Offshore wind power flexible direct current grid-connected system stability analysis method and related device

Abstract

The invention discloses a stability analysis method for an offshore wind power flexible direct current grid-connected system and a related device. The method comprises the following steps: acquiring an impedance model of each subsystem in a first interconnection system of an offshore wind power plant formed by a plurality of subsystems through flexible direct current grid connection; determining a plurality of different segmentation modes used for segmenting the first interconnection system into a plurality of second interconnection systems composed of two subsystems, and obtaining an impedance ratio matrix of each second interconnection system; and estimating the distribution condition of the characteristic root locus of the impedance ratio matrix corresponding to each second interconnection system, and analyzing the stability of the second interconnection system according to the distribution condition of the characteristic root locus. And taking the analysis result corresponding to the second interconnection system with the minimum stability margin as the stability analysis result of the operation of the offshore wind plant through the flexible direct current grid-connected system, thereby solving the problems that the influence of different segmentation points on the stability result is not considered in the prior art, and tedious characteristic value calculation needs to be carried out.

Description

Technical Field

[0001] The present application relates to the technical field of wind power generation, and in particular to a stability analysis method and related devices for an offshore wind power system connected to a flexible direct current grid. Background Art

[0002] The offshore wind power grid-connected system via flexible direct current is a high-proportion power electronic power system dominated by power electronic control, including wind power converters, modular multi-level converters and other power electronic equipment, and with multi-time scale dynamic response characteristics. In recent years, a series of electromagnetic oscillation phenomena caused by power electronic converters have occurred in many actual projects at home and abroad during operation or commissioning, which seriously affected the reliable absorption of large-scale offshore wind power and threatened the safe and stable operation of the power system. Therefore, it is of great significance to conduct in-depth research on the stability analysis of the offshore wind power grid-connected system via flexible direct current.

[0003] The existing stability analysis methods for offshore wind farms connected to the grid via flexible DC mainly include eigenvalue analysis and impedance analysis. Among them, the eigenvalue analysis method can accurately obtain the unstable resonance point and accurate oscillation information of the entire system. However, the offshore wind farm connected to the grid via flexible DC contains a large number of controllers and delay links, which play a dominant role in the stability of the system. As a result, a large number of control equations need to be considered when building the state space equation of the system, and the dimension of the state space equation of the system increases accordingly, and the difficulty of solving the eigenvalue also increases accordingly. In addition, this method requires real-time acquisition of the operating data of each component in the system. When the working conditions change, the state space equation needs to be rebuilt and the characteristic roots need to be re-solved. The whole process is relatively cumbersome and difficult to adapt to the complex and changeable new power system. In recent years, the impedance analysis method has gradually gained favor among researchers. However, most of the existing impedance analysis methods ignore the frequency coupling effect of the subsystem, and this method requires the calculation of the eigenvalues ​​of the impedance matrix at each frequency, which is computationally intensive.

[0004] In summary, the shortcomings of the prior art can be summarized as follows: first, the influence of different segmentation points on the stability result is not considered; second, cumbersome eigenvalue calculation is required. Summary of the invention

[0005] The present application provides a stability analysis method and related devices for an offshore wind power flexible direct current grid-connected system, which are used to solve the problem that the prior art does not consider the impact of different segmentation points on the stability results and requires cumbersome eigenvalue calculations.

[0006] In view of this, the first aspect of the present application provides a stability analysis method for an offshore wind power system connected to a flexible direct current grid, the method comprising:

[0007] The impedance model of each subsystem in the first interconnected system of the offshore wind farm connected to the flexible direct current grid, which is composed of multiple subsystems, is obtained by using the swept frequency impedance method;

[0008] Determine a plurality of different segmentation modes based on the topology of the first interconnected system, for respectively segmenting the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and respectively obtain an impedance ratio matrix of each of the second interconnected systems, wherein each segmentation mode includes a corresponding segmentation point;

[0009] The Gershgorin circle theorem is used to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems. The stability of the second interconnected systems is analyzed according to the distribution of the characteristic root loci, and the analysis result corresponding to the second interconnected system with the smallest stability margin is used as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

[0010] Optionally, the method of obtaining the impedance model of each subsystem in a first interconnected system consisting of multiple subsystems of an offshore wind farm connected to the grid via flexible direct current by using a swept frequency impedance method includes:

[0011] After injecting a small disturbance voltage of a positive sequence frequency into the first interconnected system, sampling voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain values ​​of a positive sequence current and a coupling current under the small disturbance voltage of the positive sequence frequency as first column data constituting a two-dimensional impedance matrix;

[0012] After injecting a small disturbance voltage of a negative sequence frequency into the first interconnected system, sampling voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain values ​​of negative sequence current and coupling current under the small disturbance voltage of the negative sequence frequency as the second column data constituting the two-dimensional impedance matrix;

[0013] The impedance model is obtained by forming a two-dimensional impedance matrix through the first column of data and the second column of data.

[0014] Optionally, the determining of several different segmentation modes based on the topology of the first interconnected system is used to respectively segment the first interconnected system into several second interconnected systems each consisting of two subsystems, and respectively obtain an impedance ratio matrix of each second interconnected system, wherein each segmentation mode includes a corresponding segmentation point, including:

[0015] Based on the topology of the first interconnected system, several different division methods are determined, and the multiple subsystems in the first interconnected system are divided into several two subsystems through each of the division methods to form several second interconnected systems. According to the impedance model, the impedance matrices of the two subsystems are respectively solved, and the impedance ratio matrices of the second interconnected systems are formed according to the solved impedance matrices.

[0016] Optionally, the using of the Gershgorin circle theorem to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems, analyzing the stability of the second interconnected systems according to the distribution of the characteristic root loci, and taking the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system, includes:

[0017] Based on the Gershgorin circle theorem, the center of each circle in the impedance ratio matrix is ​​calculated and the center Distance to point (-1, j0) and distance ;

[0018] According to the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems;

[0019] A stability margin is obtained according to the stability analysis results corresponding to each of the second interconnected systems, and the analysis result corresponding to the second interconnected system with the smallest stability margin is used as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

[0020] Optionally, the impedance matrix obtained by solving the problem forms several impedance ratio matrices of the second interconnected system, including:

[0021] Substituting each of the impedance matrices into an impedance ratio matrix formula, respectively, to calculate and obtain several impedance ratio matrices of the second interconnection system;

[0022] Wherein, the impedance ratio matrix formula is:

[0023]

[0024] In the formula, is the impedance ratio matrix, and is the impedance matrix of the two subsystems.

[0025] Optionally, the calculation of the center of each circle in the impedance ratio matrix and the center Distance to point (-1, j0) and distance ,

[0026] Based on the distance calculation formula, calculate the center of each circle in the impedance ratio matrix and the center Distance to point (-1, j0) and distance ;

[0027] Wherein, the distance calculation formula is:

[0028] .

[0029] Optionally, the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems includes:

[0030] Design several criteria, and based on the several criteria, according to the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems;

[0031] Among them, several of the criteria include:

[0032] The first criterion: When the center L 11 (f p ) and the center L 22 (f p ) The trajectory does not surround the point (-1, j0) and satisfies S 11 >|L 12 (f p )| and S 22 >|L 21 (f p )|, that is, the center trajectory does not surround (-1, j0) and all circular regions do not contain (-1, j0), at this time the system operates stably;

[0033] The second criterion: When the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable;

[0034] The third criterion: When the center L 11 (f p ) trajectory surrounds the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L 21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable;

[0035] The fourth criterion: When the center L 11 (f p ) or L 22 (f p ) trajectory surrounds the point (-1, j0), and satisfies S 11 >|L 12 (f p )|or S 22 >|L 21 (f p )|, that is, the characteristic root locus of the impedance ratio matrix will surround the point (-1, j0), and the system is unstable at this time.

[0036] A second aspect of the present application provides a stability analysis system for an offshore wind power flexible direct current grid-connected system, the system comprising:

[0037] A first acquisition unit is used to acquire, by using a swept frequency impedance method, an impedance model of each subsystem in a first interconnected system consisting of a plurality of subsystems of an offshore wind farm connected to the flexible direct current grid;

[0038] a second acquisition unit, configured to determine a plurality of different segmentation modes based on the topology of the first interconnected system, to respectively segment the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and to respectively acquire an impedance ratio matrix of each of the second interconnected systems, wherein each segmentation mode includes a corresponding segmentation point;

[0039] The analysis unit is used to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems by using the Gershgorin circle theorem, analyze the stability of the second interconnected systems according to the distribution of the characteristic root loci, and use the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

[0040] A third aspect of the present application provides a stability analysis device for an offshore wind power system connected to a flexible direct current grid, the device comprising a processor and a memory:

[0041] The memory is used to store program code and transmit the program code to the processor;

[0042] The processor is used to execute the steps of the offshore wind power flexible direct current grid-connected system stability analysis method as described in the first aspect according to the instructions in the program code.

[0043] A fourth aspect of the present application provides a computer-readable storage medium, wherein the computer-readable storage medium is used to store program code, and the program code is used to execute the offshore wind power flexible direct current grid-connected system stability analysis method described in the first aspect above.

[0044] It can be seen from the above technical solutions that this application has the following advantages:

[0045] The present application provides a method for analyzing the stability of an offshore wind power system connected to the flexible direct current grid. First, the impedance model of each part of the system is obtained by using measured data, which can adapt to the complex and changeable working conditions of the MMC converter and the wind farm station, so as to quickly and accurately obtain the impedance information of the MMC converter and the wind farm station in the power grid. Further, the frequency coupling effect of the system is considered when building the impedance model, and the impedance information of each part is represented by a two-dimensional impedance matrix. Then, different splitting points are taken to divide the system into two subsystems, and the influence of different splitting points on the stability analysis results of the whole system is considered to avoid the deviation of the stability analysis results caused by inappropriate selection of splitting points. Finally, the Gershgorin circle theorem is used to estimate the distribution of the characteristic root trajectory of the system impedance ratio matrix, and the corresponding stability analysis results are proposed for the four possible situations in the circular domain. Further, the minimum stability margin of the system under various splitting methods is used to obtain conservative stability analysis results. It avoids the disadvantage of needing to solve a large number of matrix characteristic roots and simplifies the calculation process. Thereby solving the problem that the prior art does not consider the influence of different splitting points on the stability results and needs to perform cumbersome eigenvalue calculations. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1A schematic flow chart of a method for analyzing the stability of an offshore wind power system connected to a flexible direct current grid provided in an embodiment of the present application;

[0047] Figure 2 A topological diagram of a multi-wind farm flexible DC grid-connected system provided in an embodiment of the present application;

[0048] Figure 3 The impedance matrix amplitude-frequency characteristic diagram of each part provided in the embodiment of the present application;

[0049] Figure 4 A Thevenin equivalent circuit diagram of an interconnection system provided in an embodiment of the present application;

[0050] Figure 5 A schematic diagram of impedance matrix calculation corresponding to the four segmentation methods provided in the embodiments of the present application;

[0051] Figure 6 It is the Gershgorin circle domain diagram corresponding to the segmentation method 1 provided in the embodiment of the present application;

[0052] Figure 7a is the center L of the circle in the four segmentation methods provided in the embodiments of the present application 11 trajectories;

[0053] Figure 7b is the center L of the circle in the four segmentation methods provided in the embodiments of the present application 22 trajectories;

[0054] Figure 8 This is a structural schematic diagram of a stability analysis system for an offshore wind power flexible DC grid-connected system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0055] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0056] See also Figure 1 A method for analyzing the stability of an offshore wind power system connected to a flexible direct current grid provided in an embodiment of the present application includes:

[0057] Step 101: using a swept frequency impedance method, obtain an impedance model of each subsystem in a first interconnected system consisting of multiple subsystems in which an offshore wind farm is connected to a flexible direct current grid.

[0058] It should be noted that the offshore wind farm is an interconnected system through the flexible DC grid-connected system. For the convenience of subsequent description, this application is described as the first interconnected system of the offshore wind farm through the flexible DC grid-connected system. Since the operation of the MMC converter and the direct-drive wind turbine in the offshore wind farm through the flexible DC grid-connected system is complex and changeable, this embodiment uses the swept frequency impedance method to obtain the impedance model of each subsystem in the first interconnected system. The specific implementation method is: when the first interconnected system is operating normally, the port harmonic voltage is read after injecting harmonic current into the AC outlet side of the MMC converter, and the equivalent impedance of the system at each frequency is obtained by solving the relationship between the harmonic voltage and the harmonic current, where the quotient of the harmonic voltage amplitude and the harmonic current amplitude is the amplitude of the impedance corresponding to the frequency, and the phase angle difference is the phase value of the impedance.

[0059] Step 102: determine several different segmentation methods based on the topology of the first interconnected system, for segmenting the first interconnected system into several second interconnected systems each consisting of two subsystems, and respectively obtain an impedance ratio matrix of each second interconnected system, wherein each segmentation method includes a corresponding segmentation point.

[0060] It can be understood that, in this embodiment, several different segmentation points, i.e., segmentation modes, are determined based on the topology of the first interconnected system, so as to segment the first interconnected system of the offshore wind farm originally composed of multiple subsystems and connected to the flexible direct current grid into a second interconnected system composed of two interconnected subsystems. Since there are several segmentation modes, there are also several obtained second interconnected systems, and according to the impedance matrix obtained in step 101, the impedance ratio matrix of each second interconnected system is obtained respectively.

[0061] Step 103: Use the Gershgorin circle theorem to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each second interconnected system, analyze the stability of the second interconnected system according to the distribution of the characteristic root loci, and use the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

[0062] It should be noted that the basic principle of estimating the impedance ratio matrix using the Gershgorin circle theorem is as follows:

[0063] With a 2D matrix For example, the two eigenvalues ​​λ1 and λ2 of the matrix must be located on the diagonal elements L 11 and L 22 is the center of the circle, and the non-diagonal element L 12 and L 21 In two Gershgorin circle domains with radius , the relationship between the impedance ratio matrix elements and eigenvalues ​​can be expressed as:

[0064]

[0065] The impedance ratio matrix of the interconnected system constructed in step 102 is calculated using the above method, and the circular domain corresponding to the characteristic roots of the matrix at different frequencies is estimated for subsequent analysis of the stability of the second interconnected system.

[0066] In one embodiment, step 101 includes:

[0067] Step 1011: after injecting a small disturbance voltage of a positive sequence frequency into the first interconnected system, sampling the voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain the values ​​of the positive sequence current and the coupling current under the small disturbance voltage of the positive sequence frequency as the first column data constituting the two-dimensional impedance matrix;

[0068] It should be noted that this embodiment takes into account the frequency coupling effect of the system, so a small disturbance voltage of the positive sequence frequency is first injected into the first interconnected system. , and detect the voltage and current values ​​of the voltage and current sensors of each subsystem, and obtain the Positive sequence current under disturbance and coupling current The value of , thus obtaining the first column of data of the two-dimensional impedance matrix of each subsystem:

[0069]

[0070]

[0071] In the formula, the superscripts of voltage and current represent the voltage and current values ​​obtained by the sensors of each subsystem, for example and The frequencies detected at MMC converter and wind farm i are f p The positive sequence voltage value is , where i = 1, 2, 3. The subscripts represent the positive and negative sequence components, “p” is the positive sequence component and “n” is the negative sequence component.

[0072] Step 1012: after injecting a small disturbance voltage of a negative sequence frequency into the first interconnected system, sampling the voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain the values ​​of the negative sequence current and the coupling current under the small disturbance voltage of the negative sequence frequency as the second column data constituting the two-dimensional impedance matrix;

[0073] It should be noted that, similar to the method in step 1011, a small disturbance voltage with a negative sequence frequency is injected into the first interconnected system. , and detect the voltage and current values ​​of the voltage and current sensors of each subsystem, and obtain them through Fourier transform Negative sequence current under disturbance and coupling current The value of is obtained to obtain the second column of the two-dimensional impedance matrix of each subsystem and merge it with the above matrix to obtain:

[0074]

[0075]

[0076] In the formula, the meanings of the superscripts and subscripts of the variables are the same as those in step 1011.

[0077] Step 1013: construct a two-dimensional impedance matrix using the first column of data and the second column of data, thereby obtaining an impedance model.

[0078] It can be understood that step 1011 and step 1012 have obtained the first column data and the second column data of the two-dimensional impedance matrix, so the two-dimensional impedance matrix is ​​obtained, as shown in step 1012, that is, the impedance model is obtained.

[0079] In one embodiment, step 102 includes:

[0080] Step 1021: Determine a plurality of different segmentation modes based on the topology of the first interconnected system, and segment the plurality of subsystems in the first interconnected system into a plurality of two subsystems using each segmentation mode, thereby forming a plurality of second interconnected systems;

[0081] It should be noted that the difference in the segmentation point will directly affect the result of the system stability analysis, so this embodiment will select different segmentation points to segment the subsystem, and then compare the stability of each segmentation situation. Step 101 has obtained the impedance model (impedance matrix) of the first interconnected system (MMC converter and each wind farm) by the swept frequency impedance method, and then this step determines several different segmentation methods through the topology of the first interconnected system, and the multi-terminal interconnected system (first interconnected system) is equivalent to a system in which two different subsystems are interconnected to obtain a second interconnected system. Since there are several segmentation methods, there are also several corresponding second interconnected systems.

[0082] Step 1022: Solve several impedance matrices of the two subsystems according to the impedance model, substitute each impedance matrix into the impedance ratio matrix formula, and calculate and obtain several impedance ratio matrices of the second interconnected system.

[0083] Among them, the impedance ratio matrix formula is:

[0084]

[0085] In the formula, is the impedance ratio matrix, and is the impedance matrix of the two subsystems.

[0086] It should be noted that the impedance matrices of the two subsystems in step 1021 are respectively and Furthermore, since the offshore wind farm connected to the grid via flexible DC is a typical "Z+Y" type interconnected system, the impedance ratio matrix of the interconnected system under different segmentation modes is constructed through the impedance ratio matrix formula. .

[0087] In one embodiment, step 103 includes:

[0088] Step 1031: Calculate the center of each impedance ratio matrix based on the distance calculation formula and the center Distance to point (-1, j0) and distance ;

[0089] The distance calculation formula is:

[0090]

[0091] It should be noted that according to the generalized Nyquist curve criterion, if the characteristic root locus of the impedance ratio matrix of the interconnected system does not surround the point (-1, j0), it proves that the system is stable. Therefore, when this embodiment uses the characteristic value circle domain obtained by the Gershgorin circle theorem to perform stability analysis, the distance from the center of each Gershgorin circle domain to the point (-1, j0) is first calculated. For the two-dimensional impedance ratio matrix of this embodiment, that is, solving the center and the center Distance to point (-1, j0) and distance The above step 103 has introduced the basic principle of estimating the impedance ratio matrix using the Gershgorin circle theorem, which will not be repeated here.

[0092] Step 1032: design several criteria, and based on the several criteria, and distance Analyze the stability of the second interconnected systems to obtain stability analysis results corresponding to each second interconnected system;

[0093] Among them, several criteria include:

[0094] The first criterion: When the center L 11 (f p ) and the center L 22 (f p ) The trajectory does not surround the point (-1, j0) and satisfies S 11 >|L12 (f p )| and S 22 >|L 21 (f p )|, that is, the center trajectory does not surround (-1, j0) and all circular regions do not contain (-1, j0), at this time the system operates stably;

[0095] The second criterion: When the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L 21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable;

[0096] The third criterion: When the center L 11 (f p ) trajectory surrounds the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L 21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable;

[0097] The fourth criterion: When the center L 11 (f p ) or L 22 (f p ) trajectory surrounds the point (-1, j0), and satisfies S 11 >|L 12 (f p )|or S 22 >|L 21 (f p )|, that is, the characteristic root locus of the impedance ratio matrix will surround the point (-1, j0), at which point the system is unstable;

[0098] Step 1033: obtain a stability margin according to the stability analysis results corresponding to each second interconnected system, and use the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating via the flexible DC grid-connected system.

[0099] It should be noted that, according to step 1031 and step 1032, by using the Gershgorin circle theorem and the interconnection system impedance ratio matrix L(f p ) After the stability analysis of the interconnected system is realized. Further, since the stability analysis results under different partitioning modes are different, this embodiment will regard the analysis result corresponding to the partitioning mode with the smallest stability margin as the stability result of the system operation, thereby improving the conservatism of the stability analysis result.

[0100] The following is a simulation example provided for this application:

[0101] In PSCAD / EMTDC platform, Figure 2 The multi-wind farm shown is simulated and verified by the electromagnetic transient model of the flexible DC grid-connected system, and its main parameters are shown in Table 1. The topology includes three wind farms WF1, WF2 and WF3, and Ci (i=1,2,3,4) represents each section of the cable line.

[0102] Table 1 Main parameters of wind farm and flexible DC system

[0103]

[0104] According to the method provided in the present application, a specific frequency f is first injected in series at the interface between the wind farm and the flexible DC converter. p The small disturbance voltage signal of p Voltage signal and negative sequence (f p - 2f1) voltage signal, and calculate the two-dimensional impedance matrix of the MMC converter and each wind farm station according to the formulas of steps 1021 and 1022 , , and , repeat the above steps to obtain the impedance matrix at each frequency. The impedance information is as follows: Figure 3 shown.

[0105] Secondly, the flexible DC system of multiple wind farms is divided according to the method from 1021 to 1022, and the whole system is divided into two interconnected subsystems. According to the system topology of this embodiment, four different division points are selected to divide the system. First, Figure 2 The topological structure of is represented by the Thevenin equivalent circuit, as Figure 4 As shown. The calculation formulas for each segmentation method and the corresponding impedance ratio matrix are as follows Figure 5 shown.

[0106] Then, the impedance ratio matrix under a certain segmentation mode is subjected to stability analysis according to the method from step 1031 to step 1033, and the stability analysis of the flexible DC grid-connected system of multiple wind farms is realized through the determination method of step 1032. Figure 6 As shown, the Gershgorin circle domain with point A1 as the dividing point is analyzed as an example. It can be seen from the figure that the center trajectory does not surround (-1, j0) and the radius of the circle domain is also smaller than the distance from the center of the circle to the point (-1, j0), which meets the first criterion of the situation in step 1032, and the system runs stably.

[0107] Finally, the impedance ratio matrices obtained by the other three segmentation methods are analyzed using the above method to obtain four groups of Gershgorin circle domain diagrams. In order to more clearly show the differences between the four segmentation methods, Figure 7a and Figure 7b Only the center trajectory of each segmentation method is given for comparison, among which, Figure 7a is the center L of the circle under four segmentation methods 11 The trajectory of Figure 7b is the center L of the circle under four segmentation methods 22 By comparing the Gershgorin circles corresponding to the various segmentation methods in this embodiment, it can be seen that the different segmentation points directly affect the distribution of the Gershgorin circle domain. The center trajectory corresponding to the segmentation point A1 is closer to the point (-1, j0), and the stability margin of the system is the smallest. Therefore, the stability result obtained by this segmentation method is regarded as the stability result of the operation of the system, thereby improving the conservatism of the stability analysis result.

[0108] The present application provides a method for analyzing the stability of an offshore wind power system connected to the grid via flexible direct current. First, the impedance model of each part of the system is obtained using measured data, and the impedance information of the MMC converter and wind farm in the power grid can be obtained quickly and accurately. Furthermore, the frequency coupling effect of the system is considered when building the impedance model, and the impedance information of each part is represented by a two-dimensional impedance matrix. Then, different segmentation points are taken to divide the system into two subsystems, and the influence of different segmentation points on the stability analysis results of the entire system is considered to avoid deviations in the stability analysis results due to inappropriate segmentation point selection. Finally, the Gershgorin circle theorem is used to estimate the distribution of the characteristic root locus of the system impedance ratio matrix, and corresponding stability analysis results are proposed for the four possible situations in the circular domain. Further, the minimum stability margin of the system under various segmentation methods is used to obtain conservative stability analysis results.

[0109] The above is a stability analysis method for an offshore wind power system connected to the grid via a flexible direct current (DC) circuit provided in an embodiment of the present application. The following is a stability analysis system for an offshore wind power system connected to the grid via a flexible DC circuit provided in an embodiment of the present application.

[0110] See also Figure 8 , a stability analysis system for an offshore wind power flexible direct current grid-connected system provided in an embodiment of the present application comprises:

[0111] The first acquisition unit 201 is used to acquire the impedance model of each subsystem in a first interconnected system consisting of multiple subsystems of an offshore wind farm connected to the flexible direct current grid by using a swept frequency impedance method.

[0112] A second acquisition unit 202 is used to determine a plurality of different segmentation modes based on the topology of the first interconnected system, to respectively segment the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and to respectively acquire an impedance ratio matrix of each second interconnected system, wherein each segmentation mode includes a corresponding segmentation point;

[0113] The analysis unit 203 is used to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each second interconnected system by using the Gershgorin circle theorem, analyze the stability of the second interconnected system according to the distribution of the characteristic root loci, and use the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

[0114] Furthermore, an embodiment of the present application also provides a stability analysis device for an offshore wind power system connected to a flexible direct current grid, the device comprising a processor and a memory:

[0115] The memory is used to store program codes and transmit the program codes to the processor;

[0116] The processor is used to execute the steps of the offshore wind power flexible direct current grid-connected system stability analysis method as described in the above method embodiment according to the instructions in the program code.

[0117] Furthermore, a computer-readable storage medium is provided in an embodiment of the present application, and the computer-readable storage medium is used to store program code, and the program code is used to execute the offshore wind power flexible direct current grid-connected system stability analysis method described in the above method embodiment.

[0118] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0119] The terms "first", "second", "third", "fourth", etc. (if any) in the specification of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0120] It should be understood that in this application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0121] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. 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 an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0122] 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.

[0123] In addition, each functional unit in each embodiment of the present application 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.

[0124] 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 application is essentially 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 to enable a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

[0125] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. 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 application.

Claims

1. A stability analysis method for an offshore wind power system connected to a flexible direct current grid, characterized in that: include: The impedance model of each subsystem in the first interconnected system of the offshore wind farm connected to the flexible direct current grid, which is composed of multiple subsystems, is obtained by using the swept frequency impedance method; Determine a plurality of different segmentation modes based on the topology of the first interconnected system, for respectively segmenting the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and respectively obtain an impedance ratio matrix of each of the second interconnected systems, wherein each segmentation mode includes a corresponding segmentation point; The Gershgorin circle theorem is used to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems. The stability of the second interconnected systems is analyzed according to the distribution of the characteristic root loci, and the analysis result corresponding to the second interconnected system with the smallest stability margin is used as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

2. The offshore wind power flexible direct current grid-connected system stability analysis method according to claim 1 is characterized in that: The method of using the swept frequency impedance method to obtain the impedance model of each subsystem in the first interconnected system consisting of multiple subsystems of an offshore wind farm connected to the grid via flexible direct current includes: After injecting a small disturbance voltage of a positive sequence frequency into the first interconnected system, sampling voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain values ​​of a positive sequence current and a coupling current under the small disturbance voltage of the positive sequence frequency as first column data constituting a two-dimensional impedance matrix; After injecting a small disturbance voltage of a negative sequence frequency into the first interconnected system, sampling voltage and current values ​​of each subsystem in the first interconnected system, and extracting the voltage and current values ​​to obtain values ​​of negative sequence current and coupling current under the small disturbance voltage of the negative sequence frequency as the second column data constituting the two-dimensional impedance matrix; The impedance model is obtained by forming a two-dimensional impedance matrix through the first column of data and the second column of data.

3. The method for analyzing the stability of an offshore wind power grid-connected system via flexible direct current according to claim 1, characterized in that: The determining of a plurality of different segmentation modes based on the topology of the first interconnected system is used to respectively segment the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and respectively obtain an impedance ratio matrix of each of the second interconnected systems, wherein each of the segmentation modes includes a corresponding segmentation point, including: Based on the topology of the first interconnected system, several different division methods are determined, and the multiple subsystems in the first interconnected system are divided into several two subsystems through each of the division methods to form several second interconnected systems. According to the impedance model, the impedance matrices of the two subsystems are respectively solved, and the impedance ratio matrices of the second interconnected systems are formed according to the solved impedance matrices.

4. The method for analyzing the stability of an offshore wind power grid-connected system via flexible direct current according to claim 1, characterized in that: The method of using the Gershgorin circle theorem to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems, analyzing the stability of the second interconnected systems according to the distribution of the characteristic root loci, and using the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system, includes: Based on the Gershgorin circle theorem, the center of each circle in the impedance ratio matrix is ​​calculated and the center Distance to point (-1, j0) and distance ; According to the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems; A stability margin is obtained according to the stability analysis results corresponding to each of the second interconnected systems, and the analysis result corresponding to the second interconnected system with the smallest stability margin is used as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

5. The method for analyzing the stability of an offshore wind power system connected to a flexible DC grid according to claim 3 is characterized in that: The impedance matrix obtained by solving the problem forms several impedance ratio matrices of the second interconnected system, including: Substituting each of the impedance matrices into an impedance ratio matrix formula, respectively, to calculate and obtain several impedance ratio matrices of the second interconnection system; Wherein, the impedance ratio matrix formula is: In the formula, is the impedance ratio matrix, and is the impedance matrix of the two subsystems.

6. The method for analyzing the stability of an offshore wind power grid-connected system via flexible direct current according to claim 4 is characterized in that: The calculation of the center of each circle in the impedance ratio matrix and the center Distance to point (-1, j0) and distance , Based on the distance calculation formula, calculate the center of each circle in the impedance ratio matrix and the center Distance to point (-1, j0) and distance ; Wherein, the distance calculation formula is: 。 7. The method for analyzing the stability of an offshore wind power grid-connected system via flexible direct current according to claim 4 is characterized in that: According to the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems includes: Design several criteria, and based on the several criteria, according to the distance and the distance Analyzing the stability of the second interconnected systems to obtain stability analysis results corresponding to each of the second interconnected systems; Among them, several of the criteria include: The first criterion: When the center L 11 (f p ) and the center L 22 (f p ) The trajectory does not surround the point (-1, j0) and satisfies S 11 >|L 12 (f p )| and S 22 >|L 21 (f p )|, that is, the center trajectory does not surround (-1, j0) and all circular regions do not contain (-1, j0), at this time the system operates stably; The second criterion: When the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L 21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable; The third criterion: When the center L 11 (f p ) trajectory surrounds the point (-1, j0) and S 11 <|L 12 (f p )|, or the center L 11 (f p ) The trajectory does not surround the point (-1, j0) and S 22 <|L 21 (f p )|, that is, the characteristic root locus of the system impedance ratio may surround (-1, j0), at which point the system is conservatively considered unstable; The fourth criterion: When the center L 11 (f p ) or L 22 (f p ) trajectory surrounds the point (-1, j0), and satisfies S 11 >|L 12 (f p )|or S 22 >|L 21 (f p )|, that is, the characteristic root locus of the impedance ratio matrix will surround the point (-1, j0), and the system is unstable at this time.

8. A stability analysis system for an offshore wind power flexible DC grid-connected system, characterized in that: include: A first acquisition unit is used to acquire, by using a swept frequency impedance method, an impedance model of each subsystem in a first interconnected system consisting of a plurality of subsystems of an offshore wind farm connected to the flexible direct current grid; a second acquisition unit, configured to determine a plurality of different segmentation modes based on the topology of the first interconnected system, to respectively segment the first interconnected system into a plurality of second interconnected systems each consisting of two subsystems, and to respectively acquire an impedance ratio matrix of each of the second interconnected systems, wherein each segmentation mode includes a corresponding segmentation point; The analysis unit is used to estimate the distribution of the characteristic root loci of the impedance ratio matrix corresponding to each of the second interconnected systems by using the Gershgorin circle theorem, analyze the stability of the second interconnected systems according to the distribution of the characteristic root loci, and use the analysis result corresponding to the second interconnected system with the smallest stability margin as the stability analysis result of the offshore wind farm operating through the flexible DC grid-connected system.

9. An offshore wind power flexible DC grid-connected system stability analysis device, characterized in that: The device comprises a processor and a memory: The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the offshore wind power flexible direct current grid-connected system stability analysis method described in any one of claims 1-7 according to the instructions in the program code.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium is used to store program code, and the program code is used to execute the offshore wind power flexible direct current grid-connected system stability analysis method described in any one of claims 1-7.

Images