A stability analysis method for a wind power flexible direct current transmission system

Abstract

The present application belongs to the technical field of stability analysis of flexible DC transmission system, and particularly relates to a stability analysis method of wind power through flexible DC transmission system, comprising the following steps: S1, obtaining the impedance characteristics of grid-connected inverter and MMC through injection current disturbance and voltage disturbance; S2, respectively constructing the equivalent closed-loop structure of grid-connected inverter and MMC under current and voltage disturbance to obtain the system characteristic equation; S3, substituting the system characteristic equation of S2 into the impedance characteristics of grid-connected inverter and MMC of S1, and extracting the equivalent admittance of grid-connected inverter and the equivalent impedance of MMC through Schur complement transformation; S4, converting the equivalent admittance of grid-connected inverter into equivalent impedance, making the impedance characteristic curve of grid-connected inverter and MMC, and analyzing the stability of flexible DC transmission system considering the impedance coupling of wind power and MMC. The present application can more accurately determine the stability of flexible DC transmission system considering the impedance coupling of wind power and MMC.

Description

Technical Field

[0001] This invention belongs to the field of stability analysis technology for flexible DC transmission systems, and particularly relates to a stability analysis method for wind power transmitted through a flexible DC transmission system. Background Technology

[0002] Wind power, with its advantages of stable resource conditions and high energy efficiency, has become an important direction for new energy development in recent years. As the scale and capacity of wind power projects continue to expand, the layout of wind farms is gradually shifting from short-distance, small-capacity to clustered, large-scale operations. Flexible DC transmission technology, with its advantages of flexible control, low switching losses, and high modularity, has become one of the ideal solutions for transmitting large-scale, long-distance wind farms. However, wind power transmission systems via flexible DC transmission contain a large number of power electronic devices, and the dynamic interaction between the wind farm and the MMC (Multi-Mode Control Center) may cause broadband oscillations in the system, seriously threatening the safe and stable operation of the system.

[0003] Currently, a relatively complete theoretical framework has been established for the study of broadband oscillation problems in wind power systems connected to flexible DC transmission lines. However, existing methods for modeling the impedance of converters (MMCs) or wind turbines typically only consider the converter itself or the impedance of the AC grid to which it is connected. In AC transmission scenarios, the AC impedance / admittance of MMCs and wind turbines has been proven to be affected by grid impedance, and this effect is further aggravated by decreasing grid strength or increasing PLL bandwidth. To accurately describe the changes in impedance characteristics after coupling between MMCs or wind turbines and the AC grid, some literature analyzes the transmission process of disturbance voltage / current between grid impedance and MMC from the perspective of actual physical processes, revealing the coupling effect between MMCs and grid impedance. From a mathematical perspective, some literature has proven that the calculation of the equivalent impedance of a converter-connected system considering grid impedance coupling corresponds to the Schur complement transformation of the system admittance matrix. Building upon this, the stability of multi-converter cascaded systems (Z+Z systems, Y+Y systems) was further explored. However, as a typical Z+Y interconnected system, the dynamic interaction involved in the interconnection of wind turbines and MMCs undergoes fundamental changes in both mathematical transformations and physical processes. Mathematically, the computational complexity of the Schur complement transformation of the Z+Y interconnected system matrix increases significantly, and when multiple converters exist in the system, there is a lack of relevant theoretical guidance to correctly decompose the equivalent impedance (admittance) of each converter. Physically, compared to the equivalent impedance of the power grid expressed using electrical parameters, the converter impedance needs to be calculated and its representation is more complex, making it difficult to directly incorporate into circuit equations for calculation. Therefore, the applicability of the above methods to wind power transmission systems via flexible direct current transmission requires further investigation. Furthermore, some studies, based on control theory, equate wind power transmission systems via flexible direct current transmission to multi-input multi-output systems, establishing MMC multidimensional sequence impedance models and using the generalized Nyquist criterion to determine system stability. However, stability analysis based on the generalized Nyquist criterion struggles to provide a stability margin for the system, and the complex calculation process fails to provide guidance for controller design, making it difficult to apply to practical engineering. It is evident that existing methods have not directly addressed the key issue of the dynamic interaction between wind turbines and MMCs leading to changes in their impedance characteristics. Currently, a method for accurately analyzing the stability of systems interconnected with wind turbines and MMCs is still lacking.

[0004] In summary, how to more accurately determine the stability of flexible DC transmission systems that consider the impedance coupling between wind power and MMC has become an urgent problem to be solved. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a stability analysis method for a flexible DC transmission system that transmits wind power via a flexible DC transmission system. This method can more accurately determine the stability of a flexible DC transmission system that considers the impedance coupling between wind power and MMC.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A stability analysis method for a wind power transmission system via flexible DC transmission includes the following steps:

[0008] S1. Based on the reference direction of the flexible DC transmission system, the impedance characteristics of the grid-connected inverter and MMC are obtained by injecting current disturbance and voltage disturbance.

[0009] S2. Construct the equivalent closed-loop structures of the grid-connected inverter and MMC under current and voltage disturbances respectively, and obtain the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC, which is used to characterize the parallel admittance and series impedance of the system.

[0010] S3. Substitute the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC obtained in S1, and extract the equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC through the Schur complement transformation.

[0011] S4. After converting the equivalent admittance of the grid-connected inverter into equivalent impedance, plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC.

[0012] Preferably, in S1, the impedance characteristics of the obtained grid-connected inverter and MMC are as follows:

[0013]

[0014]

[0015] In the formula, s=jω p s -2 =jω p -j2ω1;ω p ω1 is the injected perturbation frequency, j is the imaginary unit, and ω1 is the fundamental frequency;

[0016] Δv GSC For the voltage disturbance of the grid-connected inverter, Δi GSC For the current response of the grid-connected inverter; The Y-parameter matrix describes the port characteristics of the grid-connected inverter, where the diagonal elements... These represent voltage disturbances Δv and Δv, respectively. g (f p ) response to current Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p -2f1) Current response Δi ac (f pThe transfer function of -2f1), off-diagonal elements These represent voltage disturbances Δv and Δv, respectively. g (f p -2f1) Current response Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p ) response to current Δi ac (f p The transfer function of -2f1);

[0017] Δi MMC For the current disturbance of MMC, Δv MMC The voltage response of the MMC; The Z-parameter matrix describes the characteristics of the MMC port, where the diagonal elements... These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of ) and the current disturbance Δi ac (f p -2f1) to voltage response Δv gac (f p The transfer function of -2f1), off-diagonal elements These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of -2f1) and the current disturbance Δi ac (f p -2f1) to voltage response Δv gac (f p The transfer function of ).

[0018] Preferably, in S2, the process of obtaining the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC includes:

[0019] Construct the relationship between the input vector U(s) and the output vector Y(s) of a multivariable closed-loop negative feedback system:

[0020] [I+G0(s)H(s)]Y(s)=G0(s)U(s);

[0021] In the formula, I is a unit vector, G0(s) is an open-loop gain vector, and H(s) is a feedback vector;

[0022] Transform the above formula:

[0023] Y(s) = [I + G0(s)H(s)] -1 G0(s)U(s);

[0024] The transfer function matrix of the multivariable closed-loop negative feedback system is obtained as follows:

[0025] G(s) = [I + G0(s)H(s)] -1 G0(s);

[0026] This leads to the characteristic equation of the multivariable closed-loop negative feedback system:

[0027] det[I+G0(s)H(s)]=0;

[0028] By injecting a current disturbance into a flexible DC transmission system considering the impedance coupling between wind power and MMC, the characteristic equation of the system's parallel admittance is obtained:

[0029] det[I+Z MMC (s)Y GSC [(s)] = 0;

[0030] In the formula, Z MMC (s) is the Z-parameter matrix describing the impedance characteristics of the MMC, Y GSC (s) is the Y-parameter matrix describing the admittance characteristics of the grid-connected inverter;

[0031] By injecting a voltage disturbance into a flexible DC transmission system that considers the impedance coupling between wind power and MMC, the characteristic equation of the system's series impedance is obtained:

[0032] det[I+Y GSC (s)Z MMC [(s)] = 0.

[0033] Preferably, S3 includes:

[0034] Substituting the characteristic equation of the parallel admittance of the system obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC, we get:

[0035]

[0036] Then, through the Schur complement transformation, we obtain:

[0037]

[0038] when Then, the above equation is equivalent to:

[0039]

[0040] The equivalent admittance of the grid-connected inverter extracted through the Schur complement transformation is obtained as follows:

[0041]

[0042] Similarly, substituting the characteristic equation of the system's series impedance obtained from S2 into the impedance characteristics of the grid-connected inverter and MMC, and then using the Schur complement transformation, we obtain the equivalent impedance of the MMC extracted through the Schur complement transformation:

[0043]

[0044] Preferably, S4 includes:

[0045] S41. The equivalent admittance of the grid-connected inverter is converted into equivalent impedance using the following formula:

[0046] Z GSC =1 / Y GSC ;

[0047] S42. Plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC.

[0048] Preferably, in S4, when analyzing the stability of the flexible DC transmission system considering the coupling between wind power and MMC impedance, if the amplitude-frequency characteristics of the grid-connected inverter and the MMC impedance characteristic curves intersect, and the phase angle difference corresponding to the intersection point exceeds 180 degrees, then the system is unstable; otherwise, the system is stable.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] 1. Using this method, based on the reference direction of the flexible DC transmission system, the impedance characteristics of the grid-connected inverter and MMC are obtained by injecting current and voltage disturbances. Then, equivalent closed-loop structures of the grid-connected inverter and MMC under current and voltage disturbances are constructed respectively, yielding the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC. This equation characterizes the parallel admittance and series impedance of the system. Next, the obtained characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC is substituted into the impedance characteristics of the grid-connected inverter and MMC. The equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC are extracted through Schur complement transformation. Finally, the equivalent admittance of the grid-connected inverter is converted into equivalent impedance, and the impedance characteristic curves of the grid-connected inverter and MMC are plotted to analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and MMC.

[0051] Compared to existing simplified models for evaluating system stability, this method can more accurately determine the stability of flexible DC transmission systems that consider the impedance coupling between wind power and MMC. Furthermore, this method can directly utilize the Nyquist criterion for determination, making it convenient and intuitive to operate, and possessing potential for engineering applications.

[0052] In summary, this invention can more accurately determine the stability of flexible DC transmission systems that take into account the impedance coupling between wind power and MMC.

[0053] 2. This invention provides the characteristic equations of parallel admittance and series impedance of a multivariable closed-loop negative feedback system, as well as the specific construction process of these equations. It also provides the process of extracting the equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC through the Schur complement transformation, which can ensure the accuracy and effectiveness of the method and process operation.

[0054] 3. This invention provides a specific method for analyzing the stability of flexible DC transmission systems considering the impedance coupling between wind power and MMC. If the amplitude-frequency characteristics of the grid-connected inverter and the MMC impedance characteristic curves intersect at a point, and the phase angle difference corresponding to the intersection point exceeds 180 degrees, the system is unstable; otherwise, the system is stable. This analysis method allows for a convenient and more accurate determination of the stability of flexible DC transmission systems considering the impedance coupling between wind power and MMC. Attached Figure Description

[0055] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0056] Figure 1 The flowchart in the embodiment is shown below;

[0057] Figure 2 This is a structural diagram of the multivariable closed-loop negative feedback system in the embodiment;

[0058] Figure 3 This is an equivalent closed-loop structure diagram of the grid-connected inverter and MMC under current disturbance in the embodiment.

[0059] Figure 4 This is an equivalent closed-loop structure diagram of the grid-connected inverter and MMC under voltage disturbance in the embodiment. Detailed Implementation

[0060] The following detailed explanation illustrates the specific implementation methods:

[0061] Example:

[0062] like Figure 1 As shown in the figure, this embodiment discloses a stability analysis method for a wind power transmission system via flexible DC transmission, including the following steps:

[0063] S1. Based on the reference direction of the flexible DC transmission system, the impedance characteristics of the grid-connected inverter and MMC are obtained by injecting current disturbance and voltage disturbance.

[0064] In practical implementation, based on the reference direction of the flexible DC transmission system, and through the injection of current and voltage disturbances, combined with circuit principles, the impedance characteristics of the grid-connected inverter and the MMC can be expressed as follows:

[0065]

[0066]

[0067] In the formula, s=jω p s -2 =jω p -j2ω1;ω p ω1 is the injected perturbation frequency, j is the imaginary unit, and ω1 is the fundamental frequency;

[0068] Δv GSC For the voltage disturbance of the grid-connected inverter, Δi GSC For the current response of the grid-connected inverter; The Y-parameter matrix describes the port characteristics of the grid-connected inverter, where the diagonal elements... These represent voltage disturbances Δv and Δv, respectively. g (f p ) response to current Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p -2f1) Current response Δi ac (f p The transfer function of -2f1), off-diagonal elements These represent voltage disturbances Δv and Δv, respectively. g (f p -2f1) Current response Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p ) response to current Δi ac (f p The transfer function of -2f1);

[0069] Δi MMC For the current disturbance of MMC, Δv MMC The voltage response of the MMC; The Z-parameter matrix describes the characteristics of the MMC port, where the diagonal elements... These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of ) and the current disturbance Δi ac (fp -2f1) to voltage response Δv gac (f p The transfer function of -2f1), off-diagonal elements These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of -2f1) and the current disturbance Δi ac (f p -2f1) to voltage response Δv gac (f p The transfer function of ).

[0070] S2. Construct the equivalent closed-loop structures of the grid-connected inverter and MMC under current and voltage disturbances respectively, and obtain the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC, which is used to characterize the parallel admittance and series impedance of the system.

[0071] In practice, the process of obtaining the characteristic equations of a flexible DC transmission system that considers the impedance coupling between wind power and MMC includes:

[0072] Construct the relationship between the input vector U(s) and the output vector Y(s) of a multivariable closed-loop negative feedback system:

[0073] [I+G0(s)H(s)]Y(s)=G0(s)U(s);

[0074] In the formula, I is a unit vector, G0(s) is the open-loop gain vector, and H(s) is the feedback vector; the structure of the multivariable closed-loop negative feedback system is as follows: Figure 2 As shown.

[0075] Transform the above formula:

[0076] Y(s) = [I + G0(s)H(s)] -1 G0(s)U(s);

[0077] The transfer function matrix of the multivariable closed-loop negative feedback system is obtained as follows:

[0078] G(s) = [I + G0(s)H(s)] -1 G0(s);

[0079] This leads to the characteristic equation of the multivariable closed-loop negative feedback system:

[0080] det[I+G0(s)H(s)]=0;

[0081] Based on different disturbance injection methods and considering the differences in impedance characteristics between the grid-connected inverter and the MMC, equivalent closed-loop structures for the grid-connected inverter and the MMC under current and voltage disturbances are constructed respectively, as shown below. Figure 3 and Figure 4 As shown.

[0082] By injecting a current disturbance into a flexible DC transmission system considering the impedance coupling between wind power and MMC, the characteristic equation of the system's parallel admittance is obtained:

[0083] det[I+Z MMC (s)Y GSC [(s)] = 0;

[0084] In the formula, Z MMC (s) is the Z-parameter matrix describing the impedance characteristics of the MMC, Y GSC (s) is the Y-parameter matrix describing the admittance characteristics of the grid-connected inverter;

[0085] By injecting a voltage disturbance into a flexible DC transmission system that considers the impedance coupling between wind power and MMC, the characteristic equation of the system's series impedance is obtained:

[0086] det[I+Y GSC (s)Z MMC [(s)] = 0.

[0087] S3. Substitute the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC obtained in S1, and extract the equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC through the Schur complement transformation.

[0088] In practical implementation, based on the aforementioned characteristic equations of the flexible DC transmission system considering the impedance coupling between wind power and MMC, the characteristic equations of the parallel admittance of the system obtained in S2 are substituted into the impedance characteristics of the grid-connected inverter and MMC, resulting in:

[0089]

[0090] Then, through the Schur complement transformation, we obtain:

[0091]

[0092] when Then, the above equation is equivalent to:

[0093]

[0094] The equivalent admittance of the grid-connected inverter extracted through the Schur complement transformation is obtained as follows:

[0095]

[0096] Similarly, substituting the characteristic equation of the system series impedance obtained from S2 based on the injected voltage disturbance into the impedance characteristics of the grid-connected inverter and MMC, and then using the Schur complement transformation, we obtain the equivalent impedance of the MMC extracted by the Schur complement transformation:

[0097]

[0098] S4. After converting the equivalent admittance of the grid-connected inverter into equivalent impedance, plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC.

[0099] In practice, S4 includes:

[0100] S41. The equivalent admittance of the grid-connected inverter is converted into equivalent impedance using the following formula:

[0101] Z GSC =1 / Y GSC ;

[0102] S42. Plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC. Specifically, when analyzing the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC, if the amplitude-frequency characteristics of the impedance characteristic curves of the grid-connected inverter and the MMC intersect at a point, and the phase angle difference corresponding to the intersection point exceeds 180 degrees, then the system is unstable; otherwise, the system is stable.

[0103] Using this method, based on the reference direction of the flexible DC transmission system, the impedance characteristics of the grid-connected inverter and MMC are obtained by injecting current and voltage disturbances. Then, equivalent closed-loop structures of the grid-connected inverter and MMC under current and voltage disturbances are constructed respectively, yielding the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and the MMC. This equation characterizes the system's parallel admittance and series impedance. Next, the obtained characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and the MMC is substituted into the impedance characteristics of the grid-connected inverter and the MMC, and the equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC are extracted through Schur complement transformation. Finally, after converting the equivalent admittance of the grid-connected inverter into equivalent impedance, the impedance characteristic curves of the grid-connected inverter and the MMC are plotted, and the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC is analyzed. Compared with existing simplified models for evaluating system stability, this method can more accurately determine the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC. Furthermore, this method can be directly used to make judgments using the Nyquist criterion, making it convenient and intuitive to operate and possessing potential for engineering applications.

[0104] Furthermore, this invention provides the characteristic equations for the parallel admittance and series impedance of a multivariable closed-loop negative feedback system, as well as the specific construction process for these equations. It also details the extraction of the grid-connected inverter's equivalent admittance and the MMC's equivalent impedance via Schur complement transformation, ensuring the accuracy and effectiveness of the method's operational flow. In addition, this invention provides a specific method for analyzing the stability of flexible DC transmission systems considering the coupling between wind power and MMC impedance. If the amplitude-frequency characteristics of the grid-connected inverter and MMC impedance characteristic curves intersect, and the phase angle difference corresponding to the intersection exceeds 180 degrees, the system is unstable; otherwise, the system is stable. This analytical method allows for a convenient and more accurate determination of the stability of flexible DC transmission systems considering the coupling between wind power and MMC impedance.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A stability analysis method for a wind power transmission system via flexible DC transmission, characterized in that, Includes the following steps: S1. Based on the reference direction of the flexible DC transmission system, the impedance characteristics of the grid-connected inverter and MMC are obtained by injecting current disturbance and voltage disturbance. S2. Construct the equivalent closed-loop structures of the grid-connected inverter and MMC under current and voltage disturbances respectively, and obtain the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC, which is used to characterize the parallel admittance and series impedance of the system. S3. Substitute the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC obtained in S1, and extract the equivalent admittance of the grid-connected inverter and the equivalent impedance of the MMC through the Schur complement transformation. S4. After converting the equivalent admittance of the grid-connected inverter into equivalent impedance, plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and MMC. In S1, the impedance characteristics of the grid-connected inverter and the MMC are as follows: ； ； In the formula, s=jω p , =jω p -j2ω1;ω p ω1 is the injected perturbation frequency, j is the imaginary unit, and ω1 is the fundamental frequency; Δv GSC For voltage disturbances in grid-connected inverters. For the current response of the grid-connected inverter; Y GSC (s)= - , The Y-parameter matrix describes the port characteristics of the grid-connected inverter, where the diagonal elements... , These represent voltage disturbances Δv and Δv, respectively. g (f p ) response to current Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p -2f1) Current response Δi ac (f p The transfer function of -2f1), off-diagonal elements , These represent voltage disturbances Δv and Δv, respectively. g (f p -2f1) Current response Δi ac (f p The transfer function of ) and voltage disturbance Δv g (f p ) response to current Δi ac (f p The transfer function of -2f1); For the current disturbance of MMC, Δv MMC For the voltage response of MMC; Z MMC (s)= , The Z-parameter matrix describes the characteristics of the MMC port, where the diagonal elements... , These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of ) and the current disturbance Δi ac (f p -2f1) to voltage response Δv gac (f p The transfer function of -2f1), off-diagonal elements , These represent the current disturbance Δi, respectively. ac (f p ) to voltage response Δv gac (f p The transfer function of -2f1) and the current disturbance Δi ac (f p -2f1) to voltage response Δv gac (f p The transfer function of ). S4 includes: S41. The equivalent admittance of the grid-connected inverter is converted into equivalent impedance using the following formula: Z GSC =1 / Y GSC ; In the formula, Z GSC Y is the equivalent impedance of the grid-connected inverter. GSC The equivalent admittance of the grid-connected inverter; S42. Plot the impedance characteristic curves of the grid-connected inverter and the MMC, and analyze the stability of the flexible DC transmission system considering the impedance coupling between wind power and the MMC.

2. The stability analysis method for a wind power transmission system via flexible DC transmission as described in claim 1, characterized in that: In S2, the process of obtaining the characteristic equation of the flexible DC transmission system considering the impedance coupling between wind power and MMC includes: Construct the relationship between the input vector U(s) and the output vector Y(s) of a multivariable closed-loop negative feedback system: ； In the formula, I is a unit vector. H(s) is the open-loop gain vector, and H(s) is the feedback vector. Transform the above formula: ； The transfer function matrix of the multivariable closed-loop negative feedback system is obtained as follows: ； This leads to the characteristic equation of the multivariable closed-loop negative feedback system: ； By injecting a current disturbance into a flexible DC transmission system considering the impedance coupling between wind power and MMC, the characteristic equation of the system's parallel admittance is obtained: ； In the formula, Z MMC (s) is the Z-parameter matrix describing the impedance characteristics of the MMC, Y GSC (s) is the Y-parameter matrix describing the admittance characteristics of the grid-connected inverter; By injecting a voltage disturbance into a flexible DC transmission system that considers the impedance coupling between wind power and MMC, the characteristic equation of the system's series impedance is obtained: 。 3. The stability analysis method for a wind power transmission system via flexible DC transmission as described in claim 2, characterized in that: S3 includes: Substituting the characteristic equation of the parallel admittance of the system obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC, we get: ； Then, through the Schur complement transformation, we obtain: ； when Then, the above equation is equivalent to: ； The equivalent admittance of the grid-connected inverter extracted through the Schur complement transformation is obtained as follows: 。 4. The stability analysis method for a wind power transmission system via flexible DC transmission as described in claim 3, characterized in that: S3 also includes: substituting the characteristic equation of the system's series impedance obtained in S2 into the impedance characteristics of the grid-connected inverter and MMC, and then obtaining the equivalent impedance of the MMC extracted by the Schur complement transformation: 。 5. The stability analysis method for a wind power transmission system via flexible DC transmission as described in claim 1, characterized in that: In S4, when analyzing the stability of a flexible DC transmission system considering the impedance coupling between wind power and MMC, if the amplitude-frequency characteristics of the grid-connected inverter and the MMC impedance characteristic curves intersect, and the phase angle difference corresponding to the intersection point exceeds 180 degrees, the system is unstable; otherwise, the system is stable.

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