Method for limiting current for a grid forming wind turbine

Abstract

This invention relates to a method for controlling a wind power generation device to limit its current, comprising a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a line-side converter configured to supply current 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 line-side converter having an output voltage and an output current, the method comprising: determining a grid voltage reference for controlling the line-side converter, comprising a sum of first and second output voltages; managing slow dynamics by means of a grid forming controller configured to control the first output voltage to tend toward the grid voltage reference, and managing fast dynamics by means of a grid follower controller configured to control the second output voltage to tend toward the grid voltage reference; triggering a current limiter mode when the output current reaches a threshold current value; operating the grid follower controller by means of a grid current controller (GCC), the grid current controller utilizing: a proportional (P) controller when the output current is below the threshold current value and a proportional-integral (PI) controller when the output current reaches the threshold current value, maintaining the first output voltage of the grid forming controller at the output voltage level prior to triggering the current limiter during operation in the current limiter mode, and operating the line-side converter according to a combination of the output voltages from the grid forming controller and the grid follower controller.

Description

Technical Field

[0001] This invention relates to the control of power generation in a wind turbine, and more particularly to limiting the current from the converter of the wind turbine. Background Technology

[0002] To allow for a higher proportion of renewable energy sources, such as wind turbines, into the grid, requirements have been put forward to equip wind turbine power converters with grid-forming (or grid-building) characteristics similar to those of conventional synchronous generators. These requirements can be addressed, for example, by configuring renewable generation units as virtual synchronous machines (VSMs).

[0003] When a wind turbine is configured to operate as a VSM or according to other grid formation control schemes, grid disturbances (such as phase jumps) may cause the inverter controller to request peak currents that exceed the current that the system can provide.

[0004] Therefore, one problem is that wind turbines configured to operate according to grid formation control schemes have limited overcurrent capacity when responding to grid disturbances. Summary of the Invention

[0005] The object of this invention is to improve the control of a wind turbine, which includes a power converter configured to be controlled according to a grid formation control scheme to mitigate overload current problems.

[0006] According to a first aspect of the invention, a method is provided for controlling a wind power generation device to limit the current of the wind power generation device, the wind power generation device including a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a line-side converter configured to supply current to a power grid, and a DC link electrically connected to the output of the machine-side converter and the input of the line-side converter, the line-side converter having an output voltage and an output current, the method comprising: Determine the grid voltage reference used to control the line-side converter; The grid voltage reference includes the sum of the first output voltage and the second output voltage; By utilizing a grid forming controller to manage slow dynamics, the grid forming controller is configured to control the first output voltage to tend towards the grid voltage reference, and With the help of a grid follower controller to manage fast dynamics, the grid follower controller is configured to control the second output voltage to tend toward the grid voltage reference; When the output current reaches the threshold current value, the current limiter mode is triggered. The grid follower controller is operated by a grid current controller (GCC), which utilizes: A proportional (P) controller when the output current is below the threshold current value, and The proportional-integral (PI) controller operates when the output current reaches the threshold current value. During operation in current limiter mode, the first output voltage of the grid forming controller is maintained at the output voltage level before the current limiter is triggered. The line-side converter is operated based on a combination of output voltages from the grid forming controller and the grid following controller.

[0007] According to an embodiment of the present invention, the method further includes: When the output current is lower than the threshold current value, the output of the proportional-integral (PI) controller is gradually reduced until the output of the proportional-integral (PI) controller drops below the voltage threshold.

[0008] According to an embodiment of the present invention, the method further includes: Maintain current limiter mode until the voltage threshold is reached.

[0009] According to an embodiment of the present invention, the method further includes: After the current level drops below the threshold current value, the current limiter mode will be maintained for a predetermined period of time.

[0010] According to an embodiment of the present invention, the method further includes: Entering Fault Ride-through (FRT) mode, and in response to FRT mode, injecting a predetermined amount of reactive current into the grid.

[0011] According to an embodiment of the present invention, the method further includes: The amount of power supplied to the grid by the line-side converter is used as a power reference, the amount of power supplied to the grid by the line-side converter is processed, and the processed amount of power supplied to the grid by the line-side converter is used as a motor power reference.

[0012] According to an embodiment of the present invention, the processing of the magnitude of the line-side power includes one or more of the following: when the power supplied to the grid by the line-side converter undergoes a transient change, a rate of change limit is applied to the magnitude of the power supplied to the grid by the line-side converter; and the magnitude of the line-side power and / or the result of the rate of change are low-pass filtered before determining the motor power reference.

[0013] According to an embodiment of the present invention, when the motor power reference deviates from the value of the line-side power: the difference between the DC link voltage and the DC link voltage reference caused by the power difference is compensated by using the DC link voltage correction component.

[0014] According to an embodiment of the present invention, slow dynamic refers to a limited bandwidth of less than 5 Hz.

[0015] According to an embodiment of the present invention, fast dynamics refers to a frequency higher than 5 Hz.

[0016] According to an embodiment of the present invention, the grid forming controller is configured to control the output voltage to tend toward the grid voltage reference by determining the virtual synchro angle.

[0017] According to a second aspect of the present invention, a computer program is provided, comprising instructions that, when executed by a computer, cause the computer to perform the method described according to an embodiment of the present invention.

[0018] According to a third aspect of the invention, a wind power generation equipment control system is provided, arranged for limiting the current of a wind power generation equipment, the wind power generation equipment including a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a line-side converter configured to supply current 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 line-side converter having an output voltage and an output current, the wind power generation equipment control system being configured to perform the method according to an embodiment of the invention.

[0019] According to a fourth aspect of the present invention, a wind power generation device including the control system described in the third aspect is provided. Attached Figure Description

[0020] Embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein: Figure 1 A wind turbine is shown; Figure 2A An example of an electrical system for a wind turbine or power generation unit is shown; Figure 2B A control assembly is shown, which is arranged to control the generation of active and reactive power supplied to the grid at the power output of the wind turbine or power generation unit; Figure 3 An example of combined grid formation and DC link control is shown; Figure 4 An example of combined grid formation and grid following is shown; Figure 5 An example of combined grid formation and grid following with freezing functionality is shown; Figure 6 An example of a control system is shown; Figure 6 a shows an enlarged diagram of the current limiting function; Figure 7A Simulations of FRT and current limiting events are shown; Figure 7BSimulations of FRT and current limiting events are shown; Figure 8 A method for limiting generator power reference variations is shown.

[0021] Figure 9 Simulations of FRT events and DC link voltages are shown. Detailed Implementation

[0022] The voltage source behavior of grid-forming wind turbines makes their output current highly dependent on external system conditions. When large disturbances such as voltage dips or phase jumps occur at the grid connection point, conventional synchronous generators can typically provide 5-7 p.u. of overcurrent.

[0023] However, semiconductor-based converters used in wind turbines typically can only withstand overcurrents of 1.2–2 pu, limiting their ability to maintain voltage distribution during normal operation. Therefore, implementing appropriate current-limiting control methods for grid-forming wind turbines becomes crucial to effectively address and withstand these disturbances.

[0024] Figure 1 A wind turbine 100 (WTG) is shown, comprising a tower 101 and an aerodynamic rotor 102 with at least one rotor blade 103 (e.g., three blades). The aerodynamic rotor is connected to a nacelle 104, which is mounted on top of the tower 101 and adapted to drive a generator disposed within the nacelle via a transmission system. The aerodynamic rotor 102 is responsive to wind. The rotational energy of the rotor blades 103 caused by the wind is transmitted through a shaft and, in many cases (as in this example), through a gearbox to the generator. The generator has a rotor and a stator. Thus, the wind turbine 100 is capable of converting the kinetic energy of the wind into mechanical energy through the rotor blades, and subsequently into electrical energy through the generator. The generator is connected to a power converter via the stator. The power converter includes a generator-side converter and a line-side converter. The generator-side converter converts AC power from the generator to DC power, and the line-side converter converts DC power to AC power for injection into the power grid.

[0025] Figure 2A A more detailed illustration of the wind turbine (e.g.) Figure 1An example of an electrical system 200 for a wind turbine 100. The electrical system 200 includes a generator or power source 201, as described above, connected to the rotor 102 of the wind turbine 100, wherein the drive system typically includes a gearbox (not shown) connecting the rotor to the generator. The electrical 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 located between them, on which a DC link voltage Udc is present during use. The power converter 202 may also include a capacitor 207 connected to a controllable switch 206. Resistors and switches form a power dissipation device, also known as a chopper 209, for dissipating active power when needed, such as if the wind turbine is operating in islanded mode.

[0026] DC link 205 includes one or more DC link capacitors that are charged by the DC output current from machine-side converter 203 and provide DC power to line-side converter 204. The output AC power current from line-side converter 204 can be supplied to the grid or power line 220 via output inductor 210 and possibly via wind turbine transformer 208. In this example, the output AC power current is a three-phase current output. Furthermore, harmonic filter capacitor 216 can be arranged between the output leads, forming a harmonic filter together with inductor 210 to convert the square wave voltage signal from line-side converter 204 into a sinusoidal voltage signal.

[0027] Power line 220 may be a medium-voltage power bus that receives power from other wind turbines 100. Power line 220 may be connected to a high-voltage network, for example, via other transformers. Thus, power line 220 and the power systems 200 of one or more corresponding wind turbines constitute a wind power plant or wind farm, which is arranged to supply power to a public power grid for distribution. Power line 220 and the high-voltage network are generally referred to herein as the power grid.

[0028] The power converter 202 can be a full-scale converter, which is configured according to different principles, including forced commutation and grid commutation converters.

[0029] The power system 200 is shown for illustrative purposes only, and the system can be a three-phase system. However, the principles of the described implementation are applicable to single-phase and multi-phase systems.

[0030] The line-side converter 204 uses pulse width modulation (PWM) to convert DC power into AC power. The control system 250 is used to control the modulation of the line-side converter 204 and to control the active power P and reactive power Q generated by the line-side converter 204.

[0031] Figure 2AThe diagram illustrates the measurable grid voltage Ugrid, here the voltage at the low-voltage side LV of transformer 208. The grid voltage Ugrid can be used to control the converter's power output, based on the active power Pgrid determined from the grid voltage Ugrid and grid current Igrid. Reactive power Qgrid can be determined similarly from Ugrid and Igrid. Alternatively, the grid voltage Ugrid can be measured at the high-voltage side HV of the transformer and corrected according to the transformer's turns ratio, 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.

[0032] Figure 2B An example of a control component 260 is shown, arranged to control the generation of active power Pgrid and reactive power Qgrid supplied to the power grid at the power output 270 of the wind turbine 100. That is, the control component 260 may be arranged to control the output active power Pgrid and output voltage amplitude at the low-voltage side LV, or to control the output active power Pgrid and output reactive power Qgrid at the low-voltage side LV. The control component 260 (such as coordinate system transformation unit 266 and pulse width modulator 265) may form part of or receive control signals from the control system 250.

[0033] Active and reactive power references can be received from the plant controller (PPC) or the grid operator, or determined based on active and reactive power references (e.g., from the grid operator). The system shown can be used for grid forming control, for example, based on a virtual synchro angle θVSM for active power control, where the voltage magnitude is provided by reactive power control. The voltage reference is a combination of voltage magnitude and voltage angle.

[0034] As mentioned above, power converters can be controlled according to different control strategies, among which the grid-following method has historically been used. Also, as stated, one advantage of the grid-following control scheme is that, because the line-side controller controls the voltage based on the current voltage on the grid, the line-side converter will immediately react to changes occurring on the grid and adjust the current injected into the grid, even within the converter's limitations.

[0035] This invention aims to limit the current of grid-forming wind turbines, wherein a combination of grid-forming control (GFC) and grid-following (GFL) operations is implemented (see...). Figure 3 and Figure 4 ).

[0036] Figure 4The method is illustrated, in which the GFC 340 and GFL 360 operate in parallel, and the outputs of the two controllers are summed in a summing module 315, where the output voltages from the GFC and GFL are ultimately combined and sent to the line-side converter (LSC) system 310. The LSC system 310 outputs various signals, such as VLalpha-beta, used as an input to the PWM modulator 265 for converter control. The GFC 340 is responsible for managing steady-state operation and is configured with a limited bandwidth of less than 5 Hz, while the GFL 360 is specifically designed to handle the fast dynamics required for DC voltage regulation and current limiting.

[0037] Figure 3 The overall principle of a combined grid follower and grid forming controller is illustrated. In the figure, the line-side converter and its associated control are schematically shown by block 310 (also denoted as "System"). This block also represents a measured value of the DC link voltage Udc, which can be measured, for example, at the input side of the line-side converter. System block 310 may also represent power converters, generators, the grid, etc. System block 310 is also responsible for determining the magnitudes of the active power PL entering the grid and the reactive power QL injected into the grid. These magnitudes can be determined, for example, from the grid voltage Ugrid and grid current Igrid, which can be measured according to the methods described above or according to alternative voltage measurements as described. Active and reactive currents can also be determined from these magnitudes.

[0038] However, as discussed, there may be requests from, for example, grid operators, for wind turbine generators to participate in grid formation and assist in maintaining grid stability in the event of grid events affecting grid stability. This can be achieved using grid formation algorithms, which can be of different types. For example, active power Pgrid can be controlled using a virtual synchronous machine angle θVSM, such as... Figure 2B As illustrated schematically. In short, the angular acceleration of the synchrotron (the second time derivative of θVSM) corresponds to the difference between the power reference Pref, the expected power output of the wind turbine, and the grid power Pgrid, actually supplied by the wind turbine to the grid.

[0039] The synchronous generator angle θVSM can be determined based on a grid-forming converter scheme (e.g., a virtual synchronous generator control scheme). The grid-forming converter scheme models the inherent rotating mass inertia of a conventional synchronous generator. By modeling this inertia, the converter can provide improved grid stability by resisting grid frequency variations using the grid-forming converter model. That is, an increase in 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 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 simulated synchronous generator will result in an increase or decrease in the kinetic energy of the rotor 102.

[0040] The synchronizing angle θVSM can be used to transform signals from a rotating DQ coordinate system to a non-rotating coordinate system (such as the αβ or abc coordinate system) and vice versa. Based on the synchronizing angle θVSM and the voltage amplitude reference Vqref, the control signals for the desired active and reactive power are determined.

[0041] The synchronizing angle θVSM can be defined in a rotating DQ coordinate system, defined by the angular position θVSM and rotated at a frequency ωVSM. Based on the synchronizing angle θVSM, the angle of the control signal, i.e., the modulated voltage signal used for the pulse width modulator PWM 265, is determined and transformed to a non-rotating coordinate system (such as the αβ or abc coordinate system). The modulated voltage reference signal controls the active power Pgrid and reactive power Qgrid.

[0042] The coordinate system transformation unit 266 transforms the control signal from the DQ coordinate system to the αβ or abc coordinate system and determines the sinusoidal voltage reference for the PWM 265. The coordinate system transformed output signal from the coordinate system transformation unit 266 is converted by the pulse width modulator PWM 265 into a modulation signal for the line-side converter 204 to generate the desired active and reactive power and / or voltage amplitude.

[0043] A voltage amplitude reference, Vqref, is provided as a reference for the desired grid voltage or the desired reactive power Qgrid generated by converter 204. The voltage amplitude 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. Therefore, the reactive power Qgrid generated by line-side converter 204 can be controlled based on the voltage amplitude reference Vqref. The voltage reference can also be part of the grid formation.

[0044] The voltage amplitude reference Vqref can be defined in the DQ coordinate system rotating at a virtual synchronous machine speed ωVSM, which, under steady-state conditions, can be equal to the fundamental frequency of the AC power grid voltage (e.g., 50Hz). The voltage amplitude reference Vqref, or its modifications as described below, can be transformed from the DQ coordinate system to the αβ or abc coordinate system and output from the coordinate system transformation unit 266 to the pulse width modulator PWM 265, which determines the modulation signal used for the line-side converter 204. Regarding the DQ coordinate system, it should be noted that generator symbols such as Id, Iq, Ud, Uq, etc., are used in this specification, which differ from the usual motor symbols for active and reactive currents and voltages.

[0045] As described, in a conventional synchronous motor, inherent inertia can be used to stabilize the power grid. In a wind turbine, an increase or decrease in the kinetic energy of a simulated synchronous generator will result in an increase or decrease in the kinetic energy of the rotor 102. If these changes are transient, the kinetic energy demand of the rotor 102 will change accordingly, which will also lead to changes in other mechanical components.

[0046] According to the control procedures of wind turbine power plants, the power converter of the wind turbine may be required to operate as a virtual synchronous machine, at least when the grid current Igrid is below a given overcurrent threshold. If the overcurrent threshold is high, grid disturbances such as phase jumps may cause high power or torque peaks on the generator side and in the drive system, resulting in an undesirable increase in mechanical load. The object of this invention is to provide a method for mitigating overcurrent in the converter system.

[0047] Of course, there are many situations in the power grid that could trigger the current limiter. In principle, the current output of a line-side converter (LSC) is determined by the voltage difference between the grid voltage and the LSC voltage. Because the current flows through the converter's grid choke, if the grid voltage changes too rapidly, the converter controller cannot react quickly enough. Such events could be phase jumps in the grid voltage, i.e., a sudden change in the phase of the grid voltage at the terminal, usually due to a change in grid impedance caused by the disconnection or connection of branches in the grid. Another source could be fault ride-through (FRT) events, where a grid fault causes a voltage drop, or the grid voltage enters a high-voltage state. FRT events, or at least low-voltage ride-through (LVRT) events, require the power source (whether a wind turbine or a PV system) to provide rated reactive current during the fault, resulting in large reactive currents, while phase jumps typically cause a step change in active current due to the inductive impedance between the converter and the grid.

[0048] Figure 5 A more detailed embodiment of the invention is shown, which has the same Figure 4Similar modules include GFC module 540 and GFL module 560. The output Vgfc from GFC module 540 is applied to freeze module 545 before being used for the summed output. Once the current exceeds a threshold, a current limiter is triggered, and the freeze module maintains the Vgfc output level. When the current limiter is activated, the GFC output voltage is frozen, while the GFL output voltage is adjusted to modify the total output voltage sent to the modulator, thereby limiting the current. The summed output is then sent to LSC system 510.

[0049] Once the current exceeds the threshold, the current limiter is triggered. When the current limiter is activated, the GFC output voltage is maintained by the freeze module 545, which is triggered by the ActivateFreeze input. The GFC output voltage is kept at its level due to its freeze function. At the same time, the GFL output voltage 561 is adjusted accordingly and applied to the GFC output voltage 541, thereby reducing the total output voltage sent to the modulator 265 and limiting the current.

[0050] Figure 6 The complete implementation of the system is shown, in which GFL with GCC module is... Figure 6A The image is shown in an enlarged version. The mains current controller (GCC) within the GFL 560 is implemented in... Figure 6A The system has two branches, one for active current reference (Iqref) 600 and one for reactive power reference (Idref) 605. The Current Saturation Algorithm Module (CSA) 610 implements D / Q current vector limiting based on the maximum permissible amplitude (reactive current supply priority). Each branch has comparators 620 and 625, which generate current errors. During normal operation, i.e., when not in current-limiting mode, the GCC uses proportional (P) controllers 630 and 635. Selection elements 621 and 626 (which can also be considered current limiters) guide the error signal based on whether the current limit threshold has been exceeded. Active and reactive current references are selected to allow current flow generated by the GFC control action.

[0051] During current-limiting events, the GCC utilizes proportional-integral (PI) controllers 640 and 645 to enhance its control over the output current. Active and reactive current references are set in the current reference saturation algorithm to limit the total current to a maximum threshold and meet specific requirements.

[0052] When the current exceeds a specified threshold, the integral gain of the PI controllers 640 and 645 is enabled, and the integrator output is initialized with a value configured to provide fast current limiting action.

[0053] When the current drops below a specified threshold, the integral gain of the PI controller is disabled, and the integrator output is gradually reduced until the total GFL voltage drops below a specified voltage threshold, and the current limiter event ILim_Active ends. The outputs from controllers 630, 635, 640, and 645 are then guided again from the respective controllers by selection modules 631 and 636 based on the current level. Afterward, the GCC smoothly transitions from using a PI controller to using a P controller. This is in Figure 6 While not specifically reflected in the text, the switching between modules 631 and 636 ensures a smooth transition out of the rate limiting mode.

[0054] The primary objective of this system is to protect grid-forming wind turbines while maintaining grid-forming behavior unless current limiting is required, thereby enhancing operational safety. This disclosure also ensures that reactive current injection meets requirements during large disturbances.

[0055] This disclosure also ensures that reactive current injection meets requirements during large disturbances. As already mentioned, during FRT events, grid operators require power generation systems such as wind turbines to provide rated reactive current. Therefore, in Figure 6 The input IdrefFRT 605 is crucial to ensure that the system supports the grid by injecting reactive power, which serves both to maintain voltage levels and to ensure that protective relays trip by feeding in current to clear faults in the grid.

[0056] Without such current limiting methods and systems, the converter system would suffer from grid disconnection due to the combined requirements of operating in grid formation mode and injecting large amounts of reactive current, which would push the system beyond its capacity.

[0057] Figure 7A and Figure 7B Two different simulations involving fault ride-through (FRT) events are shown. Both display the converter output voltage in an alpha / beta coordinate system in the top plot, meaning that three voltage vectors have been projected into a two-vector system with a 90-degree displacement, known as the Clarke transform. The second plot shows the voltage output of the grid follower controller (560). The third plot shows the converter output current, with the threshold current value represented by a dashed line. The bottom plot shows the status of the control status flags FRT_Active and ILim_Active.

[0058] Figure 7AThe graph shows the event of a grid voltage drop at time 0.9 T1a. As the grid voltage drops, the FRT mode is activated (FRT_Active), causing the current to increase from 1 pu to exceed a threshold of approximately 1.4 pu, as the system is required to provide reactive current. The FRT mode can also involve injecting active current or a combination of active and reactive current. The current threshold is exceeded, therefore the current limiter function is also activated (ILim_Active), and thus, as the integral part of the PI controller increases its output, the GFL voltage output increases, as shown in the second graph. The large transient of the GFL voltage is due to changes in the DC voltage.

[0059] Once the DC voltage stabilizes, i.e., the voltage error decreases, the GFL voltage is minimized. The current level only momentarily reaches the threshold level, and therefore the current limiter function is deactivated at approximately 1 T2a. The GFL voltage then begins to drop again, and noise in the current signal keeps the GFL voltage output stable until the FRT event ends at 5.4 T3a, at which point the voltage recovers to approximately 1 pu, and thus the current also decreases as reactive current injection ends. The decrease in current level introduces some disturbance to the GFL voltage, and only the P controller is active here. The FRT mode remains active until 6 T4a ​​because it performs some other functions in the system, which are irrelevant to this invention.

[0060] B illustrates another event, also intertwined with the FRT event, where at time 0.9 T1b, the voltage drops, and the current increases due to reactive current injection, causing the current level to reach a threshold and the current limiter mode to activate. In this event, the current level remains at its threshold until time 5.4 T2b. Observing the GFL voltage, we can see that the voltage increases due to the integrator's action until time 5.4 T2b, at which point the FRT event ends. The current then drops below the specified threshold, and the GFL's output voltage gradually decreases until it falls below the voltage threshold, at which point the current limiter mode ILim_Active terminates. This is to ensure seamless switching between entering and exiting the current limiter mode and a smooth transition from its PI control mode to operation as a P controller.

[0061] The main objective of this method is to enhance operational safety measures by protecting grid-forming wind turbines by limiting the current to their designated reference.

[0062] Figure 7A and Figure 7B Both events indicate that the current limiter function is effective, and that, as previously explained, the higher frequency components are handled by the GFL branch of the line-side converter controller.

[0063] Figure 8A broader implementation of a converter system in a wind turbine operating in grid-forming mode is shown, where a current limiter function can be applied, but the invention is not limited to this. Figure 8 This is implemented in the system. Under 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 example, to compensate for the power consumed by auxiliary equipment of the wind turbine converter and / or losses and / or other power. According to this example, there is therefore a generator active power controller GAPC 320, which takes the power reference Pref_VMP representing the desired power output of the wind turbine and the power PL injected into the grid as inputs and outputs a machine-side active power reference PMSC_ref. Therefore, the power actually generated by the generator can be set to the desired power output and compensate for losses, etc., so that the actual injected power PL corresponds to the power reference Pref_VMP. Therefore, the machine-side active power reference PMSC_ref is used to control the generator, using the generator power to control the GPC 330 and the machine-side converter to obtain the desired power on the DC link.

[0064] The references used for active power Pref_VMP and reactive power QLref_VMP can be received from the plant controller (PPC) or the grid operator, or determined based on active and reactive power references (e.g., from the grid operator). The power references can reflect the power extracted from wind energy and can therefore vary, for example, based on the current power output of the wind turbine generator. Thus, the power reference Pref_VMP can reflect, for example, power variations caused by wind changes. In this way, power balance is also achieved on the drive system. As an alternative to providing the turbine reference power to the GAPC, either PL or Pref_VMP can be selected as the reference in the GAPC, and a combination can also be used as the power reference.

[0065] As stated, the line-side converter controls the output voltage based on a voltage input (e.g., voltage Vαβ). According to this example, this voltage input Vαβ consists of two voltage components, which will be described below.

[0066] The power PMSC supplied to the DC link by the machine-side converter is used by the grid formation control GFC 340 to determine the output voltage VGFC,αβ to be output by the line-side converter to achieve the desired power output. This can be achieved, for example, by using grid formation control with a virtual synchronous machine angle to control the active power injected into the grid. It is important to note that the output voltage component Vαβ, GFC, can be generated based on any suitable grid formation control scheme and is therefore not limited to controlling the line-side converter based on a virtual synchronous machine. For example, virtual oscillator grid formation and / or moving average filtered grid formation can be used as alternatives to controlling the line-side converter as a virtual synchronous machine. The grid formation control GFC 340 performs the necessary calculations based on the power PLref (which includes the power output by the machine-side converter) and the further component PDCIref described below. Additionally, a reactive power reference QLref is used.

[0067] Figure 3 The control based on GFC 340 is therefore configured to control the line-side converter according to a voltage reference. However, using this control alone, as is typically the case, exhibits the disadvantages explained above, because from a transient perspective, the generator is no longer decoupled from the grid in the same way as when controlled according to the grid-following control scheme, as the rest of the system must adapt to the control of the line-side converter. The generator power control GPC will adapt the power PL currently input to the grid by the line-side controller.

[0068] During normal operation, a balance will be maintained between the power output of the line-side converter and the power generated by the motor. The power reference Pref_VMP from the turbine is adhered to because it provides information about the amount of power that can be injected into the grid based on the current power extracted from wind energy. However, if a transient occurs in the grid, the grid formation control (GFC) requires maintaining the grid voltage, which will result in a transient in the current, which will be injected into the grid as a result of maintaining the voltage reference.

[0069] Therefore, the power (PL) injected into the grid by the line-side converter will also experience transient changes. As a result, changes in the output current of the line-side converter will be directly reflected in changes in the generator torque request, leading to transient changes in the generator torque request. DC links typically have very limited energy storage and therefore cannot account for sudden current changes. This means that the power supplied by the generator must be delivered to the grid immediately so that the DC link voltage can be maintained at the desired level. The DC link voltage must be kept within its lower and upper limits to keep the converter running, and therefore this can only be ensured by maintaining an energy balance between the generator and the power injected into the grid. Therefore, transients will occur in the generator power / torque.

[0070] Figure 3A reactive power control GPRC for reactive power control is also shown. A reactive power control loop is necessary to ensure that reactive power is not unnecessarily generated. Nevertheless, both active and reactive power are necessary to form the total output power, therefore it is necessary to generate reactive power, for example, for controlling virtual motor angles.

[0071] The reactive power control GPRC 370 takes a reactive power reference Qref_VMP (which can be determined in a manner similar to the active power reference) representing the desired reactive power output of the wind turbine and the reactive power QL injected into the grid as inputs. The GPRC 370 outputs a general reactive power reference QL_ref, which is provided to the grid forming control 340 and constitutes a portion of the output voltage generated by the grid forming control. The GPRC also outputs the reactive power reference Qref to the grid following control, thereby being controlled in a manner similar to the active power grid following control, and this reactive power reference Qref constitutes a portion of the output voltage component obtained from the grid following control.

[0072] Figure 8 Another embodiment according to the present invention is shown. GAPC 420 and GPC 430 are similar to... Figure 3 Therefore, we will not discuss this further. Figure 8 The implementation can be viewed as a system that allows the GFL branch 460 to process several control features. The current limiting function can be implemented as one of these features.

[0073] Regarding the DC link control DCC 450, the determination 451 of the DC link voltage error signal Udcerr (squared) and the I controller 452 for generating the integral part PDCIref of the error signal are shown. This can also be implemented without an integral part, i.e., a conventional P controller. A P controller 453 for generating the proportional part PDCPref of the error signal is also shown. These signals are referenced above. Figure 3 The use of.

[0074] The proportional power component PDCPref, output by the DC link control DCC 450, is provided to the grid following control (illustratively indicated by 460) as described above, where the power component PDCPref is first added to the power PL injected into the grid. This is done because only the total currents Id and Iq, i.e., the combination of the outputs from the grid forming control and the grid following control, can be measured. It is not possible to measure the currents specifically associated with the grid forming control and the grid following control separately. 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. The current Iq is then subtracted from the grid following reference current Igfl_ref. This results in the grid following GFL control component following the power reference Pgfl_ref; therefore, Pgfl_ref is the power representation required to correct the DC link voltage. The generated current is processed by a PI controller to generate a voltage component, which is then used by a voltage generator 461 to generate an output voltage Vαβ,GFL. This output voltage is combined with the control output Vαβ,GFC from the grid and compensates for the current output by the line-side converter so that the DC link voltage can be maintained. It should be noted that this PI controller can alternatively be a P controller.

[0075] The grid follower control 460 also demonstrates a similar generation of reactive power voltage components, which operate in the same manner and also constitute part of the grid follower control output voltage generated in the voltage generator 461.

[0076] In addition to providing a more detailed example of grid-following control, Figure 8 An example of a grid formation control algorithm 440 for determining the synchronizing angle θGFC of a virtual synchronous generator is also shown.

[0077] The synchronous machine angle θGFC is determined based on the concept of virtual synchronous machine control, which aims to generate a power response corresponding to the power response of an actual synchronous generator (including the inertia of the synchronous generator).

[0078] The power error Perr is determined as the difference between PLref as defined above and the power PL injected into the grid, and the difference between the damping power PD determined according to the virtual synchronization model.

[0079] In response to changes in grid power PL (e.g., due to a decrease in grid voltage Ugrid and a corresponding increase in grid current Igrid), the power error Perr becomes non-zero, causing the angle θVSM to increase or decrease to reduce the power error Perr. Therefore, in response to fluctuations in grid power Pgrid, for example, the synthetic inertial response becomes non-zero, causing the virtual machine to accelerate or decelerate to reach a new equilibrium state. A new equilibrium is reached when PL again follows PLref.

[0080] The virtual synchronization model includes a closed loop, in which the virtual synchronizing machine speed ωGFC from grid formation control is determined based on a combination of feedback from the damping power PD, the power reference PLref of the wind turbine's expected active power output, and the active grid power PL supplied by the wind turbine to the grid.

[0081] The inertial integral model is implemented as 1 / (2Hs) according to the example shown, where H is the inertial time constant and 1 / s is the integral in the s-domain, with Perr used as the input to the inertial integral model.

[0082] The damping power PD is determined as the difference between the grid speed ωg and the synchronous machine speed ωGFC multiplied by the damping factor Dp. The damping factor Dp suppresses the performance of the control loop for grid formation control.

[0083] The synchronous generator angle θGFC is determined based on the integral of ω0 / s over the synchronous generator speed ωGFC, where ω0 is the rated synchronous generator speed.

[0084] Figure 8 The diagram also shows a decoupling virtual impedance 470, which can be used or not, primarily in situations with a strong grid. In such cases, even a small change in the synchronous generator angle θGFC can lead to high power differentials. The virtual impedance 470 slightly alters the voltage to provide decoupling between the active and reactive power loops. This improves system stability. The diagram also shows a current limiter 480, which can be used to ensure that the current does not exceed a set limit by keeping it below that limit. This limitation is typically not required during normal operation. The virtual impedance can be determined for one or more phases. By increasing the resistance and / or reactance of the virtual impedance, the output current drawn from the line-side inverter output can be reduced. The virtual impedance can be used to reduce the acceleration of the virtual synchronous generator during overcurrent conditions, thereby reducing the output current Igrid of the line-side inverter. Figure 8 The diagram also illustrates reactive power control 490 for grid formation control, which provides voltage amplitude in a manner known per se, while active power control provides voltage angle. Thus, active and reactive power control together constitute the voltage amplitude and angle output by grid formation control. Reactive power control will not be discussed in detail here, as the invention relates to active power control, where the Uref in reactive power control can be locally set or received from an external source.

[0085] Furthermore, as described above, the present invention allows for the use of limitations on the rate of change of power / torque generated, for example, by a generator, which can be taken into account when determining the power reference used by the machine-side controller.

[0086] According to embodiments of the invention, the machine-side power reference can alternatively be set after applying a limit on the rate of change of power / torque generated by, for example, a generator, so that this can be taken into account when determining the power reference that forms the basis of the power controller. Therefore, according to embodiments of the invention, it can be further ensured that the generator and drive system are not subjected to harmful transients, or at least that transients are reduced through such control, but the grid-following control according to the invention still addresses the potential differences in DC link voltage that may arise.

[0087] Figure 9 The chart shows the changes in DC link voltage during the FRT event. The bottom chart shows the UDC voltage level and its reference voltage, where a drop in line voltage at time 0.91 (see the first chart) causes a disturbance in the UDC, and the current in the third chart triggers current limiting at time 0.925. The second chart shows the activity in the GFL voltage.

[0088] Furthermore, regarding the power shown, namely machine-side power and line-side power, as those skilled in the art will know, this power may include power components that do not constitute part of the available energy injected into the power grid. Such power components may include, for example, power losses, drive system damping power, power consumed by auxiliary components, etc.

[0089] For example, the damping power of the drive system can be AC ​​power, where, for example, a frequency of approximately 1 to 3 Hz can be used to attempt to dampen the low-frequency oscillations inherent in the drive system during operation, where such oscillations may be a result of, for example, the drive system's resonant frequency. This power component can be included in the calculations when determining the power reference level, for example, in conventional power grid forming control.

[0090] Part of the generator control function is to extract the desired average power and also to suppress the drive system to account for low-frequency oscillations caused by the drive system's resonant frequency.

[0091] Another advantage of this invention is that, regarding grid formation, there may be frequency ranges where control is not performed. This also means that it may pose difficulties for propagating, for example, drive system damping power into the grid, because the control still available is very slow, at most, for example, 5 Hz, and grid formation control would therefore be used for very slow changes in drive system damping power, for example, 1 to 2 Hz. Therefore, this may be difficult to fully account for in the grid formation control algorithm.

Claims

1. A method for controlling a wind power generating device to limit the current of the wind power generating device, the wind power generating device including a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a line-side converter configured to supply current to a power grid, and a DC link electrically connected to the output of the machine-side converter and the input of the line-side converter, the line-side converter having an output voltage and an output current, the method comprising: Determine the grid voltage reference used to control the line-side converter; The grid voltage reference includes the sum of the first output voltage and the second output voltage; By utilizing a grid forming controller to manage slow dynamics, the grid forming controller is configured to control the first output voltage to tend towards the grid voltage reference, and With the help of a grid follower controller to manage fast dynamics, the grid follower controller is configured to control the second output voltage to tend toward the grid voltage reference; When the output current reaches the threshold current value, the current limiter mode is triggered. The grid follower controller is operated by a grid current controller (GCC), which utilizes: A proportional (P) controller when the output current is below the threshold current value, and The proportional-integral (PI) controller operates when the output current reaches the threshold current value. During operation in current limiter mode, the first output voltage of the grid forming controller is maintained at the output voltage level before the current limiter is triggered. The line-side converter is operated based on a combination of output voltages from the grid forming controller and the grid following controller.

2. The method according to claim 1, further comprising: When the output current is lower than the threshold current value, the output of the proportional-integral (PI) controller is gradually reduced until the output of the proportional-integral (PI) controller drops below the voltage threshold.

3. The method according to claim 2, further comprising: Maintain current limiter mode until the voltage threshold is reached.

4. The method according to claim 1, further comprising: After the current level drops below the threshold current value, the current limiter mode will be maintained for a predetermined period of time.

5. The method according to any one of the preceding claims, further comprising: Enter Failover Trip (FRT) mode, and In response to the fault ride-through (FRT) mode, a predetermined amount of reactive current is injected into the grid.

6. The method according to any one of claims 1-5, the method further comprising: The amount of power supplied to the grid by the line-side converter is used as a power reference. Processing the magnitude of the power supplied to the grid by the line-side converter, and The processed power supplied to the grid by the line-side converter is used as a reference for motor power.

7. The method of claim 6, wherein the processing of the magnitude of the line-side power includes one or more of the following: When the power supplied to the grid by the line-side converter undergoes transient changes, a rate of change limit is imposed on the amount of power supplied to the grid by the line-side converter. Before determining the motor power reference, the magnitude and / or rate of change of the line-side power are low-pass filtered.

8. The method according to claim 6 or 7, wherein, When the motor power reference deviates from the line-side power value: The DC link voltage correction component is used to compensate for the difference between the DC link voltage and the DC link voltage reference caused by power differences.

9. The method according to claim 1, wherein slow dynamic refers to a limited bandwidth of less than 5 Hz.

10. The method of claim 1, wherein fast dynamics refers to a frequency higher than 5 Hz.

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

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

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

14. A wind power generation equipment control system arranged for limiting the current of a wind power generation equipment, the wind power generation equipment including a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a line-side converter configured to supply current 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 line-side converter having an output voltage and an output current, the wind power generation equipment control system being configured to perform the method according to any one of claims 1-11.

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

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