Stable control system and method for wind farm grid-connected through flexible HVDC transmission system

Abstract

The present application relates to the technical field of flexible direct current power transmission, and is a kind of stability control system and method for wind farm through flexible direct current power transmission system grid connection, which is aimed at the problem of oscillation instability caused by additional inertia support and frequency regulation function.The phase corrector is added to the existing control loop of wind turbine, which only works in the potential oscillation frequency range, and corrects the negative damping of potential oscillation frequency range to positive damping, thereby enhancing the stability of wind farm through flexible direct current power transmission system grid connection without affecting the existing inertia support and frequency regulation function.The design method of key parameters in the stability control system is given, which effectively enhances the practicability of the stability control system.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to a stable control system and method for wind farms connected to the grid via a flexible DC transmission system. Background Technology

[0002] With the transformation of the global energy structure and the increasing reliance on renewable energy, wind power, as a clean and sustainable energy form, is receiving increasing attention and importance. However, wind farms are typically located far from land, posing numerous challenges to power transmission. Flexible DC transmission technology based on voltage source converters, due to its high efficiency, flexibility, and strong controllability, is gradually becoming the preferred solution for grid connection of offshore wind power.

[0003] To enhance the grid support capability of flexible DC transmission systems, scholars both domestically and internationally have proposed grid-based control methods. Currently, grid-based control mainly includes power droop, power synchronization, virtual synchronization, and inertial synchronization control. Power droop, power synchronization, and virtual synchronization are suitable for converters with stable and controllable input power. Inertial synchronization control, which utilizes the converter's DC voltage to achieve autonomous synchronization with the grid, is suitable for operating conditions where the input power is intermittent and fluctuating. Furthermore, under inertial synchronization control, the converter's DC voltage can follow the grid frequency changes, acting as a frequency-locked loop. The grid frequency can be observed through the converter's DC voltage under inertial synchronization control, and the output power of wind farms can be adjusted based on this frequency signal, achieving the function of participating in grid inertia support and frequency regulation. However, this additional inertia support and frequency regulation function may cause instability in the flexible DC transmission system, leading to problems such as harmonic oscillations. Therefore, it is necessary to study related stability control systems and methods. Summary of the Invention

[0004] For wind power systems connected to the grid via flexible DC transmission, this invention proposes a stable control system based on the phase correction principle to address issues such as oscillation and instability caused by additional inertia support and frequency regulation functions. The invention also provides a parameter design method for the stable control system, which effectively enhances the stability of wind power connected to the grid via flexible DC transmission.

[0005] In view of this, the first aspect of this application provides a stable control system for a wind farm connected to the grid via a flexible DC transmission system, wherein the receiving-end converter station of the flexible DC transmission system is connected to the power grid, and the wind farm is connected to the sending-end converter station of the flexible DC transmission system; wherein the wind farm consists of multiple wind turbine units, and the wind turbine units adopt the following control structure:

[0006] Detecting the rotor speed ω of a wind turbine t ω tThe system enters the main control module of the wind turbine. The main control module employs maximum power point tracking (MPPT) control, and its output is the MPPT reference value P for the wind turbine. MPPT ;

[0007] Detecting the DC voltage u of the wind turbine converter dc u dc Entering the inertia support and frequency regulation control module, the output of the inertia support and frequency regulation control module is the change in active power ΔP. The inertia support and frequency regulation control module is also controlled by the stability controller.

[0008] ΔP and P MPPT The reference value P of the output active power of the wind turbine is generated by superposition. ref P ref The wind turbine's control loop is entered, and the output of the control loop is the wind turbine's control signal. The wind turbine's control loop adopts a grid-type control based on DC voltage autonomous synchronization.

[0009] Optionally, the inertia support and frequency adjustment control module adopts the following control structure:

[0010] Detecting the DC voltage u of the wind turbine dc u dc The system then enters the inertia support control stage. The output of the inertia support control stage then enters the inertia-side phase correction stage, and the output of the inertia-side phase correction stage is the power change. ;

[0011] u dc Subtract the rated value of DC voltage u dcn Obtain the deviation of DC voltage , The frequency regulation control stage then enters the frequency regulation side phase correction stage, and the output of the frequency regulation side phase correction stage is the power change. , Overlay The following is the change in active power ΔP.

[0012] Optionally, the transfer function G(s) of the inertia-side phase correction stage is:

[0013] ;

[0014] In the formula, T1 is the time constant of the inertia-side phase correction element, and m is the power of the inertia-side phase correction element.

[0015] Optionally, the transfer function H(s) of the frequency adjustment side phase correction stage is:

[0016] ;

[0017] In the formula, T2 is the time constant of the phase correction stage on the frequency adjustment side, and n is the power of the phase correction stage on the frequency adjustment side.

[0018] A second aspect of this application provides a stability control method for a wind farm connected to the grid via a flexible DC transmission system, the method comprising:

[0019] S1. Remove the inertia support and frequency regulation control module from the control structure of the wind turbine, establish the impedance model of the wind turbine in the rotating dq coordinate system, and verify the stability of the wind turbine.

[0020] S2. If the wind turbine can operate stably, proceed directly to step S3; if the wind turbine is unstable, optimize the parameters of the grid-type control based on DC voltage autonomous synchronization so that the wind turbine can operate stably after removing the inertia support and frequency regulation control modules.

[0021] S3. In the inertia support and frequency regulation control module, only the inertia support control link is working and the frequency regulation control link is not working. The time constant T1 of the inertia side phase correction link is set to 0 to check the stability of the wind turbine. If the wind turbine can operate stably, proceed directly to step S5.

[0022] S4. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. i Set the parameters T1 and m for the inertia-side phase correction stage; check the stability of the wind turbine at this time. If the wind turbine is unstable, reset the parameters T1 and m; if the wind turbine can operate stably, proceed to step S5.

[0023] S5. In the inertia support and frequency regulation control module, make the frequency regulation control loop work, set the time constant T2 of the frequency regulation side phase correction loop to 0, and check the stability of the wind turbine. If the wind turbine can run stably, the parameter design is complete.

[0024] S6. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. f Set the parameters T2 and n for the phase correction stage on the frequency regulation side; check the stability of the wind turbine at this time. If the wind turbine is unstable, return to step S4; if the wind turbine can operate stably, the parameter design is complete.

[0025] Optionally, parameter T1 in step S4 satisfies the following range:

[0026] .

[0027] Optionally, the parameter m in step S4 is tuned as follows:

[0028] Set m to 0 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0. Otherwise, set m to 1 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1. Otherwise, set m to 2 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2. Otherwise, set m to 3 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3. If the wind turbine still cannot operate stably, give a new value for parameter T1, set the value of m back to 0, and continue to test the stability of the wind turbine.

[0029] Optionally, parameter T2 in step S6 satisfies the following range:

[0030] .

[0031] Optionally, the parameter n in step S6 is tuned as follows:

[0032] Set n to 0 and test the stability of the wind turbine. If the wind turbine can operate stably, then n is set to 0. Otherwise, set n to 1 and test the stability of the wind turbine. If the wind turbine can operate stably, then n is set to 1. Otherwise, set n to 2 and test the stability of the wind turbine. If the wind turbine can operate stably, then n is set to 2. Otherwise, set n to 3 and test the stability of the wind turbine. If the wind turbine can operate stably, then n is set to 3. If the wind turbine still cannot operate stably, reset the value of parameter T2, set n to 0 again, and continue to test the stability of the wind turbine.

[0033] As can be seen from the above technical solutions, the present invention has the following advantages:

[0034] 1) Add a phase corrector to the existing control loop of the wind turbine. This phase corrector only works in the potential oscillation frequency range, correcting the negative damping of the oscillation frequency range to positive damping, thereby enhancing the stability of the wind farm connected to the grid via the flexible DC transmission system without affecting the existing inertia support and frequency regulation functions.

[0035] 2) The design method of key parameters in the stable control system is given, which enhances the practicality of the proposed stable control system. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1This is a schematic diagram of an embodiment of the present invention—a wind farm connected to the grid via a flexible DC transmission system;

[0038] Figure 2 This is a control block diagram of a wind turbine generator connected to the grid via a flexible DC transmission system, according to an embodiment of the present invention.

[0039] Figure 3 This is a schematic diagram of an embodiment of the present invention—an inertia support and frequency adjustment control module;

[0040] Figure 4 This is a flowchart of a stability control method for wind farms connected to the grid via a flexible DC transmission system, according to an embodiment of the present invention.

[0041] Figure 5 This is a simulation embodiment of the present invention—the unstable waveform before the addition of phase correction stages on the inertia side and frequency adjustment side;

[0042] Figure 6 This is a simulation embodiment of the present invention—a stable waveform after adding phase correction stages on the inertia side and frequency adjustment side. Detailed Implementation

[0043] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] Please see Figure 1-2 This invention provides a stable control system for a wind farm connected to the grid via a flexible DC transmission system. The receiving-end converter station of the flexible DC transmission system is connected to the power grid, and the wind farm is connected to the sending-end converter station of the flexible DC transmission system. The wind farm consists of multiple wind turbine units, and the wind turbine units employ the following control structure:

[0045] Detecting the rotor speed ω of a wind turbine t ω t The system enters the main control module of the wind turbine. The main control module employs maximum power point tracking (MPPT) control, and its output is the MPPT reference value P for the wind turbine. MPPT ;

[0046] Detecting the DC voltage u of the wind turbine converter dc u dcEntering the inertia support and frequency regulation control module, the output of the inertia support and frequency regulation control module is the change in active power ΔP. The inertia support and frequency regulation control module is also controlled by the stability controller.

[0047] ΔP and P MPPT The reference value P of the output active power of the wind turbine is generated by superposition. ref P ref The wind turbine's control loop is entered, and the output of the control loop is the wind turbine's control signal. The wind turbine's control loop adopts a grid-type control based on DC voltage autonomous synchronization.

[0048] In one embodiment, see Figure 3 The inertia support and frequency regulation control module adopts the following control structure:

[0049] Detecting the DC voltage u of the wind turbine dc u dc The system then enters the inertia support control stage. The output of the inertia support control stage then enters the inertia-side phase correction stage, and the output of the inertia-side phase correction stage is the power change. ;

[0050] u dc Subtract the rated value of DC voltage u dcn Obtain the deviation of DC voltage , The frequency regulation control stage then enters the frequency regulation side phase correction stage, and the output of the frequency regulation side phase correction stage is the power change. , Overlay The following is the change in active power ΔP.

[0051] The transfer function G(s) of the inertia-side phase correction stage is:

[0052] ;

[0053] In the formula, T1 is the time constant of the inertia-side phase correction element, and m is the power of the inertia-side phase correction element, with m taking the value of 0, 1, 2, or 3.

[0054] The transfer function H(s) of the phase correction stage on the frequency adjustment side is:

[0055] ;

[0056] In the formula, T2 is the time constant of the phase correction stage on the frequency adjustment side, and n is the power of the phase correction stage on the frequency adjustment side, with n taking the value of 0, 1, 2, or 3.

[0057] The above are embodiments of the stability control system for wind farms connected to the grid via a flexible DC transmission system provided in the present invention. The following are embodiments of the stability control method for wind farms connected to the grid via a flexible DC transmission system provided in the present invention.

[0058] Please see Figure 4 The present invention provides a stability control method for a wind farm connected to the grid via a flexible DC transmission system, comprising:

[0059] S1. Remove the inertia support and frequency regulation control module from the control structure of the wind turbine, establish the impedance model of the wind turbine in the rotating dq coordinate system, and verify the stability of the wind turbine.

[0060] S2. If the wind turbine can operate stably, proceed directly to step S3; if the wind turbine is unstable, optimize the parameters of the grid-type control based on DC voltage autonomous synchronization so that the wind turbine can operate stably after removing the inertia support and frequency regulation control modules.

[0061] S3. In the inertia support and frequency regulation control module, only the inertia support control link is working and the frequency regulation control link is not working. The time constant T1 of the inertia side phase correction link is set to 0 to check the stability of the wind turbine. If the wind turbine can operate stably, proceed directly to step S5.

[0062] S4. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. i Set the parameters T1 and m for the inertia-side phase correction stage; check the stability of the wind turbine at this time. If the wind turbine is unstable, reset the parameters T1 and m; if the wind turbine can operate stably, proceed to step S5.

[0063] S5. In the inertia support and frequency regulation control module, make the frequency regulation control loop work, set the time constant T2 of the frequency regulation side phase correction loop to 0, and check the stability of the wind turbine. If the wind turbine can run stably, the parameter design is complete.

[0064] S6. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. f Set the parameters T2 and n for the phase correction stage on the frequency regulation side; check the stability of the wind turbine at this time. If the wind turbine is unstable, return to step S4; if the wind turbine can operate stably, the parameter design is complete.

[0065] In one embodiment, parameter T1 in step S4 satisfies the following range:

[0066] .

[0067] In step S4, parameter m is tuned as follows:

[0068] Set m to 0 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0. Otherwise, set m to 1 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1. Otherwise, set m to 2 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2. Otherwise, set m to 3 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3. If the wind turbine still cannot operate stably, give a new value for parameter T1, set the value of m back to 0, and continue to test the stability of the wind turbine.

[0069] In step S6, parameter T2 satisfies the following range:

[0070] .

[0071] In step S6, the parameter n is tuned as follows:

[0072] Set n to 0 and test the stability of the wind turbine. If the wind turbine operates stably, the value of n is 0. Otherwise, set n to 1 and test the stability of the wind turbine. If the wind turbine operates stably, the value of n is 1. Otherwise, set n to 2 and test the stability of the wind turbine. If the wind turbine operates stably, the value of n is 2. Otherwise, set n to 3 and test the stability of the wind turbine. If the wind turbine operates stably, the value of n is 3. If the wind turbine still cannot operate stably, reset the value of parameter T2, set the value of n back to 0, and continue to test the stability of the wind turbine.

[0073] Furthermore, the present invention also provides the following simulation embodiments.

[0074] Simulation Example 1:

[0075] like Figure 5 As shown, a simulation embodiment of the present invention—the instability waveform before the addition of phase correction links on the inertia side and frequency regulation side—is illustrated, wherein the grid short-circuit ratio is 2, and the control coefficient K in the inertia support link of the wind turbine is... in The value is 8, and the coefficient K in the frequency regulation control loop is... dr The value is 5, the value of m in the inertia-side phase correction stage is 0, and the value of n in the frequency adjustment-side phase correction stage is 0. From Figure 5It can be seen that at 20.1 seconds, the grid frequency begins to decrease from 50 Hz, and the DC voltage of the wind turbine decreases accordingly. The active power output of the wind turbine increases, thus supporting the grid, and the wind turbine speed decreases. However, during the process of the wind turbine supporting the grid, both the active power output and DC voltage of the wind turbine exhibit oscillation and divergence, leading to wind turbine instability. This indicates that without the addition of phase correction links on the inertia side and frequency regulation side, the wind turbine is at risk of operational instability.

[0076] Simulation Example 2:

[0077] like Figure 6 As shown, a simulation embodiment of the present invention—the stable waveform after adding phase correction stages on the inertia side and frequency regulation side—indicates a grid short-circuit ratio of 2 and a control coefficient K in the inertia support stage of the wind turbine. in The value is 8, and the coefficient K in the frequency regulation control loop is... dr The value of is 5, the value of m in the inertia-side phase correction stage is 3, the value of time constant T1 is 0.5, and the value of n in the frequency adjustment-side phase correction stage is 0. From Figure 6 It can be seen that at 24 seconds, the grid frequency starts to decrease from 50 Hz, and the DC voltage of the wind turbine decreases accordingly, following the change in grid frequency. The active power output of the wind turbine increases, thus supporting the grid. The speed of the wind turbine decreases. During the process of the wind turbine supporting the grid, there is no problem of oscillation or divergence in the active power output and DC voltage of the wind turbine. The wind turbine operates stably, indicating that after adding the phase correction links on the inertia side and frequency regulation side, the wind turbine can operate stably.

[0078] In summary, the stability control system and method for wind farms connected to the grid via a flexible DC transmission system proposed in this invention add a phase corrector to the existing control loop of the wind turbine. This phase corrector only functions in the potential oscillation frequency range, correcting the negative damping in the oscillation frequency range to positive damping, thereby enhancing the stability of wind farms connected to the grid via a flexible DC transmission system without affecting the existing inertia support and frequency regulation functions. Furthermore, the design method for key parameters in the stability control system is provided, enhancing the practicality of the proposed stability control system.

[0079] In the embodiments provided by this invention, it should be understood that the disclosed systems and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0080] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0081] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0082] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0083] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A stable control system for a wind farm connected to the grid via a flexible DC transmission system, characterized in that, The receiving-end converter station of the flexible DC transmission system is connected to the power grid, and the wind farm is connected to the sending-end converter station of the flexible DC transmission system; wherein, the wind farm consists of multiple wind turbine units, and the wind turbine units adopt the following control structure: Detecting the rotor speed ω of a wind turbine t ω t The system enters the main control module of the wind turbine. The main control module employs maximum power point tracking (MPPT) control, and its output is the MPPT reference value P for the wind turbine. MPPT ; Detecting the DC voltage u of the wind turbine converter dc u dc The inertia support and frequency regulation control module outputs the change in active power. The inertia support and frequency regulation control modules are simultaneously controlled by the stability controller; With P MPPT The reference value P of the output active power of the wind turbine is generated by superposition. ref P ref The wind turbine's control loop is entered, and the output of the control loop is the wind turbine's control signal. The wind turbine's control loop adopts a grid-type control based on DC voltage autonomous synchronization. The inertia support and frequency adjustment control module adopts the following control structure: Detecting the DC voltage u of the wind turbine dc u dc The output of the inertia support control stage then enters the inertia-side phase correction stage, where the output is the power change. ; u dc Subtract the rated value of DC voltage u dcn Obtain the deviation of DC voltage , The frequency regulation control stage then enters the frequency regulation side phase correction stage, whose output is the power change. , Overlay The following is the change in active power. .

2. The stability control system for wind farms connected to the grid via a flexible DC transmission system according to claim 1, characterized in that, The transfer function G(s) of the inertial side phase correction stage is: ； In the formula, T1 is the time constant of the inertia-side phase correction stage, and m is the power of the inertia-side phase correction stage.

3. The stability control system for wind farms connected to the grid via a flexible DC transmission system according to claim 1, characterized in that, The transfer function H(s) of the phase correction stage on the frequency adjustment side is: ； In the formula, T2 is the time constant of the phase correction stage on the frequency adjustment side, and n is the power of the phase correction stage on the frequency adjustment side.

4. A stability control method for a wind farm connected to the grid via a flexible DC transmission system, characterized in that, include: S1. Remove the inertia support and frequency regulation control module from the control structure of the wind turbine, establish the impedance model of the wind turbine in the rotating dq coordinate system, and verify the stability of the wind turbine. S2. If the wind turbine can operate stably, proceed directly to step S3. If the wind turbine unit becomes unstable, the parameters of the grid-type control based on DC voltage autonomous synchronization will be optimized so that the wind turbine unit can operate stably after removing the inertia support and frequency regulation control modules. S3. In the inertia support and frequency regulation control module, only the inertia support control link is working and the frequency regulation control link is not working. The time constant T1 of the inertia side phase correction link is set to 0 to check the stability of the wind turbine. If the wind turbine can operate stably, proceed directly to step S5. S4. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. i Set the parameters T1 and m for the inertia-side phase correction stage; check the stability of the wind turbine at this time. If the wind turbine becomes unstable, reset the parameters T1 and m. If the wind turbine can operate stably, proceed to step S5; S5. In the inertia support and frequency regulation control module, make the frequency regulation control loop work, set the time constant T2 of the frequency regulation side phase correction loop to 0, and check the stability of the wind turbine. If the wind turbine can run stably, the parameter design is complete. S6. If the wind turbine unit operates unstablely, detect the oscillation frequency ω of the wind turbine unit. f Set the parameters T2 and n of the phase correction stage on the frequency regulation side; check the stability of the wind turbine at this time. If the wind turbine is unstable, return to step S4; if the wind turbine can operate stably, the parameter design is complete. The inertia support and frequency adjustment control module adopts the following control structure: Detecting the DC voltage u of the wind turbine dc u dc The output of the inertia support control stage then enters the inertia-side phase correction stage, where the output is the power change. ; u dc Subtract the rated value of DC voltage u dcn Obtain the deviation of DC voltage , The frequency regulation control stage then enters the frequency regulation side phase correction stage, whose output is the power change. , Overlay The following is the change in active power. .

5. The stability control method for wind farms connected to the grid via a flexible DC transmission system according to claim 4, characterized in that, In step S4, parameter T1 satisfies the following range: 。 6. The stability control method for wind farms connected to the grid via a flexible DC transmission system according to claim 4, characterized in that, In step S4, parameter m is tuned as follows: Set m to 0 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0. Otherwise, set m to 1 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1. Otherwise, set m to 2 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2. Otherwise, set m to 3 and test the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3. If the wind turbine still cannot operate stably, give a new value for parameter T1, set the value of m back to 0, and continue to test the stability of the wind turbine.

7. The stability control method for wind farms connected to the grid via a flexible DC transmission system according to claim 4, characterized in that, In step S6, parameter T2 satisfies the following range: 。 8. The stability control method for wind farms connected to the grid via a flexible DC transmission system according to claim 4, characterized in that, In step S6, the parameter n is tuned as follows: Let n be 0 to test the stability of the wind turbine. If the wind turbine can operate stably, then the value of n is 0; otherwise, let n be 1 to test the stability of the wind turbine. If the wind turbine can operate stably, then the value of n is 1. Otherwise, set n to 2 and test the stability of the wind turbine. If the wind turbine can operate stably, then the value of n is 2. Otherwise, set n to 3 and test the stability of the wind turbine. If the wind turbine can operate stably, then the value of n is 3. If the wind turbine still cannot operate stably, give a new value for parameter T2 and set the value of n to 0 again to continue testing the stability of the wind turbine.

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