Series-parallel resonance characteristic evaluation method and system

Abstract

The invention discloses a series-parallel resonance characteristic evaluation method and system, and belongs to the technical field of distributed new energy grid-connected power generation, and the method comprises the steps: analyzing the interaction between all units of a microgrid system based on a transfer function method; the method comprises the following steps: performing series resonance modal analysis and parallel resonance modal analysis on a micro-grid system by using a modal analysis method, and constructing a modal analysis equivalent model; extracting resonance modal information according to the modal analysis equivalent model; a resonance center is positioned based on series-parallel modal analysis, resonance types are distinguished in combination with a transfer function method, and resonance characteristic evaluation is realized. According to the method, the resonance type and the resonance center of the micro-grid system can be accurately obtained in the island operation mode, key information such as the key resonance mode and the node participation factor is extracted, and a theoretical basis is provided for subsequent resonance suppression.

Description

Technical Field

[0001] The present application relates to the field of distributed renewable energy grid-connected power generation, and in particular to a method and system for evaluating series-parallel resonance characteristics. Background Art

[0002] A multi-converter microgrid system is a highly coupled, complex system with numerous resonance points. When the background harmonics introduced by distributed renewable energy sources match the frequencies of the system's inherent resonance points, harmonic amplification occurs, leading to severe power quality degradation and even system instability. Effectively characterizing the system's resonance characteristics and extracting key oscillation information can provide theoretical support for the subsequent deployment of targeted oscillation suppression strategies and improve power quality.

[0003] Currently, effective methods for characterizing harmonic oscillations and locating oscillation sources mainly include frequency-domain impedance analysis and frequency-domain modal analysis. Existing microgrid scenarios for resonance research are relatively simple, primarily focusing on the study of resonance in microgrid grid-connected operation. Furthermore, when using frequency-domain modal analysis, only parallel resonance modes are employed, making it impossible to accurately distinguish the type of resonance. However, actual microgrid systems have numerous heterogeneous power electronic devices in islanded operation mode, with complex coupling between units. Therefore, it is necessary to conduct more comprehensive resonance mechanism research and source tracing for microgrid systems in islanded operation mode to provide a basis for the design of subsequent oscillation blocking and suppression strategies. Summary of the Invention

[0004] The present application aims to provide a method and system for evaluating the series-parallel resonance characteristics of a microgrid system in an island operation mode.

[0005] To achieve the above objectives, the technical solution of this application is:

[0006] A method for evaluating series-parallel resonance characteristics, comprising:

[0007] Analyze the interaction between the various units of the microgrid system based on the transfer function method;

[0008] Using modal analysis method, the series resonance modal analysis and parallel resonance modal analysis of the microgrid system are carried out, and the modal analysis equivalent model is constructed;

[0009] According to the modal analysis equivalent model, the resonant modal information is extracted; based on the series-parallel modal analysis, the resonance center is located, and the transfer function method is combined to distinguish the resonance type and realize the resonance characteristic evaluation.

[0010] Optionally, the constructing of a modal analysis equivalent model includes:

[0011] Set the resonant frequency search range and frequency step;

[0012] Perform parallel resonance modal analysis on the microgrid system and construct the node admittance matrix;

[0013] Perform series resonance modal analysis on the microgrid system and construct the loop impedance matrix.

[0014] Optionally, the node admittance matrix is ​​expressed as follows:

[0015]

[0016] Among them, each element in the node admittance matrix is ​​expressed as follows:

[0017]

[0018] Among them, Y i is the equivalent impedance Z of the i-th distributed generation unit in the microgrid system i The corresponding admittance, Y li is the line impedance Z of the PCC point connected to the tth distributed generation unit in the microgrid system li The corresponding admittance of g is the load admittance;

[0019] The loop impedance matrix is ​​expressed as follows:

[0020]

[0021] The meaning of each element in the loop impedance matrix is ​​as follows:

[0022]

[0023] Among them, Z i is the equivalent impedance of the i-th distributed generation unit in the microgrid system, Z li is the line impedance of the i-th distributed generation unit connected to the common connection point PCC in the microgrid system, Z g is the load impedance.

[0024] Optionally, extracting resonance modal information based on a modal analysis equivalent model; locating the resonance center based on series-parallel modal analysis, distinguishing the resonance type in combination with a transfer function method, and implementing resonance characteristic evaluation specifically includes:

[0025] Step S31: Determine whether the frequency exceeds the resonant frequency search range. If not, proceed to step S32; if yes, proceed to step S34;

[0026] Step S32: performing eigenvalue decomposition on the node admittance matrix and the loop impedance matrix;

[0027] Step S33: Calculating modal impedance and modal admittance;

[0028] Step S34: evaluating the resonance characteristics based on the series-parallel resonance modal analysis, locating the resonance center, and distinguishing the resonance type by combining the transfer function method.

[0029] Optionally, the left and right eigenvector matrices are used to perform eigenvalue decomposition on the node admittance matrix and the loop impedance matrix of the microgrid system.

[0030] Optionally, the node admittance matrix is ​​substituted into the node voltage column vector, and the node admittance matrix is ​​simplified to a matrix having only diagonal elements, where the elements of the matrix are defined as modal impedances;

[0031] Substituting the loop impedance matrix into the loop electromotive force column vector, the loop impedance matrix is ​​simplified to a matrix having only diagonal elements, and the elements of the matrix are defined as modal admittance.

[0032] Optionally, locating the resonance center includes:

[0033] Draw a curve showing the modal impedance of the microgrid system changing with frequency;

[0034] Extract the resonant mode frequency at each resonant peak in the curve;

[0035] Calculate the participation factor of each node in the parallel resonance mode and rank the node resonance sensitivity;

[0036] The resonance center of the microgrid system is determined according to the participation factor of each node in the parallel resonance mode.

[0037] Optionally, when the participation factor value is maximum, the node resonance center of the participation factor.

[0038] Optional, distinguish resonance types include:

[0039] Step S345: drawing a curve showing the change of modal impedance of the microgrid system with frequency;

[0040] Step S346: extracting the resonant mode frequency at each resonant peak in the curve;

[0041] Step S347: Calculate the participation factor of each loop in the series resonance mode;

[0042] Step S348: Determine whether the participation factor of the load impedance loop is 0. If not, proceed to step S349; if so, proceed to step S3410;

[0043] Step S349: The resonance type of the microgrid system is: resonance of the net current injected into the common connection point PCC, i.e., system resonance;

[0044] Step S3410: Determine whether the participation factor of the load impedance loop is 1. If not, proceed to step S3411; if so, proceed to step S3412;

[0045] Step S3411: The resonance type of the microgrid system is: heterogeneous or heterogeneous power generation units dominate the interactive current resonance, that is, local resonance;

[0046] Step S3412: The resonance type of the microgrid system is: the homogeneous and homogeneous power generation units dominate the interactive current resonance, that is, the local resonance.

[0047] A series-parallel resonance characteristic evaluation system comprises: one or more processors; a storage device for storing one or more programs. When the one or more programs are executed by the one or more processors, the one or more processors implement the series-parallel resonance characteristic evaluation method as described in any one of the above.

[0048] The series-parallel resonance characteristic evaluation method and system proposed in this application can accurately obtain the resonance type and resonance center of the microgrid system in the island operation mode, and extract key information such as key resonance modes and node participation factors, providing a theoretical basis for subsequent resonance suppression.

[0049] In order to make the above features and advantages of the application more obvious and easy to understand, the following embodiments are given and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a structural diagram of each distributed power generation unit in the microgrid system in island operation mode.

[0051] Figure 2 This is a flow chart of the series-parallel resonance characteristics evaluation method provided in this application.

[0052] Figure 3 Flowchart for building an equivalent model for modal analysis.

[0053] Figure 4 This is the topological diagram of the equivalent circuit of each distributed generation unit in the microgrid system under island operation mode.

[0054] Figure 5 This is a flowchart of step S3 provided in this application.

[0055] Figure 6 Figure (a) is a schematic diagram of the parallel resonance modal analysis of the microgrid system in the island operation mode in the first embodiment of the present application.

[0056] Figure 6Figure (b) is a schematic diagram of the series resonance modal analysis of a microgrid system in island operation mode consisting of two grid-following power generation units with the same control structure but different parameters.

[0057] Figure 7 A flow chart for locating the resonance center is provided for this application.

[0058] Figure 8 This is a flow chart for distinguishing resonance types provided by this application.

[0059] Figure 9 Figure (a) is a schematic diagram of the parallel resonant modal impedance curve of the microgrid system under the power generation unit combination in the second embodiment of the present application.

[0060] Figure 9 Figure (b) is a schematic diagram of the parallel resonant modal impedance curve of the microgrid system under the power generation unit combination in the second embodiment of the present application.

[0061] Figure 10 Figure (a) is a schematic diagram of the FFT analysis results of the voltage of the type A grid-following power generation unit (node ​​1) in combination d when harmonics are injected from the type A grid-following power generation unit in the second embodiment of the present application.

[0062] Figure 10 Figure (b) is a schematic diagram of the FFT analysis results of the voltage of the type B grid-following power generation unit (node ​​4) in combination d when harmonics are injected from the type A grid-following power generation unit in the second embodiment of the present application.

[0063] Figure 10 Figure (c) is a schematic diagram of the FFT analysis results of the voltage of the type C grid-following power generation unit (node ​​6) in combination d when harmonics are injected from the type A grid-following power generation unit in the second embodiment of the present application.

[0064] Figure 10 Figure (d) is a schematic diagram of the FFT analysis results of the voltage of the type D grid-following power generation unit (node ​​8) in combination d when harmonics are injected from the type A grid-following power generation unit in the second embodiment of the present application.

[0065] Figure 10 Figure (e) is a schematic diagram of the FFT analysis results of the common connection point PCC (node ​​10) voltage of combination d when harmonics are injected from a type A grid-following power generation unit in the second embodiment of the present application.

[0066] Figure 10 Figure (f) is a schematic diagram of the FFT analysis results of the voltage of the type A grid-following power generation unit (node ​​1) in combination d when harmonics are injected from the type B grid-following power generation unit in the second embodiment of the present application.

[0067] Figure 10Figure (g) is a schematic diagram of the FFT analysis results of the voltage of the type B grid-following power generation unit (node ​​4) in combination d when harmonics are injected from the type B grid-following power generation unit in the second embodiment of the present application.

[0068] Figure 10 Figure (h) is a schematic diagram of the FFT analysis results of the voltage of the type C grid-following power generation unit (node ​​6) in combination d when harmonics are injected from the type B grid-following power generation unit in the second embodiment of the present application.

[0069] Figure 10 Figure (i) is a schematic diagram of the FFT analysis results of the voltage of the type D grid-following power generation unit (node ​​8) in combination d when harmonics are injected from the type B grid-following power generation unit in the second embodiment of the present application.

[0070] Figure 10 Figure (j) is a schematic diagram of the FFT analysis results of the common connection point PCC (node ​​10) voltage of combination d when harmonics are injected from the type B grid-following power generation unit in the second embodiment of the present application. DETAILED DESCRIPTION

[0071] To make the purpose and technical solutions of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the described embodiments of 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.

[0072] This application provides a series-parallel resonance characteristic evaluation method for a microgrid system in island operation mode. Figure 1 The circuit topology of the microgrid system in island operation mode consists of m grid-following generation units and n grid-forming generation units. Each generation unit is connected to the common connection point PCC through line impedance. Specifically, the m grid-following generation units include grid-following generation units 111 to grid-following generation units 11m, and the n grid-forming generation units include grid-forming generation units 121 to grid-forming generation units 12n. Specifically, the grid-following generation units use photovoltaic power generation, while the grid-forming generation units use energy storage power generation. Please refer to Figure 2 , Figure 2 This is a flow chart of the series-parallel resonance characteristic evaluation method provided in this application. The series-parallel resonance characteristic evaluation method includes: steps S1 to S3.

[0073] Step S1: Analyze the interaction between the units of the microgrid system based on the transfer function method;

[0074] Step S2: Using the modal analysis method, perform series resonance modal analysis and parallel resonance modal analysis on the microgrid system to construct a modal analysis equivalent model;

[0075] Step S3: Extract the resonant modal information based on the modal analysis equivalent model; locate the resonance center based on the series-parallel modal analysis, and distinguish the resonance type by combining the transfer function method to realize the resonance characteristic evaluation.

[0076] In step S1, see Figure 2 In step S1, the interaction between the various units of the microgrid system is analyzed based on the transfer function method.

[0077] As an example, based on the operating characteristics of each power generation unit, the grid-following power generation unit is equivalent to a Norton circuit, and the grid-forming power generation unit is equivalent to a Thevenin circuit. According to the microgrid system topology and superposition theorem, the total output current I of m grid-following power generation units and n grid-forming power generation units connected in parallel to the common connection point is PCC Expressed as:

[0078]

[0079] Among them, Z g is the load impedance, Z ci and I ci are the equivalent impedance and equivalent current source of the i-th grid-following power generation unit, Z cli is the line impedance of the i-th grid-connected generating unit connected to the common connection point PCC; Z vi and U vi are the equivalent impedance and equivalent voltage source of the i-th grid-type power generation unit, Z vli is the line impedance of the i-th grid-connected generating unit connected to the common connection point PCC.

[0080] According to the power conversion theorem, the Thevenin equivalent circuit and the Norton equivalent circuit are essentially the same and can be converted into each other. To unify the representation, the voltage source, current source, equivalent impedance, and line impedance are treated as a unified representation as follows:

[0081]

[0082] Among them, I i is the current source of the i-th distributed generation unit in the microgrid system, Z i is the equivalent impedance of the i-th distributed generation unit in the microgrid system, Z li is the line impedance of the PCC connected to the i-th distributed generation unit in the microgrid system.

[0083] Furthermore, the total output current I PCC The expression is:

[0084]

[0085] Furthermore, according to the circuit structure of the microgrid system, the output current I of the tth distributed generation unit flowing into the common connection point PCC is PCCt It is expressed as follows:

[0086]

[0087] Among them, Y t is the equivalent impedance Z of the tth distributed generation unit in the microgrid system t The corresponding admittance is expressed as follows:

[0088] Y t =1 / Z t (1≤t≤m+n)

[0089] Y lt is the line impedance Z of the PCC point connected to the tth distributed generation unit in the microgrid system lt The corresponding admittance is expressed as follows:

[0090] Y lt =1 / Z lt (1≤t≤m+n)

[0091] U PCC is the voltage at the common connection point PCC, which is obtained based on Kirchhoff's voltage law and is expressed as follows:

[0092]

[0093] Among them, Y i is the equivalent impedance Z of the i-th distributed generation unit in the microgrid system i The corresponding admittance, Y li is the line impedance Z of the PCC point connected to the tth distributed generation unit in the microgrid system li The corresponding admittance of i ||Y li is the series admittance formula, expressed as: Y i ||Y li =Y i ·Y li / (Y i +Y li );Y g is the load admittance.

[0094] Furthermore, the voltage U at the common connection point PCC is PCC Substitute the output current I of the tth distributed generation unit flowing into the common connection point PCC PCCt In the expression of , it is expressed as follows:

[0095]

[0096] Among them, Y h Indicates Y lt / (Y t +Y lt ); According to the above formula, in the isolated operation mode of the microgrid system, the current injected into the common connection point PCC by the t-th distributed generation unit includes two parts: the equivalent output current of the t-th distributed generation unit itself and the interaction current generated by the coupling of other distributed generation units with the distributed generation unit of this application. PCCt Arrange the output current I of the tth distributed generation unit flowing into the common connection point PCC PCCt With the total output current I PCC For the parts with similar structures, the denominator structure is expressed as a closed-loop transfer function of a unit negative feedback system as much as possible, as follows:

[0097]

[0098] Furthermore, according to the physical meaning of each part, the output current I of the tth distributed generation unit flowing into the common connection point PCC is PCCtt The current is re-divided into two parts: the current injected into the common connection point PCC and the interaction current generated by coupling. The interaction current generated by coupling is further decomposed into two parts: the interaction current generated by the equivalent current source of the t-th distributed generation unit and the interaction current generated by the equivalent current sources of other generation units on this distributed generation unit.

[0099] Furthermore, the resonance types under the island operation of the microgrid system include: net current resonance injected into the common connection point PCC and interaction current resonance between distributed generation units; among them, the net current resonance injected into the common connection point PCC is system resonance, and the interaction current resonance between distributed generation units is local resonance.

[0100] As an example, the interaction between the various units of the microgrid system is analyzed based on the transfer function method, revealing the resonance mechanism and resonance characteristics of different resonance types.

[0101] In step S2, see Figure 2 In step S2, the modal analysis method is used to perform series resonance modal analysis and parallel resonance modal analysis on the microgrid system, and a modal analysis equivalent model is constructed.

[0102] Specifically, see Figure 3 , Figure 3Flowchart for constructing a modal analysis equivalent model. Constructing a modal analysis equivalent model includes the following steps:

[0103] Step S21: Setting the resonant frequency search range [f min , f max ] and frequency step Δf.

[0104] As an example, the search range of the resonance frequency f is set according to the requirements [f min , f max ] and frequency step Δf.

[0105] In one embodiment of the present application, the frequency step size Δf is 2 Hz, and the search range is 0-3000 Hz, which not only meets the accuracy data requirements but also does not take too long to solve.

[0106] Step S22: Perform parallel resonance modal analysis on the microgrid system and construct the node admittance matrix Y f .

[0107] As an example, to more comprehensively and effectively evaluate the resonant information of a microgrid system, modal analysis methods are first used to determine the singularities of the microgrid's node admittance matrix and loop impedance matrix based on the system's topological properties. This allows analysis of the possibility of structural problems in the microgrid system. Furthermore, modal analysis equivalent models are constructed at the node and loop levels of the microgrid system.

[0108] As an example, modal analysis includes the analysis of two different types of resonance: series resonance and parallel resonance. Parallel resonance is characterized by a small injection current at the parallel node causing an abnormal voltage rise. Figure 4 , Figure 4 The topology diagram of the equivalent circuit of each distributed power generation unit in the microgrid system under the isolated operation scenario is given. The equivalent circuit of each distributed power generation unit in the microgrid system under the isolated operation scenario includes m equivalent grid-following power generation units and n equivalent grid-forming power generation units. Specifically, the m equivalent grid-following power generation units include equivalent grid-following power generation units 211 to equivalent grid-following power generation units 21m, and the n equivalent grid-forming power generation units include equivalent grid-forming power generation units 221 to equivalent grid-forming power generation units 22n. The equivalent output nodes and common connection points PCC of each distributed power generation unit are numbered, where Z g Indicates the system load impedance. Figure 4 The elements of the equivalent circuit of each distributed generation unit in the microgrid system in the isolated operation scenario are uniformly processed and expressed as follows:

[0109]

[0110] Furthermore, the microgrid system node admittance matrix Y fIt is expressed as follows:

[0111]

[0112] Among them, each element in the node admittance matrix is ​​expressed as follows:

[0113]

[0114] Step S23: Perform series resonance modal analysis on the microgrid system and construct the loop impedance matrix Z L .

[0115] As an example, series resonance is a phenomenon in which a small excitation voltage in a series circuit can cause a significant amplification of the loop current. Figure 4 The elements of the equivalent circuit of each distributed generation unit in the microgrid system in the isolated operation scenario are uniformly processed and expressed as follows:

[0116]

[0117] Among them, U Li is the voltage source of the i-th distributed generation unit in the microgrid system.

[0118] Furthermore, the loop impedance matrix Z L It is expressed as follows:

[0119]

[0120] Among them, the loop impedance matrix Z L The meaning of each element is as follows:

[0121]

[0122] As an example, using the modal analysis method to perform series resonance and parallel resonance modal analysis on a microgrid system can comprehensively and effectively evaluate the resonance information of the microgrid system.

[0123] In step S3, see Figure 2 In step S3, the resonant modal information is extracted based on the modal analysis equivalent model. The resonance center is located based on the series-parallel modal analysis, and the transfer function method is used to distinguish the resonance type and realize the resonance characteristic evaluation.

[0124] Specifically, see Figure 5 , Figure 5 This is a flow chart of step S3, where step S3 includes the following steps:

[0125] Step S31: Determine whether the frequency f exceeds the resonant frequency search range [f min , f max], if not, go to step S32; if so, go to step S34.

[0126] Step S32: Calculate the node admittance matrix Y f And the loop impedance matrix Z L Perform eigenvalue decomposition.

[0127] As an example, in the parallel resonance modal analysis, the relationship between the equivalent output node voltage and the node injection current of each distributed power generation unit and the node admittance matrix Y f The following relationship is satisfied:

[0128]

[0129] Among them, [U1,U2,…,U m+n ,U pcc ] is the voltage of the output node, [I1,I2,…,I m+n ,0] is the current injected into each node.

[0130] As an example, assume that a microgrid system with m+n+1 nodes undergoes parallel resonance at frequency f, and the node voltage column vector is U f , the node injection current column vector is I f , the node admittance matrix of the microgrid system is Y f , node voltage column vector U f It is expressed as follows:

[0131]

[0132] As an example, the left and right eigenvector matrices are used to calculate the node admittance matrix Y of the microgrid system. f Perform eigenvalue decomposition, which is expressed as follows:

[0133]

[0134] Where x = m + n + 1; L f 、T f are the left and right eigenvector matrices respectively, satisfying: Λ f is the node admittance matrix Y f The diagonal matrix of eigenvalues.

[0135] Furthermore, the loop impedance matrix Z L Perform eigenvalue decomposition and assume that the microgrid system with m+n loops undergoes series resonance at frequency f, and the loop current column vector is I L , the loop electromotive force column vector is E L , the loop electromotive force column vector is E L It is expressed as follows:

[0136] E L =Z L I L

[0137] Among them, the loop current column vector I L =[I L1 ,I L2 ,…,I L(m+n) ] T , the loop electromotive force column vector E L =[U L1 -U L2 ,U L2 -U L3 ,…,U L(m+n ] T .

[0138] Furthermore, following the parallel resonance modal analysis of the node admittance matrix Y f Analysis of the loop impedance matrix Z L Perform eigenvalue decomposition to obtain the loop impedance matrix Z L The eigenvalue diagonal matrix Λ L , I will not go into details here.

[0139] Step S33: Calculate modal impedance and modal admittance

[0140] As an example, the node admittance matrix Y f Substitute the node voltage column vector U f In the equation, the node voltage column vector U f It is expressed as follows:

[0141]

[0142] As an example, define the modal voltage V f =T f U f , modal current J f =T f I f At this time, the node admittance matrix Y f Reduced to a matrix with only diagonal elements, the elements of which are defined as the modal impedances Modal voltage V f , modal current J f and modal impedance The one-to-one correspondence between them is expressed in matrix form as:

[0143]

[0144] As an example, the i-th modal voltage V fiOnly affected by the corresponding i-th modal current J fi Influence, the i-th modal impedance Considered as a proportional coefficient. When the node admittance matrix Y f With the smallest eigenvalue λ if When the corresponding i-th modal impedance Get the maximum value, at this time even if the i-th modal current J fi is very small, it will also stimulate a large modal voltage V fi Therefore, we select the minimum eigenvalue λ if is the key eigenvalue, and the modal impedance corresponding to the reciprocal of the key eigenvalue is is the dominant mode.

[0145] Furthermore, for the modal current J f Analysis, the i-th modal current J fi It is expressed as follows:

[0146] J fi =T fi1 I1+T fi2 I2+…T fi(m+n+1) I (m+n+1)

[0147] Among them, if the right eigenvector matrix T of the i-th row and the second column fi2 The value of is the largest, then when the node injects current of the same amplitude, node 2 has the largest effect on the modal current J fi The contribution of is the largest. If the right eigenvector matrix T in row i and column 1 fi1 The value of approaches or is equal to zero, then no matter how large the node injection current is at node 1, it cannot excite resonance. Therefore, the right eigenvector matrix T f Each element in represents the excitability of each node in the microgrid system to the modal current.

[0148] Furthermore, for the modal voltage V f Analysis, assuming that mode 1 resonates, the corresponding modal voltage V f1 The other modal voltages can be ignored. The observable node voltage column vector U f It is expressed as follows:

[0149]

[0150] Among them, in the first mode voltage V f1 In the case of the same size, if the left eigenvector matrix L of the ith row and second column is fi2 The value of is the largest, then the second node voltage U obtained after mapping f2 Therefore, the left eigenvector matrix L fEach element in represents the observability of the voltage of each node in the system. Since the left and right eigenvector matrices are inverse and transposed matrices of each other, the left eigenvector L f The observability reflected is related to the right eigenvector T f The reflected excitability is essentially the same. If a node shows the strongest excitability, the harmonic voltage amplitude of this node will reach the maximum and will be the easiest to detect.

[0151] As an example, assuming a parallel resonance occurs at frequency f, the dominant modal impedance is but Node voltage column vector U f It is expressed as follows:

[0152]

[0153] For simplicity, x = m + n + 1. It can be seen that the matrix diagonal elements achieve the combination of observability and excitability, so the matrix diagonal elements are defined as the dominant resonant mode impedance of the microgrid system node k. The participation factor PF (participation factor) is expressed as:

[0154] PF ki =L ki T ik

[0155] Among them, T ik is the element in the right eigenvector matrix of row i and column k, L ki is the element in the left eigenvector matrix at row k and column i, PF ki is the dominant resonant modal impedance of the k-pair microgrid system node The participation factor (PF) can be used to quantitatively analyze the contribution of each node in a microgrid system to a specific resonant mode, revealing the distribution characteristics of the resonance within the microgrid system. The PF directly reflects the node's contribution to the resonant mode, providing a theoretical basis for accurately locating the resonant source and, in turn, determining the primary excitation source and propagation path of the resonance.

[0156] Similarly, in the series resonance modal analysis, according to the loop impedance matrix Z L Calculating series modal admittance The participation factors PF of each circuit in the microgrid system will not be elaborated here.

[0157] Step S34: evaluating the resonance characteristics based on the series-parallel resonance modal analysis, locating the resonance center, and distinguishing the resonance type by combining the transfer function method.

[0158] Series resonance modal analysis and parallel resonance modal analysis are performed on the microgrid system in island operation mode to obtain their respective physical meanings. A mapping relationship between them and the resonance analysis results obtained based on the transfer function method is established. A complete method for evaluating the resonance characteristics of the microgrid system is constructed to obtain key information such as the resonance type, resonance frequency, and resonance center.

[0159] As an example, see Figure 6 , select type A grid-following power generation units and type B grid-following power generation units with the same control structure and different parameters to form a microgrid system in island operation mode, and perform series resonance modal analysis and parallel resonance modal analysis on the microgrid system. Set the number of type A grid-following power generation units to 2 and the number of type B grid-following power generation units to 1. Assuming that the line impedance from the output port of the two types of power generation units to the common connection point is equal, the parallel resonance modal analysis of the microgrid system is obtained as follows Figure 6 As shown in Figure (a), the series resonance modal analysis of the microgrid system is as follows: Figure 6 As shown in Figure (b), the contribution of each node to the dominant mode is further calculated as shown in Table 1, and the contribution of each loop to the dominant mode is shown in Table 2.

[0160] Table 1 Node participation factors of parallel resonance of microgrid system

[0161]

[0162] Table 2 Loop participation factors of series resonance of microgrid system

[0163]

[0164] The modal analysis results show that although the series resonance mode and the parallel resonance mode are consistent in resonant frequency, their resonant participating elements are not exactly the same. It is necessary to further identify the resonance type based on the similarities and differences in the resonance mechanism and participating elements between the series resonance mode and the parallel resonance mode.

[0165] Specifically, in the interactive current resonance type, the interactive current flows only between the generator units and does not flow through the branch containing the load impedance. Therefore, interactive current resonance is a local resonance. From the perspective of the microgrid system nodes, the interactive current flows from a generator unit node, passes through the line impedance to the common connection point, and then flows through the line impedance to the node of another generator unit. Therefore, the participation factor obtained when using parallel resonance modal analysis cannot accurately reflect the actual local resonance. From the perspective of the microgrid system loop, when a certain frequency resonance is dominated by the loop between two generator units, the interactive current flows only in that loop, and the loop containing the load impedance contributes minimally to the local resonance, resulting in it being unexcitable and unobservable. Further referring to Table 2, when using series resonance modal analysis, the participation factors of the load impedance loop are zero or close to zero for dominant modes 2 and 3. This indicates that mode 2 is excited by two Class A grid-following generator units and flows only in loop 1. Mode 3 is primarily excited by loop 2, where the Class A and Class B grid-following generator units are located, and is also influenced to some extent by loop 1.

[0166] In the resonance type of the net current injected into the common connection point PCC, the net current injected into the common connection point is generated by each power generation unit, and after being collected at the common connection point PCC, it is jointly injected into the branch where the load impedance is located. Therefore, the net current resonance belongs to the system resonance. From the perspective of the system node, the net current injected into the common connection point PCC flows through all nodes in the system. At this time, the parallel resonance mode participation factors are all non-zero values, which is similar to the performance results of the dominant interactive current resonance between non-similar power generation units, making it difficult to accurately distinguish the resonance type. From the perspective of the system loop, since the net current must flow through the load impedance, the loop where it is located shows strong excitability and observability for the system resonance. Please continue to refer to Table 2. When using series resonance mode analysis, the participation factor of the load impedance loop is non-zero under the dominant mode 1, indicating that the dominant mode 1 corresponds to the net current resonance injected into the common connection point PCC.

[0167] Specifically, the results of the parallel resonance modal analysis can be used to determine the optimal observation point and the highest excitation point of the microgrid system resonance by calculating the participation factor of each node, thereby locating the resonance center. However, the parallel resonance results cannot directly reflect the actual resonance of the microsystem. In the series resonance modal analysis, whether the participation factor of the loop where the load impedance is located in the dominant resonance mode is zero can be used as an important criterion for effectively distinguishing the resonance type. Therefore, the series-parallel resonance characteristic evaluation method proposed in this application includes: resonance type identification and resonance center positioning.

[0168] Specifically, see Figure 7 , Figure 7 The flowchart for locating the resonance center includes the following steps:

[0169] Step S341: Draw the modal impedance of the microgrid system Curve that changes with frequency f.

[0170] Step S342: Extract the resonant mode frequency f at the i-th resonant peak in the curve i .

[0171] Step S343: Calculate the participation factor PF of each node in the parallel resonance mode f , sorting the node resonance sensitivity.

[0172] Step S344: determining the resonance center of the microgrid system according to the participation factor of each node in the parallel resonance mode.

[0173] As an example, based on the participation factor PF f The analysis results can identify the nodes that have the most significant impact on the resonant mode. When the participation factor value of a node is the largest, it means that this node has the greatest impact on the impedance of the dominant resonant mode. The most significant influence is the best observation node and the highest excitation node of the microgrid system resonance, that is, the resonance center.

[0174] Specifically, see Figure 8 , Figure 8 A flowchart for distinguishing resonance types, which includes the following steps:

[0175] Step S345: Draw the microgrid system modal impedance Curve that changes with frequency f.

[0176] Step S346: Extract the resonant mode frequency f at the i-th resonant peak in the curve i .

[0177] Step S347: Calculate the participation factor PF of each loop in the series resonance mode L .

[0178] Step S348: Determine whether the participation factor of the load impedance loop is 0. If not, proceed to step S349; if so, proceed to step S3410.

[0179] Step S349: The resonance type of the microgrid system is: resonance of the net current injected into the common connection point PCC, that is, system resonance.

[0180] Step S3410: Determine whether the participation factor of the load impedance loop is 1. If not, proceed to step S3411; if so, proceed to step S3412.

[0181] Step S3411: The resonance type of the microgrid system is: heterogeneous or heterogeneous power generation units dominate the interactive current resonance, that is, local resonance.

[0182] Step S3412: The resonance type of the microgrid system is: the homogeneous and homogeneous power generation units dominate the interactive current resonance, that is, the local resonance.

[0183] In one embodiment of the present application, a verification method for a series-parallel resonant characteristic evaluation method is provided. A microgrid island operation model is constructed in the power system analysis software PSCAD / EMTDC. The influence of different power generation unit combinations on the system resonance characteristics in the microgrid island operation scenario is studied to prove the effectiveness of the series-parallel resonant characteristic evaluation method proposed in this application.

[0184] As an example, two types of grid-following and grid-connecting power generation units are selected to represent the actual photovoltaic and energy storage modules within a microgrid system. The parameters of the Type A and Type B grid-following power generation units remain unchanged. Meanwhile, the Type C and Type D grid-connecting power generation units, with the same structure but different parameters, are selected to represent the energy storage modules within the microgrid system. The specific parameters are shown in Table 3.

[0185] Table 3 Simulation setting parameters of Class C and Class D grid power generation units

[0186]

[0187]

[0188] Furthermore, the number of each type of power generation unit is changed, and the corresponding combinations are shown in Table 4.

[0189] Table 4 Combinations of different power generation units

[0190]

[0191] See also Figure 9 The parallel resonant modal impedance curve of the microgrid system under each power generation unit combination is as follows: Figure 9 As shown in Figure (a), the series resonant modal admittance curve of the microgrid system under each power generation unit combination is as follows Figure 9 As shown in Figure (b) in the figure, the af combination node participation factors of the microgrid system under each power generation unit combination are shown in Table 5, and the af combination loop participation factors of the microgrid system under each power generation unit combination are shown in Table 6.

[0192] Table 5 Node participation factors of microgrid system when the combination of power generation units changes

[0193]

[0194]

[0195] Table 6 Loop participation factors of the microgrid system when the combination of power generation units changes

[0196]

[0197]

[0198] Please continue to combine Figure 9 From Tables 5 and 6, we can see that the microgrid system with multiple heterogeneous power generation units connected has the following resonance characteristics:

[0199] 1) In the island operation scenario, the resonant modes of the microgrid system are dominated by the interaction between power generation units and belong to local resonance.

[0200] 2) The key resonant modes in the microgrid system exhibit multi-node coupling characteristics, and their formation requires the joint participation of two or more nodes. The independent action of a single node cannot achieve effective excitation of the resonant mode.

[0201] 3) When multiple grid-connected power generation units of the same type exist in a microgrid system, a dominant resonant mode is excited. This dominant resonant mode exhibits the following characteristics: First, its excitation source and affected entities are strictly limited to this type of grid-connected power generation unit. Second, because the generation mechanism of this resonant mode stems from the inherent characteristics of this type of grid-connected power generation unit, the corresponding resonant center frequency and resonant peak amplitude remain unchanged regardless of changes in the control structure or number of other power generation units in the microgrid system.

[0202] 4) The change in the number of grid-connected power generation units using the Vf control strategy does not change the number of key resonant modes in the microgrid system, and its impact is only limited to the central resonant frequency and amplitude of the lowest frequency resonant mode.

[0203] 5) In a microgrid system, the two resonant modes in the 800-1000Hz and 1500-1800Hz frequency bands are always excited by all nodes in the system. The lower-frequency resonant mode is dominated by Class B grid-following generation units, while the higher-frequency resonant mode is dominated by Class A grid-following generation units.

[0204] 6) For the two key resonant modes jointly influenced by all nodes, as the proportion of type A grid-following power generation units increases, the center frequency of the resonant modes decreases, the resonance peak value in the lower frequency band decreases, and the resonance peak value in the higher frequency band increases. As the proportion of type B grid-following power generation units increases, the peak value of the resonant modes decreases, the center frequency of the lower frequency band mode decreases, and the center frequency of the higher frequency band mode increases.

[0205] 7) For the same key resonant mode, the participation factors of similar power generation units are the same.

[0206] Furthermore, a microgrid model shown in combination d is constructed in the power system analysis software PSCAD / EMTDC, which includes two Class A grid-connected power generation units, one Class B grid-connected power generation unit, two Class C grid-connected power generation units, and one Class D grid-connected power generation unit. At this time, the microgrid system contains a total of 7 nodes.

[0207] See also Figure 10 On the basis of ensuring the stable operation of the microgrid system in the island operation mode, 17th, 23rd, 33rd and 36th harmonics are injected into the output ports of the type A grid-following power generation unit (nodes 1 and 2) and the type B grid-following power generation unit (node ​​4). Figure 10 Figure (a) shows the FFT analysis results of the voltage of the type A grid-following power generation unit (node ​​1) in combination d when harmonics are injected from the type A grid-following power generation unit. Figure 10 Figure (b) shows the FFT analysis results of the voltage of the type B grid-following power generation unit (node ​​4) in combination d when harmonics are injected from the type A grid-following power generation unit. Figure 10 Figure (c) shows the FFT analysis results of the voltage of the type C grid-following power generation unit (node ​​6) in combination d when harmonics are injected from the type A grid-following power generation unit. Figure 10 Figure (d) is a schematic diagram of the FFT analysis results of the voltage of the type D grid-following power generation unit (node ​​8) in combination d when harmonics are injected from the type A grid-following power generation unit. Figure 10 Figure (e) shows the FFT analysis results of the voltage at the PCC (node ​​10) of combination d when harmonics are injected from the grid-following generation unit of type A. Figure 10 Figure (f) is a schematic diagram of the FFT analysis results of the voltage of the type A grid-following power generation unit (node ​​1) in combination d when harmonics are injected from the type B grid-following power generation unit. Figure 10 Figure (g) is a schematic diagram of the FFT analysis results of the voltage of the type B grid-following power generation unit (node ​​4) in combination d when harmonics are injected from the type B grid-following power generation unit. Figure 10 Figure (h) is a schematic diagram of the FFT analysis results of the voltage of the type C grid-following power generation unit (node ​​6) in combination d when harmonics are injected from the type B grid-following power generation unit. Figure 10 Figure (i) shows the FFT analysis results of the voltage of the type D grid-following generation unit (node ​​8) in combination d when harmonics are injected from the type B grid-following generation unit. Figure 10 Figure (j) is a schematic diagram of the FFT analysis results of the common connection point PCC (node ​​10) voltage of combination d when harmonics are injected from the type B grid-following power generation unit.

[0208] Depend on Figure 10It can be seen that when harmonics of the corresponding resonant frequencies are injected into the output port of a Class A grid-following power generation unit, the 33rd and 36th resonant modes dominated by them exhibit the strongest excitation, as evidenced by the significantly higher 33rd and 36th harmonic content at node 1 compared to other nodes. Furthermore, node 4 exhibits the strongest observability for the 17th resonant mode compared to other nodes. This is because, while node 1 has some excitation for the 17th harmonic, node 4 dominates this harmonic, resulting in a slightly higher harmonic content than node 1.

[0209] When harmonics are injected into the Class B grid-following generation unit, the 17th and 23rd order resonant modes dominated by them exhibit the strongest excitation, as evidenced by significantly higher 17th and 23rd order harmonic content at node 2 compared to other nodes. Furthermore, because node 4's participation factor in the 33rd order resonant mode is close to 0 and it does not participate in the 36th order resonant mode at all, the 33rd and 36th order resonant modes are not effectively excited.

[0210] In summary, the verification results of the series-parallel resonance characteristic evaluation method are consistent with the theoretical analysis results of the resonant mode, verifying the effectiveness and accuracy of the series-parallel resonance characteristic evaluation method proposed in this application.

[0211] The present application also provides a series-parallel resonance characteristic evaluation system, comprising: one or more processors; a storage device for storing one or more programs. When the one or more programs are executed by the one or more processors, the one or more processors implement the above-mentioned series-parallel resonance characteristic evaluation method.

[0212] The series-parallel resonance characteristic evaluation method and system proposed in this application can accurately obtain the resonance type and resonance center of the microgrid system in the island operation mode, and extract key information such as key resonance modes and node participation factors, providing a theoretical basis for subsequent resonance suppression.

[0213] While the present invention has been described above with reference to preferred embodiments, this is not intended to limit the present invention. Persons skilled in the art will readily appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

[0214] Although the present application has been disclosed above with reference to the embodiments, they are not intended to limit the present application. Anyone with ordinary knowledge in the technical field may make slight changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be determined by the scope of the appended patent application.

Claims

1. A method for evaluating series-parallel resonance characteristics, characterized in that: include, Analyze the interaction between the various units of the microgrid system based on the transfer function method; Using modal analysis method, the series resonance modal analysis and parallel resonance modal analysis of the microgrid system are carried out, and the modal analysis equivalent model is constructed; According to the modal analysis equivalent model, the resonant modal information is extracted; based on the series-parallel modal analysis, the resonance center is located, and the transfer function method is combined to distinguish the resonance type and realize the resonance characteristic evaluation.

2. The series-parallel resonance characteristic evaluation method according to claim 1, wherein: The construction of the modal analysis equivalent model includes: Set the resonant frequency search range and frequency step; Perform parallel resonance modal analysis on the microgrid system and construct the node admittance matrix; Perform series resonance modal analysis on the microgrid system and construct the loop impedance matrix.

3. The series-parallel resonance characteristic evaluation method according to claim 2, wherein: The node admittance matrix is ​​expressed as follows: Among them, each element in the node admittance matrix is ​​expressed as follows: Among them, Y i is the equivalent impedance Z of the i-th distributed generation unit in the microgrid system i The corresponding admittance, Y li is the line impedance Z of the PCC point connected to the tth distributed generation unit in the microgrid system li The corresponding admittance of g is the load admittance; The loop impedance matrix is ​​expressed as follows: The meaning of each element in the loop impedance matrix is ​​as follows: Among them, Z i is the equivalent impedance of the i-th distributed generation unit in the microgrid system, Z li is the line impedance of the i-th distributed generation unit connected to the common connection point PCC in the microgrid system, Z g is the system load impedance.

4. The series-parallel resonance characteristic evaluation method according to claim 3, wherein: The method extracts the resonance modal information based on the modal analysis equivalent model; locates the resonance center based on the series-parallel modal analysis, and distinguishes the resonance type by combining the transfer function method to realize the resonance characteristic evaluation, which specifically includes: Step S31: Determine whether the frequency exceeds the resonant frequency search range. If not, proceed to step S32; if yes, proceed to step S34; Step S32: performing eigenvalue decomposition on the node admittance matrix and the loop impedance matrix; Step S33: Calculating modal impedance and modal admittance; Step S34: evaluating the resonance characteristics based on the series-parallel resonance modal analysis, locating the resonance center, and distinguishing the resonance type by combining the transfer function method.

5. The series-parallel resonance characteristic evaluation method according to claim 4, wherein: The left and right eigenvector matrices are used to perform eigenvalue decomposition on the node admittance matrix and loop impedance matrix of the microgrid system.

6. The series-parallel resonance characteristic evaluation method according to claim 4, wherein: Substituting the node admittance matrix into the node voltage column vector, the node admittance matrix is ​​simplified to a matrix having only diagonal elements, and the elements of the matrix are defined as modal impedances; Substituting the loop impedance matrix into the loop electromotive force column vector, the loop impedance matrix is ​​simplified to a matrix having only diagonal elements, and the elements of the matrix are defined as modal admittance.

7. The series-parallel resonance characteristic evaluation method according to claim 6, wherein: Locating the resonance center includes: Draw a curve showing the modal impedance of the microgrid system changing with frequency; Extract the resonant mode frequency at each resonant peak in the curve; Calculate the participation factor of each node in the parallel resonance mode and rank the node resonance sensitivity; The resonance center of the microgrid system is determined according to the participation factor of each node in the parallel resonance mode.

8. The series-parallel resonance characteristic evaluation method according to claim 7, wherein: When the participation factor value is maximum, the node resonance center of the participation factor.

9. The series-parallel resonance characteristic evaluation method according to claim 6, wherein: Identify the resonance types including: Step S345: drawing a curve showing the change of modal impedance of the microgrid system with frequency; Step S346: extracting the resonant mode frequency at each resonant peak in the curve; Step S347: Calculate the participation factor of each loop in the series resonance mode; Step S348: Determine whether the participation factor of the load impedance loop is 0. If not, proceed to step S349; if so, proceed to step S3410; Step S349: The resonance type of the microgrid system is: resonance of the net current injected into the common connection point PCC, i.e., system resonance; Step S3410: Determine whether the participation factor of the load impedance loop is 1. If not, proceed to step S3411; if so, proceed to step S3412; Step S3411: The resonance type of the microgrid system is: heterogeneous or heterogeneous power generation units dominate the interactive current resonance, that is, local resonance; Step S3412: The resonance type of the microgrid system is: the homogeneous and homogeneous power generation units dominate the interactive current resonance, that is, the local resonance.

10. A series-parallel resonance characteristic evaluation system, characterized in that: include: one or more processors; A storage device for storing one or more programs. When the one or more programs are executed by the one or more processors, the one or more processors implement the series-parallel resonance characteristic evaluation method according to any one of claims 1 to 9.

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