Stable control system and method for grid connection of wind power plant through flexible direct current power transmission system

Abstract

The invention relates to the technical field of flexible direct-current power transmission, aims at a system for wind power grid connection through flexible direct-current power transmission, and discloses a stable control system and method for grid connection of a wind power plant through a flexible direct-current power transmission system to solve the problems of oscillation instability and the like caused by additional inertia supporting and frequency adjusting functions. A phase corrector is added to an existing control loop of a wind turbine generator, the phase corrector only plays a role in a potential oscillation frequency band, negative damping of the potential oscillation frequency band is corrected into positive damping, and the stability of grid connection of a wind power plant through a flexible direct-current power transmission system is enhanced on the basis that existing inertia supporting and frequency adjusting functions are not affected. A design method of key parameters in the stability control system is given, and the practicability of the stability control system is effectively enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current power transmission, and in particular to a stable control system and method for connecting a wind farm to a grid via a flexible direct current power transmission system. Background Art

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

[0003] In order to enhance the support capability of the flexible DC transmission system for the power grid, scholars at home and abroad have proposed the use of a grid-building control method. At present, the main grid-building controls include 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. Whether inertial synchronization control can use the DC voltage of the converter to achieve autonomous synchronization with the power grid is suitable for working conditions with intermittent and fluctuating input power. In addition, under inertial synchronization control, the DC voltage of the converter can follow the changes in the grid frequency and play a role similar to a frequency-locked loop. The DC voltage of the converter under inertial synchronization control can be used to observe the grid frequency. According to the frequency signal, the output power of the wind farm can be adjusted to realize the function of participating in the grid inertia support and frequency regulation. However, this additional inertia support and frequency regulation function may cause instability in the operation of the flexible DC transmission system, leading to problems such as harmonic oscillation. Therefore, it is necessary to study related stability control systems and methods. Summary of the invention

[0004] Aiming at the system of wind power grid-connected via flexible direct current transmission, the present invention proposes a stable control system based on the phase correction principle to solve the problems of oscillation instability caused by additional inertia support and frequency regulation function, and gives a parameter design method of the stable control system, which effectively enhances the stability of wind power grid-connected via flexible direct current transmission system.

[0005] In view of this, the first aspect of the present application provides a stable control system for a wind farm connected to the grid via a flexible direct current transmission system, wherein a receiving-end converter station of the flexible direct current transmission system is connected to the power grid, and the wind farm is connected to a sending-end converter station of the flexible direct current transmission system; wherein the wind farm is composed of a plurality of wind turbines, and the wind turbines adopt the following control structure:

[0006] Detect the rotor speed of the wind turbine t ,ω tEnter the main control module of the wind turbine, the main control module adopts maximum power tracking control, and the output of the main control module is the maximum power tracking control reference value P of the wind turbine MPPT ;

[0007] Detect the DC voltage u of the wind turbine converter dc ,u dc Enter the inertia support and frequency regulation control module, the output of which is the change in active power ΔP, and the inertia support and frequency regulation control module is controlled by the stability controller at the same time;

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

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

[0010] Detect the DC voltage u of the wind turbine dc ,u dc Enter the inertia support control link, the output of the inertia support control link enters the inertia side phase correction link, and the output of the inertia side phase correction link is the power change ;

[0011] u dc Subtract the rated value of DC voltage u dcn Get the DC voltage deviation , Enter the frequency regulation control link, the output of the frequency regulation control link enters the frequency regulation side phase correction link, the output of the frequency regulation side phase correction link is the power change , Overlay The latter is the active power change ΔP.

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

[0013] ;

[0014] Wherein, T1 is the time constant of the phase correction link on the inertia side, and m is the power of the phase correction link on the inertia side.

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

[0016] ;

[0017] Wherein, T2 is the time constant of the phase correction link on the frequency regulation side, and n is the power of the phase correction link on the frequency regulation side.

[0018] A second aspect of the present application provides a stable control method for grid-connecting a wind farm via a flexible direct current transmission system, the method comprising:

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

[0020] S2. If the wind turbine can operate stably, directly proceed to step S3; if the wind turbine is unstable, optimize the parameters of the grid-type control based on autonomous synchronization of DC voltage, 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 operated, and the frequency regulation control link is not operated, and the time constant T1 of the inertia side phase correction link is set to 0, and the stability of the wind turbine is checked. If the wind turbine can operate stably, directly enter step S5;

[0022] S4. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine i , set the parameters T1 and m of the inertia side phase correction link; 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 run stably, enter step S5;

[0023] S5. In the inertia support and frequency regulation control module, the frequency regulation control link is operated, the time constant T2 of the phase correction link on the frequency regulation side is set to 0, and the stability of the wind turbine is tested. If the wind turbine can operate stably, the parameter design is completed.

[0024] S6. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine. f , set the parameters T2 and n of the phase correction link 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 completed.

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

[0026] .

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

[0028] Let the value of m be 0 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0; otherwise, let the value of m be 1 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1; otherwise, let the value of m be 2 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2; otherwise, let the value of m be 3 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3; if the wind power still cannot operate stably, re-give the value of parameter T1, take the value of m back to 0, and continue to check the stability of the wind turbine.

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

[0030] .

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

[0032] Let n be 0 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 0. Otherwise, let n be 1 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 1. Otherwise, let n be 2 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 2. Otherwise, let n be 3 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 3. If the wind turbine still cannot operate stably, re-give the value of parameter T2, take n as 0 again, and continue to check the stability of the wind turbine.

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

[0034] 1) Add a phase corrector to the existing control loop of the wind turbine. The phase corrector only works in the potential oscillation frequency range, correcting the negative damping in the oscillation frequency range into positive damping, and enhancing the stability of the wind farm connected to the grid through 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 stability control system is given to enhance the practicality of the proposed stability control system. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[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 system connected to the grid via a flexible direct current transmission system according to an embodiment of the present invention;

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

[0040] Figure 4 This is a flow chart of a stable control method for connecting a wind farm to the grid via a flexible DC power transmission system, according to an embodiment of the present invention;

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

[0042] Figure 6 This is a simulation embodiment of the present invention - a stable waveform after adding phase correction links on the inertia side and the frequency adjustment side. DETAILED DESCRIPTION

[0043] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] See also Figure 1-2 In an embodiment of the present invention, a stable control system for a wind farm connected to the grid via a flexible DC transmission system is provided. The receiving-end converter station of the flexible DC transmission system is connected to the grid, and the wind farm is connected to the sending-end converter station of the flexible DC transmission system. The wind farm is composed of multiple wind turbines, and the wind turbines adopt the following control structure:

[0045] Detect the rotor speed of the wind turbine t ,ω t Enter the main control module of the wind turbine, the main control module adopts maximum power tracking control, and the output of the main control module is the maximum power tracking control reference value P of the wind turbine MPPT ;

[0046] Detect the DC voltage u of the wind turbine converter dc ,u dcEnter the inertia support and frequency regulation control module, the output of which is the change in active power ΔP, and the inertia support and frequency regulation control module is controlled by the stability controller at the same time;

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

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

[0049] Detect the DC voltage u of the wind turbine dc ,u dc Enter the inertia support control link, the output of the inertia support control link enters the inertia side phase correction link, and the output of the inertia side phase correction link is the power change ;

[0050] u dc Subtract the rated value of DC voltage u dcn Get the DC voltage deviation , Enter the frequency regulation control link, the output of the frequency regulation control link enters the frequency regulation side phase correction link, the output of the frequency regulation side phase correction link is the power change , Overlay The latter is the active power change ΔP.

[0051] Among them, the transfer function G(s) of the inertia side phase correction link is:

[0052] ;

[0053] Wherein, T1 is the time constant of the phase correction link on the inertia side, m is the power of the phase correction link on the inertia side, and the value of m is 0, 1, 2 or 3.

[0054] Among them, the transfer function H(s) of the phase correction link on the frequency adjustment side is:

[0055] ;

[0056] Wherein, T2 is the time constant of the phase correction link on the frequency regulation side, n is the power of the phase correction link on the frequency regulation side, and the value of n is 0, 1, 2 or 3.

[0057] The above is an embodiment of a stable control system for a wind farm connected to the grid via a flexible DC transmission system provided in an embodiment of the present invention. The following is an embodiment of a stable control method for a wind farm connected to the grid via a flexible DC transmission system provided in an embodiment of the present invention.

[0058] See also Figure 4 In an embodiment of the present invention, a method for stabilizing the grid connection of a wind farm via a flexible direct current transmission system is provided, comprising:

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

[0060] S2. If the wind turbine can operate stably, directly proceed to step S3; if the wind turbine is unstable, optimize the parameters of the grid-type control based on autonomous synchronization of DC voltage, 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 operated, and the frequency regulation control link is not operated, and the time constant T1 of the inertia side phase correction link is set to 0, and the stability of the wind turbine is checked. If the wind turbine can operate stably, directly enter step S5;

[0062] S4. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine i , set the parameters T1 and m of the inertia side phase correction link; 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 run stably, enter step S5;

[0063] S5. In the inertia support and frequency regulation control module, the frequency regulation control link is operated, the time constant T2 of the phase correction link on the frequency regulation side is set to 0, and the stability of the wind turbine is tested. If the wind turbine can operate stably, the parameter design is completed.

[0064] S6. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine. f , set the parameters T2 and n of the phase correction link 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 completed.

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

[0066] .

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

[0068] Let the value of m be 0 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0; otherwise, let the value of m be 1 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1; otherwise, let the value of m be 2 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2; otherwise, let the value of m be 3 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3; if the wind power still cannot operate stably, re-give the value of parameter T1, take the value of m back to 0, and continue to check 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 set as follows:

[0072] Let n be 0 to check the stability of the wind turbine. If the wind turbine can run stably, n is 0; otherwise, let n be 1 to check the stability of the wind turbine. If the wind turbine can run stably, n is 1; otherwise, let n be 2 to check the stability of the wind turbine. If the wind turbine can run stably, n is 2; otherwise, let n be 3 to check the stability of the wind turbine. If the wind turbine can run stably, n is 3; if the wind turbine still cannot run stably, re-give the value of parameter T2, and n is 0 again, and continue to check the stability of the wind turbine.

[0073] Furthermore, the specific embodiment of the present invention also provides the following simulation embodiment.

[0074] Simulation Example 1:

[0075] like Figure 5 As shown, the unstable waveform before adding the phase correction link on the inertia side and the frequency adjustment side in a simulation embodiment of the present invention, where the short-circuit ratio of the power grid is 2, and the control coefficient K in the inertia support link of the wind turbine generator set is in The value is 8, the coefficient K in the frequency regulation control link dr The value of is 5, the value of m in the phase correction link on the inertia side is 0, and the value of n in the phase correction link on the frequency regulation side is 0. Figure 5It can be seen that at 20.1 seconds, the grid frequency began to drop from 50 Hz, the DC voltage of the wind turbine decreased accordingly, the active power output of the wind turbine increased to support the grid, and the speed of the wind turbine decreased; however, in the process of the wind turbine supporting the grid, the active power and DC voltage output by the wind turbine both showed oscillation divergence problems, and the wind turbine became unstable. This shows that before the phase correction link on the inertia side and the frequency regulation side is added, the wind turbine is at risk of unstable operation.

[0076] Simulation Example 2:

[0077] like Figure 6 As shown, a simulation embodiment of the present invention - a stable waveform after adding the phase correction link on the inertia side and the frequency adjustment side, where the grid short-circuit ratio is 2, and the control coefficient K in the inertia support link of the wind turbine generator set in The value is 8, the coefficient K in the frequency regulation control link dr The value of is 5, the value of m in the phase correction link on the inertia side is 3, the value of the time constant T1 is 0.5, and the value of n in the phase correction link on the frequency regulation side is 0. Figure 6 It can be seen that at 24 seconds, the grid frequency begins to drop from 50 Hz, and the DC voltage of the wind turbine drops accordingly, following the change in grid frequency. The active power output by the wind turbine increases to support the grid, and the speed of the wind turbine decreases. In the process of the wind turbine supporting the grid, the active power and DC voltage output by the wind turbine do not have oscillation divergence problems, and the wind turbine operates stably, indicating that after adding the phase correction links on the inertia side and the frequency adjustment side, the wind turbine can operate stably.

[0078] In summary, the present invention proposes a stable control system and method for connecting a wind farm to the grid via a flexible direct current transmission system. A phase corrector is added to the existing control loop of the wind turbine. The phase corrector only works in the potential oscillation frequency segment, and the negative damping of the oscillation frequency segment is corrected into positive damping. The stability of the wind farm connected to the grid via the flexible direct current transmission system is enhanced without affecting the existing inertia support and frequency regulation functions. A design method for key parameters in the stable control system is given to enhance the practicability of the proposed stable control system.

[0079] In the several embodiments provided by the present 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 only schematic, for example, the division of units is only a logical function division, and there may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

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

[0081] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0082] If the integrated unit is implemented in the form of 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. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0083] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions 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 power transmission system is connected to the power grid, and the wind farm is connected to the sending-end converter station of the flexible DC power transmission system; wherein the wind farm is composed of multiple wind turbines, and the wind turbines adopt the following control structure: Detect the rotor speed of the wind turbine t ,ω t Enter the main control module of the wind turbine, the main control module adopts maximum power tracking control, and the output of the main control module is the maximum power tracking control reference value P of the wind turbine MPPT ; Detect the DC voltage u of the wind turbine converter dc ,u dc Enter the inertia support and frequency regulation control module, the output of which is the change in active power ΔP, and the inertia support and frequency regulation control module is controlled by the stability controller at the same time; ΔP vs. P MPPT The reference value P of the wind turbine output active power is generated by superposition ref , P ref Enter the control loop of the wind turbine generator set, the output of the control loop is the control signal of the wind turbine generator set, and the control loop of the wind turbine generator set adopts a grid-type control based on autonomous synchronization of DC voltage.

2. The stable control system for connecting a wind farm to the grid via a flexible DC transmission system according to claim 1, characterized in that: The inertia support and frequency regulation control module adopts the following control structure: Detect the DC voltage u of the wind turbine dc ,u dc Enter the inertia support control link, the output of the inertia support control link enters the inertia side phase correction link, and the output of the inertia side phase correction link is the power change ; u dc Subtract the rated value of DC voltage u dcn Get the DC voltage deviation , Enter the frequency regulation control link, the output of the frequency regulation control link enters the frequency regulation side phase correction link, the output of the frequency regulation side phase correction link is the power change , Overlay The latter is the active power change ΔP.

3. The stable control system for connecting a wind farm to the grid via a flexible DC transmission system according to claim 2, characterized in that: The transfer function G(s) of the inertia side phase correction link is: ； Wherein, T1 is the time constant of the phase correction link on the inertia side, and m is the power of the phase correction link on the inertia side.

4. The stable control system for connecting a wind farm to the grid via a flexible DC transmission system according to claim 2, characterized in that: The transfer function H(s) of the phase correction link on the frequency adjustment side is: ； Wherein, T2 is the time constant of the phase correction link on the frequency regulation side, and n is the power of the phase correction link on the frequency regulation side.

5. A stable control method for connecting a wind farm 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 generator set, establish an impedance model of the wind turbine generator set in a rotating dq coordinate system, and verify the stability of the wind turbine generator set; S2. If the wind turbine can operate stably, directly proceed to step S3; If the wind turbine is unstable, the parameters of the grid-type control based on DC voltage autonomous synchronization are optimized so that the wind turbine 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 operated, and the frequency regulation control link is not operated, and the time constant T1 of the inertia side phase correction link is set to 0, and the stability of the wind turbine is checked. If the wind turbine can operate stably, directly enter step S5; S4. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine i , set the parameters T1 and m of the inertia side phase correction link; 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; S5. In the inertia support and frequency regulation control module, the frequency regulation control link is operated, the time constant T2 of the phase correction link on the frequency regulation side is set to 0, and the stability of the wind turbine is tested. If the wind turbine can operate stably, the parameter design is completed. S6. If the wind turbine is unstable, detect the oscillation frequency ω of the wind turbine. f , set the parameters T2 and n of the phase correction link 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 completed.

6. The method for stabilizing the connection of a wind farm to the grid via a flexible DC transmission system according to claim 5, characterized in that: In step S4, parameter T1 satisfies the following range: 。 7. The method for stabilizing the connection of a wind farm to the grid via a flexible DC transmission system according to claim 5, characterized in that: In step S4, the parameter m is set as follows: Let the value of m be 0 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 0; otherwise, let the value of m be 1 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 1; otherwise, let the value of m be 2 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 2; otherwise, let the value of m be 3 to check the stability of the wind turbine. If the wind turbine can operate stably, the value of m is 3; if the wind power still cannot operate stably, re-give the value of parameter T1, take the value of m back to 0, and continue to check the stability of the wind turbine.

8. The method for stabilizing the connection of a wind farm to the grid via a flexible DC transmission system according to claim 5, characterized in that: In step S6, parameter T2 satisfies the following range: 。 9. The method for stabilizing the connection of a wind farm to the grid via a flexible DC transmission system according to claim 5, characterized in that: In step S6, the parameter n is set as follows: Let n be 0 to check the stability of the wind turbine. If the wind turbine can run stably, n is 0. Otherwise, let n be 1 to check the stability of the wind turbine. If the wind turbine can run stably, n is 1. Otherwise, let n be 2 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 2. Otherwise, let n be 3 to check the stability of the wind turbine. If the wind turbine can operate stably, n is 3. If the wind turbine still cannot operate stably, re-give the value of parameter T2, take n as 0 again, and continue to check the stability of the wind turbine.

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