to be maintained by a particular wind turbine generator. The voltage reference can also be determined according to other criteria. For example, this may be the case if the wind turbine generator is configured to set, for example, a local electrical grid, i.e., operate in island mode, where the voltage amplitude can be adapted according to the specific needs of the connected consumers. Thus, in grid forming control, a wind turbine generator can create an electrical power grid by powering an otherwise de-energized power line.

[0019] Therefore, grid forming control is different from more conventional grid following control, where, instead, the grid following converter synchronizes to the currently dominant grid voltage amplitude and frequency, where the line side converter adjusts the output voltage to track an external voltage reference given by the currently dominant voltage on the electrical grid.

[0020] In the steady state mode, the power balance in the grid-following control substantially keeps the torque and speed of the generator constant, and thus the power output of the generator is constant. The DC-link voltage of the DC link interconnecting the machine-side converter and the line-side converter is also constant, while the voltage output by the line-side converter is variable such that the current injected into the grid is adapted in magnitude to correspond to the dominant power delivered by the generator.

[0021] In the event of a fault or contingency in the electrical grid, when the line-side converter operates according to the grid-following control, in principle it will continuously adjust the voltage to the current dominant voltage on the grid, and thus also in the case where the grid voltage deviates from the voltage that the grid aims to maintain. Therefore, the grid-following line-side converter does not support the rigidity of the grid in this case, but simply follows the voltage change when it occurs and continuously supplies the power generated by the generator to the grid at the voltage magnitude and angle at which the grid is currently operating by adjusting the injected current accordingly, such that the injected power remains substantially the same.

[0022] Therefore, when an abnormal grid event occurs while operating in the grid-following mode, the machine-side converter and thus also the generator are substantially unaffected because the generated power is continuously delivered to the DC link and forward to the grid, regardless of the dominant grid voltage. Thus, in terms of the changes occurring on the grid, the generator and other drivetrain components can be considered decoupled from the grid, and thus these components also do not experience potentially harmful voltage or torque transients due to changes in the grid voltage because the power supplied by the generator can continue to be supplied to the grid at the same power level, or alternatively or additionally, if the power cannot be injected into the grid, a DC-link chopper can be used to dissipate the energy, still keeping the machine-side unaffected (at least within the capabilities of the DC chopper). The obvious drawback of this control is the lack of support for system stability, and the more power sources connected to the grid in this way, the higher the impact.

[0023] As mentioned, the stability issue can be mitigated by the power source, alternatively, controlled according to the grid-forming control, where the line-side converter is alternatively configured to act as a grid stabilizer. Then, the line-side converter is configured to control the output voltage towards a voltage reference and maintain that voltage reference, regardless of the actual dominant grid voltage, in order to support the maintenance of the desired voltage.

[0024] This improves the stability of the power grid, but as a result, grid events (especially transient changes) may impose harmful torque transients on the wind turbine generator assembly. If something happens in the power grid, there will be an immediate current response that automatically follows when continuing to control the output of the line-side converter towards the voltage reference. This will have the result that the current injected into the power grid will instead be a direct result of the voltage control of the line-side converter.

[0025] As a result, according to this prior art solution, when the power grid undergoes transients, the current to be injected into the power grid may also exhibit transients, and when transients occur in the DC link current, this will also affect the control of the DC link voltage. The machine-side converter controls the DC link voltage, and in order to maintain the DC link voltage at the DC link voltage reference, the power request from the generator undergoes corresponding transient changes to compensate for the changes in the DC link. The result is that the transients can be transferred to the machine-side converter, and thus also to the generator and other drivetrain components that supply power to the machine-side converter. For example, the pneumatic rotor, as well as the generator and the gearbox that interconnects the pneumatic rotor and the generator, will be directly affected by such torque changes, and components of this type do not respond well to harmful torque spikes that may cause excessive wear. In addition, if subjected to, for example, excessive current spikes, the components of the power converter may break.

[0026] According to the present invention, a control method is provided that provides the desired grid-forming control while reducing excessive wear of the mechanical components as well as the converter components caused by transient events occurring on the power grid.

[0027] According to a first aspect of the present invention, a voltage reference for controlling the line-side converter is determined, which can be performed as described above regarding grid-forming control, and thus is the voltage towards which the line-side converter will control the output voltage when a voltage-changing grid event occurs. In addition, the power supplied to the power grid by the line-side converter is controlled by controlling the line-side converter according to the grid-forming control, and the grid-forming control is configured to control the output voltage towards the voltage reference. Therefore, general control of an electric machine such as a wind turbine generator is performed according to the grid-forming control in order to provide grid stability control.

[0028] The power supplied to the power grid can be fed forward to the generator control to be used as a power reference for the power control of the electric machine, and the machine-side converter and the electric machine can be controlled according to this power reference. As described above, the power reference can also be determined in other ways. This is also according to the general grid-forming control, where transient changes in the power normally supplied to the power grid will be reflected in the power reference for the power control of the generator, and harmful transients in the mechanical components may occur due to sudden changes in the requested power.

[0029] According to the present invention, such transients can be at least mitigated or partially or completely eliminated by applying a control mechanism for controlling the DC link voltage of the DC link. Typically, the DC link voltage is maintained by a power balance between the power supplied by the machine-side converter and the power consumed by the line-side converter, wherein, in order to maintain the DC link voltage, the power supplied by the machine-side converter is required to correspond to the power injected into the grid by the line-side converter, wherein, as stated, transient changes in the power to be injected into the grid will propagate to the machine (generator) side.

[0030] According to the present invention, instead of using control of the machine-side converter (and thereby the electric machine) to maintain the DC link voltage, a DC link control is performed, wherein an error between a measured DC link voltage value and a DC link reference value is derived, wherein the error is caused by a power imbalance between the line-side converter power and the machine-side converter power, for example caused by a transient event on the grid.

[0031] A DC link voltage correction component is derived in response to the error signal and added to the output of the GFC output voltage. The line-side converter is then operated using the combined DC link voltage correction component and the output voltage of the grid formation control. In this manner, control of the DC link voltage is performed in parallel with the normal grid formation control of power injected into the grid, wherein the output voltage of the grid formation control is supplemented, i.e., corrected, by the DC link voltage correction component. This addition from the DC link control at least partially compensates for transient events, eliminating the need for the generator to account for such transient changes during grid formation control.

[0032] Therefore, by adding the voltage component generated by the DC link control, the actual voltage output by the line-side converter will differ to some extent from the calculation generated by the grid forming control, but harmful transients on mechanical components can be reduced. The DC link control can be performed in parallel with the grid forming control of the voltage to be output by the line-side converter.

[0033] The DC link control may also be performed according to a grid following control algorithm, wherein the line side converter may thus be controlled by a grid forming algorithm and a grid following algorithm operating in parallel. An advantage of the present invention is that, because the grid forming control and grid following control schemes are configured to operate in parallel, a majority of the power injected into the grid may be controlled according to the grid forming control, while the grid following control may be used to reduce harmful transients that would otherwise occur during the grid forming control.

[0034] According to an embodiment of the present invention, before actually using the measured value of the power supplied to the power grid (which can be used as a power reference for the power control of the generator by feedforward, or the power reference is obtained according to other means) as the power reference of the generator, the measured value is processed. In this way, sudden changes in the power supplied to the power grid can be reduced, and thus sudden changes in the power reference of the generator can also be reduced, while still ensuring the maintenance of the DC link voltage through separate DC link voltage control.

[0035] According to an embodiment of the present invention, the processing of the power reference (such as the measured value of the line-side power) may include: when a transient change occurs in the power supplied to the power grid by the line-side converter, subjecting the measured value of the power supplied to the power grid by the line-side converter to a rate-of-change limit. In this way, it can be ensured that the transient change in the power supplied to the power grid is slowed down, so that the power reference for controlling the generator changes at a slower rate, thereby reducing the wear of mechanical components. When carried out alone, this rate-of-change control will have a negative impact on the DC link voltage, but as described above, it is compensated by the separate DC link voltage control according to the present invention.

[0036] As an alternative or supplement to controlling the rate of change of the power reference of the generator, the measured value of the line-side power and / or the result of the rate of change can be low-pass filtered before determining the generator power reference. This can further reduce the transient change of the generator power reference.

[0037] Therefore, the present invention allows the generator power reference to deviate from the measured value of the power difference between the line-side power, because this difference can be the difference between the DC link voltage and the DC link voltage reference caused by this power difference, which can be compensated using DC link voltage control. Then, the DC link control compensates for any power imbalance between the machine-side and line-side converters, thus keeping the DC link voltage constant. This allows the feedforward of the power reference of the generator to be limited in terms of rate, bandwidth, and / or peak, so that only what is allowed by the mechanism is fed into the generator power. The difference caused by the limitation will be processed by the DC link control and "returned" to the power grid. The DC link control thus allows imperfect "reference tracking".

[0038] Therefore, the DC link voltage control can be configured to affect the power and / or voltage of the power (power) supplied to the power grid by the line-side converter. In this way, by adding a voltage component that changes the output current of the line-side converter, power can be fed back into the power grid during transient events, thereby reducing the negative transient impact on mechanical components.

[0039] According to an embodiment of the present invention, the derivation of the DC link voltage correction component includes converting the error between the measured DC link voltage value and the DC link reference value into a corresponding DC link error reference power, and adding the power injected into the power grid to the DC link error reference power. The current of the grid-following control cannot be determined individually, and only the total current injected into the power grid can be measured. The addition of the total power allows the measured value of the total current to be used in the control. The resulting total power can then be divided by the dominant grid voltage to form a reference current to be used in the grid-following control, and by subtracting the total current entering the power grid from the reference current, a control current representing the DC link voltage error is obtained, and the control current can be subjected to a PI controller to form a reference voltage component to be used in the generation of the voltage correction component.

[0040] In addition, the drivetrain components typically exhibit fundamental frequency oscillations. These oscillations typically have a very low frequency. The power output of the generator can be compensated by the drivetrain damping power to suppress the drivetrain fundamental frequency oscillations, where the drivetrain damping power is compensated by supplying the drivetrain damping power to the power grid. According to the present invention, this compensation can be performed by adding the drivetrain damping power to the power reference of the machine, which thus occurs as a power imbalance between the machine and the line-side converter. This imbalance is then handled by the DC link control according to the present invention, such that it is not necessary to use grid-forming control to perform this compensation.

[0041] In addition, the grid-forming control is typically limited in terms of the allowable control frequency. The grid-forming control should not respond to changes occurring in the power grid in a specific frequency range. This range can be, for example, 5 Hz to 1000 Hz, and thus the occurrence of any fault / event in such a frequency range will not be responded to. This further emphasizes the problem with respect to the transients of the drivetrain of the wind turbine generator during the transition, as well as the problem of not being able to correctly account for the transients propagated through the components of the wind turbine generator. The use of the DC link voltage control allows for a faster and more targeted control of the DC link voltage for other frequency components compared to the use in the grid-forming control of the output voltage of the line-side converter, which provides effective control and also helps to compensate for, for example, low-frequency components such as the drivetrain damping power.

[0042] The grid-forming control of the power injected into the power grid by the line-side converter can be configured to be controlled according to various grid-forming control algorithms. For example, the line-side converter can be controlled to simulate a synchronous machine and thus, for example, be controlled according to the swing equation as well described in the art. According to an embodiment of the present invention, the grid-forming control can alternatively be performed, for example, according to virtual oscillator grid-forming and / or moving average filter grid-forming or other grid-forming methods.

[0043] The advantages of the present invention are that since the grid-forming control scheme and the grid-following control scheme can be configured to operate in parallel to perform the control of the output voltage of the power injected into the grid, these two control loops also operate during the normal operation of the wind turbine generator. This has the following advantages: in the case where the control of the wind turbine generator is to be transferred (for example, determined by a higher-level power control, which can be configured to control the total power output by multiple power sources such as wind turbine generators and possibly other power sources), it can be decided to switch the control of the wind turbine generator to the grid-forming control or the grid-following control. In this case, the wind turbine operating according to an embodiment of the present invention will already have the grid-forming control loop and the grid-following control loop already in operation, and the desired operation can be selected by simply switching the control loop that is not intended to be utilized at this time. According to an embodiment of the present invention, therefore, a time-efficient switching of the operating mode is provided. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0045] Figure 1 A wind turbine is shown;

[0046] Figure 2A An example of a power system of a wind turbine or a power generation unit is shown, in which the generator is connected to the converter via the stator;

[0047] Figure 2B A control component arranged to control the generation of the active power and the reactive power supplied to the grid at the power output of the wind turbine or the power generation unit is shown;

[0048] Figure 3 An example of the combined grid-forming and DC-link control according to the present invention is shown;

[0049] Figure 4 Another example of the combined grid-forming and DC-link control according to the present invention is shown;

[0050] Figure 5 A method of limiting the change of the generator power reference is shown. DETAILED DESCRIPTION

[0051] Figure 1Shows a wind turbine 100 (WTG) which includes a tower 101 and an aerodynamic rotor 102 having at least one rotor blade 103, such as three blades. The aerodynamic rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and is adapted to drive a generator located inside the nacelle via a drivetrain. The aerodynamic rotor 102 can be rotated by the action of the wind. The rotational energy caused by the wind on the rotor blades 103 is transmitted via a shaft and typically via a gearbox as in the present case to the generator. The wind turbine 100 is thus 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 has a rotor and a stator, the stator being connected to a power converter which includes a machine-side converter and a line-side converter. The machine-side converter converts the generator AC power into DC power, and the line-side converter converts the DC power into AC power for injection into the power grid.

[0052] Figure 2A Shows in more detail an example of a power system 200 of a wind turbine (such as Figure 1 wind turbine 100). The power system 200 includes a generator or power source 201 which, according to the above, is connected to the aerodynamic rotor 102 of the wind turbine 100, where typically the drivetrain includes a gearbox (not shown) connecting the aerodynamic rotor to the generator. The power system 200 also includes a power converter 202. The power converter 202 includes a machine-side converter 203, a line-side converter 204 and a DC link 205 therebetween, where there is a DC link voltage Udc in use. The power converter 202 may also include a resistor 207 connected to a controllable switch 206. The resistor and the switch form a power dissipation device, also called a chopper 209, for dissipating active power when needed, which may be the case, for example, if the wind turbine is operating in island mode.

[0053] The DC link 205 includes one or more DC link capacitors which are charged by the DC output current from the machine-side converter 203 and supply DC power to the line-side converter 204. The output AC current from the line-side converter 204 can be supplied to the power grid or power line 220 via an output inductor 210 and possibly via a wind turbine transformer 208. In this example, the output AC current is a three-phase current output. In addition, harmonic filter capacitors 216 can be arranged between the conductors of the output, which together with the inductor 210 form a harmonic filter which converts the square wave voltage signal from the line-side converter 204 into a sinusoidal voltage signal.

[0054] The power line 220 can be a medium-voltage power bus that receives power from other wind turbines 100. The power line 220 can be connected to a high-voltage network via an additional transformer, for example. Thus, the power line 220 and one or more power systems 200 of the corresponding wind turbines constitute a wind power plant or park arranged to supply power to the public power grid for power distribution. The power line 220 and the high-voltage network are generally referred to as the power grid or grid in this document.

[0055] The power converter 202 can be a full-scale converter configured according to different principles, including forced-commutation and line-commutation converters.

[0056] The power system 200 is only schematically shown, and the system can be a three-phase system. However, the principles of the described embodiments apply to single-phase and polyphase systems.

[0057] The line-side converter 204 converts DC power into AC power using pulse-width modulation (PWM). 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.

[0058] Figure 2A It is shown that the grid voltage Ugrid can be measured, here the voltage at the low-voltage LV side of the transformer 208. The grid voltage Ugrid can be used 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 determined similarly from Ugrid and Igrid. Alternatively, the grid voltage Ugrid can be measured at the high-voltage HV side of the transformer and corrected based on the turns ratio of the transformer, or an internal voltage amplitude reference Vqref can be used instead of the measured voltage Ugrid. In an alternative, an internal voltage amplitude reference such as Vqref, Vdqref, or Vαβref can be used to determine Pgrid. The grid current Igrid supplied to the grid can also be measured.

[0059] Figure 2B An example of a control component 260 arranged to control the generation of the active power Pgrid and the reactive power Qgrid supplied to the grid at the power output 270 of the wind turbine 100 is shown. That is, the control component 260 can be arranged to control the output active power Pgrid and the output voltage amplitude at the low-voltage side LV, alternatively to control the output active power Pgrid and the output reactive power Qgrid at the low-voltage side LV. Control components 260 such as the frame conversion unit 266 and the pulse-width modulator 265 can form part of the control system 250 or receive control signals from the control system 250.

[0060] References for active power and reactive power can be received from a power plant controller PPC or a grid operator, or determined from active and reactive power references (e.g., from a grid operator). The system shown can be used for grid-forming control, e.g., based on the virtual synchronous machine angle θVSM for active power control, and where the voltage magnitude is provided by reactive power control. The voltage reference is a combination of voltage magnitude and voltage angle.

[0061] As described above, the power converter can be controlled according to different control strategies, where historically the grid-following method has been utilized. As also mentioned, the advantage of the grid-following control scheme is that since the line-side controller controls the voltage according to the dominant voltage on the grid, the line-side converter will instantaneously react to changes occurring on the grid and adjust, for example, the current such that the amount of power injected into the grid still corresponds to the amount of power supplied to the DC link by the machine-side controller.

[0062] However, as also discussed, there may be requirements from, for example, a grid operator that a wind turbine generator participates in the formation of the grid and helps maintain the stability of the grid when grid events affecting stability occur. When the wind turbine generator operates according to a grid-forming control scheme rather than a large following control speed, the line-side converter is operated to follow a fixed voltage reference regardless of any negative stability-affecting events occurring on the grid.

[0063] This can be achieved by using a grid-forming algorithm, where such grid-forming algorithms can be of different kinds. For example, the virtual synchronous machine angle θVSM as schematically indicated in Figure 2B can be used to control the active power Pgrid. Briefly, the synchronous machine angle acceleration (the second time derivative of θVSM) corresponds to the difference between the power reference Pref for the desired power output of the wind turbine and the grid power Pgrid that the wind turbine actually supplies to the power grid.

[0064] The synchronous machine angle θVSM can be determined according to a grid-forming converter scheme such as a virtual synchronous machine control scheme. The grid-forming converter scheme models the inherent rotational mass inertia of a conventional synchronous generator. By modeling the inertia, the converter can provide improved grid stability by counteracting changes in the grid frequency through the grid-forming converter model. That is, an increase in the grid frequency leads to an increase in the kinetic energy and rotational frequency of the inertia, but the response time is determined by the inertia. Conversely, a decrease in the grid frequency leads to a decrease in the kinetic energy and frequency of the inertia, but the response time is determined by the inertia. In a wind turbine, an increase or decrease in the kinetic energy of the modeled synchronous generator results in an increase or decrease in the kinetic energy of the aerodynamic rotor 102.

[0065] The synchronous machine angle θVSM can be used to convert signals from a rotating DQ frame to a non-rotating frame, such as an αβ or abc frame, and vice versa. Based on the synchronous machine angle θVSM and the voltage magnitude reference Vqref, control signals for the desired active and reactive power are determined.

[0066] The synchronous machine angle θVSM can be defined in a rotating DQ frame defined by the angular position θVSM and rotating at a frequency ωVSM. Based on the synchronous machine angle θVSM, a control signal, i.e., the angle of the modulation voltage signal for the pulse width modulator PWM 265, is determined and converted to a non-rotating frame, such as an αβ or abc frame. The modulation voltage reference signal controls the active power Pgrid and the reactive power Qgrid.

[0067] The frame conversion unit 266 converts the control signal from the DQ frame to an αβ or abc frame and determines the sinusoidal voltage reference for the PWM 265. The frame conversion output signal from the frame conversion unit 266 is converted by the pulse width modulator PWM 265 into a modulation signal for the grid-side converter 204 in order to generate the desired active and reactive power and / or voltage magnitude.

[0068] The voltage magnitude reference Vqref is provided as a reference for the desired grid voltage or the desired reactive power Qgrid to be generated by the converter 204. The voltage magnitude reference Vqref can be determined based on the difference between the reactive power reference Qref and the actual reactive power Qgrid delivered to the grid. Thus, the reactive power Qgrid to be generated by the line-side converter 204 can be controlled based on the voltage magnitude reference Vqref. The voltage reference can also be part of grid formation.

[0069] The voltage magnitude reference Vqref can be defined in a DQ frame that rotates at the rotational speed ωVSM of the virtual synchronous machine, which can be equal to the fundamental frequency of the AC grid voltage, such as 50 Hz, under steady-state conditions. The voltage magnitude reference Vqref or its modification as described below can be converted from the DQ frame to an αβ or abc frame and output from the frame conversion unit 266 as a control signal to the pulse width modulator PWM 265, which determines the modulation signal for the grid-side converter 204. Regarding the DQ frame, it should be noted that in this specification, generator symbols such as Id, Iq, Ud, Uq, etc. are used, which are different from the general machine symbols for active and reactive current and voltage.

[0070] As described above, in a conventional synchronous machine, the inherent inertia can be used for grid stabilization purposes. In a wind turbine, an increase or decrease in the kinetic energy of the modeled synchronous generator results in an increase or decrease in the kinetic energy of the aerodynamic rotor 102. If these changes are transient, there will be corresponding transient changes in the requirements for the kinetic energy of the aerodynamic rotor 102 and thus also for other mechanical components.

[0071] According to the grid management code of a wind turbine power plant, the power converter of the wind turbine may need to operate as a virtual synchronous machine, at least for grid current Igrid below a given overcurrent threshold. If the overcurrent threshold is high, grid disturbances such as phase jumps may cause high power peaks or torque peaks in the generator side and the drivetrain, and thus an undesired increase in mechanical loads.

[0072] For example, as described above, the rotor carrying the airfoils, the generator, and the gearbox interconnecting these components may be highly sensitive to torque peak heights. Therefore, high torque peaks in the drivetrain are highly undesirable as they may provide excessive wear and reduce the expected lifespan of the components therein. Therefore, when it comes to controlling a wind turbine generator according to a grid-forming control scheme, this may pose a challenge.

[0073] According to the present invention, a method for alleviating such problems is provided, in which the grid-forming control scheme is still utilized, but the grid-forming control scheme is used in parallel to mitigate the effects of possible transients.

[0074] Figure 3 The general principle according to the present invention is illustrated. In the figure, the line-side converter and the associated control of the line-side converter are schematically shown by block 310 (also denoted as "system"). This block also represents the measurement of the DC link voltage Udc, which can be measured, for example, on the input side of the line-side converter. The system block 310 can also represent the power converter, the generator, the grid, etc. The system block 310 is also responsible for determining the measured values of the active power PL entering the grid and the reactive power QL injected into the grid. These measured values can be determined, for example, based on the grid voltage Ugrid and the grid current Igrid, which can be measured according to the above or according to alternative voltage measurement results also as described above. The active current and the reactive current can also be established based on these measured values.

[0075] During constant operating conditions, the power PL injected into the grid by the line-side converter will be substantially the same as the power PMSC provided by the machine-side converter. However, the power PL injected into the grid may need to be compensated for, for example, the power drawn by auxiliary devices of the wind power converter and / or losses and / or other power. According to this example, there is thus a generator active power controller GAPC 320, which takes as input the power reference Pref_VMP representing the desired power output of the wind turbine and the power PL injected into the grid, and outputs a machine-side active power reference PMSC_ref. Thus, the actual power to be generated by the generator can be set to the desired power output compensating for losses, etc., such that the actually injected power PL corresponds to the power reference Pref_VMP. Thus, the machine-side active power reference PMSC_ref is used to control the generator using the generator power control GPC 330 and the machine-side converter to obtain the desired power on the DC link.

[0076] As stated, the active Pref_VMP and the reactive power QLref_VMP references can be received from the power plant controller PPC or the grid operator, or they can be determined from the active and reactive power references, for example, from the grid operator. The power reference can reflect the power extracted from the wind and can thus change, for example, according to the power currently being produced by the wind turbine generator. Thus, the power reference Pref_VMP can reflect, for example, the power variations caused by the changes in the wind. In this way, the power balance on the drivetrain is also obtained. As an alternative to the turbine reference power reference GAPC, PL or Pref_VMP can be selected as the reference in the GAPC, and the combination can also be used as the power reference.

[0077] As described above, the line-side converter controls the output voltage based on a voltage input, such as the voltage Vαβ. According to this example, this voltage input Vαβ consists of two voltage components, which will be explained below.

[0078] The grid-forming controller GFC 340 uses the power PMSC supplied to the DC link by the machine-side converter to determine the output voltage VGFC,αβ to be output by the line-side converter in order to obtain a desired power output. For example, the grid-forming control using the virtual synchronous machine angle can control the active power injected into the grid. It should be noted that the output voltage components Vαβ,GFC can be generated according to any suitable grid-forming control scheme and are thus not limited to controlling the line-side converter according to a virtual synchronous machine. For example, virtual oscillator grid-forming and / or moving average filter grid-forming can be used as an alternative to controlling the line-side converter as a virtual synchronous machine. The grid-forming control GFC 340 performs the required calculations based on the power PLref that includes the power output by the machine-side converter and an additional component PDCIref described below. In addition, the reactive power reference QLref is also used.

[0079] Therefore, the control of GFC 340 according to Figure 3 is described for controlling the line-side converter according to a voltage reference. However, as is generally the case, using this control alone exhibits the disadvantages described above because the generator is no longer decoupled from the grid from a transient viewpoint in the same way as when controlled according to a grid-following control scheme, since the rest of the system must adapt to the control of the line-side converter. The generator power control GPC will adapt to the power PL currently input into the grid by the line-side controller.

[0080] During normal operation, there will be a balance between the power output by the line-side converter and the power generated by the motor. The power reference Pref_VMP from the turbine is adhered to because this reference provides information about the amount of power that can be injected into the grid according to the current power extracted by the wind. However, if a transient occurs in the grid, the grid-forming control GFC requires maintaining the grid voltage, and this will result in transients in the current injected into the grid due to maintaining the voltage reference.

[0081] Therefore, there will also be transient changes in the power PL injected into the grid by the line-side converter, and thus there will be transient changes in the torque request from the generator because the change in the current output by the line-side converter will be directly reflected by the change in the torque request of the generator. The DC link is generally very limited in terms of energy storage and thus cannot account for sudden current changes. This means that the power provided by the generator must be delivered to the grid instantaneously (immediately) so that the DC link voltage can be maintained at the desired level. The DC link voltage must be maintained constant to keep the converter operating, and thus this can only be ensured by maintaining the energy balance between the generator and the power injected into the grid. Therefore, transients will occur in the generator power / torque.

[0082] According to Figure 3The solution is to mitigate this problem by using a separate DC-link voltage control, which is implemented in the form of grid-following control. Thus, grid-forming control is combined with grid-following control, where the purpose of the grid-following control is to maintain the DC-link voltage at a desired voltage level. The DC-link voltage control DCC 350 takes the currently dominant DC-link voltage Udc (squared) as input, where this voltage can be established as described above. In addition, the DC-link voltage control DCC 350 also takes the DC-link voltage reference Udc_ref (squared) as input. This DC-link voltage control can be implemented in various ways and, for example, includes an integral controller (I-controller) and / or a proportional controller (P-controller) and / or a proportional-integral controller (PI-controller). According to this example, DCC 350 includes a PI controller, where the proportional part determines a power value PDCPref proportional to the DC-link voltage difference, and the integral part determines a power value PDCIref proportional to the time integral of the DC-link voltage difference. The integral part is the slowly varying part, which is added to the power PMSC injected into the DC-link by the machine-side converter. The slowly varying part PDCIref provides DC-link voltage stability during steady-state operation by considering the slow variations of the DC-link voltage caused by variations and power differences in the normal operation of the wind turbine. The active power reference PLref input to the grid-forming control is the sum of the power PMSC and PDCIref.

[0083] The grid-following control processes the transient variations of the DC-link voltage based on the proportional part from the DCC. As described above, these transient variations may cause undesired stress and wear on the drivetrain components. The proportional part determines a power value PDCPref proportional to the DC-link voltage difference. This power value is input to the grid-following control 360. In principle, the grid-following control 360 calculates a voltage correction component Vαβ,GFL based on the power value PDCPref and then combines it with the voltage component Vαβ,GFC calculated by the grid-forming control to form the total output voltage Vαβ to be output by the line-side converter. The voltage correction component Vαβ,GFL controls the DC-link voltage towards the power reference and thus cancels the voltage variations that the DC-link would otherwise experience.

[0084] Therefore, when, for example, a transient grid event occurs, according to this example, in a situation where the generator is usually forced to operate outside its operating limits, a difference between the DC-link voltage and the DC-link voltage reference will occur. However, since the transient does not need to be immediately propagated to the generator, these limits can now be adhered to. As described below, limits can also be imposed on the power reference PMSC_ref to further reduce the risk of potentially harmful transients in the mechanical components.

[0085] The difference that appears in the DC link voltage due to the change in the power \(P_{L}\) injected into the grid and that is not immediately accounted for by the corresponding change in the power generated by the generator is handled by the grid-following control, which is used to return the DC link voltage level \(U_{dc}\) towards the DC link voltage reference. In principle, the grid-following control feeds power back to the grid instead of requesting the generator to fully account for the rapid changes in the line-side power.

[0086] Therefore, the present invention is a slight modification to the general requirement that the line-side converter in grid-forming mode always maintains the grid voltage reference, because the component added by the grid-following algorithm will change the total voltage output by the line-side converter. However, conversely, this will reduce the stress that the generator and other drivetrain components may experience during transient events on the grid. At the same time, it is possible to avoid the converter tripping completely and becoming unavailable during the process of maintaining grid stability, and it is also possible to avoid supplying power to the grid until the operation can be reset. Therefore, a means is provided to increase the use of the converter grid for stable operation in addition to reducing harmful transients.

[0087] Typically, compared to the total power output on the grid, the power added by the grid-following algorithm and thus the impact on the total output voltage may be small, and therefore the impact on maintaining grid stability may also be small. However, in some cases, the initial power controlled by the grid-following algorithm during a grid event may be large and even exceed the power controlled by the grid-forming algorithm at that time, but this will only be the case as long as the machine-side control does not have time to adapt to the new power requirements in view of application limitations regarding the rate of change, etc. After this initial situation, the control of the output voltage will increasingly return to being controlled according to the grid-forming algorithm. As can be understood, this control can be fast, and after the initial transient, the grid-following algorithm will again only control small changes in the DC link voltage.

[0088] Figure 3 The reactive power control GPRC for reactive power control is also shown. The reactive power control loop needs to ensure that reactive power is not generated unnecessarily. In addition, both active power and reactive power are required to form the total output power, and therefore reactive power needs to be generated, for example, in order to control the virtual machine angle.

[0089] The reactive power control GPRC 370 takes the reactive power reference Qref_VMP as an input, where the reactive power reference Qref_VMP represents the desired reactive power output of the wind turbine, which can be determined in a similar manner to the active power reference, and the reactive power QL is injected into the grid. The reactive power control GPRC 370 outputs a general reactive power reference QL_ref, which is provided to the grid forming control 340 and forms part of the output voltage generated by the grid forming control. The reactive power control GPRC also outputs a reactive power reference Qref, which is input to the grid following control and thus is controlled in a similar manner to the active power grid following control, and this reactive power reference Qref forms part of the output voltage component obtained from the grid following control.

[0090] Figure 4 Another embodiment according to an embodiment of the present invention is shown. The GAPC 420 and GPC 430 are similar to Figure 3 and thus will not be discussed further. Regarding the DC link control DCC 450, the determination 451 of the DC link voltage error signal Udcerr (squared) is shown, as well as an I controller 452 for generating the integral part PDCIref of the error signal and also a P controller 453 for generating the proportional part PDCPref of the error signal. These signals are used as described above with reference to Figure 3 the use of these signals.

[0091] As described above, the proportional power component PDCPref output by the DC link control DCC 450 is provided to the grid following control (schematically indicated by 4 and 60), where the power component PDCPref is first added to the power PL injected into the grid. The reason is that only the total currents Id and Iq can be measured, i.e., the combination of the outputs from the grid forming control and the grid following control. It is not possible to separately measure the currents specifically related to the grid forming control and the grid following control. The total power obtained from this addition (forming the grid following reference power Pgfl_ref) is then divided by the voltage U to form the grid following reference current Igfl_ref. Subsequently, the current Iq is subtracted from the grid following reference current Igfl_ref. As a result, this will cause the grid following GFL control component to follow the power reference Pgfl_ref, which thus represents the power required to correct the DC link voltage. The resulting current is subjected to a PI controller for generating a voltage component, which is then utilized by the voltage generator 461 to generate the output voltage Vαβ,GFL, which will be combined with the output Vαβ,GFC from the grid forming control and compensate for the current to be output by the line side converter such that the DC link voltage can be maintained. It should be noted that this PI controller can alternatively be a P controller.

[0092] The grid-following control 460 also shows a similar generation of the reactive power voltage component, which operates in the same way and also forms part of the grid-following control output voltage generated in the voltage generator 461.

[0093] In addition to showing in more detail an example of grid-following control, Figure 4 an example of a grid-forming control algorithm 440 for determining the synchronous machine angle θGFC of a virtual synchronous generator is also shown.

[0094] The synchronous machine angle θGFC is determined based on a virtual synchronous machine control concept that aims to generate a power response corresponding to the power response from a real synchronous generator (including the inertia of the synchronous generator).

[0095] The power error Perr is determined as the difference between PLref as defined above and the power PL injected into the grid and the damping power PD determined according to the virtual synchronous model.

[0096] In response to a change in the grid power PL, for example due to a decrease in the grid voltage Ugrid and a corresponding increase in the grid current Igrid, the power error Perr becomes non-zero, which causes the angle θVSM to increase or decrease to reduce the power error Perr. Thus, in response to fluctuations in, for example, the grid power Pgrid, the combined inertia response value becomes non-zero, which causes the virtual machine to accelerate or decelerate to reach a new equilibrium condition. When PL follows PLref again, a new equilibrium is reached.

[0097] The virtual synchronous model includes a closed loop in which the virtual synchronous machine rotational speed ωGFC from the grid-forming control is determined based on a combination of the feedback of the damping power PD and the power reference PLref of the desired active power output of the wind turbine and the active grid power PL supplied by the wind turbine to the grid.

[0098] According to the example shown, the inertia integration model is implemented as 1 / (2Hs), where H is the inertia time constant and 1 / s is the integration in the s-domain, and Perr is used as the input to the inertia integration model.

[0099] The damping power PD is determined as the difference between the rotational speed ωg of the grid and the synchronous machine rotational speed ωGFC multiplied by the damping factor Dp. The damping factor Dp suppresses the performance of the control loop of the grid-forming control.

[0100] The synchronous machine angle θGFC is determined based on the synchronous machine rotational speed ωGFC according to the integration of ω0 / s, where ω0 is the rated synchronous generator speed.

[0101] Figure 4Also shown is a decoupling virtual impedance 470, which may or may not be used, and is mainly used in the presence of a strong power grid. In this case, small changes in the synchronous machine angle θGFC may result in high power differences. The virtual impedance 470 slightly changes the voltage to provide decoupling between the active and reactive power loops. This improves system stability. The figure also shows a current limiter 480, which can be used to ensure that the current does not exceed a set limit by keeping the current below such a limit. During normal operation, such a limit is usually not required. The virtual impedance can be determined for one or more phases. By increasing the resistance and / or reactance value of the virtual impedance, the output current drawn from the output of the line-side inverter can be reduced. The virtual impedance can be used to reduce the acceleration of the virtual synchronous generator during an overcurrent situation, thereby reducing the output current Igrid of the line-side inverter. Figure 4 Also shown is a reactive power control 490 for grid-forming control, which provides the voltage amplitude in a manner known per se, while the active power control provides the voltage angle. Thus, the active power control and the reactive power control together form the voltage amplitude and angle output by the grid-forming control. The reactive control will not be discussed in detail, as the present invention relates to the active power control Uref in the reactive power control, which can be locally set or received from an external source.

[0102] Furthermore, as described above, the present invention allows the use of limitations on, for example, the rate of change of power / torque generated by a generator, which can be taken into account when determining the power reference used by the machine-side controller.

[0103] According to an embodiment of the present invention, the machine-side power reference can alternatively be set after applying limitations on, for example, the rate of change of power / torque generated by a generator, such that this can be taken into account when determining the power reference that forms the basis of the machine-side controller. Thus, according to an embodiment of the present invention, it can be further ensured that the generator and the drivetrain do not undergo harmful transients, or at least the transients are reduced by this control, but where the grid-following control according to the present invention still handles the difference in the DC link voltage that this may cause.

[0104] This is schematically shown in Figure 5 where the measured value of the power PL supplied by the line-side converter to the power grid is processed before using it as the generator power reference PMSC_ref. According to the example shown, the measured value of the line-side power is subject to a rate-of-change limitation 510 such that transient changes in the power PL will not be immediately reflected in the generator power reference. A low-pass filter 520 can also be applied to the measured value before the generator power reference PMSC_ref is forwarded to the generator power control GPC. This results in a power difference, but as explained, this is handled by the grid-following control according to the present invention.

[0105] In addition, with respect to the powers shown, namely the machine-side power and the line-side power, as is known to those skilled in the art, these powers may include power components that do not form part of the available energy injected into the power grid. Such power components may include, for example, power losses, powertrain damping power, power consumed by auxiliary components, and the like.

[0106] For example, the powertrain damping power may be AC power, where, for example, a frequency of approximately 1 to 3 Hz may be utilized to attempt to suppress low-frequency oscillations inherent in the powertrain during use, where such oscillations may be, for example, the result of the powertrain resonance frequency. When determining, for example, a power reference level in a conventional power grid forming control, such power components may be included in the calculation.

[0107] Part of the function of the generator control is to extract the desired average power and also to suppress the powertrain to account for low-frequency oscillations caused by the resonance frequency of the powertrain.

[0108] According to the present invention, this can be compensated for as a component of the reference power provided to the DC link for injection into the power grid. That is, the powertrain damping power can be added to the power reference of the generator, and this will thereby cause an imbalance controlled by the DC link control. Other powers having a frequency different from the main power grid frequency can also be added to this control in a similar manner.

[0109] Another advantage of the present invention is that, with respect to power grid formation, there may be frequency intervals in which no control is performed, and this also means that it may cause difficulties in terms of propagation, such as using a very slow control that is still available up to, for example, a maximum of 5 Hz, and the power grid forming control (so it will also be used for very slow variations of the powertrain damping power, for example, at 1 to 2 Hz) to propagate the powertrain damping power into the power grid. Therefore, it may be difficult to fully account for this in the power grid forming control algorithm. The DC link voltage control can be configured to control the DC link voltage at a control frequency higher than the control frequency utilized or even allowed when controlling the output voltage of the line-side converter. This helps to control, for example, low-frequency power components.

Claims

1. A method for controlling a wind power installation to limit motor transients, the wind power installation comprising an aerodynamic rotor, an electric machine having a stator and a rotor driven by the rotor, a power converter comprising a machine-side converter connected to the stator and a line-side converter configured to supply power to the grid, and a DC link electrically connected to the output of the machine-side converter and the input of the line-side converter, the method comprising: Determining a grid voltage reference for controlling the line-side converter; Controlling the power (PLsc) supplied by the line-side converter to the grid by controlling the line-side converter using a grid-forming controller configured to control an output voltage towards the grid voltage reference; Using a power reference (PMsc) for machine power control; Controlling the machine-side converter and the electric machine according to the power reference (PMsc); Utilizing DC link voltage control to derive an error signal representing an error between a measured DC link voltage value and a DC link reference value, the error being caused by a power imbalance between the line-side converter power and the machine-side converter power; Deriving a DC link voltage correction component in response to the error signal; Adding the DC link voltage correction component to the output voltage of the grid-forming control; And Operating the line-side converter according to a combination of the output voltage from the grid-forming controller and the DC link voltage correction component.

2. The method according to claim 1, further comprising: Deriving a DC link voltage correction component for maintaining the DC link voltage in parallel with a grid-forming controller that controls the output voltage towards the grid voltage reference.

3. The method according to claim 1 or 2, wherein, Using grid-following control to derive the DC link voltage correction component, the line-side converter being controlled by the grid-forming control and the grid-following control operating in parallel.

4. The method according to any one of claims 1 - 3, the method further comprising: Using a measured value of the power supplied by the line-side converter to the grid as a power reference, Processing the measured value of the power supplied by the line-side converter to the grid, and Using the processed measured value of the power supplied by the line-side converter to the grid as a motor power reference.

5. The method according to claim 4, wherein, The processing of the measured value of the line-side power includes one or more of the following: When there is a transient change in the power supplied by the line-side converter to the grid, subjecting the measured value of the power supplied by the line-side converter to the grid to a rate-of-change limit; Low-pass filtering the measured value of the line-side power and / or the result of the rate of change before determining the motor power reference.

6. The method according to claim 4 or 5, wherein When the motor power reference deviates from the measured value of the line-side power: Using the DC link voltage correction component to compensate for a difference in the DC link voltage relative to the DC link voltage reference caused by the power difference.

7. The method according to any one of claims 1 - 6, wherein: The DC link voltage correction component is configured to affect the power and / or voltage of the power supplied by the line-side converter to the grid.

8. The method according to any one of claims 1-7, wherein The derivation of the DC link voltage correction component includes: converting the error between the measured DC link voltage value and the DC link reference value into a corresponding DC link error reference power (PDCPref), adding the power (PL) injected into the grid to the DC link error reference power (PDCPref), dividing the resulting power by the dominant grid voltage to form a reference current (Pgfl_ref), subtracting the total current entering the grid from the reference current, subjecting the result to a PI controller to form a reference voltage component, and generating the DC link voltage correction component; 9. The method according to any one of claims 1 - 8, further comprising: compensating for the power output by the motor by means of a drivetrain damping power to suppress drivetrain fundamental frequency oscillations, the drivetrain damping power being compensated by supplying the drivetrain damping power to the grid, wherein the drivetrain damping power is added to the power reference (PMsc).

10. The method according to any one of claims 1 - 9, further comprising: using a higher frequency bandwidth than that utilized in the control of the output voltage of the line - side converter to control the DC link voltage.

11. The method according to any one of claims 1 - 10, wherein: the grid - forming controller is configured to control the output voltage towards the grid voltage reference by determining a virtual synchronous machine angle.

12. A computer program comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of claims 1 - 11.

13. A computer - readable medium comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of claims 1 - 11.

14. A wind power plant control system arranged to limit motor transients, the wind power plant comprising an aerodynamic rotor, a motor having a stator and a rotor driven by the aerodynamic rotor, a machine - side converter connected to the stator, a line - side converter configured to supply power to the grid, and a DC link electrically connected to the output of the machine - side converter and the input of the line - side converter, the wind power plant control system being configured to perform the method according to any one of claims 1 - 11.

15. A wind power plant comprising the control system according to claim 14.

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