Method and system for analyzing stability of multi-followed network-structured converter system

Abstract

The application belongs to the technical field of new power system analysis, and discloses a multi-following network-structure network type converter system stability analysis method and an analysis system eq and Q ml ; joint diagonalization processing is performed on the matrices Q eq and Q ml , corresponding diagonal matrices Lambda eq and Lambda ml are obtained, n sets of characteristic parameters (lambda i , mu i ) are extracted, and it is judged whether (lambda i , mu i X RPC0 ) are all located above a critical stability curve of the following network type converter; if yes, the system is stable, otherwise, the system is unstable. Based on the above method, the stability criterion of the multi-following network-structure network type converter system can be simplified, the influence of the structure network type converter on the stability of the following network type equipment under different control modes can be determined, and thus the reactive power control strategy of the structure network type converter can be optimized, and the support capability of the structure network type converter on the power grid can be enhanced.

Description

Technical Field

[0001] This invention belongs to the technical field of novel power system analysis, and more specifically, relates to a stability analysis method and system for multi-grid-type converter systems. Background Technology

[0002] Power electronic converters, serving as grid-connection interfaces for renewable energy sources, are widely used in modern power systems, such as wind power, photovoltaic, and energy storage systems. However, grid-connected converters pose challenges to system stability due to their inability to provide effective support in terms of inertia response and grid strength. To address this issue, grid-connected converters, with their voltage source characteristics similar to synchronous machines, were proposed. These converters can autonomously construct the system's voltage and frequency, connecting to the grid at appropriate locations with voltage and frequency sufficient to support the grid. Therefore, grid-connected converters are indispensable in power systems with high renewable energy penetration rates.

[0003] Grid-type converters employ diverse reactive power control methods, each with varying impacts on system stability. Therefore, it is crucial to investigate the influence of different reactive power control methods on power system stability, enabling the optimization of reactive power control strategies for various scenarios. However, due to the multi-timescale interactions between grid-type and grid-connected converters, current methods for assessing the small-signal stability of multi-grid-connected systems are quite complex. Summary of the Invention

[0004] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a stability analysis method and system for multi-grid-structured converter systems. The purpose is to evaluate the small-signal stability of multi-grid-structured systems in a simpler way, thereby clarifying the impact of different control methods of the structured converter on the stability of the grid-type equipment.

[0005] To achieve the above objectives, this invention is proposed.

[0006] According to a first aspect of the present invention, a stability analysis method for a multi-grid converter system is provided, comprising:

[0007] Calculate the admittance matrix Q used to evaluate the stability of the mesh-type flow generator. eq and Q ml : Q ml =Q n,m Q m,n In the formula, Q n,n Q n,m Q m,n matrix Q redA block matrix with an n×n dimension at the top left, an n×m dimension at the top right, and an m×n dimension at the bottom left, and matrix Q. red The system node admittance matrix Q is obtained as an (n+m) dimensional square matrix after Kron elimination; n and m are the number of grid-type and mesh-type converters in the system, respectively, ω0 is the fundamental angular frequency, and L... f H is the filter inductor for the grid-type converter. V For the transfer function H V (s) amplitude above 20Hz, function H V (s) is the product of the transfer functions of the voltage loop and current loop PI controllers, where s represents the complex frequency domain variable;

[0008] For matrix Q eq and Q ml Perform joint diagonalization to obtain the corresponding diagonal matrix Λ eq and Λ ml Extract its diagonal elements to form n sets of feature parameters (λ). i ,μ i ), λ i ,μ i These are diagonal matrices Λ eq and Λ ml The i-th diagonal element in the array, where i = 1, 2, ..., n;

[0009] Judgment (λ) i ,μ i X RPC0 If all values ​​are above the critical stability curve of the grid-connected converter, the system is stable; otherwise, the system is unstable. RPC0 The modulus of the impedance generated by the reactive power control of the grid-type converter at a preset oscillation frequency.

[0010] According to a second aspect of the present invention, a stability analysis system for a multi-grid converter system is provided, comprising a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the method described in any of the preceding claims.

[0011] According to a third aspect of the invention, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described in any of the preceding claims.

[0012] According to a fourth aspect of the invention, a computer program product is provided, comprising a computer program or instructions that, when executed by a processor, implement the steps of the method described in any of the preceding claims.

[0013] In summary, compared with the prior art, the technical solutions conceived in this invention have the following main advantages:

[0014] This invention derives the admittance matrix Q for evaluating the stability of grid-type converters by equating high-order, complex multi-grid energy storage systems to low-order, simple converter systems and analyzing them. eq and Q ml Then, the system is subjected to joint diagonalization to decouple it into multiple subsystem equivalent models, obtaining the characteristic parameters of each subsystem. The stability of the system can then be determined by judging the relationship between the characteristic parameters of each subsystem and the critical stability curve of the grid-connected converter. This method reduces the complexity of the high-order multi-grid-connected converter system to a lower-order, simpler subsystem, lowering the analysis difficulty. Furthermore, when the grid structure or grid-connected energy storage parameters change, only the characteristic parameters need to be recalculated for system stability assessment, avoiding multiple modeling steps and significantly reducing computational load. Based on this method, the stability criteria for multi-grid-connected converter systems can be simplified, and the impact of different control methods on the stability of grid-connected equipment can be clarified. This allows for optimization of the reactive power control strategy of grid-connected converters, enhancing their support capability for the power grid. Attached Figure Description

[0015] Figure 1 This is a flowchart of the steps of a system stability analysis method according to an embodiment of the present invention;

[0016] Figure 2 This is a schematic diagram of a multi-grid-structured converter system according to an embodiment of the present invention;

[0017] Figure 3 This is an equivalent model diagram of the decoupled equivalent subsystem in the embodiments of the present invention;

[0018] Figure 4 This is a schematic diagram of the topology of the test system used in the embodiments of the present invention;

[0019] Figure 5 This is a diagram showing the position of the subsystem characteristic parameters in the stable region in an embodiment of the present invention;

[0020] Figure 6 These are frequency domain analysis diagrams of the system under three different reactive power control methods in the embodiments of the present invention. Among them, (a) is the Bode plot of the open-loop transfer function of the system under different reactive power control methods, and (b) is the Bode plot of the transfer function characterizing the reactive power-voltage droop control and reactive power-voltage integral droop control characteristics.

[0021] Figure 7 It corresponds Figure 6The electromagnetic simulation results of the grid-type converter in the embodiment are shown in the figure. (a1)-(a3) are the power waveforms of each converter when the grid-type converter is connected to nodes 1, 2, and 3, respectively. (b1)-(b3) are the power waveforms of each converter when the grid-type converter is connected to node 2 and the constant voltage, reactive voltage droop, and reactive voltage integral droop control are adopted, respectively.

[0022] Figure 8 This is a frequency domain analysis of the system under different reactive power control in another embodiment of the present invention, wherein (a) is the Bode plot of the open-loop transfer function of the system when constant voltage control and reactive power-voltage droop control are adopted, and (b) is the Bode plot of the transfer function characterizing the characteristics of reactive power-voltage droop control and the proposed reactive power control.

[0023] Figure 9 This is the corresponding invention Figure 8 The optimized reactive power control structure diagram adopted in the embodiment;

[0024] Figure 10 It corresponds Figure 8 Electromagnetic simulation results of different reactive power control methods for the grid-type converter in the embodiment, wherein (a)-(c) are the time-domain electromagnetic simulation results of the system under constant voltage control, reactive voltage droop control and the proposed control, respectively. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Example 1

[0027] This invention provides a stability analysis method for multi-grid converter systems.

[0028] like Figure 1 The diagram shown is a flowchart of the system stability analysis method in one embodiment of the present invention. The steps are described in detail below.

[0029] S1. Calculate the admittance matrix Q used to evaluate the stability of the mesh-type flowformer. eq and Q ml :

[0030] Q ml =Q n,m Q m,n In the formula, Q n,n Qn,m Q m,n

[0031] matrix Q red A block matrix with an n×n dimension at the top left, an n×m dimension at the top right, and an m×n dimension at the bottom left, and matrix Q. red The Kron elimination process is applied to the system node admittance matrix Q to obtain an (n+m) dimensional square matrix; n and m are the number of grid-type converters and network-type converters in the system, respectively.

[0032] ω0 is the fundamental angular frequency, L f H is the filter inductor for the grid-type converter. V For the transfer function H V (s) amplitude above 20Hz, function H V (s) is the product of the transfer functions of the voltage loop and current loop PI controllers, where s represents the complex frequency domain variable.

[0033] like Figure 2 The diagram shown is a schematic of a multi-grid-type converter system in one embodiment. The system has a total of n+m+k nodes, including n grid-type converter (GFL) nodes, m grid-type converter (GFM) nodes, and k other types of nodes.

[0034] Since real-world multi-grid converter systems are very complex, this invention simplifies the analysis process by establishing an equivalent model of the system and decoupling it.

[0035] The equivalent treatment includes equivalent treatment for grid-connected converters and equivalent treatment for grid-connected converters.

[0036] Equivalent treatment of grid-connected converters: n heterogeneous grid-connected converters are equivalent to n homogeneous grid-connected converters. The equivalent admittance Y of each homogeneous grid-connected converter is... PLL (s) is the weighted admittance of all heterogeneous and grid-type converters in the system, with a dimension of 2×2.

[0037] Specifically, the weights can be determined using the following method:

[0038] Let u and v be matrices respectively. The weighted weight of the i-th grid converter, corresponding to its left and right eigenvectors with minimum eigenvalue, is p. i =u i v i , where u i and v i Let S be the i-th element of vectors u and v, respectively. B Let Q be the capacity ratio matrix of GFL. eqThe calculation method is given below.

[0039] Equivalent treatment of grid-connected converters: Each grid-connected converter is equivalent to a corresponding impedance voltage source structure. The impedance voltage source structure includes an equivalent inductor and a voltage source connected in series. The equivalent inductor has an equivalent impedance, which is obtained by superimposing the inner loop control impedance and the impedance generated by reactive power control of the grid-connected converter. Its dimension is 2×2, and the equivalent impedance matrix Z is... GFM (s) can be specifically expressed as:

[0040]

[0041] In the formula, L f H is the filter inductor for a grid-type converter. V (s)=PI VCL (s)PI CCL (s), PI VCL (s) and PI CCL (s) are the transfer functions of the voltage loop and current loop PI controllers, respectively, D q Let β be the reactive power droop coefficient, and γ be parameters characterizing different reactive power control methods of the grid-type converter. β is the filter time constant, and γ is the ratio of the integral coefficient to the proportional coefficient of the reactive power PI controller. When the grid-type converter uses reactive power-voltage droop control, β = γ = 0; when the grid-type converter uses constant reactive power control, β = 0, γ ≠ 0; when the grid-type converter uses reactive power-voltage integral droop control, β ≠ 0, γ = 0. d0 The system's rated d-axis voltage, ω0 is the fundamental angular frequency, s represents the complex frequency domain variable, and Z... b (s) is the inner loop control impedance matrix, Z QV (s) is the impedance matrix generated by reactive power control.

[0042] In this invention, through theoretical analysis, n heterogeneous grid-type converters are equivalent to n homogeneous grid-type converters. The influence of the grid-type converter on the grid-type converter is concretized as an impedance voltage source structure. In this way, the high-order complex multi-grid-grid energy storage system can be equivalent to a low-order simple converter system. Based on the equivalent low-order simple converter system, frequency domain theory can be applied for stability analysis and control optimization.

[0043] Based on this equivalent model, the closed-loop characteristic equations of n isomorphic converters connected to the grid converter are determined as follows:

[0044]

[0045] Y PLL (s) is the equivalent admittance matrix of the grid converter, S B Let S be the capacity ratio matrix of GFL. MHere is the capacity ratio matrix of GFM. Let Q be the equivalent admittance matrix of the grid-type converter, and τ be the average impedance-to-inductance ratio of the line. n,n Q n,m Q m,n and Q m,m It is matrix Q red The block matrices are of order n×n, n×m, m×n and m×m, respectively.

[0046] Q is the nodal Laplace matrix after Kron elimination. Matrix Q1, Q2, Q3, and Q4 are block matrices of matrix Q, where Q1 is an n+m square matrix. Q is the nodal admittance matrix of the system.

[0047] The system's node admittance matrix Q is a matrix composed of the line admittances between every pair of all (m+n+k) nodes in the system, with dimensions (m+n+k)×(m+n+k). Krone elimination is applied to matrix Q to obtain matrix Q'. red Its dimension is (m+n)×(m+n). Let matrix Q... red Divide into 4 parts, and obtain matrix Q respectively. n,n Q n,m Q m,n and Q m,m Q n,n Q n,m Q m,n and Q m,m They are arrays Q red A block matrix with n×n dimensions in the top left corner, n×m dimensions in the top right corner, m×n dimensions in the bottom left corner, and m×m dimensions in the bottom right corner.

[0048] Based on the above equivalent model and the above closed-loop characteristic equation, the two admittance matrices Q describing the interconnection information of n isomorphic grid converters can be calculated. eq Q RPC The calculation formula is:

[0049]

[0050] remember:

[0051]

[0052]

[0053] Among them, Q eq Q is the equivalent admittance matrix of the inner loop impedance of the grid-type converter in the equivalent model. RPC (s) is the additional admittance matrix introduced by reactive power control, H V For the transfer function HV (s) Amplitude above 20Hz. Grid-type converters change Q... eq (s) and Q RPC (s) affects the stability of GFL, while Q RPC The value of (s) depends on matrix Q. ml .

[0054] Therefore, based on the above theoretical analysis, in order to conduct a stability analysis, this invention first needs to calculate the admittance matrix Q used to evaluate the stability of the mesh-type flow generator according to the above formula. eq and Q ml .

[0055] To achieve decoupling of n grid-type converters, this invention further refines the admittance matrix Q. eq and Q ml Perform joint diagonalization.

[0056] S2, for matrix Q eq and Q ml Perform joint diagonalization to obtain the corresponding diagonal matrix Λ eq and Λ ml Extract its diagonal elements to form n sets of feature parameters (λ). i ,μ i ).

[0057] λ i ,μ i These are diagonal matrices Λ eq and Λ ml The i-th diagonal element in the array, where i = 1, 2, ..., n.

[0058] Specifically, for the admittance matrix Q eq Q ml Perform joint diagonalization; the single transformation matrix of joint diagonalization is:

[0059]

[0060] Among them, U pq0 For a (p-q+1) dimensional square matrix, U pq It is an orthogonal matrix, that is Rotation angle The selection is based on the following: the matrix to be jointly diagonalized has the smallest sum of squares of the off-diagonal elements in the p-th row and q-th column after the transformation, that is:

[0061]

[0062] From this, the rotation angle can be obtained. The value can be:

[0063]

[0064] Where K = 2 is the number of matrices to be undiagonalized, a pq,1 and a pq,2 Represent matrix Q respectively eq Sum matrix Q ml The element in row p and column q. p and q are the elements that make the current |Q eq,pq |+|Q ml,pq The index of the largest element. Each time the rotation matrix U is changed... pq0 The values ​​of p and q in the matrix Q after several rotation transformations eq and Q ml The off-diagonal elements decrease gradually during iteration, eventually achieving approximate joint diagonalization. The product of all rotation matrices gives the final transformation matrix U.

[0065] Based on the above process, matrix Q eq and Q ml By performing joint diagonalization, the corresponding diagonal matrix Λ can be obtained. eq and Λ ml The diagonal matrices are all n-dimensional square matrices, which can be specifically represented as:

[0066] Λ eq =U T Q eq U;

[0067] Λ ml =U T Q ml U;

[0068] Subsequently, the diagonal matrix Λ is extracted. eq The main diagonal elements λ1~λ n And extracting the diagonal matrix Λ ml The main diagonal elements μ1 to μ n We obtain n sets of characteristic parameters (λ1, μ1) ~ (λ n ,μ n ), λ i ,μ i These are diagonal matrices Λ eq and Λ ml The i-th diagonal element in.

[0069] Based on the above process, n sets of characteristic parameters (λ1, μ1)~(λ) are obtained. n ,μ n After that, the stability of the system can be determined. That is, proceed to step S3 as follows.

[0070] S3, Judgment (λ) i ,μ i X RPC0If all values ​​are above the critical stability curve of the grid-connected converter, the system is stable; otherwise, the system is unstable.

[0071] X RPC0 The modulus of the impedance generated by the reactive power control of the grid-type converter at a preset oscillation frequency.

[0072] After obtaining n sets of characteristic parameters (λ1, μ1 ~ (λ n ,μ n After this, the system can be decoupled into n rooted network subsystems. The characteristic equations of the n rooted network subsystems can be expressed in the following form:

[0073]

[0074] In the formula, The deviation term, λ, is introduced due to the asymmetry of reactive power control. i and μ i These are the characteristic parameters of the i-th root network subsystem obtained from the calculation.

[0075] At this point, it is only necessary to construct the characteristic parameters (λ) of the decoupled subsystem. i ,μ i X RPC0 By analyzing the positional relationship between the curve and the critical stability curve of the grid-type converter, it can be determined whether the system is stable.

[0076] Among them, X RPC0 =|X RPC (jω c )|,X RPC0 The expression is as follows:

[0077]

[0078] In the formula, ω c This is the preset oscillation frequency of the grid converter, which is an empirical value and can be taken as 20Hz. d0 The system's rated d-axis voltage, ω0 is the fundamental angular frequency, and H... RPC Here, s represents the intermediate parameter, β and γ represent the complex frequency domain variable, and D represents the parameters characterizing different reactive power control modes of the grid-type converter. q This is the reactive power droop coefficient.

[0079] When the characteristic parameters (λ) of all decoupled subsystems i ,μ i X RPC0 When the line is above the critical stability curve L(a,b)=0 of the grid-connected converter, the system is stable; otherwise, the system is unstable.

[0080] In one embodiment, the critical stability curve can be determined using the following analytical expression:

[0081]

[0082] In the formula, Y PLL (jω c The equivalent admittance matrix of the grid-type converter at the preset oscillation frequency ω c The value of F(jω) is obtained by weighting the admittances of all heterogeneous and grid-type converters in the system, with the equivalent admittance matrix of the grid-type converter being the same. c ) is an intermediate parameter, and Δf is a deviation term introduced due to the asymmetry of reactive power control.

[0083] In one embodiment, further analysis can be performed based on the above theory to optimize the reactive power control of the grid-type converter.

[0084] Specifically, this technical solution also includes:

[0085] S4. Select a set of characteristic parameters (λ) that are closest to the critical stability curve of the grid-type converter. w ,μ w X RPC0 The closed-loop characteristic equation of the equivalent model of the weakest decoupled subsystem is as follows:

[0086]

[0087] In the formula, f PLL (s) and f RPC (s) are all transfer functions, f PLL (s) includes information about the GFL itself and the network structure, and physically represents the GFL through admittance λ. i Stability of accessing an infinite bus, f RPC (s) contains reactive power control information from the GFM; the product of the two is the open-loop transfer function of the weakest subsystem, Z. PLL,dd (s) and Z PLL,qq (s) is the impedance matrix The diagonal element, Y PLL (s) represents the equivalent admittance matrix of the grid-connected converter. The equivalent admittance matrix of the grid-connected converter is obtained by weighting the admittances of all heterogeneous grid-connected converters in the system. τ is the average impedance-to-inductance ratio of the line, and D... q V is the reactive power droop factor. d0 β is the rated d-axis voltage of the system, and β and γ are parameters characterizing different reactive power control modes of the grid-type converter. β is the filter time constant, and γ is the ratio of the integral coefficient to the proportional coefficient of the reactive power PI controller.

[0088] like Figure 3 The diagram shows the structural schematic of the equivalent model of the weakest decoupled subsystem.

[0089] S5. Based on the transfer function f PLL (s) and f RPC The expression for (s) is given by f. RPC (s) to perform compensation to enhance the stability of the system.

[0090] In one embodiment, the technical solution further includes: analyzing the stability of the grid-type converter system under different access locations, and taking the access location corresponding to the highest stability as the optimal access location of the grid-type converter.

[0091] Specifically, the different connection locations of the grid-type converter will affect the system node admittance matrix Q in step S1, and consequently affect the final characteristic parameter (λ). i ,μ i After changing the access location of the grid-type converter, repeat steps S1 to S3 to select the access node of the grid-type converter when the system stability margin is maximized.

[0092] Based on the above methods, reactive power control optimization can be performed on the network converter, according to the transfer function f corresponding to the weakest subsystem. PLL (s) and f RPC (s) expression, design a suitable compensation mechanism to change f RPC (s) Improve its phase margin by considering its characteristics in the unstable frequency band.

[0093] In one embodiment, the control law for unified reactive power control of the grid-type converter is:

[0094]

[0095] In the formula, Q ref and Q E These are the commanded and actual values ​​of reactive power, V0 and V, respectively. d For the rated voltage and the actual d-axis voltage, D q β is the reactive power droop coefficient, β is the filter time constant, and K is the reactive power droop coefficient. RPC For adjustable reactive power control parameters, adjust K RPC Different reactive power control methods can be implemented;

[0096] When K RPC When the value is 0, the unified reactive power control method is the existing reactive power droop control method with low-pass filter, which can suppress the subsynchronous oscillation of the grid converter;

[0097] When K RPC When = 1, the unified reactive power control method is the conventional reactive power-voltage droop control, which has the simplest control structure;

[0098] K RPC When the value is greater than 1, it can suppress the supersynchronous oscillation of the grid converter.

[0099] Example 2

[0100] This invention also relates to a stability analysis system for a multi-grid converter system, comprising a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps of the method described above.

[0101] The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. Memory can be used to store computer programs and / or modules. The processor runs or executes the computer programs and / or modules stored in memory, and accesses data stored in memory.

[0102] Example 3

[0103] The present invention also relates to a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described above.

[0104] Specifically, the memory may include high-speed random access memory, as well as non-volatile memory, such as hard disks, RAM, plug-in hard disks, smart media cards (SMC), secure digital cards (SD), flash cards, at least one disk storage device, flash memory device, or other volatile solid-state storage devices.

[0105] Example 4

[0106] This invention provides a computer program product or computer program that includes computer instructions stored in a computer-readable storage medium. A processor of a computer device reads the computer instructions from the computer-readable storage medium and executes the computer instructions, causing the computer device to perform the steps of the method described in the above embodiments of this invention.

[0107] Example 5

[0108] by Figure 4Taking the multi-grid converter system shown as an example, the effectiveness of the technical solution of the present invention under different instability modes is verified. In this system, nodes 1 to 3 are connected to the grid converter, nodes 6 to 8 are internal nodes, and node 0 is connected to an infinite power source to simulate the external power grid. The grid converter can be connected to three 37kV buses. The connection of the grid converter will add one converter node and one internal node. For the sake of rationality, these two nodes are designated as nodes 4 and 5, respectively. The main control parameters of the system are shown in Table 1. The time-domain electromagnetic simulation of the system is performed in MATLAB / Simulink.

[0109] Table 1 Main parameters of the system in the embodiment

[0110]

[0111] The effectiveness of this invention will first be verified when system instability is dominated by the current loop of the grid-connected converter.

[0112] Select the reactive power droop coefficient D q =8p.u., calculate the characteristic parameters (λ) of the weakest subsystem when the grid-type converter is connected to different nodes. i ,μ i The results obtained by setting i = 1, 2, ..., n are shown in Table 2.

[0113] Table 2 Characteristic parameters of converters with different grid configurations at the landing point

[0114]

[0115] The results show that when the grid-type converters are connected sequentially near nodes 3, 1, and 2, λ i Gradually increase; at the same time μ i X RPC0 The value also gradually increases, indicating that the reactive power control of the grid-type converter is causing increasing disturbance to converter 2.

[0116] like Figure 5 The diagram shows the positions of three sets of characteristic parameters in the stability region of the grid-type converter. When the grid-type converter is connected near node 1 or 3, the system is in an unstable state due to λ. i This is caused by λ being too small; when connected to node 2, although λ i While the reactive power control has significantly improved performance, the system's stability margin remains low, necessitating an appropriate increase in the reactive power droop factor D of the grid-type converter. q .

[0117] Then, based on the method proposed in this invention, the impact of different reactive power control methods of grid-type converters on system stability was analyzed. For example... Figure 6As shown, three different reactive power control methods were selected: constant voltage control (CVC), reactive voltage droop control (QVC), and reactive voltage-reactive power integral droop control (QVIC). G in the figure... ol,CVC and G ol,QVC The open-loop transfer functions of the system using CVC and QVC are shown in (a), respectively. Compared to constant voltage control, when the reactive voltage droop control uses D... q =8p.u., f RPC (s) A magnitude of approximately -6dB exists in the low-frequency band, causing the crossover frequency to advance earlier, resulting in a stability margin PM = 7deg. The smaller the reactive power droop coefficient, the weaker the system stability, and the more prone the system is to subsynchronous oscillations below 50Hz. However, when reactive power-voltage integral droop control is used, as shown in (b), the transfer function f... RPC (s) can be viewed as the original transfer function cascaded with the lag element H1(s), where the positive amplitude and phase of the lag element cause f RPC (s) The increase in both amplitude and phase angle in the low-frequency band is beneficial to enhancing the low-frequency stability of the grid-connected converter. Therefore, by analyzing the impact of different reactive power control methods on system stability of the grid-connected converter, the optimal reactive power control method can be determined.

[0118] Figure 7 The time-domain electromagnetic simulation results corresponding to the above stability analysis are given. GFL1-p, GFL2-p, and GFL3-p represent the active power curves of the three grid-connected converters, respectively; GFL1-q, GFL2-q, and GFL3-q represent the reactive power curves of the three grid-connected converters, respectively; (a1)-(a3) are the power waveforms of each converter when the grid-connected converters are connected to nodes 1, 2, and 3, respectively; (b1)-(b3) are the power waveforms of each converter when the grid-connected converters are connected to node 2 and respectively using constant voltage, reactive voltage droop, and reactive-voltage integral droop control. Figure 7 It can be seen that the grid-type converter exhibits the best stability when using constant voltage control, followed by reactive-voltage integral droop control, while the stability is weakest when using reactive-voltage droop control. This is consistent with the results of frequency domain theoretical analysis, thus verifying the effectiveness of this method.

[0119] Furthermore, the effectiveness of reactive power control optimization using this invention is verified when system instability is dominated by the phase-locked loop of the grid-connected converter. Figure 8 From the open-loop transfer function of the weakest subsystem after decoupling, it can be seen that under reactive power-voltage droop control, f RPC (s) It can provide positive gain in the frequency band above 50Hz, while f under constant voltage control RPCThe amplitude of (s) remains at 0, which is the reason for improving the stability of the grid-type converter. Through reactive power control optimization using this invention, a reactive power compensator H2(s) can be added to the reactive power control loop to increase f. RPC (s) Amplitude in the 50–150 Hz frequency band, such as Figure 9 As shown.

[0120] like Figure 10 As shown, (a)-(c) represent the time-domain electromagnetic simulation results of the system under constant voltage control, reactive voltage droop control, and the proposed control, respectively. When constant voltage control and reactive voltage droop control are used, the output power of the grid converter exhibits divergent and constant-amplitude oscillation states after being disturbed, respectively; in contrast, the proposed control achieves rapid convergence. The results indicate that under this instability mode, reactive voltage droop control achieves better stability than constant voltage control, and the effectiveness of the proposed control structure is verified.

[0121] The above results show that the stability analysis method proposed in this invention can characterize the actual stability of the system, and that optimizing the control of the converter based on this analysis can improve the small-signal stability of the grid-connected converter system.

[0122] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. It should be noted that the terms "in one embodiment," "for example," and "again" in this invention are intended to illustrate the invention and are not intended to limit the invention.

[0123] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A stability analysis method for a multi-grid converter system, characterized in that, include: Calculate the admittance matrix Q used to evaluate the stability of the mesh-type flow generator. eq and Q ml : Q ml =Q n,m Q m,n ; In the formula, Q n,n Q n,m Q m,n matrix Q red A block matrix with an n×n dimension at the top left, an n×m dimension at the top right, and an m×n dimension at the bottom left, and matrix Q. red The system node admittance matrix Q is obtained as an (n+m) dimensional square matrix after Kron elimination; n and m are the number of grid-type and mesh-type converters in the system, respectively, ω0 is the fundamental angular frequency, and L... f H is the filter inductor for the grid-type converter. V For the transfer function H V (s) amplitude above 20Hz, function H V (s) is the product of the transfer functions of the voltage loop and current loop PI controllers, where s represents the complex frequency domain variable; For matrix Q eq and Q ml Perform joint diagonalization to obtain the corresponding diagonal matrix Λ eq and Λ ml Extract its diagonal elements to form n sets of feature parameters (λ). i ,μ i ), λ i ,μ i These are diagonal matrices Λ eq and Λ ml The i-th diagonal element in the array, where i = 1, 2, ..., n; Judgment (λ) i ,μ i X RPC0 If all values ​​are above the critical stability curve of the grid-connected converter, the system is stable; otherwise, the system is unstable. RPC0 The modulus of the impedance generated by the reactive power control of the grid-type converter at a preset oscillation frequency.

2. The stability analysis method as described in claim 1, characterized in that, X RPC0 Determined by the following calculation formula: In the formula, ω c It is the preset oscillation frequency of the grid converter, V d0 The system's rated d-axis voltage, ω0 is the fundamental angular frequency, and H... RPC Here, s represents the intermediate parameter, β and γ represent the complex frequency domain variable, and D represents the parameters characterizing different reactive power control modes of the grid-type converter. q This is the reactive power droop coefficient.

3. The stability analysis method as described in claim 2, characterized in that, The critical stability curve of the grid converter satisfies the equation L(a,b)=0, where (a,b) are the coordinates of a point on the curve, and L(a,b) is a function of (a,b), with the following analytical expression: In the formula, Y PLL (jω c The equivalent admittance matrix of the grid-type converter at the preset oscillation frequency ω c The value of F(jω) is obtained by weighting the admittances of all heterogeneous and grid-type converters in the system, with the equivalent admittance matrix of the grid-type converter being the same. c ) is an intermediate parameter, and Δf is a deviation term introduced due to the asymmetry of reactive power control.

4. The stability analysis method as described in claim 1, characterized in that, Preset oscillation frequency ω c Use an empirical value of 18Hz to 25Hz.

5. The stability analysis method as described in claim 1, characterized in that, The method further includes: Select a set of characteristic parameters (λ) that are closest to the critical stability curve of the grid converter. w ,μ w X RPC0 The closed-loop characteristic equation of the equivalent model of the weakest decoupled subsystem is as follows: In the formula, f PLL (s) and f RPC (s) are all transfer functions, Z PLL,dd (s) and Z PLL,qq (s) is the impedance matrix The diagonal element, Y PLL (s) represents the equivalent admittance matrix of the grid-connected converter. The equivalent admittance matrix of the grid-connected converter is obtained by weighting the admittances of all heterogeneous grid-connected converters in the system. τ is the average impedance-to-inductance ratio of the line, and D... q V is the reactive power droop factor. d0 β is the rated d-axis voltage of the system, and β and γ are parameters characterizing different reactive power control modes of the grid-type converter. β is the filter time constant, and γ is the ratio of the integral coefficient to the proportional coefficient of the reactive power PI controller. Based on the transfer function f PLL (s) and f RPC The expression for (s) is given by f. RPC (s) to perform compensation to enhance the stability of the system.

6. The stability analysis method as described in claim 1, characterized in that, The method further includes: The stability of the grid-type converter system under different connection locations is analyzed, and the connection location corresponding to the highest stability is taken as the optimal connection location of the grid-type converter.

7. The stability analysis method as described in claim 1, characterized in that, The control law for unified reactive power control of grid-type converters is: In the formula, Q ref and Q E These are the commanded and actual values ​​of reactive power, V0 and V, respectively. d For the rated voltage and the actual d-axis voltage, D q β is the reactive power droop coefficient, β is the filter time constant, and K is the reactive power droop coefficient. RPC For adjustable reactive power control parameters, adjust K RPC To achieve different reactive power control methods.

8. A stability analysis system for a multi-grid converter system, comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method as described in any one of claims 1 to 7.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method as described in any one of claims 1 to 7.

10. A computer program product, comprising a computer program or instructions, characterized in that, When the computer program or instructions are executed by a processor, they implement the steps of the method as described in any one of claims 1 to 7.

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